2: A Brief Review of Clinical Neuroanatomy

To interpret the neurologic examination (NE), you must have a firm grasp of basic neuroanatomy. If you just groaned, let us distinguish two types of neuroanatomy: tongue tip neuroanatomy is used on the spot in the clinic, and fingertip neuroanatomy is researched as the need arises. This chapter clearly separates the two.

A. Two main parts of the nervous system

The nervous system consists of the central nervous system and the peripheral nervous system.

1. The central nervous system (= CNS = neuraxis) consists of the brain and spinal cord. See Fig. 2-1.

2. The peripheral nervous system (PNS) consists of:

   1. Cranial nerves (CrNs) and their ganglia

   2. Spinal nerves and their ganglia and plexuses

   3. Autonomic nerves and their ganglia, plexuses, and nerves

3. To cleanly separate the PNS from the CNS, simply snip all cranial and spinal nerves at their attachment sites to the brain and spinal cord.

B. Cutting the neuraxis into its gross subdivisions

1. To separate the brain from the spinal cord: Locate the foramen magnum shown in Fig. 2-1 and sweep a scalpel transversely across the neuraxis at the level of the foramen magnum. The plane of that cut defines the medullocervical junction.

   1. The brain is rostral to the cut.

   2. The spinal cord is caudal to the cut.

2. Definition of the brain: The whole brain consists of the brainstem, cerebellum, diencephalon, and the cerebral hemispheres. Identify them in Fig. 2-1.

3. To separate the brainstem from the rest of the brain, make the following cuts:

   1. Transect the neuraxis just as the midbrain (= mesencephalon) emerges from the base of the cerebrum (Fig. 2-1). (Technically the plane of transection runs from the postmammillary sulcus to the posterior commissure.) The rostral cut surface of the midbrain abuts on the caudal cut surface of the diencephalon (Fig. 2-1).

   2. Then, to separate the cerebellum from the brainstem, slice through the cerebellar peduncles that attach it to the pons (Figs. 2-1 and 8-3). The brainstem remains.

   3. To separate the brainstem into its three parts, the midbrain, pons, and medulla, make transverse cuts at the midbrain-pontine junction and at the pontomedullary junction (Figs. 2-1 and 2-20).

   4. The diencephalon and cerebrum remain.

4. To separate the diencephalon from the cerebrum, core it out with a wire loop: The basal ganglia and deep cerebral white matter surround the diencephalon. To detach these structures from the cerebrum, insert a wire loop from the base of the cerebrum, where the diencephalon presents as the hypothalamus with its mammillary bodies, infundibulum, and optic chiasm (Fig. 2-20). After coring out the diencephalon, the cerebrum remains.

5. Definition of the cerebrum: The cerebrum consists of two cerebral hemispheres. Sections of each hemisphere show that the cerebrum consists of:

   1. Cortex, a surface coating of alternating layers of neuronal perikarya and neuronal process (Fig. 13-10).

   2. Deep white matter, consisting of nerve fibers (Fig. 13-10).

   3. Basal ganglia (basal nuclei), comprises the corpus striatum [caudate nucleus and putamen], globus pallidus, nucleus accumbens, substantia nigra, and subthalamic nucleus, consisting of masses of neurons.

   4. Lateral ventricles, consisting of cavities filled with spinal fluid (Figs. 13-1 and 13-10).

6. Learn to reproduce Fig. 2-1. It is core knowledge.

C. Fissures and sulci of the cerebrum

The cerebral surface displays two types of crevices: fissures and sulci.

1. Landmark fissures include:

   1. The interhemispheric (longitudinal) fissure that separates the two cerebral hemispheres (Fig. 13-7).

   2. The Sylvian or lateral fissure. Its anterior part separates the temporal from the frontal lobe, and its posterior part separates the temporal from the parietal lobe (Fig. 2-2A).

2. Landmark sulci include:

   1. The central sulcus that separates the frontal and parietal lobes laterally (Fig. 2-2A).

   2. The parieto-occipital sulcus that separates the parietal and occipital lobes medially (Fig. 2-2B).

   3. The calcarine sulcus that divides the occipital lobe into superior and inferior halves (Fig. 2-2B).

D. Boundaries of the cerebral lobes

1. Each hemisphere has four major lobes, frontal, parietal, temporal, and occipital, so named because anatomists had already given these names to the overlying bones of the skull.

2. Some lobar boundaries are natural landmarks, others are arbitrary lines. Learn to draw Figs. 2-2A and 2-2B. Then test yourself with frames 3 to 12.

3. The plane of the central sulcus divides the posterior margin of the ______________________________ lobe from the anterior margin of the _____________________________ lobe.

   frontal; parietal

4. Notice in Fig. 2-2A that the hemisphere forms a U-shape around the Sylvian fissure, with the frontal and parietal lobes above the fissure and the temporal below. The Sylvian fissure extends anteriorly and posteriorly to the plane of the central sulcus (Fig. 2-2A). The anterior part of the Sylvian fissure divides the ___________________ lobe above from the ___________________ lobe below.

   frontal; temporal

5. The posterior part of the Sylvian fissure divides the ___________________ lobe above from the ___________________ below.

   parietal; temporal

6. Laterally, the occipital lobe abuts superiorly and anteriorly on the ___________________ lobe and inferiorly and anteriorly on the ___________________ lobe.

   parietal; temporal

7. Laterally, the line that divides the occipital lobe from the parietal and temporal lobes extends from the _______________________________ notch to the ___________________ notch.

   superior preoccipital; inferior preoccipital

8. Taking the midpoint of the line connecting the preoccipital notches, how do you divide the parietal from the temporal lobe? __________________________________________

   _____________________________________________.

   Draw a perpendicular line until it meets the Sylvian fissure

9. Medially, the occipital lobe is divided from the parietal lobe by a natural boundary, the ___________________ sulcus.

   parieto-occipital

10. Describe how to divide the occipital from the temporal lobes on the medial aspect of the hemisphere. ____________________________________

    _______________________________________________.

    Draw a line to connect the inferior preoccipital notch to the junction of the calcarine and parieto-occipital fissures (Fig. 2-2C)

11. Mnemonic: By knowing all the boundaries of just one lobe, you can fairly well define the boundaries of all four major lobes. The keystone lobe is the ___________________ lobe.

    parietal

12. Draw medial and lateral views of the cerebrum and demarcate and label the major fissures, sulci, and lobes. Compare your drawing with Figs.2-2A and 2-2B.

E. The olfactory and limbic lobes

1. Previous neuroanatomists have recognized an olfactory lobe, consisting of the olfactory bulbs and tracts and their immediate cortex. See the olfactory lobe (in black) shown in Fig. 2-3. This lobe contains original neural tissue that unites the hemispheres across the midline, forming the median stalk from which the two hemispheres expand outward.

2. From the medial parts of the four traditional lobes, Paul Broca (1824–1880) separated another lobe, the limbic lobe (Fig. 2-3).

3. Notice in Fig. 2-3 that the limbic lobe (stippled) forms a concentric ring around the inner ring formed by the olfactory lobe (in black). The general term for such an anatomic junction, as of the cornea with the sclera, is limbus, hence the term limbic lobe.

Ring mnemonic for all cerebral lobes: The parietal lobe is the keystone lobe. If you understand the U- or C-shape of the cerebral hemisphere around the Sylvian fissure and can draw the boundaries of the parietal lobe, you will know the boundaries of the frontal, parietal, occipital, and temporal lobes. The limbic lobe, or limbic ring, attaches the four lobes to the hemispheric hilus. The olfactory lobe comprises the hilus. The olfactory bulbs extend forward from the ring of the olfactory lobe (Fig. 2-3).

F. Functional localization in the cerebral cortex

Various areas of the cortex serve different functions. The cortex contains somatomotor areas, sensory receptive areas, association areas, and limbic areas.

1. Somatomotor cortex

   1. The primary somatomotor cortex occupies the precentral gyrus, located just in front of the central sulcus (Fig. 2-2C). Secondary and supplementary motor areas exist also.

   2. The somatomotor cortex originates (gives rise to) the pyramidal tract, the pathway that mediates volitional movements. Pyramidal tract axons also arise in the postcentral gyrus (Russell and DeMyer, 1961).

2. Sensory receptive cortex: somatosensory, auditory, and visual.

   1. The somatosensory receptive cortex occupies the postcentral gyrus, just behind the central sulcus (Fig. 2-2C).

      1. It receives the somatosensory pathways from the thalamus (diencephalon). This cortex receives the general somatosensory pathways that result in conscious appreciation: touch, pain, temperature, and the discriminative senses, such as position sense and stereognosis.

      2. The somatomotor and somatosensory cortices represent the body parts upside down (Fig. 2-2C).

   2. The auditory receptive cortex consists of transverse temporal gyri in the hidden part of the temporal lobe that forms the floor of the Sylvian fissure, buried under the parietal lobe (Fig. 2-2C). It receives the auditory pathway from the medial geniculate body of the thalamus.

   3. The visual receptive cortex occupies the superior and inferior banks of the calcarine sulcus. It receives the visual pathway from the lateral geniculate body of the thalamus.

3. Association cortex (the blank cortex in Fig. 2-2C) receives and sends pathways to and from these primary receptive sensory cortices, associating the primary sensory information with its meaning and with motor responses.

4. Limbic cortex mediates emotions and their expression in autonomic and endocrine activities and overall behavior. Phylogenetically, the limbic lobe is derived from the olfactory system, with which it retains close anatomic connections, including the amygdala, hippocampal formation, temporal lobe, and hypothalamus (Gloor, 1997).

   1. Hippocampal, thalamic, and limbic circuits also mediate recent memory. Diseases such as herpes simplex encephalitis and other limbic encephalopathies (paraneoplastic, antibody mediated, etc.) that attack the temporal lobe impair emotional expression and recent memory, as do diffuse cerebral diseases.

   2. The rabies virus strongly attacks the hippocampus. The patient (Pt) suffers hydrophobia in association with the throat spasms of the disease.

   3. Deep midline neoplasms, such as gliomas of the septum pellucidum, hippocampus-fornix, corpus callosum, and adjacent limbic lobe, also frequently cause changes in emotionality and loss of recent memory.

BIBLIOGRAPHY · Gross Subdivisions of the Neuraxis

A. Definition of a neuron

Neurons are the parenchymal cells of the nervous system. Each neuron consists of a nucleus (karyon), a cell body (perikaryon), and one or more branches.

1. Most neurons typically display two types of branches, dendrites and axons (Fig. 2-4).

   1. Dendrites convey nerve impulses toward the perikaryon.

   2. Axons convey nerve impulses away from the perikaryon.

2. Some branches of some neurons (amacrine neurons) defy simplistic classification as axons and dendrites.

B. Function of neurons

1. The basic function of neurons is communication. When stimulated, neurons produce nerve impulses that communicate with other neurons, or effector cells (gland cells or muscle cells), to activate or inhibit them.

2. The nerve impulse can be electronically recorded and measured as an action potential. The action potential, a wave of electrical depolarization, propagates along the surface of the neuronal membrane to end feet at the axonal tips. The end feet form synapses on a dendrite, perikaryon, or an axon of one or more neurons, or on an effector cell. A synapse consists of:

   1. A presynaptic membrane provided by an axonal end foot

   2. A synaptic cleft

   3. A postsynaptic membrane of a neuron or effector cell

3. The branching of dendrites increases the surface area of any given neuron to receive larger numbers of synapses. The branching of axons increases the number of synapses a given neuron can disperse to subsequent neurons or effector cells.

4. At a synapse, the axonal tip or end foot releases a chemical called a neurotransmitter. The neurotransmitter crosses the synaptic cleft to attach to receptor sites on the postsynaptic membrane that alters its electrical polarization.

   1. Excitatory neurotransmitters depolarize the postsynaptic cell, thereby promoting impulse generation.

   2. Inhibitory transmitters hyperpolarize the postsynaptic cell, thereby opposing impulse generation.

5. The simplest communication loop, called a reflex arc, involves an afferent neuron that responds to a stimulus, one central synapse that fires an efferent neuron that then activates an effector.

6. A synapse acts as a one-way valve. At a synapse impulses flow from the incoming axon to the next neuron. In Fig. 2-5, the flow of impulses is □XWY/□WXY/□YXW.

   ☑ WXY

C. The six tenets of the neuron doctrine

The doctrine asserts that the neuron is the anatomic, functional, directional, genetic, pathologic, and regenerative unit of the nervous system (Shepherd, 1991).

1. Each neuron is an anatomic unit. A continuous surface membrane separates each neuron from all other cells.

2. Each neuron is a functional unit. The neuron is the smallest unit capable of receiving, generating, and transmitting nerve impulses.

3. Each neuron is a directional unit. In general, a neuron in a circuit conducts impulses in only one direction, from dendrite to axonal tip (amacrine cells excepted).

4. Each neuron is a genetic unit. Each neuroblast develops by mitosis from a primitive precursor cell; neuroblasts then differentiate into populations of specific neurons that differ in structure, biochemistry, and connections.

5. Each neuron is a pathologic unit:

   1. Any part of a neuron separated from its perikaryon dies, although the rest of the cell may live. If the perikaryon itself dies, all branches die.

   2. Although neurons live or die as individuals, they exist as populations or species. Because of genetic differences in biochemistry and structure, the various species of neurons differ in susceptibility to disease. A pathogen such as a genetic defect, virus, or toxin may affect only one susceptible neuronal species, whereas neighboring species survive. If the pathogen affects retinal neurons, the Pt becomes blind; if it affects auditory neurons, the Pt becomes deaf.

   3. This differential susceptibility leads to an almost endless number of genetic or toxic diseases that cause systematized degeneration of specific types of neurons.

6. Each neuron is a regenerative unit. Most mature neurons cannot multiply, but some can regenerate axons.

   1. After transection of an axon, the distal part, severed from its perikaryon dies, a process called Wallerian degeneration.

   2. The surviving neuronal perikaryon of a peripheral axon may regenerate its severed axon. Peripheral nerves therefore can effectively and functionally regenerate, but effective axonal regeneration of severed major tracts of CNS axons in humans apparently does not occur. The promotion of axonal and neuronal regeneration ranks as one of the most important areas of research (Mehler and Kessler, 1999; Ochs, 1977; Tuszynski and Kordower, 1998).

7. Review Section II C until you can recite the six tenets of the neuron doctrine.

BIBLIOGRAPHY · Neuron Doctrine, Degeneration, and Regeneration

How to get to the clinic through the muck and slime of phylogenesis and a little help from set theory.

A. Somites and the segmental level of the neuraxis

1. During ontogeny and phylogeny, paired tissue masses, somites, develop along each side of the neuraxis, up to the midbrain level (Fig. 2-6A).

2. The spinal cord and brainstem, the parts of the neuraxis that send nerves to the somites or segments, constitute the somite or segmental level of the nervous system. The rostralmost nerve to a somite, CrN III, attaches to the midbrain. It is also the rostralmost motor CrN.

3. The diencephalon and cerebrum are the suprasegmental levels of the neuraxis.

4. The primitive cells of each somite differentiate into a dermatome, myotome, and sclerotome. The dermatome spreads out under the epidermis to form the dermis, the myotome differentiates into muscle, and the sclerotome becomes bone. These tissues form the extremities and parietes, or wall, of the body.

B. Spinal nerves and the theory of nerve components

1. Each pair of somites receives a pair of spinal nerves and a pair of spinal arteries. Autonomic branches of the spinal nerves innervate the underlying viscera. The spinal cord innervates approximately 30 pairs of somites through 30 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral.

2. A dorsal root and a ventral root unite to form each spinal nerve (Figs. 2-6 and 2-7).

3. The theory of nerve components states that the nerve roots and peripheral nerves convey afferent and efferent axons of specific functional types.

   1. The law of Bell and Magendie states that the dorsal roots convey sensory axons and the ventral roots convey motor axons. The internal plan of the gray matter of the segmental nervous system reflects this sensorimotor dichotomy because the dorsal horns are mainly sensory and the ventral are mainly motor.

   2. Notice that perikarya in dorsal root ganglia provide the axons of the somatosensory afferents (GSAs) from the dermatome, myotome, and sclerotome and viscerosensory afferents (GVAs) from the viscera (Fig. 2-7).

   3. Notice that perikarya in the ventral horns of the spinal gray matter provide somatic efferent (GSE) axons. GSE axons innervate the striated muscle derived from the myotomes.

   4. Notice that perikarya in the intermediolateral horn (Fig. 2-12A) provide general visceral efferent (GVE) axons to innervate glands and the smooth muscle of the viscera (via a synapse in an outlying ganglion). Virtually all spinal nerves contain all four components: GSA, GVA, GVE, and GSE.

4. Figure 2-7 shows that the GSA or GVA axons can synapse monosynaptically on their respective GSE or GVE motor neurons. Read all circuit diagrams by tracing along the course of the nerve impulses. In Fig. 2-7, start at a receptor ending, e.g., the GSA receptor. Trace the impulse centrally past the dorsal root ganglion to the motoneuron. Excitation of the motoneuron then causes contraction of the skeletal muscle fibers innervated by the GSE axon.

5. Draw and label a cross section of the spinal cord showing the dorsal and ventral horns and roots and their functional nerve components. By means of arrows, indicate the course of impulse flow, beginning at a receptor and ending at an effector. Check your drawing against Fig. 2-7.

C. Three functional types of neurons: afferent (sensory), efferent (motor), and internuncial

1. The perikaryon of all afferent neurons, except for the retina, occupy dorsal root ganglia of the spinal nerves or their counterpart ganglia on the CrNs, including the trigeminal ganglion, cochlear, and vestibular ganglia, genicu-late ganglion, olfactory ganglion, and, by analogy, the retina.

   Rule for the origin of afferent systems: All afferent systems, including smell but not vision, originate in nodules of neurons called sensory ganglia, located outside the CNS on a CrN or spinal nerve.

   1. Dispersion of afferent axons: The proximal branches of the perikarya in dorsal root ganglia enter the CNS. After entering the CNS, the afferent axons may synapse (Fig. 2-8):

      1. At the level of entry

      2. After ascending or descending

      3. After decussating

   2. The afferent axon may synapse on efferent neurons monosynaptically (Fig. 2-7) or on one or more internuncial neurons (Fig. 2-8). The number of internuncials between afferent axons and efferent neurons varies from none to countless.

2. Efferent neurons (= motoneurons = lower motoneurons)

   1. Consist of somatic and autonomic lower motoneurons (LMNs). The brainstem also contains a group of branchial efferent (BE) lower motoneurons.

   2. The perikarya of these LMNs is always within the CNS, but their axons exit into the PNS to reach their effectors.

3. The internuncial or intercalary neurons interpose themselves between the afferent and efferent pools (Fig. 2-8).

   1. Because internuncials remain entirely within the CNS and do not enter peripheral nerves, the theory of nerve components excludes them.

   2. Internuncial neurons comprise the vast majority of all CNS neurons. They provide polysynaptic circuits of greater complexity and plasticity than do the direct monosynaptic reflex arcs that can produce only simple muscular twitches (Figs. 2-8 and 7-4).

D. Somite migrations and the law of original innervation

1. The phylogenetic specializations leading to limb and head development obscure the simple serial arrangement of the somites. The proximal portions of the dermatomes attenuate as they extend into the limbs. They are stretched so thin that C4 abuts on T2 in the final state of the dermatomes (Figs. 2-9 and 2-10).

2. Notice that dermatome C7 extends to the middle finger and divides dermatomes C5 and C6 from C8 through T1. C7 then can be regarded as the "axis" of symmetry of the brachial plexus (DeMyer, 1998). Learn this fact and the following mnemonic. The information is valuable at the bedside.

   Mnemonic for dermatomal distributions (Fig. 2-10):

   1. C2 abuts on the trigeminal nerve at the interaural line, a "hood" distribution. C1 does not have a sensory branch.

   2. C3 and C4 cover the shoulders in a "cape" distribution, where a cape would rest. Therefore, remember that C2, C3, and C4 have a "hooded cape" distribution.

   3. C5 through T1: Remember the C7 mnemonic. March down the superior aspect of the arm for C5 and C6 to C7. Then march back up the inferior aspect from C7 to C8, T1, and T2.

   4. T4: the nipple level.

   5. T10: the umbilicus

   6. L1: the groin.

   7. L5: the large toe (L = large toe).

   8. S1: the small toe (S = small toe).

3. The myotomes and sclerotomes also migrate. For example, the myotomes of cervical somites 3, 4, and 5 combine to produce the diaphragm. They migrate to the level of T12 and drag their somite nerves, united as the phrenic nerves, along with them from the cervical region. Many of the extremity muscles also contain contributions from more than one myotome. Table 2-1 lists the segmental innervations of the skeletal muscles, and Table 2-2 lists the segmental levels of some clinically important reflexes. Use these tables for reference, as needed.

4. The law of original innervation: Wherever a somite derivative migrates, it retains its original somite nerve. Even when a muscle or bone receives contributions from several somites, each spinal nerve continues to innervate only the tissue that derived directly from its original somite. Only the thoracicoabdominal wall retains the nerves, muscles, ribs, and intercostal vessels in their primordial somite sequences, without migrating, because no limbs develop (Fig. 2-10).

5. Plexuses versus peripheral nerve distributions

   1. To reach their original somites, the axons of the spinal nerves may detour in the cervical, brachial, and lumbosacral plexuses to share peripheral nerves to the limbs.

   2. No plexuses develop in the thoracicoabdominal region because of the lack of limbs.

   3. The peripheral nerves distal to the plexuses have dorsal and ventral root axons from more than one spinal cord segment. A lesion in or distal to a plexus causes deficits that exceed the innervation field of one somite.

   4. By mapping the patterns of sensory or motor loss and comparing them with those shown in Figs. 2-10 and 2-11, the examiner (Ex) can decide whether the Pt suffers from a dorsal root (dermatomal), plexus, or a peripheral nerve lesion.

E. Segmental composition and motor distribution of the major peripheral nerves from the spinal plexuses

Note: Study Sections E-1 to E-3 but learn only the principal movements served by the major peripheral nerves. This material is not programmed, but you need to know it.

1. Motor distribution of the cervical plexus: Roots C1 to C4.

   1. This plexus innervates:

      1. The neck muscles that turn the head and open the jaw

      2. The trapezius muscle in conjunction with CrN XI

      3. The diaphragm

   2. The phrenic nerve is the single most important nerve in the body because it innervates the diaphragm. It arises from C3 through C5. C4 usually supplies most of its axons. The nerve may be pre fixed (C2–C4) or post fixed (C4–C6).

2. Motor distribution of the extremity nerves of the brachial plexus (roots C5 to T1): This plexus innervates a number of proximal muscles of the shoulder girdle and issues five major terminal nerves for the arms, as follows:

   1. The axillary or circumflex nerve (C5–C6) is the elevator nerve of the arm because it innervates the deltoid muscle.

   2. The musculocutaneous nerve (C5–C7) is the flexor nerve of the arm. It flexes the brachium on the shoulder and the forearm on the brachium. The only arm flexor muscle not innervated by the musculocutaneous nerve is the brachioradialis muscle, which is innervated anomalously by the radial nerve, an extensor nerve.

   3. The radial nerve (C5–C8) is the extensor nerve of the arm (elbow, wrist, and fingers), but it does not extend the distal phalanges. It also supplies the brachioradialis, which flexes the elbow.

   4. Recap: Can you name the elevator nerve of the arm, the flexor nerve of the elbow, and the extensor nerve of the elbow, wrist, and fingers?

   5. The median nerve (C6–C8) is a flexor nerve of the wrist. It innervates the long forearm muscles that flex the wrist and the fingers, and inner-vates five intrinsic hand muscles. See the LLOAF/2 mnemonic below.

   6. The ulnar nerve (C7 and C8) is a flexor nerve of the fingers

      1. In the forearm, the ulnar nerve innervates the flexor carpi ulnaris exclusively and, with the median nerve, innervates long flexors of the wrist.

      2. In the hand, the ulnar nerve innervates all interossei, lumbricals to digits 4 and 5, and the adductor pollicis.

3. The muscles acting on the digits of the hand

   1. The lumbrical muscles, innervated by median and ulnar nerves, flex the proximal phalanges and extend the distal phalanges.

   2. The interossei, innervated by the ulnar nerve, flex the proximal phalanges, extend the distal phalanges, and waggle the fingers laterally (adduct and abduct the fingers).

   3. The thumb action tests the three motor nerves that control the digits:

      1. Radial: extension of the thumb

      2. Median: abduction of the thumb and opposition of the thumb to the little finger

      3. Ulnar: adduction of the thumb

      4. Median and ulnar: flexor brevis of the thumb

      5. Chapter 7 describes clinical testing of these actions

4. Here is a mnemonic summary, if you care to learn it.

   Mnemonic for the motor innervation of the major brachial plexus nerves (formed by the union of ventral rami of spinal nerves C5–T1):

   Axillary (circumflex) C5–C6: arm elevation (deltoid).

   Musculocutaneous C5–C7: flexor nerve of the shoulder and elbow (except for the brachioradialis muscle, which is innervated by the radial nerve).

   Radial C5–C8: extensor nerve of the elbow, wrists, and fingers, except the distal phalanges, which are extended by the intrinsic hand muscles.

   Median C5–T1: flexors of the wrist; five intrinsic muscles of the hand: three thenar and lateral two lumbrical muscles. LLOAF/2 (mnemonic within a mnemonic): LL = Lateral two Lumbricals, O = Opponens pollicis, A = Abductor pollicis brevis, F = half of the Flexor pollicis brevis.

   Ulnar C7–T8–T1: flexor carpi ulnaris and all intrinsic muscles of the hand except for the opponens pollicis, abductor pollicis brevis, and half of the flexor pollicis brevis; ulnar intrinsics (the interossei, medial two lumbricals) flex metacarpophalangeal joints of fingers.

   Mnemonic within a mnemonic: The action of the thumb tests three of the long motor nerves of the brachial plexus:

   Extension: radial nerve.

   Opposition to little finger: median nerve.

   Abduction: median nerve.

   Adduction: ulnar nerve

5. Motor distribution of the lumbosacral plexus (formed from the ventral rami of spinal nerves L1–S4): This plexus provides proximal branches to the glutei and hip abductors and adductors. The major terminal nerves to the extremities are

   1. The femoral nerve (L2–L4): the extensor nerve of the knee.

   2. The obturator nerve (L2–L4): the adductor nerve of the thigh.

   3. The sciatic nerve (L4–S3): the flexor nerve of the knee and the flexor and extensor nerves of the foot and toes and the invertors and evertors of the foot. The largest of the peripheral nerves, the sciatic can be regarded as consisting of three nerves:

      1. The hamstring nerve that branches to the knee flexors

      2. The tibial nerve that branches to the posterior muscles of the calf

      3. The common peroneal nerve that branches to the anterior muscles

   4. After giving off hamstring branches, the sciatic nerve divides in the popliteal space into the tibial and peroneal nerves:

      1. The tibial nerve (L4–S3) innervates the plantar flexors of the foot and toes and the muscle that inverts the foot when the foot plantar flexes.

      2. The peroneal nerve (L4–S1) innervates the dorsiflexors of the ankle and toes and the muscle that everts the foot when the foot dorsiflexes.

   5. The pudendal nerve (S2–4), a somatic nerve, innervates the urogenital diaphragm, voluntary bowel and bladder sphincters, and sexual sensation.

   6. The pelvic splanchnic nerve (nervi erigentes), a visceral nerve, innervates the bladder and internal sphincters and is the vasomotor for erection of the penis and clitoris.

F. The axonal pathways in the spinal cord

1. Learn the nomenclature and location of the pathways of the spinal cord depicted in Figs. 2-12A to 2-12C. Figure 2-12 contains the essential information for bedside examination of a Pt with a spinal cord lesion.

   Mnemonic for the cross-sectional anatomy of the spinal cord: Divide the spinal cord into dorsal and ventral halves by a line from one denticulate ligament across to the other (Fig. 2-12A). The sensory nuclei of the gray matter, the lateral corticospinal tract, the dorsal spinocerebellar tract, and the white matter of the dorsal columns are dorsal to the denticulate plane. Everything else that has clinically testable functions is at that plane or ventral to it.

2. The pathways of the spinal cord arise from neurons in the dorsal root ganglia outside the CNS or from neurons inside the CNS. The axons of the fasciculi gracilis and cuneatus and of the dorsolateral tract of Lissauer arise from primary sensory neurons outside the CNS, in the dorsal root ganglia. All other tracts arise from neuronal perikarya within the CNS.

3. The primary axons that form the fasciculi gracilis and cuneatus travel up the dorsal columns of the cord to synapse on secondary neurons at the cervicomedullary junction (see Fig. 2-28). The secondary axons then decussate to form the medial lemniscus, which synapses on somatosensory nuclei of the thalamus that relay to the somatosensory cortex. This pathway mediates the so-called deep sensory modalities of position sense, vibration, stereognosis, pressure, two-point discrimination, and direction of movement.

4. The dorsolateral tract consists of primary axons from the dorsal root ganglia that synapse at the level of entry or after traveling up or down the cord only one or two segments. These primary axons synapse ipsilaterally in the spinal gray matter on secondary neurons whose axons decussate and ascend in the ventrolateral columns as spinothalamic tracts. These axons mediate pain and temperature (see Figs. 2-12 and 2-28).

5. The other spinothalamic tract component mediates touch. The primary axons of this pathway synapse in the dorsal horn and cross and ascend in the ventrolateral columns of the cord with the pain and temperature pathway (Fig. 2-28).

A. For clinical neurology you must know:

1. The three longitudinal and three transverse subdivisions of the brainstem

2. The location of the major tracts on cross section of the brainstem and their decussations

3. The name and number of the CrNs and their composition

4. The location of the CrN nuclei

5. The point where each CrN attaches to the neuraxis and exits from the skull

B. Three transverse and three longitudinal subdivisions of the brainstem

1. The three transverse subdivisions of the brainstem, the mesencephalon, pons, and medulla (Fig. 2-1), consist of three continuous longitudinal layers, tectum, tegmentum, and basis (Fig. 2-13).

2. The tectum (= roof, the roof of the brainstem) consists of the quadrigeminal plate that roofs the midbrain aqueduct and the anterior and posterior medullary vela that roof the fourth ventricle of the pons and medulla.

3. The tegmentum (= covering, the covering of the basis) separates the tectum from the basis. It consists of tracts and vitally important neurons grouped into various nuclei.

4. The basis (= the bottom, the bottom layer of the brainstem).

   1. The basis mesencephali, basis pontis, and basis medullae (the medullary pyramids) convey the longitudinally coursing corticofugal motor pathways, consisting of the pyramidal tract and the corticopontine tracts.

   2. In addition, the basis of the pons contains nuclei that swell it enormously. These nuclei receive the corticopontine tracts and relay them to the cerebellum. This pathway comprises the large belly of the basis pontis (Figs. 2-1, 2-16, and 2-17).

C. Cross-sectional organization of the brainstem

Learn Fig. 2-14, which shows the plan for the arrangement of gray and white matter along the length of the brainstem. Figures 2-15 to 2-18 show the regional variations, for reference.

D. The tegmental gray matter consists of CrN nuclei, supplementary motor nuclei, and reticular formation

1. The CrN nuclei are scattered along the tegmentum, toward its dorsal half. The BE nuclei are ventrolateral to the GSE nuclei (Fig. 2-14).

2. Supplementary motor nuclei occupy the ventral half of the tegmentum, or the basis in the special case of the pons.

   1. The midbrain contains the substantia nigra and red nucleus (Fig. 2-18 and Table 2-3).

   2. The basis pontis contains the pontine nuclei (Figs. 2-16 and 2-17 and Table 2-3).

   3. The medulla contains the inferior olivary nuclei (Fig. 2-15).

3. The reticular formation is a mixture of neuronal perikarya, axons, and dendrites that fills in the tegmental space not occupied by nuclei or tracts (Table 2-3 and Figs. 2-14 to 2-18). Many regions mediate motor functions.

E. The brainstem white matter (Figs. 2-14 to 2-18 and Table 2-3)

1. The brainstem transmits many short, medium, and long ascending and descending pathways. Many arise in or course to the reticular formation and supplementary motor nuclei.

2. The medial longitudinal fasciculus interconnects the vestibular nuclei with the nuclei of CrNs III, IV, and VI and the spinal cord. It runs just ventral to the aqueduct and fourth ventricle (Figs. 2-14 to 2-18 and 5-1).

3. The lemnisci (Table 2-3 and Figs. 2-14 to 2-18 and 2-28), by definition, convey axons of secondary sensory neurons that terminate in a somatosensory thalamic nucleus. The thalamic nucleus then relays to the respective sensory areas of the cerebral cortex.

4. Long descending corticofugal motor tracts, consisting of the pyramidal and corticopontine tracts, run in the basis.

5. The cerebellar pathways, consisting of afferent and efferent tracts, course along the lateral and dorsal aspects of the pons, where they form cerebellar peduncles that attach the cerebellum to the pons (Fig. 8-3).

F. Review exercise for the composition of the brainstem

1. Trace the perimeter shown in Fig. 2-14 two times on scrap paper. From memory draw in and label the white matter. Compare your drawing with Fig. 2-14.

2. On the second figure, draw in and label the gray matter and compare with Fig. 2-14.

3. From memory, draw the perimeter of the generalized brainstem cross section and insert and label the white and gray matter. Match your drawing against Fig. 2-14.

G. Functional significance of the additional neurons and tracts of the brainstem

1. The additional brainstem circuitry expands the internuncial neuronal pool beyond that available in the spinal cord gray matter. Like the spinal nuclei, the CrN nuclei mediate monosynaptic reflexes, but the additional neurons in the reticular formation, supplementary motor nuclei, quadrigeminal plate, and cerebellum provide polysynaptic circuitry for complicated brainstem reflexes controlling posture, eye and body movements, breathing, feeding, and homeostasis.

2. The vestibular nuclei and cerebellum add additional internuncial sensory circuits for the brainstem. The cochlear nuclei and superior olivary nuclei of the auditory pathway similarly add their circuitries.

A. Number and name of the cranial nerves

1. By definition, a CrN is any major nerve trunk that traverses one of the major foramina in the base of the skull. Learn Table 2-4.

2. The CrN number corresponds to the rostrocaudal sequence in which the nerve exits the base of the skull (except XI). The number very nearly duplicates the rostrocaudal sequence by which the CrNs attach to the brain (Fig. 2-20).

3. The CrN name conveys at least something about the components, function, or distribution of the nerve. Learn Table 2-5.

B. Functional summary of the cranial nerves

Learn Table 2-6. It lists the function of each CrN in the fewest words. It is the ultimate simplification. You need Tables 2-4 to 2-6 at your tongue tip.

C. Special functional components of the cranial nerves

Some CrNs retain the four general nerve components present at spinal levels (GSE, GVE, GVA, and GSA; Fig. 2-7), but three special functions, one motor and two sensory, are added, making seven possible components.

1. Special motor nuclei, special BE nuclei, innervate the skeletal muscle derived from the gill arches.

2. Special visceral afferent (SVA) axons for taste and special somatic afferent (SSA) axons for auditory and vestibular functions require special nuclei and pathways. Any given CrN may contain one or more but never all of the seven possible components (Table 2-7).

D. Mnemonic classification of cranial nerves into three sets: solely special sensory set, somatic set, and branchial set

1. From phylogenetic and embryologic data and the theory of nerve components, we can segregate the 12 pairs of CrNs into three sets:

   1. A solely special sensory set (SSSS)

   2. A somite (somatic) set

   3. A branchial set

2. The SSSS contains three CrNs: I (smell), II (vision), and VIII (hearing and equilibrium). These nerves convey no motor fibers to effectors. Three other CrNs, VII, IX, and X, mediate the one other special sense, taste.

3. The somite set (= somatic set) contains four CrNs: III, IV, VI, and XII. Because these nerves innervate cranial somite derivatives (eye and tongue muscles), they are direct homologs of the spinal nerves. All belong to the GSE category.

4. The branchial set contains five CrNs, V, VII, IX, X, and XI, that serially innervate the derivatives of the branchial (gill) arches (Fig. 2-19). Because the branchial arches develop only in the head region, only CrNs contain BE axons.

E. The SSSS of cranial nerves I, II, and VIII

1. CrN I, the olfactory nerve, consists of axonal filaments from perikarya located in the olfactory ganglia in the nasal mucosa (Fig. 9-1).

   1. This olfactory ganglion corresponds to the dorsal root ganglion of a spinal nerve.

   2. The olfactory axons penetrate the cribriform plate to synapse on the olfactory bulb. See the olfactory bulb in Fig. 2-20.

2. The so-called CrN II, the optic nerve, is neither developmentally nor histologically a peripheral nerve.

   1. The retina and optic stalks develop as evaginations from the hypothalamus of the diencephalon (Fig. 2-22). The retinas, at the distal end of the optic stalks, extend the actual wall of the neuraxis out to the body surface to detect light.

   2. Proximally, the two optic stalks remain attached to the hypothalamus as the optic chiasm (Figs. 2-20 and 2-26). Demyelinating diseases of the CNS, such as multiple sclerosis, that affect oligodendroglial myelin, attack the optic nerves, whereas a different set of diseases attack the Schwann cell myelin of true peripheral nerves.

3. CrN VIII, the vestibulocochlear nerve, conveys afferents from the cochlear apparatus for detecting sound and the vestibular apparatus for detecting motion and the pull of gravity. Of the three special sensory CrNs, only VIII has typical peripheral nerve histology.

F. The somite set of cranial nerves: III, IV, VI, and XII

1. Fate of the rostralmost cranial somites and their nerves

   1. Of the dozen somites that arise at the brainstem level, only a few survive the travail of evolution. The cranial dermatomes retrogress, but some myotomes remain. The first cervical dermatome (C1) usually also disappears. Hence, the rostralmost dermatome is at the level of C ________________.

      C2. Review Fig. 2-10 if you missed this answer

   2. Of the rostral myotomes, three survive, wondrously transformed into the ocular muscles. The law of original innervation requires these three myotomes to retain their three original nerves. The numbers of the three somite CrNs that innervate the eye muscles are _____, _______, and _______.

      III; IV; VI. (If you had trouble recalling, sort through the CrNs one by one, from I to XII. See Table 2-5.)

   3. To supply skeletal muscle derived from somites, these nerves must have a □ GVA/□ GSE/□ GVE component.

      ☑ GSE

   4. We will assume that any nerve supplying GSE fibers to a skeletal muscle also returns proprioceptive (GSA) fibers from it. Because these nerves lack individual dorsal root ganglia, their proprioceptive afferents presumably come from anastomotic branches from a collective ganglion that is on CrN V, the trigeminal (= Gasserian) ganglion.

   5. Of the somite CrNs, only III conveys GVE axons from the brainstem (Table 2-7). These GVE axons innervate the pupilloconstrictor muscle, which adjusts pupillary size, and the ciliary muscle, which adjusts lens thickness. No GVA fibers return to the neuraxis in any somite nerve.

2. Fate of the middle and caudal cranial somites: The middle cranial somites disappear, as do their nerves. The caudal cranial somites donate their myotomes to the tongue and their sclerotomes to the base of the skull, but their dermatomes disappear. Although CrN XII runs to the tongue muscles as a single trunk, it conveys axons from several somites. Because it does not innervate any visceral organs or special senses, XII has only two components, □ GSE/□ GVE/□ GVA/□ GSA.

   ☑ GSE and ☑ GSA

3. The disappearance of many of the middle group of somites accounts for the absence of the somite nuclei between CrNs XII and VI and between VI and IV. Notice this discontinuity in Fig. 2-21.

4. Review Table 2-7 and then complete Table 2-8 to summarize the components of the somatic set of CrNs.

G. The branchial set of cranial nerves…"or human face divine"

1. To the poet Milton, it was the human face divine, created in final form, by a deity and passed along from generation to generation without a history. But to the biologist, William Gregory (1929), the face evolved gradually from the branchial arches and frontonasal process (Fig. 1-10). He stated "that the real ancestral face belonged not to a precreated man but to a poor mud-sucking protochordate of pre-Silurian times; that when in some far-off dismal swamp a putrid prize was snatched by scaly forms, their facial masks already bore our eyes and nose and mouth."

2. In creating the cheeks, jaws, and ears, the branchial arches usurped the retrogressive cranial somites. Thus the face, from fish to man, recapitulates the phylogeny of the branchial arches.

3. The branchial arches remarkably resemble somites in fundamental plan.

   1. Thus, branchial arches are paired and arranged serially, and each pair receives only one pair of nerves and only one pair of arteries. See Fig. 2-19 and compare it directly with Fig. 1-10.

   2. The branchial arches produce the dermis, muscles, and bones of the cheeks, jaws, and ears (Fig. 1-10).

   3. Throughout their transformations, the branchial arch derivatives retain their original branchial nerve. Chapter 6 on clinical testing diagrams the individual CrNs. For now learn the introductory descriptions in Tables 2-5 to 2-7.

4. Complete Table 2-9 to review the branchial CrNs.

5. Complete Table 2-10 to review the three sets of CrNs.

H. Mnemonic for the attachment sites of the cranial nerves to the brain

1. Learn now, once and for all, that CrNs VI, VII, and VIII (a somite, a branchial, and a special sensory nerve) attach, in ventrodorsal order, at the pontomedullary sulcus. See Fig. 2-20.

2. By knowing this one fact and the unique attachment sites of the olfactory and optic nerves, you can fairly well deduce where all other CrNs attach:

   1. All CrNs with numbers less than VI must attach rostrally to the pontomedullary sulcus.

   2. CrNs III to V must attach caudally to CrN II and rostral to the pontomedullary sulcus (Fig. 2-20).

   3. All CrNs with numbers greater than VIII must attach caudally to the pontomedullary sulcus.

3. In Fig. 2-20, learn the labels on the two sides and color each set of CrNs with a different color. Then do the test frames 4 to 14.

4. The only CrN with an extracranial ganglion synapsing directly on a bulb that evaginates from the base of the cerebrum is number ______.

   I

5. The stalk of a diencephalic evagination forms CrN ______.

   II

6. The only SSSS CrN with true peripheral nerve histology is CrN ______. It attaches to the brainstem at the _____________________________ sulcus.

   VII; pontomedullary

7. The rostralmost somite nerve and the rostralmost motor CrN is ______. It attaches to the □ diencephalon/□ mesencephalon/□ pons.

   III; ☑ mesencephalon

8. The unique exit site of CrN IV from the tectum (dorsum) of the mesencephalon does not appear in Fig. 2-20. No other CrN attaches dorsally. The two CrNs attached to the mesencephalon are ______ and ______.

   III; IV

9. The CrN that attaches to the lateral sides of the bulging belly of the pons is ______.

   V

10. Three CrNs attach at the pontomedullary sulcus in ventrodorsal order, ______, ______, and ______.

    VI; VII; VIII

11. Three additional CrNs attach to the medulla: ______, ______, and, ______.

    IX; X; XII

12. The only CrN attaching to the spinal cord is ______.

    XI

    Note: CrN XI qualifies as a CrN because although it enters the posterior fossa through the foramen magnum, it then exits through the jugular foramen, thus meeting the definition of a CrN (Section V A-1).

13. In Fig. 2-20, notice that CrNs III, VI, and XII and the first cervical nerves that are phylogenetically homologous, attach in a parasagittal line along the brainstem. To remember this fact, draw a colored line in Fig. 2-20 from the attachment sites of CrN III through VI, and XII and the rootlets of the first cervical nerve.

14. The branchial set (CrNs V, VII, IX, X, and XI) then attach □ lateral to/□ medial to/□ in line with the somatic set.

    ☑ lateral to (Fig. 2-20)

15. Draw Fig. 2-20 from memory.

I. Location of the cranial nerve nuclei

Note: In Fig. 2-21 connect the motor CrN nuclei of the somite and branchial sets of CrN motor nuclei with different colored lines. Use the same color for the somite set as you used in Fig. 2-20.

Next complete frames 1 to 11. In thinking out the answers, sort through the CrNs one by one, from I to XII, for each question.

1. The CrN nuclei limited to the mesencephalon are ______________.

   III and IV

2. The CrN motor nuclei limited to the pons are for CrNs _______________.

   V, VI, and VII

3. The two CrN sensory nuclei that straddle the pontomedullary junction and serve CrN VIII are the ________________ and ________________ nuclei.

   cochlear; vestibular

4. The CrN motor nuclei limited to the medulla are for CrNs _____________.

   IX, X, and XII

5. The sensory nucleus limited solitarily to the medulla is called the nucleus ______________. It serves the visceral sensory functions of the medullary branchial nerves numbered _________________.

   solitarius; IX and X

6. The one CrN nucleus extending from the spinal cord to the mesencephalon belongs to CrN __________.

   V

7. The two CrN nuclei found in the rostral part of the cervical cord are (sensory) _______ and (motor) _______.

   V (sensory); XI (motor)

8. The only CrNs attaching to the brain rostral to the mesencephalon are _______ and _______.

   I; II

9. In column 1 of Table 2-11, list in rostrocaudal sequence, the numbers of the branchial CrNs. Then complete the rest of Table 2-11.

10. Describe how the sites of the nuclei and sites of brainstem attachment of the somatic set of CrNs (GSE) are located with respect to the midsagittal plane. Then describe how the branchial set of CrNs (BE) are located with respect to the somite CrNs.________________________________________

    _____________________________________________

    ______________________________________________

    __________________________________________.

    The somatic nuclei (GSE) are all paramedian, that is, adjacent to the median or midsagittal plane, and the nerves attach in a paramedian line along the brainstem. The branchial nuclei (BE) align more laterally and their nerves attach more laterally. (The actual location of the branchial nuclei with respect to the somatic nuclei is ventrolateral. See Figs. 2-14 to 2-18 and 2-21.)

11. Draw Fig. 2-21 from memory to ensure that you know the locations of the CrN nuclei.

J. An "elimination" mnemonic for learning the distribution of the branchial nerves: V, VII, IX, X, and XI

1. First, eliminate V and XI.

   1. CrNs V and XI are relatively simple in composition and distribution.

      1. Both are sensorimotor nerves and have only BE and GSA components (Table 2-1).

      2. Neither nerve conveys special senses or GVE (autonomic axons) to glands or smooth muscle (Table 2-7).

   2. CrN V has a huge GSA component for facial skin and related mucous membranes and cavities (Fig. 10-2). It conveys BE axons to the chewing muscles (Fig. 6-1) and returns proprioceptive, GSA fibers, from them.

      Mnemonic for CrN V: CrN V feels the face and chews (Fig. 6-1).

   3. CrN XI conveys BE axons to the sternocleidomastoid and trapezius muscles that turn the head and shrug the shoulders. CrN XI returns only a small contingent of proprioceptive (GSA) axons. It closely resembles the somite set.

2. Common plan of branchial CrNs VII, IX, and X.

   Note: With CrNs V and XI eliminated, three complicated CrNs, VII, IX, and X, remain. They all convey the same components and adhere to a common plan but pursue radically different peripheral courses. Compare with Tables 2-7 and 2-12.

   1. All three CrNs, VII, IX and X, convey BE axons to striated muscle derived from a gill arch.

   2. All three convey GVE (preganglionic parasympathetic) axons to a peripheral ganglion located in or near one of the large exocrine glands of the head or to glands in mucous membranes or viscera.

      1. CrN VII sends GVE axons to the lacrimal glands and to the mucous glands of the nasal mucosa via the sphenopalatine ganglia (Figs. 6-5 and 6-6) and to the submandibular and sublingual glands via the submandibular ganglia.

      2. CrN IX sends GVE axons to the parotid gland via the otic ganglion and to the glands of the pharyngeal mucosa (Fig. 6-9).

      3. CrN X sends GVE axons to the glands of the pharyngeal and laryngeal mucosa and to the smooth muscle and glands of the thoracicoabdominal viscera, as far as the splenic flexure of the colon (Fig. 6-10).

   3. All three CrNs, VII, IX and X, convey GVA fibers from the palatopharyngeal mucosa. The Xth nerve also carries afferents from the thoracicoabdominal viscera (Fig. 6-10).

   4. All three convey SVA fibers for taste: CrN VII from the anterior two-thirds of the tongue, IX from the posterior third, and X from the palatal orifice: rostrocaudal sequence.

   5. All three convey GSA fibers from the skin of the external auditory canal but no auditory or vestibular fibers. The ear twigs from these nerves underscore the complex origin of the ear from several branchial arches. The skin area of CrN V on the face and the skin twigs of VII, IX, and X demonstrate retention of the dermis of the branchial arches.

3. CrNs IX and X differ from VII in innervating palatal and pharyngeal muscles during swallowing, the carotid sinus and carotid body, and the larynx for phonation. CrN X, the vagrant nerve, is unique in wandering through the thoracicoabdominal viscera to the splenic flexure.

4. List the five nerve components of VII, IX, and X.__________________________.

   BE, GVE, GVA, GSA, and SVA (Tables 2-7 and 2-12)

5. Special features of CrN VII:

   1. By eliminating the GSA cutaneous twig to the ear and the GVA palatopharyngeal sensory innervation as clinically insignificant, we can reduce CrN VII to its BE, GVE, and SVA components, its clinically important components (Figs. 6-4 and 6-5). Read the quotation from Bell, page 231. Realize that, through this nerve, we wink, smile, frown, cry, taste, salivate, and produce mucus in the nose. This leads to a mnemonic:

      Mnemonic for remembering the functions of CrN VII: CrN VII tears, snots, tastes, salivates, and moves the face. It innervates all facial movements (except for eyelid elevation by CrN III) and mandibular movements by CrN V (Figs. 6-5 and 6-6).

   2. CrN VII travels through the parotid gland but does not innervate it (Fig. 6-5). CrN IX innervates the parotid gland.

A. Anatomic definition of the reticular formation

1. The reticular formation (RF) consists of tegmental neurons more or less loosely arranged into nuclear groups connected by countless, multisynaptic, internuncial circuits.

2. The RF extends through the tegmentum from the rostral end of the spinal cord into the caudal end of the diencephalon. RF fills the tegmental space not occupied by CrN nuclei, supplementary motor nuclei, or tracts in transit.

3. The RF receives collaterals from sensory, motor, and autonomic pathways that traverse the brainstem and returns pathways to every part of the CNS.

B. Functions of the reticular formation

1. The RF mediates functions as diverse a breathing and consciousness. A midpontine transection divides the RF into rostral and caudal halves that somewhat separate its major functions.

2. The rostral, or pontomesencephalic, half of the RF sends ascending pathways to the thalamus and cerebral cortex. This ascending reticular activating system mediates consciousness and the waking state. Chapter 12 discusses the neuroanatomy of consciousness.

3. The caudal, or pontomedullary, half of the RF mediates various vital reflexes related to breathing, feeding, homeostasis, control of the blood pressure and pulse, alimentation and elimination, and control of posture and eye movements.

   1. Destruction of the caudal half of the RF does not impair consciousness, if respiration and blood pressure are artificially maintained.

   2. The caudal part of the RF, through CrNs V, VII, IX, X, and XII, controls the oronasopharyngeal conduits for feeding, breathing, and vocalizing.

      1. Feeding-related actions include sucking, chewing, salivating, and swallowing.

      2. Breathing-related actions include phonation, sneezing, coughing, sighing, and hiccoughing. To phonate and articulate speech and yet maintain the blood CO₂ and O₂ levels requires coordination of facial, oropharyngeal, laryngeal, diaphragmatic, intercostal, and abdominal muscles.

      3. Sensory receptors innervated via CrNs IX and X and neurons located directly in the medullary RF itself control breathing and blood gases. Chapter 6 discusses the neuroanatomy of breathing.

   3. The caudal pontomedullary RF likewise mediates autonomic reflexes of the thoracicoabdominal viscera that control blood pressure, pulse, bronchial diameter, gastrointestinal motility, and elimination.

4. The pontine RF is necessary for sleep regulation. Lesions of the pontine tegmentum abolish rapid eye movement sleep and greatly reduce the amount of non-rapid eye movement sleep.

A. The four nuclear subdivisions of the diencephalon: nuclear zones consist of the epithalamus, thalamus dorsalis, thalamus ventralis (subthalamus), and hypothalamus. These structures form the roof, walls and floor of the third ventricle

(Figs. 2-22 and 13-10B)

B. The epithalamus consists of the pineal body, habenula, and membranous roof plate of the third ventricle

(Fig. 2-22)

C. The thalamus (thalamus dorsalis)

1. To know the brain, you must know the thalamus. The thalamus consists of diverse nuclei that modulate all mental, motor, and sensory functions, including cognition, memory, speech, and affective experience.

2. The thalamus receives ascending sensory pathways (except for smell); ascending motor impulses from the RF, cerebellum, and basal motor nuclei; the ascending reticular activating system; and limbic pathways.

3. Thalamocortical and corticothalamic feedback circuits link the thalamic nuclei with motor, sensory, association, and limbic areas of the cerebral cortex. Almost everything that enters or exits the cortex must submit to thalamic modulation. As a corollary, thalamic lesions can impair the mental, motor, and sensory functions of the cortex, causing, for example, dementia, loss of memory, dysphasia, and various sensory and motor deficits.

4. The thalamus contains five groups of nuclei:

   1. Sensory nuclei receive the optic tract and the medial, lateral, spinal, and trigeminal lemnisci and relay to the respective sensory areas of the cerebral cortex (Table 2-13). Only smell lacks a direct thalamic relay nucleus.

      Thalamic sensory relay mnemonic: All sensory pathways known to reach consciousness relay to the cerebral cortex through a specific thalamic sensory nucleus, except for smell.

   2. Somatomotor nuclei relay impulses from the cerebellum, RF, and basal motor nuclei to the motor cortex. These connections modulate the output of the motor cortex through the pyramidal tract.

   3. Midline and intralaminar nuclei relay ascending reticular activating impulses to the cortex at large, mediating alerting responses and consciousness.

   4. Association nuclei mediate cognition by relaying to the association cortex of the frontal, parietal, temporal, and occipital lobes (Figs. 2-2C and 2-3). Association cortex does not directly receive sensory pathways or originate axons for the pyramidal tract.

   5. Limbic nuclei interconnect with the limbic lobe.

D. The subthalamus (thalamus ventralis) consists of the subthalamic nucleus of Luys and the zona incerta that belong to the basal motor nuclei

(Fig. 2-32)

E. The hypothalamus controls homeostatic and autonomic functions and emotional expression in close collaboration with the limbic lobe, hippocampus, amygdala, periaqueductal gray matter, and pituitary gland

A. Ontogeny of cortical connections

1. The cerebral pathways are best understood from their ontogeny. An axon growing from its cortical perikaryon might form three possible types of pathways: association, commissural, and projection.

   1. Association pathways: An outgrowing axon may connect with an ipsilateral cortical neuron to associate the functions of the two neurons (Fig. 2-23).

      1. Numerous long and short association pathways course through the cerebral white matter to connect the various cortical areas of one hemisphere.

      2. The longest such association fibers extend rectilinearly or in sweeping arcs through the white matter to connect frontal, occipital, and temporal lobes and the areas in between (Fig. 2-24).

   2. Commissural pathways: An outgrowing axon that crosses the midline to end in a mirror image site on the opposite side is, by definition, a commissural fiber. The largest number of commissural fibers runs through the corpus callosum (Figs. 13-10A and 13-10B). Smaller commissures exist at all levels of the neuraxis.

   3. Projection pathways: An outgrowing cortical axon may end ipsilaterally or contralaterally on an infracortical internuncial or an efferent neuron (Fig. 2-23). Such an axon projects the cortical influence down to the next neuron in line.

      1. Axons that cross the midline to end on non-mirror image sites are called decussations, to distinguish them from commissures.

      2. Like commissures, decussations occur at all levels of the neuraxis.

      3. The term projection applies generically for efferent connections at any level, not just cortical projections.

2. In summary, outgrowing cortical axons establish pathways of three types: ____________________________, _____________________________, and _____________________________.

   associational; commissural; projectional

3. A given axon may collateralize to form all three types of connections: associational, commissural and projectional. See right side of Fig. 2-23.

4. What is the essential difference between commissures and decussations? _________________________________________________

   ___________________________________________

   _____________________.

   Commissures connect mirror image points of the cortical or nuclear gray matter of the two sides of the CNS, whereas decussations connect non-mirror image sites.

B. Review of the pathways in the cerebral white matter

1. Association pathways, long and short, connect ipsilateral cortical areas.

2. Commissural pathways connect mirror image cortical areas of the two cerebral hemispheres.

3. Cortical projection pathways connect the cortex with ipsilateral or contralateral infracortical neurons. Major cortical projections include corticostriatal, corticothalamic, pyramidal (corticobulbar and corticospinal), and corticopontine pathways.

4. Thalamocortical pathways connect the various thalamic nuclei to various cortical areas.

5. Most of the incoming and outgoing cortical pathways, including the massive corticothalamic and thalamocortical circuits, fan out from or funnel down through the internal capsule (Fig. 13-10). See the geniculocalcarine tract in Figs. 3-4 to 3-6 for an example of a fan.

6. Lesions of the white matter disconnect various cortical areas from each other and from the thalamus, resulting in cerebral disconnection syndromes of dementia, dysphasia, dyspraxia, amnesia, spatial neglect, and alterations of affect, consciousness, and cognition.

A. The contralateral representation of the visual fields

1. To localize lesions that interrupt the long CNS pathways, you must know where they decussate. A lesion rostral to a decussation causes contralateral deficits, whereas a lesion caudal to a decussation causes ipsilateral deficits.

2. To explain decussations we had best start, surprisingly, with viewing an arrow in space.

   1. The physical properties of light and the physical optics of the eye (the pinhole camera effect of the pupil) form an inverted, real image on the retina (Figs. 2-25 and 2-26).

   2. Each site on the retinal image corresponds to a site on the arrow, but each half of the retina represents the opposite half of the visual field (Fig. 2-26).

   3. The mind then creates a mental image that projects or interprets an image that falls on one half of the retina as coming from the opposite side, i.e., from the location of the real object.

3. From these facts comes an intriguing theory that, phylogenetically, the wiring diagram of the visual system depends on:

   1. The physical properties of light.

   2. The physical optics of the eye.

   3. The necessity to unite the halves of visual space so that the mind can scan the entire visual field in continuity (Santiago Ramon y Cajal).

4. Because mammals have a cerebral cortex, the pathway from the retina runs to the lateral geniculate body (a thalamic nucleus) and from it via the geniculocalcarine tract to the calcarine cortex of the occipital lobe (Fig. 2-26). This pathway conveys the image of each right or left half of the visual field in uninterrupted fashion because of the compensatory semidecussation of axons at the optic chiasm (Fig. 2-26). The crucial fact is that each hemisphere mediates the visual field from the contralateral half of space.

5. Inversion of the sensorimotor homunculus in the paracentral region

   1. The retinal image is reversed not only laterally but also vertically (Fig. 2-27). Thus, the lower half of the retina receives the top half of the visual field and projects it to the calcarine cortex in topographic order (Fig. 2-27).

   2. This inversion of the vertical arrow may account for the inverted homunculus of motor and sensory representation in the pre- and postcentral gyri (Fig. 2-27), because then the mind has only to make one inversion to make the retinal input correspond to external reality.

BIBLIOGRAPHY · Contralateral Representation of the Visual Fields

B. The contralaterality of the somatosensory pathways

1. The cerebral hemisphere that receives its visual information from the contralateral half of space also receives the somatosensory information from the contralateral half of the body. Then the association area of that hemisphere can readily integrate the visual stimuli and proprioceptive and tactile stimuli from the contralateral limbs.

2. The primary, secondary, and tertiary neuron plan of the somatosensory pathways: Start tracing a somatosensory pathway at the receptor ending from its primary neuron in a dorsal root ganglion (Fig. 2-28).

   1. The peripheral branch of a dorsal root neuron receives a stimulus representing the superficial modalities of touch, pain, or temperature or the deep modalities of form, texture, vibration, position sense, direction of movement, stereognosis, and barognosis.

   2. The central branch synapses on a secondary neuron in the spinal cord gray matter. The axon of the secondary neuron then decussates and ascends to the thalamus as a lemniscus. The critical factor in tracing a sensory pathway is the level of the secondary neuron because the decussation occurs at that level.

      1. The primary axons for the deep sensory modalities enter the spinal cord white matter and ascend through the ipsilateral dorsal column to synapse on the nuclei gracilis and cuneatus located at the cervicomedullary junction (Figs. 2-28 and 10-14). The axons from the secondary neurons in these nuclei then decussate at that level (as the internal arcuate decussation). The axons turn rostrally in the contralateral half of the brainstem, where they ascend as the medial lemniscus to the thalamus.

      2. The primary axons for the superficial modalities of pain and temperature synapse on their secondary neurons at or near the level of entry of each dorsal root into the CNS. The axons of the secondary neurons then decussate and turn rostrally in the contralateral ventrolateral quadrant of the spinal cord white matter. This pathway of second-order neurons is called the spinal lemniscus to designate that it ends in the thalamus (Figs. 2-28 and 10-6).

   3. The superficial modality of touch is mediated by dorsal and ventral column pathways. The ventral column component comprises part of the spinal lemniscus (Fig. 2-28).

      The law of the level of decussation of the somatosensory pathways: The general somatosensory pathways decussate at the level of the nucleus for the perikarya of the second-order neurons. The law holds for the spinothalamic, spinocerebellar, touch, pain and temperature, and dorsal column pathways.

3. The third-order neuron of the somatosensory pathways resides in a sensory relay nucleus of the thalamus. The thalamic neurons relay to the ipsilateral somatosensory cortex in the postcentral gyrus of the parietal lobe (Fig. 2-28). Association fibers then connect the visual and somatosensory areas with other association cortices.

4. The pathways of two special senses, smell and hearing, are partial exceptions to the contralaterality law. Their stimuli, odors and sounds as distinct from light, go around corners and over obstructions. Contrary to other sensory pathways, the ascending pathways for hearing contain about an equal number of decussated and undecussated axons (Fig. 9-7). Thus, a unilateral CNS lesion does not ordinarily cause a strongly lateralized hearing loss.

C. The lemnisci and the role of the thalamus in sensation

1. The general somatic and special sensations, except for smell, relay through a lemniscus to a specific thalamic nucleus. The thalamus then relays general somatosensory information (GSA) to the ____________________ gyrus of the ______________________ lobe.

   postcentral; parietal

2. The retina projects the special sense of vision (SSA) to the lateral geniculate body of the thalamus. In this sense, the optic tract is an analog of the lemnisci and qualifies as the optic lemniscus. Another SSA sense, hearing, projects through the lateral lemniscus that synapses in the medial geniculate body.

3. Interruption of a decussated somatosensory (GSA) pathway through a lemniscus, its thalamic nucleus, or the pathway up to and including the post-central gyrus would cause a loss of sensation □ bilaterally/□ ipsilaterally/□ contralaterally.

   ☑ contralaterally

4. Learn Table 2-13 and then do test frames C-5 to C-10.

5. The trigeminal sensory nuclei for facial sensation send axons to the thalamus via the _________________________ lemniscus.

   trigeminal

6. Somatosensory impulses from the remainder of the body run to the thalamus via the ________________________ lemniscus.

   medial (or medial and spinal lemnisci)

7. The auditory pathway runs to the thalamus via the ________________ lemniscus.

   lateral

8. In general, lemnisci contain axons from □ primary/□ secondary/□ tertiary perikarya that have ____________________ the midline.

   ☑ secondary

   crossed

9. The thalamic sensory relay nucleus for vision is the ______________________ body and that for hearing the __________________ body.

   lateral geniculate; medial geniculate

10. The perikarya of the primary neurons of a somatosensory pathway occupy ___________________________________________, whereas the tertiary neurons occupy a _________________________ nucleus.

    dorsal root ganglion; thalamic

D. Union of the lemnisci in the brainstem

1. The lemnisci unite in the brainstem to travel as one to the thalamus.

   1. The medial lemniscus proper originates in the dorsal column nuclei (nuclei gracilis and cuneatus) of the medullocervical junction (Fig. 2-28).

   2. As the medial lemniscus travels rostrally into the pons, the spinal and trigeminal lemnisci merge with it. The name medial lemniscus still applies even though its composition differs along the brainstem. In the rostral pons, the lateral lemniscus merges along the dorsolateral edge of the medial lemniscus.

2. The domain of the lemnisci on transverse section of the brainstem.

   1. No long pathway runs along the tectum, but a cerebellar pathway cuts through it.

   2. Only corticofugal tracts run through the basis.

   3. Thus all lemnisci run through the tegmentum. Trace the medial lemniscus through the tegmentum in Figs. 2-15 to 2-18.

E. Sensation as the basis of movement

1. In reaching out to pick up an object, you have to locate it and guide the action of your muscles to it. The brain can guide movement by immediate or remembered information from the visual and somatosensory systems. By decussations, these systems bring the sensory information from one side to the contralateral cerebral hemisphere.

2. The hemisphere that receives the sensory information from the contralateral half of space also contains the motor cortex that directs the volitional movements of the contralateral extremities, via the pyramidal tract.

A. Function of the pyramidal tract

1. The pyramidal tract mediates willed movements. It begins in the motor cortex, located in the precentral and, to a lesser extent, the postcentral gyri.

2. The pyramidal tract projects through the deep white matter of the cerebrum to synapse on internuncial or efferent neurons in the brainstem and spinal cord. It consists of two components, corticobulbar and corticospinal (Fig. 2-29).

   1. The corticobulbar component decussates at various levels to synapse on brainstem neurons (Fig. 2-29). Figure 5-1 shows one of the most important corticobulbar tract decussations, in the pathway for volitional conjugate horizontal gaze. Preview it at this time by tracing through it, starting from the cortex.

   2. The corticospinal component mostly decussates at the cervicomedullary junction, where the dorsal column sensory pathways also decussate (Fig. 2-29). A variable number of corticospinal axons descend ipsilaterally in the spinal cord.

B. Paresis and paralysis after pyramidal tract interruption

1. Interruption of the pyramidal tract in the cerebrum or brainstem paralyzes most volitional movement on the contralateral side of the body. Complete or nearly complete paralysis on one side is called hemiparalysis or, more commonly, hemiplegia. Incomplete paralysis on one side is called hemiparesis.

2. Interruption of a pyramidal tract rostral to its decussation causes hemiplegia □ ipsilateral/□ contralateral to the lesion.

   ☑ contralateral

3. Interruption of a pyramidal tract just caudal to its decussation causes hemiplegia □ ipsilateral/□ contralateral to the lesion.

   ☑ ipsilateral

4. Pyramidal tract interruption in the cerebrum affects both components of the pyramidal tract, the cortico_____________________________ and cortico_______________.

   bulbar; spinal

5. Pyramidal tract interruption caudal to its decussation affects only the cortico_____________________________ tract.

   spinal

C. The pyramidal tract and the concept of upper motor neurons and lower motor neurons

1. As broadly defined, upper motoneurons (UMNs) include all motor tracts that descend from the brain to the lower motor neurons (LMNs) in the brainstem and spinal cord, i.e., the pyramidal, rubrospinal, tectospinal, reticulospinal, and vestibulospinal tracts. No UMN axons leave the neuraxis.

2. As narrowly defined, UMNs indicate only the pyramidal tracts. Clinicians generally use this definition. All other motor pathways, basal motor and cerebellar, etc., belong to the extrapyramidal system.

3. LMNs are the motoneurons in the brainstem or ventral horns of the spinal cord that send their axons into the PNS. The somatomotor LMNs, branchial and somatic, activate skeletal muscles. The visceromotor (autonomic) LMNs activate smooth muscles and glands.

D. UMN (pyramidal tract) versus LMN paralysis

1. The hallmark of pyramidal tract interruption is paresis, or paralysis of volitional movements of the parts, not the individual muscles. Volitional movements, even if ostensibly simple, usually involve more than one muscle. If you flex your finger, pucker your lips, or lift your arm, more than one muscle springs into action. Movements, then, are compounded of the actions of several muscles.

2. The hallmark of LMN, or motor nerve, interruption is paralysis of only the individual muscle supplied. This type of paralysis is called LMN paralysis to differentiate it from UMN paralysis.

3. If the patient has paresis or paralysis of one muscle or a restricted set of muscles, with normal movements otherwise, the lesion involves the □ UMNs/□ LMNs.

   ☑ LMNs

4. If the patient has paresis or paralysis of movements of one side of the body, sparing the other side, the lesion most likely affects the □ UMNs/□ LMNs.

   ☑ UMNs

   1. Taking some poetic license, we may summarize this conclusion by saying that UMN lesions paralyze □ movements/□ muscles, whereas LMN lesions paralyze □ movements/□ muscles.

      ☑ movements; ☑ muscles

   2. Despite this general rule, very tiny lesions of the motor cortex can, in rare instances, paralyze restricted movements of the fingers (Kim et al., 2002).

A. The cerebellum and the motor cortex

1. The sensory system, the cerebellum, basal motor nuclei, thalamic somatomotor nuclei, and motor cortex function by modulating each other.

   1. To modulate means to control the force, velocity, distance, amplitude, and trajectory of volitional movements and the required sequence of activation of the muscles.

   2. The pyramidal tract transmits the final modulation of the motor cortex to the muscles during volitional activation. The pathways of the various components of the motor system decussate in accordance with the law of cerebral contralaterality.

2. The clinical effect of dysmodulation differs depending on whether the lesion affects the cerebellum, sensory pathways, thalamus, or the basal motor nuclei.

   1. Lesions of the cerebellum or its pathways result in uncoordinated contractions of the muscles during volitional activation. Such uncoordinated movements are called cerebellar ataxia (Chapter 8). The Pt also displays a tremor during volitional movements but not at rest.

   2. The affected parts are very floppy and hypotonic. Drunkenness serves as a prime example of all of the cerebellar signs.

3. Afferents to the cerebellum come from:

   1. Olivary nuclei of the medulla.

   2. Sensory systems, in particular the proprioceptive system (spinocerebellar tracts and vestibular system).

   3. The cerebro-cerebello-cerebral circuit. Trace this pathway in Fig. 2-30.

4. Efferents that convey cerebellar modulation to the motor cortex run from the cerebellar dentate nucleus to the somatomotor nuclei of the thalamus that relay to the cerebral motor cortex. Trace the dentato-thalamo-cortical pathway in Fig. 2-30. Notice the decussation of this pathway in the caudal midbrain (Fig. 2-30).

5. Next trace the pyramidal tract down through its decussation (Fig. 2-30). Thus, the cerebellar circuit requires three decussations to finally reach and modulate the LMNs.

   1. The cortico-ponto-cerebellar pathway decussates in the __________________.

      basis pontis

   2. The dentato-thalamic pathway decussates in the _______________________.

      caudal midbrain

   3. The corticobulbar components of the pyramidal tract decussate at levels along the brainstem, whereas the corticospinal component decussates at the ________________________ junction level.

      medullocervical

B. Laterality of cerebellar signs

1. The cerebellar pathway, with its two decussations (Fig. 2-30) and the third, pyramidal decussation, explains a clinical aphorism: Cerebral hemisphere lesions cause contralateral motor signs, but cerebellar hemisphere lesions cause ipsilateral motor signs.

2. A lesion of the one cerebellar hemisphere would cause □ ipsilateral/□ contralateral ataxia, whereas a lesion of the dentatothalamic tract rostral to its decussation in the midbrain would cause □ ipsilateral/□ contralateral ataxia.

   ☑ ipsilateral; ☑ contralateral. Examine Fig. 2-30 if you don't understand the answer.

A. Definitions

1. Refer to Fig. 2-31 and learn the subdivisions of the corpus striatum.

2. Originally the corpus striatum belonged to a group of nuclei called the basal ganglia, which also included the claustrum and amygdala. Now the amygdala is classified with the limbic system, and the function of the claustrum remains uncertain.

3. The basal motor nuclei, situated in proximity at the base of the brain, modulate somatomotor activity by means of numerous feedback circuits that ultimately relay through the thalamus to the motor cortex.

4. Taken together, the basal motor nuclei constitute the extrapyramidal system (Fig. 2-31).

5. Clinical signs of lesions of the basal motor nuclei

   1. Muscular rigidity.

   2. Bradykinesia (difficulty in initiating and slowness in executing voluntary movements).

   3. Hyperkinesias (involuntary movements) consisting of

      1. Tremor, frequently of a part at rest.

      2. Patterned involuntary movements called chorea, athetosis, and dystonia. The type and distribution of the movements depend on the nuclei or pathways interrupted (Chapter 7).

B. Connections of the basal motor nuclei and laterality of clinical signs

1. The basal motor nuclei project to the motor cortex through the ipsilateral somatomotor nuclei of the thalamus (Fig. 2-32).

2. Thus, a unilateral lesion of the basal motor connections results in motor signs expressed contralaterally through dysmodulation of the motor cortex and the crossing of the pyramidal tract (Fig. 2-32).

3. The evidence that basal motor nuclei pathways operate by modulating the output of the cerebral motor cortex comes from the fact that involuntary movements, such as chorea, disappear contralaterally after destruction of the motor cortex or the pyramidal tract. Of course, contralateral voluntary movements also disappear. Similarly, ataxia cannot appear without the voluntary movement mediated by the pyramidal tract. We come to the heretical conclusion that pyramidal tract interruption paralyzes voluntary and involuntary movements. This statement applies to the effects of acquired lesions in previously normal brains. Infants born with no pyramidal tracts do make movements, which must come from extrapyramidal sources.

4. A key difference between basal motor nuclei signs and cerebellar signs is that ataxia appears only during voluntary movements, whereas involuntary movements may be superimposed on muscles at rest or superimposed on voluntary actions. Almost all abnormal involuntary movements caused by lesions in the basal motor nuclei disappear during sleep.

5. Review the summary of the clinical syndromes of motor systems lesions in Table 2-14.

Learn Table 2-15.

1. The contralaterality law applies to the decussations of all long sensory and motor pathways: One cerebral hemisphere receives most of its somatosensory and visual sensory information from the contralateral side of the body or the contralateral half of space and in turn controls the motor activity of the contralateral side of the body (Louis, 1994).

2. Using a colored pencil, superimpose the pyramidal pathway of Fig. 2-29 onto Fig. 2-28. Then mentally trace a nerve impulse from a somatic sensory receptor to the cerebral cortex and back to the motoneurons via the pyramidal tract while noting the decussations.

3. Table 2-15 summarizes the clinical correlates of lesions at various sites in the motor system.

BIBLIOGRAPHY · Contralaterality