Chapter 5: The Spinal Cord

The spinal cord connects the brain with most of the body. It is the target of a number of disease processes, some of which (eg, spinal cord compression) are treatable but rapidly progressive if not treated. Failure to diagnose some disorders of the spinal cord, such as spinal cord compression, can be catastrophic and may relegate the patient to a lifetime of paralysis. A knowledge of the architecture of the spinal cord and its coverings, and of the fiber tracts and cell groups that comprise it, is essential.

Differentiation

At about the third week of prenatal development, the ectoderm of the embryonic disk forms the neural plate, which folds at the edges into the neural tube (neuraxis). A group of cells migrates to form the neural crest, which gives rise to dorsal and autonomic ganglia, the adrenal medulla, and other structures (Fig 5–1). The middle portion of the neural tube closes first; the openings at each end close later.

The cells in the wall of the neural tube divide and differentiate, forming an ependymal layer that encircles the central canal and is surrounded by intermediate (mantle) and marginal zones of primitive neurons and glial cells (Figs 5–1 and 5–2). The mantle zone differentiates into an alar plate, which contains mostly sensory neurons, and a basal plate, which is primarily composed of motor neurons. These two regions are demarcated by the sulcus limitans, a groove on the wall of the central canal (see Fig 5–1D). The alar plate differentiates into a dorsal gray column; the basal plate becomes a ventral gray column. The processes of the mantle zone and other cells are contained in the marginal zone, which becomes the white matter of the spinal cord (see Fig 5–2A).

An investing layer of ectodermal cells around the primitive cord forms the two inner meninges: the arachnoid and pia mater (pia) (see Fig 5–2B). The thicker outer investment, the dura mater (dura), is formed from mesenchyma.

The spinal cord occupies the upper two-thirds of the adult spinal canal within the vertebral column (Fig 5–3). The cord is normally 42 to 45 cm long in adults and is continuous with the medulla at its upper end. The conus medullaris is the conical distal (inferior) end of the spinal cord. In adults, the conus ends at the L1 or L2 level of the vertebral column. The filum terminale, consisting of pia and glial fibers extends from the tip of the conus and attaches to the distal dural sac.

The central canal is lined with ependymal cells and filled with cerebrospinal fluid. It opens upward into the inferior portion of the fourth ventricle.

Enlargements

The spinal cord widens laterally in the cervical enlargement and the lumbosacral enlargement (see Fig 5–3). The latter tapers off to form the conus medullaris. The enlargements of the cord contain increased numbers of lower motor neurons (LMNs) and provide the origins of the nerves of the upper and lower extremities. The nerves of the brachial plexus originate at the cervical enlargement; the nerves of the lumbosacral plexus arise from the lumbar enlargement.

Segments

The spinal cord consists of approximately 30 segments (see Fig 5–3 and Appendix C)—8 cervical (C) segments, 12 thoracic (T) segments (termed dorsal in some texts), 5 lumbar (L) segments, 5 sacral (S) segments, and a few small coccygeal (Co) segments—that correspond to attachments of groups of nerve roots (Figs 5–3 and 5–4). There are no sharp boundaries between segments within the cord itself.

Because the spinal cord is shorter than the vertebral column, each spinal cord segment at lower levels is located above the similarly numbered vertebral body. The relation between spinal cord segments and vertebral bodies is shown in Table 5–1 and Figure 5–4.

Longitudinal Divisions

A cross section of the spinal cord shows a deep anterior median fissure and a shallow posterior (or dorsal) median sulcus, which divide the cord into symmetric right and left halves joined in the central midportion (Fig 5–5). The anterior median fissure contains a fold of pia and blood vessels; its floor is the anterior (or ventral) white commissure. The dorsal nerve roots are attached to the spinal cord along a shallow vertical groove, the posterolateral sulcus, which lies at a short distance anterior to the posterior median sulcus. The ventral nerve roots exit in the anterolateral sulcus.

A note on terminology: In descriptions of the spinal cord, the terms ventral and anterior are used interchangeably. Similarly, dorsal and posterior have the same meaning when referring to the spinal cord and its tracts; the dorsal columns, for example, are sometimes referred to as the posterior columns.

Each segment of the spinal cord gives rise to four roots: a ventral and a dorsal root on the left and a similar pair on the right (see Fig 5–5). The first cervical segment usually lacks dorsal roots.

Each of the 31 pairs of spinal nerves has a ventral root and a dorsal root; each root is made up of 1 to 8 rootlets (Fig 5–6). Each root consists of bundles of nerve fibers. In the dorsal root of a typical spinal nerve, close to the junction with the ventral root, lies a dorsal root (spinal) ganglion, a swelling that contains nerve cell bodies that give rise to sensory axons. The portion of a spinal nerve outside the vertebral column is sometimes referred to as a peripheral nerve. The spinal nerves are divided into groups that correspond to the spinal cord segments (see Fig 5–4).

The vertebral column surrounds and protects the spinal cord and normally consists of 7 cervical, 12 thoracic, and 5 lumbar vertebrae as well as the sacrum, which is usually formed by fusion of 5 vertebrae, and the coccyx. The nerve roots exit from the vertebral column through intervertebral foramina. In the cervical spine, the numbered roots exit the vertebral column above the corresponding vertebral body. The C8 root exits between vertebral bodies C7 and T1. In the lower parts of the spine, the numbered roots exit below the correspondingly numbered vertebral body.

The spinal cord itself is shorter than the vertebral column, and it usually ends at L1–2. The anatomy of the vertebral column is discussed further in Chapter 6.

Direction of Roots

Until the third month of fetal life, the spinal cord is as long as the vertebral canal. After that, the vertebral column elongates faster than the spinal cord, so that at birth the cord extends to about the level of the third lumbar vertebra. In adults, the tip of the cord normally lies at the level of the first or second lumbar vertebra. Because of the different growth rates of the cord and spine, the cord segments are displaced upward from their corresponding vertebrae, with the greatest discrepancy in the lowest segments (see Fig 5–4). In the lumbosacral region, the nerve roots descend almost vertically below the cord to form the cauda equina (horse's tail) (see Figs 5–3 and 5–4).

Ventral Roots

The ventral (or anterior) roots constitute motor outflow tracts from the spinal cord. The ventral roots carry the large-diameter alpha motor neuron axons to the extrafusal striated muscle fibers; the smaller gamma motor neuron axons, which supply the intrafusal muscle of the muscle spindles (Fig 5–7); preganglionic autonomic fibers at the thoracic, upper lumbar, and midsacral levels (see Chapter 20); and a few afferent, small-diameter axons that arise from cells in the dorsal root ganglia and convey sensory information from the thoracic and abdominal viscera.

Dorsal Roots

The dorsal (posterior) roots are largely sensory. Each dorsal nerve root (except usually C1) contains afferent fibers from the nerve cells in its ganglion. The dorsal roots contain fibers from cutaneous and deep structures (see Table 3–2). The largest fibers (Ia) come from muscle spindles and participate in spinal cord reflexes; the medium-sized fibers (A-beta) convey impulses from mechanoreceptors in skin and joints. Most axons in the dorsal nerve roots are small (C, nonmyelinated; A-delta, myelinated) and carry information of noxious (eg, pain) and thermal stimuli.

Branches of Typical Spinal Nerves

A. Posterior Primary Division

This consists of a medial branch, which is in most instances largely sensory, and a lateral branch, which is mainly motor.

B. Anterior Primary Division

Larger than the posterior primary division, the anterior primary divisions form the cervical, brachial, and lumbosacral plexuses. In the thoracic region they remain segmental, as intercostal nerves.

C. Rami Communicantes

The rami join the spinal nerves to the sympathetic trunk. Only the thoracic and upper lumbar nerves contain a white ramus communicans, but the gray ramus is present in all spinal nerves (see Fig 5–6).

D. Meningeal or Recurrent Meningeal Branches

These nerves, also called sinuvertebral nerves, are quite small; they carry sensory and vasomotor innervation to the meninges.

Types of Nerve Fibers

Nerve fibers can be classified on the basis of their diameter and conduction velocity (Tables 3–2 and 3–3) or on a physioanatomic basis.

A. Somatic Efferent Fibers

These motor fibers innervate the skeletal muscles. They originate in large cells in the anterior gray column of the spinal cord and form the ventral root of the spinal nerve.

B. Somatic Afferent Fibers

These fibers convey sensory information from the skin, joints, and muscles to the central nervous system. Their cell bodies are unipolar cells in the spinal ganglia that are interposed in the course of dorsal roots (dorsal root ganglia). The peripheral branches of these ganglionic cells are distributed to somatic structures; the central branches convey sensory impulses through the dorsal roots to the dorsal gray column and the ascending tracts of the spinal cord.

C. Visceral Efferent Fibers

The autonomic fibers are the motor fibers to the viscera. Sympathetic fibers from the thoracic segments and L1 and L2 are distributed throughout the body to the viscera, glands, and smooth muscle. Parasympathetic fibers, which are present in the middle three sacral nerves, go to the pelvic and lower abdominal viscera. (Other parasympathetic fibers are carried by cranial nerves III, VII, IX, and X.)

D. Visceral Afferent Fibers

These fibers convey sensory information from the viscera. Their cell bodies are in the dorsal root ganglia.

Dermatomes

The sensory component of each spinal nerve is distributed to a dermatome, a well-defined segmental portion of the skin (Fig 5–8). An understanding of the dermatomes is essential for the sensory examination. It is important for all clinicians to remember the following key points:

• Because in many patients there is no C1 dorsal root, there is no C1 dermatome (when a C1 dermatome does exist as an anatomic variant, it covers a small area in the central part of the neck, close to the occiput).

• The dermatomes for C5, C6, C7, C8, and T1 are confined to the arm, and the C4 and T2 dermatomes are contiguous over the anterior trunk.

• The thumb, middle finger, and fifth digit are within the C6, C7, and C8 dermatomes, respectively.

• The nipple is at the level of T4.

• The umbilicus is at the level of T10.

The territories of dermatomes tend to overlap, making it difficult to determine the absence of a single segmental innervation on the basis of sensory testing (Fig 5–9).

Myotomes

The term myotome refers to the skeletal musculature innervated by motor axons in a given spinal root. The organization of myotomes is the same from person to person, and the testing of motor functions (see Appendix B) can be very useful in determining the extent of a lesion in the nerve, spinal cord segment, or tract, especially when combined with a careful sensory examination. Most muscles, as indicated in Appendix B, are innervated by motor axons that arise from several adjacent spinal roots. Nevertheless, lesions of a single spinal root, in many cases, can cause weakness and atrophy of a muscle.

Especially useful to the clinician will be Table 5–2 which lists segment-pointer muscles, whose weakness or atrophy may suggest a lesion involving a single nerve root or a pair of adjacent nerve roots.

Gray Matter

A. Columns

A cross section of the spinal cord shows an H-shaped internal mass of gray matter surrounded by white matter (see Fig 5–5). The gray matter is made up of two symmetric portions joined across the midline by a transverse connection (commissure) of gray matter that contains the minute central canal or its remnants. This gray matter extends the entire length of the spinal cord, and is considered to consist of columns. The ventral (or anterior) gray column (also called the ventral, or anterior, horn) is in front of the central canal. It contains the cells of origin of the fibers of the ventral roots, including alpha and gamma motor neurons ("lower" motor neurons).

The intermediolateral gray column (or horn) lies between the dorsal and ventral gray columns; it is a prominent lateral triangular projection in the thoracic and upper lumbar regions but not in the midsacral region. It contains preganglionic cells for the autonomic nervous system. Within spinal segments T1 to L2, preganglionic sympathetic neurons within the intermediolateral gray column give rise to sympathetic axons that leave the spinal cord within the ventral roots and then travel to the sympathetic ganglia via the white rami communicantes. Within spinal segments S2, S3, and S4, there are sacral parasympathetic neurons within the intermediolateral gray column. These neurons give rise to preganglionic parasympathetic axons that leave the spinal cord within the sacral ventral roots. After projecting to the pelvic viscera within the pelvic nerves, these parasympathetic axons synapse on postganglionic parasympathetic neurons that project to the pelvic viscera.

The dorsal gray column (also called the posterior,or dorsal, horn) reaches almost to the posterolateral (dorsolateral) sulcus. A compact bundle of small fibers, the dorsolateral fasciculus (Lissauer's tract), part of the pain pathway, lies on the periphery of the spinal cord.

The form and quantity of the gray matter vary at different levels of the spinal cord (Fig 5–10). The proportion of gray to white matter is greatest in the lumbar and cervical enlargements. In the cervical region, the dorsal gray column is comparatively narrow and the ventral column is broad and expansive, especially in the four lower cervical segments. In the thoracic region, both the dorsal and ventral columns are narrow, and there is a lateral column. In the lumbar region, the dorsal and ventral columns are broad and expanded.

B. Laminas

A cross section of the gray matter of the spinal cord shows a number of laminas (layers of nerve cells), termed Rexed's laminae after the neuroanatomist who described them (Fig 5–11). As a general principle, superficial laminae tend to be involved in pain signaling, while deeper laminae are involved in non-painful as well as painful sensation.

1. Lamina I

This thin marginal layer contains neurons that respond to noxious stimuli and send axons to the contralateral spinothalamic tract.

2. Lamina II

Also known as substantia gelatinosa, this lamina is made up of small neurons, some of which respond to noxious stimuli. Substance P, a neuropeptide involved in pathways mediating sensibility to pain, is found in high concentrations in laminas I and II.

3. Laminas III and IV

These are referred to together as the nucleus proprius. Their main input is from fibers that convey position and light touch sense.

4. Lamina V

This layer contains cells that respond to both noxious and visceral afferent stimuli.

5. Lamina VI

This deepest layer of the dorsal horn contains neurons that respond to mechanical signals from joints and skin.

6. Lamina VII

This is a large zone that contains the cells of the dorsal nucleus (Clarke's column) medially as well as a large portion of the ventral gray column. Clarke's column contains cells that give rise to the posterior spinocerebellar tract. Lamina VII also contains the intermediolateral nucleus (or intermediolateral cell column) in thoracic and upper lumbar regions. Preganglionic sympathetic fibers project from cells in this nucleus, via the ventral roots and white rami communicantes, to sympathetic ganglia.

7. Laminas VIII and IX

These layers represent motor neuron groups in the medial and lateral portions of the ventral gray column. The medial portion (also termed the medial motor neuron column) contains the LMNs that innervate axial musculature (ie, muscles of the trunk and proximal parts of the limbs). The lateral motor neuron column contains LMNs for the distal muscles of the arm and leg. In general, flexor muscles are innervated by motor neurons located close to the central canal, whereas extensor muscles are innervated by motor neurons located more peripherally (Fig 5–12).

8. Lamina X

This represents the small neurons around the central canal or its remnants.

White Matter

A. Columns

The spinal cord has white columns (funiculi)—dorsal (also termed posterior), lateral, and ventral (also termed anterior)—around the spinal gray columns (see Fig 5–5). The dorsal column lies between the posterior median sulcus and the posterolateral sulcus. In the cervical and upper thoracic regions, the dorsal column is divided into a medial portion (the fasciculus gracilis or gracile fasciculus) and a lateral portion (the fasciculus cuneatus or cuneate fasciculus). The lateral column lies between the posterolateral sulcus and the anterolateral sulcus. The ventral column lies between the anterolateral sulcus and the anterior median fissure.

B. Tracts

The white matter of the cord is composed of myelinated and unmyelinated nerve fibers. The fast-conducting myelinated fibers form bundles (fasciculi) that ascend or descend for varying distances. Glial cells (oligodendrocytes, which form myelin, and astrocytes) lie between the fibers. Fiber bundles with a common function are called tracts. Some tracts decussate or cross the midline from one side of the spinal cord or brain.

Descending Fiber Systems

A. Corticospinal Tract

Arising from the cerebral cortex (primarily the precentral motor cortex, or area 4, and the premotor area, or area 6) is a large bundle of myelinated axons that descends through the brain stem via a tract called the medullary pyramid and then largely crosses over (decussates) downward into the lateral white columns. These tracts contain more than 1 million axons; the majority are myelinated.

The corticospinal tracts contain the axons of upper motor neurons (ie, neurons of the cerebrum and subcortical brain stem that descend and provide input to the anterior horn cells of the spinal cord). These anterior horn cells, which project directly to muscle and control muscular contraction, are called lower motor neurons.

The great majority of axons in the corticospinal system decussate in the pyramidal decussation within the medulla and descend within the lateral corticospinal tract (Fig 5–13 and Table 5–3). These fibers terminate throughout the ventral gray column and at the base of the dorsal column. Some of the LMNs supplying the muscles of the distal extremities receive direct monosynaptic input from the lateral corticospinal tract; other LMNs are innervated by interneurons (via polysynaptic connection).

The lateral corticospinal tract is relatively new in phylogenetic terms, present only in mammals, and most highly developed in primates. It provides the descending pathway that controls voluntary, highly skilled, and fractionated movements.

In addition to the lateral corticospinal tract, which decussates and is the largest descending motor pathway, there are two smaller descending motor pathways in the spinal cord. These pathways are uncrossed.

About 10% of the corticospinal fibers that descend from the hemisphere do not decussate in the medulla but rather descend uncrossed in the anterior (or ventral) corticospinal tract and are located in the anterior white matter column of the spinal cord. After descending within the spinal cord, many of these fibers decussate, via the anterior white commissure, and then project to interneurons (which project to LMNs) but connect directly to LMNs of the contralateral side.

A small fraction (0–3%) of the corticospinal axons descend, without decussating, as uncrossed fibers within the lateral corticospinal tract. These axons terminate in the base of the posterior horn and the intermediate gray matter of the spinal cord. They provide synaptic input (probably via polysynaptic circuits) to LMNs controlling axial (ie, trunk and proximal limb) musculature involved in maintaining body posture.

B. Vestibulospinal Tracts

There are two major components to the vestibulospinal tracts. Fibers of the lateral vestibulospinal tract arise from the lateral vestibular nucleus in the brain stem and course downward, uncrossed, in the ventral white column of the spinal cord. Fibers of the medial vestibulospinal tract arise in the medial vestibular nucleus in the brain stem and descend within the cervical spinal cord, with both crossed and uncrossed components, to terminate at cervical levels. Fibers of both vestibulospinal tracts provide synaptic inputs to interneurons in Rexed's laminae VII and VIII, which project to both alpha and gamma LMNs. Fibers of the vestibulospinal tracts provide excitatory input to the LMNs for extensor muscles. The vestibulospinal system facilitates quick movements in reaction to sudden changes in body position (eg, falling) and provides control of antigravity muscles.

C. Rubrospinal Tract

This fiber system arises in the contralateral red nucleus in the brain stem and courses in the lateral white column. The tract projects to interneurons in the spinal gray columns and plays a role in motor function (see Chapter 13).

D. Reticulospinal System

This tract arises in the reticular formation of the brain stem and descends in both the ventral and lateral white columns. Both crossed and uncrossed descending fibers are present. The fibers terminating on dorsal gray column neurons may modify the transmission of sensation from the body, especially pain. Those that end on ventral gray neurons influence gamma motor neurons and thus various spinal reflexes.

E. Descending Autonomic System

Arising from the hypothalamus and brain stem, this poorly defined fiber system projects to preganglionic sympathetic neurons in the thoracolumbar spinal cord (lateral column) and to preganglionic parasympathetic neurons in sacral segments (see Chapter 20). Descending fibers in this system modulate autonomic functions, such as blood pressure, pulse and respiratory rates, and sweating.

F. Tectospinal Tract

This tract arises from the superior colliculus in the roof (tectum) of the midbrain and then courses in the contralateral ventral white column to provide synaptic input to ventral gray interneurons. It causes head turning in response to sudden visual or auditory stimuli.

G. Medial Longitudinal Fasciculus

This tract arises from vestibular nuclei in the brain stem. As it descends, it runs close to, and intermingles with, the tectospinal tract. Some of its fibers descend into the cervical spinal cord to terminate on ventral gray interneurons. It coordinates head and eye movements. The last two descending fiber systems descend only to the cervical segments of the spinal cord.

Ascending Fiber Systems

All afferent axons in the dorsal roots have their cell bodies in the dorsal root ganglia (Table 5–4). Different ascending systems decussate at different levels. In general, ascending axons synapse within the spinal cord before decussating.

A. Dorsal Column Tracts

These tracts, which are part of the medial lemniscal system, convey well-localized sensations of fine touch, vibration, two-point discrimination, and proprioception (position sense) from the skin and joints; they ascend, without crossing, in the dorsal white column of the spinal cord to the lower brain stem (Fig 5–14). The fasciculus gracilis carries input from the lower half of the body, with fibers that arise from the lowest, most medial segments. The fasciculus cuneatus lies between the fasciculus gracilis and the dorsal gray column; it carries input from the upper half of the body, with fibers from the lower (thoracic) segments more medial than the higher (cervical) ones. Thus, one dorsal column contains fibers from all segments of the ipsilateral half of the body arranged in an orderly somatotopic fashion from medial to lateral (Fig 5–15).

Ascending fibers in the gracile and cuneate fasciculi terminate on neurons in the gracile and cuneate nuclei (dorsal column nuclei) in the lower medulla. These second-order neurons send their axons, in turn, across the midline via the lemniscal decussation (also called the internal arcuate tract) and the medial lemniscus to the thalamus. From the ventral posterolateral thalamic nuclei, sensory information is relayed upward to the somatosensory cortex.

B. Spinothalamic Tracts

Small-diameter sensory axons conveying the sensations of sharp (noxious) pain, temperature, and crudely localized touch course upward, after entering the spinal cord via the dorsal root, for one or two segments at the periphery of the dorsal horn. These short, ascending stretches of incoming fibers that are termed the dorsolateral fasciculus, or Lissauer's tract, then synapse with dorsal column neurons, especially in laminas I, II, and V (Figs 5–11 and 5–16). After one or more synapses, subsequent fibers cross to the opposite side of the spinal cord and then ascend within the spinothalamic tracts, also called the ventrolateral (or anterior) system. These spinothalamic tracts actually consist of two adjacent pathways: The anterior spinothalamic tract carries information about light touch, and the lateral spinothalamic tract conveys pain and temperature sensibility upward.

The spinothalamic tracts, like the dorsal column system, show somatotopic organization (see Fig 5–15). Sensation from sacral parts of the body is carried in lateral parts of the spinothalamic tracts, whereas impulses originating in cervical regions are carried by fibers in medial parts of the spinothalamic tracts. Axons of the spinothalamic tracts project rostrally after sending branches to the reticular formation in the brain stem and project to the thalamus (ventral posterolateral, intralaminar thalamic nuclei).

C. Clinical Correlations

The second-order neurons of both the dorsal column system and spinothalamic tracts decussate. The pattern of decussation is different, however. The axons of second-order neurons of the dorsal column system cross in the lemniscal decussation in the medulla; these second-order sensory axons are called internal arcuate fibers where they cross. In contrast, the axons of second-order neurons in the spinothalamic tracts cross at every segmental level in the spinal cord. This fact aids in determining whether a lesion is in the brain or the spinal cord. With lesions in the brain stem or higher, deficits of pain perception, touch sensation, and proprioception are all contralateral to the lesion. With spinal cord lesions, however, the deficit in pain perception is contralateral to the lesion, whereas the other deficits are ipsilateral. Clinical Illustration 5–1 provides an example.

D. Spinoreticular Pathway

The ill-defined spinoreticular tract courses within the ventrolateral portion of the spinal cord, arising from cord neurons and ending (without crossing) in the reticular formation of the brain stem. This tract plays an important role in the sensation of pain, especially deep, chronic pain (see Chapter 14).

E. Spinocerebellar Tracts

Two ascending pathways (of lesser importance in human neurology) provide input from the spinal cord to the cerebellum (Fig 5–17 and Table 5–4).

1. Dorsal spinocerebellar tract

Afferent fibers from muscle and skin (which convey information from muscle spindles, Golgi tendon organs, and touch and pressure receptors) enter the spinal cord via dorsal roots at levels T1 to L2 and synapse on second-order neurons of the nucleus dorsalis (Clarke's column). Afferent fibers originating in sacral and lower lumbar levels ascend within the spinal cord (within the dorsal columns) to reach the lower portion of the nucleus dorsalis.

The dorsal nucleus of Clarke is not present above C8; it is replaced, for the upper extremity, by a homologous nucleus called the accessory cuneate nucleus. Dorsal root fibers originating at cervical levels synapse with second-order neurons in the accessory cuneate nucleus.

The second-order neurons from the dorsal nucleus of Clarke form the dorsal spinocerebellar tract; second-order neurons from the lateral cuneate nucleus form the cuneocerebellar tract. Both tracts remain on the ipsilateral side of the spinal cord, ascending via the inferior cerebellar peduncle to terminate in the paleocerebellar cortex.

2. Ventral spinocerebellar tract

This system is involved with movement control. Second-order neurons, located in Rexed's laminae V, VI, and VII in lumbar and sacral segments of the spinal cord, send axons that ascend through the superior cerebellar peduncle to the paleocerebellar cortex. The axons of the second-order neurons are largely but not entirely crossed.

Reflexes are subconscious stimulus-response mechanisms. The reflexes are extremely important in the diagnosis and localization of neurologic lesions (see Appendix B).

Simple Reflex Arc

The reflex arc (Fig 5–18) includes a receptor (eg, a special sense organ, cutaneous end-organ, or muscle spindle, whose stimulation initiates an impulse); the afferent neuron, which transmits the impulse through a peripheral nerve to the central nervous system, where the nerve synapses with an LMN or an intercalated neuron; one or more intercalated neurons (interneurons), which for some reflexes relay the impulse to the efferent neuron; the efferent neuron (usually an LMN), which passes outward in the nerve and delivers the impulse to an effector; and an effector (eg, the muscle or gland that produces the response). Interruption of this simple reflex arc at any point abolishes the response.

Types of Reflexes

The reflexes of importance to the clinical neurologist may be divided into four groups: superficial (skin and mucous membrane) reflexes, deep tendon (myotatic) reflexes, visceral (organic) reflexes, and pathologic (abnormal) reflexes (Table 5–5). Reflexes can also be classified according to the level of their central representation, for example, as spinal, bulbar (postural and righting reflexes), or midbrain.

Spinal Reflexes

The segmental spinal reflex involves the afferent neuron and its axon within a peripheral nerve and dorsal root and a motor unit at the same level (see Fig 5–18). Simple reflex reactions involve specific patterns of muscle contractions. The delay between stimulation and effect is caused by the time needed for propagation of the impulse along the nerve fibers concerned and the synaptic delay (1 ms at each synapse). For a particular reflex to be present, a reflex arc (muscle receptors, sensory axons within a peripheral nerve and dorsal root, LMN and its axon, muscle) must be intact; thus, evaluation of spinal reflexes can provide information that is highly useful in the localization of lesions.

A. Stretch Reflexes and Their Anatomic Substrates

Stretch reflexes (also called tendon reflexes or deep tendon reflexes) provide a feedback mechanism for maintaining appropriate muscle tone (see Fig 5–18). The stretch reflex depends on specialized sensory receptors (muscle spindles), afferent nerve fibers (primarily Ia fibers) extending from these receptors via the dorsal roots to the spinal cord, two types of LMNs (alpha and gamma motor neurons) that project back to muscle, and specialized inhibitory interneurons (Renshaw cells).

B. Muscle Spindles

These specialized mechanoreceptors are located within muscles and provide information about the length and rate of changes in length of the muscle. The muscle spindles contain specialized intrafusal muscle fibers, which are surrounded by a connective tissue capsule. (Intrafusal muscle fibers should not be confused with extrafusal fibers or primary muscle cells, which are the regular contractile units that provide the force underlying muscle contraction.)

Two types of intrafusal fibers (nuclear bag fibers and nuclear chain fibers) are anchored to the connective tissue septae, which run longitudinally within the muscle and are arranged in parallel with the extrafusal muscle fibers. Two types of afferent axons, Ia and II fibers, arise from primary (or annulospinal) endings and secondary (or flower-spray) endings on the intrafusal fibers of the muscle spindle. These afferent axons carry impulses from the muscle spindle to the spinal cord via the dorsal roots. The muscle spindle and its afferent fibers provide information about both muscle length (the static response) and the rate of change in muscle length (the dynamic response). The static response is generated by nuclear chain fibers; the dynamic response is generated by nuclear bag fibers. After entering the spinal gray matter, Ia afferents from the muscle spindle make monosynaptic, excitatory connections with alpha motor neurons.

The muscle spindles are distributed in parallel with the extrafusal muscle fibers. Lengthening or stretching the muscle distorts the sensory endings in the spindle and generates a receptor potential. This causes the afferent axons from the muscle spindle (Ia afferents) to fire, with a frequency that is proportionate to the degree of stretch (Fig 5–19). Conversely, contraction of the muscle shortens the spindles and leads to a decrease in their firing rate.

Deep tendon reflexes are concerned with resisting inappropriate stretch on muscles and thus contribute to the maintenance of body posture. The Ia fibers from a muscle spindle end monosynaptically on, and produce excitatory postsynaptic potentials in, motor neurons supplying extrafusal muscle fibers in the same muscle. Lengthening of a muscle stretches the muscle spindle, thereby causing a discharge of an afferent Ia fiber in the dorsal root. This, in turn, activates alpha motor neurons running to the muscle, causing the extrafusal muscle fibers to contract so that the muscle will shorten.

In addition to monosynaptically exciting the alpha motor neurons involved in the stretch reflex, Ia afferents project, via inhibitory interneurons, to antagonistic muscle groups. This action provides for reciprocal inhibition, whereby flexors are excited and extensors are inhibited (or vice versa) in a coordinated manner.

C. Alpha Motor Neurons

Extrafusal muscle fibers, responsible for muscle contraction, are innervated by large anterior horn neurons termed alpha motor neurons. When alpha motor neurons fire, action potentials propagate, via axons in the ventral roots and peripheral nerves, to the motor end-plate, where they have an excitatory effect and produce muscle contraction. The axons of alpha motor neurons have diameters of 12–20 μm, and transmit action potentials rapidly, with conduction velocities of 70–120 m/s, so that they rapidly reach their target muscles.

D. Gamma Motor Neurons

Each muscle spindle contains, within its capsule, 2 to 10 small intrafusal fibers. Intrafusal muscle fibers receive their own innervation from gamma motor neurons, which are small, specialized motor neurons whose cell bodies are located in the ventral horn (Fig 5–20). Gamma motor neurons have relatively small axons (in the Aγ groups, 3–6 μm in diameter) that make up about 25% to 30% of the fibers in the ventral root. Firing in gamma motor neurons excites the intrafusal muscle fibers so that they contract. This action does not lead directly to detectable muscle contraction, because the intrafusal fibers are small. Firing gamma motor neurons, however, does increase tension on the muscle spindle, which increases its sensitivity to overall muscle stretch. Thus, the gamma motor neuron/intrafusal muscle fiber system sets the "gain" on the muscle spindle. The firing rates of gamma motor neurons are regulated by descending activity from the brain. By modulating the thresholds for stretch reflexes, descending influences regulate postural tone.

E. Renshaw Cells

These interneurons, located in the ventral horn, project to alpha motor neurons and are inhibitory. Renshaw cells receive excitatory synaptic input via collaterals, which branch from alpha motor neurons. These cells are part of local feedback circuits that prevent overactivity in alpha motor neurons.

F. Golgi Tendon Organs

A second set of receptors, the Golgi tendon organs, is present within muscle tendons. These stretch receptors are arranged in series with extrafusal muscle fibers and are activated by either stretching or contracting the muscle. Group Ib afferent fibers run from the tendon organs via the dorsal roots to the spinal gray matter. Here, they end on interneurons that inhibit the alpha motor neuron innervating the agonist muscle, thus mediating the inverse stretch reflex (see Fig 5–18). This feedback arrangement prevents overactivity of alpha motor neurons.

G. Clinical Correlations

If the alpha motor neuron fibers in a ventral root or peripheral nerve are cut or injured, the muscle's resistance to stretching is reduced. The muscle becomes weak and flaccid and has little tone.

Examination of deep tendon reflexes can provide valuable diagnostic information. Loss of all deep tendon reflexes, for example, can suggest a polyneuropathy (eg, Guillain–Barré syndrome), while loss or reduction of one particular deep tendon reflex (eg, loss of the knee jerk on one side) suggests injury to the afferent or efferent nerve fibers in the nerves or roots supplying that reflex.

The large extensor muscles that support the body are kept constantly active by coactivation of alpha and gamma motor neurons. Transection of the spinal cord acutely reduces muscle tone below the level of the lesion, indicating that supraspinal descending axons modulate the alpha and gamma motor neurons. In the chronic phase after transection of the spinal cord, there is hyperactivity of stretch reflexes below the level of the lesion, producing spasticity. This condition is a result of the loss of descending, modulatory influences. Spasticity can be disabling and is often treated with baclofen, a gamma-aminobutyric acid agonist. In some patients, however, the increased extension tone in spastic lower extremities is useful, providing at least a stiff-legged spastic gait after damage to the corticospinal system (eg, after a stroke).

H. Polysynaptic Reflexes

In contrast to the extensor stretch reflex (eg, patellar, Achilles tendon), polysynaptic, crossed extensor reflexes are not limited to one muscle; they usually involve many muscles on the same or opposite side of the body (Fig 5–21). These reflexes have several physiologic characteristics:

1. Reciprocal action of antagonists

Flexors are excited and extensors inhibited on one side of the body; the opposite occurs on the opposite side of the body.

2. Divergence

Stimuli from a few receptors are distributed to many motor neurons in the cord.

3. Summation

Consecutive or simultaneous subthreshold stimuli may combine to initiate the reflex.

Propriospinal axons, located on the periphery of the spinal gray matter, are the axons of local circuit neurons that convey impulses upward or downward, for several segments, to coordinate reflexes involving several segments. Some researchers refer to these axons as the propriospinal tract.

Lesions in the motor pathways, the muscle or its myoneural junction, or the peripheral nerve all result in disturbances of motor function (see Fig 5–20; see also Chapter 13). Two main types of lesions—of the upper and lower motor neurons—are distinguished in spinal cord disorders (Table 5–6).

Lower-Motor-Neuron Lesions

A lower motor neuron, the motor cell concerned with striated skeletal muscle activity, consists of a cell body (located in the anterior gray column of the spinal cord or brain stem) and its axon, which passes to the motor end-plates of the muscle by way of the peripheral or cranial nerves (Fig 5–22). Lower motor neurons are considered the final common pathway because many neural impulses funnel through them to the muscle; that is, they are acted on by the corticospinal, rubrospinal, olivospinal, vestibulospinal, reticulospinal, and tectospinal tracts as well as by intersegmental and intrasegmental reflex neurons.

Lesions of the LMNs may be located in the cells of the ventral gray column of the spinal cord or brain stem or in their axons, which constitute the ventral roots of the spinal or cranial nerves. Lesions can result from trauma, toxins, infections (eg, poliomyelitis, which can affect purely lower motor neurons), vascular disorders, degenerative processes, neoplasms, or congenital malformations affecting LMNs in the brain stem or spinal cord. Compression of ventral root axons (ie, the axons of LMNs in the spinal cord) by herniated intervertebral disks is a common cause of LMN dysfunction. Signs of LMN lesions include flaccid paralysis of the involved muscles (see Table 5–6); muscle atrophy with degeneration of muscle fibers after some time has elapsed; diminished or absent deep tendon reflexes (hyporeflexia or areflexia) of the involved muscle; and absence of pathologic reflexes (discussed next). Fasciculations and fibrillations may be present.

Upper-Motor-Neuron Lesions

Damage to the cerebral hemispheres or lateral white column of the spinal cord can produce signs of upper-motor-neuron lesions. These signs include spastic paralysis or paresis (weakness) of the involved muscles (see Table 5–6), little or no muscle atrophy (merely the atrophy of disuse), hyperactive deep tendon reflexes, diminished or absent superficial reflexes, and pathologic reflexes and signs, especially the extensor plantar reflex (Babinski's sign) (Fig 5–23).

Upper-motor-neuron lesions are commonly seen as a result of strokes, which can damage upper motor neurons in the cortex, and infections or tumors, which injure upper motor neurons either in the brain or as they descend in the spinal cord. The corticospinal, rubrospinal, and reticulospinal tracts lie close together or overlap within the lateral white column. Interruption of the corticospinal tract is usually accompanied by interruption of the other two tracts, resulting in spasticity and hyperreflexia. Isolated lesions of the corticospinal tract are rare; when these lesions occur, they cause loss of fine motor control (eg, loss of dexterity of the individual fingers) but tend to spare axial muscle groups (ie, those located proximally in the limbs) that control gross trunk and limb movement.

Disorders of Muscle or Neuromuscular Endings

Abnormal muscle tissue may be unable to react normally to stimuli conveyed to it by the LMNs. This effect may manifest as weakness, paralysis, or tetanic contraction caused by disturbances in the muscle itself or at the neuromuscular junction. Myasthenia gravis and the myasthenic syndrome (Lambert–Eaton myasthenic syndrome) are disorders of the neuromuscular junction that present with weakness. The muscular dystrophies and inflammatory myopathies (such as polymyositis) are typical disorders of muscle, characterized by muscular dysfunction (weakness in the presence of apparently normal nerve tissue). Sensory function is normal in these disorders.

Localization of Spinal Cord Lesions

In localizing spinal cord lesions, it is important to ask the following questions:

1. At what level does the abnormality begin (ie, is there a sensory level below which sensation is impaired)? Is motor function impaired below a specific myotomal level?

2. Which tracts are involved?

3. On which side are they located?

4. Which sensory modalities are involved (all modalities, suggesting involvement of the lateral and dorsal columns; vibration and position sense, suggesting dorsal column dysfunction; or dissociated loss of sensibility for pain and temperature, suggesting a lesion involving the spinothalamic fibers, possibly in the central part of the cord where they cross)?

A segmental lesion (a lesion involving only some segments of the spinal cord) injures motor neurons at the site of injury (causing LMN dysfunction at that level) and also injures descending tracts (producing upper-motor-neuron dysfunction below the site of injury).

Types of Spinal Cord Lesions

Several typical sites of pathologic lesions in the spinal cord produce characteristic syndromes:

1. A small central lesion can affect the decussating fibers of the spinothalamic tract from both sides without affecting other ascending or descending tracts. As a result, these lesions can produce dissociated sensory abnormalities with loss of pain and temperature sensibility in appropriate dermatomes but with preserved vibration and position sense. This occurs, for example, in syringomyelia (see the following section) (Fig 5–24A).

2. A large central lesion involves, in addition to the pain and temperature pathways, portions of adjacent tracts, adjacent gray matter, or both. Thus, there can be LMN weakness in the segments involved, together with upper-motor-neuron dysfunction and, in some cases, loss of vibratory and position sense at levels below the lesion (Fig 5–24B).

3. A dorsal column lesion affects the dorsal columns, leaving other parts of the spinal cord intact. Thus, proprioceptive and vibratory sensation are involved, but other functions are normal. Isolated involvement of the dorsal columns occurs in tabes dorsalis, a form of tertiary syphilis (see later discussion), which is rare at present because of the availability of antibiotics (Fig 5–24C).

4. An irregular peripheral lesion (eg, stab wound or compression of the cord) involves long pathways and gray matter; functions below the level of the lesion are abolished. In practice, many penetrating wounds of the spinal cord (stab wounds, gunshot wounds) cause irregular lesions (Fig 5–24D).

5. Complete hemisection of the cord produces a Brown–Séquard syndrome (see later discussion; Figs 5–24E and 5–25).

   Lesions outside the cord (extramedullary lesions) may affect the function of the cord itself as a result of direct mechanical injury or secondary ischemic injury resulting from the compromise of the vascular structures or vasospasm.

6. A tumor of the dorsal root (such as a neurofibroma or schwannoma) involves the first-order sensory neurons of a segment and can produce pain as well as sensory loss. Deep tendon reflexes at the appropriate level may be lost because of damage to Ia fibers (Fig 5–24F).

7. A tumor of the meninges or the bone (extramedullary masses) may compress the spinal cord against a vertebra, causing dysfunction of ascending and descending fiber systems (Fig 5–24G). Tumors can metastasize to the epidural space, causing spinal cord compression. Herniated intervertebral disks can also compress the spinal cord. Spinal cord compression may be treatable if diagnosed early. Suspected spinal cord compression thus requires aggressive diagnostic workup on an urgent basis.

Spinal Cord Compression

Spinal cord compression—due, for example, to an extramedullary tumor such as meningioma, neurofibroma, or metastatic cancer, an epidural abscess, or a ruptured intervertebral disc- can injure the spinal cord and can rapidly progress to irreversible paraplegia or quadriplegia if not promptly diagnosed and treated.

Spinal cord compression should be suspected in any patient with weakness, numbness or sensory loss in the legs. A "sensory level," that is, impaired sensation below a specific dermatomal level, or the presence of Babinski reflexes and hyper-reflexia in the lower extremities supports the diagnosis (although in the acute phase of spinal cord compression, spinal shock can produce transient hyporeflexia below the lesion). Bowel or bladder dysfunction may be present. Pain over the spinal column, or tenderness on mild percussion, provides further support for the diagnosis. If the lesion compresses the conus medullaris or cauda equina, there may be sensory loss in a "saddle" distribution and hyporeflexia. Spinal cord compression is surgically treatable but can rapidly progress to irreversible paraplegia if not treated. Imaging of the spine is required on an urgent basis in any patient in whom spinal cord compression is suspected.

Syringomyelia

Syringomyelia presents a classical clinical picture, characterized by loss of pain and temperature sensation at several segmental levels, although the patient usually retains touch and pressure sense as well as vibration and position sense (dissociated anesthesia) (Fig 5–26). Because the lesion usually involves the central part of the spinal cord and is confined to a limited number of segments, it affects decussating spinothalamic tracts only in these segments and results in a pattern of segmental loss of pain and temperature sense. When this type of injury occurs in the cervical region, there is a cape-like pattern of sensory loss. If the lesion also involves the ventral gray matter, there may be LMN lesions and atrophy of the denervated muscles.

Tabes Dorsalis

Tabes dorsalis, a form of tertiary neurosyphilis, is now rare, but was common in the pre-antibiotic era, and is characterized by damage to the dorsal roots and dorsal columns. As a result of this damage, there is impairment of proprioception and vibratory sensation, together with loss of deep tendon reflexes, which cannot be elicited because the Ia afferent pathway has been damaged. Patients exhibit "sensory ataxia." Romberg's sign (inability to maintain a steady posture with the feet close together, after the eyes are closed, because of loss of proprioceptive input) is usually present. Charcot's joints (destruction of articular surfaces as a result of repeated injury of insensitive joints) are sometimes present. Subjective sensory disturbances known as tabetic crises consist of severe cramping pains in the stomach, larynx, or other viscera.

Brown–Séquard Syndrome

This syndrome is caused by hemisection of the spinal cord as a result of, for example, bullet or stab wounds, syringomyelia, spinal cord tumor, or hematomyelia. Signs and symptoms include ipsilateral LMN paralysis in the segment of the lesion (resulting from damage to LMNs) (see Fig 5–25); ipsilateral upper-motor-neuron paralysis below the level of the lesion (resulting from damage to the lateral corticospinal tract); an ipsilateral zone of cutaneous anesthesia in the segment of the lesion (resulting from damage to afferent fibers that have entered the cord and have not yet crossed); and ipsilateral loss of proprioceptive, vibratory, and two-point discrimination sense below the level of the lesion (resulting from damage to the dorsal columns). There is also a contralateral loss of pain and temperature sense below the lesion (resulting from damage to the spinothalamic tracts, which have already decussated below the lesion). Hyperesthesia may be present in the segment of the lesion or below the level of the lesion, ipsilaterally, or on both sides. In practice, "pure" Brown–Séquard syndromes are rare because most lesions of the spinal cord are irregular.

Subacute Combined Degeneration (Posterolateral Sclerosis)

Deficiency in intake (or metabolism) of vitamin B₁₂ (cyanocobalamin) may result in degeneration in the dorsal and lateral white columns. There is a loss of position sense, two-point discrimination, and vibratory sensation. Ataxic gait, muscle weakness, hyperactive deep muscle reflexes, spasticity of the extremities, and a positive Babinski sign are seen.

Spinal Shock

This syndrome results from acute transection of, or severe injury to, the spinal cord from sudden loss of stimulation from higher levels or from an overdose of spinal anesthetic. All body segments below the level of the injury become paralyzed and have no sensation; all reflexes below the lesion, including autonomic reflexes, are suppressed. Spinal shock is usually transient; it may disappear in 3 to 6 weeks and is followed by a period of increased reflex response.