The rest of this chapter examines the brain and spinal cord with the aim of understanding the motor pathways and their spinal terminations. This discussion begins with the cerebral cortex—the highest level of the movement control hierarchy—and proceeds caudally to the spinal cord, following the natural flow of information processing in the motor systems.
The Cortical Motor Areas Are Located in the Frontal Lobe
Similar to each sensory modality, multiple cortical areas serve motor control functions (Figure 10–7). Four separate motor areas have been identified in the frontal lobe: the primary motor cortex and three premotor areas—the supplementary motor area, the premotor cortex, and the cingulate motor area. These areas are anatomically and functionally different. Each has several distinct subregions. All together, there are more than one dozen distinct cortical motor areas. These frontal motor areas all receive input from the ventral lateral and ventral anterior thalamic nuclei (see Figure 2–14), but to varying extents. The ventral lateral nucleus is the principal thalamic relay nucleus for the cerebellum and the ventral anterior nucleus, for the basal ganglia. These nuclei have complex subdivisions and the nomenclature used for animal studies—where connections are defined precisely—differs from that used to describe the human thalamus, where neurosurgical procedures are done. Until specific functions can be ascribed to the various nuclei, it is best to consider them simply and collectively as the motor thalamus. Additionally, some of the premotor areas receive information from the medial dorsal nucleus, which serves more integrative and cognitive functions.
Lateral (A) and medial (B) views of the human brain, indicating the locations of the primary motor cortex, premotor cortex, supplementary motor area, and cingulate motor area. The primary somatic sensory cortex is also shown.
The premotor cortical regions integrate information from diverse sources
The premotor cortical areas receive information from parietal, prefrontal, and other motor areas and, in turn, use this information to help plan movements. Whereas damage to the primary motor cortex produces weakness and incoordination, premotor damage produces apraxias, a motor planning disorder in which there is a loss of the ability to produce learned purposeful movements, even though the person is physically capable of making the movement. The supplementary motor area is located primarily on the medial surface of the cerebral hemisphere, in area 6 (Figure 10–7). Its specific contribution to movement control has not been identified, although research shows it may be important in planning bimanual movements. The premotor cortex is located laterally in area 6 (Figure 10–7A). The premotor cortex has at least two distinct motor fields with separate sets of connections and distinctive functions: the dorsal and ventral premotor cortices; each area is further functionally subdivided. The dorsal premotor cortex uses visual information from the external world around us to help control reaching. By contrast, the ventral premotor cortex uses visual information about objects of interest for grasping. Surprisingly, this ventral subregion becomes active not only when we move, but also when we watch others move. Animal studies show that it contains mirror neurons, which discharge action potentials when an animal makes a movement and when the animal watches someone else perform the same movement. The ventral premotor cortex may also be important in understanding the meaning of movements and in learning by imitation. The cingulate motor areas are found on the medial surface of the cerebral hemisphere, in cytoarchitectonic areas 6, 23, and 24, deep within the cingulate sulcus (Figure 10–7B). Curiously, the cingulate motor areas are located in a cortical region that is considered part of the limbic system, which is important for emotions. Although its function is not yet elucidated, this motor area may play a role in motor behaviors that occur in response to emotions and drives.
The primary motor cortex gives rise to most of the fibers of the corticospinal tract
The primary motor cortex, cytoarchitectonic area 4, receives input from three major sources: the premotor cortical regions, the somatic sensory areas (in the parietal lobe), and the motor thalamic nuclei. The cytoarchitecture of the primary motor cortex is different from that of sensory areas in the parietal, temporal, and occipital lobes (see Figure 2–19). Whereas the sensory areas have a thick layer IV and a thin layer V, the primary motor cortex has a thin layer IV and a thick layer V. Recall that layer IV is the principal input layer of the cerebral cortex, where most of the axons from the thalamic relay nuclei terminate, and that layer V is the layer from which descending projections originate (see Figure 2–17). In the primary motor cortex, thalamic axons terminate in most of the cortical layers.
The primary motor cortex, like the somatic sensory cortex (see Chapters 4 and 5), is somatotopically organized (Figure 10–8A). Somatotopy in the primary motor cortex can be revealed by transcranial magnetic stimulation, a noninvasive method for activating cortical neurons, or by functional imaging, such as functional magnetic resonance imaging (fMRI) (see Chapter 2). Regions controlling facial muscles (through projections to the cranial nerve motor nuclei; see Chapter 11) are located in the lateral portion of the precentral gyrus, close to the lateral sulcus. Regions controlling other body parts are—from the lateral side of the cerebral cortex to the medial side—neck, arm, and trunk areas. The leg and foot areas are found primarily on the medial surface of the brain. The motor representation in the precentral gyrus forms the motor homunculus; it is distorted in a similar way as the sensory homunculus of the postcentral gyrus (see Figure 4–9). Arm and leg areas contribute preferentially to the lateral corticospinal tract, and neck, shoulder, and trunk regions to the ventral corticospinal tract (Figure 10–8B). The face area of the primary motor cortex projects to the cranial nerve motor nuclei and thus contributes axons to the corticobulbar projection (see Chapter 11). Interestingly, stimulation of premotor cortical areas rarely produces a movement. Rather, it disrupts the production of an ongoing movement, suggesting that the stimulus altered ongoing firing of neurons important for movement planning.
Somatotopic organization of the primary motor cortex (A). B. The descending pathways by which these areas of primary motor cortex influence motor neurons are indicated. (A, Adapted from Penfield W, Rasmussen T. The Cerebral Cortex of Man: A Clinical Study of Localization. New York, NY: Macmillan; 1950.)
The Projection From Cortical Motor Regions Passes Through the Internal Capsule En Route to the Brain Stem and Spinal Cord
The corona radiata is the portion of the subcortical white matter that contains descending cortical axons and ascending thalamocortical axons (Figure 10–9A). The corona radiata is superficial to the internal capsule, which contains approximately the same set of axons but is flanked by the deep nuclei of the basal ganglia and thalamus (see Figure 2–15). The internal capsule is shaped like a curved fan (Figure 10–9A), with three main parts: (1) the rostral component, termed the anterior limb, (2) the caudal component, termed the posterior limb, and (3) the genu (Latin for "knee"), which joins the two limbs (Figures 10–9A). The anterior limb is rostral to the thalamus, and the posterior limb is lateral to the thalamus (Figure 10–9C).
A. Three-dimensional view of fibers in the white matter of the cerebral cortex. The regions corresponding to the internal capsule, basis pedunculi, and pyramid are indicated. The corona radiata is the portion of the white matter beneath the gray matter of the cerebral cortex. B. MRI from a patient with a stroke in the posterior limb of the internal capsule. Degeneration can be followed back (or retrograde) toward the precentral gyrus and forward (or anterograde) toward the brain stem. C. Myelin-stained horizontal section through the internal capsule. Note that the thalamus extends rostrally as far as the genu. The head of the caudate nucleus and the putamen are separated by the anterior limb of the internal capsule. The fiber constituents and somatotopic organization of the internal capsule are indicated. F, face; A, arm; T, trunk; L, leg. (A, Adapted with permission from Parent A. Carpenter's Human Neuroanatomy, 9th ed. Williams & Wilkins; 1996. B, Courtesy of Dr. Adrian Danek, Ludwig Maximilians University, Munich, Germany; Danek A, Bauer M, Fries W. Tracing of neuronal connections in the human brain by magnetic resonance imaging in vivo. Eur J Neurosci. 1990;2:112-115.)
Each cortical motor area sends its axons into a slightly different part of the corona radiata and internal capsule. Within the internal capsule, the descending motor projection from the primary motor cortex to the spinal cord courses in the posterior part of the posterior limb. The location of this projection is revealed in an MRI scan from a patient with a small lesion confined to the posterior limb of the internal capsule (Figure 10–9B). The pathway can be seen in this scan because degenerating axons produce a different magnetic resonance signal from that of normal axons. Retrograde degeneration can be followed back toward the cortex, and anterograde degeneration can be followed into the brain stem. The approximate location of the corticospinal projection in the posterior limb is shown in Figure 10–9C (labeled A, T, and L, for projections controlling muscles of the arm, trunk, and leg). The projection to the caudal brain stem, via the corticobulbar tract, descends rostral to the corticospinal fibers in the genu and posterior limb. Clinically, sufficient numbers of corticobulbar fibers are located in the genu, so that lesion of that structure disrupts facial muscle control (fibers labeled F for face in Figure 10–9C). Most of the path of the descending motor projection within the brain can be followed in a coronal section through the cerebral hemispheres, diencephalon, and brain stem (Figure 10–10A), and in an MRI from another patient who had a stroke in the posterior limb of the internal capsule (Figure 10–10C). The diffusion tensor image from a healthy person (Figure 10–10C) shows the white matter course of descending cortical fibers from the arm and leg areas of motor cortex.
A. Myelin-stained coronal section through the posterior limb of the internal capsule. Note that the component of the internal capsule is identified as the posterior limb in this section because the thalamus is medial to the internal capsule. B. Magnetic resonance imaging scan from a patient with an internal capsule lesion. Coronal slice through the posterior limb of the internal capsule showing bright vertically oriented band extending from the lesion caudally into the pons. This band corresponds to degenerated axons in the internal capsule, basis pedunculi, and pons. C. Diffusion tensor image of the corticospinal tracts from the arm and lege regions of a healthy person. (B, Courtesy of Dr. Jesús Pujol; from Pujol J, Martí-Vilalta JL, Junqué C, Vendrell P, Fernández J, Capdevila A. Wallerian degeneration of the pyramidal tract in capsular infarction studied by magnetic resonance imaging. Stroke. 1990;21:404-409.)
The descending projections from the premotor areas also course within the internal capsule but rostral to those from the primary motor cortex. This separation of the projections from primary and premotor cortical areas is clinically significant, especially for the supplemental motor area, which courses most rostral. Because of the high density of corticospinal axons, patients with a small posterior limb stroke can exhibit severe signs because of damage to these axons. Typically, however, they can recover some function, such as strength. This recovery is mediated in part by the spinal projections from the premotor cortical regions that are rostral to the injury. Corticopontine axons, which carry information to the cerebellum for controlling movements, and corticoreticular axons, which affect the reticular formation and reticulospinal tracts, are also located in the internal capsule. The internal capsule also contains ascending axons as well as other descending axons. The thalamic radiations are the ascending thalamocortical projections located in the internal capsule (Figure 10–9A). The ascending projections from the ventral anterior and ventral lateral nuclei of the thalamus course here, as do those from many of the other thalamic nuclei that project to the frontal and parietal lobes.
Small strokes tend to damage one or another contingent of internal capsule axons because of the particular vascular distributions in the region of the internal capsule (see Figure 3–6). The anterior choroidal artery supplies the posterior limb, where the projections from the primary motor cortex are located. Branches from the anterior cerebral artery or the lenticulostriate branches (anterior and middle cerebral artery) supply the anterior limb and genu.
The Corticospinal Tract Courses in the Base of the Midbrain
The entire internal capsule appears to condense to form the basis pedunculi of the midbrain (Figures 10–9A and 10–11A). The basis pedunculi contains only descending fibers and therefore is smaller than the internal capsule. Each division of the brain stem contains three regions from its dorsal to ventral surfaces: tectum, tegmentum, and base (Figure 10–11A). In the rostral midbrain, the tectum consists of the superior colliculus. The midbrain base is termed the basis pedunculi. Together, the tegmentum and basis pedunculi constitute the cerebral peduncle.
A. Myelin-stained transverse section through the rostral midbrain. The composition of axons in the basis pedunculi and the somatotopic organization of the corticospinal fibers are shown on the right. B. Transverse slice through the midbrain (horizontal slice through central hemispheres) showing site of degeneration. C. Transverse magnetic resonance imaging (MRI) scan through the midbrain of an 8-year-old child with cerebral palsy, produced by a perinatal lesion of the cerebral hemisphere. The MRI scan shows damage to the right cerebral cortex and underlying white matter and degeneration of the basis pedunculi. The area of the degenerated basis pedunculi in this patient was about half that of the other side. She had severe motor impairments of the right arm, especially for highly skilled hand movements. (B, Courtesy of Dr. Jesús Pujol; from Pujol J, Martí-Vilalta JL, Junqué C, Vendrell P, Fernández J, Capdevila A. Wallerian degeneration of the pyramidal tract in capsular infarction studied by magnetic resonance imaging. Stroke. 1990; 21:404-409. C, Courtesy of Dr. Etienne Olivier, University of Louvain; Duqué J, Thonnard JL, Vandermeeren Y, et al. Correlation between impaired dexterity and corticospinal tract dysgenesis in congenital hemiplegia. Brain. 2003; 126:1-16.)
Corticospinal tract axons course within the middle of the basis pedunculi, flanked medially and laterally by corticopontine axons (see Chapter 13) and other descending cortical axons (Figure 10–11A). The location of these axons can be seen on an MRI scan from a patient with a lesion of the posterior limb of the internal capsule (Figure 10–12B). Figure 10–12C shows atrophy in the cerebral peduncle on the side in which the patient, an 8-year-old child, suffered from hemiplegic cerebral palsy produced by damage to the motor cortex and the underlying white matter earlier during childhood.
Myelin-stained section through the pons (A) and MRI through approximately the same level from a person with unilateral internal capsule lesion (B). Note, ventral is down in both images. A. Myelin-stained section through the pons, showing the locations of the motor pathways. B. Transverse slice through the pons and cerebellum (horizontal slice through central hemispheres) showing site of degeneration. (B, Courtesy of Dr. Jesús Pujol; from Pujol J, Martí-Vilalta JL, Junqué C, Vendrell P, Fernández J, Capdevila A. Wallerian degeneration of the pyramidal tract in capsular infarction studied by magnetic resonance imaging. Stroke. 1990;21:404-409.)
The rostral midbrain is a key level in the motor system because three nuclei that subserve motor function are located here: the superior colliculus, the red nucleus, and the substantia nigra. Neurons from the deeper layers of the superior colliculus (Figure 10–11) give rise to the tectospinal tract, a medial descending pathway. The red nucleus (Figure 10–11; also Figure 10–10B, dark oval structures medial to the degenerating cortical fibers), the origin of the rubrospinal tract, is a lateral descending pathway that begins primarily in the magnocellular division of this nucleus. The other major component of the red nucleus, the parvocellular (or small-celled) division, is part of a multisynaptic pathway from the cerebral cortex to the cerebellum (see Chapter 13). The tectospinal and rubrospinal tracts decussate in the midbrain. The substantia nigra is a part of the basal ganglia (see Chapter 14). Substantia nigra neurons that contain the neurotransmitter dopamine degenerate in patients with Parkinson disease.
The Pontine and Medullary Reticular Formation Gives Rise to the Reticulospinal Tracts
In the pons, the descending cortical fibers no longer occupy the ventral brain stem surface but rather are located deep within the base (Figure 10–12A, B). The pontine nuclei receive their principal input from the cerebral cortex via the corticopontine pathway. The corticopontine pathway is an important route by which information from all cerebral cortex lobes influences the cerebellum (see Chapter 13).
The reticular formation comprises a diffuse collection of nuclei in the central brain stem (see Figures 2–8, 2–9, 2–10, 2–11, 2–12). Neurons in the pontine and medullary reticular formation (Figures 10–12A and 10–13A, B) give rise to the reticulospinal tracts. (Few reticulospinal neurons originate from the midbrain.) Experiments in laboratory animals suggest that the reticulospinal tracts control relatively automatic motor responses, such as simple postural adjustments, stepping when walking, and rapid corrections of movement errors. When these automatic responses must occur during voluntary movements, such as maintaining an upright posture when reaching to lift something heavy, the corticoreticulo-spinal pathway is engaged.
A. Myelin-stained section through the medulla, showing the locations of the motor pathways. B and C. Myelin-stained transverse sections through the decussation of the internal arcuate fibers, or mechanosensory decussation (B), and the pyramidal, or motor, decussation (C). Arrows in C indicate the pattern of decussating corticospinal fibers. The solid arrow indicates an axon coursing within the portion of the tract shown in the section. The dashed arrow corresponds to a decussating axon a bit rostral or caudal to this level.
The Lateral Corticospinal Tract Decussates in the Caudal Medulla
The path of the descending cortical fibers into the medulla can be followed in the sagittal section shown in Figure 10–14. The numerous fascicles of the caudal pons collect on the ventral surface of the medulla to form the pyramids (Figures 10–13 and 10–14). The axons of the lateral and ventral corticospinal tracts, which originate primarily from the ipsilateral frontal lobe, are located in each pyramid. This is why the terms corticospinal tract and pyramidal tract are often—but inaccurately—used interchangeably. These terms are not synonymous because the pyramids also contain corticobulbar and corticoreticular fibers that terminate in the medulla. Damage to the corticospinal system produces a characteristic set of motor control and muscle impairments (see section below on brain stem and spinal lesions) that are sometimes called pyramidal signs.
A. Ventral view of the brain stem showing the path of the corticospinal tract. Brackets show the rostrocaudal levels of the somatic sensory and motor decussations. B. Myelin-stained sagittal section (close to the midline) through the brain stem.
Lateral corticospinal tract axons descend in the pyramid and most decussate in the caudal medulla, within fascicles. One fascicle of decussating axons is cut in the section shown in Figure 10–13C (solid line). Another group from the other side (located just rostrally or caudally) would likely decussate along the path shown by the dotted line. The rubrospinal tract, which had crossed in the midbrain, maintains its dorsolateral position. Here, at the medulla–spinal cord junction, the crossed lateral corticospinal tract axons join the rubrospinal axons and descend in the lateral column (Figures 10–6 and 10–13C). These are the two lateral motor pathways. The reticulospinal, vestibulospinal, and tectospinal tracts remain medially located and assume a more ventral position as they descend in the spinal cord. Note that ventral corticospinal tract axons remain ipsilateral within, traveling to the spinal cord along with the vestibulospinal tract and the tectospinal tract.
The Intermediate Zone and Ventral Horn of the Spinal Cord Receive Input From the Descending Pathways
The lateral corticospinal tract is located in the lateral column, revealed by the zone of degeneration in the lumbar cord from an individual who had a lesions of the internal capsule prior to death (Figure 10–15). (Note that the ventral corticospinal tract descends only as far as the cervical spinal cord. Thus, there are no degenerating fibers in the ventral column.) The brain stem pathways are located in both the lateral and ventral columns (Figure 10–6). The motor pathways terminate within the spinal gray matter. As discussed in Chapter 4, the dorsal horn corresponds to Rexed's laminae I through V, and the ventral horn corresponds to laminae VI and IX (Figure 10–16A). From the perspective of the motor systems, we further distinguish the intermediate zone, which corresponds to laminae VI and VII, from the remaining portions of the ventral horn proper, corresponding to laminae VIII-IX. The intermediate zone contains many important interneurons for movement control. The motor nuclei are located in lamina IX. Lamina X surrounds the spinal cord central canal. We focus on the terminations of the corticospinal tract, because its functions are the clearest. The premotor, primary motor, and primary somatic sensory cortical regions all have spinal projections in the corticospinal tract, but their target laminae differ in complex ways. Corticospinal tract axons, as well as the other motor pathways, synapse on interneurons and motor neurons.
Myelin-stained section through the lumbar spinal cord from an individual who had an internal capsule stroke before death. Region showing degeneration in the lateral column (lightly stained) corresponds to the location of the lateral corticospinal tract. Note that the ventral corticospinal is not present at this level.
A. Schematic drawing of the general organization of the spinal cord gray matter and white matter. Note the three classes of interneuron: propriospinal neuron, segmental interneuron, and commissural neuron. B. Drawing of a single spinal segment, showing columns of motor nuclei, running rostrocaudally within the ventral horn.
There are three kinds of spinal cord interneurons: segmental interneurons, commissural interneurons, and propriospinal neurons. Segmental interneurons have a short axon that distributes branches ipsilaterally within a single spinal cord segment to synapse on motor neurons and other interneurons (Figure 10–16A). In addition to receiving input from the descending motor pathways, segmental interneurons receive convergent input from different classes of somatic sensory receptors for the reflex control of movement. Segmental interneurons are located primarily in the intermediate zone and the ventral horn. Commissural interneurons have axons that distribute bilaterally for coordinating the actions of muscles on both sides of the body during walking and for maintaining balance. Propriospinal neurons have an axon that projects for multiple spinal segments before synapsing on motor neurons (Figure 10–16A), and are important for upper-lower limb coordination.
The lateral and medial motor nuclei have different rostrocaudal distributions
The motor neurons that innervate a particular muscle are located within a column-shaped nucleus that runs rostrocaudally over several spinal segments. These column-shaped nuclei of motor neurons collectively form lamina IX (Figures 10–16B and 10–17, inset). Nuclei innervating distal limb muscles are located laterally in the gray matter, whereas those innervating proximal limb and axial muscles are located medially (Figure 10–3). The medial motor nuclei are present at all spinal levels (illustrated schematically as a continuous column of nuclei in Figure 10–17, inset), whereas the lateral nuclei are present only in the cervical enlargement (C5-T1) and the lumbosacral enlargement (L1-S2). A single motor neuron innervates multiple muscle fibers within a single muscle. Collectively, all fibers innervated by a single motor neuron are termed a motor unit.
Approximate locations of the medial and lateral motor nuclei are shown at four spinal cord levels: cervical (A), thoracic (B), lumbar (C), and sacral (D). The inset shows the columnar organization of the medial and lateral motor nuclei. The medial column, which contains motor neurons that innervate proximal and axial muscles, runs throughout the entire spinal cord. Motor nuclei that contain the motor neurons that innervate individual muscles also have a columnar shape but are narrower and course for a shorter rostrocaudal distance. The lateral column contains motor neurons that innervate lateral (distal) muscles. This column is present in the cervical and lumbosacral enlargements only. As for the medial column, motor neurons that innervate individual muscles form narrower and shorter columns.
In the spinal cord, autonomic preganglionic motor neurons are also arranged in a column (see Chapter 15) and, together with the motor nuclei, have a three-dimensional organization similar to that of the brain stem cranial nerve nuclei columns (see Chapter 6). The longitudinal organization of the somatic and autonomic motor nuclei and the cranial nerve nuclei underscores the common architecture of the spinal cord and brain stem.