Spinal Cord : Anatomy and Myelopathies

Spinal Cord

The spinal cord is the downward continuation of the medulla oblongata. It extends from the upper border of the atlas to end in a tapering extremity, the conus medullaris , opposite the lower border of the first lumbar vertebra, or at the level of the intervertebral disk between the upper two lumbar vertebrae (see Plate 2-1 ). From the conus, a slender, median, fibrous thread, the filum terminale , is prolonged as far as the back of the coccyx. The dura mater and arachnoid (and therefore the subarachnoid space) extend down to the level of the second sacral vertebra. Although generally cylindric, the cord is slightly flattened anteroposteriorly and shows cervical and lumbar enlargements that correspond to segments involved in supplying nerves to the upper and lower limbs. The nerve supply to the upper limb involves the fourth cervical to second thoracic spinal cord segments, and that to the lower limb, the third lumbar to third sacral spinal cord segments.

Meninges . The cord is surrounded by dura, arachnoid, and pia mater, which are continuous with the corresponding layers of the cerebral meninges at the foramen magnum. The spinal dura mater , unlike the cerebral, consists only of a meningeal layer that is not adherent to the vertebrae; it is separated from the boundaries of the vertebral canal by an epidural space containing fatty areolar tissue and many veins. The spinal and cranial subarachnoid spaces are continuous and contain cerebrospinal fluid. The pia mater closely invests the cord; on each side, it sends out a series of 22 triangular processes, the denticulate ligaments , which are attached to the dura mater and thus anchor the cord (see Plate 2-2 ). The spinal cord is considerably smaller than the vertebral canal; the meninges, the cerebrospinal fluid and the epidural fatty tissue and veins combine to cushion it against jarring contacts with its bony and ligamentous surroundings.

Spinal Nerves . There are 31 pairs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal) of symmetrically arranged spinal nerves , attached to the cord in linear series by anterior and posterior nerve rootlets, or filaments, which coalesce to form the nerve roots. Each posterior spinal nerve root possesses an oval enlargement, the spinal (sensory) ganglion .

In early embryonic life, the cord is as long as the vertebral canal, but as development proceeds, it lags behind the growth of the vertebral column. Consequently, the cord segments move upward in relation to the vertebrae, and the nerve roots, originally horizontal, assume an increasingly oblique direction from above downward as they proceed to their foramina of exit. In the adult, except in the upper cervical region, the cord segments lie at varying distances above the corresponding vertebrae. For clinical purposes, it is customary to localize them in relation to the vertebral spinous processes. In the lower cervical region, the vertebral spines are one lower in number than the corresponding cord segments; in the upper thoracic region, two lower in number; and in the lower thoracic region, three lower in number. For example, the fourth thoracic spinous process is approximately level with the sixth thoracic cord segment. The lumbar, sacral, and coccygeal segments of the cord are crowded together and occupy the space approximately opposite the ninth thoracic to the first lumbar vertebrae. These alterations of the cord segments relative to the vertebral segments explain why the cervical enlargement (C4 to T2) lies approximately opposite the corresponding vertebrae, whereas the lumbar enlargement (L3 to S3) lies opposite the last three thoracic vertebrae. The nerve roots attached to the lower part of the cord descend to their points of exit as the cauda equina , named for their resemblance to the tail of a horse.

Spinal Membranes and Nerve Roots

Meninges . The spinal cord is enveloped by meninges, which, at the level of the foramen magnum, are directly continuous with those surrounding the brain.

The external, tough, fibrous dura mater continues downward as far as the second sacral vertebra, where it ends blindly. It is separated from the wall of the vertebral canal by an epidural space containing fatty areolar tissue and a plexus of veins. The dura ensheathes the anterior and posterior spinal nerve roots, which lie close together when they pierce it; then the roots unite almost immediately to form a spinal nerve, and the dural sheath fuses with the epineurium. Between the dura mater and arachnoid is a potential subdural space , which normally contains the merest film of lymphlike fluid.

The spinal arachnoid is loose and tenuous and also ends at the level of the second sacral vertebra. It is separated from the pia mater by the subarachnoid space , which is traversed by delicate mesothelial septa and contains cerebrospinal fluid. The spinal nerve roots, up to the points at which they penetrate the dura mater, are loosely enclosed in arachnoid.

The pia mater is a thin layer of vascular connective tissue that intimately invests the spinal cord and its nerve roots. Below the conus medullaris, it is continuous with the slender filum terminale that descends in the midst of the cauda equina, pierces the terminal parts of the dura and arachnoid, and ends by blending with the connective tissue behind the first segment of the coccyx. On each side, the pia is attached to the dura by 22 pointed processes, the denticulate ligaments.

Nerve Roots . The spinal cord is a segmented structure, and this is indicated by the regular attachments of the pairs of spinal nerves. As explained earlier, the cord and vertebral segments coincide in early embryonic life, but the vertebral canal eventually becomes longer than the cord so that most of the spinal nerves run obliquely downward to their points of exit.

The nerve filaments, or rootlets, are attached to the cord along its anterolateral and posterolateral regions. The anterior (ventral) filaments emerge in two or three irregular rows. They are composed predominantly of efferent fibers, which are the axons of cells in the anterior columns, or horns, of gray matter, and they carry motor impulses to the voluntary muscles. In the thoracic and upper lumbar regions, the filaments also contain preganglionic sympathetic fibers, which are the axons of lateral columnar, or cornual, cells. The posterior (dorsal) filaments are attached in a regular series along a shallow groove, the posterolateral sulcus, and are collections of the central processes of nerve cells located in the spinal ganglia of the related dorsal nerve roots. The lateral cell processes pass on in spinal nerves and their branches to peripheral receptors, and they convey afferent impulses back to the spinal cord from somatic, visceral, and vascular sources.

The spinal cord shows an anterior median fissure and a shallow posterior median sulcus from which a median septum of neuroglia extends forward for 4 to 6 mm. The cord is divided into symmetric halves by the fissure, sulcus, and septum. The lines of attachment of the anterior and posterior nerve filaments are used to demarcate the white matter in each half of the cord into anterior, lateral, and posterior columns , or funiculi .

Arteries of Spinal Cord and Nerve Roots

The spinal cord is supplied by multiple radicular arteries , which form the anterior spinal and two posterior spinal arteries .

The radicular arteries arise from the lateral spinal arteries, which traverse the intervertebral foramina at each vertebral segment. Regardless of their origin, the many small radicular arteries pass medially to supply the anterior and posterior nerve roots. Most do not reach the spinal cord. However, some of the larger arteries reach the dura mater, where they give off small meningeal branches and then divide into ascending and descending branches to form the spinal arteries. The larger radicular arteries, which supply both the nerve roots and the spinal cord, are called radiculomedullary arteries to distinguish them from those radicular arteries that supply only the nerve roots.

The anterior spinal artery lies within the pia and runs the entire length of the spinal cord in the midline. It usually originates in the upper cervical region at the junction of the two anterior spinal branches that arise from the intracranial portion of the vertebral artery. Six to ten feeders—the anterior radiculomedullary arteries —contribute to it throughout its length, branching upward and downward. Occasionally, in the thoracic region, the anterior spinal artery narrows to such a degree that it is discontinuous. Blood from the anterior spinal artery is distributed to the anterior two thirds of the substance of the spinal cord via central ( or sulcocommissural) branches and penetrating branches from the pial plexus .

The cervical and first two thoracic segments of the spinal cord are supplied by radiculomedullary arteries that arise from branches of the subclavian artery . Variability is common, and the branches may arise from either the right or the left (often alternately) to join the anterior spinal artery at an angle of 60 degrees to 80 degrees. Not uncommonly, one anterior radiculomedullary branch arises from the vertebral artery and accompanies the C3 nerve root, one branch arises from one of the branches of the costocervical trunk (often the deep cervical artery) and accompanies the C6 root, and one branch arises from the superior intercostal artery and accompanies the C8 root.

The midthoracic region of the spinal cord (T3 to T7) usually receives only one radiculomedullary artery, which accompanies the T4 or T5 nerve root. Consequently, this section of the cord is characterized by its poor blood supply, and the anterior spinal artery may not be continuous at this level.

The thoracolumbosacral part of the spinal cord (T8 to the conus medullaris) derives its main arterial supply from the artery of Adamkiewicz, which arises from a left intercostal (or lumbar ) artery in 80% of individuals. In 85% of instances, it reaches the cord with a nerve root between T9 and L2; in the 15% of cases in which it reaches the cord between T5 and T8, it is supplemented by a radiculomedullary artery (the artery of the conus medullaris) arising more inferiorly. The artery of Adamkiewicz has a large anterior and a smaller posterior branch. On reaching the anterior aspect of the spinal cord, the anterior branch ascends a short distance and then makes a hairpin turn to give off a small ascending branch and a larger descending branch, which drops to the level of the conus medullaris, where it forms an anastomotic circle with the terminal branches of the two posterior spinal arteries.

The cauda equina is accompanied and supplied by one or two branches from the lumbar, iliolumbar , and lateral and median sacral arteries . These branches also ascend to contribute to the anastomotic arterial circle around the conus medullaris.

The central (sulcocommissural) branches of the anterior spinal artery pass back into the anterior median fissure to supply the central parts of the spinal cord. At the anterior commissure, the branches turn alternately right and left to supply the corresponding halves of the cord, except in the lumbar enlargement, where the left and right branches arise from a common trunk. The terminal branches ascend and descend within the cord, supplying overlapping territories. There are 5 to 8 central arteries for each centimeter length of the spinal cord in the cervical region, 2 to 6 in the thoracic region, and 5 to 12 in the lumbosacral area. Branches from each central artery overlap with those from adjacent arteries. The central arteries supply the anterior commissure and adjacent white matter of the anterior columns, anterior horns, bases of the posterior horns, Clarke's columns, corticospinal tracts, spinothalamic tracts, anterior parts of the gracile and cuneate fasciculi, and the region around the central canal.

The posterior spinal arteries are paired arteries coursing on the posterolateral aspects of the entire length of the spinal cord, although they may become discontinuous at times. Each originates from the intracranial portion of the corresponding vertebral artery , and receives contributions from 10 to 23 posterior radiculomedullary arteries . The posterior spinal arteries distribute blood to the posterior third of their respective sides of the cord.

In the cervicothoracic region , the posterior spinal arteries receive one, and sometimes two, tributaries at each segment. Between the T4 and T8 levels , there are usually two or three posterior radiculomedullary branches, while in the thoracolumbar region , there are several feeders, one of which may be the posterior radicular branch of the artery of Adamkiewicz .

Pial Arterial Plexus . Small pial branches arise from the spinal arteries and ramify and interconnect on the surface of the cord to form a pial plexus. Penetrating branches of the plexus are radially oriented to supply the outer part of the substance of the cord; they follow the principal sulci of the cord (the posterior median sulcus and the posterior intermedian sulcus) to reach the anterior and posterior horns. The peripheral pial branches supply the outer portions of the posterior horns, most of the posterior columns, and the outer portion of the white matter of the periphery of the spinal cord.

There is some degree of overlap in the distribution of the peripheral and central arteries at the capillary level, but they do not anastomose at the arterial level, and hence both types are, in effect, end arteries .

Veins of Spinal Cord, Nerve Roots, and Vertebrae

Two plexuses of veins, external and internal, extend along the entire length of the vertebral column and form a series of moderately distinct rings around each vertebra. The plexuses anastomose freely with each other, receive tributaries from the vertebrae, ligaments, and spinal cord and are relatively devoid of valves. Consequently, changes in the pressure of intrathoracic or cerebrospinal fluid may produce variations in the volume of blood, especially in the internal vertebral venous plexuses.

The external vertebral plexus consists of anterior and posterior parts, which anastomose freely. The veins forming the anterior external plexus lie in front of the vertebral bodies, from which they receive venous tributaries and through which they communicate with the basivertebral veins. The posterior external plexus is a network located over the vertebral laminae and extending around the spinous, transverse, and articular processes. In the upper cervical region, the posterior plexus communicates with the occipital veins and, via these, with the mastoid and occipital emissary veins. The posterior plexus also communicates with the vertebral and deep cervical veins, and a few channels pass through the foramen magnum to the dural sinuses in the posterior cranial fossa.

The internal vertebral plexus is formed by networks of veins lying in the epidural space within the vertebral canal. The networks are arranged in anterior and posterior groups, which are interconnected by many smaller oblique and transverse channels. The anterior internal plexus consists of longitudinal veins lying on the posterior surfaces of the vertebral bodies and intervertebral disks found on each side of the posterior longitudinal ligament. Interconnecting branches lie between the ligament and the vertebral bodies and receive the basivertebral veins. The longitudinal veins in the posterior internal plexus are smaller than their anterior counterparts. They are located on each side of the median plane in front of the vertebral arches and ligamenta flava. They anastomose with the veins of the posterior external vertebral plexus via small veins that pierce the ligaments and pass between them.

The basivertebral veins resemble the cranial diploë, and tunnel through the cancellous tissue of the vertebral bodies. They converge to form a comparatively large, single (occasionally, double) vein that emerges through the posterior surface of the vertebral body to end, via openings guarded by valves, in the transverse interconnections of the anterior internal vertebral plexus. The basivertebral veins also drain into the anterior external plexus through openings in the front and sides of the vertebral body.

The veins of the spinal cord resemble the related arteries in their distribution and form a tortuous plexus in the pia mater (see Plate 2-5 ). Intrinsic veins from the anteromedial region of the spinal cord and radial veins from the anterior funiculus drain into the anterior median spinal (longitudinal)vein, sometimes duplicated. Capillaries and venules from the rest of the spinal cord drain by radial veins into the coronal veins on the posterior and lateral surface of the spinal cord. These superficial veins drain, in turn, by the anterior and posterior medullary veins , sometimes called radicular veins, which accompany the nerve roots and radicular or radiculomedullary arteries. The medullary veins unite with radicular veins draining the nerve roots and with branches from the anterior and posterior internal vertebral plexuses to form the intervertebral veins . Above, the spinal veins communicate with veins draining the medulla oblongata and the inferior surface of the cerebellum through the foramen magnum.

The intervertebral veins drain most of the blood from the spinal cord and from the internal and external vertebral venous plexuses. They accompany the spinal nerves through the intervertebral foramina and end in the vertebral, posterior intercostal, subcostal, lumbar, and lateral sacral veins. Their orifices are usually protected by valves.

Principal Fiber Tracts of Spinal Cord

The spinal cord consists of a core of gray matter , surrounded by an outer fiber layer, the white matter. The gray matter consists of the cell bodies and dendrites of spinal neurons and the axons and axon terminals issuing from them or ending upon them (see Section 1 , Normal and Abnormal Development, in Part I). The white matter consists of the axons of longitudinally running fiber tracts. The outlines of the gray and white matter are different at different spinal levels ( Plate 2-6 ). The white matter is relatively massive in the cervical region and declines progressively in bulk in the lower levels. The gray matter is most highly developed in the cervical and lumbar enlargements, where it is made up of the neurons involved in the sensory and motor functions of the arms and the legs.

The schematic cross section in the lower part of the illustration shows the location of the principal fiber tracts within the spinal white matter. As indicated by the colors, the tracts can be divided into ascending (blue) and descending (red) pathways linking the spinal cord with the brain, and propriospinal (purple) pathways made up of fibers interconnecting different levels within the spinal cord itself.

The ascending pathways include the fasciculus gracilis and fasciculus cuneatus (part of the medial lemniscus system), which convey fine discriminative sensation from the lower and upper parts of the body, respectively. Less discriminative, higher-threshold sensations are carried by the anterior and lateral spinothalamic tracts ; the latter is particularly important in conveying the sensations of pain and temperature. Other ascending pathways, which are more closely involved in reflex activity and motor control, include the posterior and anterior spinocerebellar tracts and the spino-olivary, spinotectal, and spinoreticular tracts.

The descending pathways are divided into two groups. The first group includes the corticospinal tracts and the rubrospinal tract. It terminates preferentially in the posterolateral regions of the spinal cord, which contain the neurons controlling the distal muscles of the limbs. Damage to these pathways results in loss of fine-fractionated control of the extremities. The second group includes the anterior and lateral reticulospinal tracts , the tectospinal tract , the lateral and medial vestibulospinal tracts , and the interstitiospinal tract (from the interstitial nucleus of Cajal and pretectal area) that runs in the medial longitudinal fasciculus and terminates preferentially in the anteromedial regions of the spinal cord. These regions contain the neurons controlling axial and proximal limb muscles and regulate posture and righting. In addition to their motor action, both sets of descending pathways also include fibers that modulate sensory transmission by spinal pathways.

Propriospinal Pathways . Some of the propriospinal pathways consist of afferent fibers, which enter the spinal cord via the posterior roots and then ascend or descend in the oval bundle, comma tract, posterolateral fasciculus (of Lissauer), fasciculus gracilis, or fasciculus cuneatus to terminate on spinal neurons at other levels of the spinal cord. Other propriospinal fibers originate from interneurons in the spinal gray matter itself. Collectively, propriospinal fibers are important in mediating spinal reflexes and coordinating activity at different levels of the spinal cord.

Somesthetic System of Body

Neural pathways conveying somatosensory information to the cerebral cortex can be divided into two major systems: posterior and anterolateral. The posterior pathways are involved in mediating fine tactile and kinesthetic sensations, whereas the anterolateral pathways conduct impulses for pain and temperature and for touch and deep pressure.

The posterior funiculus, made up of the fasciculus gracilis and the fasciculus cuneatus , carries fibers that signal discriminative touch or pressure, muscle length and tension, and joint position. Some afferent fibers, principally those from quickly adapting cutaneous receptors, ascend the entire length of the spinal cord to synapse directly with neurons in the gracile and cuneate nuclei , which relate to the lower and upper parts of the body. Other fibers leave the posterior columns and either activate spinal neurons for reflex purposes or project upward in the posterolateral funiculus. In humans, most of these secondary ascending fibers also end in the gracile or cuneate nuclei, although a small number of axons sometimes terminate in the upper cervical segments. All of the above nuclei send their axons via the medial lemniscus to the contralateral ventral posterolateral (VPL) nucleus of the thalamus, which projects to the somatosensory regions of the cerebral cortex.

The relay neurons in the gracile, cuneate, and VPL nuclei and the neurons of the primary somatosensory cortex are activated by a single sensory modality over a restricted receptive field. The receptive fields of neurons within each nucleus are arranged in an orderly fashion and give rise to a somatotopic representation of the body surface. Thus a high degree of specificity and order is maintained throughout the pathway.

Anterolateral Funiculus . Two somatosensory pathways ascend in the anterolateral spinal white matter: the lateral and anterior spinothalamic tracts and the smaller spinoreticulothalamic pathway. The spinothalamic tracts arise from neurons in the regions of the posterior horn of the spinal cord that correspond to laminae I, IV, V, and VI of Rexed (see Plate 2-13 ). Most axons cross in the anterior white commissure at about the level of their cell bodies and ascend in the contralateral lateral and anterior funiculi, although a few fibers ascend ipsilaterally. The spinothalamic axons end principally in the VPL nucleus and in the posterior nuclear group and intralaminar nuclei. Some spinothalamic neurons (especially those in lamina I) respond only to strong, noxious stimuli, but most of these neurons are excited by the activity of a wide variety of afferent fibers related to touch, pressure, vibration, and temperature sense. All spinothalamic neurons have large, unilateral receptive fields and transmit information about a wide variety of peripheral stimuli but with less specificity than is shown by neurons in the posterior spinal pathways.

The spinoreticulothalamic pathway (not shown) begins with neurons in the regions corresponding to laminae I and V to VIII, which ascend in the lateral and anterior funiculi to activate neurons in the brainstem reticular formation , which, in turn, project to the intralaminar nuclei of the thalamus. The spinoreticulothalamic neurons respond to the same stimuli as spinothalamic neurons, but tend to have large, bilateral receptive fields. This fact, together with the nonspecific nature of the intralaminar nuclei, suggests that this pathway is involved with poorly localizable pain sensation and is more important in generalized arousal reactions than in discriminative processing of sensation.

Lesions . Because the principal pathways of the posterior and anterolateral columns cross in the medulla and in the spinal cord, and because each pathway transmits specific modalities, damage from spinal cord lesions presents specific and characteristic deficits. Posterior column destruction results in ipsilateral loss of discriminatory touch and vibration sense, as well as loss of position sense below the level of the lesion. Anterolateral column interruption produces contralateral loss of pain and temperature sense accompanied by diminished touch sense below the lesion.

Corticospinal (Pyramidal) System: Motor Component

The corticospinal tract arises from wide regions of the cerebral cortex and is involved in multiple functions. It contributes to the control of somatosensory inputs and to motor activity. The motor component of the corticospinal ( pyramidal) tract originates primarily in the cells of layer V in the primary motor cortex of the precentral gyrus (area 4) and projects to motor neurons and interneurons concerned with motor control throughout the central nervous system (CNS). Only the direct connections, by which cortical neurons excite motor neurons in the motor nuclei of the brainstem and spinal cord, are shown. Other illustrations show the projections of the motor cortex to the basal ganglia, thalamus, red nucleus (see Plate 2-9 ), reticular formation (see Plate 2-11 ), and intermediate spinal gray matter (see Plate 2-12 ).

The direct motor component of the pyramidal tract runs from the precentral gyrus through the posterior limb of the internal capsule and into the midbrain, where it gives slips to the oculomotor, trochlear, and abducens nuclei. It then enters the pons, where it gives off fibers to the trigeminal motor and facial nuclei, which control the muscles of the face. From the pons, the tract continues through the medullary pyramids, giving off fibers to the nuclei of the ninth, tenth, eleventh, and twelfth cranial nerves. The major part of the tract then crosses to the opposite side of the brainstem at the pyramidal decussation , and the crossed fibers continue to all levels of the spinal cord as the lateral corticospinal tract . A smaller group of uncrossed fibers continues to the cervical spinal cord as the anterior (direct) corticospinal tract . The fibers end by synapsing with motor neurons in the anterior horn of the spinal cord (see Plate 2-13 ).

The pyramidal tract exhibits a somatotopic organization throughout its course. The homunculus at the top of the illustration indicates the orderly topographic arrangement of areas within the precentral gyrus, from which muscles in various parts of the body can be activated. The area controlling the face lies most laterally, with the areas related to the hand, arm, trunk, and hip following, in order, toward the midline. The areas representing the leg continue downward along the medial aspect of the cortex. Within each area, movements involving distal muscles are represented posteriorly, and proximal muscles, anteriorly. The initial somatotopic organization at the cortex persists in the arrangement of fibers along the course of the tract (see Plate 2-14 ). The control of voluntary movements probably relates, however, to distributed networks that are capable of modification rather than to discrete representations. There appears to be considerable plasticity of representations and cell properties in the primary motor cortex, probably related to the horizontal neuronal connections in the cortex. The primary motor cortex is not a simple static motor control structure but contains a dynamic substrate that participates in motor learning.

Lesions of the motor cortex may produce discrete pareses, depending upon the type and size of the lesion and its somatotopic location. Irritative lesions of the cortex can lead to abnormal movements and ultimately to jacksonian seizures as the irritative focus spreads. Damage to the internal capsule produces contralateral paralysis, along with cranial nerve involvement.

In general, pyramidal tract disturbances produce an initial flaccid paralysis and areflexia, followed by spastic paralysis and hyperactive reflexes. Brainstem lesions cause paralysis contralateral to the lesion, accompanied by ipsilateral or contralateral cranial nerve deficits, depending on the level of the lesion. Spinal cord damage to the tract is usually accompanied by alterations in the autonomic and sensory systems.

Rubrospinal Tract

The red nucleus (so-called because of its reddish color in the fresh brain) is situated in the midbrain. It receives a large number of fibers from the contralateral cerebellum and the ipsilateral cerebral cortex and, in turn, has a major projection to the spinal cord, the rubrospinal tract. Knowledge of the rubrospinal tract in humans is limited. It seems to arise predominantly from the large neurons of the caudal part of the red nucleus, is arranged somatotopically, and extends the entire length of the spinal cord, influencing alpha and gamma motor neurons. The predominant target of its action is the motor apparatus controlling the distal muscles of the contralateral limbs, although the tract also acts to inhibit the action of cutaneous and muscle afferent fibers on spinal neurons. Within the brainstem, fibers branch from the rubrospinal tract to terminate in the facial nucleus (control of facial muscles), the lateral reticular nucleus (cerebellar afferent relay), and the gracile and cuneate nuclei (control of afferent input) (see Plate 2-7 ). In addition to being the source of the rubrospinal tract, the red nucleus sends fibers to the ipsilateral inferior olive (cerebellar afferent relay) and medial reticular formation (see Plate 2-11 ).

As shown in the illustration, rubrospinal fibers decussate almost immediately on leaving the red nucleus to descend through the lateral part of the brainstem to the spinal cord. In the cord, the tract lies in the posterolateral funiculus , just anterior to the lateral corticospinal tract. The distal branches of the rubrospinal fibers terminate in the intermediate regions and anterior horn (laminae V, VI, and VII) of the spinal gray matter (see Plate 2-12 ).

The rubrospinal tract influences the motor neurons in the anterior horns, primarily through its action on inhibitory or excitatory interneurons, but in primates some fibers end directly on anterior horn motor neurons. The predominant pattern of rubrospinal action is to facilitate flexor motor neurons and thus excite limb flexor muscles and to inhibit the corresponding extensor muscles via interneurons. However, a number of rubrospinal fibers have the opposite action. This allows a wide variety of movements to be executed by the selective activation of appropriate groups of rubrospinal neurons. The rubrospinal tract may thus be responsible for much of the relatively fine control of the extremities—discriminative movement that is retained when the pyramidal tract is damaged. In animals, lesions involving both the pyramidal and rubrospinal tracts result in a much greater deficit in distal movement than that obtained from a lesion of either tract alone.

Rubrospinal control of afferent input to the spinal cord takes the form of presynaptic inhibition acting at the central posterior horn terminals of fibers from Golgi tendon organs and cutaneous receptors.

The two major sources of the input that controls the activity of rubrospinal neurons are the cerebellum and the cerebral cortex . The cerebellar projection to the red nucleus consists primarily of fibers from the interposed (emboliform and globose) nuclei, which cross in the decussation of the superior cerebellar peduncle (brachium conjunctivum) to excite the red nucleus neurons of the opposite side. Neurons of the red nucleus are also excited by branches of small pyramidal cells from the ipsilateral motor cortex (see Plate 2-8 ). Afferents from the motor and premotor cerebral cortex synapse on their distal dendrites and from the cerebellum on their proximal dendrites and cell bodies. In addition, activity in pyramidal tract axons from giant neurons in the same cortical region exerts an opposite, inhibitory effect on rubrospinal neurons via inhibitory interneurons. The input as well as the output of the red nucleus is somatotopically organized . Thus rubrospinal fibers projecting to the lumbar part of the spinal cord originate from neurons in the lateral part of the nucleus. This same region receives input from regions of the cerebellar deep nuclei and motor cortex related to control of the lower limbs. Conversely, the medial part of the red nucleus, which contains neurons projecting to cervical levels of the spinal cord, receives input from cerebellar and cerebral regions responsible for control of the arms. This pattern of organization allows for the selective activation of individual extremities by different groups of rubrospinal neurons.

Vestibulospinal Tracts

The vestibular system is involved in the control of balance. The vestibular nuclei consist of four major groups of neurons—the superior, medial, lateral, and inferior vestibular nuclei —situated in the posterolateral part of the pons and medulla oblongata. Three of these neuronal groups, the medial, lateral (Deiters), and inferior (descending) vestibular nuclei—comprise the major central termination of the vestibular afferent fibers that supply the otolithic organs (utricle and saccule) of the labyrinth. Vestibular afferent fibers supplying the semicircular canals end primarily in the superior, medial, and lateral vestibular nuclei, but many fibers also terminate in the vestibulocerebellum. In addition to these vestibular afferent impulses, the vestibular nuclei also receive input from the spinal cord, cerebellum, reticular formation, and higher centers.

The known output pathways from the vestibular nuclei include projections to the spinal cord, oculomotor nuclei, cerebellum and reticular formation. Vestibular activity also reaches the thalamus, superior colliculus and other higher centers, but the exact pathways are not known.

Vestibulospinal Tracts . The illustration shows the projections of vestibular neurons to the spinal cord via the lateral vestibulospinal tract (LVST) and medial vestibulospinal tract (MVST) . These two tracts, which lie in the anterior and anteromedial funiculi (see Plate 2-12 ), act primarily on the motor apparatus that controls the proximal muscles and therefore are important in the regulation of postural equilibrium.

The LVST is uncrossed and originates primarily from the lateral vestibular nucleus . Some of its constituent fibers extend the entire length of the spinal cord, whereas others extend only part of this distance; they may branch to innervate several regions as they descend. The lateral nucleus is somatotopically organized : neurons projecting to the lower (hindlimb) levels of the spinal cord are located in the posterior and distal portion of the nucleus, and neurons ending at higher levels are situated more anteriorly and rostrally. The former region receives a heavy projection from the cerebellar vermis, whereas the latter region receives a heavy input of vestibular afferent fibers. The LVST ends in lamina VIII and parts of lamina VII; it acts on alpha and gamma neurons.

The predominant action of the LVST is to produce the contraction of extensor (antigravity) muscles and the relaxation of flexor muscles. In the case of neck, trunk, and some lower limb extensor muscles, contraction is produced in part by direct (monosynaptic) excitation of motor neurons. The excitation of other limb extensor muscles and the inhibition of flexor muscles are mediated by pathways that include spinal interneurons.

The MVST , which projects bilaterally to the cervical cord, is involved in reflex adjustments of the head and axial muscles to vestibular stimulation. It contains fibers that originate primarily in the medial vestibular nucleus and produce direct inhibition of motor neurons controlling neck and axial muscles. The tract seems to stop in the midthoracic region. The two vestibulospinal tracts are important factors in vestibular reflex reactions that are triggered by the movement of the head in space. Particularly significant in this regard is the strong vestibular action on the neck muscles, which helps to stabilize the position of the head. However, these tracts and the reticulospinal tracts (see Plate 2-11 ) also appear to play a much wider role in the control of the proximal musculature.