The spinal cord and dorsal root ganglia innervate most of the body. Afferent sensory fibers enter the spinal cord through the dorsal roots of spinal nerves; motor and other efferent fibers leave by way of the ventral roots (the Bell-Magendie law). Signals originating in sensory endings initiate reflexes within the spinal cord and are relayed to the brain stem and cerebellum to contribute to circuits that influence motor performance and other functions. Sensory signals are also sent rostrally to the thalamus and cerebral cortex, where they enter conscious experience and may elicit immediate or delayed behavioral responses. Motor neurons in the spinal cord are excited or inhibited by impulses originating at various levels of the brain, from the medulla to the cerebral cortex. As the tracts of the spinal cord are identified, references are made to components of the brain that are discussed in later chapters. When the central nervous system (CNS) is described by regions, it is necessary to probe ahead of the region under immediate consideration. An appreciation of the major systems is acquired step by step. The general sensory and motor systems are reviewed in Chapters 19 and 23, respectively.
Gross Anatomy of the Spinal Cord and Nerve Roots
The spinal cord is a cylindrical structure, slightly flattened dorsoventrally, located in the spinal canal of the vertebral column. Protection for the cord is provided not only by the vertebrae and their ligaments but also by the meninges and a cushion of cerebrospinal fluid (CSF).
SPINAL CANAL AND MENINGES
The innermost meningeal layer is the thin pia mater, which adheres to the surface of the spinal cord. The outermost layer, the thick dura
mater (or simply, dura) forms a tube extending from the level of the second sacral vertebra to the foramen magnum at the base of the skull, where it is continuous with the dura around the brain. The arachnoid lies against the inner surface of the dura, forming the outer boundary of the fluid-filled subarachnoid space. The spinal cord is suspended in the dural sheath by a denticulate ligament on each side. This ligament is made of pia-arachnoid tissue and is in the form of a ribbon attached to the cord midway between the dorsal and ventral roots (Fig. 5-1). The lateral edge of the denticulate ligament is serrated and is attached at 21 points to the dural sheath at intervals between the foramen magnum and the level at which the dura is pierced by the roots of the first lumbar spinal nerve. An epidural space, filled with fatty tissue that contains a venous plexus, intervenes between the dura and the wall of the spinal canal. The epidural space caudal to the second sacral vertebra also contains the roots of the most caudal spinal nerves.
FIGURE 5-1 Dorsal view of the cervical enlargement of the spinal cord, showing the attachments of the denticulate ligament.
SEGMENTS OF THE SPINAL CORD, ROOTS, AND VERTEBRAL COLUMN
The segmental nature of the spinal cord is demonstrated by the presence of 31 pairs of spinal nerves, but there is little indication of segmentation in its internal structure. Each dorsal root is broken up into a series of rootlets that are attached to the spinal cord along the corresponding segment (Fig. 5-2). The ventral root arises similarly as a series of rootlets.
Each spinal nerve divides into a dorsal and a ventral primary ramus. The dorsal primary ramus supplies the skin of the back and muscles that are attached at both ends to parts of the vertebral column. In the cervical, brachial, and lumbosacral plexuses, ventral primary rami join, exchange fibers, and branch into the mixed nerves that carry motor and sensory axons to the skin and muscles of the lateral and ventral trunk and the limbs. The numeric relations of spinal nerves and vertebrae are explained in Table 5-1.
EMBRYOLOGY AND GROWTH
The early development of the spinal cord from the neural tube and the caudal eminence is described in Chapter 1. Segments of the neural tube (neuromeres) correspond in position with segments of the vertebral column (scleromeres) until the third month of fetal development. The vertebral column elongates more rapidly than the spinal cord during the remainder of fetal life. By the time of birth, the caudal end of the spinal cord is opposite the disk between the second and third lumbar vertebrae. A slight difference in growth rate continues during childhood, such that the adult's cord ends opposite the disk between the first and second lumbar vertebrae (Fig. 5-3). This is an average level; the caudal end of the cord may be as high as the twelfth thoracic or as low as the third lumbar vertebral body. The subarachnoid space caudal to the end of the spinal cord is known as the lumbar cistern.It contains CSF and is traversed by the roots of lumbar and sacral nerves.
The rostral shift of the spinal cord during development determines the direction of spinal nerve roots in the subarachnoid space. As shown in Figure 5-3, spinal nerves from C1 through C7 leave the spinal canal through the intervertebral foramina above the corresponding
vertebrae. (The first and second cervical nerves lie on the vertebral arches of the atlas and axis, respectively.) The eighth cervical nerve passes through the foramen between the seventh cervical and first thoracic vertebrae because there are eight cervical cord segments and seven cervical vertebrae. From that point caudally, the spinal nerves leave the canal through foramina immediately below the pedicles of the corresponding vertebrae.
FIGURE 5-2 A segment of the spinal cord, showing the dorsal and ventral rootlets and roots, sensory ganglia, and mixed spinal nerves. (Used with permission from Moore KL, Dalley AF. Clinically Oriented Anatomy, 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2006.)
TABLE 5-1 Numbering of Spinal Nerves and Vertebrae*
SPINAL AND VERTEBRAL LEVELS
The dorsal and ventral roots traverse the subarachnoid space and pierce the arachnoid and dura mater. At this point, the dura becomes
continuous with the epineurium. After passing through the epidural space, the roots reach the intervertebral foramina, where the dorsal root ganglia are located. The dorsal and ventral roots join immediately distal to the ganglion to form the spinal nerve. The length and obliqueness of the roots increase progressively in a rostrocaudal direction because of the increasing distance between cord segments and the corresponding vertebral segments (see Fig. 5-3). The lumbosacral roots are, therefore, the longest and constitute the cauda equina in the lower part of the subarachnoid space. The cord ends as the conus medullaris, which tapers rather abruptly into a slender filament called the filum terminale. The caudal 3 cm of the spinal cord contains most of the segments that communicate with the lower limb and perineum. Immediately below the conus medullaris are all the segmental nerve roots below L1.
FIGURE 5-3 Relation of segments of the spinal cord and spinal nerves to the vertebral column. The vertebral bodies are on the right side, and the dorsal spines of the vertebrae on the left.
The filum terminale lies in the middle of the cauda equina and has a distinctive bluish color that distinguishes it from the white nerve roots. It consists of pia mater surrounding neuroglial elements and is a vestige of the spinal cord of the embryonic tail. The filum terminale picks up a dural investment opposite the second segment of the sacrum, and the resulting coccygeal ligament attaches to the dorsum of the coccyx.
The spinal cord is enlarged in two regions for innervation of the limbs. The cervical enlargement includes segments C4 to T1, with most of the corresponding spinal nerves forming the brachial plexuses for the nerve supply of the upper limbs. Segments L2 to S3 are included in the lumbosacral enlargement, and the corresponding nerves constitute most of the lumbosacral plexuses for the innervation of the lower limbs.
Internal Structure of the Spinal Cord
The surface of the spinal cord is marked by longitudinal furrows. The deep ventral median fissure contains connective tissue of the pia mater and the anterior spinal artery and its branches. The dorsal median sulcus is a shallow midline
furrow. Many textbooks describe a dorsal septum, supposedly composed of pial tissue, that extends from the base of this sulcus almost to the gray matter. In fact, there is no collagenous connective tissue in the dorsal midline of the cord; the “dorsal septum” does not exist.
Lumbar Disks and Spinal Nerves
All intervertebral foramina are slightly rostral to the levels of the intervertebral disks. If the nucleus of a lumbar disk herniates laterally through its fibrous outer ring, the protrusion presses on a spinal nerve that has not yet left the spinal canal. For example, herniation of the disk between vertebrae L4 and L5 results in compression of spinal nerve L5 or S1.
It is helpful when examining a patient with a possible spinal cord or nerve root lesion to determine the location of the cord segments in relation to vertebral spines, vertebral bodies, and intervertebral disks. The corresponding levels are shown in Figure 5-3.
GRAY MATTER AND WHITE MATTER
In transverse sections, the gray matter has a roughly H-shaped outline (Figs. 5-4, 5-5, and 5-6). The small central canal is lined by ependyma, and its lumen may be obliterated in places. The gray matter on each side consists of dorsal and ventral horns and an intermediate zone. A lateral horn, containing preganglionic sympathetic neurons, is added in the thoracic and upper lumbar segments.
There are three main categories of neurons in the spinal gray matter. Motor cells of the ventral horn supply the skeletal musculature and consist of alpha and gamma motor neurons, whose functions are described in Chapter 3. The cell bodies of tract cells, whose axons constitute the ascending fasciculi of the white matter, are located mainly in the dorsal horn. The cells, involved in local circuitry, are called interneurons, even though many of them have quite long axons (see under Fasciculus Proprius, later).
It may be necessary to insert a needle into the subarachnoid space to obtain a sample of CSF for analysis or for other reasons. A spinal lumbar puncture is the preferred method: the needle is inserted between the dorsal spines of the third and fourth lumbar vertebrae to enter the lumbar cistern without risk of damaging the spinal cord. In the midline of the lumbar cistern, the needle does not touch the lumbosacral nerve roots.
The white matter consists of three funiculi (see Figs. 5-4, 5-5, and 5-6). (These are often called “columns,” but this word is more appropriate for longitudinally aligned arrays of neuronal cell bodies in the gray matter.) The dorsal funiculus (posterior column) is bounded by the midline and the dorsal gray horn. It consists of a gracile fasciculus, present throughout the length of the cord, and above the midthoracic level is also a laterally placed cuneate fasciculus. The remainder of the white matter consists of lateral and ventral funiculi, between which there is no anatomical demarcation. Axons decussate in the ventral white commissure. The dorsolateral tract (tract of Lissauer) occupies the interval between the apex of the dorsal horn and the surface of the cord. The white matter consists of partially overlapping bundles (tracts or fasciculi) of fibers, as described later.
Although the general pattern of gray matter and white matter is the same throughout the spinal cord, regional differences are apparent in transverse sections (see Figs. 5-4, 5-5, and5-6). For example, the amount of white matter increases in a caudal-to-rostral direction because fibers are added to ascending tracts, and fibers leave descending tracts to terminate in the gray matter. The main variation in the gray matter is its increased volume in the cervical and lumbosacral
enlargements for innervation of the upper and lower limbs. The lateral horn of gray matter is characteristic of the thoracic and upper lumbar segments. Caudal to S2, the ventral median fissure is shallow, so the left and right ventral horns blend together in a wide band of gray matter ventral to the central canal.
FIGURE 5-4 Seventh cervical segment. (Transverse section stained by Weigert's method for myelin, X6.)
FIGURE 5-5 Second thoracic segment. (Weigert's stain, X7.)
NEURONAL ARCHITECTURE OF SPINAL GRAY MATTER
As with other parts of the CNS, the spinal gray matter is composed of several neuronal populations. The cell types are classified according to their appearances under the microscope, and
it has been found that cells of the same type are usually clustered together into groups. Because the architecture of the spinal gray matter is essentially the same along the length of the cord, the populations of similar neurons occur in long columns. When viewed in transverse sections of the spinal cord, many of the cell columns appear as layers, especially within the dorsal horn. Ten layers of neurons are recognized, known as the laminae of Rexed. Before the laminae were described in 1952, names were given to many of the cell columns, but they were used differently by different authors, and confusing synonyms existed. The laminar scheme is summarized in Figure 5-7. Descriptions of the laminae can be found in the extended chapter available online.
FIGURE 5-6 First sacral segment. (Weigert's stain, X7.)
The spinal gray matter is organized in the following way. Sensory axons of dorsal roots end predominantly in the dorsal horn. Impulses concerned with pain, temperature, and touch reach the tract cells, most with cell bodies in the deeper laminae of the dorsal horn, from which the spinothalamic tract originates. The sensory information transmitted to the brain, especially for pain, is subject to modification (editing) by interaction with other modalities of sensation and by impulses that reach the dorsal horn by way of various descending pathways. Lamina II, the substantia gelatinosa, contains interneurons that have a prominent role in modifying the perception of pain (see Chapter 19). Motor neurons (lamina IX) supply the skeletal musculature. With the intervention of interneurons, the motor neurons usually come under the influence of dorsal root afferents for spinal reflexes and of several descending tracts for the control of motor activity by the brain. Of the columns of motor neurons that constitute lamina IX, those supplying axial musculature are present in the medial part of the ventral horn, and those supplying the limbs are located more laterally. Distinctive columns of motor neurons include the phrenic and accessory nuclei in the cervical segments (motor neurons for the phrenic and accessory nerves) and the nucleus of Onuf (innervation of pelvic floor muscles) in the sacral cord. Distinctive cell columns in the thoracic and upper lumbar segments (formally included with lamina VII) are the nucleus dorsalis, which gives rise to the dorsal spinocerebellar tract, and the intermediolateral cell column, which consists of preganglionic sympathetic neurons. The midsacral segments contain a less conspicuous intermediolateral column, the sacral autonomic nucleus.Scattered spinal border cells, at the
gray-white interface of the ventral horn in the lumbar segments, contribute to the ventral spinocerebellar tracts.
FIGURE 5-7 Positions of cytoarchitectonic laminae in the spinal gray matter at three levels of the human spinal cord. Roman numerals in blue are for laminae that receive input from dorsal roots; red is for the lamina that contains motor neurons. Lamina VII, which contains named cell columns, is colored yellow.
Each dorsal root branches into six to eight rootlets as it approaches the spinal cord, and the axons become segregated into two divisions within each rootlet (Fig. 5-8). The lateral division contains most of the unmyelinated (group C) axons and some thin myelinated (group A) axons. These axons enter the dorsolateral tract (of Lissauer), where they divide into ascending and descending branches, each giving off collaterals that enter the dorsal horn. Most of these fibers terminate in their own or in immediately adjacent segments, synapsing with interneurons and with tract cells that give rise to spinothalamic fibers. Most of the tract cells are in the nucleus proprius in the deeper laminae of the dorsal horn.
The medial division of dorsal root fibers, for modalities of sensation other than pain and temperature, consists largely of myelinated axons, including all the large-caliber, rapidly conducting sensory fibers. These enter the spinal white matter medial to the dorsal horn where, similar to those of the lateral division, they divide into ascending and descending branches. The descending branches run caudally within the dorsal funiculi for varying distances and eventually terminate in the dorsal horn. (Some of the long descending fibers of the dorsal funiculi are in distinct bundles, the septomarginal fasciculus and the interfascicular fasciculus, whose positions are indicated in Fig. 5-9.) Many of the ascending sensory fibers in the dorsal funiculus terminate in the gracile and cuneate nuclei in the medulla. At the other extreme, axons from the medial division of the dorsal root enter the gray matter at their own segmental levels; such fibers are conspicuous in lamina IV of the dorsal horn (see Figs. 5-4 and 5-6). Primary sensory axons conveying signals from muscle spindles have some branches that terminate on motor neurons and are involved in the stretch reflex. Some of the synaptic arrangements
in the dorsal gray horn are summarized in Figure 5-8.
FIGURE 5-8 Neuronal circuitry of the dorsal horn of the spinal gray matter, showing afferent fibers in the medial (blue) and lateral (black) divisions of the dorsal root. Principal cells of the spinal cord are shown in red, and an interneuron of the substantia gelatinosa is green. Compare with Figure 5-11.
The columns of cells constituting lamina IX contain motor neurons of two types, named after the diameters (and therefore the conduction velocities) of their axons. The alpha motor neurons supply the ordinary (extrafusal) fibers of striated skeletal muscles. The smaller gamma motor neurons are less numerous and supply the intrafusal fibers of the neuromuscular spindles. The surfaces of both motor neuron types are densely covered with synaptic terminals, which release either excitatory or inhibitory transmitter substances. Each alpha motor neuron receives at least 20,000 synaptic contacts. The sources of the afferents are numerous; some are from descending tracts of the spinal cord, and others are branches of axons of primary afferent neurons. The greatest numbers, however, are from intrinsic cells of the spinal gray matter, which behave physiologically as interneurons. The interneurons are located mainly in lamina VII. They receive their afferents from one another, from descending tracts, and from dorsal root ganglion neurons concerned with all modalities of sensation.
A special type of interneuron, from the physiological standpoint, is the Renshaw cell, which receives excitatory synaptic input from branches of the axons of nearby motor neurons. The branched axon of a Renshaw cell forms inhibitory synaptic junctions on motor neurons, including the same ones that are presynaptic to the Renshaw cell itself. By inhibiting nearby motor neurons, the Renshaw cell circuitry focuses
motor commands onto the muscles supplied by the most frequently firing motor neurons. The circuitry of the ventral horn is summarized in Figure 5-10.
FIGURE 5-9 Major tracts of the spinal white matter at midcervical level. Ascending tracts (blue) are on the left; descending tracts (red) are on the right. The stippled areas adjacent to the gray matter indicate propriospinal fibers.
TRACTS OF ASCENDING AND DESCENDING FIBERS
The spinal white matter is divided into longitudinally aligned funiculi, whose positions have already been described. Each funiculus contains tracts of ascending and descending fibers. The positions of the tracts have been approximately determined from clinical and pathological studies and from comparison of these clinical data with the more exact information obtained from animal studies. Most neuroanatomy and clinical neurology textbooks contain diagrams such as Figure 5-9, showing the positions of the major tracts. It is important to realize that the precise positions of some tracts are not known with certainty and that the territories of the different tracts overlap.
The most important component of each dorsal funiculus is a large body of ascending axons derived from neurons located in the dorsal root ganglia. Other ascending fibers are axons of neurons in the dorsal horn. The ascending fibers are all ipsilateral. They are especially concerned with the discriminative qualities of sensation, including the ability to recognize changes in the positions of tactile stimuli applied to the skin and conscious awareness of movement and of the positions of joints. It was formerly thought that conscious appreciation of vibration required the integrity of the dorsal funiculi, but clinical observations indicate that this is not so. Both the dorsal and the lateral funiculi conduct impulses initiated by vibratory stimuli.
As the spinal cord is ascended, axons are added to the lateral side of each dorsal funiculus. Consequently, in the upper cervical cord, the lowest levels of segmental innervation are represented in the most medial part of the gracile fasciculus, and the uppermost levels of segmental innervation are represented in the most lateral part of the cuneate fasciculus. These two fasciculi end, respectively, in the gracile and cuneate nuclei, which are located dorsally in the medulla. As a useful approximation, the gracile fasciculus and nucleus may be said to deal with sensations from the lower limb, and the cuneate fasciculus
and nucleus may be said to deal with sensations from the upper limb. The orderly arrangement of different levels of the body in the dorsal funiculi is an example of somatotopic lamination in a tract. As will be seen, comparable lamination also exists in other tracts of the spinal cord and brain.
FIGURE 5-10 Neuronal circuitry of the ventral horn of the spinal gray matter, showing afferents (blue) to alpha (α) and gamma (γ) motor neurons (red). Large arrows point to the sites of termination of axons of descending tracts from the brain. Interneurons are green.
It is convenient to describe the dorsal and ventral halves of the lateral funiculus separately.
The most conspicuous tract in the dorsal half of the lateral funiculus is the lateral corticospinal tract, which consists of axons of neurons in the cortex of the frontal and parietal lobes of the contralateral cerebral hemisphere. These fibers pass through the internal capsule, the basis pedunculi of the midbrain, the pons, and the medullary pyramid before they decussate and enter the lateral funiculus of the cord. Corticospinal fibers from the frontal cortex terminate mainly in the intermediate gray matter and the ventral horn. Those from the parietal lobe end in the dorsal horn. The somatotopic lamination of the lateral corticospinal tract is such that fibers destined for the lowest levels of the spinal cord are the most laterally placed.
Experiments with animals indicate that a reticulospinal component of the dorsolateral funiculus arises in the nucleus raphes magnus in the reticular formation of the medulla and terminates in laminae I, II, and III. These unmyelinated fibers, constituting the raphespinal tract in the most dorsal part of the lateral funiculus, contain histochemically demonstrable quantities of serotonin, which they probably use as a neurotransmitter. The raphespinal tract modifies the transmission from the dorsal horn of impulses initiated by noxious stimuli, which produce painful sensations. Unmyelinated hypothalamospinal
fibers, similarly located, arise from the paraventricular nucleus of the hypothalamus and end among the preganglionic autonomic neurons in segments T1 to L3 and S2 to S4. Some hypothalamospinal axons contain the peptide oxytocin.
The largest body of ascending fibers in the dorsal part of the lateral funiculus is the superficially located dorsal spinocerebellar tract, which is present only above level L3. Its axons arise from the cells of the nucleus thoracicus (Clarke's column) in the same side of the spinal cord and terminate ipsilaterally in the cortex of the cerebellum, which they enter by way of the inferior cerebellar peduncle.
Several tracts are present in the ventral half of the lateral funiculus. The largest is the spinothalamic tract, which consists of the ascending axons of neurons located in the gray matter of the opposite half of the cord. The cells of origin are mostly in the nucleus proprius of the dorsal horn (laminae IV and V-VI). The axons cross the midline in the ventral white commissure close to the central canal and then traverse the ventral horn to enter the ventrolateral and ventral funiculi. The fibers of the spinothalamic tract end in thalamic nuclei. As they pass through the brain stem, some of these axons give off collateral branches to the reticular formation in the medulla and pons and to the periaqueductal gray matter in the midbrain. The spinothalamic tract conducts impulses concerned with tactile, thermal, and painful sensations. Its fibers are somatotopically arranged, with those for the lower limb lying most superficially and those for the upper limb lying closest to the gray matter. Distinct ventral and lateral spinothalamic tracts (for touch and for pain and thermal sensation, respectively) were formerly recognized, but there seems to be little justification for such a subdivision. The functions of the spinothalamic fibers are discussed in more detail inChapter 19.
The ventral spinocerebellar tract is located superficially in the ventrolateral funiculus. It arises from the base of the dorsal horn and from the spinal border cells of the ventral horn of the lumbosacral segments and consists largely of crossed fibers. The tract ascends as far as the midbrain and then makes a sharp turn caudally into the superior cerebellar peduncle. The fibers cross the midline for a second time within the cerebellum before ending in the cerebellar cortex. Thus, both spinocerebellar tracts convey sensory information (mainly proprioceptive) from one lower limb to the same side of the cerebellum. The other ascending components of the ventral half of the lateral funiculus are small. The axons composing the spinotectal tract (also known, more accurately, as the spinomesencephalic tract) originate in the same parts of the gray matter as the spinothalamic fibers, cross the midline, and then project rostrally to the periaqueductal gray matter, the superior colliculus, and various nuclei in the reticular formation of the midbrain. The spinoreticular tract is traditionally described as including crossed fibers that terminate in the pontine reticular formation and uncrossed fibers that end in the medullary reticular formation. In addition, many spinothalamic fibers have collateral branches that synapse with neurons of the reticular formation. These projections from the spinal cord to the brain stem form part of the ascending reticular activating system (see Chapter 9) and may also be involved in the perception of pain and of various sensations that originate in internal organs. It is customary to indicate a small spino-olivary tract in the human spinal cord, but its existence in primates is uncertain.
The ventrolateral funiculus also contains descending reticulospinal fibers. These are present also in the ventral funiculus, under which heading they are described below.
The long tracts in this part of the spinal white matter are all descending ones. The ventral corticospinal tract comprises a small proportion of the corticospinal fibers—those that did not cross the midline in the lower part of the medulla. Most ventral corticospinal fibers decussate at segmental levels and terminate next to those of the larger lateral corticospinal tract. In a few people, most of the corticospinal fibers fail to decussate in the medulla and, therefore, descend ipsilaterally in the ventral funiculus or, rarely, in the ventrolateral fasciculus.
The vestibulospinal tract is uncrossed. It arises from the lateral vestibular nucleus (of Deiters) in the medulla and descends in the
ventrolateral and ventral white matter of the spinal cord, close to the surface (see Fig. 5-9). In the upper cervical cord, its fibers are located in the most medial part of the lateral funiculus. They then move medially so that in the lower cervical segments, they are close to the margin of the ventral median fissure. In the thoracic cord, the tract moves into a more lateral location in the ventral funiculus, among the axons that form the ventral rootlets, and it maintains this position at more caudal levels. Most vestibulospinal axons terminate in the medial part of the ventral horn. The tract's function is to mediate equilibratory reflexes, which are triggered by the activity of the vestibular apparatus of the internal ear and put into effect chiefly by the axial musculature and the extensors of the limbs.
Reticulospinal tracts originate in several nuclei of the reticular formation (see Chapter 9) of the midbrain, pons, and medulla. Most end by contacting interneurons in the ventral horn at all levels but most abundantly in the cervical segments. In the human spinal cord, reticulospinal fibers are present throughout the ventral funiculus and the ventral half of the lateral funiculus. The majority of the reticulospinal fibers are from the same side of the brain stem, and some of these axons cross the midline ventral to the central canal. Many reticulospinal fibers shift from the ventral into the lateral funiculus as they proceed down the spinal cord. The reticulospinal tracts constitute one of the descending pathways through which the brain directs and controls the activity of motor neurons. Whereas the corticospinal tract is concerned mainly with skilled volitional movements, the reticulospinal tracts control ordinary activities that do not require constant conscious effort. Other reticulospinal fibers influence the autonomic nervous system. The descending bundle of the lateral horn is a population of such axons that run alongside the lateral horn in the upper seven or eight thoracic segments. Clinical evidence indicates that these fibers, which probably originate ipsilaterally in the pons, are excitatory to the preganglionic sympathetic neurons that control blood vessels and sweat glands throughout the body. Degeneration studies (see Chapter 4) of human material support the preceding account of reticulospinal fibers and do not support the traditional notion of separate and distinct medullary and pontine reticulospinal tracts.
The remaining tracts of the ventral funiculus are small. The descending component of the medial longitudinal fasciculus (also called the medial vestibulospinal tract, in which case the vestibulospinal tract previously described is designated as lateral) arises in the medial vestibular nucleus in the medulla. It is involved in movements of the head required for maintaining equilibrium and probably does not descend below the upper cervical levels of the spinal cord. The few fibers that constitute the tectospinal tract from the contralateral superior colliculus do not descend below this level, either.
The fasciculus proprius, a zone containing both myelinated and unmyelinated fibers, is present in all the funiculi immediately adjacent to the gray matter (see Fig. 5-9). It containspropriospinal (spinospinalis) fibers, which connect different segmental levels of the gray matter. The shorter axons are closer to the gray matter than the longer fibers. Propriospinal fibers run both rostrally and caudally and have collateral branches that end in the gray matter near their own cell bodies, providing the functional equivalent of interneurons for reflexes within segments. Some neurons with axons that ascend in the fasciculus proprius extend for almost the whole length of the spinal cord and serve as necessary components of intersegmental spinal reflexes. Descending propriospinal fibers seldom extend over more than two segments of the spinal cord.
Certain neuronal connections in the spinal cord form the bases of spinal reflexes. The stretch reflex and the flexor reflex are examples.
The stretch reflex has a two-neuron or monosynaptic reflex arc (Fig. 5-11). Slight stretching of a muscle stimulates the sensory endings in neuromuscular spindles, and the resultant excitation reaches the spinal cord by way of primary sensory neurons that have large (group A) axons. The proximal branches of these axons in the dorsal funiculus give off collateral branches that excite alpha motor
neurons, causing the stretched muscle to contract. This is an important postural reflex. The neuromuscular spindles are delicate monitors of change in the length of the muscle, and the stretch reflex alters tension in such a way as to maintain a constant length. The stretch reflex forms the basis of the clinical tendon jerk tests that are part of physical examinations. A sharp tap on the tendon causes synchronous discharges from the spindles in the muscle, with prompt reflex contraction. A diminished or absent tendon jerk indicates disease affecting either the afferent or the efferent neurons of the stretch reflex. Exaggerated jerks indicate loss of inhibition of motor neurons by activity in tracts descending from the brain.
FIGURE 5-11 Afferent (blue) and efferent (red) limbs of the monosynaptic stretch reflex arc.
In addition to the simple monosynaptic stretch reflex, a response with longer latency occurs when a voluntarily contracting muscle is stretched. Physiological studies indicate that this slower reflex, which is most easily elicited in the hand, passes through the somatosensory and motor areas of the cerebral cortex.
The tension on a muscle is monitored by Golgi tendon organs. When the tension reaches a certain level, a distinct increase takes place in the discharge from these receptors. The resulting action potentials reach interneurons in the spinal gray matter, which in turn inhibit alpha motor neurons. Relaxation of the muscle follows. This reflex can prevent excessive tension on the muscle and tendon. When a muscle is abnormally contracting (spasm or spasticity), passive stretching can induce relaxation by stimulating the Golgi tendon organs.
The flexor reflex is also protective. It consists of the withdrawal of a limb in response to a painful stimulus. At least three neurons are involved, so this is a polysynaptic reflex (Fig. 5-12). The cutaneous receptors are free nerve endings that respond to potentially injurious stimuli, and the proximal branches of the afferent fibers synapse in the dorsal horn with interneurons. These end on alpha motor cells in several segments because a withdrawal response requires the action of groups of muscles. Some neurons in the dorsal horn have axons that decussate and contact neurons in the contralateral ventral horn to stimulate, in a fully developed response, extension of the contralateral limb: the crossed extensor reflex.
Reflexes in Infancy
FIGURE 5-12 Afferent (blue) and efferent (red) neurons of the flexor reflex arc, which include an interneuron (green).
Anatomical and Clinical Correlations
Lesions of the spinal cord result from trauma, degenerative and demyelinating disorders, tumors, infections, and impairment of blood supply. The following notes on selected lesions show the necessity of understanding the intrinsic anatomy of the spinal cord to interpret signs and symptoms.
Testing for impairment or loss of cutaneous sensation is an important part of the neurological examination; it is particularly useful in detecting the site of a lesion that involves the spinal cord or nerve roots. The distribution of cutaneous areas (dermatomes) supplied by the spinal nerves is shown in Figure 5-13. Cutaneous areas supplied by adjacent spinal nerves overlap. For example, the upper half of the area supplied by T6 is also supplied by T5, and the lower half by T7. There is, therefore, little or no sensory loss after interruption of a single spinal nerve or dorsal root. The overlapping of dermatomes contrasts with the sharp delineation of the areas supplied by cutaneous nerves, which are formed in the limb plexuses by the mingling of fibers from various segmental nerve roots.
Reflex contraction of muscles is also used in testing for the integrity of segments of the cord and of the spinal nerves. The segments involved in four commonly tested stretch or tendon reflexes are as follows: biceps reflex, C5 and C6; triceps reflex, C6 to C8; quadriceps reflex (knee jerk), L2 to L4; and gastrocnemius reflex (ankle jerk), S1 to S2.
Before specific pathological conditions are mentioned, it should be noted that a distinction is made between the effects of a lesion involving motor neurons as opposed to those involving descending motor pathways. Destruction or atrophy of lower motor neurons (in the present context, those of the ventral horn) results in flaccid paralysis of the affected muscles, diminished or absent tendon reflexes, and progressive atrophy of the muscles deprived of motor fibers. The term upper motor neuron lesion is regularly used clinically but leaves much to be desired. The lesion may be in the cerebral cortex or in another part of the cerebral hemisphere, in the brain stem, or in the spinal cord. Thus, the term upper motor neuron is a collective term including all the descending pathways that control the activities of the neurons that supply the muscles. The following signs are associated with an upper motor neuron lesion after the acute effects have worn off: varying degrees of voluntary paralysis, which is most severe in the upper limb; a positive Babinski's sign (i.e., upturning of the great toe
and spreading of the toes on stroking the sole); and spasticity with exaggerated tendon reflexes.
FIGURE 5-13 Cutaneous distribution of spinal nerves (dermatomes).
SPINAL CORD TRANSECTION
The spinal cord may be damaged by penetrating wounds (caused by stabbing or gunfire) or by spinal fracture or dislocation (especially from traffic accidents or diving into shallow water). Complete transection results in loss of all sensibility and voluntary movement below the lesion. The patient is tetraplegic (quadriplegic), with both arms and both legs paralyzed, if the upper cervical cord is transected, or paraplegic(both legs paralyzed) if the transection is between the cervical and lumbosacral enlargements. During an initial period of spinal shock, lasting from a few days to several weeks, all somatic and visceral reflex activity is abolished. On return of reflex activity, spasticity of muscles and exaggerated tendon reflexes occur. The lower limbs assume positions of flexion because the vestibulospinal tract (which stimulates extensors) is one of the transected descending pathways. Bladder and bowel functions are no longer under voluntary control.
The events that occur after partial transection of the spinal cord depend on the size and location of the lesion. Hemisection, although unusual in the literal sense, is an instructive lesion anatomically. The neurological signs caudal to
the hemisected region of the cord constitute the Brown-Séquard syndrome. Position sense, tactile discrimination, and the feeling of vibration are lost on the side of the lesion because of interruption of the dorsal and dorsolateral funiculi. Anesthesia for pain and temperature are provided on the opposite side because of interruption of the spinothalamic tract. Light touch is not much affected because of essentially bilateral conduction in the dorsal and lateral funiculi. Whereas the patient is hemiplegic (left or right upper and lower limbs paralyzed) if the lesion is in the upper cervical cord, hemisection of the thoracic cord results in paralysis of the leg (monoplegia). The paralysis is ipsilateral to the lesion and of the upper motor neuron type.
The immediate treatment of patients with incomplete spinal cord transection is largely directed to protection against further damage from fractured or dislocated vertebrae and suppression of an aggressive and destructive inflammatory reaction that occurs within the cord for several days after the injury. Long-term management includes prevention of bedsores in insensitive skin, avoidance of urinary tract infections, and maximization of any preserved motor functions.
Potentially curative treatments, such as ways to induce severed axons to grow across the scarred site of an injury, have been a goal of research for at least 100 years. Recent efforts include extracting olfactory ensheathing cells (see Chapters 2 and 17) from the olfactory mucosa of the patient's nose. These pluripotential glial cells can be propogated in culture and introduced into the injured spinal cord. In laboratory animals, the grafted cells appear to facilitate axonal growth, and with the recipient of the graft being also the donor, the cells are not rejected. Clinical trials of this procedure are in progress.
The following degenerative diseases also illustrate the anatomical basis of neurological signs. In subacute combined degeneration, bilateral demyelination and loss of nerve fibers in the dorsal and dorsolateral funiculi occur. The principal causative factor is vitamin B12deficiency, and the disorder is typically encountered in association with pernicious anemia. The lesion results in loss of the senses of position, discriminative touch, and vibration. The gait is ataxic (i.e., without coordination) because the patient is unaware of the position of his or her legs.
Amyotrophic lateral sclerosis (also called motor neuron disease) is a bilateral degenerative disease. The degenerative process is largely restricted to the motor system, affecting the corticobulbar and corticospinal tracts (and perhaps other descending motor pathways) along with motor nuclei of cranial nerves and ventral horn motor cells. A combination of upper and lower motor neuron clinical signs are present, with the latter predominating in the terminal stages of the disease. Poliomyelitis is caused by a virus that infects motor neurons, killing many of them. Paralysis is of the lower motor neuron type and affects the muscles supplied by the infected neurons. Correlation of clinical data with postmortem findings in poliomyelitis has been the chief source of knowledge of the distribution in the human ventral horn of motor neurons that supply individual muscles.
Syringomyelia differs from the disorders already mentioned in that neuronal degeneration is not the primary pathological change. Central cavitation of the spinal cord is present, usually beginning in the cervical region, with a glial reaction (gliosis) adjacent to the cavity. Decussating fibers for pain and temperature in the ventral white commissure are interrupted early in the disease. The cavitation and gliosis spread into the gray and white matter as well as longitudinally, leading to variable signs and symptoms, depending on the regions involved. The classical clinical picture is that of “yokelike” anesthesia for pain and temperature over the shoulders and upper limbs accompanied by lower motor neuron weakness and consequent wasting of the muscles of the upper limbs. Spread of the cavitation and glial reaction into the lateral funiculi may result in voluntary paresis of the upper motor neuron type, particularly affecting the lower limbs.
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