The Nervous System - Advanced Version / The Spinal Cord

submitted by Dr. Gary Farr Last Updated June, 24, 2002

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The spinal cord is an elongated cylindrical structure, about 45 centimetres (18 inches) long, that extends from the medulla oblongata of the hindbrain to a level between the first and second lumbar vertebrae of the backbone. The terminal part of the spinal cord is called the conus medullaris. Associated with local regions of the spinal cord and imposing upon it an external segmentation are 31 pairs of spinal nerves, each of which receives and furnishes one dorsal and one ventral root. On this basis the spinal cord is divided into the following nerves: 8 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 1 coccygeal. Spinal nerve roots emerge via intervertebral foramina; lumbar and sacral spinal roots, descending for some distance within the subarachnoid space before reaching the appropriate foramina, produce a group of nerve roots about the conus medullaris known as the cauda equina. Two enlargements of the spinal cord are evident: (1) a cervical enlargement (C5 through T1), which provides nerve supply for the upper extremity, and (2) a lumbosacral enlargement (L1 through S2), which supplies the lower extremity.

A cross section of the spinal cord reveals long tracts of myelinated nerve fibers (known as white matter) arranged around the periphery of a symmetrical, butterfly-shaped cellular matrix of gray matter (see figure to right).

 The gray matter forms three pairs of horns throughout most of the spinal cord: (1) the dorsal horns, composed of sensory neurons; (2) the lateral horns, well defined in thoracic segments and composed of visceral neurons; and (3) the ventral horns, especially large in the cord enlargements and composed of motor neurons.
 

The white matter forming the ascending and descending spinal tracts falls in three paired funiculi, or sectors: the dorsal or posterior funiculi, lying between the dorsal horns; the lateral funiculi, lying on each side of the spinal cord between the dorsal-root entry zones and the emergence of the ventral roots; and the ventral funiculi, lying between the ventral median sulcus and each ventral-root zone.
 

Gray matter

The cellular gray matter has a “cytoarchitectural lamination” in which nine laminae are customarily indicated by Roman numerals (see figure, above). Laminae I to V, forming the dorsal horns, receive sensory input. Lamina VII forms the intermediate zone at the base of all horns. Lamina IX is composed of clusters of large (alpha) motor neurons, whose axons innervate striated muscle, and small (gamma) motor neurons, which innervate contractile elements of the muscle spindle. Axons of both alpha and gamma motor neurons emerge via the ventral roots. Laminae VII and VIII have variable configurations, and lamina VI is present only in the cervical and lumbosacral enlargements. In addition, cells surrounding the central canal of the spinal cord form an area often referred to as lamina X.

All primary sensory neurons that enter the spinal cord originate in ganglia that are located in the intervertebral foramina. Peripheral processes of the nerve cells in these ganglia convey sensation from various receptors, and central processes of the same cells enter the spinal cord dorsolaterally as bundles of nerve filaments. Fibers conveying specific forms of sensation follow separate pathways. Impulses concerned with pain and noxious stimuli largely end upon cells in parts of laminae I and II, while impulses associated with tactile sense end in lamina IV or on processes of cells in that lamina. Signals from stretch receptors (i.e., muscle spindles and tendon organs) end upon cells in parts of laminae V, VI, and VII; collaterals of these fibers involved in the stretch reflex project into lamina IX.

Virtually all parts of the spinal gray contain interneurons, which connect various cell groups. Many interneurons have short axons distributed locally, but some have axons that extend for several spinal segments. Some interneurons may modulate or change the character of signals, while others play key roles in transmission and in patterned reflexes.

Ascending spinal tracts

Sensory tracts ascending in the white matter of the spinal cord arise either from cells of spinal ganglia or from intrinsic neurons within the gray matter that receive primary sensory input.

Dorsal column

The largest ascending tracts, the fasciculi gracilis and cuneatus, arise from spinal ganglion cells and ascend in the dorsal funiculus to the medulla. The fasciculus gracilis receives fibers from ganglia below thoracic 6, while spinal ganglia from higher segments of the spinal cord project fibers into the fasciculus cuneatus. The fasciculi terminate upon large nuclear masses (the nuclei gracilis and cuneatus) in the medulla. Cells of these nuclei give rise to fibers that cross completely and form the medial lemniscus; the medial lemniscus in turn projects to the ventrobasal nuclear complex of the thalamus. In this way, the dorsal column/medial lemniscal system conveys signals associated with tactile, pressure, and kinesthetic (or positional) sense to sensory areas of the cerebral cortex.

Spinothalamic tracts

Fibers concerned with pain, thermal sense, and light touch enter the lateral-root entry zone and then ascend or descend near the periphery of the spinal cord before entering superficial laminae of the dorsal horn—largely parts of laminae I, IV, and V. Cells in these laminae then give rise to fibers of the two spinothalamic tracts. Those crossing in the ventral white commissure (ventral to the central canal) form the lateral spinothalamic tract, which, ascending in the ventral part of the lateral funiculus, conveys signals related to pain and thermal sense. The anterior spinothalamic tract arises from fibers that cross the midline in the same fashion but ascend more anteriorly in the spinal cord; these convey impulses related to light touch. At medullary levels the two spinothalamic tracts tend to merge and cannot be distinguished as separate entities. Many of the fibers, or collaterals, of the spinothalamic tracts end upon cell groups in the reticular formation, while the principal tracts convey sensory impulses to relay nuclei in the thalamus.

Spinocerebellar tracts

Impulses from stretch receptors are carried by large-diameter fibers that synapse upon cells in deep laminae of the dorsal horn or in lamina VII. The posterior spinocerebellar tract arises from the dorsal nucleus of Clarke and ascends peripherally in the dorsal part of the lateral funiculus. The anterior spinocerebellar tract ascends on the ventral margin of the lateral funiculus. Both tracts transmit signals to portions of the anterior lobe of the cerebellum and are involved in mechanisms that automatically regulate muscle tone without reaching consciousness.

Descending spinal tracts

Tracts descending to the spinal cord are concerned with voluntary motor function, muscle tone, reflexes and equilibrium, visceral innervation, and modulation of ascending sensory signals. The largest and most important, the corticospinal tract, originates in broad regions of the cerebral cortex. Smaller descending tracts, which include the rubrospinal tract, the vestibulospinal tract, and the reticulospinal tract, originate in discrete and diffuse nuclei in the midbrain, pons, and medulla. Most of these brain-stem nuclei themselves receive input from the cerebral cortex, the cerebellar cortex, deep nuclei of the cerebellum, or some combination of these.

In addition, autonomic tracts, which descend from various nuclei in the brain stem to preganglionic sympathetic and parasympathetic neurons in the spinal cord, constitute a vital link between the centres that regulate visceral functions and the nerve cells that actually effect changes.

Corticospinal tract

Universally regarded as the single most important tract concerned with skilled voluntary activity, the corticospinal tract originates from pyramid-shaped cells in the premotor, primary motor, and primary sensory cortex. Containing about one million fibers, it forms a significant part of the posterior limb of the internal capsule and is a major constituent of the crus cerebri in the midbrain. As the fibers emerge from the pons, they form compact bundles on the ventral surface of the medulla, known as the medullary pyramids. In the lower medulla about 90 percent of the fibers of the corticospinal tract decussate and descend in the dorsal part of the lateral funiculus of the spinal cord. Of the fibers that do not cross in the medulla, approximately 8 percent cross in cervical spinal segments. As the tract descends, fibers and collaterals are given off at all segmental levels, synapsing upon interneurons in lamina VII and upon motor neurons in lamina IX. Approximately 50 percent of the corticospinal fibers terminate within cervical segments.

Rubrospinal tract

The rubrospinal tract arises from cells in the caudal part of the red nucleus, an encapsulated cell group in the midbrain tegmentum. Fibers of this tract decussate at midbrain levels, descend in the lateral funiculus of the spinal cord (overlapping ventral parts of the corticospinal tract), enter the spinal gray, and terminate on interneurons in lamina VII. Through these crossed rubrospinal projections, the red nucleus exerts a facilitating influence on flexor alpha motor neurons and a reciprocal inhibiting influence on extensor alpha motor neurons. Because cells of the red nucleus receive input from the motor cortex (via corticorubral projections) and from globose and emboliform nuclei of the cerebellum (via the superior cerebellar peduncle), the rubrospinal tract effectively brings flexor muscle tone under the control of these two regions of the brain.

Vestibulospinal tract

The vestibulospinal tract originates from cells of the lateral vestibular nucleus, which lies in the floor of the fourth ventricle. Fibers of this tract descend the length of the spinal cord in the ventral and lateral funiculi without crossing, enter laminae VIII and IX of the anterior horn, and end upon both alpha and gamma motor neurons, which innervate ordinary muscle fibers and fibers of the muscle spindle (see below Functions of the human nervous system: Movement). Cells of the lateral vestibular nucleus receive facilitating impulses from labyrinthine receptors in the utricle and from fastigial nuclei in the cerebellum. In addition, inhibitory influences upon these cells are conveyed by direct projections from Purkinje cells in the anterior lobe of the cerebellum. Thus, the vestibulospinal tract mediates the influences of the vestibular end organ and the cerebellum upon extensor muscle tone.

A smaller number of vestibular projections, originating from the medial and inferior vestibular nuclei, descend ipsilaterally in the medial longitudinal fasciculus only to cervical levels. These fibers exert excitatory and inhibitory effects upon cervical motor neurons.

Reticulospinal tract

The reticulospinal tracts arise from relatively large but restricted regions of the reticular formation of the pons and medulla—the same cells that project ascending processes to intralaminar thalamic nuclei and play an important role in maintaining alertness and the conscious state. The pontine reticulospinal tract arises from aggregations of cells in the pontine reticular formation, descends ipsilaterally as the largest component of the medial longitudinal fasciculus, and terminates among cells in laminae VII and VIII. Fibers of this tract exert facilitating influences upon voluntary movements, muscle tone, and a variety of spinal reflexes. The medullary reticulospinal tract, originating from reticular neurons on both sides of the median raphe, descends in the ventral part of the lateral funiculus and terminates at all spinal levels upon cells in laminae VII and IX. The medullary reticulospinal tract inhibits the same motor activities that are facilitated by the pontine reticulospinal tract. Both tracts receive input from regions of the motor cortex.

Autonomic pathways

Descending fiber systems concerned with visceral and autonomic activities emanate from collections of cells at various levels of the brain stem. For example, hypothalamic nuclei project to visceral nuclei in both the medulla and spinal cord; in the spinal cord these direct hypothalamospinal projections terminate upon cells of the intermediolateral cell column in thoracic, lumbar, and sacral segments. Preganglionic parasympathetic neurons originating in the oculomotor nuclear complex in the midbrain project not only to the ciliary ganglion but also directly to spinal levels. Some of these fibers reach lumbar segments of the spinal cord, most of them terminating in parts of laminae I and V. Pigmented cells in an area of the rostral pons known as the isthmus form a blackish blue collection visible in gross brain sections; known as the locus ceruleus, these cells are rich in norepinephrine and distribute this neurotransmitter widely to all regions of the brain and spinal cord. Fibers from the locus ceruleus descend to spinal levels without crossing and are distributed to terminals in the anterior horn, the intermediate zone, and the dorsal horn. Other noradrenergic cell groups in the pons, near the motor nucleus of the facial nerve, project uncrossed noradrenergic fibers that terminate in the intermediolateral cell column (that is, lamina VII of the lateral horn). Postganglionic sympathetic neurons associated with this system have direct effects upon the cardiovascular system. Cells in the nucleus of the solitary tract project crossed fibers to the phrenic nerve nucleus (in cervical segments 3 through 5), the intermediate zone, and the anterior horn at thoracic levels; these innervate respiratory muscles.

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