It might be helpful if an overview of the anatomy of the spinal cord and associated structures that is a distillation of more detailed texts is provided first (see ref. 2). The spinal cord, which is divided into 31 segments, is ensheathed by the spinal canal that is formed by the bodies, pedicles, and spinous processes of individual vertebra. Each spinal segment (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal) gives rise to a pair of spinal nerves that are joined to their segments by a pair of posterior (dorsal in quadrupeds) and anterior (ventral in quadrupeds) roots (Fig. 1). The spinal cord is partitioned into peripherally oriented white matter that is organized into tracts containing bundles of axons and centrally oriented gray matter with a "butterfly-shaped" appearance (Figs. 2 and 3). White matter contains myelinated and unmyelinated axons traveling to (posterior roots) and from (anterior roots) the spinal cord and to (ascending tracts) and from (descending tracts) the brain. Gray matter contains neuronal cell bodies, their dendrites and axons, as well as neuroglia, and is functionally organized into the laminae of Rexed (see Gray Matter and Fig. 2).
Cell bodies of sensory neurons are located in ganglia that sit on the posterior (dorsal) roots of spinal nerves. Correlations between neuronal size and electrophysiological properties of their cell bodies and axons have resulted in a classification into large clear neurons (type A) and smaller dark neurons (type B) (3,4). The processes of these pseudounipolar afferent neurons conduct somatic and visceral impulses from the periphery to the spinal cord (Fig. 1). Nerve fibers of somatic afferents carry exteroceptive (pain, temperature, and touch) and proprioceptive (body position, muscle tone, and movement) input from sensory receptors in the body wall, muscle, tendons, and joints.
Nerve fibers of visceral afferents carry enteroceptive (degree of filling or stretch of alimentary tract, bladder, and blood vessels) input from the viscera.
Cell bodies of efferent motor neurons are located in lamina IX of the anterior (ventral) horn of the spinal cord (Fig. 2) and conduct somatic impulses from the spinal cord to the periphery through myelinated axons in anterior (ventral) roots. Somatic efferents of a and y motor neurons innervate striated muscle. Anatomically, the situation for visceral efferents is more complicated and is dependent on whether the efferents are part of the sympathetic or parasympathetic divisions of the autonomic nervous system. Cell
bodies of efferent preganglionic autonomic neurons are located in the interomediolat-eral cell column of lamina VII that extends from the last cervical segment to the last lumbar segment (sympathetic) or in the midbrain, medulla, and sacral segments 2 through 4 (parasympathetic). Postganglionic autonomic cell bodies are either in paravertebral or prevertebral ganglia (Fig. 1). Myelinated preganglionic and unmyelinated postganglionic visceral efferents innervate smooth and cardiac muscle, and regulate glandular secretion.
In the spinal cord, the "butterfly-shaped" gray matter consists of a pair of posterior (dorsal) horns and a pair of anterior (ventral) horns connected by the gray commissure that contains the central canal. The size of the gray matter and thus, the spinal cord is larger in the cervical and lumbar regions, where it contains the numerous large motor neurons that control the movements of the arms and legs. Aside from regional variation in size and the presence of large motor neurons in the anterior horn, the gray matter of the spinal cord is a microscopically homogenous mixture of neurons and their processes, neuroglia cells, and blood vessels. Despite this homogeneity, the posterior and anterior horns have been functionally subdivided into distinct regions known as the laminae of Rexed (Fig. 2).
The posterior horns contain laminae I-VI and most receive afferent input from the periphery. Lamina I, also known as the posterior marginal nucleus, contains neurons whose axons contribute to the ascending spinothalamic tract and respond to input from primary afferents that sense pain and temperature. Lamina II, the substantia gelatinosa, receives afferents from Lissauer's fasciculus that convey information about pain, temperature, and touch. Laminae III and IV, the nucleus proprius, contain interneurons that relay touch and pressure sensations. The dendrites of these neurons extend up into lamina II and their axons contribute to the contralateral spinothalamic tract. As with laminae II and IV, interneurons in lamina V supply projections to the contralateral spinothalamic tract. In addition, they receive input from afferent neurons that respond to both painful and nonpainful stimuli. Located at the base of the posterior horns in the cervical and lumbar enlargements, lamina VI receives input from central processes of primary sensory neurons.
The anterior horn contains laminae VII-IX, with the gray commissure, lamina X, surrounding the central canal and continuous with laminae VII on each side of the spinal cord. Also, known as the intermediate zone, lamina VII contains interneurons, such as those in the nucleus dorsalis of the thoracic and upper lumbar regions that are associated with the posterior spinocerebellar tract. The interneurons found at the base of the anterior horn that synapse with axons of the descending tracts include lamina VIII. Lamina IX contains cell bodies of a and y motor neurons whose axons constitute efferent motor output to striated muscle.
Surrounding the gray matter, the white matter of the spinal cord is a homogeneous mixture of myelinated and unmyelinated axons, neuroglia cells, and blood vessels. It can be divided into three general regions: the posterior (dorsal), lateral and anterior (ventral) funiculi. Funiculi are further functionally subdivided into tracts or fasciculi (Fig. 3). Spinal cord tracts either carries peripheral input to the brain through a sequence of primary and secondary neurons that constitute the ascending tracts, or direct central input from the brain to the spinal cord in descending tracts that modify efferent output. The white commissure carries impulses of crossed tracts from one side of the spinal cord to the other.
In the white matter, there are three main ascending tracts: the posterior columns (tracts), the spinothalamic tracts, and the spinocerebellar tracts (Fig. 3). The posterior columns contain central processes of sensory neurons in spinal ganglia that carry input from receptors in muscles, joint capsules, and skin. These fibers ascend directly to the medulla oblongata where they synapse with secondary neurons. The posterior column contains two fasciculi: the medial fasciculus gracilis that receives central processes of primary sensory neurons in the lumbosacral region; and the lateral fasciculus cuneatus that receives central processes from neurons in the cervical and thoracic regions. Posterior tract axons convey information about well-localized touch, movement, and position senses.
The ascending spinothalamic tract is divided into the lateral and anterior components, although this division is not clearly defined. Free nerve endings throughout the body convey impulses to the sensory neurons in spinal ganglia through thinly myeli-nated (A 5) and unmyelinated (C) fibers. Short central processes of these neurons enter the spinal cord through Lissauer's fasciculus, ascend one segment and then synapse with secondary neurons in laminae I, IV, and V, whose axons cross the midline in the anterior white commissure before ascending to the thalamus in the contralateral spinothalamic tract. Secondary neurons of the lateral spinothalamic tract convey sensations of pain and temperature that are subject to modification by emotion and experience. Secondary neurons of the anterior spinothalamic tract receive input from central processes of sensory neurons with peripheral receptors in hairless skin that project throughout laminae IV-VII. These secondary neurons also cross through the anterior white commissure before ascending and conveying sensations of light, poorly localized touch.
The ascending spinocerebellar tracts have posterior and anterior components. In the posterior spinocerebellar and cuneocerebellar tracts, myelinated fibers carry input from muscle spindles (Ia), golgi tendon organs (Ib), and skin (II) in the lower and upper body, respectively. It is conveyed by central processes of neurons in spinal ganglia, which ultimately synapse with neurons in the nucleus dorsalis of lamina VII or the cuneate nucleus of the medulla. Axons of these secondary neurons consist the posterior spin-ocerebellar and cuneocerebellar tracts and terminate on mossy fibers in the cerebellar cortex. These tracts convey muscle-spindle or tendon-organ related information with resolution to the level of a single muscle fiber of a muscle-tendon complex. The anterior and rostral spinocerebellar tracts convey similar input from the lower and upper body, respectively, to the cerebellum, whereas secondary neurons in lamina VII cross to contralateral tracts through the gray commissure before ascending with postural information relating to an entire limb.
The main descending tracts of spinal cord white matter are the corticospinal, rubrospinal, tectospinal, reticulospinal, and vestibular tracts (Fig. 3). These tracts carry information from neurons originating in the cerebral cortex, midbrain, medulla, and vestibular apparatus, respectively. As a portion of the upper motor neuron pool, axons of neurons in the cerebral cortex contribute to the corticospinal tracts. They descend directly to terminate on interneurons at the base of the posterior horn and modify sensory input on anterior motor horn neurons in lamina IX or on adjacent interneurons. Cortical and subcortical neurons decussate at the level of the medullary pyramids and then travel in the lateral and anterior corticospinal tracts. Corticospinal tract function concerns voluntary control relating to manipulation of objects (upper limbs) and locomotion (lower limbs). Neurons of the rubrospinal tract originate in the red nucleus of the mid-brain and their axons descend in the spinal cord where they intermingle with corti-cospinal tracts as far as the thoracic level. Rubrospinal terminal fibers synapse with interneurons in laminae V, VI, and VIII that project to a and y motor neurons.
Rubrospinal tract activity is regulated by sensory input provided to the cerebellum through the spinocerebellar tracts and controls the tone of flexor muscle groups.
The remaining descending tracts run in the anterior funiculus. The tectospinal tract originates in the superior colliculus of the midbrain, crosses the midline in the peri-aqueductal gray area and descends in the anterior funiculus, where it intermingles with the medial longitudinal fasciculus. Tectospinal terminal fibers project to interneurons in laminae VI, VII, and VIII that synapse on motor neurons of the anterior horn to help blend the interaction of visual and auditory stimuli with postural reflex movements. Axons of the vestibulospinal tract are derived from neurons in the lateral vestibular nucleus that receive afferent input from both the vestibular apparatus of the inner ear and the cerebellum. Vestibulospinal fibers descend ipsilaterally through the anterolateral spinal cord, where they synapse with interneurons in laminae VII and VIII that project to motor neurons in the anterior horn. The vestibulospinal tract helps control basic posture or stance by facilitating activity in all extensor muscles. The axons of the two uncrossed reticulospinal tracts originate from several levels in the reticular core of the brainstem and terminate on interneurons in laminae VII and VIII that project to a and y neurons of the anterior horn. Reticulospinal tracts modify motor and sensory functions of the spinal cord and facilitate motor and cardiovascular responses (pontine reticulospinal tract) or inhibit motor and cardiovascular responses (medullary reticulospinal tract).
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