The spinal cord is that part of the CENTRAL NERVOUS SYSTEM encased in the vertebral column—the spine or backbone. It is continuous with the MEDULLA OBLONGATA rostrally. The gross anatomical and internal microscopic features of the spinal cord vary over its length as a consequence of differences in the degree of innervation of the limbs and viscera. The spinal cord is divided into segments grouped according to the relationships of the SPINAL NERVES to the vertebrae. In humans, there are 31 segments divided among 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal spinal nerves. Because of the differential growth of the vertebral column and the spinal cord, the DORSAL and VENTRAL ROOTS of the lower spinal cord are elongated in the vertebral canal forming the CAUDA EQUINA (Latin, horse’s tail). Two spinal cord enlargements are related to innervation of the limbs. The cervical enlargement (C4–T1) innervates the arms (the corresponding spinal nerves form the brachial PLEXUS) and the lumbosacral enlargement (L2–S4) innervates the legs and PERINEUM (the corresponding spinal nerves form the lumbar and sacral plexuses). The thoracic spinal cord has a smaller cross-section because it innervates the skin and muscles of the thorax and back, and the internal organs, which are less densely innervated than the limbs. In cross-section, the spinal cord consists of WHITE MATTER surrounding grey matter resembling the shape of a butterfly, a shape that varies at different segmental levels. The white matter is divided into dorsal, dorsolateral, lateral and ventromedial funiculi or columns consisting of myelinated and unmyelinated (see MYELIN) ascending and descending axons that connect the spinal cord with higher levels of the NEURAXIS.
The different functional pathways in the white matter cannot be distinguished microscopically but they are segregated within specific areas. The DORSAL FUNICULUS contains ascending sensory axons that originate in the periphery and synapse in the DORSAL COLUMN NUCLEI OF THE MEDULLA OBLONGATA (the GRACILE NUCLEUS and the CUNEATE NUCLEUS). These axons carry sensations of fine, discriminative TOUCH and vibration. The DORSOLATERAL FUNICULUS contains afferent sensory fibres mediating PAIN and temperature sensations and descending axons from the BRAINSTEM.
The periphery of the lateral funiculus contains axons ascending to higher levels of the neuraxis including the brainstem (spinoreticular pathways), CEREBELLUM (spinocerebellar pathways) and THALAMUS (spinothalamic pathways). The inner parts of the lateral funiculus contain axons descending from the CEREBRAL CORTEX (the lateral corticospinal tract) and RED NUCLEUS of the MESENCEPHALON (rubrospinal tract) that modulate spinal cord activity, especially movements of the DISTAL limb musculature. The ventral funiculus contains axons descending from several structures, including the cerebral cortex, the RETICULAR FORMATION, VESTIBULAR COMPLEX and TECTUM. These latter pathways are important in regulating basic movements and posture. PROPRIOSPINAL pathways that interconnect different levels of the spinal cord, especially between the cervical and lumbar enlargements, are located around the borders of the GREY MATTER. In the region of the CENTRAL CANAL, the remnant of the spinal ventricular system (see VENTRICLES), the anterior white commissure is formed by crossing axons of the spinothalamic tract.
The grey matter of the spinal cord is divided into dorsal and ventral horns with an intermediate zone in between and, at thoracic levels, a lateral horn containing sympathetic preganglionic neurons. Two main schemes are regularly used to describe the organization of the grey matter. In the classical neuroanatomical approach, differences in their CYTOARCHITECTURE are used to define nuclei. In a second approach, the grey matter is divided into 10 layers (REXED’S LAMINAE). This system is popular because of the ease with which it can be correlated with physiological and functional differences within the spinal cord and because it is applicable across species. The 10 laminae and the equivalent spinal nuclei are from dorsal to ventral: I—posteromarginal nucleus, II—substantia gelatinosa, III and IV—nucleus proprius, V and VI—deeper layers of the dorsal horn, VII—intermediate zone, VIII—non-motoneuronal areas of the ventral horn, IX—motor neurons and X—grey matter surrounding the central canal of the spinal cord. Two groups of neurons are present only at thoracic levels of the cord: the sympathetic preganglionic neurons of the intermediolateral and intermediomedial nuclei in lamina VII and the nucleus dorsalis of Clarke which projects to the cerebellum. Sacral parasympathetic preganglionic neurons are found in lamina VII of the sacral cord.
The functions of the many descending and PROPRIOBULBAR pathways of the spinal cord are best understood by considering how they terminate in relationship to SENSORY NEURONS, INTERNEURONS and MOTOR NEURONS. Thus, axons terminating in the dorsal horn modulate sensory processing. Axons terminating in the ventromedial grey matter regulate posture and gross locomotion, those in the ventrolateral grey matter of the ventral horn regulate proximal movements of the limbs, and those in dorsolateral regions of the ventral horn regulate distal movements, according to the somatotopical organization of motor neurons.
DAVID A.HOPKINS
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