Anatomy & Physiology

Anatomy & Physiology Study Guide

Chapter 14: Spinal Cord and Spinal Nerves The nervous system has input pathways that route sensations, and processing centers that prioritize and distribute information. The brain and spinal cord make up the central nervous system, and the cranial nerves and spinal nerves constitute the peripheral nervous system. 14.1 Gross Anatomy of the Spi al Cord The adult spinal cord measures approximately 45 cm (18 in.) in length and has a maximum width of roughly 14 mm (0.55 in.). The posterior (dorsal) surface of the spinal cord bears a shallow longitudinal groove called the posterior median sulcus . The ventral surface has the anterior median fissure, which is a deep rooted groove along the spinal cord. The amount of gray matter is largest in segments of the spinal cord dedicated to the sensory and motor control of the limbs. Inferior to the lumbar enlargement, the spinal cord becomes tapered and conical inferior to the lumbar enlargement. This region is the conus medullaris . The filum terminal extends as a lean fibrous strand from the inferior tip of the conus medullaris. The whole spinal cord can be separated into three segments on the basis of the origins of the spinal nerves. Each segment is identified with a letter and number, the same method used to identify vertebrae. Every spinal segment is paired with a pair of dorsal root ganglia . These ganglia contain the cell bodies of sensory neurons. The axons of the neurons shape the dorsal roots, which bring sensory information into the spinal cord. Distal to each dorsal root ganglion, the sensory and motor roots are connected together into a single spinal nerve . Spinal nerves are classified as mixed nerves and contain both afferent (sensory) and efferent (motor) fibers. The 31 pairs of spinal nerves are recognized by their connection with adjacent vertebrae. Each spinal nerve inherits its name from the vertebra directly superior to it. Thus, spinal nerve T1 emerges directly inferior to vertebra T1, spinal nerve T2 follows vertebra T2, and so forth. The first pair of cervical spinal nerves has a different path as C1 passes between the skull and the first cervical vertebra. The adult spinal cord extends only to the level of the first or second lumbar vertebra; thus, dorsal and ventral roots of spinal segments L2 to S5 extend inferiorly, past the inferior tip of the conus medullaris. The filum terminale and the long ventral and dorsal roots, in gross dissection, resembled a horse's tail. As a result, early anatomists called this complex the cauda equina .

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