The Spinal Cord
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| Figure 35-10 Human spinal cord and its
protection. Two vertebrae show position of the spinal cord, emerging spinal nerves, and the sympathetic trunk. The cord is wrapped by three layers of membrane (meninges) between two of which lies a protective bath of cerebrospinal fluid. |
The brain and spinal cord compose the central nervous system. During early embryonic development, the spinal cord and brain begin as an ectodermal neural groove, which by folding and enlarging becomes a long, hollow neural tube (Figure 8-12,). The cephalic end enlarges to form brain vesicles, and the rest becomes the spinal cord. Unlike any invertebrate nerve cord, the segmental nerves of the spinal cord (31 pairs in humans) are separated into dorsal sensory roots and ventral motor roots. The sensory nerve cell bodies are gathered together into dorsal root (spinal) ganglia. Both dorsal (sensory) and ventral (motor) roots meet beyond the spinal cord to form a mixed spinal nerve (Figure 35-10).
The spinal cord is enclosed by the central spinal canal and is additionally wrapped in three layers of membranes called meninges (men-injeez; Gr. meningos, membrane). In cross section the cord shows two zones (Figure 35-10). An inner zone of gray matter, resembling in shape the wings of a butterfly, contains the cell bodies of motor neurons and interconnecting interneurons (described in the following text). An outer zone of white matter contains bundles of axons and dendrites linking different levels of the cord with each other and with the brain.
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