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A cross section of the spinal cord reveals the characteristic butterfly shape of the gray matter of the spinal cord surrounded by white matter. The gray matter changes shape throughout the spinal cord depending on the neural requirements of a given region. For example, the cervical and lumbar enlargements that provide innervation to the limbs have much more gray matter than a segment of spinal cord taken from the thoracic region that supplies a segment of the trunk. Regardless of location, the cord consists of a pair of dorsal horns and a pair of ventral horns of gray matter. The dorsal horns are made of the cell bodies upon which the axons carrying sensory information from the periphery synapse. The ventral horns are composed of the cell bodies of the motor neurons innervating skeletal muscles. A useful mnemonic for remembering the relationships in the spinal cord is: SAME-DAVE (sensory-afferent, motor-efferent; dorsal-afferent, ventral-efferent). In the thoracic region, the upper lumbar region and in segments of the sacral region of the spinal cord, there is also a lateral horn of gray matter. In the thoracic and lumbar regions of the spinal cord, the lateral horn contains autonomic cell bodies for sympathetic nervous system innervation to smooth muscles, cardiac muscles and glands. In the sacral region of the spinal cord, the lateral horn contains autonomic cell bodies for parasympathetic innervation to pelvic organs. The role of the lateral horn and autonomic innervation will be discussed in another module, however, it is helpful to recognize that the lateral horns exist at this point. |