The spinal cord extends from the medulla oblongata to between the first and second lumbar vertebrae. It has a central area of gray matter surrounded by white matter. The gray matter projects toward the back and the front as the dorsal and ventral horns. The white matter contains the ascending and descending tracts (fiber bundles) that carry impulses to and from the brain.
Thirty-one pairs of spinal nerves connect with the spinal cord (Fig. 1). These nerves are grouped in the segments of the cord as follows:
•Cervical: 8
•Thoracic: 12
•Lumbar: 5
•Sacral: 5
•Coccygeal: 1
Each nerve joins the cord by two roots. The dorsal, or posterior, root carries sensory impulses into the cord; the ventral, or anterior, root carries motor impulses away from the cord and out toward a muscle or gland.
A simple response that requires few neurons is a reflex. In a spinal reflex, impulses travel through the spinal cord only and do not reach the brain. An example of this type of response is the knee-jerk reflex used in physical examinations. Most neurologic responses, however, involve complex interactions among multiple neurons (interneurons) in the CNS.
FIGURE 1. Spinal cord from the side, showing the divisions of the spinal nerves.
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