Anatomy and Physiology

• Transmission of nerve impulses. Neurons in the white matter of the spinal cord transmit sensory signals from peripheral regions to the brain and transmit motor signals from the brain to peripheral regions.

• Spinal reflexes. Neurons in the gray matter of the spinal cord integrate incoming sensory information and respond with motor impulses that control muscles (skeletal, smooth, or cardiac) or glands.

The spinal cord is an extension of the brainstem that begins at the foramen magnum and continues down through the vertebral canal to the first lumbar vertebra (L ₁). Here, the spinal cord comes to a tapering point, the conus medullaris. The spinal cord is held in position at its inferior end by the filum terminale, an extension of the pia mater that attaches to the coccyx. Along its length, the spinal cord is held within the vertebral canal by denticulate ligaments, lateral extensions of the pia mater that attach to the dural sheath.

The following are external features of the spinal cord (see Figure 1):

• Spinal nerves emerge in pairs, one from each side of the spinal cord along its length.

• The cervical nerves form a plexus (a complex interwoven network of nerves—nerves converge and branch).

• The cervical enlargement is a widening in the upper part of the spinal cord (C ₄–T ₁). Nerves that extend into the upper limbs originate or terminate here.

• The lumbar enlargement is a widening in the lower part of the spinal cord (T ₉–T ₁₂). Nerves that extend into the lower limbs originate or terminate here.

• The anterior median fissure and the posterior median sulcus are two grooves that run the length of the spinal cord on its anterior and posterior surfaces, respectively.

• The cauda equina are nerves that attach to the end of the spinal cord and continue to run downward before turning laterally to other parts of the body.

• There are four plexus groups: cervical, brachial, lumbar, and sacral.The thoracic nerves do not form a plexus.

A cross section of the spinal cord reveals the following features, shown in Figure 2:

• Roots are branches of the spinal nerve that connect to the spinal cord. Two major roots form the following:

• A ventral root (anterior or motor root) is the branch of the nerve that enters the ventral side of the spinal cord. Ventral roots contain motor nerve axons, transmitting nerve impulses from the spinal cord to skeletal muscles.

• A dorsal root (posterior or sensory root) is the branch of a nerve that enters the dorsal side of the spinal cord. Dorsal roots contain sensory nerve fibers, transmitting nerve impulses from peripheral regions to the spinal cord.

• A dorsal root ganglion is a cluster of cell bodies of a sensory nerve. It is located on the dorsal root.

• Gray matter appears in the center of the spinal cord in the form of the letter H (or a pair of butterfly wings) when viewed in cross section:

• The gray commissure is the crossbar of the H.

• The anterior (ventral) horns are gray matter areas at the front of each side of the H. Cell bodies of motor neurons that stimulate skeletal muscles are located here.

• The posterior (dorsal) horns are gray matter areas at the rear of each side of the H. These horns contain mostly interneurons that synapse with sensory neurons.

• The lateral horns are small projections of gray matter at the sides of H. These horns are present only in the thoracic and lumbar regions of the spinal cord. They contain cell bodies of motor neurons in the sympathetic branch of the autonomic nervous system.

• The central canal is a small hole in the center of the H crossbar. It contains CSF and runs the length of the spinal cord and connects with the fourth ventricle of the brain.

• White columns (funiculi) refer to six areas of the white matter, three on each side of the H. They are the anterior (ventral) columns, the posterior (dorsal) columns, and the lateral columns.

• Fasciculi are bundles of nerve tracts within white columns containing neurons with common functions or destinations:

• Ascending (sensory) tracts transmit sensory information from various parts of the body to the brain.

• Descending (motor) tracts transmit nerve impulses from the brain to muscles and glands.