In adults the spinal cord ends at vertebral level L2, and the nerve roots continue as the cauda equina to exit at the appropriate vertebral level. The level of spinal cord termination allows a lumbar puncture to be performed with greater safety at spinal level L4-5.
Cell bodies for sensory neurons carrying information to the spinal cord are located in dorsal root ganglia, which are located outside the spinal canal.
Sensory information enters the spinal cord on the dorsal side.
Alpha motor neurons from segments C3, C4 and C5 travel via the phrenic nerve to innervate the diaphragm.
Alpha motor neurons originate from lamina IX, with those innervating extensor muscles more ventral than those innervating flexor muscles and those innervating distal muscles more lateral than those innervating proximal muscles.
Gamma motor neurons, which innervate intrafusal muscle fibers necessary in the muscle stretch reflex arc, also originate from lamina IX.
Autonomic fibers originate from the intermediolateral region of the gray matter. In the thoracic and lumbar spinal cord these are preganglionic sympathetic neurons and in the sacral regions they are preganglionic parasympathetic neurons. No autonomic neurons arise from the spinal cord at the cervical level.
Axons that ascend in the posterior column of white matter have their cell bodies in the dorsal root ganglia. These neurons mediate vibration, position sense, two-point discrimination, and feeling for shape, pattern, or direction of stimulus on the skin.
The dorsal nucleus of Clarke, located in lamina V between T1 and L2, receives unconscious sensory information about leg position. It has a vital role in coordination of walking.
Bladder control and micturition require coordination of signaling from somatic innervation and from parasympathetic both derived from S2 – S4 and from sympathetic innervation derived from T11-L2.
Supplemental alpha motor neurons from segments S1 – S4 innervate the anal and urethral sphincters, as well as muscles necessary for male sexual function.
INTRODUCTION The spinal cord contains the first synapse in all sensory pathways from the body and back of the head. It also contains all the motor neurons that innervate skeletal muscles of the body. (However, it does not have a role in sensory perception from the face or in motor control of facial muscles.) It also plays a vital role in the control of urination and the bladder.
EXTERNAL VIEW The spinal cord is located in the vertebral canal. At birth the cord extends the full length of the spinal column, but in the adult it extends from the foramen magnum to the L1 or L2 vertebral level. Most of the cells in the spinal cord are present at birth, and it does not grow substantially. However, since the bony vertebral column continues to grow for another 20 years, the adult spinal cord ends at the level of the L1 or L2 vertebra. As a result the spinal nerves associated with progressively lowers levels of the cord must travel further downward within the canal to exit at the appropriate intervertebral foramen. This collection of long spinal nerves extending beyond the conus medullaris, or tapering end of the spinal cord, is called the cauda equina, or “tail of the horse.”
By way of review, the spinal cord is divided into 31 segments:
Dorsal and ventral roots exit the spinal cord at each spinal level. Dorsal roots carry sensory information to the spinal cord; their cell bodies are located in the dorsal root ganglia. The ventral roots carry motor and autonomic inform from the spinal cord; their cell bodies are located in the gray matter of the spinal cord. Both dorsal (somatosensory) and ventral (motor) roots fan into tiny rootlets that attach to the cord along a vertical line at the dorsolateral and ventrolateral surfaces of the cord. Before exiting the spinal canal, the dorsal and ventral roots join a short distance to form a spinal nerve.