Spinal Cord and Nerves

 Adult spinal cord is 18 inches long and 14 mm wide  Spinal cord does not continue in length with the vertebrae; stops at L1 and L2  Entire spinal cord can be divided into 31 segments on the basis of the origins of the spinal nerves

Amount of gray matter is greatest in those areas that deal with sensory and motor neuron control of the limbs. Inferior to the lumbar the spinal cord becomes tapered and conical. This region ends in branching off of the nerves in an area called the cauda equina (horse’s tail).

 Every spinal segment is associated with a pair of dorsal root ganglia. The ganglia contain the cell bodies of sensory neurons. The axons of the neurons make up the dorsal roots. Ventral roots contain axons of motor neurons. The sensory and motor roots are bound together into a single spinal nerve.

Dorsal root Ventral root

 Spinal nerves are classified as mixed nerves – contain both afferent (sensory) and efferent (motor) fibers.  Spinal cord continues to enlarge and elongate until an individual is 4 years old.  After that time vertebral column continues to grow, but spinal cord does not. Vertebra growth moves the intervertebral foramen and nerves further from their original positions, thus elongating the dorsal and ventral roots.

 Spinal meninges is a series of specialized membranes surrounding the spinal cord to provide protection (shock absorption)  Consists of three layers:  (1) dura mater  (2) arachnoid  (3) pia mater  These layers are continuous with the cranial meninges that surround the brain.

Dura mater Arachnoid Pia mater

 Between the dura mater and the walls of the vertebral canal lies the epidural space, a region with loose connective tissue and adipose  Anesthetics are often injected into the epidural space. The drug should only affect spinal nerves in the area of the injection. Result is an epidural block-temporary sensory loss or motor paralysis (depending on anesthetic) used to control pain during childbirth.

 A narrow region between the arachnoid and the pia mater is called the subarachnoid space that is filled with cerebrospinal fluid (CSF). The withdrawal of the CSF occurs in a procedure called a spinal tap. Spinal taps are done to diagnose CNS infection, severe back pain, disc problems, and some types of strokes.

 Every segment of the spinal cord is attached to a pair of spinal nerves. A series of connective tissue layers surrounds each spinal nerve and continues along its branches.

 Epineurium surrounds several bundles of fascicles  Perineurium surrounds a fascicle  Endoneurium surrounds individual myelin sheaths and axons

 A reflex is a rapid, automatic response to a specific stimuli.  Most reflexes occur in the spinal cord rather than the brain for faster response time. The wiring of a single reflex is called a reflex arc.