These emerge through the intervertebral foramina on the sides of adjacent vertebrae.
The uppermost pair passes between the occipital bone and the atlas.
The lowest pair passes through the intervertebral foramen in the coccyx.
Structure Of A Typical Spinal Nerve:
Common to spinal nerves at all levels : Roots
Each spinal nerve has two nerve roots (except the first, which has no sensory root).
The root in the front, called anterior or ventral root, transmits impulses from the spinal cord to the muscles.
The root in the back, known as the posterior or dorsal root, carries sensory information (about touch, position, pain, and temperature) from the body (receptors) to the spinal cord.
Structure of a typical spinal nerve
The Anterior Root Of Each Spinal Nerve :
Made up of nerve fibers which come from the anterior gray column of the spinal cord.
These nerve fibers are axons of neurons having cell bodies in anterior gray horn of spinal cord.
Consists solely of motor nerve fibers supplying ultimately a skeletal muscle fiber.
Also Called Efferent Root As Fibers Are Conveying Messages Away From CNS.
The Posterior Root Of Each Spinal Nerve:
Consists of fibers which lead to the posterior gray column of the cord.
These fibers are all sensory fibers bringing impulse from some peripheral receptors.
Also called afferent root as fibers are conveying messages towards CNS.
On the posterior root there is a small bulge called the posterior root ganglion. It is produced by the cell bodies of the pseudo unipolar neuron with the peripheral process innervating the receptor and central process entering the cord.
Within the spinal canal or vertebral canal the anterior and posterior roots unite to form the trunk of a spinal nerve which is a mixed nerve having both sensory and motor fibers.
Each spinal nerve leaves the vertebral canal through intervertebral foramen and divides into two branches called rami.
Branches Of The Spinal Nerves:
Posterior Primary Ramus:
Smaller of the two branches.
Contains both motor and sensory fibers.
Curves sharply backwards.
Divides into many small branches to supply the muscles and the skin of the back (dorsal body wall).
Branches Of The Spinal Nerves :
Anterior Primary Ramus:
The larger of the two branches.
Contains both motor and sensory nerve fibers.
Runs forward in the tissues, dividing into many small branches which serve the muscles and the skin of anterolateral body wall including limbs.
Fate of anterior primary ramus:
In the lower neck region and the lumbar region the anterior rami of the spinal nerves are very large indeed. In each of these regions they fuse together in groups in a complicated way to form nerve plexuses. From these plexuses spring the series of large nerves which supply motor nerve fibers to the muscles, and sensory nerve fibers to the skin of the arms and the legs.