(1) Coming off of the posterior and anterior sides of the spinal cord are the
posterior (dorsal) and anterior (ventral) roots of the spinal nerve. An enlargement on
the posterior root is the posterior root ganglion. A ganglion is a collection of neuron cell
bodies, together, outside the CNS.
(2) Laterally, the posterior and anterior roots of the spinal nerve join to form
the spinal nerve trunk. The spinal nerve trunk of each spinal nerve is located in the
appropriate intervertebral foramen of the vertebral column. (An intervertebral foramen
is a passage formed on either side of the junction between two vertebrae.)
(3) Where the spinal nerve trunk emerges laterally from the intervertebral
foramen, the trunk divides into two major branches. These branches are called the
anterior (ventral) and posterior (dorsal) primary rami (ramus, singular). The posterior
primary rami go to the back. The anterior primary rami go to the sides and front of the
body and also to the upper and lower members.
b. Neurons of a "Typical" Spinal Nerve. A nerve is defined above as a
collection of neuron processes. Thus, neuron processes are the components that make
up a nerve. These processes may belong to any of several different types of
neurons: afferent (sensory), efferent (motor), and visceral motor neurons of the ANS.
(1) The afferent neuron and the efferent neuron are the two types we will
consider here. An afferent neuron is one which carries information from the periphery to
A = toward
FERENT = to carry
An efferent neuron is one which carries information from the CNS to a muscle or gland.
E = away from
FERENT = to carry
(2) The afferent neuron is often called the sensory neuron because it carries
information about the senses to the CNS. The efferent neuron is often called the motor
neuron because it carries commands from the CNS to cause a muscle to act.
(3) A stimulus acts upon a sensory receptor organ in the skin or in another
part of the body. The information is carried by an afferent (sensory) neuron through
merging branches of the spinal nerve to the posterior root ganglion. The afferent
(sensory) neuron's cell body is located in the posterior root ganglion. From this point,
information continues in the posterior root to the spinal cord. The efferent (motor)
neuron carries command information from the spinal cord to the individual muscle of the