In vertebrates, each spinal nerve arises from the spinal cord by two roots:
a dorsal (posterior in human anatomy) and a ventral (anterior in human anatomy) which join to form the spinal nerve as they
pass through the wall of the vertebral column. The dorsal roots contain
all the sensory nerve fibers, their
cell bodies being in a ganglion on the
course of each root; in some vertebrates they may also contain a few motor
fibers. Through the ventral roots pass the motor fibers, their cell bodies
being in the spinal cord.
|A sensory neuron has its cell body in the dorsal
root ganglion. One end of the neuron travels out to the periphery
through the peripheral nerve (dorsal and ventral ramus), while the
other end travels into the spinal cord through the dorsal root. Inside
the spinal cord, sensory neurons carrying pain signals (nociceptors)
typically have a synapse just past the dorsal root entry zone where
they pass their electrical signal on to another neuron. Other sensory
neurons may continue up to the base of the brain before arriving at
their first synapse. Motor neuron cell bodies are in the ventral horn
of the spinal cord. The axons of these neurons travel outward through
the ventral root.
nerves can also be classified into dorsal and ventral roots, which however
do not join, remaining separate nerves. Cranial dorsal roots have ganglia and contain, besides sensory, numerous motor fibers running to the face,
jaw, and (in fish) gill muscles, and motor fibers of the parasympathetic
system. Cranial ventral roots contain some sensory (proprioceptor) fibers.