Figure 2-4. Spinal nerve.
a. A stimulus acts upon a sensory receptor. The information is carried by an
afferent (sensory) neuron through the merging branches of the spinal nerve that has
been affected. The information is carried through the posterior root ganglion and
posterior root to the spinal cord. Once the information reaches the spinal cord, it
ascends the appropriate fiber tract to the designated area of the brain.
b. Motor information (commands) from the brain will descend along the
appropriate fiber tract within the spinal cord until the appropriate spinal nerve is
innervated. The efferent (motor) neurons carry the command from the spinal cord to the
The simplest reaction of the human nervous system is the reflex. A reflex is an
automatic reaction to a stimulus. The pathway from the receptor organ to the reacting
muscle is called a reflex arc (figure 2-5). The pathway of a reflex arc contains five