Human Physiology, 14th edition (2016)

238 Chapter 8

Figure 8.28 Activation of somatic motor neurons. Somatic motor neurons may be stimulated by spinal association
neurons, as shown here, or directly by sensory neurons in a reflex arc that doesn’t involve the brain. The spinal association neurons
and motor neurons can also be stimulated by association neurons (called upper motor neurons) in the motor areas of the brain. This
affords voluntary control of skeletal muscles.

Association neuron

Spinal cord

Ventral

root

Spinal

nerve

Somatic

motor neuron

Sensory

neuron 

Skeletal muscle

Upper motor neuron

(association neuron

in brain)

Cell body

of neuron Dorsal

root

Dorsal root

ganglion

cord. This produces two “roots” of each nerve. The dorsal root
is composed of sensory fibers, and the ventral root is composed
of motor fibers ( fig.  8.28 ). An enlargement of the dorsal root,
the dorsal root ganglion, contains the cell bodies of the sensory
neurons. The motor neuron shown in figure  8.28 is a somatic
motor neuron that innervates skeletal muscles; its cell body is
not located in a ganglion but instead is contained within the gray
matter of the spinal cord. The cell bodies of some autonomic
motor neurons (which innervate involuntary effectors), however,
are located in ganglia outside the spinal cord (the autonomic sys-
tem is discussed separately in chapter 9).

Reflex Arc

The functions of the sensory and motor components of a
spinal nerve can be understood most easily by examining a

simple reflex; that is, an unconscious motor response to a

sensory stimulus. Figure  8.28 demonstrates the neural path-

way involved in a reflex arc. Stimulation of sensory receptors

evokes action potentials that are conducted into the spinal cord

by sensory neurons. In the example shown, a sensory neuron

synapses with an association neuron (or interneuron), which,

in turn, synapses with a somatic motor neuron. The somatic

motor neuron then conducts impulses out of the spinal cord to

the muscle and stimulates a reflex contraction. Notice that the

brain is not directly involved in this reflex response to sensory

stimulation. Some reflex arcs are even simpler than this; in a

muscle stretch reflex (the knee-jerk reflex, for example) the

sensory neuron synapses directly with a motor neuron. Other

reflexes are more complex, involving a number of association

neurons and resulting in motor responses on both sides of the

spinal cord at different levels. These skeletal muscle reflexes