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Figure 3.17 The Functional Divisions of the Nervous System
Nerves are differentiated according to their function. A sensory (or afferent) neuron carries
information from the sensory receptors, whereas a motor (or efferent) neuron transmits
information to the muscles and glands. An interneuron, which is by far the most common type of
neuron, is located primarily within the CNS and is responsible for communicating among the
neurons. Interneurons allow the brain to combine the multiple sources of available information to
create a coherent picture of the sensory information being conveyed.
The spinal cord is the long, thin, tubular bundle of nerves and supporting cells that extends down
from the brain. It is the central throughway of information for the body. Within the spinal cord,
ascending tracts of sensory neurons relay sensory information from the sense organs to the brain
while descending tracts of motor neurons relay motor commands back to the body. When a
quicker-than-usual response is required, the spinal cord can do its own processing, bypassing the
brain altogether. A reflex is an involuntary and nearly instantaneous movement in response to a
stimulus. Reflexes are triggered when sensory information is powerful enough to reach a given
threshold and the interneurons in the spinal cord act to send a message back through the motor