Chapter 13
The Peripheral Nervous System and Reflex Activity
513
13
and provide its readiness to initiate a voluntary act. Te con-
scious cortex then chooses to act or not act, but the groundwork
has already been laid.
Check Your Understanding
12.
What are varicosities and where would you find them?
13.
Which parts of the nervous system ultimately plan and
coordinate complex motor activities?
For answers, see Appendix H.
PART 4
Reflex Activity
The Reflex Arc
Name the components of a reflex arc and distinguish
between autonomic and somatic reflexes.
Many of the body’s control systems are reflexes, which can be
either inborn or learned.
An
inborn
(
intrinsic
)
reflex
is a rapid, predictable motor re-
sponse to a stimulus. It is unlearned, unpremeditated, and in-
voluntary, and is built into our neural anatomy. Reflexes prevent
us from having to
think
about all the little details of staying
upright, intact, and alive—helping us maintain posture, avoid
pain, and control visceral activities.
For instance, what happens when you splash a pot of boiling
water on your arm? You are likely to drop the pot instantly and
involuntarily even before you feel any pain. Your response is
triggered by an inborn spinal reflex without any help from the
brain. In many cases we are aware of the final response of a basic
reflex (you know you’ve dropped the pot of boiling water). In
other cases, reflex activities go on without any awareness on our
part. Tis is typical of many visceral reflexes, which are regu-
lated by the subconscious lower regions of the CNS, specifically
the brain stem and spinal cord.
Te second type of reflex, a
learned
(
acquired
)
reflex,
results
from practice or repetition. ±ake, for instance, the complex se-
quence of reactions that occurs when an experienced driver drives
a car. Te process is largely automatic, but only because substan-
tial time and effort were expended to acquire driving skills.
In reality, the distinction between inborn and learned reflexes
is not clear-cut and most inborn reflex actions can be modified
by learning and conscious effort. For instance, if a 3-year-old
child was standing by your side when you scalded your arm,
you most likely would set the pot down (rather than just letting
go) because you consciously recognized the danger to the child.
Recall the discussion in Chapter 11 about serial and paral-
lel processing of sensory input. What happens when you scald
your arm is a good example of how these two processing modes
work together. You drop the pot before feeling any pain, but the
pain signals picked up by the interneurons of the spinal cord
are quickly transmitted to the brain, so that within the next few
seconds you do become aware of pain, and you also know what
happened to cause it. Te withdrawal reflex is serial processing
mediated by the spinal cord, and pain awareness reflects simul-
taneous parallel processing of the sensory input.
Components of a Reflex Arc
As you learned in Chapter 11, reflexes occur over highly specific
neural paths called reflex arcs. All reflex arcs have five essential
components
(Figure 13.15)
:
1
Receptor:
Site of the stimulus action.
2
Sensory neuron:
±ransmits afferent impulses to the CNS.
3
Integration center:
In simple reflex arcs, the integration
center may be a single synapse between a sensory neuron
and a motor neuron (
monosynaptic reflex
). More complex
reflex arcs involve multiple synapses with chains of interneu-
rons (
polysynaptic reflex
). Te integration center for the re-
flexes we will describe in this chapter is within the CNS.
4
Motor neuron:
Conducts efferent impulses from the inte-
gration center to an effector organ.
5
Effector:
Muscle fiber or gland cell that responds to the ef-
ferent impulses (by contracting or secreting).
Reflexes are classified functionally as
somatic reflexes
if they
activate skeletal muscle, or as
autonomic (visceral) reflexes
if
they activate visceral effectors (smooth or cardiac muscle or
glands). Here we describe some common somatic reflexes me-
diated by the spinal cord. We will consider autonomic reflexes
in later chapters along with the visceral processes they help to
regulate.
Spinal Reflexes
Compare and contrast stretch, flexor, crossed-extensor, and
tendon reflexes.
Spinal reflexes
are somatic reflexes that are mediated by the spi-
nal cord. Many spinal reflexes occur without the direct involve-
ment of higher brain centers. Generally, these reflexes are even
present in animals whose brains have been destroyed as long as
the spinal cord is still functional.
1
2
3
4
5
Receptor
Sensory neuron
Integration center
Motor neuron
Effector
Spinal cord
(in cross section)
Interneuron
Stimulus
Skin
Figure 13.15
The five basic components of all reflex arcs.
The
reflex arc illustrated is polysynaptic.
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