The Peripheral Nervous System and Reﬂex Activity
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
What are varicosities and where would you ﬁnd them?
Which parts of the nervous system ultimately plan and
coordinate complex motor activities?
For answers, see Appendix H.
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.
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
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
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
Site of the stimulus action.
±ransmits aﬀerent impulses to the CNS.
In simple reﬂex arcs, the integration
center may be a single synapse between a sensory neuron
and a motor neuron (
). More complex
reﬂex arcs involve multiple synapses with chains of interneu-
). Te integration center for the re-
ﬂexes we will describe in this chapter is within the CNS.
Conducts eﬀerent impulses from the inte-
gration center to an eﬀector organ.
Muscle ﬁber or gland cell that responds to the ef-
ferent impulses (by contracting or secreting).
Reﬂexes are classiﬁed functionally as
activate skeletal muscle, or as
autonomic (visceral) reﬂexes
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
Compare and contrast stretch, ﬂexor, crossed-extensor, and
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.
(in cross section)
The ﬁve basic components of all reﬂex arcs.
reﬂex arc illustrated is polysynaptic.