Page 240 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of the Nervous System / 225

components, described in Chapter 10. imagine a horse relaxing and dozing on her
For a specific example of these concepts,
These two divisions of the motor outflow
VetBooks.ir of the ANS are complementary to one feet (Table 11‐2). Under these conditions,
another and tend to have more‐or‐less
her heart experiences a relatively high
opposite effects on their targets, which are parasympathetic and low sympathetic
smooth muscle, cardiac tissue, and glands. activity to maintain a low resting heart
Normally, the regulation of ANS activity rate. Compare this to the same horse exert­
occurs below the level of consciousness. ing herself in a hard gallop. To increase
However, emotional reactions (such as fear heart rate, parasympathetic nerve activity
or excitement) and input from the cerebral is reduced to permit an increase in heart
cortex also affect ANS activity. rate, and sympathetic nerves to the heart
Most organs that are innervated by the are activated as exertion (and therefore the
ANS have both sympathetic and parasym­ demand on the cardiovascular system)
pathetic innervation, and the behavior of becomes greater.
that organ at any point in time is a sum of Changes in ANS activity often occur as
the activity in the two divisions. Under the expression of reflexes. For example,
conditions of meeting immediate stressors the diameter of the pupil of the eye is in
(e.g., the physical effort involved in run­ part controlled by an ANS reflex initiated
ning), sympathetic activity is dominant by light detected by the retina. The sen­
and parasympathetic activity is minimal. sory limb of the reflex is the retina and
Conversely, when the organism is at rest optic nerve. Input via this sensory limb
or processing a meal, parasympathetic activates interneurons which in turn affect
activity dominates. The targets of the ANS the motor limb of the reflex, in this case
respond as appropriate to these two physi­ the oculomotor nerve’s parasympathetic
ologic states. axons. These innervate the smooth muscle

Table 11-2. A Synopsis of some Effects of Sympathetic and Parasympathetic Activity

Sympathetic effects Parasympathetic effects
Increase heart rate Decrease heart rate
Increase strength of heart contraction Decrease strength of heart contraction
Dilation of airways Constriction of airways
Increase blood flow to skeletal mm Reduce blood flow to skeletal mm
Decrease blood flow to viscera Increase blood flow to viscera
Decrease gut contraction Increase gut contraction
Decrease digestive gland secretion Increase digestive gland secretion
Relaxation of bladder wall Contraction of bladder wall
Dilation of pupil Constriction of pupil
Release of epi from adrenal gland Increased lacrimal gland secretion