182
Chapter 6
synapses with postganglionic neurons (
Figure 6–45
, num-
bers 1 and 4). Other possible paths the sympathetic fi bers
might take are shown in Figure 6–45, numbers 2, 3, and 5.
Due in part to differences in their anatomy, the over-
all activation pattern within the sympathetic and parasym-
pathetic systems tends to be different. The close anatomical
association of the sympathetic ganglia and the marked diver-
gence of presynaptic sympathetic neurons make that division
tend to respond as a single unit. Although small segments are
occasionally activated independently, it is thus more typical for
increased sympathetic activity to occur body-wide when cir-
cumstances warrant activation. The parasympathetic system,
in contrast, exhibits less divergence, and thus it tends to acti-
vate specifi c organs in a pattern fi nely tailored to each given
physiological situation.
In both the sympathetic and parasympathetic divisions,
acetylcholine is the neurotransmitter released between pre- and
postganglionic neurons in autonomic ganglia (
Figure 6–46
).
In the parasympathetic division, acetylcholine is also the neu-
rotransmitter between the postganglionic neuron and the
effector cell. In the sympathetic division, norepinephrine is
usually the transmitter between the postganglionic neuron
and the effector cell. We say “usually” because a few sympa-
thetic postganglionic endings release acetylcholine (e.g., sym-
pathetic pathways that regulate sweating). At many autonomic
synapses, one or more cotransmitters are stored and released
with the major neurotransmitter. These include ATP, dopa-
mine, and several of the neuropeptides, all of which seem to
play a relatively small role.
In addition to the classical autonomic neurotransmitters
just described, there is a widespread network of postganglionic
neurons recognized as nonadrenergic and noncholinergic.
These neurons use nitric oxide and other neurotransmitters
to mediate some forms of blood vessel dilation and to regulate
various gastrointestinal, respiratory, urinary, and reproductive
functions.
Many of the drugs that stimulate or inhibit various com-
ponents of the autonomic nervous system affect receptors for
acetylcholine and norepinephrine. Recall that there are several
types of receptors for each neurotransmitter. The great major-
ity of acetylcholine receptors in the autonomic ganglia are
nicotinic receptors. In contrast, the acetylcholine receptors on
smooth muscle, cardiac muscle, and gland cells are muscarinic
receptors. The cholinergic receptors on skeletal muscle fi bers,
innervated by the
somatic
motor neurons, not autonomic neu-
rons, are nicotinic receptors (
Table 6–10
).
One set of postganglionic neurons in the sympathetic
division never develops axons. Instead, they form an endocrine
gland, the
adrenal medulla
(see Figure 6–46). Upon activa-
tion by preganglionic sympathetic axons, cells of the adrenal
medulla release a mixture of about 80 percent epinephrine and
20 percent norepinephrine into the blood (plus small amounts
of other substances, including dopamine, ATP, and neuropep-
tides). These catecholamines, properly called hormones rather
than neurotransmitters in this circumstance, are transported
via the blood to effector cells having receptors sensitive to
them. The receptors may be the same adrenergic receptors that
are located near the release sites of sympathetic postgangli-
onic neurons and are normally activated by the norepinephrine
released from these neurons. In other cases, the receptors may
be located in places that are not near the neurons and are there-
fore activated only by the circulating epinephrine or norepi-
nephrine. The overall effect of these catecholamines is slightly
different due to the fact that some adrenergic receptor subtypes
have a higher affi nity for epinephrine (e.g.,
β
2
), whereas others
have a higher affi nity for norepinephrine (e.g.,
α
1
).
Table 6–11
is a reference list of the effects of autonomic
nervous system activity, which will be described in later chap-
ters. Note that the heart and many glands and smooth mus-
cles are innervated by both sympathetic and parasympathetic
fi bers; that is, they receive
dual innervation.
Whatever effect
one division has on the effector cells, the other division usu-
ally has the opposite effect. (Several exceptions to this rule
are indicated in Table 6–11.) Moreover, the two divisions are
Sympathetic trunk
(chain of sympathetic ganglia)
Spinal cord
(dorsal side)
Gray matter
White matter
To
collateral
ganglion
1
2
3
4
5
Preganglionic
neuron
Postganglionic
neuron
Sympathetic
ganglion
Figure 6–45
Relationship between a sympathetic trunk and spinal nerves
(1 through 5) with the various courses that preganglionic
sympathetic neurons (solid lines) take through the sympathetic
trunk. Dashed lines represent postganglionic neurons. A mirror
image of this exists on the opposite side of the spinal cord.
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