The preganglionic motor neurons of the sympathetic
system (shown in black) arise in the spinal cord. They pass into
sympathetic ganglia which
are organized into two chains that run parallel to and on either side of the
spinal cord.
The preganglionic neuron may do one of three things in the
sympathetic ganglion:
synapse with postganglionic neurons (shown in
white) which then reenter the spinal nerve and ultimately pass out to the sweat
glands and the walls of blood vessels near the surface of the body.
pass up or down the sympathetic chain and finally synapse
with postganglionic neurons in a higher or lower ganglion
leave the ganglion by way of a cord leading to special ganglia (e.g. the solar plexus) in the viscera. Here it may synapse with postganglionic sympathetic neurons running to the smooth muscular walls of the viscera. However, some of these preganglionic neurons pass right on through this second ganglion and into the adrenal medulla. Here they synapse with the highly-modified postganglionic cells that make up the secretory portion of the adrenal medulla.
The neurotransmitter of the preganglionic sympathetic neurons is acetylcholine (ACh). It stimulates action potentials in the postganglionic neurons.
The neurotransmitter released by the postganglionic neurons
is noradrenaline (also
called norepinephrine).
The action of noradrenaline on a particular gland or muscle
is excitatory is some cases, inhibitory in others. (At excitatory
terminals, ATP may
be released along with noradrenaline.)
- The release of noradrenaline
- stimulates heartbeat
- raises blood pressure
- dilates the pupils
- dilates the trachea and bronchi
- stimulates the conversion of liver glycogen into glucose
- shunts blood away from the skin and viscera to the skeletal muscles, brain, and heart
- inhibits peristalsis in the gastrointestinal (GI) tract
- inhibits contraction of the bladder and rectum
- and, at least in rats and mice, increases the number of AMPA receptors in the hippocampus and thus increases long-term potentiation (LTP).
In short, stimulation of the sympathetic branch of the
autonomic nervous system prepares the body for emergencies: for "fight or
flight" (and, perhaps, enhances the memory of the event that triggered the
response).
Activation of the sympathetic system is quite general
because
- a single preganglionic neuron usually synapses with many postganglionic neurons;
- the release of adrenaline from the adrenal medulla into the blood ensures that all the cells of the body will be exposed to sympathetic stimulation even if no postganglionic neurons reach them directly.
The Parasympathetic Nervous System
The main nerves of the parasympathetic system are the tenth
cranial nerves, the vagus nerves. They originate in
the medulla oblongata. Other preganglionic parasympathetic neurons also
extend from the brain as well as from the lower tip of the spinal cord.
Each preganglionic parasympathetic neuron synapses with just
a few postganglionic neurons, which are located near — or in — the effector
organ, a muscle or gland. Acetylcholine (ACh) is the neurotransmitter
at all the pre- and many of the postganglionic neurons of the parasympathetic
system. However, some of the postganglionic neurons release nitric oxide
(NO) as their neurotransmitter.
Loewi found that these two responses would occur in a second
frog heart supplied with a salt solution taken from the stimulated heart.
Electrical stimulation of the vagus nerve leading to the first heart not only
slowed its beat but, a short time later, slowed that of the second heart also.
The substance responsible was later shown to be acetylcholine. During
sympathetic stimulation, adrenaline (in the frog) is released.
- Parasympathetic stimulation causes
- slowing down of the heartbeat (as Loewi demonstrated)
- lowering of blood pressure
- constriction of the pupils
- increased blood flow to the skin and viscera
- peristalsis of the GI tract
In short, the parasympathetic system returns the body
functions to normal after they have been altered by sympathetic stimulation. In
times of danger, the sympathetic system prepares the body for violent activity.
The parasympathetic system reverses these changes when the danger is over.
The vagus nerves also help keep inflammation under
control. Inflammation stimulates nearby sensory neurons of the vagus. When
these nerve impulses reach the medulla oblongata, they are relayed back along
motor fibers to the inflamed area. The acetylcholine from the motor neurons
suppresses the release of inflammatory cytokines, e.g., tumor
necrosis factor (TNF), from macrophages in the inflamed tissue.
Although the autonomic nervous system is considered to be involuntary, this is not entirely true. A certain amount of conscious control can be exerted over it as has long been demonstrated by practitioners of Yoga and Zen Buddhism. During their periods of meditation, these people are clearly able to alter a number of autonomic functions including heart rate and the rate of oxygen consumption. These changes are not simply a reflection of decreased physical activity because they exceed the amount of change occurring during sleep or hypnosis.
