and the stellate ganglion in the upper thoracic region. Connected to the ganglion
are plexus that pass to the organs. The cardiac plexus via the stellate ganglion
supplies the heart and lungs. The solar plexus is connected with the lower thoracic
spinal nerves and supplies sympathetic fibers to the stomach, intestines, adrenals
and other viscera. The heart is supplied by sympathetic nerves arising mainly in the
neck, because the heart develops initially in the cervical region and later migrates
into the thorax taking its nerves down with it.
The neurotransmitter of the preganglionic sympathetic neurons is acetylcholine
(ACh). It stimulates action potentials in the postganglionic neurons, affecting
their targets through adrenergic receptors. 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 maybe 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 tract, inhibits
contraction of the bladder and rectum and inhibits the immune system to save
energy. Stimulation of the sympathetic branch of the autonomic nervous system
prepares the body for fight-or-flight. This emergency response is controlled by the
hypothalamus and amygdala through the HPA axis. 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.
One important exception to the activating response of the sympathetic
system is that the alimentary adrenergic nerves “inhibit” the activity of the
gastrointestinal tract while activity in the cholinergic (parasympathetic) supply
results in “activation” of the gastric and intestinal systems. This is because during
the adrenaline induced fight-or-flight response or during demanding activity,
the blood and energy is needed by the brain and muscles, leaving digestive and
eliminative functions until times of rest and relaxation. Hormones produced by
the outer region of the adrenal cortex regulate the body’s metabolism, blood
composition, and even body shape. The inner region produces hormones that are
the body’s first line of defense against stress, whether it be physical or emotional.
This inner region of the adrenals is called the adrenal medulla and is considered to
be part of the sympathetic nervous system. Adrenaline and norepinephrine act as
neurotransmitters when they are released by neurons, and as hormones when they
are produced by suprarenal glands.
ParasYmPatHetic
The parasympathetic or reposing side of the autonomic nervous system promotes
relaxation, sleep, growth and repair. It is sometimes called the “trophotropic”