The
peripheral nervous system (PNS) consists of the
nerves and
ganglions outside of the
CNS and is organized into two main
systems:
Somatic system organization
The
somatic system (SNS) controls
skeletal muscles under our conscious control. It consists of
motor neurons that travel from the
CNS to
skeletal muscle effectors. The
cell bodies of motor neurons are in the
CNS and the
axons of the
motor neurons travel directly to the
effectors.
Autonomic system organization
The
autonomic system (ANS) controls involuntary
functions of various types of
effectors:
smooth muscles,
cardiac muscles,
glands and other
tissues. The
nerves of the autonomic
system also travel from the
CNS to
effectors, but form a chain of two
motor neurons that
synapse inside a
ganglion in the PNS. A
ganglion is a
structure of the
nervous system containing
neuron cell bodies found
outside of the CNS.
In the chain of two
motor neurons the
cell body of the
preganglionic motor neuron is found in the
CNS and its
axon extends to the
ganglion.
The
cell body of the
postganglionic motor neuron is found in the
ganglion and its
axon extends to the
effector.
The
autonomic nervous system is divided into two
divisions: the
parasympathetic and
sympathetic divisions. The two divisions of the ANS oppose each other in a "tug of war" for control.
Organs and
tissues in the body are said to be under the
tone of one of the divisions at any given time.
Parasympathetic division
The
parasympathetic division is often called the "
rest and digest"
division of the ANS because it promotes energy-saving
functions like relaxation/rest and the digestion of food.
The
parasympathetic division is also called the
craniosacral division because its
nerves exit the
CNS in the
cranial and
sacral regions.
The
nerves of the
parasympathetic division have their
preganglionic motor neuron cell bodies in the
brain nuclei of the
cranial nerves and in
sacral nerves in the
spinal cord. The
axons of the preganglionic neurons travel to
terminal ganglia, which are found very close to the
effector. The
cell body of the
postganglionic motor neuron is found in the terminal
ganglion and its
axon travels a very short distance to the
effector.
Sympathetic division
The
sympathetic division is often called the "
fight or flight"
division of the ANS because it promotes energy-consumiong
functions like increased metabolic activity and exercise.
The
sympathetic division is also called the
thoracolumbar division because its
nerves exit the
CNS in the
thoracic and
lumbar regions.
The
nerves of the
sympathetic division have their
preganglionic motor neuron cell bodies in the
grey matter of the spinal cord in the
thoracic and
lumbar regions.
There are two types of
sympathetic nerves and
ganglion organizations:
Splanchnic nerves lead to
collateral ganglia near to
effectors. The preganglionic neurons of
splanchnic nerves synapse with
ganglionic neurons which will then travel to the
effectors.
Other
nerves lead to
chain ganglia in the
sympathetic chain, which is a series of ganglia lying parallel and
lateral to the
spinal cord on both sides. The
presynaptic neurons can
synapse with
postganglionic neurons in the
chain ganglion either immediately after exiting the
spinal cord, or may lead to a chain
ganglion superior or
inferior to the point of exit before synapsing. The postganglionic neurons then lead to the
effectors.
PNS organization
Figure 8.4.1 - PNS Organization. In the Somatic system a single motor neuron travels from the CNS to effectors. In the Autonomic system, a preganglionic motor neuron synapses with a postganglionic motor neuron. The postganglionic motor neuron then travels to the effectors. 
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1 - Anatomy and Physiology Basics