The ANS is organized into two major subdivisions--the sympathetic and
parasympathetic nervous systems. The first of these is also known as the
thoraco-lumbar outflow. The second is also known as the cranio-sacral outflow.
If one of these subdivisions stimulates an organ, the other will inhibit it. The
interplay of the two subdivisions helps visceral organs to function within a stable
Under conditions of stress, the sympathetic nervous system mobilizes all of the
energy-producing structures of the body. For example, it makes the heart beat faster.
Later, as equilibrium is restored, the parasympathetic nervous system has the opposite
The neurons are alined in sequences to form circuits. The transmission of
information along a neuron is electrochemical in nature. Crossing the gap between one
neuron and the next is a chemical called a neurotransmitter. (para 12-14)
Neurons are able to concentrate negative ions inside and positive ions outside of
the cell membrane. When the neuron is not actually transmitting, this process produces
When the polarity of ions is disrupted by a stimulus, that location on the cell
membrane is said to be depolarized. The restoration of the original polarity is called
repolarization. At the same time, adjacent areas are depolarized. Thus, there is a wave
of depolarization/repolarization along the length of the neuron. (para 12-16)
The speed of an impulse is proportional to the thickness of the neuron process.
Together, the gap and the "connecting" membranes between two successive
neurons are called the synapse. The gap itself is called the synaptic cleft. Containing
specific amounts of neurotransmitter are synaptic vesicles in the terminal bulb of the
first neuron. When an impulse reaches the bouton, the vesicles are stimulated to
release their neurotransmitter. This substance passes through the presynaptic
membrane, across the synaptic cleft, and to the postsynaptic membrane. Since this
process consumes much energy, the bouton contains many well-developed
The neuromuscular junction is the "connection" between a motor neuron and a
striated muscle fiber. It is nearly identical to a synapse. However, the surface of the
postsynaptic membrane is in a series of longitudinal folds. This greatly increases the
surface area receptive to the ACH.
The group of striated muscle fibers innervated by one motor neuron is called the
motor unit. Fewer muscle fibers per motor unit result in finer movements. More muscle
fibers per motor unit result in coarser movements. (para 12-19)