Here is a lateral view of the left
side of a human brain.
On the basis of their phylogenetic
and ontogenetic origin, neuroanatomists generally classify
the structures of the brain as shown below, from the most
evolved to the most primitive.
(Place your cursor over the name
of each structure to see where it is located on the sagittal
view of the brain).
The
structures listed below are often considered to constitute
the limbic system.
This system is involved in olfaction, emotions, learning,
and memory. The limbic system was introduced as a concept
by Paul MacLean in 1952 and was long considered the seat
of the emotions. Though some of the structures included
in this system are in fact involved in some emotional responses,
we now know that it does not correspond exactly to any
of the multiple emotional systems in the brain.
The
autonomic nervous system is controlled mainly by the
hypothalamus and is in turn divided into two sets of nerves:
the sympathetic and parasympathetic nervous systems. Each
of these two systems has a distinct anatomical location
and communicates with its target organs through other neurons
located in ganglia.
SYMPATHETIC NERVOUS SYSTEM
The sympathetic nervous system
goes into action to prepare the organism for physical
or mental activity. When the organism faces a major
stressor, it is the sympathetic nervous system
that orchestrates the fight-or-flight
response. It dilates the bronchi and the pupils,
accelerates heart rate and respiration, and increases
perspiration and arterial blood pressure, but reduces
digestive activity. Two neurotransmitters are primarily
associated with this system: epinephrine and norepinephrine.
PARASYMPATHETIC NERVOUS SYSTEM
The activation of the parasympathetic
nervous system causes a general slowdown in the
body’s functions in order to conserve energy.
Whatever was dilated, accelerated, or increased
by the sympathetic nervous system is contracted,
decelerated, or decreased by the parasympathetic
nervous system. The only things that the parasympathetic
nervous system augments are digestive functions
and sexual appetite. One neurotransmitter is primarily
associated with this system: acetylcholine.
