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.