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The nervous system can be divided into several connected systems that function
The Nervous
System is divided into:
The Central Nervous System and the Peripheral Nervous System
Let's break the central nervous system and the peripheral nervous system into more
Central Nervous System
The central nervous system is divided into two
parts: the brain and the spinal cord. The
average adult human brain weighs 1.3 to 1.4
kg (approximately 3 pounds). The brain
contains about 100 billion nerve cells
(neurons) and trillons of "support cells" called
glia. The spinal cord is about 43 cm long in
adult women and 45 cm long in adult men and
weighs about 35-40 grams. The vertebral
column, the collection of bones (back bone)
that houses the spinal cord, is about 70 cm
long. Therefore, the spinal cord is much
shorter than the vertebral column.
(Brain and
Peripheral Nervous System
The peripheral nervous system is divided into two major parts: the somatic
nervous system and the autonomic nervous system.
1. Somatic Nervous System
The somatic nervous system consists
of peripheral nerve fibers that send
sensory information to the central
nervous system AND motor nerve
fibers that project to skeletal muscle.
The picture on the left shows
the somatic motor system. The cell body is located in either the brain
or spinal cord and projects directly to a skeletal muscle.
2. Autonomic Nervous
The autonomic nervous
system is divided into three
parts: the sympathetic
nervous system, the
parasympathetic nervous
system and the enteric
nervous system. The
autonomic nervous system controls smooth muscle of the viscera (internal organs)
and glands.
This picture shows the general organization of the autonomic nervous system. The
preganglionic neuron is located in either the brain or the spinal cord. This
preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron
then projects to the target organ.
Notice that the somatic nervous system has only one neuron between the central
nervous system and the target organ while the autonomic nervous system uses two
The enteric nervous system is a third division of the autonomic
nervous system that you do not hear much about.
The enteric nervous system is a meshwork of nerve fibers that
innervate the viscera (gastrointestinal tract, pancreas, gall
Here is a quick look at one way to divide the brain.
From a top view, notice how the brain is
divided into two halves, called hemispheres.
Each hemisphere communicates with the other
through the corpus callosum, a bundle of nerve
fibers. (Another smaller fiber bundle that
connects the two hemispheres is called the
anterior commissure).
Brain Structures
Cerebral Cortex
Voluntary movement
The word "cortex" comes from the Latin word
for "bark" (of a tree). This is because the cortex
is a sheet of tissue that makes up the outer layer
of the brain. The thickness of the cerebral cortex
varies from 2 to 6 mm. The right and left sides of
the cerebral cortex are connected by a thick band
of nerve fibers called the "corpus callosum." In
higher mammals such as humans, the cerebral
cortex looks like it has many bumps and grooves. A
bump or bulge on the cortex is called a gyrus (the
plural of the word gyrus is "gyri") and a groove is
called a sulcus (the plural of the word sulcus is
"sulci"). Lower mammals, such as rats and mice, have
very few gyri and sulci.
The word "cerebellum" comes from the Latin
word for "little brain." The cerebellum is
located behind the brain stem. In some ways,
the cerebellum is similar to the cerebral
cortex: the cerebellum is divided into
hemispheres and has a cortex that surrounds
these hemispheres.
Brain stem
Heart Rate
Blood Pressure
Body Temperature
Circadian Rhythms
Sensory processing
Limbic System
The brain stem is a general term for the area
of the brain between the thalamus and spinal
cord. Structures within the brain stem
include the medulla, pons, tectum, reticular
formation and tegmentum. Some of these
areas are responsible for the most basic
functions of life such as breathing, heart rate and blood
The hypothalamus is composed of several different
areas and is located at the base of the brain.
Although it is the size of only a pea (about 1/300 of the
total brain weight), the hypothalamus is responsible for
some very important functions. One important function
of the hypothalamus is the control of body temperature.
The hypothalamus acts as a "thermostat" by sensing
changes in body temperature and then sending signals
to adjust the temperature. For example, if you are too
hot, the hypothalamus detects this and then sends a
signal to expand the capillaries in your skin. This
causes blood to be cooled faster. The hypothalamus
also controls the pituitary.
The thalamus receives sensory information and
relays this information to the cerebral cortex. The
cerebral cortex also sends information to the thalamus
which then transmits this information to other areas of
the brain and spinal cord.
The limbic system (or the limbic areas)
is a group of structures that includes
the amygdala, the hippocampus, basil
ganglia, ventral tegmental area, and
cingulate. These areas are important
for controlling the emotional response
to a given situation. The hippocampus
is also important for memory.
The hippocampus is one part of the limbic
system that is important for memory and
Basal Ganglia
The midbrain includes structures such as
the superior and inferior colliculi and red
nucleus. There are several other areas
also in the midbrain.
The basal ganglia are a group of
structures, including the globus pallidus,
caudate nucleus, subthalamic nucleus,
putamen and substantia nigra, that are
important in coordinating movement.
Eye Movement
Body Movement
Here is where some of these Brain areas are located:
Nerve Function and Drug Action: Simplified
There are millions of cells in the brain. This picture depicts two nerve cells (neurons) and
their important components. Nerve Cell One is on the top, Nerve Cell Two is on the
The large portion of Nerve Cell One is the working part of the cell, also known as the presynaptic
area. The presynaptic area is at the end of a sending fiber called an axon, which begins outside
the boundaries of the picture in a cell body called the soma. Inside the soma are manufacturing
chemicals known as enzymes that manufacture chemicals called neurotransmitters.
These neurotransmitters pass down the axon under the influence of a small electrical current
called an action potential. The neurotransmitters are packaged in what look like cellophane
envelopes (called vesicles). These vesicles release their contents (neurotransmitters) into the
space between the two cells (the synapse), under the influence of small concentrations of calcium
Once in the synapse, one of four things can happen to the neurotransmitters. They either 1)
activate an excitatory receptor (on the left), causing Nerve Cell Two to be more likely to fire, 2)
activate an inhibitory receptor (middle), causing Nerve Cell Two to be less likely to fire, 3) are
"gobbled up" (metabolized) by a monster enzyme (right), or 4) are taken back up into Nerve Cell
One (reuptake), repackaged, and sent on down the nerve cell for use later on.
Inside Nerve Cell One is another monster enzyme, known as MAO, which gobbles up the
neurotransmitter molecules that accidentally leak out of the vesicles. Outside and above the
nerve membrane is a small molecule known as chloride ion, which is necessary for the proper
integrity of the vesicle membrane.
1. Under each of the receptor "ghosts" is a small rectangle containing globules of
substances known as G proteins that are the beginning of a chemical and electrical
cascade of events that make Nerve Cell Two more likely (excitation) or less likely
(inhibition) to fire and carry the message of Nerve Cell One to the next nervous system