FREE ESSAY ON THE NERVOUS SYSTEM

THE NERVOUS SYSTEM

The nervous system is the most complex part of the body, as they govern our thoughts,
feelings, and bodily functions. It is an important factor in science because it can lead
to new discoveries for cures or diseases. The studies of the nervous system helped lower
death rates from heart disease, stroke, accidents, etc. 
The nervous system is a network of neurons (nerve cells) that that sends information to
the brain to be analyzed. Neurons live both in and outside the central nervous system.
Understanding how the neurons work is vital to understanding the nervous system. 
Neurons
The neuron has two important structures called the dendrite and axon, also called nerve
fibers. The dendrites are like tentacles that sprout from the cell and the axon is one
long extension of the cell. The dendrites receive signals from other neurons, while the
axon sends impulses to other neurons. Axons can extend to more than a meter long. Average
sized neurons have hundreds of dendrites; therefore it can receive thousands of signals
simultaneously from other neurons. The neuron sends impulses by connection the axon to
the dendrites of another nerve cell. The synapse is a gap between the axon and the
adjacent neuron, which is where data is transmitted from one neuron to another.
The neuron is negatively charged and it bathes in fluids that contain positively charged
potassium and sodium ions. The membrane of the neuron holds negatively charged protein
molecules. The neuron has pores called ion channels to allow sodium ions to pass into the
membrane, but prevent the protein molecules from escaping (potassium ions can freely pass
through the membrane since the ion channels mostly restrict sodium ions). When a neuron
is stimulated (not at rest), the pores open and the sodium ions rush in because of its
attraction to the negatively charged protein molecules, which makes the cell positively
charged. As a result, potential energy is released and the neurons send electrical
impulses through the axon until the impulse reaches the synapse of any neurons near it.
Once the signal is sent, the ion balances out and becomes at rest. 
The electrical impulse that runs down the axon releases a chemical called acetylcholine,
only one of many chemicals that transmits signals across the synapse. These substances
are called neurotransmitters because they transmit data from one neuron to another. Once
the chemical binds to the dendrites of another neuron, it is converted back to an
electrical impulse, which is brought to the cell body. The impulse is then sent to
another neuron, and the process repeats until the nerves are at rest. The effect of the
signals depends on what the target is. If the target of the signal is a muscle cell, the
effect might be a muscle contraction. 
The speed of the electrical impulse depends on the size of the nerve fiber. In small
nerves, the rate it transmits impulses is from a half to two meters a second. The larger
the diameter of the nerve fiber, the higher rate of conducting impulses. There is less
electrical resistance in thick fibers. When nerve impulse jumps from one node (gaps in
nerve fibers) to the next, it is called saltatory conduction. Saltatory conduction
conducts faster because it contains an insulator that prevents leakage of currents. The
rate of conduction is 2 to 120 meters a second. Not all nerves conduct impulse
electrochemically. Some impulses jump from nerve to nerve, bypassing the synapse. 
Unlike other cells, once neurons are lost, they can't be regenerated. Fortunately, there
are about 10 billion neurons and they have other cells to aid them in carrying messages
to other nerves. But if nerves are severed, the nerve fibers can regenerate if the two
ends are reattached precisely. However, restored functions may produce different actions
because the nerves might not be connected to the right channel. 
There are three main parts of the nervous system: the central, peripheral, and autonomic
nervous system. The brain and spinal cord makes up central nervous system, the spinal and
cranial nerves form the peripheral nervous system, and the autonomic nervous system is
made from various glands and muscles.
Central Nervous System
The brain and spinal cord forms the Central Nervous System. The spinal cord gathers
information from the neurons and sends it to the brain, but not all nerve impulses get
sent to the brain. Only a few impulses reach the brain, and an even smaller number
reaches the part of the brain where they cause awareness. The brain and spinal cord is
made up of gray and white matter, as well as the various nerve cells. The brain is
divided into three parts: the forebrain, midbrain, and hindbrain.
The forebrain is called the cerebrum, the largest and most developed part of the human
brain. It is divided by the longitudinal sulcus, a deep crevice that separates the
forebrain into the left and right cerebral hemispheres. Each hemispheres control the
motor (movement) and sensory (sight, smell, hearing, taste, and touch) functions of the
other side of the body. 
The two hemispheres are divided into four lobes: the frontal, parietal, occipital, and
temporal lobe. The frontal lobe is involved with muscle control from head to toe. The
parietal lobe obtains sensory information from the skin and muscles. The occipital lobe
receives information from vision, and the temporal lobe controls speech. Although the
four lobes do different jobs, they communicate with each other to allow the brain to
coordinate a response. The cerebrum combines the mixtures of sight, sound, smell, and
movement and sends the information to many body parts at once. 
The midbrain is an inch of nerve fibers underneath the forebrain, which receives messages
from the ear, eyes, and cerebrum. The midbrain helps coordinate movement and muscle tone.
It also controls eye movements and reflexes in the eyes, like making the pupils smaller
or larger. 
The hindbrain is made up of the cerebellum and parts of the brainstem. The cerebellum is
involved in controlling bodily position, muscular coordination, and emotions. It contains
neurons called Purkinje cells, which is connected to more than 100,000 nerve fibers and
makes more connections than any other type of nerve cells. 
The medulla oblongata is located at the lowest part of the brainstem. It controls
reflexes like heartbeat, breathing, and swallowing. Any messages being carried to the
muscles pass through the medulla. Located at the rear of the medulla is a group of
neurons that acts as the brain's warning system. The medulla also organizes sleeping and
waking by stopping messages being sent to the brain when we are sleeping, and sends it
through the brain when we awake. 
Three membranes surround the brain: the dura, arachnoid, and pia. Between the arachnoid
and pia is the cerebrospinal fluid. Cerebrospinal fluid is a clear fluid that protects
the brain while the head is moving. The brain produces 500 milliliters of these fluids a
day. 
The spinal cord is located below the brain and is connected to the brainstem. It is a
cylinder of 31 spinal nerves about 18 inches long. The spinal cord is made up of gray and
white matter, with the white matter surrounding the gray. The spinal cord is encased in
cerebrospinal fluid and wrapped in three layers of membrane. 
All messages sent to and from the brain travels through the spinal cord because the brain
can't read what its neurons cannot contact. The spinal cord processes sensory
information, like temperature, to the brain. The spinal cord is also necessary for
movement of the body and reflexes that doesn't involve the brain. Any damage to the
spinal cord will cause paralysis and anesthesia because its connection to the brain is
severed. Damage to the cord is permanent because the neurons can't be regenerated. 
Peripheral Nervous System
The peripheral nervous system is an extension of the central nervous system and includes
43 nerves containing afferent and efferent fibers. Afferent fibers carry signals to the
central nervous system and efferent fibers carry signals away from the central nervous
system to the body. The peripheral nervous system also has 31 pairs of spinal nerves and
12 pairs of cranial nerves. These nerves are responsible for the five senses. 
The 12 cranial nerves located in the brainstem. The skull has holes in the base to allow
nerves to travel to parts of the body. The olfactory nerves control the sense of smell.
Optic nerves have nerve fibers that transmit information about images. Oculomotor,
trochlear, and abducens and nerves contain motor nerves that move the eyeball, eyelid,
and change the pupil size. The trigeminal nerves transmit information from the head and
face and controls mouth movements. The facial nerves control muscle movements in facial
expressions. Vestibulocochlear nerves carries information about body balance from the
inner ear and transmit information about sound. Glossopharyngeal nerves provide
information about taste. Vagus nerves transmit information from major organs to the
brain. The spinal accessory nerves regulate movements of the shoulder and neck muscles.
The hypoglossal nerves move the tongue while eating or talking. 
On both sides of the spinal cord are the spinal nerves, which are divided into two
bundles. One is a sensory fiber that detects touch while the other is made up of motor
fibers. Sensory fibers carry signals from the skin, muscles, and other parts of the body
to the spinal cord. Motor fibers carry signals from the spinal cord to muscles and
glands. The sensory fibers enter the spinal cord towards the dorsal root, which is in the
back of the spine. The motor fibers exit towards the ventral root, the front side of the
spinal cord. 
Autonomic Nervous System
The autonomic nervous system is also known as the involuntary nervous system because it
controls activities of the body unconsciously. Examples of involuntary functions are
controlling heart rate, blood circulation, respiration, and digestion. Autonomic
functions are also influenced by the person's emotions. 
The autonomic nervous system is divided into two parts: sympathetic and the
parasympathetic system. The sympathetic system sends impulses that speed up the body's
response to pain, anger, and fear. The parasympathetic system controls involuntary
functions such as secretion and digestion. Both of these systems work in the reverse
order to balance out the other. For example, the sympathetic system speeds up the
heartbeat, while the parasympathetic system slows down heartbeat. 
The most important element in the autonomic nervous system is the hypothalamus, which
helps control the internal body, such as temperature, water balance, and food intake. The
hypothalamus sends messages to the autonomic nervous system and it takes the appropriate
action. For example, in cold temperatures the hypothalamus sends messages that the body
is cold. The sympathetic system creates goose bumps by contracting the skin. 
The sympathetic system activates the adrenal medulla, a gland that helps humans release
stress. It releases two chemicals (adrenaline and noradrenaline) into the bloodstream,
which transports them to all tissues of the body. Adrenaline excites the heart to
increases muscle strength, like the reaction that comes from anxiety. Noradrenaline
constricts blood vessels and helps transmit nerve signals. These chemicals are vital to
many autonomic activities.
Although the autonomic nervous system acts automatically, it is possible to have control
of some autonomic functions. Biofeedback is teaching a person to control body functions
like reducing heart rate. The benefits are that it can be used to relieve headache by
moving blood away from the head to lessen pressure or by lowering high blood pressure.
The fact that the body's automatic functions can be affected by the mind greatly
contributes to the understanding of the autonomic nervous system. 
In conclusion, the nervous system is an important part of science because understanding
it can help save lives. Millions were saved from heart attacks, strokes, etc. from
treating the nervous system. Understanding about the nervous system is also necessary for
psychologists, physicians, and neurologists. Future experiments of the nervous system can
benefit the human race by producing cures for presently incurable diseases. 
Bibliography
1. Clayman, Charles; The Brain and Nervous System; ? 1991 Dorling Kindersley, Ltd., and
American Medical Association; published by Reader's Digest Association, Pleasantville,
New York.
2. Nathan, Peter; The Nervous System; ? 1969 by Peter Nathan; published by Penguin Books,
England. 
3. The Human Brain: Controlling the Body; ? 1998 Knowledge Adventure, Inc. ;
http://www.letsfindout.com/subjects/body/rfibrain.html
4. The Nervous System; ? 1992, 94, 97, 98, MJ Farabee;
http://gened.emc.maricopa.edu/bio/bio181/biobk/biobooknerv.html
5. The Spinal Cord: Sensation and Movement; ? 1998 Knowledge Adventure, Inc.;
http://www.letsfindout.com/subjects/body/rfispinl.html