BRAIN
SPINAL CORD
NERVES
All three (3) of these parts work together to tell our
bodies what to do. For example:
The brain says, "Hey, I want to exercise!"
So, it sends a message to the spinal cord, who
then gives the message to the nerves, who
then deliver the message to the rest of your body.
Our nervous system does a lot more than help us move.
It also helps us with our five senses:
sight,hearing,smell,taste, and touch
When you breathe in, your nose sucks up smells with the air. The smell
travels up your nostrils and the smells are picked up by a patch of smell
sensors in your nasal cavity, the hollow space inside your nose. Nerve
fibers carry messages to the brain, and then your brain can tell you what
you are smelling.
We use our mouths to eat, but it's the nervous system that sends messages
from our taste buds on our tongues to help us taste.
The human nervous system is broken down into two major divisions, the
central nervous system and the peripheral nervous system. The central
nervous system consists of the brain and spinal cord. The brain serves
as regulatory center through which the activities of the whole body
are integrated and controlled. The brain receives sensory impulses
that provide information about the body's internal and external state.
In response the brain sends out messages to enable the appropriate
response. (Gerrig 216) The brain is also a center of sensations. . The
brain interprets how perceptions of the environment are received from
sensory organs and generates sensations such sight, hearing, smell,
taste, and touch. . The brain is the seed of consciousness. That is
the state of awareness of oneself and one's surroundings. . The brain
is also the source of voluntary acts. . The brain is the seat of the
emotions. It is the brain that decides whether we feel happiness,
sadness, rage,
A number of diseases can significantly affect the proper functioning of the
nervous system. Parkinson's disease, Huntington's disease, myasthenia
gravis, and amyotrophic lateral sclerosis (commonly known as Lou Gehrig's
disease) are some of the more severe diseases affecting the nervous system.
Strokes, which are related to circulatory disorders, also may have
permanent effects on the nervous system. Certain plant derivatives, such as
belladonna, cocaine, and caffeine, have a variety of stimulatory,
inhibitory, and hallucinatory effects on the nervous system.
Huntington's disease, hereditary, acute disturbance of the central nervous
system usually beginning in middle age and characterized by involuntary
muscular movements and progressive intellectual deterioration; formerly
called Huntington's chorea. The disease is sometimes confused with chorea
or St. Vitus's dance, which is not hereditary. It attacks the cells of the
basal ganglia, clusters of nerve tissue deep within the brain that govern
coordination.
The onset is insidious and inexorably progressive; no treatment is known.
Psychiatric disturbances range from personality changes involving apathy
and irritability to manic depressive or schizophreniform illness. Motor
manifestations include flicking movements of the extremities, a lilting
gait, and motor impersistence (inability to sustain a motor act such as
tongue protrusion).
In 1993 the gene responsible for the disease was located; within that gene
a small segment of code is, for some reason, copied over and over. Genetic
counseling is extremely important, since 50% of the offspring of an
affected parent inherit the gene, which inevitably leads to the disease.
myasthenia gravis , chronic disorder of the muscles characterized by
weakness and a tendency to tire easily. It is caused by an autoimmune
attack on the acetylcholine receptor of the post synaptic neuromuscular
junction. The initiating event leading to antibody production is unknown.
The disease is most common between the ages of 20 and 40 and more frequent
in women. The muscles of the neck, throat, lips, tongue, face, and eyes are
primarily involved. Exertion quickly brings on difficulty in swallowing,
chewing, and talking. The eyelids may droop, and there are visual
disorders. Myasthenia gravis is transmitted passively to fetuses from
infected mothers, a syndrome call neonatal myasthenia. Congenital
myasthenia is a rare autosomal recessive disorder of neuromuscular
transmission beginning in childhood, usually with ophthalmoplegia. Life-
threatening myasthenic crisis, in which the diaphragm is affected and the
patient has respiratory failure, occurs in 10% of the patients. Treatment
of the disease includes the use of cholinesterase inhibitors, thymectomy,
corticosteroids, and immunosuppressive agents and plasmapheresis (see
apheresis). Prolonged rest is likely to restore some of the muscle
function; restricted activity at all times and complete rest during periods
of aggravation of the illness are necessary.
In vertebrates the system has two main divisions, the central and the
peripheral nervous systems. The central nervous system consists of the
brain and spinal cord. Linked to these are the cranial, spinal, and
autonomic nerves, which, with their branches, constitute the peripheral
nervous system. The brain might be compared to a computer and its memory
banks, the spinal cord to the conducting cable for the computer's input and
output, and the nerves to a circuit supplying input information to the
cable and transmitting the output to muscles and organs.
The nervous system is built up of nerve cells, called neurons, which are
supported and protected by other cells. Of the 200 billion or so neurons
making up the human nervous system, approximately half are found in the
brain. From the cell body of a typical neuron extend one or more outgrowths
(dendrites), threadlike structures that divide and subdivide into ever
smaller branches. Another, usually longer structure called the axon also
stretches from the cell body. It sometimes branches along its length but
always branches at its microscopic tip. When the cell body of a neuron is
chemically stimulated, it generates an impulse that passes from the axon of
one neuron to the dendrite of another; the junction between axon and
dendrite is called a synapse. Such impulses carry information throughout
the nervous system. Electrical impulses may pass directly from axon to
axon, from axon to dendrite, or from dendrite to dendrite.
So-called white matter in the central nervous system consists primarily of
axons coated with light-colored myelin produced by certain neuroglial
cells. Nerve cell bodies that are not coated with white matter are known as
gray matter. Nonmyelinated axons that are outside the central nervous
system are enclosed only in a tubelike neurilemma sheath composed of
Schwann cells, which are necessary for nerve regeneration. There are
regular intervals along peripheral axons where the myelin sheath is
interrupted. These areas, called nodes of Ranvier, are the points between
which nerve impulses, in myelinated fibers, jump, rather than pass,
continuously along the fiber (as is the case in unmyelinated fibers).
Transmission of impulses is faster in myelinated nerves, varying from about
3 to 300 ft (1-91 m) per sec.
Both myelinated and unmyelinated dendrites and axons are termed nerve
fibers; a nerve is a bundle of nerve fibers; a cluster of nerve cell bodies
(neurons) on a peripheral nerve is called a ganglion. Neurons are located
either in the brain, in the spinal cord, or in peripheral ganglia. Grouped
and interconnected ganglia form a plexus, or nerve center. Sensory
(afferent) nerve fibers deliver impulses from receptor terminals in the
skin and organs to the central nervous system via the peripheral nervous
system. Motor (efferent) fibers carry impulses from the central nervous
system to effector terminals in muscles and glands via the peripheral
system.
The peripheral system has 12 pairs of cranial nerves: olfactory, optic,
oculomotor, trochlear, trigeminal, abducent, facial, vestibulo-cochlear
(formerly known as acoustic), glossopharyngeal, vagus, spinal accessory,
and hypoglossal. These have their origin in the brain and primarily control
the activities of structures in the head and neck. The spinal nerves arise
in the spinal cord, 31 pairs radiating to either side of the body: 8
cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
The nervous system includes the brain and the nerves that reach out to the
rest of the body, acting as a two way street carrying information back and
forth. The brain and the rest of the nervous system are composed of many
different types of cells, but the main one is a cell called the neuron. All
sensations, movements, thoughts, memories, and feelings are the result of
signalneurons consist of three parts. The cell body contains the nucleus,
where most of the molecules that the neuron needs to survive and function
are manufactured. Dendrites extend out from the cell body like the branches
of a tree and receive messages from other nerve cells. Signals then pass
from the dendrites through the cell body and may travel away from the cell
body down an axon to another neuron, a muscle cell, or cells in some other
organ. The neuron is usually surrounded by many support cells. Some types
of cells wrap around the axon to form an insulating sheath. This sheath can
include a fatty molecule called myelin, which provides insulation for the
axon and helps nerve signals travel faster and farther. Axons may be very
short, such as those that carry signals from one cell in the cerebral
cortex to another cell less than a hair's width away. Or axons may be very
long, such as those that carry messages from the brain all the way down the
spinal cord. The longest axon in the body is the sciatic nerve which goes
from the base of the spinal cord all the way down our leg. The space
between an axon and the dendrites of another neuron is called the synapse.
Cells communicate with each other by sending chemicals into the synapse
that are picked up by the next cell and passed on.
The spinal cord starts at the base of the skull and runs down the middle of
the spinal column, inside the vertebrae. At each vertebra nerves exit the
spinal cord and reach out to the rest of the body. If the spinal cord is
cut, by an injury or disease, the messages stop at the point of injury and
the functions of the nervous system below that point stops.
s that pass through neurons.
The nervous system is the master controller of all cells, tissues and
organs. Nerves control the heart, lungs, immune system, endocrine system,
as well as our thoughts and other cognitive processes. Effectively, the
nervous system is in charge of directing and overlooking all bodily
functions - keeping us alive and healthy, fighting off diseases and
infections, and healing us after we have sustained injury.
Many people are surprised to learn that the spine plays a key role in
protecting the nervous system. In fact, the main function of the spinal
column (in addition to providing movement for the torso) is to encase and
protect the spinal cord and nerve roots. Without this protection it's
unlikely that we would survive a relatively small slip and fall injury.
As the spinal cord descends from the brain, spinal nerve roots peel away
from the spinal cord at each vertebral level and exit through openings made
by adjacent vertebrae. These small protective pathways are called
intervertebral foramen or IVFs and permit safe exit of the delicate nerve
roots to the rest of the body. Because of the location of the IVF,
herniated or bulging spinal discs, subluxated vertebrae, arthritic bony
growths, and inflammatory biochemicals from nearby injured tissues commonly
irritate or impinge upon the spinal nerve roots.
When there is irritation and interference to a nerve, messages or impulses
traveling along that nerve can get scrambled. Some signals become only
slightly altered while others may completely fail to reach their
destination. As this process continues, those cells, tissues and organs
which depend on the affected nerves for communication become less effective
in performing their many important tasks and become less able to maintain
their optimal health. Ultimately, the affected tissues can deteriorate,
degenerate and become nonfunctional and diseased.
Sight.
The eye is the organ of vision. It has a complex structure consisting of a
transparent lens that focuses light on the retina. The retina is covered
with two basic types of light-sensitive cells-rods and cones. The cone
cells are sensitive to color and are located in the part of the retina
called the fovea, where the light is focused by the lens. The rod cells are
not sensitive to color, but have greater sensitivity to light than the cone
cells. These cells are located around the fovea and are responsible for
peripheral vision and night vision. The eye is connected to the brain
through the optic nerve. The point of this connection is called the "blind
spot" because it is insensitive to light. Experiments have shown that the
back of the brain maps the visual input from the eyes.
The brain combines the input of our two eyes into a single three-
dimensional image. In addition, even though the image on the retina is
upside-down because of the focusing action of the lens, the brain
compensates and provides the right-side-up perception. Experiments have
been done with subjects fitted with prisms that invert the images. The
subjects go through an initial period of great confusion, but subsequently
they perceive the images as right side up.
The range of perception of the eye is phenomenal. In the dark, a substance
produced by the rod cells increases the sensitivity of the eye so that it
is possible to detect very dim light. In strong light, the iris contracts
reducing the size of the aperture that admits light into the eye and a
protective obscure substance reduces the exposure of the light-sensitive
cells. The spectrum of light to which the eye is sensitive varies from the
red to the violet. Lower electromagnetic frequencies in the infrared are
sensed as heat, but cannot be seen. Higher frequencies in the ultraviolet
and beyond cannot be seen either, but can be sensed as tingling of the skin
or eyes depending on the frequency. The human eye is not sensitive to the
polarization of light, i.e., light that oscillates on a specific plane.
Bees, on the other hand, are sensitive to polarized light, and have a
visual range that extends into the ultraviolet. Some kinds of snakes have
special infrared sensors that enable them to hunt in absolute darkness
using only the heat emitted by their prey. Birds have a higher density of
light-sensing cells than humans do in their retinas, and therefore, higher
visual acuity.
Color blindness or "Daltonism" is a common abnormality in human vision that
makes it impossible to differentiate colors accurately. One type of color
blindness results in the inability to distinguish red from green. This can
be a real handicap for certain types of occupations. To a colorblind
person, a person with normal color vision would appear to have extrasensory
perception. However, we want to reserve the term "extrasensory perception"
for perception that is beyond the range of the normal.
Hearing.
The ear is the organ of hearing. The outer ear protrudes away from the head
and is shaped like a cup to direct sounds toward the tympanic membrane,
which transmits vibrations to the inner ear through a series of small
bones. The inner ear, or cochlea, is a spiral-shaped chamber covered
internally by nerve fibers that react to the vibrations and transmit
impulses to the brain via the auditory nerve. The brain combines the input
of our two ears to determine the direction and distance of sounds.
The human ear can perceive frequencies from 16 cycles per second, which is
a very deep bass, to 28,000 cycles per second, which is a very high pitch.
In addition, the human ear can detect pitch changes as small as 3
hundredths of one percent of the original frequency in some frequency
ranges. Some people have "perfect pitch", which is the ability to map a
tone precisely on the musical scale. Bats and dolphins can detect
frequencies higher than 100,000 cycles per second.
Taste.
The receptors for taste, called taste buds, are situated chiefly in the
tongue, but they are also located in the roof of the mouth and near the
pharynx. They are able to detect four basic tastes: salty, sweet, bitter,
and sour. The tongue also can detect a sensation called "umami" from taste
receptors sensitive to amino acids. Generally, the taste buds close to the
tip of the tongue are sensitive to sweet tastes, whereas those in the back
of the tongue are sensitive to bitter tastes. The taste buds on top and on
the side of the tongue are sensitive to salty and sour tastes. At the base
of each taste bud there is a nerve that sends the sensations to the brain.
The sense of taste functions in coordination with the sense of smell. The
number of taste buds varies substantially from individual to individual,
but greater numbers increase sensitivity. Women, in general, have a greater
number of taste buds than men. As in the case of color blindness, some
people are insensitive to some tastes.
Smell.
The nose is the organ responsible for the sense of smell. The cavity of the
nose is lined with mucous membranes that have smell receptors connected to
the olfactory nerve. The smells themselves consist of vapors of various
substances. The smell receptors interact with the molecules of these vapors
and transmit the sensations to the brain. The nose also has a structure
called the vomeronasal organ whose function has not been determined, but
which is suspected of being sensitive to pheromones that influence the
reproductive cycle. The smell receptors are sensitive to seven types of
sensations that can be characterized as camphor, musk, flower, mint, ether,
acrid, or putrid. The sense of smell is sometimes temporarily lost when a
person has a cold. Dogs have a sense of smell that is many times more
sensitive than man's.
Touch.
The sense of touch is distributed throughout the body. Nerve endings in the
skin and other parts of the body transmit sensations to the brain. Some
parts of the body have a larger number of nerve endings and, therefore, are
more sensitive. Four kinds of touch sensations can be identified: cold,
heat, contact, and pain. Hairs on the skin magnify the sensitivity and act
as an early warning system for the body. The fingertips and the sexual
organs have the greatest concentration of nerve endings. The sexual organs
have "erogenous zones" that when stimulated start a series of endocrine
reactions and motor responses resulting in orgasm.
The nervous system can be broken into two(2) parts, the central nervous
system (CNS) and the peripheral nervous system (PNS). Further more, the
brain and the spinal cord makes up the CNS, while the sensory nerves and
the motor nerves makes up the PNS. The PNS is composed of the sense organs
(e.g. the eye, the ear, touches nerve cells, taste buds, and olfactory
nerve cells). The somatic nervous system and the autonomic nervous system
are the parts of the motor nerves.
The nervous system acts like a telephone system. Information is transmitted
from and to the brain, the brain receiving information from the sensory
nerves, and from the motor nerves. Information about our environment is
received by the sensory nerves, then sent to our brain. At the same time,
information about our bodies (e.g. we are hungry) is received by the motor
nerves, and it too is sent to the brain.
Our brain controls how our body acts. Voluntary movement is initiated by
our brain, and sent to the somatic nervous system, which controls our
biceps, triceps and other voluntary muscles. Involuntary movement, such the
beating of our hearts, does not need our brain to work. But we when we run,
our bodies need more oxygen, so our brain tells the autonomic nervous
system that controls our heart to beat faster. One very easy difference to
see between the two nervous system's is that the autonomic nervous system
system act much faster.
The Sense Organs
Our sensory system is composed of many different types of nerve cells.
Nerve receptors are the nerve cells that receive information about our
environment (e.g.: pain, cold, heat, sweet taste, bitter taste, etc.). The
following table defines important terms used to describe our sensory
nerves.
Sense Cell
Function
Mechanoreceptors
Nerve receptor cells that detect different senses of touch, and hearing.
Meissner's corpuscles
Nerve receptor cells that tells the brain the shape and feel of an object
in the hand, or the touch of a kiss, always adjusting to a constantly
changing environment, which is why the brain eventually ignores clothing
that you are wearing.
Pacinian corpuscle
Nerve receptor cells that detects pressure, telling the brain when a limb
has moved. After the brain has told a limb, such as an arm, to move into
the correct position.
Free Nerve endings
Nerve receptor cells that inform the brain about pain, and are located over
the entire body.
Thermoreceptors
Nerve receptor cells that detect changes in temperature.
Ruffini's end-organ
A type of nerve receptor cell that detects heat.
End bulb of Krause
A type of nerve receptor cell that detects cold.
Chemoreceptors
Nerve receptor cells that detect chemicals in food, drink, or in the air.
Odor
A collection of chemicals drifting in the air.
Olfactory bulb
A collection of nerve receptor cells that detect chemicals drifting in the
air.
Olfactory tract
The nerve cells that send information about odor to the brain. They act
like a phone line.
Taste buds
Nerve receptor cells that detect different types of flavors.
Photoreceptors
Nerve receptor cells that detect light, depth, and color.
Retina
A layer in the back of the eye that contains the eye's photoreceptors.
Rods
The photoreceptor that detects light the best. There are 20X more rods than
cones.
Cones
The photoreceptor that detects color.
At the heart of the nervous system is the brain. The brain can be broken
into five (5) parts, the Brain stem, the thalamus, the hypothalamus, the
cerebellum, and the cerebrum. The following table defines the roles of each
part of the brain.
Anatomy of the Brain
|Section of Brain|Function|
|Brain stem|The heart, and the most primitive part of the|
||brain. It is here that the brain controls the|
||body's most basic functions, such as our heart|
||rate, respiration, blood pressure, swallowing,|
||coughing, sneezing, and vomiting.|
|Thalamus |Just like the phone jack in the back of your|
||computer, this is the main information pathway|
||between the brain and the spinal cord and |
||cerebrum.|
|Hypothalamus|This is the control center for our body's |
||homeostasis. It is this part of the brain that|
||controls our body temperature, appetite, thirst |
||for water (keeping us hydrated), desire for salt |
||(keeping our pH steady), and endocrine system|
||(hormones). |
|Cerebellum|This section of our brain keeps our movements|
||smooth and coordinated, helping maintain our|
||posture, muscle tone, and equilibrium (balance). |
|Cerebrum |This is what gives humans intellect, the power of|
||speech, and memory.|
A nerve cell, or neuron acts exactly like a phone line or a computer cable.
It conducts electrons, just like wires and electricity. For these electrons
to be able to move from one cell to another, they must pass through across
a gap or space lying between two nerve cells called a synapse. But
electrons cannot move across a gap. So a neurotransmitter carries these
electrons across the synapse, in the form of Na+ and K+. As the
neurotransmitter move across the synapse, they are caught by a receptor
protein on the next nerve cell.
Nerve cells never touch each other, or other cells. If they did, the
nervous system would short out like an unprotected wire. So nerve cells are
coated by a myelin sheath, which insulates it from other cells in our body,
exactly like the plastic coating on an electrical wire, or computer cable.
This myelin sheath prevents the nerve cells from shorten out, and keeps our
bodies working properly.
The nervous system controls our lives and all of our body functions. Every
action and every response to anything is coordinated by the nervous system,
be it physical, mental or emotional. Internal changes are also coordinated
by the nervous system, for example in response to changes in body
temperature to maintain homeostasis. The basic function of the brain is the
reception of stimuli by its special sensory cells. To carry out this
function, the system's special sensory cell called the neuron must be very
diverse. Either just simple general functions or very efficient complex
commands. The system transfers information throughout the body through
impulses called nerve impulses. These impulses are received and sent also
by the neurons, you can now proceed to the next part regarding the neuron
for more detailed information.
Our five senses provide sensory input when they react to stimuli by
transmitting an action potential sent to the Central Nervous System. These
are classified into five types of sensory receptors, the mechanoreceptors,
photoreceptors, chemoreceptors, thermoreceptors and electroreceptors.
Mechanoreceptors detect and respond to hearing, balance and stretching.
Photoreceptors are, in one word, eyes. Chemoreceptors are associated with
smell, taste and the digestive and circulatory systems. Thermoreceptors
detect and respond to heat and cold. Electroreceptors detect and respond to
electricity.
The ear converts sound waves into vibrations that vibrate a
liquid in the inner ear. These in turn make hairs, also in the inner ear,
vibrate. A sensory dendrite then creates action potentials defined by these
vibrations which are sent along the auditory nerve to the brain. Loud
sounds can cause damage to these hair cells by creating large vibrations
that tear them.
These hairs also serve the purpose of detecting orientation and
gravity. They are situated along three planes and respond to fluctuations
of the liquid mentioned earlier. Hence we are able to maintain our balance
and sense if we are falling or not.
Have you ever wondered why things seem brighter in the corner of
your eye than in the centre or that they seem more colourful in the centre
than in the corner? Well, there are two types of photoreceptor cells that
work together to produce our vision. Rods, found more towards the edge of
the eye, detect light intensity while cones, found further in the centre,
detect colour. This explains the phenomena above.
When light enters the eye, it causes a chemical to decompose
(relax, this does no harm) and action potentials defined by these chemical
reactions are created and sent to the occipital lobe (there are four lobes
of the cerebrum, mentioned in the brain.) via the optic nerve.
||
|Disorders of the |
|Nervous System o |
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There are many diseases that affect the nervous system. These however don't
just appear on their own, they include genetic malformations, poisoning,
vascular disorders, degeneration, inflammations, metabolic defects, and
tumors. All of which affect either the neuron or the supporting elements.
Vascular disorders, such as cerebral hemorrhage or other forms of stroke,
are among the most common causes of paralysis and other neurologic
complications. These diseases sometimes come due to outside climates or
age. For instance, multiple sclerosis is very common in temperate areas as
a degenerative disease but very rare in the tropics.
A very common reason for a disorder of the system are bacterias, parasites,
and viruses. For example, meningitis, which is the infection of the
meninges investing the brain and spinal cord, can be caused by such
pathogens. On the other hand, one specific virus causes rabies. Some
viruses causing neurological ills affect only certain parts of the nervous
system. For example, the virus causing poliomyelitis commonly affects the
spinal cord; viruses causing encephalitis attack the brain.
Inflammations of the nervous system are named according to the part
affected. Myelitis is an inflammation of the spinal cord; neuritis is an
inflammation of a nerve. It may be caused not only by infection but also by
poisoning, alcoholism, or injury. Tumors originating in the nervous system
usually are composed of meningeal tissue or neuroglia (supporting tissue)
cells, depending on the specific part of the nervous system affected, but
other types of tumor may metastasize to or invade the nervous system. In
certain disorders of the nervous system, such as neuralgia, migraine, and
epilepsy, no evidence may exist of organic damage. Another disorder,
cerebral palsy, is associated with birth defects.
The Central Nervous System is made up of the brain and the spinal cord.
They are protected by the skull and the vertebrae and insulated by fluid
and tissue. It is called central because it is where most of the signals go
to and come from. It could be compared to the CPU of a computer or the city
hall of a city.
The brain and spinal cord are discussed thoroughly in the next two
sections, The Brain and The Spinal Cord.
The cerebrum, cerebellum and medulla oblongata make up the brain.
The cerebrum is part of the forebrain, which is also made up of the
diencephalon. The thalamus relays nerve messages and the hypothalamus
regulates many processes needed for homeostasis and is the link between the
nervous system and the endocrine system. These two are make up the
diencephalon.
The cerebrum is the controller of sensory data, motor functions,
intelligence, reasoning, learning and memory. It is divided two
hemispheres, left and right. These are connected by the corpus callosum.
The cerebral cortex, which is divided into four lobes, the occipital,
temporal, parietal and frontal lobes, cover the cerebrum. None of these
function alone.
The occipital lobe and temporal lobe handle sight, hearing and
language. The parietal lobe and frontal lobe handle sense organs, muscles,
speech and thought.
The cerebellum controls movements, posture and balance and is a
section of the hindbrain, albeit the brain stem.
Vital processes such as heartbeat and breathing are controlled by the
medulla oblongata. It also provides reflexes for swallowing and vomiting,
coughing and sneezing and hiccuping (now you and I know why we hiccup all
of a sudden).
Some sections of the midbrain and hindbrain are in the brain stem,
which is just on top of the spinal cord. The midbrain connects the
forebrain and hindbrain and provides eye reflexes.
The spinal cord is the object linking the brain to the rest of the body.
Bony vertebrae that make up the vertebral column protect it. Inside the
spinal cord are cell bodies and dendrites with interneuronal axons
surrounding them. They carry messages to and from the brain. Reflexes not
immediately involving the brain are controlled by the spinal cord.
The image beside you here explains where several parts of the body are
attached to the spinal cord. The image here is courtesy of the Spinal Cord
Injury ring.
The Peripheral Nervous System is only made up of nerves and links the
Central Nervous System with the rest of the body. The nerves in the
Peripheral Nervous System are divided into two types, the cranial nerves
and the spinal nerves. Information is passed to and from the brain and
spinal cord by these two types of nerves, respectively. There are two main
components of the Peripheral Nervous System, sensory pathways and motor
pathways. Sensory pathways transmit input from stimuli to the Central
Nervous System. Motor pathways pass information to muscles and glands.
Two subdivisions of the Peripheral Nervous System occur, the somatic
and autonomic systems.
The Autonomic Nervous System controls internal organs, and is divided into
two subsystems, the Sympathetic Nervous System and the Parasympathetic
Nervous System. These two control the same organs except that they are the
reverse of each other, needed for homeostasis (ensuring a stable internal
environment to provide an organism with a certain degree of independence
from variations in external environments, like maintaining a constant body
temperature).
The motor neurons of the Autonomic Nervous System are not connected to
their targets but are instead connected to other motor neurons that link
them to their targets.
The Somatic Nervous System controls muscles and external sensory
receptors like the skin. Muscles and glands are effectors, which convert
the nerve impulses received into a form of action.
The Somatic Nervous System provides an automatic and involuntary
reaction to a stimulus. Examples of these are the knee-jerk that causes the
lower leg to jerk up after tapping just below the knee, blinking in
response to a sudden bright light and balance in response to something that
causes you to be off balance. These reactions are not conscious.
The motor neurons of the Somatic Nervous System are not connected to
their targets but are instead connected to other motor neurons that link
them to their targets.
Signals that inhibit action potentials cannot be passed through the
Somatic Nervous System. This makes it different from the Autonomic Nervous
System.
You giggle. You dance. You weep. You worry. You belch. You blink. You
wonder. You remember.
And your brain - all 3 blobby pounds - makes it all happen. By receiving
and routing messages from nerves throughout your body, your brain makes it
possible for you to do what you do and to be who you are.
The constant communication between brain and nerves occurs through
electrical impulses and chemical interactions. Your brain receives
countless impulses from the body's network of nerves. After it interprets
some of the messages, your brain sends information to specialized parts of
the brain for storage as memory and fires instructions to your fingers,
legs, mouth or other body parts. By processing, sorting, filing and
responding to incoming information, your brain gives meaning to the world
around you.
The brain and spinal cord make up your central nervous system. The
peripheral nervous system extends from your spinal cord and branches out to
the tips of your fingers and toes. Nerves continuously gather and transmit
information from both inside and outside your body, busily sending messages
to and receiving input from your brain.
Your brain
The human brain contains billions of nerve cells arranged in patterns that
coordinate thought, emotion, behavior, movement and sensation. The largest
part of the brain, the cerebrum, is divided into two hemispheres - the left
and the right. A mass of fibers called the corpus callosum connects them.
Although each hemisphere of the brain has certain distinct functions, both
work together. The left hemisphere controls the right side of your body,
and the right hemisphere controls the left side.
Each cerebral hemisphere is divided into four separate areas (lobes). Each
lobe is responsible for different activities.
. Occipital lobes. These are located at the back of your brain and
receive and process visual information.
. Parietal lobes. These lie in front of the occipital lobes and process
information about senses such as taste, temperature and touch.
Parietal lobe processing includes specific information about the
position of your body parts, the space around your body and your
relationship to this space.
. Temporal lobes. These are located on each side of your brain. Both are
important in processing memory information. The temporal lobes are
also important to the sense of hearing. And the dominant temporal lobe
- in the left cerebral hemisphere for right-handed people - is of
special importance in speech and language functions.
. Frontal lobes. These make up the largest part of the cerebrum. Part of
this area is responsible for control of voluntary movement. Part of
the left frontal lobe controls the action of the right side of the
body, and part of the right frontal lobe controls the action of the
left side of the body. The frontal lobes are also responsible for
organizational skills, problem solving, some use of language,
attention and understanding. These lobes also help regulate emotion
and behavior.
The cortex, which covers your cerebrum, is a layer of tissue less than a
quarter-inch thick. Grayish brown and wrinkled in appearance, this layer is
commonly called gray matter. The cerebral cortex does most of the
information processing in your brain. All of the grooves, bulges and folds
(convolutions) in the cortex allow a very large surface area to fit inside
the skull, thus increasing the amount of information that can be processed.
The human brain has more convolutions than any other animal brain.
In addition, there are basal ganglia deeper in the cerebrum. These ganglia
play a critical role in relaying messages between different areas of the
brain.
The limbic system, also located in the deeper regions of the brain, is
responsible for emotions and is composed of the following structures:
. Hypothalamus. It controls many body functions and urges - such as
eating, sleeping and sexual behavior - and regulates body temperature.
. Thalamus. It acts as an information relay center, sorting and sending
messages to and from other parts of the brain.
. Amygdala (uh-MIG-duh-luh). It governs such emotions as anger and fear
and triggers your response to danger, a reaction commonly called the
fight-or-flight response.
. Hippocampus (hip-o-KAM-pus). It plays an important role in forming,
storing and retrieving your memories.
The medulla oblongata, pons and midbrain make up the brainstem, which is
responsible for regulating some of your most basic survival functions, such
as breathing and heart rate. The cerebellum coordinates movements.
All of the areas of the brain work together. Connecting fibers and nerve
pathways allow the areas of the brain to share information and coordinate
functions.
Protecting your brain
Your bony skull and vertebrae protect your brain and spinal cord. In
addition, three layers of membranes (meninges) surround the brain and
spinal cord: the outermost layer is the dura mater, the middle layer is the
arachnoid and the innermost layer is the pia mater.
Cerebrospinal fluid, located between the arachnoid and pia mater, acts as a
shock absorber and further protects your brain and spinal cord from injury.
Feeding your brain
Your arteries provide oxygen and nutrients that are critical to the
functioning of your brain. Despite its relatively small size and weight,
your brain uses 20 percent of the heart's output of blood and 20 percent of
the oxygen consumed when your body is at rest. Blood is brought to the
brain by the paired vertebral and carotid arteries, which extend up through
your neck from your aorta. These large arteries then divide into smaller
ones that supply blood to all regions of your brain.
Nerves
Peripheral nerves run from the spinal cord to all other parts of your body.
The spinal cord acts as a central communication network that can transmit
signals back and forth between your brain and the farthest reaches of your
peripheral nervous system.
Connected directly to your brain are cranial nerves. These nerves leave the
brain to control muscles in your face, eyes, tongue, ears and throat.
Cranial nerve I, for example, is responsible for your sense of smell, and
cranial nerve II is responsible for your vision. Cranial nerves go directly
to the cerebral hemispheres. They also convey sensations from the cerebral
hemispheres back to the brain.
Your brain and nervous system at work
The basic unit of your brain and nervous system is a nerve cell (neuron).
The billions of neurons allow different parts of your body to communicate.
They do so by generating electric impulses - messages - and relaying them
near and far, to and from your brain and the rest of your body. Surrounding
neurons are the glial cells that act as bodyguards - protecting, nourishing
and supporting neurons. The human brain has about 12 billion neurons and 50
billion glial cells.
Extending from nerve cells are branches called dendrites, which receive
incoming messages from other nerve cells. Another branch called an axon
carries outgoing signals from the cell body to other neurons or to other
cells. That's how neurons communicate so fast and so efficiently.
Covering some axons is a white fatty substance called myelin. Myelin helps
insulate the axons and speeds the transmission of impulses.
All neurons communicate with other nerve cells or body cells through
electric impulses. Something stimulates a neuron, which then passes the
message to another neuron. The message is passed from one neuron to the
next.
It works like this: Within a neuron, the impulse travels through the cell
body to the tip of the axon where there are tiny sacs containing
neurotransmitters, chemicals that act as data messengers. The arrival of an
electric impulse causes the release of the neurotransmitters into the
synapse, the tiny gap between two nerve cells. In the synapse, the
neurotransmitters bind to receptors on a receiving cell. This allows the
impulse to enter the receiving cell and pass on to that cell's axon. Once
some neurotransmitters have done their job, they're taken up into the
synapse and return to their cell of origin, where some of them may be
reused. This process is repeated from neuron to neuron as the impulse
travels to its destination. Other neurotransmitters are metabolized.
Neurons require energy from nutrients found in circulating blood, such as
glucose and oxygen, a process called metabolism. Neurons are constantly
repairing themselves so that they can continue to function properly.
And when the neurons function properly, you giggle, dance, weep, worry,
belch, blink, wonder and remember.
