Download Advanced Biology\AB U14 Nervous System

The Nervous System
Objectives:
• Explain the function of the major regions of the brain.
• Describe the components of a neuron and their function.
• Predict which neurotransmitter and/or which system
(parasympathetic or sympathetic) nervous system would be
activated for a given stimuli.
• Evaluate the function of the sodium-potassium pump in
neurotransmission.
• Explain the relationship between disorders like, multiple sclerosis,
epilepsy, Alzheimer's, etc. and the brain.
• Describe the role certain nutrients and certain environmental toxins
play in brain health.
• Distinguish between various eye, ear, and nose disorders and their
causes/treatments.
Vocabulary:
neuron * axon * dendrite * synapse * neurotransmitters * myelin *
Schwann cells * peripheral nervous system * central nervous system *
cerebrum * cerebral cortex * cerebellum * brain stem * corpus callosum *
frontal, parietal, occipital, temporal lobes * ganglion * olfactory, optic, and
auditory nerves * medulla oblongata * hypothalamus * meninges *
limbic system *
gray and white matter * cerebrospinal fluid * sensory
and motor neurons * interneuron * somatic and autonomic nervous system *
parasympathetic and sympathetic nervous system * nodes of Ranvier *
resting and action potentials * multiple sclerosis * sciatic nerve *
depolarization * reflex arc * meningitis * solar plexus * cornea * sclera *
aqueous humor * pupil * lens * choroid * vitreous humor * retina *
rods * cones * retinal * rhodopsin * blind spot * iridology *
pigments * myopia/nearsightedness * cataracts * astigmatism *
presbyopia/farsightedness * macular degeneration * semicircular canals *
Meniere’s disease * vertigo * pinna * tympanic membrane * maleus *
incus * stapes * auditory nerve * tinnitus * eustachian tube * anosmia *
olfactory nerve * gustation
Single nerve cells are referred to as neurons. Neurons consist of
numerous dendrites, which receive impulses from other neurons or
directly from stimuli such as when touching things. Neurons also
consist of a cell body, which contains the cell nucleus, and an axon,
which carries an impulse away from the cell body. Many axons in our
bodies are covered by a white, fatty substance called myelin. Myelin is
produced by Schwann cells and protects and insulates the nerve. The
myelin sheath has gaps called the nodes of Ranvier. These nodes
allow faster conduction of impulses. In multiple sclerosis, there is
scarring and destruction of the myelin sheath. This results in
numbness/weakness, memory loss, and possible paralysis.
There is a gap, or space, between the axon of one neuron and the
dendrites of another. This gap is called a synapse. At the synapse,
neurotransmitters (chemical messengers) are released which will
impact the next neuron’s dendrites. These neurotransmitters are what
make each nervous response unique. That is, they will determine
whether or not the next neuron will be stimulated or inhibited. Many
drugs used to control neurological disorders act on neurotransmitters.
Often the drugs either block the transmitters or prolong their effect in
the body. Neurons are organized into nerves.
Nerve conduction involves the separation of charges across the
nerve cell membrane. The sodium-potassium pump constantly pumps
2 potassium ions into the cell and 3 sodium ions out of the cell. In
addition, sodium filters back into the cell slowly while potassium flows
out fairly rapidly. This creates a positive external environment and an
internal environment that is less positive, or relatively negative. This
build-up occurs when the nerve is not stimulated and is therefore
referred to as the resting potential. However, when a nerve is
stimulated, sodium gates open, allowing sodium to rush into the cell,
depolarizing it. This is called an action potential. Depolarization along
a neuron occurs in a domino or wave like motion. While the cell
re-polarizes, it cannot “fire” another nerve impulse. This is the basis of
some pain therapies that involve excessive nerve stimulation. For
example, to control phantom limb pain.
The nervous system is divided into 2 parts: the central nervous
system, which consists of the brain and the spinal cord, and the
peripheral nervous system, which consists of all the nerves extending
from the brain and spinal cord.
The Central Nervous System
The brain weighs about 3 pounds (1.4 kg). The brain uses about
20% of our oxygen intake in order to nourish its 100 billion neurons.
The brain has 3 major regions:
1)
The cerebellum - is the second largest region of the brain. It is
inferior to (below) the cerebrum at the back of the head. The
cerebellum controls coordination of muscles, balance, etc. Much
of its control on muscles is subconscious.
2)
The brain stem includes the pons and the medulla oblongata
which regulates breathing, heart rate, blood pressure, and
numerous involuntary or semi-involuntary functions such as
sneezing, coughing, swallowing, etc. In addition, messages
from/to the body pass through here but most neurons cross to
opposite sides. This means the right side of the brain controls
most of the left side of the body and vice versa. Serious injury to
the brain stem usually results in death.
3) Cerebrum - (largest brain region) consists of 2 hemispheres. These 2
halves of the brain communicate with each other through a group of nerves
called the corpus callosum. In general, the left side of the cerebrum gets
credit for verbal skills, math, and logic. The right hemisphere is credited
with artistic ability, intuition, and spatial reasoning. Occasionally, severe
cases of epilepsy, or other brain disorders, require cutting this nerve
connection in order to control seizures.
The outer layer of the cerebrum, the cerebral cortex, is highly folded, or
convoluted. Scientists believe that the folding of the cerebrum is an
important indicator of intelligence. Folding increases the surface area of the
brain for more neuronal interactions in a given space. The outer layer of the
cerebral cortex is referred to as “gray matter”. The inner area is called white
matter. The white coloration is from the myelin sheath covering the inward
facing axons. The cell bodies lie in the outer layer of the cortex and have a
gray appearance because they do not have myelin.. In general, this is the
color pattern for the entire brain. However, the spinal cord is just the
opposite.
The cerebrum has deep grooves which divide the cerebrum into the frontal,
parietal, temporal, and occipital lobes. Overall, the cerebrum is considered
the area for intelligence/learning, emotion, memory, sensory data, voluntary
muscle control, etc. Each lobe has specific duties too. For example, the
parietal lobe helps make sense of and reacts to sensory signals, the
temporal deals with hearing and making sense of speech, the frontal lobe
with intellect, smell, speech/the mouth, the occipital lobe deals with sight.
1)
2)
3)
4)
Other Parts of the Brain
The thalamus acts as a relay center for incoming sensory input so
that it is sent to the correct part of the cerebrum.
The hypothalamus links the nervous system to the endocrine
system (hormonal system), For example, it controls body
temperature through communication with the pituitary gland which
then hormonally signals the thyroid when more thyroid hormone is
needed to help produce body heat. The hypothalamus not only
controls body temperature, but also influences water balance,
heartbeat, and blood pressure. The hypothalamus also interacts
with the cerebral cortex to control emotions, desires, hunger/thirst,
etc. and the emotions associated with memories stirred up by
specific smells, etc. This interaction forms a partnership often
referred to as the limbic system.
Twelve cranial nerve pairs which include the auditory nerve
(hearing), the olfactory nerve (smell), the optic nerve (sight), all of
the facial muscles, etc.
The pineal gland is NOT part of the nervous system but is located in
the head and controls the hormone melatonin (sleep/wake cycle).
The brain and spinal cord are so important to survival that they are
protected by a triple layer of membranes called the meninges. Within
the meninges flows cerebrospinal fluid. This not only cushions the
brain and spinal cord but supplies nutrients and contains white blood
cells to prevent infection. Unfortunately, some bacteria or viruses can
sometimes get into the cerebrospinal fluid and cause swelling of the
meninges (meningitis) and brain. Bacterial meningitis is often fatal if
not treated quickly. Symptoms may include sudden, severe headache,
stiff neck, and fever.
Encephalitis is another illness that can cause swelling of the brain
due to a viral infection, recent vaccination, etc. Symptoms include
lethargy, apathy, etc.
Most medications do not easily cross the blood-brain barrier even
though viruses and many environmental toxins can sneak in “the back
door”.
Neurotoxins
Numerous environmental substances can act as neurotoxins. The brain is 60%
lipid (fat) so anything that is fat soluble tends to be attracted to the brain if it can
cross the brain barrier. Mercury can and it is fat soluble. Hg also binds to the
lipid bi-layer of cell membranes, stiffening the membrane and making it difficult
for nutrients to enter and wastes to leave the cell. The speech centers of the
brain are especially affected by mercury and excess salivation is common.
Mercury exposure can cause neurofibillary tangles and amyloid plaques.
Both of these are seen in Alzheimer’s patients. Alzheimer’s disease is also far
more prevalent in people who are homozygous for the APO-E4 gene. People
who inherit APO-E2 can excrete 2 mercury atoms out of brain cells at a time
and the APO -E3 gene will help excrete 1 mercury atom out of the brain, but
the APO-E4 gene does not promote excretion of mercury out of brain cells at
all! This allows a build-up of mercury and, therefore, brain damage. In
addition, children with autism often are found to be homozygous for APO-E4.
Besides nerves, sodium and calcium channels can also be adversely affected.
Cilantro and chlorella in the diet can help remove mercury. High sulfur
supplements like MSM and foods like garlic or certain protein sources, like
eggs and whey, can help bind mercury too. Selenium blocks Hg to some
extent. Testosterone increases Hg neurotoxicity.
Aspartame and monosodium glutamate (MSG) can open the calcium
channels in the brain. This excites neurons to death. Both zinc and
magnesium can help the brain resist damage from these by helping the
calcium channels remain closed.
Other brain disorders include epilepsy and ALS, or Lou Gehrig’s
disease. ALS is related to the body’s inability to make super oxide
dismutase (SOD), a very important anti-oxidant associated with a cell’s
ability to detoxify.
Epilepsy is a seizure disorder. It can be caused by an injury to the
head or it can be inherited. Magnesium “wasting” has been associated
with some inherited epilepsy cases. In severe seizure disorders, the
corpus callosum between the brain hemispheres must be cut to control
grand mal seizures. Mild seizures often appear as though the person is
daydreaming, or “tuned out”.
The Peripheral Nervous System
Thirty one pairs of nerves travel through the spinal column to innervate the
body. Any injury to the spinal cord or the nerves as they exit the spinal column,
can result in pain or paralysis. Probably the best known nerve is the sciatic
nerve. It is the largest nerve. It exits between lumbar vertebrae, travels behind
the hips and down the back of the leg. When giving IM injections, nurses aim
for the upper, outer quadrant of the buttocks to avoid hitting the sciatic nerve
with their needle. In general, the higher up along the spine that a spinal cord
injury occurs, the more organs that are likely to be affected or the greater the
paralysis.
Although much of our daily nerve stimulation is carried to the brain for
processing, sometimes quick responses are needed. For example, when you
touch a hot stove, you need to respond quickly. Instead of registering the hot
stimulus in the brain and thinking about what to do, a reflex arc is activated. In
this case, a sensory neuron receives the stimuli (excess heat) and carries the
impulse to the spinal cord. There an interneuron transmits the impulse to a
motor neuron that stimulates the muscle to quickly pull your hand away.
In general, sensory neurons receive a stimulus, an interneuron in the spinal
cord or the brain receives the signal and a motor neuron tells the body what to
do. The motor neuron may act on a muscle or a gland or another organ in the
body to get the desired response.
The peripheral nervous system is sometimes divided as follows:
1) The Somatic Nervous System - includes the cranial (12 pairs) and
spinal nerves (31 pairs) used to control voluntary movement of skeletal
muscles.
2) The Autonomic Nervous System - includes those nerves that control
involuntary functions like heart rate and breathing. The autonomic
nervous system is subdivided into the :
a) Sympathetic Nervous System - is our “fight or flight” response.
This includes increasing respiration and heart rates and turning off
digestion as oxygenated blood is diverted to the muscles to
protect us in potentially harmful situations. Often, adrenalin levels
will rise in these situations.
b) Parasympathetic Nervous System - operates during relaxed
episodes. It increases digestive activities and returns functions in
the body to normal, resting rates.
The peripheral nervous system has numerous clusters of nerve cells
called ganglia. A bunching of ganglia and nerve cells is called a nerve
plexus. The best known is the solar plexus which is located just behind
the stomach. It controls the stomach, gall bladder, liver, pancreas,
spleen, and duodenum. Ganglia and their plexus groups are part of the
sympathetic nervous system. Therefore, stress, activating the
sympathetic nervous system, can cause upset
stomach, etc.
The senses are considered part of the peripheral
nervous system.
The Senses
The Eye: As light enters the eye through the cornea (the transparent
area over the pupil - a modification of the sclera, or whites of our eyes)
it is bent toward the retina. Light continues past the cornea, through
the aqueous humor, a fluid filled area that: 1) helps bend incoming
light toward a focal point on the retina, 2) helps the eyeball keep its
shape, and 3) helps nourish the cornea and lens. Light passes through
the pupil (an opening) to the lens. The lens helps focus light on the
retina. The lens is adjusted by muscles which help shape the lens for
far or near focusing. Behind the lens lies the vitreous humor, which
makes up the bulk of the eye. This, too can help bend light toward the
retina. When light hits the retina, which contains rods and cones, the
light stimulates the rods and/or cones (both are photoreceptive
neurons). This creates an action potential down the optic nerve to the
occipital lobe of the cerebrum where neural input creates “vision”.
Since there are no rods or cones where the optic nerve attaches to the
retina, this area cannot register visual stimuli and is called the “blind
spot”.
The rods can operate in low light but only register black and white
images. The cones need higher light levels to become active but can
register color. The cones are not active in low light so we cannot see
color in dim light. There are 3 types of cones that interact to give us the
full spectrum of color. If all 3 cone types (blue, red, & green) are not
present or if there is a problem with their corresponding pigments
(pigments are compounds that absorb different wavelengths, i.e. colors,
of light), a person will be either partially or fully color blind. Color
blindness for red and green is an X-linked genetic disorder. However,
“blue” color blindness problems are genetically autosomal.
Retinal, which is made from vitamin A (retinoic acid), joins with an
enzyme called “opsin” to make rhodopsin. Retinal is what actually
absorbs light for vision. A deficiency in vitamin A can cause poor night
vision or even total blindness.
The iris of our eye gives us our eye color, but more importantly, it is a
circular, smooth muscle that controls the size of the opening (pupil) and
therefore, the amount of light that enters the eye.
A defect in the shape of the cornea, lens, or the humors can prevent
light from focusing on the fovea centralis of the retina. This causes
vision problems. An inability to contract and relax the muscle to
reshape the lens effectively, can also affect the ability to focus. Eye
“strain” is usually from focusing on nearby objects for too long, keeping
the muscle contracted to the point of fatigue.
Eye Disorders
Myopia (nearsightedness) occurs when light is focused in front of the
retina because the eyeball is elongated from front to back. A concave
lens can correct the problem.
Presbyopia (farsightedness) occurs when the eyeball is too short from
front to back so light rays want to converge behind the retina. A convex
lens can correct this problem.
Astigmatism is when the curve of the eye is rough, rather than smooth.
This causes the light rays to scatter unequally. There is an inherited
tendency for astigmatism. An uneven lens corrects this.
Cataracts occur due to a clouding of the lens. This is due to a change
in the lens proteins. Oxidative damage from too much sun exposure,
cigarette smoking, high sugar diets, etc. over many years can alter
these proteins just like heat makes an egg white become opaque.
Antioxidants like vitamin C, E, A, and zinc and taurine can help lower
cataract risk.
Macular degeneration occurs when the macula (located in the middle of
the retina and receiving the most intense focused light), grows excess
blood vessels which can leak and cause internal swelling. This makes
it difficult to see clearly. A laser is sometimes used to seal off leaky
blood vessels. Antioxidants might help prevent this disorder.
The eyes are considered by many to be the “windows” into our
internal health. For example, a particular retinal problem in the eye is
indicative of diabetes (diabetic retinopathy). There are whole areas of
study called sclerology (study of the whites of the eye) and iridology.
Iridology is the study of the iris and all of its patterns and markings.
Certain structures called lacunae and certain colorations might indicate
problems internally with specific organs. Good iridologists claim to be
able to even tell if you were a C-section or natural birth and at about
what age certain traumas occurred in your life.
Full pigmentation of the iris doesn’t occur until about age 7.
In older people, a whitish cloud near the top of the iris or encircling the
whole iris is sometimes called a senility or cholesterol ring. “Bread
crumbs” in the eye might indicate inability to absorb iron. This can
indicate a kidney problem since the kidney makes a hormone that helps
with iron absorption. Large pupils, not due to drugs, or pulsing of pupils
when a light is shined in them can indicate adrenal problems.
The Ear: is involved in both hearing and balance.
Balance:
The vestibule and the perpendicular semicircular canals are the part of
the (inner) ear involved in balance. Both contain hair cells. The hairs
are surrounded by a fluid. Motion causes the fluid to move and
stimulate the hairs which stimulate nerve cells that help us determine
our position. The movement of calcium carbonate granules (called
otoliths) in the fluid helps us sense acceleration and deceleration. An
inner ear infection can cause us to feel dizzy and off balance due to
swelling and irritation surrounding the hair cells. Meniere’s disease
causes vertigo, a spinning, drunk-like sensation due to damage in the
inner ear. The damage can be from a virus, etc. Al Shepard was
grounded from space for many years until he had a “procedure” done
that relieved his symptoms. Motion sickness can be caused by the
inner ear not sensing motion as you sit still in a car, etc. but the eyes
seeing the motion. This sends conflicting messages to the brain.
Hearing:
Sound waves, or compressional waves, are detected by our ears.
The amplitude (energy) of the wave determines its perceived loudness.
The frequency (number of waves per second) determines its pitch.
The outer part of our ears is called the pinna. It helps direct the
sound waves into the auditory canal to the eardrum (tympanic
membrane) which vibrates like a speaker diaphragm. This causes the
maleus (hammer), incus (anvil), and stapes (stirrup) bones in the
middle ear to move. They amplify the incoming sound by 20. The
vibrations then travel to the “oval window” membrane next to the inner
ear. The vibration of the oval window is transferred to fluids in the inner
ear and to the basilar membrane and nearby hair cells which signal
action potentials down the auditory nerve.
Tinnitus, or ringing in the ears, can be caused by numerous things,
including a B12 deficiency, nerve damage from loud noise, aspirin,
antibiotics (especially gentamicin), high insulin, high blood pressure, or
poor circulation, and even by an imbalance in the sex hormones!
Popping sensations in the ear are due to a pressure build-up behind
the tympanum (ear drum) being released. Air enters from outside the
ear through the pinna but can enter on the other side of the eardrum
through the eustachian tube. The eustachian tubes connect in the back
of the mouth just to the side of where the nose joins the throat.
Yawning or chewing can help open the tube up to allow air flow,
relieving pressure.
Infants and young children are far more prone to ear infections
because the eustachian tube is nearly level as it flows from the mouth
to the middle ear. This allows fluid from the mouth to enter the tube
and get into the ear. As we grow, our head shape changes and the
eustachian tube now angles down toward the mouth for better
drainage.
Loud noise above 80 decibels can cause hearing loss. This can be
partial (only to certain frequencies or below certain amplitudes) or total.
Deafness can be genetic (very common in white German Shepards),
due to infections like meningitis, and numerous other problems.
The Nose: contains chemoreceptors. Although smell and taste are
related so much so that a stuffy nose can sometimes make food seem
tasteless, not all people born with no sense of smell (anosmia) suffer
from an inability to taste things.
Protein “olfactory” receptors high up in the nose are stimulated by
certain chemicals that we inhale and this stimulates the olfactory nerve
which carries the signal to the cerebral cortex.
The Tongue: Taste, or gustation, involves chemoreceptors located on
the taste buds. The 4 primary tastes are: sweet, salty, bitter, and sour.
The sweet sensation is concentrated near the tip of the tongue, bitter
near the back, sour toward the middle edges, and salt is close by but
further forward. Our individual genetics affect how we perceive
different foods. Ex. One gene makes the difference between broccoli
tasting bitter or mild. And, thiourea, PTC, etc. can only be detected by
some people.