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.

Other Parts of the Brain

1) The thalamus acts as a relay center for incoming sensory input so

that it is sent to the correct part of the cerebrum.

2) 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.

3) Twelve cranial nerve pairs which include the auditory nerve (hearing), the olfactory nerve (smell), the optic nerve (sight), all of the facial muscles, etc.

4) 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”.

NeurotoxinsNumerous 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.