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Nervous System Guide for Potential Doctors
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Homeostasis
CNS: neurons, brain, spinal cord
PNS: somatic (voluntary) nervous system, autonomic (involuntary) nervous system
Sense organs
Introduction
As the most complex system, the nervous system serves as the body control center and
communications electrical-chemical wiring network. As a key homeostatic regulatory and
coordinating system, it detects, interprets, and responds to changes in internal and
external conditions. The nervous system integrates countless bits of information and generates
appropriate reactions by sending electrochemical impulses through nerves to effector
organs such as muscles and glands. The brain and spinal cord are the central nervous system
(CNS); the connecting nerve processes to effectors and receptors serve as the peripheral
nervous system (PNS). Special sense receptors provide for taste, smell, sight, hearing, and
balance. Nerves carry all messages exchanged between the CNS and the rest of the body.
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CNS: neurons, brain, spinal cord
The neuron transmits electric signals like an electric wire. The perikaryon (cell body) is the
neuron central part. Dendrites, short branches, extend from the neuron. These input
channels receive information from other neurons or sensory cells (cells that receive
information from the environment). A long branch, the axon, extends from the neuron as
its output channel. The neuron sends messages along the axon to other neurons or
directly to muscles or glands.
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Neurons must be linked to each other in order to transmit signals. The connection between two
neurons is a synapse. When a nerve impulse (electrical signal) travels across a neuron to the
synapse, it causes the release of neurotransmitters. These chemicals carry the nerve signal
across the synapse to another neuron.
Nerve impulses are propagated (transmitted) along the entire length of an axon in a
process called continuous conduction. To transmit nerve impulses faster, some axons are
partially coated with myelin sheaths. These sheaths are composed of cell membranes
from Schwann cells, a type of supporting cell outside the CNS. Nodes of Ranvier (short
intervals of exposed axon) occur between myelin sheaths. Impulses moving along
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myelinated axons jump from node to node. This method of nerve impulse transmission is
saltatory conduction.
The brain has billions of neurons that receive, analyze, and store information about internal and
external conditions. It is also the source of conscious and unconscious thoughts, moods, and
emotions. Four major brain divisions govern its main functions: the cerebrum, the
diencephalon, the cerebellum, and the brain stem.
The cerebrum is the large rounded area that divides into left and right hemispheres (halves) at a
fissure (deep groove). The hemispheres communicate with each other through the corpus
callosum (bundle of fibers between the hemispheres). Surprisingly, each hemisphere controls
muscles and glands on the opposite side of the body. Comprising 85 percent of total brain weight,
the cerebrum controls language, conscious thought, hearing, somatosensory functions
(sense of touch), memory, personality development, and vision.
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Gray matter (unmyelinated nerve cell bodies) composes the cerebral cortex (outer portion of
the cerebrum). Beneath the cortex lies the white matter (myelinated axons). During embryonic
development, the cortex folds upon itself to form gyri (folds) and sulci (shallow grooves) so
that more gray matter can reside within the skull cavity.
The diencephalon forms the central part of the brain. It consists of three bilaterally symmetrical
structures: the hypothalamus, thalamus, and epithalamus. The hypothalamus 'master
switchboard' resides in the brain stem upper end. It controls many body activities that
affect homeostasis (maintenance of a stable internal environment in the body).
The hypothalamus is the main neural control center (brain part that controls endocrine
glands). The pituitary gland lies just below the hypothalamus. The pituitary gland is a small
endocrine gland that secretes a variety of hormones (organic chemicals that regulate the body's
physiological processes). When the hypothalamus detects certain body changes, it releases
regulating factors (chemicals that stimulate or inhibit the pituitary gland). The pituitary
gland then releases or blocks various hormones. Because of this close association
between the nervous and endocrine systems, together they are called the neuroendocrine
system.
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The hypothalamus also regulates visceral (organ-related) activities, food and fluid intake,
sleep and wake patterns, sex drive, emotional states, and production of antidiuretic
hormone (ADH) and oxytocin. The pituitary gland produces both these hormones.
The thalamus is a relay and preprocessing station for the many nerve impulses that pass through
it. Impulses carrying similar messages are grouped in the thalamus, then relayed to the
appropriate brain areas.
The epithalamus is the most dorsal (posterior) portion of the diencephalon. It contains a
vascular network involved in cerebrospinal fluid production. Extending from the epithalamus
posteriorly is the pineal body, or pineal gland. Its function is not yet fully understood; it is thought
to control body rhythms.
At the rear of the brain is the cerebellum. The cerebellum is similar to the cerebrum: each has
hemispheres that control the opposite side of the body and are covered by gray matter and
surface folds. In the cerebellum, the folds are called folia; in the cerebrum, sulci. The vermis
(central constricted area) connects the hemispheres. The cerebellum controls balance,
posture, and coordination.
The brain stem connects the cerebrum and cerebellum to the spinal cord. Its superior portion,
the midbrain, is the center for visual and auditory reflexes; examples of these include blinking
and adjusting the ear to sound volume. The middle section, the pons, bridges the cerebellum
hemispheres and higher brain centers with the spinal cord. Below the pons lies the medulla
oblongata; it contains the control centers for swallowing, breathing, digestion, and
heartbeat.
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The reticular formation extends throughout the midbrain. This network of nerves has widespread
connections in the brain and is essential for consciousness, awareness, and sleep. It also filters
sensory input, which allows a person to ignore repetitive noises such as traffic, yet awaken
instantly to a baby's cry.
The spinal cord is a continuation of the brain stem. It is long, cylindrical, and passes through a
tunnel in the vertebrae called the vertebral canal. The spinal cord has many spinal segments,
which are spinal cord regions from which pairs (one per segment) of spinal nerves arise. Like the
cerebrum and cerebellum, the spinal cord has gray and white matter, although here the white
matter is on the outside. The spinal cord carries messages between the CNS and the rest of the
body, and mediates numerous spinal reflexes such as the knee-jerk reflex.
Meninges, three connective tissue layers, protect the brain and spinal cord. The outermost
dura layer forms partitions in the skull that prevents excessive brain movement. The
arachnoid middle layer forms a loose covering beneath the dura. The innermost pia layer
clings to the brain and spinal cord; it contains many tiny blood vessels that supply these
organs.
Another protective substance, cerebrospinal fluid, surrounds the brain and spinal cord. The
brain floats within the cerebrospinal fluid, which prevents against crushing under its own
weight and cushions against shocks from walking, jumping, and running.
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PNS: somatic (voluntary) nervous system, autonomic (involuntary) nervous system
The peripheral nervous system includes sensory receptors, sensory neurons, and motor neurons.
Sensory receptors are activated by a stimulus (change in the internal or external environment).
The stimulus is converted to an electronic signal and transmitted to a sensory neuron. Sensory
neurons connect sensory receptors to the CNS. The CNS processes the signal, and transmits a
message back to an effector organ (an organ that responds to a nerve impulse from the CNS)
through a motor neuron.
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The PNS has two parts: the somatic nervous system and the autonomic nervous system.
The somatic nervous system, or voluntary nervous system, enables humans to react
consciously to environmental changes. It includes 31 pairs of spinal nerves and 12 pairs of
cranial nerves. This system controls movements of skeletal (voluntary) muscles.
Thirty-one pairs of spinal nerves emerge from various segments of the spinal cord. Each spinal
nerve has a dorsal root and a ventral root. The dorsal root contains afferent (sensory) fibers that
transmit information to the spinal cord from the sensory receptors. The ventral root contains
efferent (motor) fibers that carry messages from the spinal cord to the effectors. Cell bodies of the
efferent fibers reside in the spinal cord gray matter. These roots become nerves that innervate
(transmit nerve impulses to) muscles and organs throughout the body.
Twelve pairs of cranial nerves transmit from special sensory receptors information on the
senses of balance, smell, sight, taste, and hearing. Cranial nerves also carry information from
general sensory receptors in the body, mostly from the head region. This information is
processed in the CNS; the resulting orders travel back through the cranial nerves to the skeletal
muscles that control movements in the face and throat, such as for smiling and swallowing. In
addition, some cranial nerves contain somatic and autonomic motor fibers.
The involuntary nervous system (autonomic nervous system) maintains homeostasis. As
its name implies, this system works automatically and without voluntary input. Its parts include
receptors within viscera (internal organs), the afferent nerves that relay the information to
the CNS, and the efferent nerves that relay the action back to the effectors. The effectors
in this system are smooth muscle, cardiac muscle and glands, all structures that function
without conscious control. An example of autonomic control is movement of food through
the digestive tract during sleep.
The efferent portion of the autonomic system is divided into sympathetic and
parasympathetic systems. The sympathetic nerves mobilize energy for the 'Fight or Flight'
reaction during stress, causing increased blood pressure, breathing rate, and bloodflow to
muscles. Conversely, the parasympathetic nerves have a calming effect; they slow the
heartbeat and breathing rate, and promote digestion and elimination. This example of
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intimate interaction with the endocrine system is one of many that explain why the two systems
are called the neuroendocrine system.
The relationship between sensory and motor neurons can be seen in a reflex (rapid motor
response to a stimulus). Reflexes are quick because they involve few neurons. Reflexes
are either somatic (resulting in contraction of skeletal muscle) or autonomic (activation of
smooth and cardiac muscle). All reflex arcs have five basic elements: a receptor, sensory
neuron, integration center (CNS), motor neuron, and effector.
Spinal reflexes are somatic reflexes mediated by the spinal cord. These can involve higher brain
centers. In a spinal reflex, the message is simultaneously sent to the spinal cord and brain. The
reflex triggers the response without waiting for brain analysis. If a finger touches something hot,
the finger jerks away from the danger. The burning sensation becomes an impulse in the sensory
neurons. These neurons synapse in the spinal cord with motor neurons that cause the burned
finger to pull away. This spinal reflex is a flexor, or withdrawal reflex.
The stretch reflex occurs when a muscle or its tendon is struck. The jolt causes the
muscle to contract and inhibits antagonist muscle contraction. A familiar example is the
patellar reflex, or knee-jerk reflex, that occurs when the patellar tendon is struck. The
impulse travels via afferent neurons to the spinal cord where the message is interpreted. Two
messages are sent back, one causing the quadriceps muscles to contract and the other
inhibiting the antagonist hamstring muscles from contracting. The contraction of the
quadriceps and inhibition of hamstrings cause the lower leg to kick, or knee-jerk.
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Sense organs
The sense organs are highly specialized structures that receive information from the environment.
These organs contain special sense receptors ranging from complex structures, such as eyes
and ears, to small localized clusters of receptors, such as taste buds and olfactory epithelium
(receptors for smell).
Smell and taste are chemical senses, which contain chemoreceptors that respond to
chemicals in solution. Food chemicals dissolved in saliva stimulate taste receptors in taste
buds. The nasal membranes produce fluids that dissolve chemicals in air. These chemicals
stimulate smell receptors in olfactory epithelium. The chemical senses complement each
other and respond to many of the same stimuli.
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Photoreceptors, which include rods and cones, in back of the eye respond to light energy.
Rods provide dim-light, black-and-white vision, and are the source of peripheral vision.
Cones operate in bright light and provide color vision. Cones are most concentrated at the
back center of each eye. Rods are more numerous than cones, and surround the cones.
Information from the rods and cones travels via the optic nerve into the brain for
interpretation.
The ear has two specialized functions: sound wave detection and interpretation of the head
position in space. Sound waves enter the outer ear through the external auditory canal (ear
canal) and strike the tympanic membrane (eardrum). Vibration of the eardrum moves three
ossicles (small bones) inside the middle ear, which in turn stimulate the organ of Corti (hearing
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receptor in the inner ear). Impulses travel from the organ of Corti through the
vestibulocochlear nerve to be interpreted by the brain.
The ear also contains equilibrium (sense of balance) receptors. The vestibular apparatus, a
group of equilibrium receptors in the inner ear, sense movement in space. Maculae receptors
in the vestibule monitor static equilibrium (head position with respect to gravity when the
body is still). Cristae receptors in the semicircular canals monitor dynamic equilibrium
(movement). Impulses from the vestibular apparatus travel along the vestibulocochlear
nerve to appropriate brain areas. These centers start responses that fix the eyes on objects
and stimulate muscles to maintain balance.
Mechanoreceptors respond to mechanical energy forces: touch, pressure, stretching, and
movement. Ranging in complexity from free nerve endings beneath the skin to more complex
tactile receptors at the bases of hair, mechanoreceptors change shape when pushed or pulled.
Different types of skin receptors sense different environmental stimuli. Free nerve endings sense
pain. Specialized receptors such as Merkel's discs and Meissner's corpuscles sense touch.
Pacinian corpuscles sense deep pressure. Naked nerve endings are thought to be
responsible for sensing temperature.
Other types of sensory receptors provide the brain information on the body. Interoreceptors in
body organs inform the CNS about internal conditions such as hunger and pain.
Proprioceptors in joints, tendons, and muscles detect changes in position of skeletal
muscles and bones. This information allows humans to be aware the positions of their trunk and
limbs without having to see them.
Nervous System Disorders
Stroke
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What is a stroke?
A stroke, also called a "brain attack," happens when brain cells die because of inadequate
blood flow. A stroke is considered to be a cardiovascular disease and a neurological
disorder.
What causes a stroke/brain attack?
Most strokes are caused by the blockage of an artery in the neck or brain, and the rest by
bleeding into or around the brain. When brain cells die, function of the body parts they
control becomes damaged or destroyed. This may include paralysis, speech problems,
memory and reasoning deficits, coma, and possibly death.
What happens during a stroke?
According to the American Heart Association:
A brain attack occurs when a blood vessel bringing oxygen and nutrients to the brain bursts
or is clogged by a blood clot or some other particle.
When the brain doesn't get the needed blood flow, because of a rupture or blockage, it is
deprived of oxygen. Thus, nerve cells cannot properly function and die within minutes.
And when nerve cells cannot function, the part of the body controlled by these cells cannot
function either. The devastating effects of stroke are often permanent because dead brain
cells are not replaced.
Alzheimer's Disease
What is Alzheimer's disease?
Alzheimer's disease is a progressive, neurodegenerative disease that occurs in the brain and
often results in the following:
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impaired memory, thinking, and behavior
confusion
restlessness
personality and behavior changes
impaired judgment
impaired communication
inability to follow directions
language deterioration
impaired visuospatial skills
emotional apathy
With Alzheimer's disease, motor function is often preserved.
When Alzheimer's was first identified by German physician, Alois Alzheimer, in 1906, it was
considered a rare disorder. Today, with one in 10 persons over age 65, and nearly half of
persons over age 85, affected, Alzheimer's disease is recognized as the most common
cause of dementia (a disorder in which mental functions deteriorate and breakdown).
How is Alzheimer's different from other forms of dementia?
Alzheimer's disease is distinguished from other forms of dementia by characteristic changes
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in the brain that are visible only upon microscopic examination during autopsy. Brains
affected by Alzheimer's disease often show presence of the following:
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fiber tangles within nerve cells (neurofibrillary tangles)
clusters of degenerating nerve endings (neuritic plaques)
Another characteristic of Alzheimer's disease is the reduced production of certain brain
chemicals necessary for communication between nerve cells, especially acetylcholine, as
well as norepinephrine, serotonin, and soma-tostatin.
Fetal alcohol syndrome
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Overview
Symptoms
Treatment
Prevention
Definition:
Fetal alcohol syndrome is the manifestation of specific growth, mental, and physical birth defects associated
with the mother's high levels of alcohol use during pregnancy.
Alternative Names:
Alcohol in pregnancy; Drinking alcohol during pregnancy
Causes, incidence, and risk factors:
Alcohol use or abuse by the pregnant woman subjects her to the same range of risks that alcohol has in the
general population. However, it poses extreme and unique risks to the fetus and is associated with fetal
alcohol syndrome (FAS).
Timing of alcohol use during pregnancy is also of importance. Alcohol use during the first trimester is more
damaging than during the second trimester, which is, in turn, more damaging than use in the third trimester.
Alcohol ingested by a pregnant woman easily passes across the placental barrier to the fetus. Because of
this, drinking alcohol can adversely affect the development of the baby.
A pregnant woman who drinks any amount of alcohol is at risk, since a "safe" level of alcohol ingestion
during pregnancy has not been established. However, larger amounts appear to cause increased problems.
Multiple birth defects associated with "classical" fetal alcohol syndrome are more commonly associated with
heavy alcohol use or alcoholism.
Fetal alcohol syndrome consists of the following abnormalities:
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Intrauterine growth retardation: growth deficiency in the fetus and newborn in all parameters -- head
circumference , weight, height
Delayed development with decreased mental functioning (mild to severe)
Facial abnormalities including small head (microcephaly); small maxilla (upper jaw); short, up-turned nose;
smooth philtrum (groove in upper lip); smooth and thin upper lip; and narrow, small, and unusual-appearing
eyes with prominent epicanthal folds
Heart defects such as ventricular septal defect (VSD) or atrial septal defect (ASD)
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Limb abnormalities of joints, hands, feet, fingers, and toes
Cocaine intoxication
Definition:
Cocaine is a powerful central nervous system stimulant with potent cardiovascular (heart and blood vessel)
side effects. Signs of intoxication typically begin with enlarged pupils, euphoria, agitation, and increased
heart rate and blood pressure.
With higher doses, symptoms can progress to sweating, tremors, confusion, hyperactivity, seizures, stroke ,
cardiac arrhythmias (irregular heart beats), and sudden death.
Barbiturate intoxication and overdose
Definition:
Barbiturates are a type of depressant drug that causes relaxation and sleepiness. In relatively low doses,
barbiturates and alcohol have very similar clinical syndromes of intoxication.
However, excessive and prolonged dosages of barbiturate drugs, such as phenobarbital, may produce the
following chronic symptoms: memory loss, irritability, changes in alertness, and decreased interpersonal
functioning. Barbiturates may also cause an acute overdose syndrome, which is life-threatening.
Alternative Names:
Intoxication - barbiturates
Causes, incidence, and risk factors:
Barbiturate abuse is still a major addiction problem in the population, although it has been partly replaced by
addiction to other depressant drugs more commonly prescribed, such as benzodiazepines.
Though most people who take these medications for seizure disorders or pain syndromes do not abuse
them, many abusers start by abusing medication prescribed for them or for other family members.
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Rubric Scores
3
The guide booklet contains:
Written
 an explanation of how the human body maintains homeostasis;
 an explanation of the nervous systems role in homeostasis;
 a description of the function of the nervous system;
 a description of how a nerve impulse is transmitted;
 a description of the central nervous system;
 an explanation of the division of the peripheral nervous system;
 a description of the five sensory receptors;
 a description of the classes of drugs that affect the nervous system;
as well as
 vocabulary used and defined in the written section – nervous
tissue, feedback inhibition, neuron, cell body, dendrite, axon, myelin
sheath, resting potential, action potential, threshold, synapse,
neurotransmitter, central nervous system, meninges, cerebellum,
cerebrum, brain stem, thalamus, hypothalamus, reflex, drug,
stimulant, depressant, addiction, fetal alcohol syndrome, drug
abuse,
Pictures, Diagrams, Tables, and Charts
 nervous system structure and divisions;
 neurons;
 resting neuron membrane;
 action potential along an axon;
3
The booklet has a title page, table of contents, evidence of organization,
however, it can be better. The pictures, diagrams, tables or charts are
informational, however, they may lack clarity with production, labels or captions.
A potential doctor can find some information for learning and understanding the
structures, functions, and homeostasis of the nervous system, but will probably
go to additional sources.