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THE NERVOUS SYSTEM
Chapter 33
STRUCTURE OF NERVOUS SYSTEM
Section 1
NEURONS
 Neurons
are specialized cells that help you
gather information about your environment,
interpret the information and react to it.
 Neurons make up an enormous
communication network in your body called
your nervous system.
 A neuron consists of 3 main regions:
1.
Dendrites: receive impulses from other neurons
and send impulses to the cell body.

2.
3.
A single neuron may have many dendrites.
Cell Body: nucleus of the neuron
Axon: carries the nerve impulse from the cell
body to other neurons and muscles
NEURON
NEURONS CONT…
 There
are 3 kinds of neurons:
Sensory Neurons
2. Interneurons
3. Motor Neurons
1.
A
reflex arc is a nerve pathway that consists
of a sensory neuron, an interneuron and a
motor neuron. (The brain is not involved).
AN ACTION POTENTIAL AKA NERVE IMPULSE
 The
minimum stimulus to cause an action
potential to be produced is a threshold.
 Threshold: Not all stimuli cause an impulse.
A stimulus below the threshold has no effect on
the neuron.
 Some people have higher thresholds for pain,
heat or other stimuli. This means they can
tolerate a stronger stimulus before their nervous
system reacts with an impulse.

potentials are described as being “all or
nothing” meaning a nerve impulse is either
strong enough to travel along the neuron or its
not strong enough.
 Action
AN ACTION POTENTIAL CONTINUED…
 When
a stimulus reaches threshold,
channels in the plasma membrane open.

Na+ ions rapidly move into the cytoplasm
causing temporary reversal in electrical charges.


The inside of the cell is now positive, which causes other
channels to open.
K+ leave the cell through these channels,
restoring + charges outside the cell.
SPEED OF AN ACTION POTENTIAL
 Depend
upon the axons
in a neuron.
 Most neurons are
covered with a lipid layer known as the myelin
sheath.

The Myelin Sheath both insulates and speeds up
transmission of action potentials through the
axon.
 Other
structures along the axon are gaps
(nodes) in the myelin sheath known as nodes
of ranvier.

These gaps allow the impulses to travel faster
than if they travelled along the entire length of
the neuron.
THE SYNAPSE
A
small gap exists
b/w the axon of one
neuron and the
dendrite of another
neuron, this is called
a synapse.
 At the end of an
axon, small sacs
called vesicles
carrying
neurotransmitters
fuse with the plasma
membrane and
release a
neurotransmitter by
exocytosis.
THE SYNAPSE CONT…
A neurotransmitter is a chemical that diffuses
across a synapse and binds to receptors on the
dendrite of a neighboring neuron.
 An nt causes channels to open on the neighboring
cell and creates a new action potential.

ORGANIZATION OF THE NERVOUS SYSTEM
Section 2
THE CENTRAL NERVOUS SYSTEM
 The
nervous system consists of 2 major
divisions:


Central Nervous System (CNS): brain/spinal
cord
Peripheral Nervous System (PNS): sensory
neurons and motor neurons
 The
CNS is made mostly of interneurons.
 The CNS’s function is to coordinate all the
body’s activities.
 The CNS relays messages, processes
information, and analyzes responses.
THE BRAIN
Over 1 billion neurons are
found in the brain.
 It is called the control
center of the body.

THE BRAIN—CEREBRUM

The cerebrum is the
largest part of the brain



Broken into 2 halves
called hemispheres.
Hemispheres connected
by a bundle of nerves.
Functions:





Thought
Voluntary movement
Language
Reasoning
Perception
MEMORY
http://faculty.washington.edu/chudler/stm0.html
 Short Term Memory
 Trials 1-6

THE BRAIN—CEREBELLUM
Known as the “little brain”
 Functions:
 Movement
 Balance
 Posture


The brain stem connects
the brain to the spinal cord.

Medulla Oblongata: relays
signals b/w the brain and spinal
cord.


Controls breathing rate, heart
rate and blood pressure.
Pons: relays signals b/w the
cerebrum and cerebellum.

Helps control breathing rate.
THE BRAIN—BRAIN STEM
The brain stem is a general
term for the area of the
brain b/w the thalamus and
spinal cord.
 Structures within the brain
stem include the medulla
oblongata, pons, etc.
 Functions:
 Breathing
 Heart Rate
 Blood Pressure

THE BRAIN—HYPOTHALAMUS
 Its
about the size of a
fingernail.
 Functions:





Body Temperature
Emotions
Hunger
Thirst
Circadian Rhythms
(Sleep)
THE SPINAL CORD
The spinal cord is a nerve column that extends from
the brain to the lower back.
 It is protected by the vertebrae.
 Spinal nerves extend from the spinal cord to parts
of the body and connect them to the CNS.
 Reflexes are processed by the spinal cord.

THE PERIPHERAL NERVOUS SYSTEM

Contains all the nerves
throughout the body.

A nerve is actually a
bundle of axons.
12 cranial nerves lead
to and from the brain.
 31 spinal nerves.

THE SOMATIC NERVOUS SYSTEM
 Nerves
in the somatic
nervous system relay
information from external
sensory receptors (skin) to
the CNS, and motor nerves
relay info from the CNS to
skeletal muscles.
 Most reflexes (quick
response to change in an
environment) go only to the
spinal cord, not to the brain.
THE AUTONOMIC NERVOUS SYSTEM
The ANS carries
impulses from the CNS
to the heart and other
internal organs.
 The body responds
involuntary.

THE AUTONOMIC NERVOUS SYSTEM

There are two
branches of the ANS:

Sympathetic:
Activated in times of
emergency or stress
when the heart rate and
breathing rate increases.
 Fight or flight response


Parasympathetic

Activated when the body
is relaxed.
Central Nervous
System (CNS)
Somatic
(voluntary)
Relays info to and from
skin to skeletal muscles.
Sympathetic
Controls organs in time of stress
Peripheral Nervous
System (PNS)
Autonomic
(involuntary)
Relays info to internal
organs.
Parasympathetic
Controls organs when the
body is at rest.
THE SENSES
Section 3
TASTE & SMELL
 Caused
by sensory neurons.
 Stimulated by chemicals.
 Specialized receptors located high in the nose
respond to chemicals in the air and send the
info to the olfactory bulbs in the brain.
 Taste buds are areas of specialized chemical
receptors on the tongue that detect the tastes
of sweet, sour, salty and bitter.
 **Tip try holding your nose while eating; you’ll
find out that your food looses much of its
flavor.**
TASTE & SMELL
TASTE
SMELL
SIGHT

Light enters the eye through the cornea.


Cornea: transparent durable layer of the eye.
The cornea focuses the light that enters
through the pupil.

The pupil is regulated by muscles in the iris, which is
the colored part of the eye.
Behind the iris is the lens.
 The lens inverts the image and projects it
onto the retina.
 From the retina the image travels through
the vitreous humor.



V.R. is a gelatin-like liquid b/w the lens and the retina.
The retina contains numerous receptors
called rods and cones.


Rods: light sensitive (black & white)
Cones: bright light (colors)
These receptors send action potentials to
the brain via the neurons in the optic nerve.
 The brain interprets the signals from the
retina and forms a visual image.

SIGHT
HEARING
Vibrations called sound waves cause particles in the
air to vibrate.
 1. Sound waves enter the auditory or ear canal
causing the eardrum to vibrate.
 2. These vibrations travel through 3 bones in the
middle ear—malleus (hammer), incus (anvil), stapes
(stirrup)
 3. As the stapes vibrates it causes the oval window to
move back & forth.
 4. Now at the inner ear, the cochlea, which is filled
with fluid and lined with tiny hair cells, vibrates the
fluid along the hair cells.
 5. The hair cells respond by generating nerve
impulses in the auditory nerve and transmit them to
the brain.

HEARING
BALANCE
 Found
in the inner ear are structures that
control balance.
 Semicircular canals transmit information about
body position and balance to the brain.
 Like the cochlea they are filled with fluid and
contain hair cells.
 When the position of your head changes, fluid
moves through the canals. This causes the
hair cells to bend, which sends nerve
impulses to the brain.
 The brain is able to determine your position
and whether your body is still or in motion.
BALANCE
TOUCH
 Many
sensory receptors that respond to
temperature, pressure and pain are found in
the epidermis and dermis layers of the skin.
 Distribution of receptors is not uniform in all
areas of the body.
 Fingers detect light touch
 Feet detect heavy pressure
 Pain receptors exist throughout the body
except for the brain.
TOUCH
Light
Touch
Vibration
&
pressure
EFFECTS OF DRUGS
Section 4
HOW DRUGS WORK
A
drug is a substance, natural or artificial, that
alters the function of the body.
 Examples include: antibiotics, cocaine,
marijuana, caffeine, nicotine and alcohol.
 Not all drugs affect the nervous system.
 However those that affect the nervous system
affect it by:
Increase the amount of a neurotransmitter
 Block the receptor site on a dendrite
 Prevent a neurotransmitter from leaving a synapse
 Imitate a neurotransmitter

HOW DRUGS WORK
Many drugs affect the nervous system influence the
level of a neurotransmitter called Dopamine.
 Dopamine is a neurotransmitter found in the brain
that is involved with the control of body movements
and other functions.

CLASSES OF COMMONLY ABUSED DRUGS

Stimulants are drugs that increase alertness
and physical activity.

Nicotine:
Is found in cigarette and cigar smoke which increases the
amount of dopamine released in a synapse.
 Nicotine also constricts blood vessels, raising blood
pressure and causes the heart to work harder.
 It also causes lung cancer.


Caffeine:
Is the most used and abused stimulant
 Found in coffee, tea, soft drinks and chocolate
 Makes users feel awake and alert.
 Temporarily raises epinephrine (adrenaline) level in the
body giving quick bursts of energy, but quickly wears off.

CLASSES OF COMMONLY ABUSED DRUGS
 Depressants:
drugs tend to slow down the
CNS. These drugs can lower blood pressure,
interrupt breathing and slow heart rate.

Alcohol
Most widely abused drug
 Results in feeling of relaxation and sluggishness
 Short term use impairs judgment, coordination and reaction
time.
 Long term use includes reduction in brain mass, liver
damage, stomach and intestinal ulcers as well as high
blood pressure.


Inhalants
Chemical fumes that influence the nervous system
 Short term use effect intoxication (nausea and vomiting)
 Long term use effect memory loss, hearing loss, vision
problems, peripheral nerve damage, and brain damage.

CLASSES OF COMMONLY ABUSED DRUGS

Illegal Drugs
 Amphetamines and cocaine prevent dopamine from being
reabsorbed so it remains in the synapse.
 Cocaine


Amphetamines


Results in disturbances in heart rhythm, heart attacks, chest
pains, respiratory failure, strokes, seizures, headaches,
abdominal pain and nausea.
Results in rapid heart rate, irregular heartbeat, increased blood
pressure, and irreversible stroke producing damage.
Marijuana
Most used illegal drug in the U.S.
 Tetrahydrocannabinol (THC) binds to receptors on neurons in
the brain, which produces an effect of intense pleasure.
 Short term problems include memory and learning loss,
anxiety, paranoia and panic attacks.

TOLERANCE & ADDICTION
 Tolerance
occurs when a person needs more
and more of the same drug in order to get the
same effect.

The dosage increases because the body becomes
less responsive to the drug.
 Addiction
is the psychological and/or
physiological dependence on a drug.


Emotionally disturbed
Physical health deteriorates
SENSORY NEURONS

Send impulses from receptors in the skin and sense
organs to the brain and spinal cord. (Sensory signal
interneurons)
INTERNEURONS

Found in the spinal cord and brain. They carry the
impulse to motor neurons. (Interneurons signal
motor neurons)
MOTOR NEURONS

Carry the impulse away from the brain and spinal
cord to a gland or muscle which creates a
response.
A NERVE IMPULSE

A nerve impulse is an electrical charge traveling the
length of a neuron.




When a neuron is not
sending a signal, it is "at
rest"
At rest the inside of the
neuron is negative relative
to the outside.
The concentrations of the
different ions attempt to
balance out on both sides of
the membrane, but cannot
because of the cell’s
selective permeability.
At rest, Potassium ions (K+)
can cross through the
membrane easily. Chloride
ions (Cl-)and Sodium ions
(Na+) have a more difficult
time crossing. The
negatively charged protein
molecules (A-) inside the
neuron cannot cross the
membrane.
A NEURON AT REST
A NEURON AT REST




In addition to these selective ion channels, there is
a pump that uses energy to move 3 Sodium (Na+) ions
out of the neuron for every 2 Potassium (K+) ions it puts
in.
Finally, when all these forces balance out, and the
difference in the voltage between the inside and outside
of the neuron is measured, you have the resting
potential.
The resting potential of a neuron is about -70 mV
(mV=millivolt) - this means that the inside of the neuron
is 70 mV less than the outside.
At rest, there are relatively more sodium ions outside the
neuron and more potassium ions inside the neuron.