Transcript Nervous System I

The Nervous System
rev 10-11
• Receives information and produces a
meaningful, quick output.
• To do this, the nervous system
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–
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quickly sorts through our memory bank
decides the probable meaning of the input
integrates the information
Provides a quick response
• So, the nervous system controls and
integrates all other body systems and
functions
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What are the characteristics of the nervous system
that allow us to do this?
1. It must receive information from our senses.
2. It integrates information.
-Integration is the process of taking different
pieces of information from different sources,
making sense of all of it at the same time, and
coming up with an action plan.
3. The nervous system is fast; it can do this within
tenths of a second.
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The CNS-Central Nervous System (the brain and
spinal cord)
– is the integrating and command center of the
nervous system
– It receives and interprets incoming sensory
information and produces motor responses
The PNS-peripheral nervous system is the part of
the nervous system outside the CNS.
– it contains the communication lines that link all
parts of the body to the CNS
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The PNS consists of
• 12 pairs of cranial nerves: carry impulses
between brain and body
• 31 pairs of spinal nerves: connect to spinal
cord via dorsal and ventral roots
– Dorsal root has sensory neurons and
transmit information TO the cord
– Ventral root has motor neurons that
transmit information FROM the cord to
the body
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• the Peripheral NS has 2 functional subdivisions
– the sensory or afferent division carries
impulses TO the CNS
• keeps the CNS informed of events going on
inside and outside of the body
– The motor or efferent division carries
impulses FROM the CNS
• this division enables us to respond to stimuli
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The Motor Division can be further subdivided into 2
parts:
• the Somatic nervous system
– Voluntary: controls voluntary and involuntary
skeletal muscle movements
• Motor neurons are activated either by
conscious control from the brain or by an
involuntary response called a reflex
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Reflex Division:
– Spinal reflexes
• Spinal reflexes are involuntary, automatic
responses handled primarily by the spinal
cord and spinal nerves
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AND the Autonomic nervous system (ANS)
• this division regulates involuntary activities; the
activity of smooth muscles, cardiac muscles, and
glands (regulates anything that occurs
automatically in the body)
• Requires 2 neurons to transmit information from
the CNS to a “target” cell
– Preganglionic neurons: cell bodies of the first neurons
lie within the CNS
– The axons of these go to postganglionic neurons
which lie outside the CNS
• Postganglionic axons extend to wherever in our
body the target glands or organs are located
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The ANS is further sub-divided into the
• SYMPATHETIC NERVOUS SYSTEM which
mobilizes body systems during emergency
situations.
• Origin: thoracic or lumbar regions of the
spinal cord
• Function: releases neurotransmitters
epinephrine and norepinephrine for fight-orflight reaction; opposes parasympathetic
division
• reduces blood flow to organs that do not help
with an immediate disaster
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AND the PARASYMPATHETIC
NERVOUS
SYSTEM which conserves our energy and
predominates during relaxing
– Origin: brain or sacral area of spinal cord
(craniosacral)
– Function: releases acetylcholine to relax the
body; opposes sympathetic division
• In most organs, the actions of the sympathetic and
parasympathetic divisions have opposite effects.
• The two divisions counterbalance each other’s
activities to maintain homeostasis.
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Composition of the Nervous System
Nerves, Neurons, Neuroglia
The Nervous System is composed of
Nerves which consists of the axons of many neurons
wrapped together in a sheath of connective tissue
Neurons are cells which are specialized for
communication.
• Classification of neurons
– Sensory or Afferent neurons carry (sensory)
information from receptors TO the CNS
– Motor (Efferent) Neurons carry messages
AWAY FROM the CNS to the muscles and
the glands
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– Interneurons or Association Neurons are
located in the CNS and conduct impulses
within the CNS. They receive information
from sensory neurons, integrate the input, and
then deliver the information to other neurons.
• are multipolar neurons
• Neuroglia (or Glia) are the supporting and
protecting cells of the nervous system. We will
speak more of these later.
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Neurons
• All neurons have
• a cell body (contains the nucleus)
• an axon (long slender tube of cell membrane;
specialized to carry electrical impulses)
– Axons of sensory neurons originate from a dendrite
– Axons of interneurons and motor neurons originate from a
cone shaped area of the cell body called the axon hillock
– At its other end, the axon branches into slender extensions
called axon terminals and the end of this is called an
axon bulb
• dendrites (typically slender extensions of the cell
body; receive stimuli)
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Types of Neurons
Unipolar Neurons have a single process which
emerges from the cell body
• this process divides into a proximal and distal
branch
– One branch behaves as an afferent branch and
the other behaves as an efferent branch
• All unipolar neurons are sensory
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Unipolar
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Bipolar Neurons
• have 2 processes emerging from a round cell body
• processes extend from opposite sides of the cell
body
• found only in some of the special sense organs
where they act as receptor cells
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Types of Neurons
Bipolar Neuro
Bipolar
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Multipolar Neurons
• have 3 or more processes
• are the most common neuron type in humans and
major neuron type in the CNS
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Types of Neurons
Multipolar
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Interneurons or Association Neurons
• are a multipolar neuron
• located in the CNS
• conduct impulses within the CNS
• are the connecting link between sensory and motor
neurons
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Some neurons have a myelin sheath
• the myelin sheath
– is a fatty wrapping around the axon which provides
insulation to the axon and thus saves it energy
– it speeds impulse transmission by allowing a leaping
pattern of transmission called saltatory conduction
– The impulse jumps from one Node of Ranvier to
another
– Between neighboring Schwann cells are short,
uninsulated gaps called Nodes of Ranvier
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Myelin
• in the peripheral nervous system is formed from
Schwann Cells which wrap around the axon
It helps damaged or severed axons of peripheral
nerves regenerate
•in the CNS is formed by oligodendrocytes
•The oligodendrocyte sheath degenerates once the
axon it protects is damaged or destroyed
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Neuroglial cells
• Provide physical support to neurons
• Protection to neurons
• Help maintain concentrations of chemicals in the
fluid surrounding them
• Neuroglial cells DO NOT generate or transmit
impulses
• Example: myelin sheath
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Impulse Transmission-Summary
The function of a neuron is to transmit information from one
part of the body to another.
• This is done in the form of electrical impulses.
• An impulse arrives at the dendrite
• When the impulse is strong enough, it depolarizes the
membrane and the impulse is transmitted along the axon
• When the impulse reaches the axon terminals, the
information needs to be converted to another form of
energy in order for the information to be transmitted to its
target (i.e. a muscle, a gland, or another neuron)
• A chemical, called a neurotransmitter, is released which
allows the impulse to jump the synapse, or space, between
the 2 cells
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Impulse Transmission- Definitions
• Resting or Membrane Potential: a small difference in
voltage across the cell membrane; the cell is normally
negatively charged.
– This allows the neuron to be ready to respond more quickly than it
could if it were electrically neutral.
– Think about a car battery. It retains a charge so that the
car will start as soon as the key is turned
– Unlike most body cells, neurons can alter the electrical
charge across the neurolemma. The membrane potential
alternates between -70 and +30 millivolts. Charge
differences are controlled by the movement of sodium and
potassium ions entering and leaving the neuron
• Action Potential: changes in the electrical activity of the
nerve of sufficient intensity to reach the threshold
necessary to move an electrical impulse down the axon
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• Threshold: the level of stimulus a neuron needs in order
to fire
• All or nothing phenomenon: once the threshold level is
reached, the nerve transmits an impulse
• Depolarization: moving the negative charge within the
axon closer to zero
• Na+ (sodium) moves into the cell via sodium channels
which open
• Repolarization: K+ (potassium) moves out of the cell;
Na+ channels close and the reversal of the membrane
polarity triggers opening of the K+ channels so the K+
moves out of the cell. Loss of K+ means that the interior
of the axon becomes negative again and the resting
potential is restored
• Refractory period: The part of the axon that has already
fired is unable to fire again so the impulse must move
forward (This prevents the impulse from moving
backwards.)
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Movie Clip
• http://brainu.org/files/movies/action_potential_car
toon.swf
• http://highered.mcgrawhill.com/sites/0072495855/student_view0/c
hapter14/animation__transmission_across_a
_synapse.html
• http://www.sumanasinc.com/webcontent/animatio
ns/content/actionpotential.html
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Action potential Overview:
Abstracted from
http://soe.ucdavis.edu/ss0708/eghbalis/Notes/u12Notes.html
The functioning of the neuron is dependent on the separation
of positive and negative ions, keeping the negative charge
on the inside and the positive charge on the outside.
Neurons are typically at a resting state or resting potential:
the amount of positive ions on one side and negative ions
on the other side of the plasma membrane remains the
same, creating a -70millivolt potential difference.
How can the charge inside the cell be negative if the cell contains
positive ions? In addition to the K+, negatively charged protein
and nucleic acid molecules also inhabit the cell; therefore, the
inside is negative as
compared to the outside.
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In order for the neuron to send a message, there needs to be a
change in the electrical charge, causing an impulse to be
sent down the axon. This process is called an action
potential.
• A stimulus causes voltage-gated sodium channels to open
causing sodium ions to move into the cell. Because
positive charges are coming inside, the inside of the
membrane less negative (more positive).
• Depolarization:When the impulse becomes strong
enough, (-55mV), an action potential begins. As sodium
ions enter the axon, more sodium channels open causing
even a greater influx of sodium ions.
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Peak:
• By the time the peak is reached (usually +50mV) the
sodium channels have already begun closing, reducing the
rise in the potential. As this happens, the voltage-gated
potassium channels open.
• Repolarization: The voltage gated potassium channels are
open and potassium ions move out of the cell. This causes
neurons to return to the negative membrane potential.
• Hyperpolarization: The potassium channels will begin to
close but they don't close fast enough and thus there is an
overshoot of depolarization, making the membrane more
negative than -70mV.
• Refractory Period: During the next few seconds, there is
a refractory period and during this time no action potential
can take place
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The following graph demonstrates how the electric potential
of the membrane changes during an action potential.
Initially, the cell is at rest (-70 mV).
• If there is enough increase in the voltage (to -55 mV) then
an action potential takes place. If it's anything lower than
that, nothing happens. This is an all or none response
(you either get a signal, or not, there is no such thing as a
weak signal).
• If the voltage reaches -55 mV, the cell membrane will
depolarize and the voltage will increase rapidly to above
0mV. This is an action potential.
•
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• Then the membrane repolarizes (returns to
normal) as potassium ions are released.
• There is an over release of potassium ions, making
the membrane more negative than the resting
potential. Until the membrane returns to the
resting potential, it is in that refractory period
where the neuron cannot be stimulated again
(cannot send another message).
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– Meninges--3 protective membranes of
connective tissue enclose the brain and spinal
cord
• Dura mater is the outermost and strongest
layer
• Arachnoid mater is the middle layer; it has
spidery extensions which secure it to the
innermost layer.
• Pia mater is the innermost layer and clings
tightly to the CNS.
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Cerebrospinal fluid (CSF): is found within and
around the brain and spinal cord. It
• functions as a liquid shock absorber
• serves as a blood-brain barrier by isolating
the CNS from infections
• aids in providing nutrients for cells and
removing waste products from cells
• by floating the brain, the CSF effectively
reduces brain weight by 97% and prevents
the brain from crushing under its own weight
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• CSF is formed from blood plasma
– it is made in the choroid plexuses in the
ventricles of the brain
– much of the fluid is found in the subarachnoid
space
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Spinal Cord
• The spinal cord is located in the vertebral column.
• It is approximately the width of the thumb except
at the cervical and lumbar areas.
– Cervical enlargement is where the nerves for
the shoulder and arms enter and exit the cord.
– Lumbar enlargement is where the nerves for
the legs enter and exit the cord.
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• The spinal cord does not extend to the end of the
vertebral column. It ends at the level of L2 (the area
between the first and second lumbar vertebrae).
• The cord ends in a tapering cone shape which is
called the conus medularis.
• The lumbar and sacral nerves angle sharply
downward and travel through the vertebral canal
before they exit through intervertebral foramina.
• The collection of these nerves is called the cauda
equina because they resemble a horse’s tail.
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The outer portion of the spinal cord consists primarily of bundles
of axons called nerve tracts.
– These axons are usually myelinated so they have a white
appearance and are called “white matter.”
• White matter is made up of ascending (sensory) and
descending (motor) nerve tracts of myelinated axons.
– Near the center of the cord is an area called the “gray
matter”. It is occupied primarily by cell bodies, dendrites
and axons of neurons which are not myelinated.
• Is in the shape of an “H” or a butterfly; this is where all
synapses between sensory, association, and motor
neurons takes place.
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Spinal Cord Anatomy
• The Central Canal is where the CSF circulates.
• The Ventral (or anterior) Root is where
MOTOR axons exit the cord.
• The Dorsal (or posterior) Root is where
SENSORY axons enter the cord.
• The Dorsal Root Ganglion is where sensory
nerve cell bodies are located.
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The Reflex Arc
Many of the body’s control systems belong to a
general category known as reflexes.
• A reflex is a rapid, predictable motor response to a
stimulus.
– It is automatic, involuntary, and protective.
– Both the spinal cord and the brain are reflex
centers.
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Reflex arcs include the following components:
1. The receptor is the where the stimulus begins.
2. The sensory neuron transmits the afferent impulses to
the CNS.
3. An association neuron receives the information and
causes an instantaneous impulse to be transmitted to a
motor neuron.
4. The motor neuron sends an impulse to the effector
organ (organ which will cause a response).
5. The effector is the muscle or gland that responds in
a characteristic way.
This allows us to respond to a stimulus instantly and
without thinking.
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• In more complex reflexes, some information goes
to the brain to allow it to integrate information.
This will allow you to learn from the situation.
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The Brain and the Spinal Cord
The CNS controls and processes all information
received by the body.
• Protection of the CNS
– Bone protects it from physical injury
– the brain is covered by the skull
– the spinal cord is surrounded by the
vertebrae
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The Brain
• The brain is the central command center of the
body
– Receives information in the form of action
potentials from various nerves and the spinal
cord, integrates it and generates the appropriate
response.
• 3 major anatomical and functional divisions of the
brain have been identified:
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Brain: Major Divisions
• Hindbrain: coordinates basic, automatic, and vital
functions
• Midbrain: helps coordinate muscle groups and
responses to sight and sound
• Forebrain: receives and integrates sensory input
from the external environment and determines most
of our complex behavior
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Hindbrain: movement and automatic functions
• Connected to the spinal cord
• Medulla Oblongata: controls automatic functions
of internal organs
– cardiovascular center- regulates heart rate
and blood pressure
– respiratory center-controls rate and depth
of breathing
– other centers which coordinates reflexes
such as coughing, vomiting, swallowing,
and sneezing
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– All information passing between the higher
areas of the brain and spinal cord must pass
through the medulla
– Motor nerves from one side of the forebrain
cross over to the other side of the body in the
medulla
• Left side of brain controls right side of body
• Right side of brain controls left side of body
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Cerebellum
• Coordinates basic movements below the level
of conscious control
– Stores and produces whole sequences of
skilled movements
– Receives sensory input from many sources
– excessive alcohol disrupts normal
functioning of the cerebellum
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• Pons
– Aids information flow
• connects the higher brain centers and the
spinal cord
• its respiratory nuclei work with the
respiratory centers of the medulla in
regulating respiration
• coordinates the information flow between
the cerebellum and higher brain centers
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• The Midbrain—functions relate to vision and hearing
– Visual and auditory sensory inputs pass through the
midbrain before being relayed to higher brain centers
– Coordinates movements of the head in response to
vision and hearing
– controls movement of the eyes and pupil size
– monitors the unconscious movement of skeletal
muscles so their actions are smooth and coordinated
The reticular formation extends through the medulla,
the pons, and the midbrain.
– works with the cerebellum to coordinate muscle activity
to maintain posture, balance and muscle tone
--The Reticular Activating System, within the reticular
formation, is responsible for maintaining our level of
wakefulness
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The forebrain and diencephalon: emotions and
conscious thought
Important areas are the cerebrum, thalamus, hypothalamus,
and limbic system
– Also includes 2 glands: the pineal gland and the pituitary gland
• Determines our most complex behavior including emotions
and conscious thought.
Hypothalamus and Thalamus maintain homeostasis and
process information
• Hypothalamus is a small region at the base of the forebrain
that coordinates some automatic functions including
regulating homeostasis due to monitoring of sensory
signals
• Also controls the pituitary gland
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• Thalamus: located just above the hypothalamus
– Is primarily a receiving, processing and transfer
center; it sends signals to the cerebrum to be
interpreted.
• Limbic System is a group of neuronal pathways
which connect parts of the thalamus,
hypothalamus and cerebrum.
– involved in emotions and memory.
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• Cerebrum deals with higher functions and is
most highly developed
– is divided into left and right “cerebral
hemispheres” by the longitudinal fissure
– each hemisphere controls the opposite side of
the body
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• In the middle of the hemispheres is the corpus
callosum which joins the 2 hemispheres and
enables them to communicate and share
information
• Below the corpus callosum in each hemisphere
are the lateral ventricles which secrete CSF
• The outer layer of the cerebrum is called the
cerebral cortex and is primarily gray matter
• The inner portion of the cerebrum is primarily
white matter
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– surface tissue of the cerebrum is covered
with sulci (grooves) and gyri (ridges)
which increase the surface area for
information exchange
Each hemisphere is further divided into 4 lobes:
the frontal, parietal, temporal, and occipital
lobes
– all 4 lobes are involved in memory storage
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• The frontal lobes initiate motor activity and are
responsible for speech, conscious thought, and
personality.
– these lobes may be further divided into the prefontal
lobes or cortex which are the intellectual center
• the premotor cortex
– skilled repetitive activities (typing) and conditioned
reflexes (Pavlov’s dog)
• the primary motor cortex which initiates voluntary
motor activity of the arms, legs, trunk and face
• In the dominant hemisphere only is our primary
speech center
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• The parietal lobes house somatosensory cortex
– interpret sensory information from the skin and from
proprioceptors in the muscles and joints.
– integrate different sensory inputs to allow us to
interpret sensory information i.e. reaching into your
pocket and being able to interpret the coins in it without
using visual cues.
• The occipital lobes house the primary visual
cortex and the visual association area
• The temporal lobes interprets auditory
information and is responsible for perceptual
judgment
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Cranial Nerves
• Twelve pairs of cranial nerves arise from the brain
• They have sensory, motor, or both sensory and
motor functions
• Each nerve is identified by a number (I through
XII) and a name
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Memory: Storing and Retrieving Information
Memory has 2 stages: short term and long term
• Short term: working memory, information from previous
few hours
• Long term: information from previous days to years
The brain manages the 2 types of memory differently.
STM goes into the limbic system and triggers a burst of action
potentials so we can remember information for a few
minutes.
LTM: if information important, it is transmitted to your
cerebral cortex for storage as LTM. Neurons undergo a
permanent change and create additional synapses so we can
remember and retrieve information.
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Psychoactive Drugs
• Action: affects higher brain functions (consciousness,
emotions, or behavior); drugs influence the actions of brain
neurotransmitters; can cross the blood brain barrier
– Methamphetamine (crystal meth), cocaine, crack, alcohol,
nicotine, heroin
– When the body releases neurotransmitters, their effects are
typically short because the neurotransmitter remains in the
synapse for a brief period of time
– These drugs block the reabsorption of the neurotransmitters
so they remain in the synapse and stimulate the body again
and again
• Dopamine is one of the most important neurotransmitters
in areas of the brain associated with pleasure
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– But, as the neurotransmitter reuptake is blocked, the
body releases less and less and the “good feeling”
disappears
• Psychological dependence: user craves the feeling
associated with the drug
• Tolerance: takes more of the substance to achieve the
same affect
• Addiction: the need to continue obtaining and using a
substance; no free choice
• Withdrawal: physical symptoms that occur upon
stopping the drug
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Disorders of the Nervous System
Autoimmune Disorders:
Multiple Sclerosis (MS):
• An autoimmune disease that mainly affects young adults
– The sheaths of myelinated neurons in the brain and
spinal cord degenerate and form hardened (sclerotic)
scar tissue. These areas can’t effectively insulate the
neurons and so impulse transmission is slowed and
disrupted and the nerves are also damaged.
– People with MS experience a variety of symptoms
depending on which areas of the CNS are damaged.
• Symptoms include visual disturbances, weakness,
loss of muscular control and sensation, and urinary
incontinence
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Multiple Sclerosis
• Cause: unknown. Thought to be disorder of the
immune system or genetic tendency
– Thought that a virus attacks immune system so it
perceives myelin as a threat
• Course: can be mild to severe
• Diagnosed: MRI, Evoked Potential nerve test to
determine the speed of impulses traveling through
nerves; examine CSF to see if any cell abnormalities
• Disease-modifying drugs: Interferons or Copazone
to reduce frequency of relapses, Avonex or
Betaseran to help decrease disability
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Amyotrophic Lateral Sclerosis (ALS)
– Similar to MS but the sclerotic areas begin in
areas of the spinal cord involved in the motor
control of skeletal muscles
– Primary symptom is progressive weakening and
wasting of skeletal muscles, especially those
responsible for breathing
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Trauma:
• Concussion: caused by a violent blow to the head or
neck
– Usually see a short loss of consciousness due to a
disruption of the electrical activity of brain
neurons.
– After regaining consciousness, person may have
blurred vision, confusion, nausea and vomiting
– Typically concussions don’t have permanent
damage unless there is a subdural hematoma:
bleeding into the space between the meninges
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• Bleeding increases pressure within
headcompresses brain tissue and disrupts
function of brain
– Symptoms: drowsiness, headache and
weakness of 1 side of body
• Treatment: surgery for immediate relief of
pressure and repair of bleeding blood vessels
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• Spinal cord injuries: will impair sensation and function
below the level of injury
– Paraplegia or quadriplegia; can be fatal; will always
cause problems with bladder and bowel control
Infections: Brain and spinal cord typically do not get
infected due to the blood-brain barrier
• Encephalitis: inflammation of the brain; typically
caused by a virus
– Causes: breathing in respiratory droplets,
contaminated food, insect bite, skin contact
– Symptoms: inflammation of brain tissuecerebral
edema, headache, fever, fatigue, hallucinations,
confusion, disturbances in speech, memory or
behavior, epileptic seizures
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• Treatment: Hospitalization with intravenous medications –
antiviral or antibiotics, anti-seizure to prevent seizures,
steroids to reduce brain swelling
• Acute phase usually lasts for 1-2 weeks; fever and
symptoms may gradually or suddenly disappear; some
people may take several months to recover although in
severe cases there may be residual disabilities
• Meningitis: Inflammation of the meninges; can be
viral or bacterial
– Symptoms: headache, fever, nausea and vomiting,
light sensitivity, stiff neck
– Treatment: hospitalization. If viral, mild symptoms
and will improve in few weeks
– If bacterial, can be fatal; IV antibiotics needed
ASAP
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Rabies: infectious viral brain disease
• Transmitted to humans by direct contact, either bite or
lick over broken skin
– Virus attacks the sensory neurons in the bite region then
travels to the spinal cord, then to the brain where it
multiplies and kills cells
• Symptoms: swollen lymph glands, painful swallowing,
vomiting, choking, spasms of throat and chest muscles,
fever, becomes irrational. Death within 2-20 days
• Treatment: wash wound thoroughly ASAP, go to
emergency room or doctor, have animal tested, receive
rabies immunization ASAP
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Neural and synaptic transmission disorders: action
potentials can’t be properly sent. Symptoms depend
on which nerves are affected
• Epilepsy: recurring episodes of abnormal electrical
activity in brain
– Seizure triggers: fatigue, stress, flashing lights
– Seizures vary widely due to which part of brain is
affected
– Treatment: EEG, medications
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• Parkinson’s disease: progressive degenerative illness;
loss of dopamine releasing neurons in the area of the
midbrain that coordinates muscle movements; can’t
perform smooth, coordinated movement
– Symptoms: stiff joints, muscle tremors in hand,
loss of mobility, depression and other mental
impairments
– Treatment: L-dopa, a drug which the body converts
to dopamine.
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Dementia:
• Loss of mental functions that is severe
enough to interfere with a person’s daily life
• Is not a disease itself but is a group of
symptoms
• Alzheimer’s disease is a common cause of
dementia
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• Alzheimer’s disease: disorder of mental impairment,
especially memory due to a shortage of the
neurotransmitter acetylcholine. Primarily affects
neurons in the limbic system and frontal lobe. See
plaques in brain tissue and abnormal, tangled neurons.
– Symptoms: Progresses from memory lapses to
severe memory loss, especially of STM. LTM
affected very slowly. Disorientation, dementia,
personality changes, loss of ability to function
independently
– Treatment: medications which increase the brain’s
production of acetylcholine
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Brain tumors: abnormal growth in or on the brain
• Can be cancerous or benign
– Problems due to increased pressure within the
brain
– Symptoms: headache, vomiting, visual
impairment, confusion, muscle weakness,
difficulty speaking, seizures
– Treatment: Surgical removal, radiation and
chemotherapy.
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