Download Biology 30 NERVOUS SYSTEM

Biology 30
NERVOUS SYSTEM
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Nervous System Overview
The Neuron
Reflex Arc
The Action Potential
The Synapse / Neurotransmitters
Nervous System Diseases
Drugs
PNS
CNS
The Brain
The nervous system is responsible for
maintaining homeostasis by responding quickly
and efficiently to internal and external
stimuli.
The
nervous system has 5 general functions:
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1. Reception – receiving information from the external and
internal environments
2. Conduction – the passage of information to specific parts
of the brain / body
3. Interpretation – organizing sensory input into events that
incorporate past experience and present sensation
4. Organization – coordinating a thought or action n
response to internal or external stimulus
5. Transmission – sending information required to execute
a reaction to stimulus
Nervous System Organization
The Neuron
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The neuron (nerve cell) is
the basic unit of the
nervous system. It
consists of several basic
parts: the cell body, the
dendrites, the axon, the
mylelin sheath, and the
nodes of Ranvier. Some
myelinated nerves are
covered with a membrane
called neurilemma, which
promotes regeneration of
damaged nerve cells in
some organisms. Glial
cells such as Schwann
cells, nourish and support
neurons.
There are 3 types of nerve cells:
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Sensory Neurons – (afferent)
have long dendrites and short
axons
- carry nerve impulses from
the sensors to the central
nervous system
Motor Neurons – (efferent)
have short dendrites and long
axons
- carry nerve impulses from
the central nervous system to
the effectors (muscles and
glands)
Interneurons – short
dendrites and long or short
axons (may be multipolar)
- carry nerve impulses within
the central nervous system
(brain and spinal chord)
Reflex Arc
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Reflexes are automatic, quick, involuntary
responses to internal or external stimuli.
They follow a special pathway of conduction,
called a reflex arc, that does not immediately
involve the brain. This allows quicker reaction
times to potentially harmful stimulus such as
touching a hot stove.
http://www.brainviews.com/abFiles/AniPatellar.htm
The reflex arc also involves
a one-way flow of
information. Sensory
neurons may stimulate a
number of inter-neurons,
which take impulses to
different parts of the central
nervous system. This is
why we are usually
conscious of stimuli that we
reflexively react to.
Action Potential
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The Action Potential is an electrochemical
event, whereby a nerve impulse is
transmitted down the length of the axon.
 The action potential occurs in a series of
stages, each marked by a specific chemical
event.
1. Resting Potential
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Voltage is (-65 mV) when a cell is resting, it must be in a polarized
state where there is an unequal distribution of + and – charges
across the membrane. This is achieved because [Na+] is higher on
the outside, while [K+] and [Cl-] concentrations are higher on the
inside. The different concentrations of Na+ and K+ are maintained
by a sodium / potassium pump. Gates for Na+ and K+ are closed
in this state.
http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_ho
mework/lect2.dcr
2. Stimulation / Depolarization
 (+ 20 mV) stimulation by a change in pH, pressure, or an
electrical stimulus cause the Na+ gates to open, and Na+
ions rush into the cell. This causes the membrane to
become depolarized, with the outside of the cell being
less positive than the inside.
3. Re-polarization
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(-70 mV) the change in electrical potential causes the
opening of the K+ channels. This results in K+ ions
rushing out of the cell, restoring the polarized state
(except the ion concentrations are reversed from the
resting state).
4. Refractory period
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(-65 mV) before the neuron can fire again, the
original resting potential must be restored. Na+
ions are pumped out of the cell and K+ ions are
pumped back into the cell, using ATP energy
The Action Potential
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In order for a nerve impulse to occur, the nerve must
be adequately stimulated, that is a minimum number of
sodium channels must open.
The Action Potential in Action
Neuron Action Potential
Threshold level – minimum depolarization that must be
reached before sufficient Na+ gates open to continue the
action potential
All or None Response – if the threshold level is not
reached, the action potential will not occur at all. If the
threshold is reached or exceeded a full action potential
will result.
Saltatory Action – the speed of the nerve impulse is
increased by the impulse jumping from node of Ranvier
to node of Ranvier, this is how myelinated neurons are
able to conduct impulses faster than non-myelinated
neurons
Propagation of the Action Potential
The intensity of the nervous response is
determined by:
1. the number of neurons that fire simultaneously
2. the frequency at which the neurons fire
3. the threshold level of different neurons (lower
threshold neurons are more likely to fire, and are
found in more “sensitive” areas
The Synapse and Neurotransmitters
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Once the action potential has reached the end of
the axon, the electrical impulse cannot continue by
the same mechanism. The information must be
chemically carried to the next axon, where the
action potential will be initiated again. The
electrical impulse stimulates the release of a
neurotransmitter from the synaptic vesicles in the
pre-synaptic membrane. The neurotransmitter will
diffuse across the synaptic cleft, and be picked up
by receptors on the post-synaptic membrane.
The Synapse
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Neurotransmitter substances – are chemicals that are
stored in the synaptic vesicles of the axon, and are released
across the synaptic cleft. These substances may continue
or inhibit the nerve impulse. They can be either excitatory
or inhibitory.
1. excitatory neurotransmitters – cause the
movement of Ca2+ ions into the cell causing
depolarization or the opening of Na+ channels to
cause depolarization
2. inhibitory neurotransmitters – prevent
depolarization by blocking Na+ channels, opening
K+ channels (causing hyper-polarization), or moving
Cl- ions into the neuron
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Summation – whether or not a neuron fires depends upon
the net effect of excitatory and inhibitory neurotransmitters
received by the neuron. If there is adequate excitation to
reach the threshold, the neuron will fire.
Integration – the degree of sensation felt, or the degree of
response created by the brain depends on the number of
neurons that fire within a nerve bundle, and how the brain
organizes all incoming stimuli.
Let’s look at nerve conduction overall…
http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/mo
dules_homework/lect3.dcr
Neurotransmitters
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There are 9 universally recognized
neurotransmitters: aspartate, glycine, GABA,
glutamate, dopamine, nor-epinephrine,
epinephrine, seratonin, and acetylcholine. Once a
neurotransmitter is released, it must act quickly
before it is broken down by enzymes or is reabsorbed by the pre-synaptic vesicles.
Some of the more common neurotransmitters
(and their enzymes) include:
Nor-epinephrine – (NE) an excitatory neurotransmitter in the
autonomic nervous system, responsible for the fight or flight reflex
 Dopamine – an excitatory neurotransmitter often associated with
behavioral states and muscle contraction
 - broken down by a class of enzymes called MAO inhibitors
 GABA – an inhibitory neurotransmitter in the CNS, may be involved
with promoting “responsible” and appropriate behavior
 Seratonin – an excitatory neurotransmitter in the with behavioral
states such as mood, sleep, attention, learning and memory
 - broken down by a class of enzymes called monoamine oxidases
MAO’s
 Acetylcholine – (Ach)an excitatory neurotransmitter in both the CNS
and PNS, often responsible for skeletal muscle contraction
 - broken down by the enzyme acetylcholinesterase (AchE)
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Once a neurotransmitter has conveyed the message to
the next neuron, its action must be blocked. This can
occur by the following mechanisms.
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1. Degradation by enzymes in the synaptic cleft
2. Re-uptake by the pre-synaptic membrane
3. Diffusion out of the synaptic cleft
4. Inability to bind due to competitive inhibitors
Drugs often interfere with the normal neurotransmitter
function.
 Close to Home Animation: Cocaine
Chemical Diseases of the Nervous
System
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Parkinson’s Disease: wide-eyed, unblinking expression, involuntary
tremor, muscle rigidity, shuffling gait
- due to dopamine deficiency, or the malfunction of dopamine
receptors
Alzheimer’s Disease: characterized by loss of memory, senility,
deterioration of cells in the basal nuclei, presence of tangles and
plaques
- possibly due to a malfunction of acetylcholine
- seems to be linked to a gene located on chromosome #21
Schizophrenia: delusions, random thoughts, disjointed thoughts,
sensory hallucinations
- may be the result of excessive activity of brain neurotransmitters
Chemical Diseases Cont.
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Huntington’s Disease: progressive deterioration of the
nervous system that leads to writhing movements, insanity
and eventually death
- seems to be caused by the malfunction of the inhibitory
neurotransmitter GABA
Depression: low affect, feeling blue, lack of or excessive
sleep and eating patterns
- seems to be linked to malfunctions in dopamine and
seratonin, perhaps caused by an excess of monoamine
oxidase enzymes
The Effects of Drugs
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A drug is – anything that is not food that alters the normal
bio-chemistry of the body in some way.
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Drugs affect the reticular activating system (RAS) in the
brain. This system sorts out incoming stimuli to the brain,
and affects the limbic system which creates pleasant or
unpleasant sensations, feelings or emotions. Some drugs
block or promote the activity of certain neurotransmitters.
Here are some examples.
Some examples…
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Alcohol: - seems to block GABA’s ability to cause Cl- uptake,
leads to lack of coordinated response, and loss of normal social
inhibitionsClose to Home Animation: Alcohol
Marijuana: may have an impact on the activity of seratonin in the
brain, not physically addicting, however this is a gateway drug and
may be psychologically addicting
Cocaine: blocks the re-uptake of dopamine, causing an adrenaline
like effect from the dopamine, as dopamine levels increase in the
synapse, the body produces less, thus making cocaine very
physically addicting
Heroine: binds to the body’s receptors for endorphins which block
the “pain neurotransmitters” called substance P, when the drug is
removed, there is a flood of pain neurotransmitter causing a
physical dependence and addiction to the drug.
Close to Home Animation: Heroin
Peripheral Nervous System (PNS)
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The PNS is composed of:
Cranial nerves – 12 pairs of sensory, motor and mixed
nerves that control the face, neck and shoulders.
Spinal Nerves – 31 pairs of nerves that emerge from the
spinal cord by two roots (branches) (one pair for each
segment)
Dorsal root nerves – contain sensory neurons and ganglia
Ventral root nerves – contain motor neurons
All other nerves not part of the CNS
The PNS is subdivided into two major
parts:
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The Somatic Nervous System – contains all the nerves that serve
the musculo-skeletal system and the sensory organs. The actions
are generally conscious and deliberate.
The Autonomic Nervous System – contains all the nerves that
serve the internal organs. The actions are unconscious and
automatic. The autonomic nervous system is also divided into the:
– A. Sympathetic nervous system – responsible for the fight or
flight response
dilation of the pupils, increased heart rate, increased breathing rate,
slowed digestion, enhanced performance, increase in blood sugar
– B. Parasympathetic nervous system – responsible for the
relaxation response (after fight or flight)
– http://itc.gsw.edu/faculty/gfisk/anim/autonomicns.swf
Central Nervous System (CNS)
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Is primarily responsible for the processing and
organization of information.
The CNS consists of two major structures:
– 1. The Brain
– 2. The Spinal Cord
The Spinal Cord
Central Cavity – contains cerebrospinal fluid
Dorsal Root Ganglion – entry of sensory neurons to spinal
cord and CNS, ganglion is the collection of cell bodies
 Ventral Root – exit of motor neurons from the spinal cord
 White Matter – contains myelinated nerve cells
 Grey Matter – contains un-myelinated nerve cells
 Meninges – 3 protective membranes surrounding the brain
(dura mater, arachnoid, pia mater)
 Cerebrospinal Fluid – circulates between the inner and
middle layers in the central canal and spinal cord. Provides
protection, nutrient / waste exchange, etc.
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Spinal Cord Functions
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center for reflex action
provides a pathway for communication between the brain
and peripheral nerves
The Brain
The Unconscious Brain
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Midbrain – reflex center for head movements in
response to visual stimuli, connects cerebrum to other
parts of the brain
Hindbrain – important for autonomic functions required
for survival
cerebellum – responsible for muscle co-ordination,
posture, coordinated muscle movement and balance
medulla oblongota – controls heartbeat, respiration,
blood pressure, reflex center for vomiting, sneezing,
hiccupping, coughing and swallowing
pons – connects the cerebrum to other parts of the brain,
regulates breathing rate
The Conscious Brain (Forebrain)
Localization of Function
Frontal Lobe – controls voluntary muscle movement,
concentration, problem solving, judgement of behavioral
consequences, conscious thought, higher intellectual
processes
 Temporal Lobe –responsible for hearing and smell,
association areas interpret sensory experiences such as
visual scenes, music, etc.
 Parietal Lobe – responsible for perceptions of touch,
temperature, pressure, pain, etc from the skin, also includes
association areas for understanding speech, thoughts and
feelings.
 Occipital Lobe – responsible for vision, combining
images with other sensory experiences
 The Secret Life of the Brain : 3-D Brain Anatomy
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Other parts of the brain
Thalamus – central relay station for sensory impulses
traveling upward from other parts of the spinal cord and
brain to the cerebrum
 Hypothalamus – contains neuro-secretory cells that
produce some hormones, controls thirst, hunger, and
controls many of the pituitary hormones
 Limbic System – includes the frontal lobes, temporal
lobes, thalamus and hypothalamus, controls pain / pleasure
feelings, emotions, memory and learning
 Corpus Callosum – structure that connects the two halves
of the brain (left and right), allowing for integrated
thoughts and coordinated responses
 Left brain – verbal, linguistic dominant
 Right brain – spatial, artistic, visual dominant
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Viewing the Brain
PET – Positron Emission Tomography
– measures emissions from radioactively
labeled chemicals that have been injected
into the bloodstream and uses the data to
produce two- or three-dimensional
images of the distribution of the
chemicals throughout the brain and body.
SPECT-Single Photon Emission
Computed Tomography
– Similar to PET, this imaging procedure
also uses radioactive tracers and a
scanner to record data that a computer
uses to construct two- or threedimensional images of active brain
regions.
MRI-Magnetic Resonance Imaging
- uses magnetic fields and radio waves
to produce high-quality two- or three
dimensional images of brain structures
without injecting radioactive tracers.
EEG-Electroencephalography
- uses electrodes placed on the scalp to
detect and measure patterns of
electrical activity emanating from the
brain.
CT-Computed Tomography Scan
- use a series of X-ray beams passed
through the head. The images are then
developed on sensitive film. This
method creates cross-sectional images
of the brain and shows the structure of
the brain, but not its function.