Transcript Slide 1

Nervous System
Overview
• 2 ways body communicates to send
signals.
• 1st - endocrine system - hormones.
• 2nd - nervous system - no chemical
signaling – impulses - travel through
network of cells to get to
destination.
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http://www.drstandley.com/images/nervous5.bmp
• Impulses allow organisms to receive
and respond to stimuli in
environment.
• Controls all functions of life ability to move, think, breathe, etc.
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Functional unit
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• Functional unit of nervous system neurons.
• Specialized cell designed to
transmit electrochemical signals
called action potentials (nerve
impulses).
• Signals formed by altering of
voltage across plasma membrane.
• Basic part of neuron’s structure cell body, dendrites, axon.
• Cell body contains nucleus, most
organelles - site of protein
synthesis.
• Dendrites project from body and
receive chemical information from
other neurons; carry this
information to cell body.
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http://www.nida.nih.gov/JSP/MOD3/images/NEURON2.gif
• Axon - projection of neuron that
transmits information to cell body
to target cells.
• Has to be long enough to carry
action potential from central
nervous system to extremities.
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Fig. 48.2
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Resting potential
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• Action potential requires that
voltage manipulated across
membrane.
• Altered by moving ions back and
forth across membrane (ions
charged).
• Cells have voltage across plasma
membrane generated through
actions of protein called Na+/K+
ATPase.
http://openwetware.org/images/thumb/a/a6/Action-potential.jpg/300px-Action-potential.jpg.png
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• Hydrolysis of ATP for energy protein pumps sodium ions out of
cell and potassium into cell.
• Activity essential in maintaining
osmotic balance of cells.
• Some potassium leaks back out of
cell through potassium channel.
http://scienceblogs.com/clock/upload/2006/08/Potassium-Channel-2-2004.JPG
• Potassium channel is ion channel
that selectively allows potassium
ions to flow down K+ gradient
established by ATPase.
• Resting potential about -70mVolts most positive ions on outside of cell.
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Action potential
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• Most cells maintain membrane
potential at resting potential.
• Membrane excited - potential
changed - allows information to be
carried via action potential.
• Neurons, muscle cells have ion
channel proteins in plasma
membrane that open to allow ions
through.
http://www.unm.edu/~lkravitz/MEDIA2/Action%20Potential.jpg
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• Happens in response to decrease in
membrane potential.
• Protein that does this in neurons voltage-gated sodium channel.
• When membrane potential more
negative than resting potential,
(from -70mVolts to -90mVolts),
membrane hyperpolarized.
http://courses.washington.edu/conj/membrane/chan.gif
• When membrane potential less
negative than usual, (-70mVolts to 50mVolts or 0mVolts) membrane
depolarized.
• Voltage-gated sodium channels
closed at resting potential - do not
let ions through membrane.
• Change in membrane potential
causes voltage-gated sodium
channels to open to allow sodium
through.
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• If membrane voltage becomes less
negative than resting potential (70mVolts to -50mVolts) voltagegated sodium channels in membrane
will open.
• Voltage at which voltage-gated
channels open - threshold potential.
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• When channels open, sodium
diffuses freely through channel to
cross membrane from outside of
cell into cytoplasm.
• Opening of channels in one region of
membrane, entry of sodium through
channels causes membrane
depolarization (membrane less
polarized, moving toward 0)
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• After voltage-gated sodium
channels opened and depolarization
complete, channels close rapidly
again, allowing membrane voltage to
normal potential (millisecond).
• Return of voltage to normal
negative state - repolarization.
• Section of membrane depolarizes,
triggers threshold for voltagegated channels in next section of
membrane to depolarize.
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• Action potential moves along length
of axon in wavelike manner until it
reaches end of neuron at synapse.
• Some vertebrate – neurons have
myelin - surrounds axon, allows
action potentials to travel more
quickly.
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• Myelin formed by glial cells - wrap
plasma membrane around axons,
insulating it.
• Small spaces between myelin nodes of Ranvier.
• Myelinated neurons - action
potential jumps from 1 node to
another node bypassing insulated
myelin regions where no ions cross
membrane.
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• Allows action potential to travel
much quicker - can jump forward
instead of traveling whole length of
axon - saltatory conduction.
• Larger neurons carry action
potentials more quickly.
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Size and frequency of action
potentials
• Every action potential in neuron same size.
• Once membrane reaches threshold
for depolarization - fully depolarize
- all-or-nothing response.
• Either neuron fires an action
potential or it doesn’t.
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• Strength of stimulus does not
change size of action potential
depolarization or duration of
depolarization.
• Strength of stimulus determines
intensity of action potential.
• Light touch will trigger less
frequent response.
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• Action potential same in strength
but frequency of action potentials
change with intensity of stimulus.
• When action potential passes
through section of membrane cannot carry action potential again
immediately.
• Must first finish depolarizing, then
repolarize.
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• Limit to frequency of action
potential firing in neuron refractory period.
• Places upper limit on number of
action potentials that can pass
through neuron in unit of time.
• Action potentials carried in one
direction, from cell body to end of
axon.
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The synapse
• When action potential reaches end
of neuron - neurotransmitters
released to communicate with next
cell across small gap between cells synapse.
• 2 types of synapses, chemical and
electrical.
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• At chemical synapse - action
potential reaches end of axon,
comes in contact with rounded
terminal filled with vesicles - have
neurotransmitters in them.
• Include seratonin, dopamine,
acetylcholine, and glutamate.
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• When action potential signals it,
voltage-gates calcium channels open
and allow calcium into cell.
• Calcium causes some vesicles to
fuse with plasma membrane and
release contents into synapse.
• Synapse and target cell together synaptic cleft.
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• Neurotransmitter diffuses across
synaptic cleft and binds to
receptors on target cell plasma
membrane.
• When bound, receptors will open ion
channels - allows specific ions
through membrane in response to
neurotransmitter.
• Ions cause response in postsynaptic
cell (cell after synapse)
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Fig. 48.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Each neurotransmitter has specific
receptors that it interacts with at
synapse; each receptor opens
channel that allows specific ion
through.
• Excitatory neurotransmitter binds
to receptor that depolarizes
membrane of postsynaptic cell.
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• Acetylcholine used with skeletal
muscle, diffuses across synaptic
cleft - binds to receptors for it.
• Receptors - ligand-gated ion
channels that bind Ach - open to
allow sodium ions to diffuse into
cell.
• When sodium enters - depolarizes
plasma membrane of target muscle
cell.
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• If depolarization of target reaches
threshold - action potential will be
initiated in muscle cell membrane
by voltage-gated channels, and will
be sent throughout muscle cell
membrane, which triggers muscle
cell to contract.
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• More action potentials that reach
muscle and more muscle cells
involved, stronger muscle
contraction.
• Neurotransmitter can bind to
receptor -opens to allow chloride to
enter postsynaptic membrane
causing hyperpolarization.
• Membrane potential moves away
from threshold for triggering an
action potential.
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• If neurotransmitter inhibits - more
difficult for action potential to
start in target cell.
• Most common - GABA.
• Neuron can form synapses with
many neurons; release
neurotransmitter to alter
membrane potential of target cell.
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http://huguenard-lab.stanford.edu/beta3/gaba.jpg
• Information from all synapses
neuron interacts with combined in
cell body of neuron in summation single neuron processes information
from all of its stimulating neurons.
• Then decides whether or not to
initiate an action potential itself.
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• If changes in potential of neuron
cause it to reach threshold
depolarization to open voltagegated channels, it will fire an action
potential.
• If not, neuron will not fire an action
potential.
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http://www.unm.edu/~lkravitz/MEDIA2/Action%20Potential.jpg
• To turn off signal • Once neurotransmitter released
into synaptic cleft, continues to
bind to postsynaptic receptors
unless removed from synapse.
• One way - neurotransmitter to
diffuse into surrounding fluid.
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• Another way - enzyme that
degrades neurotransmitter.
• Acetylocholinesterase acts on
acetylcholine to inactivate it.
• Pesticides, nerve gas inactivate this
enzyme.
• 3rd way - take neurotransmitter
back up into cells at synapse.
• Happens with
norepinephrine,seratonin.
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Organization of nervous system
• As complexity of organism
increases, complexity of nervous
system also increases.
• Simple organisms can respond to
simple stimuli, more complex
organisms can discern stimuli
(i.e.shades of color)
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Invertebrate nervous systems
• Protozoa - single celled, no nervous
system.
• Receptors that respond to stimuli
(heat, light, chemicals).
• Sponges - multicellular - have
almost no response to environment,
no nerves.
• Cnidarians - network of cells nerve net, located between inner
and outer layers of cells of bodies.
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• Annelids - primitive nervous system
consisting of ventral nerve cord and
anterior brain of fused ganglia.
• Arthropods - better developed
nervous system - specialized sense
organs, including sight and hearing
organs.
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Human nervous system
• Central nervous system - brain,
spinal cord.
• Brain contains all functions beyond
simple reflexes - consists of outer
portion containing neuronal cell
bodies (gray matter), inner portion
containing axons (white matter).
• CNS processes information, sends
response out to body through
neurons.
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http://www.medem.com/MEDEM/images/ama/ama_brain_stroke_lev20_thebraineffectsstroke_01.gif
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Human brain
• 1Cerebral cortex – all voluntary motor
activity - initiates responses of motor
neurons present within spinal cord.
• Controls higher functions (memory,
creative thought).
• Cortex divided into hemispheres (left
and right), with some specialization of
function between them.
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• 2Olfactory lobe – center of
reception and integration of
olfactory input.
• 3Thalamus – nervous impulses and
sensory information relayed and
integrated in this section as impulse
travels to and from cerebral
cortex.
• 4Hypothalamus – hunger, thirst,
pain, temperature regulation, water
balance controlled here.
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• 5Cerebellum – muscle activity
coordinated, modulated.
• 6Pons –relay center for cortical
fibers on their way to cerebellum.
• 7Medulla oblongata –controls vital
physiological functions - breathing,
heart rate, gastrointestinal activity
- has receptors for CO2 levels.
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Fig. 48.20
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Spinal cord is part of central
nervous system.
• Route axons to travel out of brain.
• Serves as center for reflex actions
- do not involve brain.
• Dorsal horn of spinal cord is
entrance point for sensory nerve
fibers whose cell bodies are
contained within dorsal root
ganglion.
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• Ventral horn contains cell bodies of
motor neurons - initiate muscular
contractions.
• Lower sections of brain perform
more primitive functions (spinal
cord, medulla, cerebellum);
forebrain and cortex more
advanced.
• Cortex and forebrain important in
evolution of vertebrates.
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Peripheral nervous system
• Carries nerves from CNS to target
tissues in body.
• 12 cranial nerves (head and
shoulders), 31 spinal nerves (rest of
body).
• Cranial nerves exit from brainstem,
spinal nerves exit from spinal cord.
• 2 divisions: somatic and autonomic.
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Somatic nervous system
• Innervates skeletal muscle,
responsible for voluntary
movement.
• Motor neurons release
acetylcholine (neurotransmitter ACh) onto ACh receptors in skeletal
muscle.
• Causes depolarization of skeletal
muscle leading to muscle
contraction.
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• Somatic nervous system also
important in reflexes.
• 2 types of reflexes: monosynaptic
(one synapse between sensory
neuron and motor neuron) and
polysynaptic (sensory neurons
synapse with > 1 neuron)
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• Example of monosynaptic - kneejerk response.
• When patella hit, stretch receptors
sense this - action potentials sent
up sensory neuron into spinal cord.
• Sensory neuron synapses with
motor neuron in spinal cord stimulates leg muscles to contract,
causing leg to move.
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• Example of polysynaptic is
withdrawal reflex.
• Person steps on nail - injured leg
withdraws in pain, while other leg
extends to retain balance.
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Autonomic nervous system
• Regulates involuntary functions of body.
• Innervates heart and blood vessels, GI
tract, urinary system, respiratory
system, muscles of eye.
• Innervates glands and smooth muscle
(not skeletal muscle).
• Made up of sympathetic and
parasympathetic nervous systems.
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Sympathetic nervous system
• System uses epinephrine as its
transmitter.
• Activates body for emergency
situations and actions (fight or
flight response).
• Strengthens heart contractions,
increases heart rate, dilates pupils
and bronchioles, constricts vessels
feeding digestive tract.
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• Adrenal gland regulated by this
system.
• Produces epinephrine in response to
stimulation - produces many of
same fight or flight responses.
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Parasympathetic nervous system
• Acetylcholine - primary
neurotransmitter for this system.
• System deactivates or slows down
activities of muscles and glands.
• Constricts pupils, slows down heart
rate, constricts bronchioles, dilates
vessels of digestive tract.
• Principle nerve - vagus nerve.
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