Download Text 4-Nervous system: Organization and Physiology

4-Nervous system: Organization and Physiology
ORGANIZATION
AXON ION GRADIENTS
ACTION POTENTIAL
(axon conduction)
GRADED POTENTIAL
(cell-cell communication
at synapse)
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SYNAPSE STRUCTURE & FUNCTION
NEURAL INTEGRATION
Q: What is the nervous system?
A network of billions of nerve cells linked together in a highly
organized fashion to form the rapid control center of the body
In the brain, roughly 100 billion (1011) neurons and 100 trillion
(1014) synapses (connections between nerve cells)
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Q: What does the nervous system do?
Functions include:
–Integrating center for information coming into the body from the periphery
or internally; sensation
–Generation of movement
–Regulation of many body functions
–Locus of much of what makes us human – thought self-awareness, etc.
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NERVOUS STRUCTURE ORGANIZATION
Two major divisions:
1.Central Nervous System (CNS)
-Brain & spinal cord
2.Peripheral Nervous System (PNS)
-Nervous system outside of the
brain and spinal cord
-Carries information to and from the
CNS
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NERVOUS STRUCTURE ORGANIZATION
Two divisions to PNS:
i) Somatic; voluntary (afferent and
efferent)
-12 pairs of cranial nerves
-31 pairs of spinal nerves
ii) Autonomic; visceral;
involuntary; (efferent)
-Sympathetic component (“fight/
flight”; thoraco-lumbar)
-Parasympathetic component
(“rest/digest”; cranio-sacral)
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MANY different types, shapes, and sizes
Functional types
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Among all types of neurons, myelinated neurons
conduct action potentials most rapidly
Schwann cells:
axons of PNS
Oligodendrocytes:
axons of CNS
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Neuroglia
Neural Crest!!!!
Q: What is function of neuroglia?
Schwann cells:
axons of PNS
Oligodendrocytes:
axons of CNS
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“Spines” on dendrites: change shape and strength of
connection with other nerve cells in response to
learning
Signal
Signal
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Brain Structure
Major Landmarks:
Forebrain
-Cerebrum
-Diencephalon
Corpus callosum
Brainstem
-Midbrain
-Pons
-Medulla oblongata
Cerebellum
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Cranial nerves: 12
pairs
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Spinal Nerves
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Spinal Nerves
Spinal nerve structure:
(simple version)
-“gray matter” = nerve cell
bodies
-“white matter” = nerve cell
axons
Anterior view of one
vertebra and the nearby
section of the spinal cord
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CNS
CNS = brain +
spinal cord; all
parts of
interneurons are in
the CNS
PNS
PNS: (1) afferent neurons (their activity
“affects” what will happen next) into the CNS &
(2) efferent neurons (“effecting” change:
movement, secretion, etc.) projecting out of
the CNS.
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Contrast autonomic and somatic
components of the nervous system
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Contrast autonomic and somatic
components of the nervous system
Voluntary
Command:
MOVE!
Involuntary
Command:
Rest/Digest
Involuntary
Command
FIGHT!
FLIGHT!
Skeletal
Muscle
Contraction
Heart,
smooth
muscle,
glands,
etc.
Heart,
smooth
muscle,
glands,
etc.
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Schematic diagram of the mammalian
autonomic nervous system
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Another schematic diagram of the
mammalian autonomic nervous system
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Spinal Nerves
Q: What integrates the afferent and efferent signals?
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Evolution of the
spinal nerve pattern
Not all vertebrates have
the mammalian pattern
Contrast autonomic and somatic
components of the nervous system
Voluntary
Command:
MOVE!
Involuntary
Command:
Rest/Digest
Involuntary
Command
FIGHT!
FLIGHT!
Skeletal
Muscle
Contraction
Heart,
smooth
muscle,
glands,
etc.
Heart,
smooth
muscle,
glands,
etc.
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Physiology of the nervous system
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Physiology of the nervous system
Only a very thin shell of charge difference is
needed to establish a membrane potential.
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Membrane Channels
K+ ion
Shut
Open
From Above
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Membrane Channels
• Change conformation in response to voltage change in the
surrounding membrane: “voltage gated”
• Change conformation in response to binding by an ion or other
compound: “ligand gated”
• Are selective in which ions pass through the pore in the center
• Amino acid charges around the pore can attract specific ions
• May have 3 states:
deactivated (closed),
activated (open),
inactivated (closed)
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Sodium – Potassium Pump
(Na+/K+-ATPase)
• 3 Sodium ions moved out
• 2 Potassium ions moved in
• Uses ATP to power protein conformational change
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The size of a graded
potential is proportional
to the size of the stimulus.
Graded potentials decay
as they move over
distance.
Membrane Potential (mV)
Graded potentials can be
excitatory (an action
potential is more likely to
occur), or inhibitory
where an action potential
is less likely.
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Action potentials
~100mV
An action potential is
an “all-or-none”
sequence of changes
in membrane potential
(and an example of
positive feedback).
Action potentials result
from an all-or-none
sequence of changes
in ion permeability due
to the operation of
voltage-gated Na+ and
K + channels.
The rapid opening
of voltage-gated
Na+ channels
allows rapid entry
of Na+
The slower
opening of
voltage-gated K+
channels allows K
+ exit
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Flash animations:
1.Voltage gated channels
in the axon membrane
“links” on site
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Action potentials
Four action potentials, each the result of a stimulus
strong enough to cause depolarization, are shown. Note
that all are the same height.
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Action potential propagation down the axon
The propagation of the action potential from
the dendrites to the axon-terminal end is
typically one-way because the absolute
refractory period follows along in the “wake”
of the moving action potential; the AP starts
at the neuron initial segment.
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Action potential propagation down the axon in
myelinated axons
Saltatory conduction: action potentials jump from one node to the
next as they propagate along a myelinated axon.
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Remember the synapse … when the action potential
arrives at the axon terminals …
The synapse is the point of
communication between
two neurons.
Chemical synapses have a
synaptic cleft (about 10 –
20 nm wide) and
neurotransmitter diffuses
across the cleft to bind to
receptors on the
postsynaptic neuron
membrane.
Chemical synapses are
one-directional.
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NOTE:
1. The role of voltage-gated
calcium channels
2. Vesicles with
neurotransmitter
3.Neurotransmitter binding to
postsynaptic receptors (often
ligand-gated ion channels)
4.Re-uptake and enzymatic
breakdown of neurotransmitter
5. At an excitatory synapse nonselective ion channels open and
ions, mostly Na+ move down
the gradient 6. At an inhibitory
synapse Cl- and K+ channels
open
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1.EPSP: excitatory
postsynaptic potential
Key point: An action
potential in a presynaptic
neuron results in a
graded potential in the
postsynaptic neuron.
2. IPSP: inhibitory
postsynaptic potential
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Neural integration
•Real neurons receive as many as 200,000 synapses each
•Ion flows from all inputs summate or average at the initial segment
•An action potential in the postsynaptic neuron occurs if the membrane
potential at the initial segment reaches threshold
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