Download 5-Nervous system II: Physiology of Neurons

5-Nervous system II: Physiology of Neurons
AXON ION GRADIENTS
ACTION POTENTIAL
(axon conduction)
GRADED POTENTIAL
(cell-cell communication
at synapse)
SYNAPSE STRUCTURE & FUNCTION
NEURAL INTEGRATION
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
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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https://www.youtube.com/watch?v=6qS83wD29PY
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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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Action potentials mediated by voltage
gated channels in the axon membrane
https://www.youtube.com/watch?v=W2hHt_PXe5o
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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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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.
Ca++
Chemical synapses are
one-directional.
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https://www.youtube.com/watch?v=WhowH0kb7n0
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Excitatory vs Inhibitory Influences: Neural Integration
But also Ach,
serotonin
and others
EPSP: excitatory
IPSP: inhibitory
postsynaptic potential postsynaptic potential
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
(GABA and glycine are
the main
neurotransmitters)
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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 non-selective ion channels open and ions,
mostly Na+ move down the gradient
6. At an inhibitory synapse Cl- channels open
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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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