Transcript Nerve and muscle signalling

Sci2 Lect. 3- Action Potential
© Dr Bill Phillips 2002, Dept of Physiology
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Need for amplified signalling
Depolarisation initiates neuronal signalling
Voltage gated sodium channels
Voltage gated potassium channels
Ratio of permeabilities
Na + Channel inactivation
Information is encoded in by AP frequency
Need for amplified signalling
Muscle fibre
Mot or neurone
Nucleus
Local circuit current spread
depolarisation along membrane
Inward
current
The membrane has electrical properties that affect
the spread of changes in membrane potential
Membrane has
electrical capacitance
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Ion channels in the
membrane have resistance
but allow leakage of local
circuit currents out of the membrane
The axoplasm is narrow
and has resistance
impedes the depolarising
current down the membrane
Electrical components of the
resting membrane
Axon Equivalent circuit
+
Vm
Dist ance along dendrit e or axon
Depolarisation initiates neuronal
signalling
• Membrane is depolarised when there is a
net inward current
• Depolarisation is usually initiated by
opening of ligand gated cation channels at
an excitatory synapse
• Depolarisation can also be triggered
artificially, say by applying an electrical
stimulus (Prac class)
Voltage gated sodium channels
Vm
Hodgkin Cycle
PNa
PK
Voltage-gated sodium channels
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Closed when the membrane is polarised
Begin to open as the membrane depolarises
Selectively permeable just to Na+
Begin to inactivate as the membrane
depolarises
• Inactivation shuts off the Na+ current flow
• ‘voltage gates’ and ‘inactivation gates’
Hodgkin Cycle
• Stimulus depolarisation opens a small
fraction of voltage-gated Na+ channels
• Increase in inward current through these
Na+ channels further depolarises the
membrane
• As membrane depolarises more, a greater
fraction of the Na + channels open leading
to more depolarisation etc
Voltage-gated Na+ channels can exist in multiple states
depending upon the recent history of depolarisation
CLOSED
depolarisation
OPEN
time
repolarised
INACT IVAT ED
time
depolarised
Voltage gated potassium channels
Vm
PNa
Slow opening of
voltage-gated K+ channels
PK
Voltage gated potassium channels
• Mostly closed when the membrane is
polarised
• Begin to open as the membrane depolarises
• Do not inactivate
• Fraction of channels open increases
proportionately with depolarisation
Ratio of permeabilities
Vm
PNa
PK
Ratio of permeabilitiesDuring the action potential:
• Stimulus begins to open voltage-gated Na+
channels
• If sufficient, the Hodgkin Cycle begins
• As the membrane rapidly depolarises further, Na +
channels begin to inactivate and K + channels
begin to open
• Shutting off of the inward Na + current and
increase in the outward K + current repolarises the
cell
Limits to Action Potential
Frequency
• Voltage-gated Na+ channels stay inactivated
for a fraction of a millisecond after the
action potential preventing a second AP
• After the action potential there is a brief
hyperpolarisation (period of reduced
excitability). Effect?
Information is encoded by AP
frequency
• When they become depolarised most
neurons fire a ‘train’ of several to hundreds
of action potentials (spikes)
• The frequency of spikes within a trains
usually encodes the intensity of the
sensation or instruction
• Trains of spikes are usually interspersed by
periods of silence