BIOL273 Lecture Notes - Skeletal Muscle

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3 Feb 2013
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1d) The action potential
Different from graded potential:
i. amplitude in a single neuron not graded, always the same size (size doesn't
depend on the triggering event) , change in membrane potential of 100mV (-70mV to
+30mV)
ii. Action potentials' amplitude does not decrease as they travel across neuron's
membrane = suitable for long-d signal transmission
- also has same duration
iii. graded potentials arise from ligand-gated channels in the membrane while action
potentials arise from voltage-gated channels (opens when receives pulse of electricity)
- Na+ and K+
- open in response of depolarization (depolarization of a certain size = -70
must be brought to -55mV i.e. -15mV in size) of membrane (won't open in
sub-threshold = smaller than = -15mV)
- Na+ channels open instantly
- K+ channels open after a delay of about half a millisec
- Na+ channel has two gates (illustrated in notebook)
- transmission of signal down the length of an axon
- when Na+ ions flood into cell (opening of the Na+ channels), cell
depolarizes, adjacent voltage-gates channels pop open at the
same time (slightly afterwards) = positive feedback (inflow of
Na+ > voltage > Na+ channels opens > inflow of Na+ > more
Na+ channels open > wave of depolarization sweeps down
entire length of axon
- amplitude and duration is the same because it depends on two constant
factors:
a) the RMP
b) length of time that the Na+ channels are open
* Refractory periods: make the neuronal membrane much
less responsive to stimulation immediately after an action
potential has begun. > no new action potential can be started
during the absolute refractory period, no matter how strong the
stimulus
* absolute refractory period
* relative refractory period: action potential can be initiated, but
only by a stimulus much stronger than normal because
some of the Na+ channels have reset to original
condition of being responsive but some K+ channels are
still open (outward flow of positive charge of the cell so
more Na+ channels will have to be open than normal to
compensate)
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