2.7 (2) Action Potential (Dr. Talbot)
Know the basic terminology associated with changes in membrane potential $$$ (Resting potential, Hyperpolarization, Depolarization, Threshold potential, AP, Graded potential)
1.) Resting potential = potential maintained across membrane of excitable cells (neurons, muscle)
2.) Hyperpolarization = Vm more negative than rest
3.) Depolarization = Vm less negative (or positive) than rest
4.) Threshold potential = the level of depolarization that triggers an AP
5) Action potential = a rapid, large regenerative depolarization; it is a process and describes a change over time.
– Depolarizing stimulus of sufficient intensity induces AP
– voltage-gated ion channels
– NO decay
6) Graded potential = Amplitude of voltage deflection is variable and dependent upon stimulus intensity.
– Magnitude of voltage deflection proportional to rate of current flow.
– Can occur along axonal domain as long as depolarization is less than threshold
Know the different membrane domains in the neuron,
1) Somatodendritic domain (basal)
– cell body & dendrites
– Ligand-gated ion channels & GPCRs
– Graded response – passive spread
2) Axonal domain (apical)
– Voltage-gated NaK ion channels
– All or none response
– AP starts at axon hillock!
Know the ionic basis of an action potential in a neuron
– Conduction signal at axon hillock -> opens voltage-gated channesl -> Na+ rushes into cell and depolarizes membrane
1) *Presence of voltage-gated Na+ and K+ channels
2) Maintenance of resting Vm (-)
– Function of Vm change over time
Know which ion channels are responsible for which parts of the action potential profile
1) Resting state: ALL voltage-gated Na+ and K+ channels closed
2) Depolarization phase: voltage-gated Na+ channels open, INCREASE in Na+ permeability (Na+ rushes in)
3) Repolarization phase: voltage-gated Na+ channels inactive and K+ channels open (K+ exits)
4) Hyperpolarization phase: K+ channels slowly close and Na+ channels transition from inactive back to closing
NB: – Voltage-gated Na+ channels open quickly, inactivate quickly. Voltage-gated K+ channels open slowly, inactivate slowly. – Not just change in Vm that induces response but the response of an entire population of channels
K= koala = slow creatures
What is inactivation and why is it important? Know how inactivation relates to the refractory period.
Inactivation of voltage-gated Na+ channels means that channel is unable to respond to depolarization (change in Vm). It is voltage insensitive.
Relation to refractory (unresponsive) period: Inactive voltage-gated channels ensure refractory periods (membranes unresponsive to further depolarizations)
– Two gates in voltage-gated Na channel: voltage sensitive gate and inactivation particle
– Closed voltage-gated Na+ channels are voltage sensitive
–Absolute refractory period – no AP EVER.
– Relative refractory period – smaller than normal heigh AP possible (and requires a larger than normal stimulus)
– Magnitude of AP is a function of the number of Na+ channels that are open
Know how positive feedback is used to help develop an action potential (Hodgkin Cycle)
– Example of positive feedback– depolarization reinforced
– Any stimulus that depolarizes membrane enough above threshold that causes voltage-gated Na+ channels to open will further cause other neighboring Na+ to open.
Outside mechanism = spontaneous inactivation of the channels
– Depolarization in one place will cause depolarization in another
Know how action potentials are propagated down an axon
1) Passive spread of depolarization – change in Vm spreads in all directions from point source without requiring proteins (electronic spread) but magnitude decreases with distance.
2) AP propagate down an axon without decay – stimulus above threshold will create AP that propagates w/o decay bc a new AP is regenerated at each point along the membrane (Amplitudes constant and maximal)
3) Nodes of Ranvier and Myelin Sheath move AP by saltatory conduction – AP regenerated at each node as Vm depolarizes again
Clinical: MS attacks myelin sheath
1) Axonal diameter – the larger the diameter, the lower the cytoplasmic resistance
2) Whether axon is myelinated or not (by Schwann cells) More myelinated axon = increased membrane resistance
Passive spread of change in potential governed by:
1) Shape of cell
2) Resistance to ion flow across membrane and cytoplasm (Length constant directly related to membrane resistance, inversely related to resistance of cytoplasm) Rmem generally > Rcyto
3) Capacitance of membrane
NB: Length constant determines how far signal will move before decay from max value by 63%
Blocks fast voltage-gated Na+ channel of neurons and striated muscle. Hyperpolarization will occur at that single point but single will not be propagated.
– K+ efflux will always cause inhibitory post synaptic potentials (IPSP)
– Cl is usually inhibitory PSP
– NA+ influx -; EPSP