What is an Action Potential?
- large, rapid, reversible change in membrane potential
- once initiated, propagates across entire excitable membrane in nerves
Where does an Action Potential occur?
- starts at axon hillock
- propagates along axon
- terminates at synaptic knobs
What does the production of an AP require?
- depolarising threshold stimulus (i.e. strong enough graded potential)
- presence of voltage-gated Na+ and K+ channels
What is the relationship of the AP and depolarising stimulus?
Size of AP is independent of the depolarising stimulus
Why does the membrane become depolarized after the channels open?
Rapid influx of Na+ ions
What is the Action Potential sequence?
- neuron in resting state (-70mV)
- depolarization to threshold (-60mV)
- activation of Na+ channels, rapid depolarization (+10mV)
- inactivation of Na channels & activation of K channels (+30mV)
- return to normal permeability (-90mV)
What is the (Absolute) Refractory Period?
No stimulus is capable of generating an AP (AP is unresponsive to second stimulus).
- Na+ channels are open/closed and inactivatable
What is the Relative Refractory Period?
stronger than normal stimulus (>10mV) is required to generate an AP
(AP can respond to second stimulus before it has fully recovered from first)
What happens in the 2 phases of the Relative Refractory Period?
A: not all Na+ channels are activatable
B: membrane is hypolarised (membrane potential < resting membrane potential)
What does an AP result in?
- increased Na+
- decreased K+
What does the Na+/K+ pump do to restore the RMP?
rapidly restores ionic balance by:
- pumps Na+ out
- pumps K+ in
