Biology Times 07.2019

Biology Times (^19)
July
revert (return) to the resting level (-70 mV).
The direction of the electrical gradient for
Na+ is reversed during the overshoot, limiting
Na+ influx.
After-hyper polarising phase (undershoot)
Voltage-gated sodium channels come to
resting state (inactivation gate opens and
activation gate closes) (Fig 6-e). Efflux of
K+ continues (voltage-gated potassium
channels are not only slow to open but also
slow to close). Consequently, the membrane
potential becomes even more negative
(-90mV) than it normally is at rest. The
membrane potential returns to its original
resting state as the voltage-gated potassium
channels close.
The action potential is the entire rapid
change in potential from threshold to
peak and then back to resting. In a neuron,
an action potential lasts for only 1
millisecond.
The Refractory Periods
The period of time after an action potential
begins during which it is impossible to
trigger another action potential in response to
a normal threshold stimulus is called the
refractory period.
During absolute refractory period, even
a very strong stimulus cannot trigger
a second action potential. This period
coincides with the period during which the
voltage-gated Na+channels are either already
activated (depolarizing phase) or inactivated
(repolarizing phase). They cannot be
reopened unless they come to resting state.
The relative refractory period is the time
during which a second action potential can