In: Anatomy and Physiology
Describe in detail how an action potential is generated in a neuron. In your answer, identify all the phases and specify the membrane potential and the events occurring at each phase.
GENERATION OF ACTION POTENTIAL:
1. Neurons or nerve cells are excitable and secretory cells. Its membrane potential changes in the presence or absence of stimulus.
2. When a cell is not producing any stimulus, it is said to be in resting state and membrane potential is referred to as resting potential. The axoplasm of neuron contains high number of K+ ions and less number of Na+ ions whereas the fluid outside the axon consists of low concentration of potassium ions and higher concentration of sodium ions thereby creating a ionic gradient. The plasma membrane contains large no. of K+ leaky channels which allows the flow of potassium ions outside the axon leaving behind unbalanced negetive charge inside creating a membrane potential referred to as resting potential which is about -70mV. A nerve cell that exhibits a membrane potential is said to be polarized.
3. If the net inward flow of positively charged ions increases compared to resting state, the membrane becomes depolarized. This is caused by the opening of voltage gated Na+ channel due to which sodium ions move inside and the negative membrane potential becomes less negative i.e. -70mV to +30mV within a fraction of second. these rapid and large changes in membrane potential is called as action potential.
4. The voltage gated Na+ channels have an automatic inactivating mechanism which causes the channel to reclose rapidly even though membrane is still depolarized. The voltage gated Na+ channels remain in this inactivated state unable to reopen until a miliseconds after the membrane potential returns to its inital negative value i.e. -70mV. This is referred as the repolarizing phase.
5. Following the repolarizing phase, there may be an after- hyperpolarizing phase during which the membrane potential temporarily becomes more negative than resting state i.e. -90mV. This happens due to the opening of voltage gated K+ channel and closure of voltage gated sodium channel which increases the outward flow of positively charged K+ ions.
6. Refractory period- the period of time after an action potential begins during which an excitable cell cannon generate another action potential in response to a normal threshold stimulus is celled the refractory period.
7. An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed as threshold potential ( typically between -50 to -55mV) .