Stages / Phases of Action Potential:
1. All cells have a membrane potential; however, only certain kinds of cells, including neurons and muscle cells, have the ability to generate changes in their membrane potentials. Collectively these cells are called excitable cells. The membrane potential of an excitable cell in a resting (unexcited) state is called the resting potential, and a change in the resting potential may result in an active electrical impulse.
2 Neurons have special ion channels, called the gated ion channels, that
ahoy the cell to change its membrane potential in response to stimuli the cell receives. If the stimulus opens a potassium channel, an increase in efflux of potassium will occur, and the membrane potential will become more negative. Such an increase in the electrical gradient across the membrane is called a hyperpolarization. If the channel opened by the stimulus is a sodium channel, an increased influx of sodium will occur, and the membrane potential will become .less negative. Such a reduction in the electrical gradient is called a depolarization. Voltage changes produced by stimulation of this type are called graded potentials because the magnitude of change (either hyperpolarization or depolarization) depends on the strength of the stimulus: A larger stimuls will open more channels and will produce a larger change in permeability.
- In an excitable cell, such as a neuron, the response to a depolarizing
stimulus is graded with stimulus intensity only up to, a particular level of depolarization, called the threshold potential. If a depolarization reaches the threshold, a different type of response, called an action potential, will be triggered.
- The action potential is the nerve impulse. It is a nongraded all-or-none event, meaning that the magnitude of the action potential is independent of the strength of the depolarizing stimulus that produced it, provided the depolarization is sufficiently large to reach threshold. Once an action potential is triggered, the membrane potential goes through a stereotypical sequence of changes.
- During the depolarizing phase, the membrane polarity briefly reverses, with the interior of the cell becoming positive with respect to the outside. This is followed rapidly by a steep repolarizing phase, during which the membrane potential returns to its resting level. Fig. 2.5.
- There may also be a phase, called the undershoot, during which the membrane potential is more negative than the normal resting potential. The whole event is typically over within a few milliseconds.
Role of gated ion channelgein the action potential:
The action potential arises because the plasma membranes of excitable cells have special voltage-gated channels. These ion channels have gates that open and close in response to changes in membrane potential. Fig. 2.4, 2.5
Two types of voltage-gated channels contribute to the action potential: potassium channels and sodium channels.
Each potassium channel has ,a single gate that is voltage-sensitive; it is closed when resting and opens slowly in response to depolarization.
By contrast, each sodium channel has two voltage-sensitive gates
(i) an ‘activation gate, that is closed when resting and responds to depolarization by opening rapidly, and
(ii) an inactivation gate, that is open when resting and responds to depolarization by closing slowly.
In the membrane’s resting state, the inactivation gate is open but the activation gate is closed, so the channel does not allow Na + to enter the neuron. Upon
depolarization the activation gate opens quickly, causing an influx of Na, which depolarizes the membrane further, opening more voltage-gated sodium channels and causing still more depolarization. This inherently explosive process. example of positive feed back, continues until all the sodium channels at the stimulated site of the membrane are open.
Two factors underlie the rapid repolarizing phase of the action potential as membrane potential is returned to rest. First, the sodium channel inactivation gate, which is slow to respond to changes in voltage, has time to respond to depolarization by closing, returning sodium permeability to its low resting level. Second, potassium channels whose voltage-sensitive gates respond relatively slowly to depolarization, have had time to open. As a result, during repolarization, K+ flows rapidly out of the cell, helping restore the internal negativity of the resting neuron. The potassium channel gates are also the main cause of the undershoot, or hyperpolarization, which follows the repolarizing phase. Instead of returning immediately to their resting position, these relatively slow-moving gates remain open during the undershoot, allowing potassium to keep flowing out of the neuron. The continued potassium outflow makes the membrane potential more negative. During the undershoot, both the activation gate and the inactivation gate of the sodium channel are closed. If a second depolarizing stimulus arrives during this period, it will be unable to trigger an action potential because the inacthiation gates have not had time to reopen after the preceding action potential. This period when the neuron is insensitive to depolarization is called the refractory period, and it sets the limit on the maximum rates at which action potentials can be generated. Fig. 2.6