- Action potential is electrical impulse transmitted through nervous tissue to transfer information’s.
- As we said, action potential will not be generated without presence of (membrane potential).
- As a preview; membrane potential is (polarization) of membrane by presence of negative charge in and positive charge out at rest, and it is created by diffusion of ions (mainly Na+ and K+) through leak channels.
- In action potential, the inside suddenly become positive, as to say, depolarization, and then return to negative (repolarization) and both occur with in (5/10.000) of second , these changes in membrane potential, occur in response to stimulus applied to nerves.
Types of stimuli which may produce action potential
- Electrical (change in potential).
- Mechanical (Touch,).
- Physical (heat, cold,….).
- Chemical (e.g. chemical substance on the skin will cause itching.
Mechanism to produce action potential
- As we said membrane potential is (-90) and it is variable number from one cell to cell, for example it may be (-70).
1- When the stimulus applied to neuron, it will cause (slight) opening of Na+ gate and result in few Na+ flow (current) from out to in.
2- This flow of Na+ from out to in will cause decrease of negativity of inside [because positive charge is entering (Na+)], let’s say from (-70) to (-50), and this (-50) is considered as a (threshold) for opening of the Na+ channel gate.
3- When membrane potential reach the threshold (-50), the Na+ gates will open and the flow of Na+ toward inside will change the inside to positive value and that’s what we call (depolarization), and this Na+ flow is called (Na+ influx).
4- When the inside of the cell become positive let’s say, Na+ gates will close gradually…. And this is the end of depolarization.
5- In the same time, when inside cell become positive, and when Na+ gates close, K+ gates will open to allow K+ to flow from inside to outside (K+ efflux) and this is the beginning of repolarization from the end of depolarization.
6- When K+ flow to outside, it is the positive charge that going out from inside cell to outside, and by this returning the cell to negative polarization (as it was)… up to now the goal is achieved (electrical impulse represented as a change from negative to positive and again return to negative) and these above changes will occur along the nerve (from the stimulus to the end of the fiber).
7- Some body may ask, is this (Na+ influx) and (K+ efflux) will change their concentration difference of these ions between outside and inside?
Answer: we need 100.000 action potential (above changes), so that the concentration difference is affected significantly some body also may ask, if this 100.000 action potential occurred, that’s means concentration difference of (Na+) and (K+) will be affected and No more action potential occur after that number?
Answer: No…… that’s not true….. because we have the famous (Na+ – K+ pump) which act after each action potential to transport (Na+) out and bring (K+) in.
- Action potential begins from the threshold.
- The stimulus if bring membrane potential to threshold, then will be action potential, and if the stimulus is weak and couldn’t bring membrane potential to threshold, there will be No action potential, but only the membrane potential will change, let’s say, from (-70) to (-60) or from (-70) to any value below (-50) [which the threshold] and this is called … (local response)…. As to say the stimulus is weak and produces slight changes in membrane potential in its site only and not action potential that generalized to the end of the fiber.
- Above point will lead us to a very important phenomena called (all or none), and that’s mean,… either there is action potential or not … we don’t have half action potential or fractions of action potential, either the stimulus is strong enough to bring membrane potential to the threshold and action potential will be generated, or Not.
- Local response may be graded as to say for e.g.: change from (-70) to (-65) or to (-60) or (-55).. etc., while in action potential there is No graded pattern.
- when K+ gates open in the beginning of (repolarization), the K+ efflux will make inside cell loose positive charge and gradually the inside will become negative until it reach the [resting membrane potential] and after it reached it, K+ gates remains opened for a millisecond and allowing to slight more K+ to flow out and by this …… the membrane potential will exceed slightly the resting state (-70) and let’s say may reach (-80) for a (millisecond) and then return to resting state (-70) at this time K+ gates closed. And this extra negativity is called hyperpolarization and (caused by delay of K+ gate closure) .
- In depolarization, the Na+ conductance (flow) will increase up to 5000 fold than resting and by this exceeding K+ conductance, and in repolarization, K+ conductance will in increase gradually until it exceed Na+ conductance [when the Na+ conductance begin to fade in repolarization.
- Summary of events
- Resting membrane potential rise to (threshold) by the effect of a stimulus (by flowing few Na+ to inside through Na+ gates).
- From the (threshold), the depolarization begin, caused by full opening of Na+ gates and (influx of Na+) and making inside positive.
- In the end of (depolarization), Na+ gates close and K+ gates open, by this, a phase of repolarization begin (caused by K+ out flow) and membrane potential reach resting state and exceed it slightly (hyperpolarization) for a millisecond period and then K+ gates close and membrane potential will be resting membrane potential again.
* Hyperpolarization caused by delay of K+ gates closure for a millisecond.
- Na+ – K+ pump will restore the normal ionic distribution after action potential
- Transfer of action potential from stimulus to the end of the fibers called [propagation of action potential.
- some body may ask if the action potential present and in the same time we applied a second stimulus, is it possible to generate a second action potential while the first one is not completed yet ?….. the answer is in following
1- Absolute refractory period: this period extend from beginning of depolarization to the end or slightly before end of repolarization.
In this period if you apply a second stimulus, there will be no action potential whatever the stimulus is strong, and this is because Na+ gates are closed and no force can open them
2- Relative refractory period: in this period (extend from small portion of end of (repolarization) to the end of (hyperpolarization).
In this period, we can generate a second action potential after first but; we need stronger than usual stimulus to do this and this is because Na+ gates are closed but be opened by strong stimulus.