Action PotentialĪn action potential is auto propagating, not graded (once they produce they will be always of the same strength in the same excitable cell). Local potential shows phenomenon of summation. It is graded potential which depends upon the strength of stimulus it may be small, slightly large or very large. ![]() Local potential is a non-propagating, small fluctuation in membrane potential. A wave of depolarization is spread out as a Na + influx wave and wave of depolarization is following as a K + efflux wave. So “action potential” is a wave of depolarization followed by repolarization sweeping over the membrane and act as a signaling mechanism. In this way wave of depolarization travels over the membrane. Propagation of this depolarization occurs by the spread of local currents in adjacent areas of membrane which bring the resting potential of the adjacent part to threshold potential. During this time the membrane potential becomes very close to the K + equilibrium potential.ĭepolarization in one part of the membrane occurs by the huge inward positive current. Hyperpolarizing Afterpotential (Undershoot)įor a while voltage dependent K + channels remain open even after achieving the resting membrane potential. ![]() This leads to re-establishment of negative resting membrane potential. So, potassium efflux increases and membrane potential tends to approach K+ equilibrium potential. Just after depolarization of membrane, voltage dependent K + channels open and Na + channels closes. More Na + influx abruptly stopped and membrane may not achieve the equilibrium potential of Na +. +65mv), inactivation gate of sodium channels gets closed. Just before the membrane potential really reach the Na + equilibrium potential (e.g. As sodium keeps on moving, membrane potential becomes less negative until membrane potential depolarizes even overshoot occurs. At threshold potential, voltage gated sodium channels open and suddenly a lot of sodium moves inside and try to bring the membrane potential towards its own Na+ equilibrium potential. If the stimulus is strong enough, then resulting influx of sodium will take the resting potential to threshold potential. Mechanically operated Na+ channels can be activated by any stimulus from outside, which allows sodium to move inside of the cell. Polarization of Cell Membrane Depolarization of Cell Membrane This small proportional inward current is neutralized by K + efflux hence fluctuation in RMP graph disappears. Diminutive inward current cause slight fluctuation in RMP but it does not reach the threshold potential. If we apply a very little stimulus which is not enough to reach the threshold potential, this stimulus is known as sub threshold stimulus. What is Sub Threshold Stimulus & Potential? Remember these mechanical stimulated sodium channels are very different to voltage gated sodium channels. This sodium influx is not capable to produce action potential across the membrane to reach threshold point. These channels allow sodium influx upon touch (mechanical stimulus). There are touch sensitive sodium channels present in the cell membrane of neurons. How Mechanical Stimulus Dependent on Sodium (Na + ) Channels In most of the cells RMP is between -90 to -70mv. In a non-stimulated neuron, RMP is very near to K + equilibrium potential. ![]() As membrane is not absolutely permeable to K + and impermeable to Na +, so it makes the RMP a little less negative than equilibrium potential of K +. In this way intra cellular environment becomes more and more negative. Due to this reason K + keeps on diffusing out until this diffusion potential of K + come near to equilibrium potential. In resting condition, neuronal membranes are permeable to K + but not significantly permeable for Na +. There is also K + leaking channels through which potassium is leaking out all the time. Like all other cells, neurons have Na + /K + -ATPase that pump throw three sodium outside of the cell and accumulate two Potassium ions into the cell. How Resting Membrane Potential (RMP) is Achieved? Master Medical Sciences with crystal clear concepts.Īction potential is rapidly propagating electrochemical changes in the cell membrane after excitation or stimulus. ![]() Not a member? Sign up now to access 800+ videos on Basic Medical Sciences & Clinical Medicine. Action Potential Watch Free Online Medical Video Lecture on The Action Potential PART 1
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