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The Action Potential

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Definition of Action Potential

Action potential is rapidly propagating electrochemical changes in the cell membrane after excitation or stimulus.

How Resting Membrane Potential (RMP) is Achieved?

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. There is also K+ leaking channels through which potassium is leaking out all the time. In resting condition, neuronal membranes are permeable to K+ but not significantly permeable for Na+. Due to this reason K+ keeps on diffusing out until this diffusion potential of K+ come near to equilibrium potential. In this way intra cellular environment becomes more and more negative. 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 a non-stimulated neuron, RMP is very near to K+ equilibrium potential.

In most of the cells RMP is between -90 to -70mv.

How Mechanical Stimulus Dependent on Sodium (Na+ ) Channels

There are touch sensitive sodium channels present in the cell membrane of neurons. These channels allow sodium influx upon touch (mechanical stimulus). This sodium influx is not capable to produce action potential across the membrane to reach threshold point.

Remember these mechanical stimulated sodium channels are very different to voltage gated sodium channels.

What is Sub Threshold Stimulus & 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. Diminutive inward current cause slight fluctuation in RMP but it does not reach the threshold potential.

This small proportional inward current is neutralized by K+ efflux; hence fluctuation in RMP graph disappears.

Polarization of Cell Membrane

Depolarization of Cell Membrane

Mechanically operated Na+ channels can be activated by any stimulus from outside, which allows sodium to move inside of the cell. If the stimulus is strong enough, then resulting influx of sodium will take the resting potential to threshold potential. 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. As sodium keeps on moving, membrane potential becomes less negative until membrane potential depolarizes even overshoot occurs. 

Just before the membrane potential really reach the Na+ equilibrium potential (e.g. +65mv), inactivation gate of sodium channels gets closed. More Na+ influx abruptly stopped and membrane may not achieve the equilibrium potential of Na+.

Repolarization of Cell Membrane

Just after depolarization of membrane, voltage dependent K+ channels open and Na+ channels closes. So, potassium efflux increases and membrane potential tends to approach K+ equilibrium potential. This leads to re-establishment of negative resting membrane potential.

Hyperpolarizing Afterpotential (Undershoot)

For a while voltage dependent K+ channels remain open even after achieving the resting membrane potential. During this time the membrane potential becomes very close to the K+ equilibrium potential.

Propagation of Action Potential

Depolarization in one part of the membrane occurs by the huge inward positive current. 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. In this way wave of depolarization travels over the membrane.

So “action potential” is a wave of depolarization followed by repolarization sweeping over the membrane and act as a signaling mechanism. A wave of depolarization is spread out as a Na+ influx wave and wave of depolarization is following as a K+ efflux wave.

Local Potential vs. Action Potential

Local Potential

Local potential is a non-propagating, small fluctuation in membrane potential. It is graded potential which depends upon the strength of stimulus; it may be small, slightly large or very large. Local potential shows phenomenon of summation.

Action Potential

An action potential is auto propagating, not graded (once they produce they will be always of the same strength in the same excitable cell). Cell undergoes action potential has been always stereotyped and do not show the phenomenon of summation.

All or None Principal

This is absolutely inappropriate that weak stimuli will produce little action potential and strong stimulus produces great action potential. Once a stimulus reaches the threshold, all the membranes will undergo action potential.

Action potential shows phenomenon of all or none. Either there is no action potential by the stimulus or whole membrane will undergo depolarization if the stimulus reaches the threshold.

What is Absolute Refractory Period?

The time period during which next action potential cannot be elicited even in the presence of any stimulus is called absolute refractory period.

The voltage gated Na+ channels open during depolarization whereas these channels become inactive during repolarization.

These channels will not open until they pass through this inactive stage. Absolute refractory period is the time when most of Na+ channels are stuck in inactive stage. Recovery of these Na+ channels from the inactive stage back to resting stage (which is achieved at resting membrane potential) is time dependent as well as voltage dependent.

What is Relative Refractory Period?

A time period when normal stimulus unable to elicit action potential, while certain strong stimulus may produce depolarization. During this time period, part of the membrane is hyperpolarized for a while (due to excessive loss of K+) and only strong stimulus may elicit an action potential.

Accommodation in Action Potential

A situation when RMP becomes more near to the threshold potential, but no depolarization takes place, is called accommodation.

If an excitable cell is present in hyperkalemic state, the RMP moves near to the threshold potential for longer time, it may lead to the closure of inactivation gates of the Na+ channels. If you give stimulus, RMP may reach the threshold potential, but depolarization will not occur because most of Na+ channels are stuck in inactive state.

Communication Between the Neurons

Once action potential moves across the neuronal membrane, it leads to release of neurotransmitters. These neurotransmitters open the ligand gated Na+ channels which eventually produce Na influx in second neuronal membrane that may lead to threshold potential and depolarize the membrane. In this way depolarization wave travels across the one neuronal membrane to another.

Information travels as electrical signals over the membrane while as chemical signals between the neurons. The cell can be depolarized by mechanical, chemical or even electrical signal.

What is Excitable Tissues/Cells?

Neuronal cells having characteristics that if given appropriate stimulation, a wave of depolarization and repolarization runs on the surface.

Every patch of membrane depolarizes by inward Na+ current which triggers the depolarization in the next area and previous patch undergoes repolarization by the outward K+ current. In this way each part of neuron successively undergoes depolarization and repolarization.

Factors Determining Velocity of Action Potential:

Conduction velocity of action potential depends upon various factors.

1. Diameter of Nerve Fibers

With the increasing diameter of nerve fibers, internal resistance to conduction decreases. The nerve fiber with wide diameter conducts fast; and the fiber having narrow diameter conducts slow.

2. Myelination of Nerve Fibers

Schwann cells are the special cells which are folded around the axons. Schwann cell contains multiple layers of membranes rich in sphingomyelin lipids around particular areas of axons. These lipid layers behave as an insulator.

3. Nodes of Ranvier

In heavy myelinated axons having gaps where axon membranes are exposed to extracellular fluid. These are called nodes of ranvier. As we know nerve conduction in myelinated axons is in fashion of saltatory conduction, due to which action potential jumps from the one node to another. Hereby node of ranvier plays important role in fast conduction

4. Saltatory Conduction in Myelinated Axons

It is another mechanism to enhance conduction velocity within axon system.

Upon stimulus depolarization occurs in one part of membrane by heavy inward current, then this current moves within the axon towards the next node of ranvier where it takes RMP to threshold potential and this nodal area undergoes depolarization. Thus depolarizing current in one node triggers the depolarization in the next nodal area. In this way, action potential jumps from one node to the next node. This type of conduction is called saltatory conduction.

In myelinated axon fibers conduction velocity may reach up to 100m/sec whereas in non myelinated fibers velocity is around 0.25m/sec.

5. Fast and Slow Sensations

Sensations (e.g. sharp pain and fine touch etc) travelling through myelinated fibers reach CNS with high velocity. Sensations (e.g. tickling, itching, dull pain, temperature, sexual sensation etc) travelling through poor myelinated fibers reach CNS with slow velocity.

6. Myelination in Central Nervous System (CNS) and Peripheral Nervous System (PNS)

Myelination of axons in CNS is done by oligodendrocytes; and Schwann cells are responsible for myelination in PNS. Many diseases of CNS and PNS are result of defective myelination. Multiple sclerosis is a disease of defective myelination in CNS and guillain-barré syndrome is an example of PNS pathology.

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