Action potential in a Neuron
In a nerve cell, communication take place by the process of excitation and conduction of electrical impulses.These impulses are the language of neurons.The stimulus which cause excitation can be electrical, chemical or mechanical.Once the nerve is excited it can produce two types of physicochemical disturbance or potential changes.One is local non-propagated potential and another is propagated disturbance which is called action potential.The electrical activity can be studied in detail by using micro electrode, electronic amplifiers and Cathode ray oscilloscope, for recording potential changes.In this article we are discussing about the changes in potentials that occur to a neuron during the time of a propagating action potential.
Before we learn about action potential we must understand some basic characteristics of neuron and how we record potential changes.Suppose two electrodes are connected using a suitable amplifier to Cathode ray oscilloscope and placed on the surface of axon, no potential change can be observed.But if one electrode is placed in the interior of the axon, a constant potential difference is observed, with the inside negative related to the outside of the axon at rest.This potential recorded in a resting neuron across the membrane is called Resting Membrane potential and it is usually about -70mV.At this point we must understand that electrical events in neurons are rapid, being measure in milliseconds (ms) and the potential change are too small and measured in millivolts (mV).
Now let us take the case where a stimulus is given to neuron using electrodes resulting in conduction of impulse.A characteristics series of potential changes occurs which is known as action potential, when the impulse passes the external electrode.To understand this potential changes we have to use a graph.The graph given below is based on standard text books in which the proportions of the tracing are changed or distorted in order to illustrate the various components of action potential.
In the beginning we can see that recording is showing -70mV which is nothing but the resting membrane potential.When a stimulus is applied, a brief irregular deflection occurs in baseline.This is called stimulus artifact.Though the reason for the stimulus artifact is the current leakage from the stimulating electrode to recording electrode, it helps us to know the point of application of stimulus in the graph.After stimulus artifact there is an isopotential interval (no change in potential) called latent period which ends with the start of action potential.It corresponds to the time the impulse take to travel from the point of stimulation to the recording electrode.One application of the latent potential is that, if we know the duration of latent period from the graph and the distance between the electrodes, we can very well calculate the speed of conduction of impulse in the axon.
As said earlier, with the end of latent period, the events of action potential begins.At the beginning there is an initial depolarization or potential change of 15mV and after that rate of depolarization increases.The point at which the change in rate occurs is called the firing level or sometimes the threshold.Then we can see that recording of the oscilloscope rapidly reaches and overshoots the zero potential line to approximately +35mV.It then reverses and falls rapidly towards the resting level.The falling back is due to the processes of re-polarization.When re-polarization is about 70% completed, the rate decreases and the tracing approach the resting membrane potential level more slowly.In the graph, what we see as a sharp rise and rapid fall are the spike potential of the neuron and the slower fall at the end of the recording is the after-depolarization.Also we can see that after reaching the resting level,the tracing overshoots slightly in the hyper polarizing direction to form a small but prolonged after-hyperpolarization.With this action potential is completed.
There is an important features of action potential that we need to discuss.It is a law called ‘All or None law’.This law is about a property of action potential which is related to the intensity of the stimulus applied.There is a particular intensity of stimulus at which action potential occurs, which is called threshold intensity.If the intensity of the stimulus is less than the threshold, then action potential won't take place.If the intensity is at the threshold or above the threshold, the action potential will take place at the same amplitude and form, as long as the experimental conditions remain the same.How much we increase the intensity above the threshold, the action potential remain the same.Thus it is a all or none condition.
Thus we have understood the characteristics features of action potential and also the potential changes occurring in the neurons.The reason for this electrical activity is the presence and movement of electrical charges across the membrane in both direction through specific channels or gates.Sodium and potassium ions play a very important role in these conduction.Thus in the form of action potential the neurons carry impulses and communicate and coordinate the activities of the organism.
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