Membrane Potential at Rest
Organic tissue is characterized by electrical charge in it. The cell membrane, known as sarcoplasmatic membrane, has electrochemical mechanisms that manage to keep negative charges inside, and positive charges outside. The accumulation of charges creates an electrical field across the membrane, which as any electrical field, is characterized by an electric potential. Each living cell is characterized by this potential, which is known as membrane potential. Its value at rest is different from the value during excitation.
Purpose of the Membrane Potential
The membrane potential acts as a filter. If the stimulation is small it cannot penetrate it and nothing happens. If the stimulation is large enough, it can overcome the membrane potential, penetrate inside the cell and activate it. Therefore it filters out signals that are not strong enough.
Threshold Level
The value of the electric potential, which determines whether signals are strong enough or not to be further transmitted, is the threshold value. Both muscular tissue and motoneurones have a threshold potential of -55 mV (milli-Volt). However, their rest values are different: -70 mV for nervous tissue and - 90 mV for muscular tissue. This is the reason why it’s easier to stimulate muscle through their nerves.
Action Potential
When a stimulus decreases the membrane potential below its threshold value the cell membrane inverts its polarity. That is, as soon as the membrane potential is lowered from -55 mV to a value closer to zero, the membrane triggers an automatic ion-exchange mechanism across, which switches the membrane potential from negative to positive. This polarity inversion is called Action Potential.
Purpose of the Action Potential
The Action Potential acts as the messenger of a nervous signal. The polarity inversion switches the membrane of the next cell below its threshold level; this in turn causes another action potential in the next cell, and so on as in a chain reaction.
Sequence of Action Potential Generation
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[li]At rest the membrane potential is -70 mV.
[/li][li]External preturbation, i.e. stimulus, changes the membrane potential to -55 mV.
[/li][li]Beyond the threshold value the ion exchange mechanism triggers polarity inversion, i.e. the action potential, which is transmitted along the nervous fiber.
[/li][li]The action potential excites the membrane of the next cell, propagating the action potential mechanism to the target fiber.[/ul]
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What if the Threshold is not Reached?
If the initial stimulus does not reach the threshold value, there is no transmission of action potential, and the stimulus causes only a local effects.
In the next post we’ll take a look at Electrostimulation-Induced Contraction