The neurone’s resting membrane potential represents its stability, when not stimulated or firing. The cell’s charge is –70 mV, the cell is negatively charged whereas the outside is less. The membrane controlling ions moving in and out the cell through ion channels using ion pumps (sucking in or extruding the ions) creates a negative voltage due to unequal distribution of ions:
- The protein anions, which are too large to pass the semi-permeable axon’s membrane.
- Sodium cations (Na+) are pumped out of the cell. The channels allowing the sodium back into the cell are shut during the resting membrane potential.
- Sodium and potassium (K+) ions lead to an excess of positively charged ions outside the cell, although some chlorine anions, (Cl-), are also present there.
Every neuron has its threshold: it is believed to be around –55mV in mammals, although it can vary from one neurone to another. If the stimulus is strong enough for the neurone to reach its threshold, the resting membrane potential will change to the action potential condition. We can then observe a chain reaction: when the action potential occurs, parts of the axon membrane become depolarised, causing a sodium channel to open, enabling the ion pumps to suck in sodium cations into the axon, leading to the neighbouring ion channels to open, therefore more and more sodium surges in. As a result, the neurone’s voltage changes from –70mV to +50mV (approximately). Becoming temporary positively charged, the action potential lasting for about one millisecond, the neuron is then able to fire. Within milliseconds, the ion channels close, therefore stopping sodium coming in. The excess of sodium resulted in the potassium cations to be pushed out of the cell, thus restoration of the electrical balance. In order to fire again, the neurone has to come back to its resting membrane potential, otherwise no reaction will happen: this is called the all-or-none law. (Although Charles Sherrington suggested an action potential being able to occur even when the stimulus is below the threshold: if the stimuli are repeated consecutively, they build up at the synapse until the accumulation reaches the threshold and enables action potential to take place.)
When there is action potential, as seen above, a propagation of events occurs. We will see now how the stimulus is propagated. Once the threshold reached, the impulse travels along the axon. This would happen very slowly if the axon would lack or be deprived from Myelination. Indeed, myelin enables the transmission to leap from node to node, instead of travelling across the entire parts of the axon. Myelination is believed to increase the transmission by approximately 120 times. Once the stimulus reaches the presynaptic axon terminal, it gets transmitted, passing through the synapse, to the postsynaptic receptive region: the dendrites of the next neuron. This carries on until the messenger impulse reaches its goal.
Understanding how this brief change in the neuronal conductance – the action potential – occurs enables us to appreciate its importance in human behaviour, especially concerning related diseases (multiple sclerosis due to a lack of myelin…).
BIBLIOGRAPHY and REFERENCE
- Biopsychology
- Dictionary of Psychology
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Gleitman, H., Fridlung, A.J. and Reisberg, D. (1999). Psychology, 5th Edition. Norton
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White, L. (2002). An Introduction to biopsychology. Lecture in Kingston University.