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1.3. Action Potentials
Introducing the Action Potentional
The action potential is vital to the daily functioning of the human body. When nerves are at rest, they have the RMP, but when they need to communicate or process information, they need to conduct action potentials.
When people talk about an 'electrical impulse' or 'nerve impulse', what they are really referring to is an action potential. Explanations and accompanying diagrams are used below to explain the formation and conduction of an action potential along a neuron. Essentially, an action potential is an ionic Mexican wave!
When people talk about an 'electrical impulse' or 'nerve impulse', what they are really referring to is an action potential. Explanations and accompanying diagrams are used below to explain the formation and conduction of an action potential along a neuron. Essentially, an action potential is an ionic Mexican wave!
The Five Stages of the Action Potential
Stage 1
An action potential must be generated by a stimulus. For example, when you place your hand on a railing, the Na+ ion stretch receptors at the tips of the sensory neurons in your hand change shape, opening their Na+ ion channels. With the Na+ ion channels open, Na+ ions begin to enter the neuron slowly. Remember that the neuron, in its resting state, is much more negative inside than outside (the RMP across the neuronal membrane is -70mV), so the influx of sodium ions is making the inside of the neuron more positive.
Side Note
What I have described here is a sensory cell that has stretch receptors embedded in it, such as a Pacinian corpuscle. However, not all neurons are equal. Sometimes, neurons synapse with each other in a big chain, or pathway, and, if this is the case, then our neuron will receive an action potential from another single neuron or multiple neurons. In this instance, the action potentials arrive in our neuron and congregate at the axon hillock (see Figure 1.1.1.), and then Stage 2 continues as below.
Stage 2
The influx of Na+ ions into the neuron occurs at a fairly slow rate until the threshold for an action potential is reached. The threshold value is around -55mV for Na+ ions. Once the threshold value is reached, many more Na+ ion channels open and a huge number of Na+ ions flood into the neuron. In fact, so many Na+ ions flood into the neuron that the membrane potential rises to +30mV. This means that the inside of the neuron is now more positive compared to the outside. The process of the neuron becoming more positive inside is called depolarisation. If the threshold value is not reached (maybe due to a weak stimulus), there will be no action potential.
Stage 3
At this stage, the electrical potential is so positive inside the neuron that Na+ ion channels begin to close, and the K+ ion channels begin to open. K+ ions begin to leak out of the neuron, and so the neuron gradually becomes more negative. Eventually, it reaches the resting membrane potential value of -70mV. This process of becoming more negative again is called repolarisation. Actually, the K+ ion channels stay open for slightly longer than they should, meaning the electrical potential reaches -70mV but then continues to become even more negative. The electrical potential eventually reaches -90mV. This 'overshoot' of K+ ion outflow and its excessive negative state is called hyperpolarisation. This has the beneficial effect of preventing another action potential from being generated too soon and becoming confused with the current action potential.
Stage 4
Once -90mV is reached, the K+ ion channels close. The Na+/K+ ATPase pump then quickly restores the balance of ions so the RMP of -70mV is achieved again. The neuron is now ready to conduct another action potential.
An action potential must be generated by a stimulus. For example, when you place your hand on a railing, the Na+ ion stretch receptors at the tips of the sensory neurons in your hand change shape, opening their Na+ ion channels. With the Na+ ion channels open, Na+ ions begin to enter the neuron slowly. Remember that the neuron, in its resting state, is much more negative inside than outside (the RMP across the neuronal membrane is -70mV), so the influx of sodium ions is making the inside of the neuron more positive.
Side Note
What I have described here is a sensory cell that has stretch receptors embedded in it, such as a Pacinian corpuscle. However, not all neurons are equal. Sometimes, neurons synapse with each other in a big chain, or pathway, and, if this is the case, then our neuron will receive an action potential from another single neuron or multiple neurons. In this instance, the action potentials arrive in our neuron and congregate at the axon hillock (see Figure 1.1.1.), and then Stage 2 continues as below.
Stage 2
The influx of Na+ ions into the neuron occurs at a fairly slow rate until the threshold for an action potential is reached. The threshold value is around -55mV for Na+ ions. Once the threshold value is reached, many more Na+ ion channels open and a huge number of Na+ ions flood into the neuron. In fact, so many Na+ ions flood into the neuron that the membrane potential rises to +30mV. This means that the inside of the neuron is now more positive compared to the outside. The process of the neuron becoming more positive inside is called depolarisation. If the threshold value is not reached (maybe due to a weak stimulus), there will be no action potential.
Stage 3
At this stage, the electrical potential is so positive inside the neuron that Na+ ion channels begin to close, and the K+ ion channels begin to open. K+ ions begin to leak out of the neuron, and so the neuron gradually becomes more negative. Eventually, it reaches the resting membrane potential value of -70mV. This process of becoming more negative again is called repolarisation. Actually, the K+ ion channels stay open for slightly longer than they should, meaning the electrical potential reaches -70mV but then continues to become even more negative. The electrical potential eventually reaches -90mV. This 'overshoot' of K+ ion outflow and its excessive negative state is called hyperpolarisation. This has the beneficial effect of preventing another action potential from being generated too soon and becoming confused with the current action potential.
Stage 4
Once -90mV is reached, the K+ ion channels close. The Na+/K+ ATPase pump then quickly restores the balance of ions so the RMP of -70mV is achieved again. The neuron is now ready to conduct another action potential.
Finally…
Stage 5
These ion movements begin at receptors in a sensory neuron and once the first set of Na+ ion channels open, a chain reaction is set off. Neighbouring Na+ ion channels open and, as mentioned before, the ion movements occur like a wave along the length of the axon until it reaches the presynaptic terminal.
These ion movements begin at receptors in a sensory neuron and once the first set of Na+ ion channels open, a chain reaction is set off. Neighbouring Na+ ion channels open and, as mentioned before, the ion movements occur like a wave along the length of the axon until it reaches the presynaptic terminal.
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