Action potential

Definition

Action potentials are nerve signals. Neurons generate and conduct these signals along their processes in order to transmit them to the target tissues. Upon stimulation, they will either be stimulated, inhibited, or modulated in some way. 

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Steps

But what causes the action potential? From an electrical aspect, it is caused by a stimulus with certain value expressed in millivolts [mV]. Not all stimuli can cause an action potential. Adequate stimulus must have a sufficient electrocal value which will reduce the negativity of the nerve cell to the threshold of the action potential. In this manner, there are subthreshold, threshold, and suprathreshold stimuli. Subthreshold stimuli cannot cause an action potential. Threshold stimuli are of enough energy or potential to produce an action potential (nerve impulse). Suprathreshold stimuli also produce an action potential, but their strength is higher than the threshold stimuli. 

So, an action potential is generated when a stimulus changes the membrane potential to the values of threshold potential. The threshold potential is usually around -50 to -55 mV. It is important to know that the action potential behaves upon the all-or-none law. This means that any subthreshold stimulus will cause nothing, while threshold and suprathreshold stimuli produce a full response of the excitable cell. 

Is an action potential different depending on whether it’s caused by threshold or suprathreshold potential? The answer is no. The length and amplitude of an action potential are always the same. However, increasing the stimulus strength causes an increase in the frequency of an action potential. An action potential propagates along the nerve fiber without decreasing or weakening of amplitude and length. In addition, after one action potential is generated, neurons become refractory to stimuli for a certain period of time in which they cannot generate another action potential.

Phases

From the aspect of ions, an action potential is caused by temporary changes in membrane permeability for diffusible ions. These changes cause ion channels to open and the ions to decrease their concentration gradients. The value of threshold potential depends on the membrane permeability, intra- and extracellular concentration of ions, and the properties of the cell membrane. 

An action potential has three phases: depolarization, overshoot, repolarization. There are two more states of the membrane potential related to the action potential. The first one is hypopolarization which precedes the depolarization, while the second one is hyperpolarization, which follows the repolarization.

Hypopolarization is the initial increase of the membrane potential to the value of the threshold potential. The threshold potential opens voltage-gated sodium channels and causes a large influx of sodium ions. This phase is called the depolarization. During depolarization, the inside of the cell becomes more and more electropositive, until the potential gets closer the electrochemical equilibrium for sodium of +61 mV. This phase of extreme positivity is the overshoot phase.

After the overshoot, the sodium permeability suddenly decreases due to the closing of its channels. The overshoot value of the cell potential opens voltage-gated potassium channels, which causes a large potassium efflux, decreasing the cell’s electropositivity. This phase is the repolarization phase, whose purpose is to restore the resting membrane potential. Repolarization always leads first to hyperpolarization, a state in which the membrane potential is more negative than the default membrane potential. But soon after that, the membrane establishes again the values of membrane potential.

After reviewing the roles of ions, we can now define the threshold potential more precisely as the value of the membrane potential at which the voltage-gated sodium channels open. In excitable tissues, the threshold potential is around 10 to 15 mV less than the resting membrane potential.

Refractory period

The refractory period is the time after an action potential is generated, during which the excitable cell cannot produce another action potential. There are two subphases of this period, absolute and relative refractoriness. 

Absolute refractoriness overlaps the depolarization and around 2/3 of repolarization phase. A new action potential cannot be generated during depolarization because all the voltage-gated sodium channels are already opened or being opened at their maximum speed. During early repolarization, a new action potential is impossible since the sodium channels are inactive and need the resting potential to be in a closed state, from which they can be in an open state once again. Absolute refractoriness ends when enough sodium channels recover from their inactive state.

Relative refractoriness is the period when the generation of a new action potential is possible, but only upon a suprathreshold stimulus. This period overlaps the final 1/3 of repolarization. 

Propagation of action potential

An action potential is generated in the body of the neuron and propagated through its axon. Propagation doesn’t decrease or affect the quality of the action potential in any way, so that the target tissue gets the same impulse no matter how far they are from neuronal body.

The action potential generates at one spot of the cell membrane. It propagates along the membrane with every next part of the membrane being sequentially depolarized. This means that the action potential doesn’t move but rather causes a new action potential of the adjacent segment of the neuronal membrane.

We need to emphasize that the action potential always propagates forward, never backwards. This is due to the refractoriness of the parts of the membrane that were already depolarized, so that the only possible direction of propagation is forward. Because of this, an action potential always propagates from the neuronal body, through the axon to the target tissue.

The speed of propagation largely depends on the thickness of the axon and whether it’s myelinated or not. The larger the diameter, the higher the speed of propagation. The propagation is also faster if an axon is myelinated. Myelin increases the propagation speed because it increases the thickness of the fiber. In addition, myelin enables saltatory conduction of the action potential, since only the Ranvier nodes depolarize, and myelin nodes are jumped over. 
In unmyelinated fibers, every part of the axonal membrane needs to undergo depolarization, making the propagation significantly slower. 

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Synapse

A synapse is a junction between the nerve cell and its target tissue. In humans, synapses are chemical, meaning that the nerve impulse is transmitted from the axon ending to the target tissue by the chemical substances called neurotransmitters (ligands). If a neurotransmitter stimulates the target cell to an action, then it is an excitatory neurotransmitter. On the other hand, if it inhibits the target cell, it is an inhibitory neurotransmitter.

Depending on the type of target tissue, there are central and peripheral synapses. Central synapses are between two neurons in the central nervous system, while peripheral synapses occur between a neuron and muscle fiber, peripheral nerve, or gland.

Each synapse consists of the:

• Presynaptic membrane – membrane of the terminal button of the nerve fiber 
• Postsynaptic membrane – membrane of the target cell 
• Synaptic cleft – a gap between the presynaptic and postsynaptic membranes

Inside the terminal button of the nerve fiber are produced and stored numerous vesicles that contain neurotransmitters. When the presynaptic membrane is depolarized by an action potential, the calcium voltage-gated channels open. This leads to an influx of calcium, which changes the state of certain membrane proteins in the presynaptic membrane, and results with exocitosis of the neurotransmitter in the synaptic cleft.

The postsynaptic membrane contains receptors for the neurotransmitters. Once the neurotransmitter binds to the receptor, the ligand-gated channels of the postsynaptic membrane either open or close. These ligand-gated channels are the ion channels, and their opening or closing will cause a redistribution of ions in the postsynaptic cell. Depending on whether the neurotransmitter is excitatory or inhibitory, this will result with different responses. 

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Related articles

Articles within this topic:

• Chemical synapses
• Ion channels and gradients
• Ligand-gated ion channels
• Mechanically-gated ion channels
• Membrane potential
• Neuropeptides
• Neurotransmitters
• Postsynaptic potentials
• Refractory periods
• Saltatory Conduction
• Synapse
• Voltage-gated ion channels