Action Potentials, Refractory Period, and Summation

Need help preparing for the Biology section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Action Potentials, Refractory Period, and Summation of Nerve Cells/Neurons. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
Action potentials are signals that neurons send down their axons to downstream targets. These signals can be initiated from presynaptic neurons, and are essential for cell-to-cell communication. Recall that presynaptic neurons release neurotransmitters that bind to receptors on the postsynaptic neuron. These receptors open ion channels that cause the influx of positive charges. This process increases the membrane potential, which is known as an excitatory postsynaptic potential (EPSP). Likewise, a signal can decrease the membrane potential, inhibiting an action potential. These types of signals are known as inhibitory postsynaptic potentials (IPSP).
In terms of EPSPs, in order to fire an action potential, the membrane potential must exceed the threshold amount or threshold potential. For most neurons, the threshold potential is between -50 mV and -55 mV. When the cell does receive enough excitatory input to reach the threshold, then the neuron will fire an action potential. This regulatory process is known as the “all or none” mechanism. If the cell reaches the threshold, an action potential will fire. If it does not, then an action potential will not fire.
Some neurotoxins, such as tetrodotoxin from the pufferfish, saxitoxin from aquatic organisms found in seawater, or dendrotoxin from the black mamba snake, can block action potentials. In the case of the pufferfish, the effect occurs by inhibition of voltage-gated sodium channels. In terms of the black mamba snake, action potential inhibition occurs via inhibition of voltage-gated potassium channels.
Phases of the Action Potential
At the start of the action potential, the cell is at rest, so the membrane potential is the resting membrane potential of approximately -70 mV. Then, there is a stimulus. The more stimuli there are, the higher the membrane potential will increase. However, if the threshold potential is not reached, nothing will happen. However, if the stimulus is strong enough to reach the threshold, the cell will enter depolarization.
During depolarization, the membrane potential increases significantly in a positive direction. When the membrane reaches -55 mV, the voltage-gated sodium channels open, and sodium becomes the most permeable ion across the membrane. Sodium will then rush into the cell, making the cell more positive, which is why the membrane potential increases so significantly during depolarization. It is important to note that the voltage-gated sodium channels have two different gates. The gate involved in depolarization is gate M or the M gate, and the gate involved in repolarization is gate H or the H gate.
Then, the membrane will repolarize, and the voltage-gated sodium channels will inactivate. Inactivation of these channels is due to the closure of the H gate. Also, when the H gate closes, it cannot be opened again by changing the membrane potential. In other words, when the sodium channels are inactivated, they cannot be opened. Also, when the membrane potential reaches a more positive value, voltage-gated potassium channels will open. The opening of the voltage-gated potassium channels causes potassium to rush out of the cell, making the membrane more negative, which is what occurs during repolarization. Also, the membrane potential becomes so negative that it drops below the resting membrane potential, hyperpolarization.
During hyperpolarization, the voltage-gated sodium channel is going to reset back to its initial state, with an open H gate and a closed M gate. In this way, the sodium channel is no longer inactivated. It is important to note that in this state, the M gate can be opened once again by changing the membrane potential. For the H gate, however, that is not the case. If the H gate is closed, it cannot be opened. Also, during hyperpolarization, the voltage-gated potassium channels are slowly closing as well, which is why the membrane potential drops below the resting membrane potential. However, once the voltage-gated potassium channels close fully, the membrane potential will be able to recover and return to the resting membrane potential.
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[@sagalmohamed7527]

Thank you! super helpful

[@MedSchoolCoachMCATPrep]

You're welcome!