Author: Matthew

The relative refractory period is the time frame in which it is more difficult than normal to fire an action potential. An action potential can be fired, but the neuron requires a greater stimulus. This period occurs when the cell is hyperpolarized. Therefore, a new signal will have to overcome the gap between the resting and threshold potentials along with the amount the cell is hyperpolarized. This period ends when the sodium-potassium pumps return the cell to resting potential. Compare to: absolute refractory period.

(“Refractory period”) 

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The absolute refractory period is the time frame in which a neuron cannot fire another action potential. This is for one of two reasons. First, the voltage-gated sodium channels could already be opened. The channels are either opened or closed; there is no difference in magnitude during depolarization. Second, the voltage-gated sodium channels could be inactivated. This occurs during repolarization and means that no stimulation will cause the sodium channels to reopen. Towards the end of repolarization, once below the threshold potential, the sodium channels transition to a normal, closed conformation. Compare to: relative refractory period.

(“Refractory period”) 

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The refractory period is the time frame in which a neuron can’t be stimulated to fire an action potential like normal. The refractory period can be split into two separate periods: the absolute refractory period and the relative refractory period. The refractory period serves to prevent the neuron from being excited again too quickly or from being excited by the remnants of the previous stimulation signal.

(“Refractory period”) 

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Hyperpolarization occurs following repolarization. Voltage-gated potassium channels stay open past the end of repolarization to a final membrane potential between -80 mV and -90 mV. This occurs in order to interrupt the positive feedback loop of the action potential. By overshooting the resting potential, it becomes tougher to quickly re-fire an action potential. Thanks to this, action potentials can’t reverb back down an axon in the opposite direction. The axon is hyperpolarized during the relative refractory period.

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Repolarization is the process by which the neuron regains its negative resting membrane potential. Repolarization starts between +30 and +40 mV. In this range, voltage-gated sodium channels will close and voltage-gated potassium channels will open. This allows positively-charged potassium ions that are in high concentration in the cell to leave. The membrane potential will drop past the resting potential before potassium channels begin to close. This occurs around -80 mV and makes up the beginning of hyperpolarization.

 

 

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Voltage-gated potassium channels are a type of voltage-gated ion channel. These potassium channels line the axon of a neuron. They open when the membrane potential in the surrounding environment reaches about +30 mV. Potassium ions will flow out of the cell through the newly opened channels. This causes repolarization. Despite resting potential lying at -70 mV, the potassium channels will shut closer to -80 mV. This causes hyperpolarization.

(“Voltage-gated potassium channel”, 2016) 

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Voltage-gated sodium channels are a type of voltage-gated ion channel. These sodium channels in the axon of a neuron remain closed until the membrane potential rises to -55 mV. Following reaching threshold potential, the sodium channel opens and allows sodium ions to enter the cell. The sodium channel closes again when the region depolarizes to about +30 mV. The sodium channels become inactivated during repolarization. This means that no stimulus may reopen the channels. Sodium channels may be reopened during the relative refractory period, but this requires a greater-than-normal stimulus.

(“Voltage-gated sodium channel”) 

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A positive feedback loop is a cycle in which the factor that initiates the cycle is generated again at the end of the cycle, restarting the loop. In neuron signaling, the cycle of rapid membrane depolarization during the action potential is a positive feedback loop. Here, the reception of a signal from a previous cell depolarizes the region around the first set of voltage-gated sodium channels along the axon. This causes the sodium channels to open, allowing sodium ions in. This further depolarizes the surrounding area, causing the next set of sodium channels to open. This continues in a chain down the axon.

(“Positive feedback,” 1999) 

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Neurotransmitters are chemicals released by a neuron to send a signal on to the next cell in line. Neurotransmitters are collected into vesicles following the firing of an action potential and are transported out into the synaptic cleft. Once in the synaptic cleft, the neurotransmitter diffuses across the gap and binds to receptor proteins on the receiving cell. Acetylcholine is an example of a neurotransmitter. Acetylcholine is involved in stimulating muscles to contract.

(“Nervous system”) 

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The synaptic cleft is the region between the axon terminus of the signaling neuron and the receiving region of the next cell. The next cell doesn’t have to be another neuron, but it can be. Following an action potential in the signaling neuron, neurotransmitter molecules are released by that neuron into the synaptic cleft. The neurotransmitter molecules diffuse across the cleft to stimulate receptor proteins on the receiving cell.

(“Nervous system”) 

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