Author: Matthew

Voltage-gated ion channels are proteins that permit or prevent the passage of ions into the cell. In the neuron, sodium and potassium channels are of greatest interest. “Voltage-gated” refers to the capacity to change conformation based on membrane potential. This means that a voltage-gated channel will open at certain membrane voltages and close again at a different voltage.

(“Voltage-gated ion channels”, 2016) 

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Depolarization is the process by which the neuron‘s membrane potential increases positively. Since the neuron normally sits at a potential of -70 mV, increasing the potential towards 0 mV decreases the total polarity of the cell. During an action potential, rapid depolarization occurs after the cell initially depolarizes enough to reach threshold potential. After reaching threshold potential, the voltage-gated sodium channels at the base of the axon open to allow the highly concentrated sodium ions outside of the cell to spill in. This then depolarizes the environment around the adjacent sodium channels upstream along the axon. This causes those sodium channels to open. The action potential continues in this manner by way of a positive feedback loop involving depolarization and the opening of sodium channels to further depolarize the cell. At the peak of depolarization, the neuron reaches a membrane potential of +30 mV. Repolarization follows depolarization.

 

 

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The sodium-potassium pump is an enzyme complex that exchanges sodium and potassium ions across the membrane of the neuron.  The sodium-potassium pump uses ATP to send 3 sodium ions out of the cell in exchange for taking in 2 potassium ions. Without the pump, the gradient would eventually come to equilibrium since some sodium and potassium will naturally diffuse across the membrane. The sodium-potassium pump runs constantly in order to maintain the resting potential of the neuron. The pump also helps reestablish the resting potential following hyperpolarization.

(“Scheme sodium-potassium pump”)

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Threshold potential is the minimum potential difference that must be reached in order to fire an action potential. For most neurons in humans, this lies at -55 mV, so a signal to a resting cell must raise the membrane potential from -70 mV. The signal will have to overcome an even greater potential difference to reach threshold if the cell is hyperpolarized. Changes in potential that don’t reach -55 mV cause no change in the cell. Changes that exceed -55 mV do not differ in effect from those that just reach -55 mV since firing an action potential is an all-or-nothing event.

(“Action potential”)

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An action potential is the process by which a signal travels down an axon. The initiation of an action potential is called “firing.” An action potential is fired after the threshold potential is reached due to stimulation at the dendrites. After reaching threshold potential, rapid depolarization occurs at the start of the axon. Depolarization here causes depolarization in the adjacent region of the axon in a positive feedback loop. In this way, the depolarization proceeds as a wave down the length of the axon, causing neurotransmitter to be released at the terminus of the axon. Repolarization occurs following depolarization and the relative refractory period in which the cell is hyperpolarized inhibits re-firing of the action potential temporarily. An action potential cannot be fired during the absolute refractory period.

 

                                   

(“Action potential,” 2016) 

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The Nodes of Ranvier are the regions on a myelinated axon between the myelin sheaths. These regions contain the voltage-gated ion channels and sodium-potassium pumps needed to conduct an action potential. To see the role of the nodes of Ranvier in conducting an action potential, see: saltatory conduction.

 

 

 

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Saltatory conduction is the process by which an action potential travels down an axon by jumping between the nodes of Ranvier. The presence of myelin sheaths prevents sodium channels from being present in the internodal region, but the myelin helps conduct the depolarization signal along to the next node. Due to saltatory conduction, an action potential travels faster down a myelinated axon than down an un-myelinated axon.

(“Saltatory conduction”, 2016) 

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Schwann cells are a part of the nervous system. They myelinate neurons after attaching to the axon. This is a reversible process and recent research has shown that the myelination of neurons is changed throughout the lifetime of the organism.

(“Conduction in a myelinated nerve fiber”, 2012) 

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Myelin sheaths are multilayered membrane extensions off of the axon. Neurons with myelin on their axons are referred to as myelinated and the process of developing a myelin sheath is myelination. Myelin is formed by Schwann cells attaching to the axon. Myelin helps to speed up the propagation of an action potential along the axon by the process of saltatory conduction (“Myelin: an Overview,” 2015). Multiple sclerosis is a disease in which myelin degrades in the patient. This leads to weakness, numbness, muscle stiffness, and even issues with thinking (“Multiple Sclerosis,” 2016).

(“Conduction in a myelinated fiber”, 2012)

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In a cell, a chemical potential is developed when there is a difference in concentration of a chemical across a membrane. In neuron signaling, chemical potentials develop due to the high concentrations of sodium outside of the cell and high concentrations of potassium inside the cell. The potential pushes sodium to enter the neuron and pushes potassium to leave. This chemical potential is high enough that potassium will leave the cell during repolarization. It will develop a new electrical membrane potential as a result.

(“Basis of membrane potential 2”, 2011)

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