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Resting and Action Potential


Neurons send messages electrochemically through the use of ions, chemicals with an electrical charge. Called semi-permeable membranes, they control which ions pass through and which do not. Those who cannot pass must use membrane proteins to cross the membrane. These functions are essential to action potentials.


Concentration Gradients

Concentration gradients are the basis of action and resting potentials. In simple terms, it is the difference in the concentration of a substance between two areas. In this context, it is the difference of ion concentrations between the inside and outside of a neuron. Ions will always move in the direction that balances the gradient.

For example, if the inside of a neuron has more K- ions than the outside, then there is a higher concentration of K- inside the cell and the inside is negatively charged. The K- ions would move towards the outside of the cell in order to make the inside and outside concentrations equal.


Membrane potential

Membrane potential is the difference between the amount of electrical charge inside and and outside the neuron. An average neuron has a resting membrane potential of about -70 millivolts (mV). This difference in electrical charge is caused by ions inside and outside the cell. When a neuron is at rest, the amount of Na+ (sodium) and Cl+ (chlorine) ions are at a higher concentration outside the cell and K- (potassium) and A- (organic anions) are at a higher concentration on the inside. As a result, the inside of neuron cells are more negatively charged than the outside.


K- ions are able to move freely through the cell membrane through channel proteins, and as mentioned before, will move towards the area with a lower concentration. However, leaving this to happen will change the electrical charge of the neuron. Therefore, the cell uses a sodium-potassium pump to maintain negative charge. Through the use of carrier proteins who move the ions in and out of the cell, three Na+ ions are pumped out while two K- ions are pumped in. Since more positive ions are pumped out than in, the cell can remain negatively charged.




Action potential

An action potential is when a neuron sends signals or information down to an axon, away from the cell body. The action potential is an explosion of electrical activity produced by a depolarizing current. This means that a stimulus has occurred to cause the resting potential to move toward 0 mV. As the depolarization reaches about -55 mV, a neuron will fire an action potential. This level is called the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. When the threshold level is reached, an action potential of a fixed size will always fire. Therefore, no matter the type of neuron the size of the action potential will remain the same. The neuron either does not reach the threshold or a full action potential is fired - this is known as the "ALL OR NONE" principle.


Action potentials are caused when different ions cross the neuron membrane. A stimulus first opens sodium channels (Na+) as there are more sodium ions on the outside. The creation of the sodium channels allows Na+ to stream into the cell, making the neuron more positive as it becomes depolarized.



To return to its original state, potassium channels once again open, allowing K+ to rush out of the cell. At the same time, the sodium channels begin to close. This causes the action potential to go back toward -70 mV (repolarization). When the neuron reaches -70 mV again (hyperpolarization) and returns to its resting state, it is ready to fire another action potential.





Phases of Action Potential

The action potential of a neuron consists of four phases: hyperpolarization, depolarization, overshoot, and repolarization.

  • Depolarization: also known as the rising phase, caused when Na+ rushes through the open voltage-gated sodium channels into a neuron

  • As additional sodium rushes in, membrane potential reverses its polarity

  • Overshoot: when the membrane potential is positive (in respect to the outside)

  • Is the region of the action potential between the 0 mV level and peak amplitude

  • Repolarization: the closing of sodium ion channels and the opening of potassium ion channels

  • occurs after the cell reaches its highest voltage from depolarization

  • Hyperpolarization: when the membrane potential becomes more negative at a certain spot on the neuron's membrane


Fun Facts

  1. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential.

  2. The giant axon of the squid can be 100 to 1000 times larger than a mammalian axon. The giant axon innervates the squid's mantle muscle. These muscles are used to propel the squid through the water.


Summary

  • An action potential is when a neuron sends a signal or information down to an axon

  • The movement of ions within the neuron causes action potentials

  • A threshold of -55 mV must be reached for an action potential to be sent

  • There are four main phases of action potential: depolarization, overshoot, repolarization and hyperpolarization

  • When neurons are not sending a signal it is "at rest."

  • Membrane potential refers to the difference in electrical charge between the inside and outside of a neuron

  • The resting membrane potential of a neuron is approximately -70 mV


References


What is an action potential?. Molecular Devices. (n.d.). Retrieved November 15, 2020, from https://www.moleculardevices.com/applications/patch-clamp-electrophysiology/what-action-potential#gref.


B, C. (2016). Resting Potential. BioNinja. Retrieved January 30, 2022, from https://ib.bioninja.com.au/standard-level/topic-6-human-physiology/65-neurons-and-synapses/resting-potential.html.


Cherry, K. (2021, November 19). Action Potential and How Neurons Fire. verywellmind. Retrieved December 4, 2021, from https://www.verywellmind.com/what-is-an-action-potential-2794811.


Neuroscientifically Challenged. (2014, July 25). 2-Minute Neuroscience: Membrane Potential. Youtube. Retrieved December 4, 2021, from https://www.youtube.com/watch?v=tIzF2tWy6KI.

University of Washington. (n.d.). Lights, Camera, Action Potential. Neuroscience for Kids. Retrieved November 15, 2020, from http://faculty.washington.edu/chudler/ap.html.





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