Written by Elif Gulce Batgi
Stepping into the realm of biology, few phenomena can match the complexity and beauty of the language of the nervous system. Neurons are the structures that help us perceive the universe via a combination of electrical and chemical impulses. Yet, scientists are still at the beginning of uncovering how they perform the complex evaluations that underlie our behaviors (1, 2). While likening human physiology to a machine beyond our control, scientists are making strides in illuminating the mechanisms of these machinery structures, such as the impulses reaching our brains and making sense to us. It is common knowledge that inputs received via our peripheral nervous system pass through neurons and reach relevant parts of the brain. The electrical part of this communication is known as an action potential, which is a rapid change in voltage across a membrane (3). The ability of neurons to transmit signals at remarkable speeds is a testament to the complexity of the nervous system's language.
(Image Credit: https://www.science.org/do/10.1126/article.31529/abs/2009102921.jpg.)
Before the Action Potential
Since the action potential is transmitted via electricity, the charge of the inside and the outside of the neuron changes through the axon. When the cell isn't sending signals, the inside of the neuron is negatively charged relative to the positive charge outside the cell (4). The situations of being charged negatively or positively are determined by ions, which are electrically charged atoms. Calcium contains two positive charges, sodium and potassium contain one positive charge, and chloride contains a negative charge (4). The difference between these charges is called the resting membrane potential, which means there are no opened channels that allow ions to move into the neuron. The resting potential of the average neuron is around -70 millivolts, demonstrating that the inside of the cell is 70 millivolts less than the outside of the cell (4). Until a signal comes to the neuron from the post-synaptic neuron to the pre-synaptic (receiver) neuron through dendrites via neurotransmitters, the neuron remains in the same state.
During the Action Potential
When neurotransmitter molecules bind to receptors located on the dendrites of the receiver neuron, ion channels are partially opened (5). The moment ion channels are partially open, sodium diffuses into the cell, shifting the negatively charged part of the membrane toward a less-negative polarization (6). Once the cell reaches a certain threshold (measuring approximately -60mV), sodium channels are fully opened (6). The sodium channels play a significant role in generating the action potential in excitable cells and activating transmission along the axon (4). Additionally, action potentials either happen or don’t; a partial firing of a neuron can't be discussed (4). The principle is known as the "all or none" law, similar to the binary system used in computer sciences. Sodium ions continue to leak through the channels, which depolarizes the neuron instantly to an action potential at about +55 mV. Depolarization activates sodium channels in adjacent parts of the membrane, allowing the impulse to move along the axon (5). At this time, the channels are inactivated and no longer able to flux ions (3). Then the repolarization begins as voltage-gated potassium channels (Kv) open. Although Kv has approximately the same threshold voltage as Na (sodium channels), the kinetics of Kv is much lower (3). Therefore, there is an opening of the slower Kv channels that is coincident with the inactivation of the faster Nav channels (3).
The flow of potassium ions out of the cell causes a decrease in membrane potential towards the cell’s resting voltage. When the membrane potential falls below the threshold, both the Nav and the Kv begin to close. However, the Kv has slow kinetics and remains open slightly longer than needed to return the cell to the resting membrane voltage. The brief dip in the membrane potential below the normal resting voltage is termed hyperpolarization. At this point, the sodium channels will return to their resting state, meaning they are ready to open again if the membrane potential again exceeds the threshold potential (5). Eventually, the extra K+ ions diffuse out of the neuron through the potassium leakage channels, returning the cell from its hyperpolarized state to its resting membrane potential (5).
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After the Action Potential
At the end of the firing, the neuron enters a refractory period for a millisecond, during which another action potential isn’t possible (4). During this period, the neuron returns to its resting potential. Once the neuron is "recharged," it becomes possible for another action potential to occur and convey an impulse through neurons.
(Image Credit: https://www.moleculardevices.com/sites/default/files/images/page/what-is-action-potential.jpg.)
From thirst to solving mathematical problems, an action potential is a significant function that helps us meet our needs in milliseconds. Throughout its way on the axon, ion channels create the best environment in the neuron for an impulse to be conveyed quickly. In a day, millions of action potentials fire involuntarily, electrically and, chemically, as a result of a complex but machinery nervous system.
References:
Neuroanatomy, Neuron action potential - StatPearls - NCBI bookshelf. (2021, August 11). National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK546639/.
Lall, S. (2022, May 31). How do neurons communicate (so quickly)? MIT McGovern Institute. https://mcgovern.mit.edu/2019/02/28/ask-the-brain-how-do-neurons-communicate/.
Physiology, action potential - StatPearls - NCBI bookshelf. (2022, May 15). National Center for Biotechnology Information. https://www.ncbi.nlm.nih.gov/books/NBK538143/.
What happens before, during, and after an action potential? (2005, October 19). Verywell Mind. https://www.verywellmind.com/what-is-an-action-potential-2794811.
Action potential | Biology for majors II. (n.d.). Lumen Learning – Simple Book Production. https://courses.lumenlearning.com/wm-biology2/chapter/action-potential/.
Action potential. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/action-potential.