Are you tired of struggling to understand action potentials, but don’t know where look for clear information?
There is a great deal of talk in the press about the electrical circuits of the brain and whether they can be duplicated by computers. This post will describe how neurons electrically signal along their axons and will explain how the neuron’s flow of electricity is different from electrical flow in a copper wire.
Illustrations vs. real neurons
It is common practice in teaching human anatomy to use drawings. It is good to remember though that the illustrations of neurons lack much detail of the real cell.
In the central nervous system, brain and spinal cord, neurons are packed closely together. In brain engineers modeled the fluid filled space around neurons and described it as a network of pores and tunnels less than 100 nanometers across. Such tight cellular packing creates a setting where chemical communication occurs across very small spaces.
Neuron electrical signals travel across the cell the membrane
Electrical current in physiology consists of a stream of atoms called ions. Ions possess either a positive or a negative electrical charge. The quality and quantity of an ion’s charge is governed by the lack of a match between its number of protons (positive particles) and electrons (negative particles).
The difference between the ion concentration of the fluid in the small spaces surrounding neurons and the ion concentration of the neuron’s cytoplasm drives the flow of ions across the cell membrane when there are open holes in the membranes called channels.
Of course, the axon’s membrane channels are only open part of the time. Their opening and closing is a tightly regulated process. For electrical signaling by a neuron’s axon, the important ions are sodium, Na+, and potassium, K+. When a neuron is at rest the ion channels are closed and current does not flow.
What makes axon membrane channels open?
The molecules that form the channels that open and close in a cell membrane are called proteins. Proteins are made of 20 unique molecular building blocks called amino acids strung together in a long chain. Because of the different size of, and charge carried by, individual amino acids protein chains fold over on themselves. In doing so each amino acid searches for space and a comfortable local electrical environment.
When there is a change in the surrounding electrical environment a protein must readjust its amino acids and therefore its shape. In one electrical environment a protein may form an effective barrier between the fluid inside and outside the cell. In another electrical environment the same protein may open up a channel that allows ions to flow through it into the fluid on the opposite side of the membrane.
For neuron axons, a change in the electrical field of their membrane forces membrane channels into an open state. While all cell membranes have an electrical field on their inside and outside surface, not all cells have proteins that open a hole in the membrane when the electrical field changes.
An electrical field force is called an electrical potential. In the case of cell membranes, an electrical potential is created by the positive and negative ions lined up on the inside and outside of the membrane.
What is a transmembrane potential?
The transmembrane potential is a useful tool to quantitate the relative difference in the electrical potential on the two sides of a cell membrane. By definition a voltage is a difference in electrical potential between two points. When a membrane’s potential is different on its two sides, there is a voltage across the membrane. The voltage across a cell membran is called a transmembrane potential and it is expressed for cells in millivolts.
The transmembrane potential of inactive axons, those not transmitting signals, is called the transmembrane resting potential. The transmembrane resting potential of most axons is in the range of -60 to -90 millivolts. By convention in physiology the positive or negative quality of a cell membrane potential is always stated as inside electrical potential relative to outsides electrical potential.
Mechanics of the axon’s signaling process—action potentials
Voltage sensitive Na+ and K+ protein channels are closed to the flow of ions at the axon’s negative transmembrane resting potential. When the transmembrane potential moves away from its negative resting value to a sufficiently less negative voltage, the channel proteins shift their amino acid configuration into an open state allowing ions to flow from the fluid where they are in high concentration to the fluid where they are in low concentration. The positive change in transmembrane potential at the initial segment of the axon is initiated by events within the cell body.
There are separate voltage-sensitive channel proteins for Na+ and K+. The channel for Na+ opens first when the transmembrane potential becomes more positive. This allows sodium to flow into the cell where its concentration is low. The entry of Na+ makes the electrical potential on the inside of the membrane less negative than before the channel opened. This moves the transmembrane potential further from its negative resting value.
The change of the transmembrane potential to a much more positive value eventually opens the K+ voltage-sensitive channel protein. Potassium ion then flows out of the cell because the extracellular fluid is low in K+. The loss of positive charge on the inside of the membrane as K+ leaves moves the transmembrane potential back toward its resting negative value. As K+ leaves, the Na+ channel closes and the flow of Na+ stops. When the transmembrane potential is sufficiently negative the K+ channel also closes.
Propagation of action potentials
When Na+ enters an axon during an action potential, it diffuses away from its channel in all direction. Some of it travels toward the end of the axon away from the cell body, the axon terminal. This excess of Na+ makes the transmembrane potential of the next segment of axon membrane more positive and opens Na+ channels there initiating another action potential. Na+ diffusing back toward the cell body cannot depolarize the transmembrane potential enough to re-open those Na+ channels because the K+ channels are still open.
The sequence of ion channels events in the axon insures that once an action potential is triggered by the cell body it can only proceed along the axon in one direction, from the cell body to the axon terminal. While action potentials proceed in only one direction, however, the ion currents that enter the axon flow in all directions. The back flow of ions through the axon is important for informing the cell body of the level of activity in the axon.
Opening and closing of axon ion channels occurs very quickly. The entire sequence of an action potential takes about 3 one thousandths of a second. Even so, signals can come too quickly from the cell body for the axon to accommodate. There is an absolute refractory period during which the voltage sensitive channel proteins cannot respond and produce another action potential. Some axons also have a relative refractory period during which a much larger than normal membrane depolarization is necessary to elicit a new action potential.
Axon currents vs. copper wire conduction of electricity
Axon electrical current is not a constant flow of electrical particles along the axon’s length. Rather it is a series of membrane electrical potential changes that open voltage sensitive ion channels across discrete sections of the axon membrane. Flow of incoming Na+ causes the series of changes in transmembrane potential to repeat itself sequentially along the axon membrane from the cell body to the axon terminal.
When the depolarization of the transmembrane potential reaches the axon terminal it sets in motion an entirely different sequence of events that will be discussed in a later post.
In contrast, electricity moves along the length of a copper wire flowing in only one direction driven by the potential difference between the ends of the wire.
If you would like to know more about electricity in the brain, I have a new book you may like to read titled “Inside the Closed World of the Brain, How brain cells connect, share and disengage and why this holds he key to Alzheimer’s disease.” For a FREE copy of the first chapter click here.
Do you have questions?
This can be a confusing subject. Please put your questions in the comment box or send me an email at DrReece@MedicalScienceNavigator.com. I read and reply to all comments and email.
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Margaret Thompson Reece PhD, physiologist, former Senior Scientist and Laboratory Director at academic medical centers in California, New York and Massachusetts and CSO at Serometrix LLC is now CEO at Reece Biomedical Consulting LLC.
Dr. Reece is passionate about helping students, online and in person, pursue careers in life sciences. Her books “Physiology: Custom-Designed Chemistry” (2012), “Inside the Closed World of the Brain” (2015) and the workbook (2017) companion to her course “30-Day Challenge: Craft Your Plan for Learning Physiology” are written for those new to life science. More about Dr. Reece’s books can be found at https://www.amazon.com/author/margaretreece.
Dr. Reece offers a free 30 minute “how-to-get-started” phone conference to students struggling with human anatomy and physiology. Schedule an appointment by email at DrReece@MedicalScienceNavigator.com.by