An introduction to action potentials

academics biology neuroscience
By Erin M.

Have you ever wondered how our brains work? Our every thought, every emotion, and every movement are generated by our brain through a vast network of cells called neurons. Neurons make connections and talk to each other through electrical signals called action potentials. 

First, let’s briefly talk about neurons. Neurons are the main type of cell in the central nervous system (so, the spinal cord and the brain) that transmits information. There are three main parts of a neuron: the soma, or cell body, the dendrites which receive information from other neurons, and the axon, which send information to other neurons. 




The electrical signal that is send down the axon is the action potential!

So, how exactly do action potentials work on a cellular level? This process can be broken down into three steps: 

Step 1: REST

Action potentials are deeply dependent on the movement of charged particles (called ions) in and out of the axon membrane. Ions move across the membrane because of an electrochemical gradient. The most important thing to know about gradients is that both sides naturally want to equilibrate (equal ion charge on both sides). In order to decrease the gradient and bring it to zero, ions will move from areas of high concentration to areas of low concentration. 

There are two main ions at play during an action potential, potassium (K+) and sodium (Na+). When a neuron is at rest, there is more Na+ outside the cell and more K+ inside the cell. In addition, the inside of the cell has a negative charge relative to the outside. 

Image from Kahn Academy

The charge on the inside of the cell relative to the outside is called the membrane potential. When a neuron is at rest, the membrane potential is -70mV, meaning inside of a resting neuron is about 70mV less positive than the outside.

 

 

Because of the electrochemical gradient, Na+ wants to move inside the cell, whereas K+ wants to move outside the cell. However, these ions cannot move through the cell membrane without the help of ion channels. Neuron membranes have both Na+ and K+ ion channels that only open under certain conditions. In this case, the membrane must reach a specific voltage. When a neuron is at rest, Na+ and K+ channels are closed. 

Step 2: DEPOLARIZATION

So now that you know what happens when a neuron is at rest, let’s talk about what happens when an action potential is triggered, a step called depolarization. First, a stimulus happens. You bump up against a table or walk out into the sun and feel the warmth on your skin. This causes fast Na+ ion channels to open. When this happens, Na+ will begin to enter the cell (moving from high to low concentration) because of the chemical and electrical gradients. Because Na+ has a positive charge, the membrane potential will start to rise from its -70mV resting value. If the stimulus is strong enough and enough Na+ enters the cell, the membrane potential will rise past -55mV and trigger the action potential to start. Anything below -55 will not trigger an action potential and is instead called a graded potential. 

Once that threshold is met, it sets off a series of events. First, more Na+ ion channels open quickly. There are a LOT of these, causing Na+ to RUSH into the cell and for the membrane potential to increase even more and become positive. 

Step 3: REPOLARIZATION

After the membrane potential reaches its peak, Na+ channels close and K+ ion channels open, causing K+ to begin flowing outside of the cell. The loss of positive K+ ions begins to reduce the membrane potential back down into the negative, and this flow of ions is even likely to overshoot and become too negative (less than -70 mV), which is called hyperpolarization. 

 

So, those are the three major steps of an action potential! 1. Rest, 2. Depolarization, and 3. Repolarization.  At this point, there is a high concentration of Na+ inside of the cell and a high concentration of K+ outside of the cell, the opposite of the resting state. In order to get back to a resting state, the cell uses a protein called the sodium potassium pump. This protein moves 3 Na+ ions outside of the cell and 2 K+ ions inside the cell, against their concentration gradients. Because the ions are moving against the gradient, this action takes energy, in this case, ATP. this pump is always working to swap the ions back around, ending the action potential and truly bringing the cell back to its original resting state.  




Erin received her Bachelor of Science in Biology from Elon University and completed her PhD in Neuroscience at the University of Virginia in 2020. While at UVA, she also received specialized pedagogy training.

Comments

topicTopics
academics study skills MCAT medical school admissions SAT college admissions expository writing English strategy MD/PhD admissions writing LSAT GMAT physics GRE chemistry biology math graduate admissions academic advice law school admissions ACT interview prep test anxiety language learning career advice premed MBA admissions personal statements homework help AP exams creative writing MD test prep study schedules computer science Common Application mathematics summer activities history philosophy secondary applications organic chemistry economics supplements research grammar 1L PSAT admissions coaching law psychology statistics & probability dental admissions legal studies ESL CARS PhD admissions SSAT covid-19 logic games reading comprehension calculus engineering USMLE mentorship Spanish parents Latin biochemistry case coaching verbal reasoning AMCAS DAT English literature STEM admissions advice excel medical school political science skills French Linguistics MBA coursework Tutoring Approaches academic integrity astrophysics chinese gap year genetics letters of recommendation mechanical engineering Anki DO Social Advocacy algebra art history artificial intelligence business careers cell biology classics data science dental school diversity statement geometry kinematics linear algebra mental health presentations quantitative reasoning study abroad tech industry technical interviews time management work and activities 2L DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay athletics business skills cold emails finance first generation student functions graphing information sessions international students internships logic networking poetry proofs resume revising science social sciences software engineering trigonometry units writer's block 3L AAMC Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian JD/MBA admissions Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser campus visits cantonese capacitors capital markets central limit theorem centrifugal force chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function fellowships freewriting genomics harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports letter of continued interest linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics music music theory nervous system