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.