You may have heard that nerve cells (or neurons) in the brain use electrical activity to communicate with one another. The proteins responsible for creating these electrical signals are called ion channels. How do neurons use these electrical signals to communicate with one another in a meaningful way?
A good way to think about the brain is that it is wired into circuits. These brain circuits are not unlike the electrical circuits that power our home, computers, and cell phones. This means that within these circuits, an individual neuron must be able to send and receive electrical signals to and from its neighbors. The electrical signals used to transmit information down the length of a neuron are called action potentials.
Brain communication: the action potential
Neurons use the electricity harnessed from the opening of ion channels to generate an action potential. Normally, when a neuron is not active, it rests at a negative resting state (or, a hyperpolarized membrane potential). This means that ion channels are closed and the neuron is not actively sending any signals to other neurons. Don’t worry, these voltages aren’t large enough to zap you! Neurons operate in a range between -90 millivolts and +40 millivolts, which is thousands of times smaller than the voltages that power circuits in our homes!
A neuron can become activated with the opening of certain ion channels, particularly ones that allow sodium ions into the neuron. Each time a sodium ion flows into the neuron, the membrane potential becomes slightly more positively charged, or depolarized. Once a specific threshold of depolarization is reached, a huge number of sodium channels open all at once and the cell’s membrane potential moves up to +20 mV.
Interestingly, another major type of ion channel, called a potassium channel, becomes activated on a slight delay compared to when sodium channels open. When potassium channels open, a large amount of positively-charged potassium quickly exits the neuron. This exit of potassium causes the membrane potential returns to its negative resting state. This allows the membrane to become hyperpolarized back to where it began. Now the cycle can start all over when another signal tells the neuron it is time to act. This whole cycle, from -70 mV to +20mV and back again, is the definition of an action potential. Action potentials quickly travel down the neuron in a single direction. Once they reach the end, they help generate a different type of chemical signal that tells the next neuron to generate an action potential of its own. And thus, the information continues to travel through the circuit.
An action potential is an all-or-none response. This means that if the threshold voltage is not reached, the neuron will remain silent and no action potential will be fired. In most neurons, a signal from a neighboring neuron causes the opening of sodium channels to help this signal initiate. However, in certain cell types (such as Purkinje neurons), action potentials can happen spontaneously, and all the time – sometimes even hundreds of times per second!
Why do neuronal action potentials matter for cerebellar ataxia?
Researchers who study mouse models of ataxia have noticed that Purkinje neuron action potentials can undergo big changes during disease. Depending on the type of cerebellar ataxia, Purkinje neuron action potentials may take on a different shape or even disappear completely. Either of these situations could make it difficult, or even impossible, for a neuron to send proper signals to its neighbors. Some researchers suggest that improving action potential firing might be one way to improve ataxia symptoms. For this reason, identifying drugs that improve action potential firing is a major area of therapeutic research in ataxia.
Membrane potential: the electrical voltage of a cell’s outer membrane. Changes in membrane potential are controlled by the opening and closing of many different types of ion channels.
Hyperpolarized: a negatively-charged membrane potential. A neuron usually rests at -70 mV when it is silent. It returns to that voltage after an action potential is completed.
Depolarized: a positively-charged membrane potential. This usually occurs when sodium channels open during the early part of an action potential. This cuases the cell quickly jumps up to +20 mV.
Snapshot written by David Bushart and edited by Celeste Suart.