Absolute Refractory Period: Definition & Significance

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson is on the absolute refractory period, a time when a neuron cannot fire another impulse. This lesson will explain the mechanism behind the absolute refractory period and the function.

What is the Absolute Refractory Period?

Do you ever wonder how feelings and sensations get from the environment to your brain? The answer is your brain cells, called neurons! Neurons communicate by sending messages between each other, using electrical and chemical signals. Neurons have a maximum amount of signals, or impulses, they can send per unit time. The time that they must rest, and not send another impulse, is called the absolute refractory period.

How Do Neurons Communicate?

Neurons send messages using electrical and chemical signals. The message starts when a neuron receives chemicals, called neurotransmitters at the dendrites. The neurotransmitters cause the neuron to become more positive inside the cell. This is called depolarizing. If a neuron depolarizes enough, a signal, called an action potential is sent down the axon towards the synaptic terminal, where it will send the signal to the next neuron.

Action potential being sent down a neuron
Action potential

How Does the Action Potential Happen?

The axon conducts the electrical signal using channel proteins that allow positive ions in, or out of the cell. First, when an axon receives enough stimuli to fire an action potential voltage-gated sodium channels open. Sodium floods into the cell because there is more sodium outside the cell than inside. Think of it like a concert. Everyone waits outside the venue, and when the doors finally open, all the concertgoers rush into the building. The sodium is the concertgoers and the doors are the sodium channel. Below is an image of sodium rushing through voltage-gated sodium channels as they open. Sodium is yellow and potassium, another ion we will see later, is purple.

Voltage-gated sodium channels allow sodium into the axon
Voltage gated sodium channels

After a specific period of time, the first voltage-gated sodium channels slam shut, preventing any more sodium from coming into the cell. Keeping with our concert analogy, this is when the band starts playing and late comers missed their chance to get into the show. The doors to the show close, and there is no more entry. The doors, again, are like our sodium channels and the concertgoers are like the sodium.

Voltage-gated sodium channels closing
Voltage gated sodium channels closing

At the same time, voltage-gated potassium channels open. These channels let the positive ion potassium flow out of the cell. This makes the axon more negative and resets the cell for another action potential. We call this repolarizing. This is like when our concert ends and the concertgoers rush out of the venue. The venue resets and is ready for the next show. Below is an image of a voltage-gated potassium channel opening. The potassium is shown as the dark blue circles.

Voltage gated potassium channel

This process repeats over and over down the axon until it reaches the synaptic terminal. There, the message is converted into a chemical signal and sent to the next neuron.

The synaptic terminal where an electrical signal is converted to a chemical signal
Synaptic terminal

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