What is happening when a neuron fires?
Answer
Neural transmission, the process by which cells of the nervous system send signals throughout the body, is very important to understand when studying anatomy and physiology of the brain. A lot goes into creating a neural impulse, but it doesn’t have to be confusing! This guide will break neural transmission into a few simple steps.
Attached at the bottom of this page are a few diagrams that go along with this explanation for visual support.
Important vocabulary:
- Na+: Sodium ion, which is found at the axon hillock and the Nodes of Ranvier of the neuron.
- Ca+: Calcium ion, which is found at the terminal buttons at the end of the axon.
- K+: Potassium ion, which is found at the Nodes of Ranvier
- mV: millivolts; the measure of electrical charge within the neuron.
Step “0”: The Neuron at Rest
At rest, the neuron is semipermeable
- Semipermeable means that ions (Na+, Ca+, K+) can travel (almost) freely between the inside and outside of the cell.
There are two types of gates/channels that determine what can and cannot enter/exit the cell.
- Ligand-gated/ionotropic receptors: open and close in the presence of specific chemicals.
- For example, an ionotropic gate may only open in the presence of Na+, but would not allow K+ to enter.
- Voltage-gated receptors: open and close in response to change in electrical charge.
- For example, if the charge of the cell becomes +60 mV, certain gates may be triggered to open.
Two gradients are present in the neuron at rest.
- Chemical gradient: also called a concentration gradient; in neurons at rest, there is a higher concentration of Na+ outside the cell, and a higher concentration of K+ inside the cell.
- Electrical gradient: refers to the electrical charge of the cell; the resting potential of the neuron is -70 mV.
- Chemical and electrical gradients go hand in hand. Movement of ions into and out of the cell will affect the resting potential (electrical charge) of the cell.
- Local potentials: even at rest, ions are constantly moving into and out of the cell. This means that the charge of the cell is constantly changing, and these small changes of electrical charge are called local potentials.
- It is important to note that the cell remains around -70 mV, even when these potentials occur.
- Action potentials: when a local potential is strong enough that the cell’s charge becomes -55 mV, an action potential occurs. The action potential is what starts the firing of the neuron.
- This is an “all or nothing” response; -56 mV isn’t enough. It must be at least -55 mV or nothing happens.
Step 1: Depolarization
The cell at rest is polarized, meaning there are opposite charges on the inside and outside of the cell.
- It is negative on the inside, and positive on the outside.
However, the occurrence of an action potential sets off a domino effect that causes more Na+ to flood into the cell. As Na+ enters the cell, it becomes less negative.
- Once enough Na+ has flooded into the cell to change the charge to -55 mV, the cell becomes depolarized.
- As the action potential travels down the axon, the neuron becomes more permeable and depolarized.
- The Nodes of Ranvier, the exposed parts of the axon, allow Na+ to enter the cell to continually depolarize it.
- The myelin sheath that covers segments of the neuron is still necessary to ensure that the action potential travels down the axon quickly.
Depolarization of the cell continues until so much Na+ has flooded into the cell that its charge is +60 mV.
Step 2: Repolarization
Depolarization occurs until enough Na+ enters the cell to make the charge +60 mV.
- This causes Na+ channels to close and K+ channels to open.
- Since the K+ channels open, K+ leaves the cell.
- The cell is now polarized, but opposite than it was at rest; there is more K+ on the outside and more Na+ on the inside, so the inside is positive and the outside is negative.
- Remember that at rest, there was more K+ on the inside and more Na+ on the outside, so it was negative inside and positive outside.
- This can get confusing, so now is where diagrams come in handy to visualize what is happening.
To return to its resting state, the neuron has calcium/potassium pumps. These move ions into and out of the cell until it once again has a charge of -70 mV, and the Na+ and K+ is where it belongs. The cell is now repolarized.
- The neuron has passed the action potential along to the next neuron, which will repeat the exact same cycle that this neuron has just undergone.
- Refractory period: a short period of time after an action potential occurs when the cell is incapable of producing another action potential.
Step 3 (...or step “0”?): The Neuron at Rest
After repolarization and the refractory period, the neuron has returned to the state that it was in when the cycle began!
- There is once again a higher concentration of Na+ outside of the cell, and a higher concentration of K+ inside the cell.
- The cell is at its resting charge of -70 mV.
The neuron is ready to fire again as soon as another action potential occurs