Neuron action potentials: The creation of a brain signal

Your body has nerves that connect your brain to the rest of your organs and muscles, just like telephone wires connect homes all around the world. When you want your hand to move, your brain sends signals through your nerves to your hand telling the muscles to contract. But your nerves don’t just say “hand, move.” Instead your nerves send lots of electrical impulses (called action potentials) to different muscles in your hand, allowing you to move your hand with extreme precision.
Neurons are a special type of cell with the sole purpose of transferring information around the body. Neurons are similar to other cells in that they have a cell body with a nucleus and organelles. However, they have a few extra features which allow them to be fantastic at transferring action potentials:
  • dendrites: receive signals from neighboring neurons (like a radio antenna)
  • axon: transmit signals over a distance (like telephone wires)
  • axon terminal: transmit signals to other neuron dendrites or tissues (like a radio transmitter)
  • myelin sheath: speeds up signal transmission along the axon
Illustration of the neuron with the dendrites, myelin sheath, axon, and axon terminus labelled.

Concentration gradients

Concentration gradients are key behind how action potentials work. In terms of action potentials, a concentration gradient is the difference in ion concentrations between the inside of the neuron and the outside of the neuron (called extracellular fluid).
Illustration demonstrating a concentration gradient along an axon.
If we have a higher concentration of positively charged ions outside the cell compared to the inside of the cell, there would be a large concentration gradient. The same would also be true if there were more of one type of charged ion inside the cell than outside. The charge of the ion does not matter, both positively and negatively charged ions move in the direction that would balance or even out the gradient.

Resting membrane potential

Neurons have a negative concentration gradient most of the time, meaning there are more positively charged ions outside than inside the cell. This regular state of a negative concentration gradient is called resting membrane potential. During the resting membrane potential there are:
  • more sodium ions (Na+^+) outside than inside the neuron
  • more potassium ions (K+^+) inside than outside the neuron
The concentration of ions isn’t static though! Ions are flowing in and out of the neuron constantly as the ions try to equalize their concentrations. The cell however maintains a fairly consistent negative concentration gradient (between -40 to -90 millivolts). How?
  • The neuron cell membrane is super permeable to potassium ions, and so lots of potassium leaks out of the neuron through potassium leakage channels (holes in the cell wall).
  • The neuron cell membrane is partially permeable to sodium ions, so sodium atoms slowly leak into the neuron through sodium leakage channels.
  • The cell wants to maintain a negative resting membrane potential, so it has a pump that pumps potassium back into the cell and pumps sodium out of the cell at the same time.

How action potentials work

Action potentials (those electrical impulses that send signals around your body) are nothing more than a temporary shift (from negative to positive) in the neuron’s membrane potential caused by ions suddenly flowing in and out of the neuron. During the resting state (before an action potential occurs) all of the gated sodium and potassium channels are closed. These gated channels are different from the leakage channels, and only open once an action potential has been triggered. We say these channels are “voltage-gated” because they are open and closed depends on the voltage difference across the cell membrane. Voltage-gated sodium channels have two gates (gate m and gate h), while the potassium channel only has one (gate n).
  • Gate m (the activation gate) is normally closed, and opens when the cell starts to get more positive.
  • Gate h (the deactivation gate) is normally open, and swings shut when the cells gets too positive.
  • Gate n is normally closed, but slowly opens when the cell is depolarized (very positive).
Voltage-gated sodium channels exist in one of three states:
  1. Deactivated (closed) - at rest, channels are deactivated. The m gate is closed, and does not let sodium ions through.
  2. Activated (open) - when a current passes through and changes the voltage difference across a membrane, the channel will activate and the m gate will open.
  3. Inactivated (closed) - as the neuron depolarizes, the h gate swings shut and blocks sodium ions from entering the cell.
Voltage-gated potassium channels are either open or closed.
There are three main events that take place during an action potential:
  1. A triggering event occurs that depolarizes the cell body. This signal comes from other cells connecting to the neuron, and it causes positively charged ions to flow into the cell body. Positive ions still flow into the cell to depolarize it, but these ions pass through channels that open when a specific chemical, known as a neurotransmitter, binds to the channel and tells it to open. Neurotransmitters are released by cells near the dendrites, often as the end result of their own action potential! These incoming ions bring the membrane potential closer to 0, which is known as depolarization. An object is polar if there is some difference between more negative and more positive areas. As positive ions flow into the negative cell, that difference, and thus the cell’s polarity, decrease. If the cell body gets positive enough that it can trigger the voltage-gated sodium channels found in the axon, then the action potential will be sent.
  2. Depolarization - makes the cell less polar (membrane potential gets smaller as ions quickly begin to equalize the concentration gradients) . Voltage-gated sodium channels at the part of the axon closest to the cell body activate, thanks to the recently depolarized cell body. This lets positively charged sodium ions flow into the negatively charged axon, and depolarize the surrounding axon. We can think of the channels opening like dominoes falling down - once one channel opens and lets positive ions in, it sets the stage for the channels down the axon to do the same thing. Though this stage is known as depolarization, the neuron actually swings past equilibrium and becomes positively charged as the action potential passes through!
  3. Repolarization - brings the cell back to resting potential. The inactivation gates of the sodium channels close, stopping the inward rush of positive ions. At the same time, the potassium channels open. There is much more potassium inside the cell than out, so when these channels open, more potassium exits than comes in. This means the cell loses positively charged ions, and returns back toward its resting state.
  4. Hyperpolarization - makes the cell more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily hyperpolarizes, or gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to reestablish the resting state.

Refractory Periods

Action potentials work on an all-or-none basis. This means that an action potential is either triggered, or it isn’t – like flipping a switch. A neuron will always send the same size action potential. So how do we show that some information is more important or requires our attention right now? The answer lies in how often action potentials are sent – the action potential frequency.
When the brain gets really excited, it fires off a lot of signals. How quickly these signals fire tells us how strong the original stimulus is - the stronger the signal, the higher the frequency of action potentials. There is a maximum frequency at which a single neuron can send action potentials, and this is determined by its refractory periods.
  • Absolute refractory period: during this time it is absolutely impossible to send another action potential. The inactivation (h) gates of the sodium channels lock shut for a time, and make it so no sodium will pass through. No sodium means no depolarization, which means no action potential. Absolute refractory periods help direct the action potential down the axon, because only channels further downstream can open and let in depolarizing ions.
  • Relative refractory period: during this time, it is really hard to send an action potential. This is the period after the absolute refractory period, when the h gates are open again. However, the cell is still hyperpolarized after sending an action potential. It would take even more positive ions than usual to reach the appropriate depolarization potential than usual. This means that the initial triggering event would have to be bigger than normal in order to send more action potentials along. Relative refractory periods can help us figure how intense a stimulus is - cells in your retina will send signals faster in bright light than in dim light, because the trigger is stronger.
Refractory periods also give the neuron some time to replenish the packets of neurotransmitter found at the axon terminal, so that it can keep passing the message along. While it is still possible to completely exhaust the neuron’s supply of neurotransmitter by continuous firing, the refractory periods help the cell last a little longer.
##Consider the following One of the main characteristics that differentiates an action potential from a different kind of electrical signal called graded potentials is that the action potential is the major signal sent down the axon, while graded potentials at the dendrites and cell body vary in size and influence whether an action potential will be sent or not. Graded potentials are small changes in membrane potential that are either excitatory (depolarize the membrane) or inhibitory (hyperpolarize the membrane). Many excitatory graded potentials have to happen at once to depolarize the cell body enough to trigger the action potential.
Graded PotentialsAction Potentials
At the dendrites and cell bodyAt the axon
Excitatory or inhibitoryAlways excitatory
Smaller in sizeLarger voltage difference
Triggered by input from the outsideTriggered by membrane depolarization
Many can happen at onceOnly one at a time
Can come in different sizesAll-or-none

Attribution:

This article is licensed under a CC-BY-NC-SA 4.0 license. https://creativecommons.org/licenses/by-nc-sa/4.0/

Additional references:

Membrane Potential During an Action Potential graph. OpenStax College CC 3.0 Unported
Loading