How does a signal pass through a neuron?
How big is a million?
What about a hundred million?
What about a billion?
What about a hundred billion?!
That’s a freakishly big number! A billion of anything is bound to take up a lot of space right?
Well not really.
As you read the intro to this article, the hundred billion neurons were firing electrical and chemical signals to each other in your brain.
Well not all hundred billion, the physical action of reading only stimulates the cerebral cortex. So, you know, only sixteen billion neurons. Oh, and fun fact, the stimulation and processing of the words by your neurons takes only about two hundred and 50 milliseconds!!
But now the question is how are my brain and your brain and everybody’s brain able to take in information (like the words on a page) and process it so quickly?
Well, that is exactly the question we are about to answer.
This is a neuron:
When a neuron is stimulated enough, it fires a signal that propagates down its axon. They only have one signal they can send and it only goes at one speed and in one direction. You can’t just send back the information your neuron processed.
HOW A SIGNAL TRAVELS THROUGH A NEURON (2.0)
Before we get into the actual portion of the signal propagating down the neuron it’s important to note that every neuron has a resting membrane potential of -70mv.
But what does that mean?
Basically, this is the stable electrical charge difference along the membrane of the cell.
Positives charges want to be with negative charges and negative charges want to be with positive charges.Because these opposite charges attract, barriers are needed to keep them separated until the cell is ready to use the energy that their attraction creates.
Their separation creates potential
Outside of the membrane there are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). On the inside there are positively charged potassium ions (K+) and anions (negatively charged proteins in the case of the neuron cell) .
The difference in electrical charge develops because of the different groupings of ions in the inside or outside of the membrane
A.N. An ion is an atom that has either gained or lost an electron rendering its overall charge positive or negative
Since there are more sodium ions on the outside than there are potassium ions on the inside, the cell’s interior has an overall negative charge or also called it’s resting membrane potential which sits at around — 70mv. When neurons aren’t processing a signal they are said to be at their resting membrane potential — or in other terms, the neuron is very polarized.
Now keep this word in mind, it is going to be very important later.
But all the positive and negative ions didn’t come to this nice and clean arrangement all on it’s own. It was formulated by the sodium — potassium pump (Na — K pump) .
The Na-K pump uses ATP hydrolysis to (as the name indicates) pump three Na+ ions out of the cell and two K+ ions into the cell.It keeps homeostasis (-70mv) within the neuron. However the Na-K pump is only active when the neuron is at rest (i.e. no signals propagating down it’s axon)
Okay okay, now that we have all this information, what do all these fancy words mean in terms of signal propagation down the cell.
This is where the action potential comes in.
In simple terms, the action potential is the mode in which neurons transmit electrical signals. It is a momentary reversal of membrane potential. And when I say momentary I mean momentary. Like 1 milisecond momentary.
The binding a neurotransmitter released by the axon terminals to the dendrites of the neuron have an effect on the resting membrane potential in a process called depolarization. This just means it makes the charge of the membrane more polar (moves closer to zero). If this depolarization of the neuron reaches a certain threshold — 55mv), the neuron enters a state called the threshold membrane potential.
A.N : A neuron can only reach the threshold membrane potential if enough charges or neurotransmitters are firing through the dendrites. If the signal is not strong enough it will have several failed attempts it cannot cross that threshold and in consequence returns to the resting potential of -70mv
If and when this threshold is reached, a large number of sodium channels (using ATP molecules) open up allowing positive sodium ions in the cell. Therefore, the more positive charges (Na+ ions) the more positively charged the cell will be.
The increased influx of positive charges into the inside of the cell (also called the intracellular space) causes a massive depolarization of the neuron, eventually reaching zero in a phase called the rising phase.
The charge of the neuron keeps rising to a maximum of +40. Once it reaches this maximum, the Sodium Channels close and Potassium channels open allowing positive K+ ions to flow out of the cell, reducing the overall charge of the neuron.
As we can see in the photo, the potassium pump opens allowing potassium ions to flow from the inside of the cell to the outside rendering it positive.
The loss of the positive K+ ions promotes repolarization (falling phase)
As the positive ions flow , the neuron’s charge drops down to the resting potential (-70mv). However, often, the charge of the neuron shoots past this equilibrium in a phase called hyperpolarization and enters a refractory period. Typically, the neuron’s membrane potential shoots down to about -70mv.
If the neuron is receiving signals through the dendrites while still in the state of hyperpolarization, it is near impossible to get it to fire again.
Once the neuron reaches hyperpolarization, the Potassium channels close and the neuron goes back to the -70mv state it was at before.
This signal reaches it’s final destination of the axon terminals where it can be passed down to the dendrites of a bunch of other neurons
Aaaaaah.
Okay that was a lot of information. Here is a short recap
TL;DR
HOW A SIGNAL TRAVELS THROUGH A NEURON
- The signal (or neurotransmitter gets passed from the axon terminal of one or several other neurons into the dendrite of another neuron
- When neurons are at rest they have a charge of -70mv (polarized neuron)
- When enough signals reach the neuron it passes a threshold of -55mv (state of depolarization due to influx of positive sodium ions coming in from the sodium potassium pump
- The charge of the neuron eventually reaches zero (rising phase)
- The sodium potassium pump opens up once again and lets positive potassium ions out of the cell rendering it more negative (repolarization or falling phase)
- The neuron goes past the -70mv threshold in a phase called hyperpolarization
- The potassium channels close and the neuron returns to it’s resting membrane potential of -70mv
- Finally, the signal goes down to the axon terminals to be passed to a different neuron
