What Are Neurons and How Do They Work?

Imagine yourself in the middle of a game of  catch . A player throws the ball towards you, and as the ball jets closer , you quickly  recognize where to position your hand in order to catch the ball safely. Have you ever wondered  how your brain is able to quickly make that decision ?

How does information flow so quickly  from your eyes to your brain and back to your hand in order to successfully catch the ball? All information from our environment— in this case, information from the eye about the  size and speed of the ball coming towards you— is converted by nerve cells or neurons into  electrical signals that are carried to the brain. Similarly, information from our brain to our  body, such as an order to move our hand towards the ball’s trajectory, is also sent in the form  of these signals.

These electric signals are called action potentials or nerve impulses.  Because these signals are instantaneous, our brain is able to help us quickly detect  and respond to things in our environment, both good and bad. It is thanks to the speedy  nature of such signals that you can immediately recognize your favo rite song, or quickly pull  your foot away when you step on a pin by accident!

So how do neurons send these  signals or “fire” action potentials? The human brain contains tens of billions  of neurons that are connected to each other. Each neuron has a cell body with several branches. 

The smaller branches— called dendrites— receive signals that come together and then the total  signal is transmitted through the longest branch— the axon. Similar to how a wire carries  electricity from a switch to an electric bulb, the axon carries nerve impulses  from a cell body to the next cell. The outer or extracellular surface of a neuron is  bathed in a ‘salty’ solution of positively charged Sodium and negatively charged Chloride ions.

The  inner surface has positively charged Potassium ions and proteins that are negatively charged. The  negatively charged proteins inside the cell are not balanced by an equal number of positive ions,  making the inner surface of the neuron slightly more negative than the outside. This gives  rise to an “electrical gradient”.

Furthermore, there is a higher concentration of potassium ions  inside, giving rise to a “diffusion gradient” that forces them to flow outside. Thus,  there is both an electrical gradient and a chemical gradient— an electrochemical gradient—  forcing ions from inside the cell to go out. Each neuron, when at rest, maintains a voltage  difference arising from the electrochemical gradient, just as a battery maintains a  certain voltage between its poles.

This voltage of approximately -70mV is referred  to as the “resting potential” of a neuron. The cell membrane of the  neuron contains “ion channels”. These are pores formed using proteins that  selectively allow certain ions to pass through, similar to how an airport security guard checks  visitors and only allows those with a ticket to proceed to their gates .

Some of these channels  are special, in that they open and close only at a certain voltage; these are called voltage-gated  ion channels. Ion channels allow ions in and out, causing the voltage of the cell to  move away from its resting potential. At any given time, a neuron keeps receiving small  amounts of incoming signals, which results in small fluctuations in the membrane voltage.

These  are called “graded potential”, but they are not strong enough to result in the firing of a neuron. A neuron fires or generates a nerve impulse only when the total sum of input signals is large  enough to cross a “threshold potential” of -55mV. This begins because it triggers  voltage-gated Sodium channels to open.

These channels, when opened, cause a rush of  many Sodium ions into the cell from outside. This causes “depolarization” of the  neuron, or a reversal of charge in its inner surface from negative to positive. The voltage difference continues to become more positive.

When the voltage reaches 30 mV, it  causes voltage-gated Potassium channels to open. These channels allow Potassium ions to move from  inside to outside, slowly “repolarizing” the cell or restoring the more negative voltage difference.  However, the voltage- gated Potassium ion channels are much slower to close, causing the voltage  to drop below the resting potential of -70mV.

This is called “hyperpolarization”. After a neuron fires in this way, its voltage would be drained, like a  rechargeable battery after prolonged us e. A reusable battery can be “recharged” by  passing an external current through it.

The neurons outsource this job to “pumps” located  on the membrane by spending energy currency called ‘ATP’. These pumps act like revolving doors in  buildings; they let out more Sodium ions from inside the cell, while simultaneously bringing  in fewer Potassium ions. This brings the neuron back to its resting potential.

This entire event of neuron firing follows certain rules: (1) It is an all or none event. Nerve impulses are much like digital signals.  They occur in the same magnitude, regardless of how strong or weak the stimulus is.

However, the  threshold voltage must be crossed to generate it. (2) They are unidirectional. Once a segment of the membrane becomes “depolarized”, it passes the signal forward to the  next segment.

Ion channels, after finishing their job of passing the signal, will pull down their  shutters and ‘take a break’, becoming inactive for a short while. This is called their “refractory  period”. Therefore, a signal cannot return to them and has no choice but to travel forward  to the next segment on the membrane.

The process of generating a nerve impulse is  fairly quick. However, in certain situations where this is not fast enough, our body uses special  neurons with a coating called a myelin sheath to speed up signalling. This coating covers the  surface of the axon, while leaving small spots called “Nodes of Ranvier” open.

In such covered  axons, the signal doesn’t have to travel through the entire length of the axon, but only through  these gaps, making the signal travel faster. This type of “jumpy” signal transmission  is called “saltatory” conduction. In our sense organs, these nerve impulses  are caused in response to a sensation, such as odo r, taste, touch, sound  or light.

For example, in the eye, a nerve impulse is generated in response to light  reaching the cells in the retina, which are then passed on through the optic nerve to the part  of the brain that processes visual information. The nerve impulse can also have a variety  of effects based on where it is sent . When you accidentally step on a pin on the ground,  nerves in the spinal cord respond by sending a nerve impulse to your leg muscles, asking  it to contract, thereby pulling your leg away from the pin in a fraction of a second.

The brain constantly makes sense of the pattern, frequency and area of origin  of the incoming nerve impulses to interpret them , and then responds by making  appropriate commands to the relevant body parts. In a way, n erve impulses or action potentials  are a digital language of the brain itself!