What is a Neuron?

A Neuron is an electrically excitable cell that processes and transmits information through electrical and chemical signals.

The Wikipedia article titled ‘Neuron’ provides a detailed picture of a neuron. I’ll describe a neuron in short.

Neurons are diverse (hundreds of types are known (ref)), but as an introduction, we take up a ‘typical’ neuron. 

Neuron structure and Information transfer. Source: 

(http://kvhs.nbed.nb.ca/gallant/biology/neuron_structure.html)

A neuron has three parts: Dendrites, Soma (the cell body), and Axon. The soma is usually compact; the axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely, getting thinner with each branching, and extending their farthest branches a few hundred micrometers from the soma. The axon leaves the soma at a swelling called the axon hillock, and can extend for great distances (upto 2m in tall people (ref)), giving rise to hundreds of branches.

The key to neural function is the synaptic signalling process, which is partly electrical and partly chemical. 

The electrical part is dependent on the neuron’s membrane potential. Almost all the cells have a membrane potential, i.e. the electrical potential on the inside of the cell is different from the outside (We speculate that the membrane potential arose as a evolutionary solution to the ‘osmotic problem’ (ref)). There are ion channels and ion pumps on the membrane which can lead to changes in the membrane potential.

The chemical part occurs at the synapse. The chemical activity is mediated by neurotransmitters which are released in response to an action potential, which will be described in the next section. These lead to either a excitation in the membrane potential, or an inhibition, dependent on the type of neurotransmitter and other factors.

Neurotransmitter flow at the Synapse. Source:
(http://www.scq.ubc.ca/wp-content/uploads/2007/05/synapse.jpg)

Neural Signalling

When neurotransmitters released from the presynaptic neuron reach the postsynaptic neuron, they initiate a reaction which leads to the opening of the Na+ channels. As the resting potential of a neuron is negative (around -70 mV), Na+ flows into the cell, and the membrane potential increases. As the membrane potential is increased, sodium ion channels keep opening up, allowing the entry of sodium ions into the cell. This is followed by the opening of potassium ion channels that permit the exit of potassium ions from the cell. The inward flow of sodium ions increases the concentration of positively charged cations in the cell and causes depolarisation, where the potential of the cell is higher than the cell's resting potential. The sodium channels close at the peak of the action potential, while potassium continues to leave the cell. The efflux of potassium ions decreases the membrane potential or hyperpolarizes the cell. For small voltage increases from rest, the potassium current exceeds the sodium current and the voltage returns to its normal resting value, typically −70 mV. However, if the voltage increases past a critical threshold, typically 15 mV higher than the resting value, the sodium current dominates. This results in a runaway condition whereby the positive feedback from the sodium current activates even more sodium channels. Thus, the cell fires, producing an action potential (ref). This is the ‘spike’ we talk about in Spiking Neural Networks (SNNs).

The profile of an Action Potential, or a Spike, with the ion dynamics involved.

Although action potentials are generated locally on patches of excitable membrane, the resulting currents can trigger action potentials on neighbouring stretches of membrane, precipitating a domino-like propagation without decay. 

When the action potential reaches an axon terminal, it initiates an influx of Ca++ ions, which in turn leads to a release of neurotransmitters from the synaptic vesicles, which go on and excite or inhibit the corresponding postsynaptic neuron.

Facts about Neurons

1. Most of the neurons do not reproduce. The neurons that are created during development of the brain are created via stem cells (ref).
2. There are ~100 billion neurons in the human brain, with ~10,000 synaptic connections per neuron.
3. The cells that look after the maintenance of neurons in the brain are known as Glia. There are 1000 times as many glial cells as there are neurons.