_"It is the little grey cells, mon ami, on which one must rely."_
It is these little grey cells that M. Hercule Poirot speaks of that have galvanised our dash to the top of the food chain and also help us solve the occasional murder mystery.
We call them neurons.
Neurons are the cells that send messages all over our body, allowing us to do virtually everything that we do, from breathing and eating to solving that hard math assignment.
It is important to understand a neuron’s structure, since that is what facilitates its function -- to fire electrical signals throughout our nervous system. These signals may act upon other neurons or muscles and organs, prompting them to perform a function – such as contraction and relaxation, as in the case of muscles.
The neuron consists of a cell body or soma which houses the nucleus of the cell and is responsible for the energy to drive the activities of the cell. The cell body also possesses small processes known as dendrites. These are primarily responsible for receiving incoming signals from other neurons. Dendrites receive both excitatory signals, which prompt the neuron to fire, and inhibitory signals, which prevent it from firing. The neuron’s decision to fire or not results from this electrical tug-of-war between the competing signals.
The way the neuron fires is through a longer process from the cell body known as the axon. The axon is primarily responsible for the transmission of impulses away from the neuron towards the dendrites of other neurons. They can be really long, sometimes even around a meter in length! Axons sometimes have a discontinuous layer of insulating fat known as the myelin sheath. Gaps in the myelin sheath are called Nodes of Ranvier. This helps in increasing the speed of transmission of the nerve impulses, as impulses jump from node to node instead of traversing the whole distance.
Impulses travel through a neuron using electrical signals. These signals arise from a difference in charge created by ion channels, which act as "invite only highways" for sodium and potassium ions. This charge difference is manifested as a change in potential difference across the neuron membrane – called an action potential, the impulse that moves along the axon.
This still doesn’t explain how neurons communicate with each other. Our nerves employ usage of specialised chemicals called neurotransmitters - the FedEx of our nervous system! These transmit the signal through a gap between neurons called a synapse, and cause excitation or inhibition of the next neuron. Axon terminals are the endpoints of the axon where neurotransmitters are released.
Neurotransmitters (and other important compounds) are synthesised in the cell body, by clusters of ribosomes (protein factories) called Nissl’s Granules.