The Nervous System - Neurones

The Nervous System – Neurones

Neurones are specialised nerve cells that conduct electrical impulses, acting as the fundamental unit of the nervous system.
These cells have adapted to their function by having long extensions, ideal for carrying impulses over distances, and branched endings to carry signals to many different cells.

The main parts of a neurone are the cell body, which contains the nucleus, the long axon, and the highly branched dendrites.
The cell body contains a nucleus, and comes equipped with various organelles such as mitochondria and rough endoplasmic reticulum necessary for protein production, and cell sustenance.
Axons are long thread-like extensions of the cytoplasm covered by a protective sheath with the function of transmitting nerve impulses away from the cell body.
Dendrites, on the other hand, are shorter and more branched, and they receive signals from other neurons and conduct them toward the cell body.

There are three primary types of neurones: sensory neurones, motor neurones, and interneurones (also known as relay neurones).
Sensory neurones carry impulses from sensory receptors to the central nervous system.
Motor neurones carry impulses from the central nervous system to effectors such as muscles or glands.
Interneurones are found within the central nervous system, processing and transmitting impulses between sensory and motor neurones.

Neurones transmit information in the form of electrical impulses.
Transmission of an impulse involves a reversal of electrical charge across the neurone’s membrane, a process known as an action potential.
This change in charge is achieved by the influx and efflux of ions across specific ion channels in the neurone’s membrane.
Resting potential is the state of a neurone when it is at rest, where the inside of the neuron is negatively charged in relation to the outside.
An action potential begins when a stimulus causes the voltage-gated sodium channels to open, allowing positively-charged sodium ions into the neurone. This process is called depolarisation.
Following depolarisation, the voltage-gated sodium channels close, and voltage-gated potassium channels open instead, resulting in an efflux of positive potassium ions from the neurone. This is known as repolarisation.
The short period after the action potential where the neurone cannot fire another impulse is called the refractory period. This period ensures that impulses travel in one direction only.

Some neurones are myelinated, meaning they have a myelin sheath that covers the axon, insulating it and speeding up the transmission of the electrical impulses.
The myelin sheath is primarily composed of lipids, with tiny protein inclusions.
Between the insulating myelin sheath, there are gaps known as nodes of Ranvier, which help facilitate rapid conduction of the nerve impulse, a process called saltatory conduction.

Information transmission between neurones is carried out at specialised structures called synapses, where the electrical signal from one neurone is converted into a chemical signal to pass on the information to the next neurone.
The gap between two neurones is called the synaptic cleft.
The neuron sending the signal is the presynaptic neurone, while the one receiving the signal is the postsynaptic neurone.
When an action potential reaches the end of a presynaptic neurone, it stimulates the release of chemical messengers known as neurotransmitters from synaptic vesicles.
The neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic neuron, leading to the generation of a new action potential in this neuron. This process is called synaptic transmission.
Some neurotransmitters, like dopamine and serotonin, are associated with mood and emotion. They are reset back into the presynaptic neuron through reuptake, and are then stored or broken down and removed.
Other neurotransmitters such as acetylcholine (ACh) may be broken down by enzymes such as acetylcholinesterase in the synaptic cleft after transmission. This process prevents continuous firing of the postsynaptic neuron and allows for another potential transmission.