NEURONES

Neurones are the basic structural units of the nervous system. In nervous tissues, they are closely packed together with little intercellular space and held together by connective tissue.
Neurones are highly specialized for transmitting electric impulses. They have therefore lost their ability to regenerate, a basic property of cells.

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Structure of Neurones:

There are billions of neurones in our body. None of them are identical, each neurone however is composed of the same basic parts:

• a cell body, with

• one or more short extensions or processes, called dendrons, each of which branches to form many fine dendrites

• a long process, the axon, which branches at its free end to end in synaptic knobs

Nerves:
In vertebrates nerves appears as a white thread when seen by the naked eye, but in reality, a nerve consists of hundreds of nerve fibres, both axons and dendrons, held together by connective tissue. Some nerves carry only sensory or motor fibres; most however are mixed nerves carrying both sensory and motor fibres. The whitish appearance of nerves and nerve fibres is due to the fatty material surrounding them.

Synapses:
Neurones are not connected to one
another directly. There are slight gaps (about 20 nm across) between the axon endings (synaptic knobs) of one neurone and the dendrites or the surface of the cell body of the neighbouring neurone. These junctions between the neighbouring neurones are known as synapses.

Types of neurones

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• unipolar neurones, those with one process only, found mainly in invertebrates

• bipolar neurones, those with two separate processes such as the neurones in the retina of the vertebrate eye ; and

• multipolar neurones, those with more than two processes, such as most of the vertebrate neurones.

In vertebrates, neurones found are:

• sensory (afferent) neurones

• motor (efferent) neurones, and

• relay neurones which connect the pathways of sensory and motor impulses, and are found mainly in the central nervous system.

Transmission of Nerve Impulses

Nerve impulse is not a flow of electrons like an electric current. It is a wave of electrical activity travels along the neurone. In other for this to happen a neurone must be able to generate electricity.

Resting Neurone

A neurone that is not transmitting an impulse known as a resting neurone. A resting neurone actively generating electricity. It does this by actively pumping sodium ions (Na+) out of the cell through the cell membrane, leavingt the chloride ions (CI-). As each sodium ion pumped out, a potassium ion (K+) is pumped into the cell. The potassium ions leak out of the cell again, but the sodium ions cannot move into the cell as the sodium gates in the membrane are closed. This results in an overall net positive charge on the outer surface of the neurone and a negative charge inside the neurone.

Stimulation of Neurone

In response to stimuli, a stimulated neurone are acts in an all or-none manner I.e It either transmits impulse or it does not, the intensity of impulse produced is the same no matter how strong the stimulus.

Transmission Along Nerve Fibres
When a stimulus exceeds the neurone's threshold, the following events occur at the site of stimulation:

• The sodium channels or gates in the cell membrane open causing sodium ions to rush into the cell;
• Which immediately wipes out the potential difference at this site causing depolarization
  Then, the interior of the cell momentarily develops a slight net positive charge with respect to the membrane's outer surface, causing the polarity to be reversed
• On depolarization, the sodium gates in the membrane immediately close simultaneously the potassium gates in the membrane open causing potassium ions to flow out and reestablish the original resting potential of the membrane.

This whole process is known as the action potential (pulse) and takes place within a thousandth of a second.

Transmission Across Synapses:
A nerve impulse passes across synaptic gaps chemically and not electrically as it does along a nerve fibre.
When an electric impulse reaches the end of an axon, it stimulates the synaptic knobs to secrete neurochemicals such as acetylcholine into the synaptic gaps. The released neurochemical binds to special receptor sites on the adjacent neurone's membrane, causing the sodium gates to open. This triggers off an impulse on the adjacent neurone in the same way as described above.
This is known as firing a second neurone. The neurochemical is quickly destroyed by enzymes, so that the neurone membrane at this site can reestablish its resting potential. Neurochemicals can only move from the axon's synaptic knobs to the target cell's membrane. So impulses are always transmitted in one direction only.

REFERENCES:

https://qbi.uq.edu.au/brain/brain-anatomy/what-neuron

https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/overview-of-neuron-structure-and-function

https://explorable.com/types-of-neurons