2. Neurons

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Nerve cells, or neurons, receive, conduct, and transmit signals. They carry signals inwards, from sense organs to the central nervous system (CNS), which consists of the brain and the spinal cord. In the CNS, neurons signal from one to another through networks of enormous complexity, to analyse, interpret, and respond to the signals. Neurons can be extremely elongated.

From the CNS, neurons extend processes outward to convey signals for action to muscles and glands. Based on their roles, the neurons found in the human nervous system can be divided into three classes: 
1) sensory neurons - receive impulses and transmit them from the sense organs to the CNS.
2) interneurons - connect sensory and motor neurons and interpret the impulse; only in the brain and spinal cord.
3) motor neurons - carry impulses from the CNS to muscles or glands.

 Neurons, like other cells, have a cell body. Various processes extend from the cell body. These include many short, branching processes, known as dendrites, and a separate process that is typically longer than the dendrites, known as the axon.
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Due to the short branching, dendrites provide a large surface area to receive signals from axons of other neurons. Axons conduct signals away from the cell body towards target cells. An axon divides at its far end into several branches which are also known as axon terminals. These allow simultaneous transmission of the impulse to many target cells.

 In this way, neurons are stimulated by signals from other neurons. This initiates a change in the membrane potential. In order to transmit the signal, the change in membrane potential has to spread to the axon. However, this becomes weaker with increasing distance from the source. This is solved by the active signalling mechanism.

 This mechanism ensures that a local electrical stimulus of sufficient strength triggers an explosion of electrical activity in the plasma membrane, and it is rapidly propagated along the membrane of the axon.

 References

Anon (n.d.) Neurons | Boundless Psychology. [Online]. Available from: https://courses.lumenlearning.com/boundless-psychology/chapter/neurons/ [Accessed: 11 April 2020].

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