Nerve Impulses

Nerve impulses, or action potentials, are electrical charges that travel along neurons. These are responsible for all our body’s functions from the blinking of our eyes to the beating of our hearts.

Neurons, or nerve cells, are cells that transmit nerve impulses. A typical neuron consists of a cell body, a large number of shorter extensions called dendrites, and one long extension called the axon.
Dendrites receive impulses, or signals, from other neurons and conduct them towards the cell body. The impulse is then transmitted down the axon to other neurons or to muscles or glands.

Structure of a Neuron

The cell body contains the nucleus and other typical cell organelles.
The axon is a long tube-like structure that carries impulses away from the cell body.
The myelin sheath is a fatty layer around the axon that insulates it and speeds up the transmission of nerve impulses.
Nodes of Ranvier are small gaps in the myelin sheath which speed up the transmission of the impulse by forcing the impulse to ‘jump’ across the gaps.

Propagation of Nerve Impulses

An impulse begins when a neuron is stimulated by another neuron or by a stimulus in the environment.
This stimulation causes a rapid change in voltage across the cell membrane, initiating the nerve impulse.
This voltage change is the result of a sudden movement of positive ions into and out of the neuron - a process called depolarization and repolarization respectively.
The nerve impulse travels down the axon in a series of small steps, jumping from Node of Ranvier to Node of Ranvier - a process called saltatory conduction.

Synapses and Neurotransmitters

A synapse is a juncture between two neurons or between a neuron and a muscle or gland cell.
At the synapse, the electrical signal, or nerve impulse, is converted into a chemical signal in the form of a neurotransmitter.
Neurotransmitters transmit the impulse across the synaptic gap to the next neuron or to a muscle or gland cell.

The All-or-Nothing Principle

The nerve impulse is an all-or-nothing event. It either fires fully or it does not fire at all.
Once a neuron fires, it must rest for a short period of time before it can fire again. This rest period is called the refractory period.

This is just a brief overview of nerve impulses. For a more in-depth understanding, make sure to read up on specific topics, practise questions and revise often!