Structure and Function: Nerve Cells

Neurons are nerve cells that originate, process, transmit, and receive nerve impulses. They are connected to other neurons or to cells in muscles, organs, or glands. Nerve impulses travel electrically along the neuron and are transmitted by chemical messengers (neurotransmitters) to the next neuron across a tiny gap, called a synapse, between the neuron and the adjacent cell, which is known as the target cell. In addition to neurons, the nervous system contains large numbers of other types of cell, called neuroglia, which protect, nourish, and support neurons.

Neurons

In addition to features common to all cells, such as a nucleus, neurons have specialized projections, known as nerve fibres (axons), that carry nerve signals. Neurons in the brain form densely packed clusters. Neurons in the spinal cord and around the body form long communication tracts.

Neuron structure

A typical neuron has one or two long nerve fibres and many dendrites. Nerve fibres carry signals away from a neuron’s cell body, and dendrites carry signals toward it.

Neural connection

This magnified image shows two neurons. The nerve fibre of one neuron links to the cell body of the other.

Conduction of nerve signals

Nerve impulses travel along neurons in the form of electrical signals. These signals cross the synapses (tiny gaps) between one neuron and the next in chemical form before being transmitted again in electrical form. Signals are also chemically transmitted to other target cells, such as those in muscles, which make appropriate responses.

Electrical and chemical conduction

When an electrical signal arrives at the end of a nerve fibre, it triggers the release of neurotransmitter molecules, which transmit the signal in chemical form to the next cell.

How neurotransmitters work

More than 50 neurotransmitters have been identified. Their task is to carry nerve impulses across the synapse (a tiny gap) between neurons and target cells. Neurotransmitters either stimulate or inhibit electrical impulses in target cells.

Crossing the synapse

Arrival of a nerve impulse stimulates the release of neurotransmitters from vesicles. They pass across the synapse and open channels in the target cell. Charged particles can then enter and trigger a second impulse.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

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