Neurones: Structures and Functions

Neurones (also written neurons*) are nerve cells.

Nerve cells together with neuroglia form the tissues of the nervous system (i.e. nervous tissue).

There are many different types of nerve cells. Here are notes about words used to refer to some functional types of neurones e.g. motor neurones vs. relay neurones, and some structures of neurones, e.g. axons, dendrites, synapses.

Figure (1): Motor Neurone

Figure (2): Sensory Neurone

Figure (3): Relay Neurone

The following table lists key terms (words) in alphabetical order and refers to the diagram above.

Term

Function(s)

Structure(s)

Axon

A nerve fibre: a single process extending from the cell body of a neurone and carrying nerve impulses away from it.

Dendrite

Carries nerve impulses from adjacent neurons into the cell body.

One of the shorter branching processes of the cell body of a neurone. All dendrites have synaptic knobs at the ends, which are the "connections" to adjoining nerves.

Mixed Nerves

Contain both motor and sensory nerve fibres running to / from a particular region of the body.

Examples include most large nerves such as the brachial nerves, and all the spinal nerves.)

 

Motor Neurone =
"Efferent Neurone"

Transmitting impulses (electrical signals for communication within the body) from the Central Nervous System (which is sometimes referred to by the abreviation CNS, and) which consists of the brain and spinal cord, to muscles & glands elsewhere in the body.

See Figure (1) in the diagram above.

Myelin Sheath

Three key functions of Myelin Sheath are:

  • Protection of the nerve fibre.

  • Insulation of the nerve fibre.

  • Increases the rate of transmission of nerve impulses.

A complex material formed of protein and phospholipid (fat) that is laid down as a sheath around the axons of certain neurons.

Nerve

A nerve is a bundle containing 100s to 1000s of axons (thread-like conductors) plus the associated connective tissue and blood vessels.

See the diagrams above.
Figures (1), (2) and (3).

Neurilemma

The sheath of the axon of a nerve fibre. The neurilemma of a medullated fibre contains myelin laid down by Schwann cells.

Neurone

A cell specialized to transmit electrical nerve impulses and so carry information from one part of the body to another.

"Neurone" = "Nerve Cell"

Each neurone has an enlarged portion the cell body (perikaryon), containing the nucleus; from the body extend several processes (dendrites) through which impulses enter from their branches. A longer process, the nerve fibre (axon), extends outwards and carries impulses away from the cell body. This is normally unbranched except at the nerve ending.

The point of contact of one neurone with another is known as a synapse.

Nodes of Ranvier

Key functions of Nodes of Ranvier include:

  • Allowing nutrients and waste products to enter / leave the neurone.

  • Allowing nerve impulses to move along the neurone through a process of de-polarisation and re-polarisation of the nerve membrane.

Gaps that occur at regular intervals in the myelin sheath of medullated nerve fibres, between adjacent Schwann cells.

Relay Neurone

Located within the Central Nervous System (CNS), relay neurons transmit the electrical impulses generated by the stimuli to other nerves.

See Figure (3) in the diagram above.

Schwann Cells

Schwann cells are the cells that lay down the protective myelin sheath around the axon of medullated nerve fibers.

Each Schwann cell protects one length of axon, around which it twists as it grows - enveloping the axon.

Sensory Neurone

Transmit impulses inwards from sense organs to the Central Nervous System (CNS).

See Figure (2) in the diagram above.

Synapse

Reaching a synapse, an impulse causes the release of a neurotransmitter, which diffuses across the gap and triggers an electrical impulse in the next neurone. (Some brain cells have more than 15,000 synapses.)

The minute gap across which nerve impulses pass from one neurone to the next, at the end of a nerve fibre.

* neuron vs neurone Some sources, including dictionaries and reference books, use the spelling "neuron" while others use the spelling "neurone". E.g. Oxford concise Colour Medical Dictionary (1998 copy), has an entry for neurone (nerve cell), some popular recent A-Level Biology texts use "neurones", Neuroanatomy: An Illustrated Colour Text (2010 edition) lists Neurones together with many sub-topics under the same main entry in its index. Other texts incl. some American books use "neuron". Both spellings are widely understood to have the same meaning. Both versions are used on this website for the benefit of visitors who, statistics indicate, search using both.

See also the pathway of a nerve impulse and the list of parts of the central nervous system.

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