The Nervous System - Key Terms

Acetylcholine - One of a group of chemicals, called neurotransmitters, released by neurones. It diffuses across the synapse between adjacent neurones and so passes an impulse from one neurone to the next.

Action potential -  Changes that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes.

Autonomic nervous system - Part of the nervous system, controlling the muscles and glands, that is not under voluntary control.

Axon - A process extending from a neurone that conducts action potentials away from the cell body.

Cholinesterase - Enzyme that breaks down and therefore inactivates the neurotransmitter, acetylcholine in the synapse.

Dendrite -  A process usually branched, extending from the cell body of a neurone, which conducts impulses towards the cell body .

Depolarisation - Temporary reversal of changes on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted.

Effector - An organ that responds to stimulation by a nerve impulse resulting in a change or response.    

Generator potential - Depolarisation of the membrane of a receptor cell as a result of a stimulus. 

Motor neurone - Neurone that transmits action potentials from the central nervous system to an effector.

Myelin - A fatty substance that surrounds axons and dendrites in certain neurones. 

Neurone - A nerve cell, comprising a cell body, axon and dendrites, which is adapted to conduct action potentials.   

Neurotransmitter - One of a number of chemicals that are involved in communication between adjacent neurones or between nerve cells and muscles. 

Node of Ranvier - A gap in the myelin sheath that surrounds tha axon of a neurone.

Reflex arc - the nerve pathway in the body taken by an action potential that leads to a rapid, involuntary response to a stimulus.

Refractory period - Period during which the membrane of the axon of a neurone cannot be depolarised and no new action potential can ne initialted.

Repolarisation - Return to the resting potential in the axon of a neurone after an action potential.

Resting potential - the difference in electrical charge maintained across the membrane of the axon of a neurone when not stimulated.

Saltatory conduction - Propogation of a nerve impulse along a myelinated dendron or axon in which the action potential jumps from one node of Ranvier to the next.

Schwann cell - Cell around a neurone whose cell-surface membrane wraps around the dendron or axon to form the myelin sheath.

Sensory neurone - A neurone that transmits an action potential from a sensory receptor to the central nervous system.

Stimulus - A detectable alteration in the internal or external environment of an organism that produces some change in that organism.

Synapse - A junction between neurones in which they do not touch but have a narrow gap, the synaptic cleft, across which a neurotransmitter can pass.

Threshold level - The minimum intensity that a stimulus must reach in order to trigger an action portential in a neurone.