association neuron

neuron

a nerve cell; any of the conducting cells of the nervous system, consisting of a cell body, containing the nucleus and its surrounding cytoplasm, and the axon and dendrites.

Neurons are highly specialized cells having two characteristic properties: irritability, which means that they are capable of responding to stimulation; and conductivity, which means that they are able to conduct impulses. They are composed of a cell body (the neurosome or perikaryon), containing the nucleus and its surrounding cytoplasm, and one or more processes (nerve fibers) extending from the cell body.

The processes are actually extensions of the cytoplasm surrounding the nucleus of the neuron. A nerve cell may have only one such slender fiber extending from its body, in which case it is classified as unipolar. A neuron having two processes is bipolar, and one with three or more processes is multipolar. Most neurons are multipolar, this type of neuron being widely distributed throughout the central nervous system and autonomic ganglia. The multipolar neurons have a long single process called an axon and several branched extensions called dendrites. The dendrites receive stimuli from other nerves or from a receptor organ, such as the skin or ear, and transmit them through the neuron to the axon. The axon conducts the impulses to the dendrite of another neuron or to an effector organ that is thereby stimulated into action.

Many processes are covered with a layer of lipid material called myelin. Peripheral nerve fibers have a thin outer covering called the neurilemma.

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adrenergic neuron

nerve cells which secrete norepinephrine as a neurotransmitter; they are mostly sympathetic postganglionic nerves plus some within specific brainstem foci.

association neuron

see internuncial neuron (below).

cholinergic neuron

nerves which synthesize the neurotransmitter acetylcholine in their terminals; they include α-motor neurons of the spinal cord, cranial nerves innervating skeletal muscle, preganglionic sympathetic and postganglionic parasympathetic neurons.

internuncial neuron

neurons found in the brain and spinal cord that conduct impulses between neurons such as from afferent to efferent neurons. Called also association neurons or interneurons.

Golgi n's

see golgi neurons.

lower motor n's

see lower motor neuron.

motor neuron

see motor neuron.

neurosecretory neuron

neurons of the hypothalamus that receive nervous impulses from higher centers and translate them into the regulation of hormone secretion.

nonadrenergic-noncholinergic (NANC) n's

release nitric oxide as a neurotransmitter.

parvicellular neuron

in the hypothalamus; regulate the secretion of adenohypophyseal hormones via releasing and inhibiting factors.

postganglionic n's

neurons whose cell bodies lie in the autonomic ganglia and which relay impulses beyond the ganglia to the effector organ.

preganglionic n's

neurons whose cell bodies lie in the central nervous system and whose efferent fibers terminate in the autonomic ganglia.

neuron transmission

the transmission of impulses along axons by means of electrical impulses and across synapses by neurotransmitters, especially norepinephrine and acetylcholine.

upper motor n's

see upper motor neuron.

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association neuron

Interneuron, internuncial neuron A nerve cell confined to the CNS, which conveys impulses in an arc from sensory to motor neurons