(noor'on‵ )

[Gr. neuron, nerve, sinew]

The structural and functional unit of the nervous system; a nerve cell. A neuron consists of a perikaryon (cell body) and its processes, an axon and one or more dendrites. Neurons function in the initiation and conduction of impulses. They transmit impulses to other neurons or cells by releasing neurotransmitters at synapses. In addition, a neuron may release neurohormones into the bloodstream. SYN: nerve cell.

SEE: illus.

neuronal,

(noor''ŏn-ăl )

afferent n.A neuron that conducts sensory impulses toward the brain or spinal cord. SYN: sensory neuron.

alpha motor n.Lower motor neuron

association n.Interneuron.

associative n.A neuron that mediates impulses between a sensory and a motor neuron.

bipolar n.A neuron that bears two processes.

central n.A neuron confined entirely to the central nervous system.

commissural n.A neuron whose axon crosses to the opposite side of the brain or spinal cord.

efferent n.A neuron whose axon carries impulses away from the brain or spinal cord.

gamma motor n.A neuron that originates in the anterior horns of the spinal cord and transmits impulses through type A gamma fibers to intrafusal fibers of the muscle spindle for muscle control.

ganglion n.A neuron of the retina that receives impulses from bipolar neurons. Axons of ganglion neurons converge at the optic disk to form the optic nerve.

SEE: retina for illus.

internuncial n.Interneuron.

lower motor n.A motor neuron that originates in the ventral (anterior) horns of the gray matter of the spinal cord or the somatic motor nuclei of the brainstem and terminates in skeletal muscles. Lesions of these neurons produce flaccid paralysis of the muscles they innervate. SYN: alpha motor neuron; lower motoneuron.

mirror n.Any of a group of neurons that become active both when an animal moves in a certain way and when the animal observes others performing the same action. Learning by simulation or imitation is thought to be a function of the system of mirror neurons in the brain.

motor n.A neuron that carries impulses from the central nervous system either to muscle tissue to stimulate contraction or to gland ular tissue to stimulate secretion.

multipolar n.A neuron with one axon and many dendrites. SYN: multipolar cell.

peripheral n.A neuron whose process constitutes a part of the peripheral nervous system. SYN: peripheral motoneuron.

peripheral motor n.A motor neuron that transmits impulses to skeletal muscle. SYN: peripheral motoneuron.

postganglionic n.A neuron of the autonomic nervous system whose cell body lies in an autonomic ganglion and whose axon terminates in a visceral effector (smooth or cardiac muscle or gland s). SYN: postganglionic cell.

preganglionic n.A neuron of the autonomic nervous system whose cell body lies in the central nervous system and whose axon terminates in a peripheral ganglion, synapsing with postganglionic neurons. SYN: preganglionic cell.

sensory n.Afferent neuron.

serotonergic n.A neuron that uses serotonin as its neurotransmitter.

unipolar n.A neuron whose cell body bears one process.

upper motor n.A motor neuron (actually an interneuron) found completely within the central nervous system that synapses with or regulates the actions of lower motor neurons in the spinal cord and cranial nerves. Lesions of these neurons produce spastic paralysis in the muscles they innervate. SYN: upper motoneuron.