motor neuron

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Related to autonomic motor neuron: autonomic nervous system

neuron

 [noor´on]
a highly specialized cell of the nervous system, having two characteristic properties: irritability (ability to be stimulated) and conductivity (ability to conduct impulses). They are composed of a cell body (called also neurosome or perikaryon), containing the nucleus and its surrounding cytoplasm, and one or more processes (nerve fibers) extending from the body. Called also nerve cell. adj., adj neuro´nal.  

The nerve fibers 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 is widely distributed throughout the central nervous system and autonomic ganglia. The multipolar neurons have a 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 to action. Many processes are covered with a layer of lipid material called the myelin sheath. Peripheral nerve fibers have a thin outer covering called neurilemma.
Types of Neurons. Neurons that receive stimuli from the outside environment and transmit them toward the brain are called afferent or sensory neurons. Those that carry impulses in the opposite direction, away from the brain and other nerve centers to muscles, are called efferent neurons, motor neurons, or motoneurons. Another type, the interneuron, found in the brain and spinal cord, conducts impulses from afferent to efferent neurons.
Synapses. The point at which an impulse is transmitted from one neuron to another is called a synapse. The transmission is chemical in nature; that is, there is no direct contact between the axon of one neuron and the dendrites of another. The cholinergic nerves (parasympathetic nervous system) liberate at their axon endings a substance called acetylcholine, which acts as a stimulant to the dendrites of adjacent neurons. In a similar manner, the adrenergic nerves (sympathetic nervous system) liberate epinephrine or related substances. The synapse may involve one neuron in chemical contact with many adjacent neurons, or it may involve the axon terminals of one neuron and the dendrites of a succeeding neuron in a nerve pathway. There are many different patterns of synapses.
Receptor End-Organs. The dendrites of the sensory neurons are designed to receive stimuli from various parts of the body. These dendrites are called receptor end-organs and are of three general types: exteroceptors, interoceptors, and proprioceptors. The exteroceptors are located near the external surface of the body, receive impulses from the skin, and transmit information about the senses of touch, heat, cold, and other factors in the external environment. The interoceptors are located in the internal organs and receive information from the viscera, e.g., pressure, tension, and pain. The proprioceptors are found in muscles, tendons, and joints and transmit “muscle sense,” by which one is aware of the position of one's body in space.
Neurons and Effectors. The axons of motor neurons form synapses with skeletal fibers to produce motion. These junctions are called motor end-plates or myoneural junctions. The axon of a motor neuron divides just before it enters the muscle fibers and forms synapses near the nuclei of muscle fibers. These motor neurons are called somatic efferent neurons. Visceral efferent neurons form synapses with smooth muscle, cardiac muscle, and glands.
Upper and lower motor neurons. From Damjanov, 2000.
Golgi n's
1. (type I): pyramidal cells with long axons, which leave the gray matter of the central nervous system, traverse the white matter, and terminate in the periphery.
2. (type II): stellate neurons with short axons in the cerebral and cerebellar cortices and in the retina.
motor neuron motoneuron.
postganglionic n's neurons whose cell bodies lie in the autonomic ganglia and whose purpose is to relay impulses beyond the ganglia.
preganglionic n's neurons whose cell bodies lie in the central nervous system and whose efferent fibers terminate in the autonomic ganglia.

mo·tor neu·ron

a nerve cell in the spinal cord, rhombencephalon, or mesencephalon characterized by having an axon that leaves the central nervous system to establish a functional connection with an effector (muscle or glandular) tissue; somatic motor neuron directly synapse with striated muscle fibers by motor endplates; visceral motor neuron or autonomic motor neuron (preganglionic m. neurons), by contrast, innervate smooth muscle fibers or glands only by the intermediary of a second, peripheral, neuron (postganglionic m. neuron) located in an autonomic, or visceral motor, ganglion.
See also: motor endplate, autonomic (visceral motor) division of nervous system.

motor neuron

n.
A neuron that conveys impulses from the central nervous system to a muscle, gland, or other effector tissue.

motor neuron

one of various efferent nerve cells that transmit nerve impulses from the brain or from the spinal cord to muscular or glandular tissue. According to location, some kinds of motor neurons are the peripheral motor neurons and the upper motor neurons. Also called motoneuron. Compare sensory nerve. See also nervous system.

mo·tor neu·ron

(mō'tŏr nūr'on)
A nerve cell in the spinal cord, rhombencephalon, or mesencephalon characterized by an axon that leaves the central nervous system to establish a functional connection with an effector (muscle or glandular) tissue; somatic motor neurons directly synapse with striated muscle fibers by motor endplates; visceral motor neurons or autonomic motor neurons (preganglionic motor neurons), by contrast, innervate smooth muscle fibers or glands only by the intermediary of a second, peripheral, neuron (postganglionic or ganglionic motor neuron) located in an autonomic ganglion.
See also: motor endplate, autonomic division of nervous system
Synonym(s): motoneuron, motoneurone.

motor neuron

a nerve cell, the fibre of which conducts a stimulus from the CENTRAL NERVOUS SYSTEM to the effector organ such as STRIATED MUSCLE or a gland, which is then stimulated into activity.

Motor neuron

Nerve cells within the central nervous system that carry nerve impulses controlling muscle movement.

neuron

Structural unit of the nervous system consisting of the nerve cell body and its various processes, the dendrites, the axon and the ending (also called bouton, end foot or axon terminal). There are many types of neurons within the nervous system; some transmit afferent nerve impulses to the brain (e.g. those carrying information from the photoreceptors to the visual cortex), or to the spinal cord (e.g. those carrying information from the receptors in the skin to the spinal cord). They are called sensory neurons. Others transmit efferent motor nerve impulses to a muscle (e.g. those carrying information from the Edinger-Westphal nucleus to the sphincter pupillae and ciliary muscles). These are called motor neurons. Other neurons carry nerve impulses from one neuron to another (internuncial neurons). Note: also spelt neurone. See action potential; synapse.

mo·tor neu·ron

(mō'tŏr nūr'on)
A nerve cell in the spinal cord, rhombencephalon, or mesencephalon characterized by an axon that leaves the central nervous system to establish a functional connection with an effector (muscle or glandular) tissue.
Synonym(s): motoneuron, motoneurone.

motor neuron

a neuron having a motor function; an efferent neuron conveying motor impulses.

motor neuron disease
equine motor neuronn. disease
(EMND) a syndrome of mature horses of uncertain etiology, but possibly an oxidative disorder associated with insufficient green forage in the diet and a deficiency of vitamin E. Characterized by generalized weakness, muscle fasciculations and weight loss, despite a normal appetite, that is caused by neurogenic muscle atrophy. There is also a distinctive retinopathy.
lower motor neuron n's
peripheral neurons whose cell bodies lie in the ventral gray columns of the spinal cord and whose terminations are in skeletal muscles. Called also LMN. See also lower motor neuron.
peripheral motor neuron n's
neurons in a peripheral reflex arc that receive impulses from interneurons and transmit them to voluntary muscles.
upper motor neuron n's
neurons in the cerebral cortex that conduct impulses from the motor cortex to the motor nuclei of the cerebral nerves or to the ventral gray columns of the spinal cord. Lesions of the upper motor neuron interrupt the inhibitory effect that upper motor neurons have on lower motor neurons, resulting in exaggerated or hyperactive reflexes. This is called also extensor rigidity. See also upper motor neuron. Called also UMN.
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