Motor neuron

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Neuron: Motor neuron
Medulla oblongata - posterior - cn xii - very high mag.jpg
Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.
LocationVentral horn of the spinal cord, some cranial nerve nuclei
FunctionExcitatory projection (to NMJ)
NeurotransmitterUMN to LMN: glutamate; LMN to NMJ: ACh
MorphologyProjection neuron
Presynaptic connectionsPrimary motor cortex via the Corticospinal tract
Postsynaptic connectionsMuscle fibers and other neurons
NeuroLex IDnifext_103
 
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Neuron: Motor neuron
Medulla oblongata - posterior - cn xii - very high mag.jpg
Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.
LocationVentral horn of the spinal cord, some cranial nerve nuclei
FunctionExcitatory projection (to NMJ)
NeurotransmitterUMN to LMN: glutamate; LMN to NMJ: ACh
MorphologyProjection neuron
Presynaptic connectionsPrimary motor cortex via the Corticospinal tract
Postsynaptic connectionsMuscle fibers and other neurons
NeuroLex IDnifext_103

A motor neuron (or motoneuron) is a nerve cell (neuron) that originates in the motor region of the cerebral cortex or the brain stem, whose cell body is located in the spinal cord and whose fiber (axon) projects outside the spinal cord to directly or indirectly control muscles[citation needed]. Motor neurons are efferent nerves (also called effector neurons), that carry signals from the spinal cord to the muscles to produce (effect) movement.[1] Examples of motor neurons are primary motor neurons, alpha motor neurons, beta motor neurons and gamma motor neurons.

A single motor neuron may innervate many muscle fibres (muscle cells), and a muscle fibre can undergo many action potentials in the time taken for a single muscle twitch (fasciculation). As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through summation or tetanus. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the somatic nervous system arrive before the end of the twitch. The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. On the other hand, tetanus is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau.[2]

Anatomy and physiology[edit]

Branch of NSPositionNeurotransmitter
Somaticn/aAcetylcholine
ParasympatheticPreganglionicAcetylcholine
ParasympatheticGanglionicAcetylcholine
SympatheticPreganglionicAcetylcholine
SympatheticGanglionicNorepinephrine*
*Except fibers to sweat glands and certain blood vessels
Motoneuron neurotransmitters

According to their targets, motor neurons are classified into three broad categories[citation needed]:

Somatic motor neurons, which originate in the central nervous system, project their axons to skeletal muscles [3] (such as the muscles of the limbs, abdominal, and intercostal muscles), which are involved in locomotion .

Special visceral motor neurons, also called branchial motor neurons, which directly innervate branchial muscles (that motorize the gills in fish and the face and neck in land vertebrates).

General visceral motor neurons (visceral motor neurons for short) which indirectly innervate cardiac muscle and smooth muscles of the viscera ( the muscles of the arteries): they synapse onto neurons located in ganglia of the autonomic nervous system (sympathetic and parasympathetic), located in the peripheral nervous system (PNS), which themselves directly innervate visceral muscles (and also some gland cells).

In consequence:

It could be argued that, in the command of visceral muscles, the ganglionic neuron, parasympathetic or sympathetic, is the real motor neuron, being the one that directly innervates the muscle (whereas the general visceral motor neuron is, strictly speaking, a preganglionic neuron).[citation needed] But, for historical reasons, the term motor neuron is reserved for the CNS neuron.

All vertebrate motor neurons are cholinergic, that is, they release the neurotransmitter acetylcholine. Parasympathetic ganglionic neurons are also cholinergic, whereas most sympathetic ganglionic neurons are noradrenergic, that is, they release the neurotransmitter noradrenaline. (see Table)

Function[edit]

The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motor neuron releases a flood of neurotransmitters that bind to postsynaptic receptors and triggers a response in the muscle fiber which leads to muscle movement.

Somatic motor neurons[edit]

Somatic motor neurons are the alpha efferent neurons, beta efferent neurons, and gamma efferent neurons. They are called efferent to indicate the flow of information from the central nervous system (CNS) to the periphery.

In addition to voluntary skeletal muscle contraction, alpha motor neurons also contribute to muscle tone, the continuous force generated by noncontracting muscle to oppose stretching. When a muscle is stretched, sensory neurons within the muscle spindle detect the degree of stretch and send a signal to the CNS. The CNS activates alpha motoneurons in the spinal cord, which cause extrafusal muscle fibers to contract and thereby resist further stretching. This process is also called the stretch reflex.

Motor units[edit]

A single motor neuron may synapse with one or more muscle fibers.[4] The motor neuron and all of the muscle fibers to which it connects is a motor unit. Motor units are split up into 3 categories:[4]

See also[edit]

References[edit]

  1. ^ Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth
  2. ^ Russell, Peter (2013). Biology - Exploring the Diversity of Life. Toronto: Nelson Education. p. 946. ISBN 978-0-17-665133-6. 
  3. ^ Silverthorn, Dee Unglaub (2010). Human Physiology: An Integrated Approach. Pearson. p. 398. ISBN 978-0-321-55980-7. 
  4. ^ a b c d e Purves D, Augustine GJ, Fitzpatrick D, et al., editors: Neuroscience. 2nd edition, 2001 [1]