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Motor neuron

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Motor neuron
Details
LocationVentral horn of the spinal cord, some cranial nerve nuclei
ShapeProjection neuron
FunctionExcitatory projection (to NMJ)
NeurotransmitterUMN to LMN: glutamate; LMN to NMJ: ACh
Presynaptic connectionsM1 via the Corticospinal tract
Postsynaptic connectionsMuscle fibers and other neurons
Identifiers
MeSHD009046
NeuroLex IDnifext_103
TA98A14.2.00.021
TA26131
FMA83617
Anatomical terms of neuroanatomy

In neurology, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons. Motor neurones are neurones that carry signals from the spinal cord to the muscles to produce movement. [1]

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Function

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.

  • In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory.
  • For vertebrates, however, the response of a muscle fiber to a neurotransmitter can only be excitatory, in other words, contractile. Muscle relaxation and inhibition of muscle contraction in vertebrates is obtained only by inhibition of the motor neuron itself. Although muscle innervation may eventually play a role in the maturation of motor activity. This is why muscle relaxants work by acting on the motoneurons that innervate muscles (by decreasing their electrophysiological activity) or on cholinergic neuromuscular junctions, rather than on the muscles themselves.

Somatic motor neurons

Somatic motoneurons are further subdivided into two types: alpha efferent neurons and gamma efferent neurons. (Both types 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 motoneurons 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.

Gamma motoneurons regulate the sensitivity of the spindle to muscle stretching. With activation of gamma neurons, intrafusal muscle fibers contract so that only a small stretch is required to activate spindle sensory neurons and the stretch reflex.

Motor units

Acording to Purves D, et al [2].A single motor neuron may synapse with one or more muscle fibers. 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: slow motor units, fast fatiguing motor units, and fast fatigue-resistant motor units.

  • Slow motor units are used to stimulate small muscle fibres which contract very slowly and provide small amounts of energy but are very resistant to fatigue, so they are used to sustain muscular contraction such keep the body upright.
  • The Fast Fatiguing motor units are used to stimulate larger muscle groups which apply large amounts of force but fatigue very quickly. They are used for tasks that requires large brief bursts on energy such as jumping or running.
  • The Fast Fatigue-Resistant motor unit stimulate moderate sized muscles groups that don't react as fast as the FF motor units, but can be sustained much longer, as implied by the name, and provide more force that the S motor unit.

See also

References

  • Sherwood, L. (2001). Human Physiology: From Cells to Systems (4th ed.). Pacific Grove, CA: Brooks-Cole. ISBN 0-534-37254-6.
  • Marieb, E. N.; Mallatt, J. (1997). Human Anatomy (2nd ed.). Menlo Park, CA: Benjamin/Cummings. ISBN 0-8053-4068-8.
  1. ^ Schacter D.L., Gilbert D.T., and Wegner D.M. (2011) Psychology second edition. New York, NY: Worth
  2. ^ Purves D, Augustine GJ, Fitzpatrick D, et al., editors: Neuroscience. 2nd edition, 2001 [1]