Motoneuron

under the term Motoneuron or motor neuron are summarized the efferent nerve courses, which are responsible for all courses of motion the musculature of the body inside four and therefore.

One differentiates between

• the lower Motoneuron (“more lower motoneuron “, LMN or 2. Motoneuron) and
• the upper Motoneuron (“more upper motoneuron “, UMN or 1. Motoneuron).

lower Motoneuron (LMN)

the LMN is the actual pulse generator for muscles. The Zellkörper of its Neurone for the trunk and the member masses are appropriate for the grey substance of the Rückenmarks in the vent ral horn (called with humans also front horn ). These motor root cells form the so-called motor core column over the entire length of the Rückenmarks. In each back Mark segment axons leave the eddy channel over the Spinalnerv. This pulls, dividing into several branches, to the motor end plates to muscles of its supply area (Myotom). For the musculature of the head the Zellkörper lie in the motor cores (Nuclei motorii) of the cranial nerves.

The LMN is the efferent (implementing) thigh of all movements and reflexes.

A strong damage of the LMN leads to a total failure of the associated musculature (paralysis) and to a complete loss of the reflexes. If only an individual back Mark segment is concerned, it comes due to the Plexus formation of the Spinalnerven however only to partial loss features. A decreased reflex can be also an indication of a partial damage of the back Mark segment.

upper Motoneuron (UMN)

the UMN is responsible for the conscious release of the movement (Willkürmotorik) and steers the body attitude. Its origin lies in the brain. The UMN does not pull directly to the muscles, but to the LMN, its actions are always obtained thus always over the LMN in back Marks.

One differentiates with respect to the UMN:

• Pyramidales system and
• Extrapyramidales system

Pyramidales system (HP)

the Pyramidale system is responsible for the Feinmotorik. It begins in the motor Cortex (Gyrus praecentralis), thus in the cerebral cortex. From there the fibers run over the white substance of the Rückenmarks, usually without change-over to the LMN. HP is particularly well developed with humans, while it plays a subordinated role with animals only.

The main part HP is the pyramid course (Tractus corticospinalis). It is both sides visible at the lower surface of the Medulla oblongata as shallow longitudinal bulge (pyramid , Pyramis). In the pyramid crossing (Decussatio pyramidum), at the transition between Nachhirn and back Marks, crosses 70 to 90 per cent of the courses as Tractus corticospinalis lateralis on those in each case different side (“against-laterally”), which remaining run as Tractus corticospinalis anterior paramedian in the front strand of the Rückenmarks and cross segmental in the front horn of the against-lateral side of the Rückenmarks. The extent of the crossing is however different with the individual mammal kinds. With humans and also with the dog the majority of the fibers crosses. With hoof animals only about half of the courses crosses.

HP predominantly pulls to the interneurons of the Rückenmarks and steers via these the motor root cells of the LMN in back Marks.

A one-sided damage of the Pyramidalen of system (e.g. by an impact accumulation) leads with humans and Primaten usually to a complete paralysis (paralysis) of the opposite side (due to the pyramid crossing). With many mammals the losses are far less dramatic, since that is not so important HP with them. Here “only” attitude disturbances of the neck arise and the loss of the position reactions, even if one removes the entire motor Cortex of a side. The kind-typical movement samples are hardly changed, since they predominantly proceed from the extrapyramidalen system and thus with other brain parts.

Extrapyramidales system (EPS)

it rises both from the Cortex and from numerous other central areas of the brain. It steers the rougher courses of motion and ensures by interconnecting among other things with the Kleinhirn, the optical reflex center and the equilibrium cores for the harmony of the movements and correction of the body attitude. It steers all automatic movements (e.g. feeling-stressed gesturing) and affects the tonus of the musculature. With the animals it is however, more important contrary to humans, many for the Willkürmotorik than HP.

The EPS is polysynaptisch, has thus always several switching places (synapses). The change-over takes place in the cores of numerous brain sections. The most important stations in the extrapyramidalen system are more ruber the Nucleus and the Formatio reticularis, since only these pull directly to the LMN. Most important neurotransmitter of the EPS is Dopamine.

Damage of the extrapyramidalen system is predominantly characterized by the omission of the restraining (inhibitorischen) influences on the UMN with humans. Thus it comes to illnesses with a strongly increased movement such as Chorea Huntington or park in on illness. In addition reflexes can be increased due to the missing Hemmwirkung of inhibitorischer Interneurone on the UMN with EPS damage (Hyperreflexie). With not Primaten usually halflateral pareses ( Hemiparese) with disturbed attitude and position reactions arise. The damage before the Nucleus lies more ruber however hardly gives it to disturbances in the motion.

A back Mark injury with paraplegia draws usually also a damage of the extrapyramidalen courses in back Marks. Since the pyramidalen courses are impaired at the same time, no more reassuring impulses arrive, and the tonus and/or at the muscle. the tonus strengthening reflex elbow of back Marks can intersperse itself. Consequence is frequent Spastik.

see also

basal ganglia, Babinski reflex, flabby paralysis

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