Motor neuron
The term motor neuron or motor neuron are efferent (exporting) neurons summarized that the muscles of the body innervate and thus are based on active contractions of skeletal muscles.
One differentiates that
- upper motoneuron (engl. upper motoneuron , UMN or 1. motoneuron ) and the
- lower motoneuron (engl. Lower motoneuron , LMN or 2. motor neuron ).
The flow of information in most movements or control of muscle tension goes from the brain via the upper motor neurons to the lower neurons to the muscles. An exception is, for example, the reflex arc .
Upper motor neuron (UMN)
The UMN is responsible for the conscious triggering of movement ( voluntary motor skills ) and also controls posture. Its cell bodies , the giant Betz cells , are located in the motor cortex in the brain . The axons form the pyramidal tract . They never pull directly to the muscles, but always to the LMN, so the actions of the UMN are always mediated via the LMN in the spinal cord.
In a broader sense, the UMN - in addition to the pyramidal tracts described - also includes extrapyramidal motor tracts that start at other parts of the brain.
Lower motor neuron (LMN)
The LMN is the actual pulse generator for the muscles. The cell bodies of the lower motor neurons for the muscles of the trunk and limbs as well as parts of the neck muscles are located in the anterior horn (better known as the ventral horn in four-legged animals ) of the gray matter of the spinal cord . These motor root cells form what is known as the motor core column over the entire length of the spinal cord. In each spinal segment leaving axons via the respective spinal nerve to the spinal canal . This pulls, dividing into several branches, to the motor end plates of the muscles of its supply area ( myotome ). For the striated muscles of the head, the cell bodies are located in the motor nuclei ( nuclei motorii ) of the cranial nerves .
The LMN is the efferent limb of all movements and reflexes . The nerve fibers of the LMNs can be divided into two types:
- The Aα fibers innervate the extrafusal skeletal muscle fibers and are responsible for muscle contraction.
- The Aγ fibers innervate the intrafusal skeletal muscle fibers and regulate the sensitivity of the length receptors.
Damage to the LMN leads to a failure of the associated muscles and - if a sufficient number of motor neurons are affected - to a loss of strength ( paresis ) and failure of the relevant reflexes. If only a single spinal cord segment is affected, the plexus formation of the spinal nerves can also lead to only partial failure symptoms. A decreased reflex can also be a sign of damage to sensitive neurons.
literature
- Martin Trepel: Neuroanatomy . Urban & Fischer, 3rd edition 2003. ISBN 3-437-41297-3 .
- Franz-Viktor Salomon: nervous system, systema nervosum . In: Salomon / Geyer / Gille (Hrsg.): Anatomy for veterinary medicine . Enke Stuttgart, 2004, pp. 464-577. ISBN 3-8304-1007-7 .