Motor neuron

The term motor neuron or motor neuron efferent neurons are summarized, which innervate the muscles of the body and thus are based on active contractions of skeletal muscles.

A distinction is

• Lower motor neuron ( engl. lower motoneuron, LMN or second motor neuron ) and the
• Upper motor neuron (English upper motoneuron, UMN or motor neuron 1 ).

Lower motor neuron ( LMN )

The LMN is the actual driving force for the muscles. The cell bodies of lower motor neurons for the muscles of the trunk and limbs and parts of the neck muscles are located in the anterior horn (better than the ventral horn in four-legged animals ) represented by the gray matter of the spinal cord. This motor root cells form the so-called core-column motor over the entire length of the spinal cord. In each spinal segment to the respective spinal nerve axons leave the spinal canal. This draws to sharing into several branches, the motor endplates of the muscles of his coverage area ( myotome ). For the striated muscles of the head of the cell bodies lie in the motor nuclei ( nuclei motorii ) of the cranial nerves.

The LMN is the efferent (Executive ) leg 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 A? 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 in a loss of power ( paresis ) and failure of the relevant reflexes. When only a single spinal cord segment is concerned, it can also lead to only partial deficits due to the plexuses of the spinal nerves. A reduced reflex can also be signs of damage to sensitive neurons.

Upper motor neuron ( UMN )

The UMN is responsible for the deliberate release of the movement ( voluntary movement ) and also controls the posture. Its cell body, the Betz giant cells are located in the motor cortex in the brain. The axons form the corticospinal tract. Never pull on the muscles, but always to the LMN, the actions of the UMN are always already mediated by the LMN in the spinal cord.

More broadly, is meant by the UMN - in addition to those described pyramidal tracts - also extrapyramidal pathways that take their starting point in other parts of the brain.