Motor unit

A motor unit comprises a single motor neuron together with all of this innervated muscle fibers and provides for the control of arbitrary as involuntary motor activity of skeletal muscle is the smallest functional unit dar.

The force development ( contraction strength ) of a muscle is determined by the number of activated ( " recruited " ) determines motor units and their properties: the higher the arbitrary tension of the muscle, the more motor units are innervated and contract. The number of innervated by a motor unit muscle fibers vary according to the function of each muscle and the force required for this step:

• Small motor units supply about 100-300 muscle fibers and allow a fine force gradation (eg, external eye muscles and muscles of the fingers ). Older information about smaller motor units of 15-25 fibers (eg in the platysma muscle superior oblique muscle, musculus opponens pollicis ) were collected in part due to stillbirths and are not detected in healthy people.
• Large motor units supply up to 2000 muscle fibers and allow only a rough force gradation (eg four -headed knee extensor muscle - quadriceps femoris).

The properties of the innervating motor neurons are closely matched to the properties of the muscle fibers supplied. There is a continuum of small motor neurons, which have a small cell soma and a relatively thin axon and a small number of muscle fibers cater to large motor neurons, which have a large cell soma and a relatively thick axon and provide a large number of muscle fibers. The muscle fibers are adapted to the discharge frequency of the associated motor neuron in terms of its metabolism. When a muscle contraction leads to a well-ordered activation (also called recruitment ) of initially small motor neurons to the large at maximum force development (see also Hennemansches principle). A second mechanism, which is used by the nervous system to control the development of force is to vary the discharge rate of each motor unit. This process is also referred to as summation.

Determination method

In animals, the number of muscle fibers per motor unit is normally counted in specially pretreated histological section. For this purpose, a nerve fiber is excited, so that the associated muscle fibers of the motor unit work and so consume their glycogen stores. It is a preparation with PAS staining ( glycogen ) is then made ​​and the bright fibers in the section are counted.

On human samples are taken from the muscle, sectioned, and counted the muscle fibers on histological section and divided by the number of myelinated nerve fibers entering minus the estimated number of nerve fibers to muscle spindles by biopsy.

The muscle fibers of a motor unit are not in groups but are in the muscles evenly on a cross-sectional area of up to 1 cm ² distributed. This results in a uniform contraction, even with the activation of fewer motor units.