Human movement is the result of an interaction between the nervous system and the muscles – the nervous system produces signals which tell the muscles to switch on and with how much force. In order to coordinate movement efficiently, these fibres are sub-divided into functional units. Enter the motor unit.
Motor units consist of a motor neuron (located in the spinal cord and brain stem) and the muscle fibres it innervates. The fibres that are housed in a single motor unit cover a cross-sectional area of 15% of the muscle and are mixed in with fibres from other motor units.
Several motor units within a single muscle control the force of the muscle contraction. This group of motor units within a single muscle is called the motor pool.
The human body houses around 250 million muscle fibres, but with roughly 420,000 motor neurons at our disposal, innervation must be distributed accordingly.
The amount of muscle fibres that are innervated by each individual motor neuron varies from muscle to muscle and is dictated by the function of that particular muscle.
For example, motor neurons which control fibres in the eye are responsible for less than ten fibres due to the complex and precise movements that are required of them. In contrast, the motor neurons of larger muscle groups such as the glutes and leg muscles may innervate 2000-3000 muscle fibres at one time due to their requirement of high force output.
The number of motor units contained in a single muscle also depends on the complexity and delicateness of the task which is performed by the muscle.
We can look at two muscles which perform very different functions in the body for an example; the first dorsal interosseous (finger) and the medial gastrocnemeous (calf).
The first dorsal interosseous contains 41,000 muscle fibres which are controlled by 120 motor units, while the medial gastrocnemeous has over one million muscle fibres which are controlled by just 580 motor units. This averages roughly 340 muscle fibres per motor unit in the finger muscle and 1,800 fibres per motor unit in the lower leg muscle (6).
So, the fewer fibres in a motor unit, the more precise the muscle action can be.Follow MuayThaiScholar