Motor unit - what is it?

Motor or motor unit is a group of fibers that are innervated by one motor neuron. The number of fibers entering into one unit may vary depending on the function of the muscle. The smaller movements it provides, the smaller the motor unit and the less effort it takes to excite it.

Motor units: their classification.

There is an important point in studying this topic. There are criteria by which any motor unit can be characterized. Physiology as a science, distinguishes two criteria:

• The rate of contraction in response to the impulse;
• Speed of fatigue.

Accordingly, based on these indicators, we can distinguish three types of motor units.

1. Slow, not fatiguing. Their motoneurons contain a lot of myoglobin, which has a high affinity for oxygen. Muscles that have a large number of slow motoneurons are called red because of their specific color. They are necessary for maintaining the human pose and keeping it in balance.
2. Fast, tired. Such muscles are able to perform a large number of contractions in a short period of time. Their fibers contain a lot of energy material, from which by means of oxidative phosphorylation it is possible to obtain ATP molecules.
3. Fast, resistant to fatigue. These fibers contain few mitochondria, and ATP is formed due to the cleavage of glucose molecules. These muscles are called white, because they lack myoglobin.

Units of the first type

The motor unit of the first type or the slow unattenant, occurs most often in large muscles. Such motoneurons have a low excitation threshold and speed of conduction of a nerve impulse. The central process of the nerve cell in its terminal branch branches and innervates a small group of fibers. The frequency of discharges arriving at slow motor units is from six to ten pulses per second. Motoneuron can maintain such a rhythm for several tens of minutes.

The strength and speed of reduction of motor units of the first type is one and a half times less than in other types of motor units. The reason for this is the low rate of ATP formation and slow calcium ion yields on the outer membrane of the cell for binding to troponin.

Units of the second type

The motor unit of this type has a large motoneuron with a thick and long axon, which innervates a large bundle of muscle fibers. These nerve cells have the highest excitation threshold and high speed of nerve impulses.

With maximum muscle tension, the frequency of nerve impulses can reach fifty per second. But motoneuron is not able to maintain this speed for a long time, therefore it quickly becomes tired. The strength and speed of contraction of muscle fibers of the second type is higher than in the previous one, since the number of myofibrils is greater in it. The fibers contain many enzymes that break down glucose, but less mitochondria, myoglobin protein and blood vessels.

Units of the third type

The motor unit of the third type refers to the fast, but fatigue-resistant muscle fibers. According to its characteristics, it should occupy an intermediate value between the first type of motor units and the second. Muscle fibers of such muscles are strong, fast and hardy. For the extraction of energy, it can use both aerobic and anaerobic pathways.

The ratio of fast and slow fibers is genetically determined and can differ for different people. That's why someone is good at running long distances, someone easily overcomes a sprint hundred meter, and someone more suits weightlifting.

Reflex to stretching and motoneuronic pool

When stretching any muscle, slow fibers are the first to react. Their neurons generate discharges of up to ten pulses per second. If the muscle continues to stretch, the frequency of the generated impulses will increase to fifty. This will lead to a reduction in the motor units of the third type and will increase the strength of the muscle tenfold. With further stretching, motor fibers of the second type are connected. This multiplies the strength of the muscle four or five times.

The motor muscular unit is controlled by motoneuron. The aggregate of nerve cells that make up one muscle is called the motor neuron pool. In one pool, neurons from different, in terms of qualitative and quantitative manifestations, of motor units can be simultaneously located. Because of this, the areas of muscle fibers are involved in work not simultaneously, but as the tension and speed of nerve impulses increase.

"Principle of magnitude"

The motor unit of a muscle, depending on its type, is reduced only when a certain threshold load is reached. The order of excitation of motor units is stereotyped: first small motor neurons are shortened, then the nerve impulses gradually reach large ones. This regularity in the middle of the twentieth century was noticed by Edwood Hennemann. He called it the "principle of magnitude".

Brown and Bronk for half a century before published their work on the study of the principle of the operation of muscle units of different types. They suggested that there are two ways to control contractions of muscle fibers. The first is to increase the frequency of nerve impulses, and the second is to involve as many motor neurons as possible in the process.