A motor or motor unit is a group of fibers that are innervated by one motor neuron. The number of fibers included in one unit may vary depending on muscle function. The smaller movements it provides, the smaller the motor unit and the less effort is needed 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:
- contraction rate in response to an impulse;
- fatigue rate.
Accordingly, based on these indicators, three types of motor units can be distinguished.
- Slow, not tiring. Their motor neurons contain a lot of myoglobin, which has a high affinity for oxygen. Muscles that have a lot of slow motor neurons are called red because of their specific color. They are necessary to maintain a person’s posture and keep him in balance.
- 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 energetic material, from which ATP molecules can be obtained using oxidative phosphorylation .
- Fast, resistant to fatigue. These fibers contain few mitochondria, and ATP is formed due to the breakdown 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 slow, fatigue, is most often found in large muscles. Such motor neurons have a low threshold of excitation and the speed of the nerve impulse. The central process of the nerve cell in its terminal section 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 this rhythm for several tens of minutes.
The strength and speed of contraction of the motor units of the first type is one and a half times less than that of other types of motor units. The reason for this is the low rate of ATP formation and the slow release of calcium ions to the outer membrane of the cell for binding to troponin.
Units of the second type
The motor unit of this type has a large motor neuron with a thick and long axon that innervates a large bundle of muscle fibers. These nerve cells have the highest threshold of excitation and the highest speed of nerve impulses.
With maximum muscle tension, the frequency of nerve impulses can reach fifty per second. But the motor neuron is not able to maintain such a speed for a long time, so it quickly gets tired. The strength and speed of contraction of muscle fiber of the second type is higher than that of the previous one, since the number of myofibrils in it is greater. The fibers contain many enzymes that break down glucose, but fewer mitochondria, myoglobin protein, and blood vessels.
Units of the third type
The motor unit of the third type is 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. The muscle fibers of such muscles are strong, fast and hardy. It can use both aerobic and anaerobic pathways to produce energy.
The ratio of fast and slow fibers is genetically determined and may vary in different people. That is why someone is good at long-distance running, someone easily overcomes the sprinting hundred meters, and weightlifting is more suitable for someone.
Tensile Reflex and Motor Neural 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, then 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 increase muscle strength by ten times. With further stretching, motor fibers of the second type will be connected. This will increase muscle strength by another four to five times.
The muscular motor unit is driven by a motor neuron. The collection 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, motor units can simultaneously be located. Because of this, sections of muscle fibers are included in the work not simultaneously, but as the voltage and speed of nerve impulses increase.
"The principle of quantity"
The motor unit of the 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 reduced, then nerve impulses gradually get to large ones. This pattern was noted in the mid-twentieth century by Edwood Hennemann. He called it the "principle of magnitude."
Brown and Bronk half a century before published their works on the study of the principle of work of different types of muscle units. They suggested that there are two ways to control muscle fiber contractions. The first of these is to increase the frequency of nerve impulses, and the second is to involve as many motor neurons as possible in the process.