Motor or motor unit is a group of fibers that are innervated by a single motor neuron. The number of fibers included in one unit may vary depending on the function of the muscle. The smaller the 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 the study of this topic. There are criteria by which any motor unit can be characterized. Physiology as a science distinguishes two criteria:
- speed of contraction in response to impulse conduction;
- fatigue speed.
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 large number 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, fatigued. 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 ATP molecules can be obtained using oxidative phosphorylation.
- Fast, fatigue resistant. 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
Motor unit of the first type or slow tireless, found most often in large muscles. Such motoneurons have a low threshold of excitation and the speed of the nerve impulse. The central process of the nerve cell branches in its terminal section and innervates a small group of fibers. The frequency of discharges to slow motor units is from six to ten impulses per second. The motor neuron can maintain this rhythm for several tens of minutes.
The strength and speed of contraction of 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 cell membrane 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.
At maximum voltagemuscles, the frequency of nerve impulses can reach fifty per second. But the motor neuron is not able to maintain such a speed of conduction for a long time, therefore it quickly gets tired. The strength and speed of contraction of the muscle fiber of the second type is higher than that of the previous one, since the number of myofibrils in it is greater. Fiber contains many enzymes that break down glucose, but fewer mitochondria, myoglobin protein and blood vessels.
Third type units
Motor unit of the third type refers to 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. They can use both aerobic and anaerobic pathways to extract energy.
The ratio of fast and slow fibers is genetically determined and may differ from person to person. That is why someone is good at long-distance running, someone easily overcomes the sprint hundred meters, and someone is more suitable for weightlifting.
Stretch reflex and motor neuron pool
When stretching any muscle, slow fibers are the first to react. Their neurons fire 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 contraction of the third type of motor units and increase the strength of the muscle tenfold. Atfurther stretching will connect motor fibers of the second type. This will multiply the strength of the muscle by another four to five times.
The motor muscle unit is controlled by a motor neuron. The set of nerve cells that make up one muscle is called the motor neuron pool. One pool can simultaneously contain neurons from different qualitative and quantitative manifestations of motor units. Because of this, sections of muscle fibers are not activated at the same time, but as the tension and speed of nerve impulses increase.
Principle of magnitude
The motor unit of a muscle, depending on its type, contracts only when a certain threshold load is reached. The order of excitation of motor units is stereotypical: first, small motor neurons contract, then nerve impulses gradually reach large ones. This pattern was noticed in the middle of the twentieth century by Edwood Henneman. He called it the “principle of magnitude.”
Brown and Bronk half a century before published their works on the study of the principle of operation of muscle units of different types. They suggested that there are two ways to control the contractions of muscle fibers. The first of them is to increase the frequency of nerve impulses, and the second is to involve as many motor neurons as possible in the process.
