Motor units

Innervation ratio and muscle precision

The number of muscle fibers per motor unit varies greatly depending on the function performed by a specific muscle. This ratio is called the innervation ratio.

There is a strict physiological pattern: the more precise and delicate movements a muscle must perform, the more motor units it contains, and consequently, the fewer muscle fibers each of them includes.

• Exquisite precision: In the extraocular muscles of the eye, a single motor neuron innervates only 3–4 muscle fibers (ratio 1:3). This allows the eye to perform extremely precise micromovements.
• Gross strength: In large muscles, such as the latissimus dorsi, a single motor neuron can innervate up to 300 fibers (ratio 1:300), since power is important here rather than fine regulation.

The set of all muscle fibers making up a single motor unit is called a muscle unit. Its most important feature is histochemical homogeneity. This means that all fibers controlled by a single neuron always belong to the same type (e.g., Type I only, Type IIA only, or Type IIB only). In turn, all motor neurons controlling an entire muscle form a motor neuron pool.

Classification of motor unit types

Depending on the size of the motor neuron, impulse conduction velocity, and the characteristics of the muscle fibers themselves, all motor units (MUs) are divided into three main types.

1. Type I (S-type: Slow — slow fatigue-resistant) This is the basis of our physical endurance and posture. The motor neuron has a small cell body (soma) and a thin axon, resulting in a low conduction velocity. The muscle fibers are red and of small diameter. They are of the oxidative type, making them extremely rich in mitochondria and myoglobin. The threshold of excitation for Type S is the lowest: they are activated first under a weak stimulus. They contract slowly, develop low force, but are capable of working for hours without fatiguing.

2. Type IIA (FR-type: Fast Fatigue-Resistant) They occupy an intermediate position. The neuron body and axon thickness are medium. The fibers are of the oxidative-glycolytic type. They have a medium excitation threshold and medium contraction amplitude. They work faster than slow units and demonstrate moderate fatigue resistance during short-term exertion.

3. Type IIB (FF-type: Fast Fatigable) (Often designated as IIX in modern literature). Responsible for maximum "explosive" power. A large motor neuron possesses a thick axon providing a very high signal conduction velocity. It innervates large-diameter white (pale) fibers (glycolytic type) containing abundant glycogen and little myoglobin. The excitation threshold is very high: they are activated last, only under extremely powerful stimuli. Contraction is lightning-fast and amplitude is maximal, but such units deplete extremely quickly.

Regulation of muscle tone

Muscle tone is necessary to maintain human posture in space. This function is carried out primarily through the activity of "red" muscle fibers (Type I). Evolutionarily, humans have more of these fibers in extensor muscles, which act as antigravity muscles maintaining upright posture.

Red fibers are characterized by a high myoglobin content, a low conduction velocity, and a slow, smooth development of contraction. A complex receptor apparatus participates in the formation and continuous regulation of tone. It includes:

1. Muscle spindles.
2. Tendon organs (Golgi tendon organs).
3. Joint capsule receptors.

Henneman's size principle

The fundamental physiological law governing the neuromuscular apparatus is Henneman's size principle (Henneman's Size Principle), or the law of motor unit recruitment.

As muscle contraction force increases, motor units are recruited in a strict and invariable order: from the smallest to the largest. MU-I $\rightarrow$ MU-IIA $\rightarrow$ MU-IIB

Physiological significance: This principle ensures ideal smoothness of movement and maximum energy conservation by the body. Small low-threshold neurons (Type I) have a higher input resistance. As a result, they depolarize significantly faster even with weak synaptic input. At the same time, large high-threshold neurons (Type IIB) require very powerful and prolonged stimulation to reach the activation threshold and send a signal to their white fibers.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Histochemical and metabolic features of muscle fibers (oxidative and glycolytic types of metabolism).
• Mechanisms of the receptor apparatus: muscle spindles and Golgi organs.
• Dependence of excitation conduction velocity on motor neuron axon diameter.
• Pathophysiology of motor units in spinal cord diseases.

Other topics in the "Motor Control" section

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