Innervation Ratio and Muscle Precision
The number of muscle fibers supplied by a single motor unit varies widely depending on the specific function of the muscle. This ratio is called the innervation ratio.
There is a strict physiological rule: the more precise and delicate movements a muscle performs, the greater the number of motor units it contains, and consequently, the fewer muscle fibers each unit supplies.
- Exquisite precision: In extraocular muscles, a single motoneuron innervates only 3–4 muscle fibers (ratio 1:3). This allows the eye to perform precise micro-movements.
- Gross power: In large muscles, such as the latissimus dorsi, a single motoneuron can innervate up to 300 fibers (ratio 1:300), where sheer power takes precedence over fine regulation.
The collection of all muscle fibers belonging to a single motor unit is called a muscle unit. Its hallmark feature is histochemical homogeneity. This means that all fibers innervated by a given neuron always belong to the same type (e.g., exclusively Type I, Type IIA, or Type IIX/IIB). Conversely, all motoneurons controlling an entire muscle form a motoneuron pool.
Classification of Motor Unit Types
Based on motoneuron size, conduction velocity, and the characteristics of the muscle fibers themselves, motor units are divided into three main types.
1. Type I (S-type: Slow, fatigue-resistant) This forms the basis of physical endurance and posture. The motoneuron has a small cell body (soma) and a thin axon, resulting in a low conduction velocity. Muscle fibers are red and of small diameter. They are oxidative fibers, making them exceptionally rich in mitochondria and myoglobin. The threshold of excitation for Type I is the lowest: they are activated first under weak stimulation. They contract slowly, generate low force, but can operate for hours without fatigue.
2. Type IIA (FR-type: Fast, fatigue-resistant) These occupy an intermediate position. The neuronal cell body and axon thickness are medium. The fibers are oxidative-glycolytic. They have an intermediate excitation threshold and moderate contraction amplitude. They operate faster than slow units and display moderate fatigue resistance during short-term exertion.
3. Type IIX (FF-type: Fast fatigable) (Often designated as Type IIB in older literature). These are responsible for maximal, "explosive" power. A large motoneuron features a thick axon ensuring very high signal conduction velocity. It innervates large-diameter white (pale) fibers (glycolytic type) containing abundant glycogen and minimal myoglobin. The excitation threshold is very high: they are recruited last, only under massive stimulation. Contraction is lightning-fast with maximal amplitude, but these units deplete extremely quickly.
Regulation of Muscle Tone
Muscle tone is essential for maintaining posture in space. This function is carried out primarily through the activity of "red" muscle fibers (Type I). Evolutionarily, humans possess a higher proportion of these fibers in extensor muscles, which act as antigravity muscles maintaining an upright posture.
Red fibers are characterized by a high myoglobin content, low propagation velocity of excitation, and a slow, smooth development of contraction. A complex receptor apparatus participates in generating and continuously regulating tone, including:
- Muscle spindles.
- Tendon organs (Golgi tendon organs).
- Joint capsule receptors.
Henneman's Size Principle
A fundamental physiological law of the neuromuscular apparatus is Henneman's Size Principle, which governs the recruitment of motor units.
As muscle contraction force increases, motor units are recruited in a strict and invariant order: from smallest to largest. MU-I $\rightarrow$ MU-IIA $\rightarrow$ MU-IIX
Physiological significance: This principle ensures ideal smoothness of movement and maximal energy conservation by the body. Smaller, low-threshold neurons (Type I) have a higher input resistance. Consequently, they depolarize significantly faster even with weak synaptic drive. Meanwhile, large, high-threshold neurons (Type IIX) require very powerful and prolonged stimulation to reach their activation threshold and fire signals to their white fibers.