What are skeletal muscle fibers?

Speaking of bones, everyone knows that they are in everyone's body. Whether old or young, everyone has bones of different sizes. It’s just that because everyone’s body shape or physique is different, the bones in the body are also different. The same bones are different, which means that the skeletal muscle fibers are also different. Based on this situation, everyone wants to know what skeletal muscle fibers are like?

Morphological characteristics

1. Structural characteristics

Generally speaking, the diameter of type I muscle fibers in most human skeletal muscles is slightly smaller than that of type II muscle fibers, and the sarcoplasmic reticulum of type II muscle fibers is twice as developed as that of type I muscle fibers. Therefore, the Ca2 uptake capacity of the sarcoplasmic reticulum of type II muscle fibers is greater than that of type I muscle fibers, thereby accelerating the reaction speed of type II muscle fibers; the number of mitochondria in type I muscle fibers is more and the diameter is larger than that of type I muscle fibers. At the same time, the capillaries around type I muscle fibers are more than those around type II muscle fibers, and the myofibril content in type II muscle fibers is more than that in type I muscle fibers, which means that the muscle fibers contain more myosin cross bridges and can generate greater contractile force during contraction. Morphological characteristics of different types of skeletal muscle fibers

2. Neural innervation

Characteristic muscle type Type I Type IIa Type IIb

Average muscle fiber area/um2 1730 2890

Motor unit 540·μ-1 440·μ-1 750·μ-1

Axonal conduction velocity/m·s-1 8.5 100 100

How many capillaries are there?

Mitochondrial content high, medium, low

Sarcoplasmic reticulum (SR): The SR of type II muscle fibers is twice that of type I muscle fibers.

The Z band of type I muscle fibers is wider than that of type II muscle fibers.

Connective tissue type I muscle fibers have more collagen fibers than type II muscle fibers

Different types of skeletal muscle fibers are innervated by motor neurons of different sizes. Large motor neurons innervate type II muscle fibers, which have thicker axons and faster nerve impulse conduction speed (>90m·s-1);

3. Muscle fiber area The size of muscle fiber area depends on the diameter of the muscle fiber and is affected by age, training and muscle fiber type. Generally speaking, from birth to the end of puberty, the area of ​​muscle fibers increases linearly with age. The difference in area between the two different types of muscle fibers in humans is small, and there are large individual differences.

Metabolic characteristics

1. Metabolic substrates

The triglyceride content of human skeletal muscle is between 5 and 15 mmol·kg-1 wet weight, among which the triglyceride content of slow muscle is higher than that of fast muscle; the muscle glycogen content is between 50 and 90 mmol·kg-1 wet weight, and there is no significant difference in muscle glycogen content between different types of muscle fibers and their subtypes.

2. Metabolic enzyme activity

The activity of enzymes involved in anaerobic oxidation in FT fiber is higher than that in FT fiber. For example, the Ca2+-activated myosin ATPase activity of FT fibers is 2.5 times higher than that of ST fibers, the myokinase activity is 1.5 times that of FT fibers, and the lactate dehydrogenase activity is 4 times higher than that of ST fibers. On the contrary, the activity of enzymes involved in aerobic oxidation in ST fibers is 30%-50% higher than that in FT fibers.

Physiological characteristics

The physiological function of muscles is contraction. The differences in physiological characteristics of different types of muscle fibers are mainly manifested in changes in muscle contraction speed, tension, and fatigue resistance.

1. Contraction speed

Studies have found significant differences between fast and slow muscle fibers. Fast-twitch muscle fibers contract quickly, and their impulse conduction speed is much faster than that of fast-twitch muscle fibers. It is related to factors such as motor neuron control, high myofibril ATPase activity, strong anaerobic metabolic capacity, and strong ability of sarcoplasmic reticulum to release and recover Ca2+.

2. Contraction force

The strength of muscle contraction depends on the cross-sectional area of ​​the muscle and is affected by factors such as muscle fiber type. Most studies believe that the contraction force of animal fast muscles is significantly greater than that of slow muscles. However, the differences in contractile strength between different types of muscle fibers in humans are not fully understood. When comparing the percentage of muscle fiber types and the relationship between muscle contraction strength, it was found that people with a high percentage of fast muscle fibers in their muscles also have greater contraction strength.

3. Anti-fatigue

Both animal and human experiments have shown that slow-twitch fibers are more resistant to fatigue than fast-twitch fibers, so fast-twitch fibers are more susceptible to fatigue than fast-twitch fibers.

What are skeletal muscle fibers like? Many friends may not have known much about them before, but after reading these contents, everyone must have known a lot. The human body is very amazing, whether it is the body structure or the fibers contained in the body. Only when you understand it can you take better care of your health.