Skeletal Muscle

The organisation of skeletal muscles

Skeletal muscles come in various shapes, sizes, and arrangements. Some skeletal muscles are broad in shape, and some are narrow. In some skeletal muscles, the fibres are arranged parallel to the muscle axis; in others, they are oblique or more complicated.

Muscle is a highly specialised type of tissue. It is composed of highly differentiated cells whose main properties are responding to nervous stimulation and being able to contract. Skeletal muscles are composed of thousands of cylindrical muscle cells called muscle fibres, or myofibers. These muscle fibres are multinucleated (meaning they have more than one nucleus) due to the fusion of their predecessors (myoblasts, i.e. the muscle cells) during embryonic development.

Usually, a muscle spans a joint and is attached to bones by tendons at both ends. One of the bones remains relatively fixed or stable while the other end moves due to muscle contraction.

Ultrastructure of skeletal muscle fibres

The different parts of the muscle fibre often have names different from their counterparts in normal cells. Each myofiber is surrounded by a cell surface membrane, just like any other cell. But this membrane is called sarcolemma in myofibers.

In addition, the cytoplasm in muscle fibres is called the sarcoplasm, and the endoplasmic reticulum is called the sarcoplasmic reticulum (SR).

Some extensions of the sarcolemma penetrate the centre of the muscle fibre. These deep tube-like projections are called transverse system tubules or T tubules in short.

The sarcoplasm of myofibers contains mitochondria, myofibrils, and sarcoplasmic reticulum. The mitochondria act like powerhouses within the myofiber and carry out aerobic respiration to generate the ATP needed for muscle contraction. The sarcoplasmic reticulum contains protein pumps on its membrane that transport Ca²⁺ ions into its lumen. The sarcoplasmic reticulum plays an imperative role in muscle contraction by storing Ca²⁺ and releasing it when the myofiber is stimulated.

Myofibrils

Myofibrils mainly consist of two types of protein filaments: thin actin and thick myosin myofilaments.

Actin is a family of globular proteins that polymerise to form two long thin filaments twisted around each other. On the other hand, myosin molecules are molecular motors shaped similar to long rod-like tails with bulbous heads, resembling golf clubs. These two filaments are arranged in a particular order, giving them a striated appearance and creating different types of bands and lines (Figure 3).

The table below describes and defines the different brands and lines in Figure 3.

Table 1. Bands and lines in the myofibrils.

Skeletal muscles in action

Skeletal muscles attach to bones either directly or through fibrous connective tissue. Skeletal muscles have various functions, including:

1. Maintaining posture.
2. Stabilizing bones and joints.
3. Control of internal movements and locomotion.
4. Heat generation (by shivering).

How do skeletal muscles get bigger?

The number of muscle fibres (myofibers) remain constant from infancy to adulthood. In other words, myofibers do not divide, unlike in other tissues. Instead, skeletal muscles grow by increasing the number of contractile proteins they contain. The process of cells growing by increasing their content instead of dividing is called hypertrophy.

Muscular hypertrophy is triggered when muscles are exercised and pushed beyond their limit, causing minor injuries in the muscle fibres. During recovery, the muscle fibres begin to heal, and it is in this period, the contractile content within them is increased.

There are other factors imperative to muscle growth. These include:

• Nutrition (vitamins and essential minerals).
• Rest and recovery.
• High protein diet.

Continued exercise is necessary for maintaining the size and strength of our muscles. If a muscle is not used, it will atrophy and get smaller and weaker. In other words, use it or lose it.

Types of skeletal muscle fibres

There are three types of fibres in skeletal muscles:

1. Slow-twitch fibre (type I).
2. Fast-oxidative-twitch fibre (type IIa).
3. Fast-glycolytic-twitch fibres (IIb).

Each of these fibres has its properties and is effective at a particular type of activity. For instance, slow-twitch fibres are highly resistant to fatigue and thus form the majority of muscle fibres in muscles responsible for maintaining posture. Fast-twitch fibres, however, become fatigued quickly but generate greater force during contraction. Article of fast-twitch fibres dives into different properties and examples of these muscle fibre types.

Skeletal muscle spasm

Spasms of skeletal muscles are most common and often arise due to:

1. Overuse and muscle fatigue.
2. Dehydration.
3. Electrolyte abnormalities.

The skeletal muscle spasms occur abruptly and are painful. They are also short-lived and may be relieved by gently stretching and massaging the muscle.

If muscle spasms are excruciating, if they do not resolve or recur, medical care should be accessed to look for other possible underlying causes.