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Skeletal Muscle

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Biology

Skeletal muscles allows you to perform different types of movements. They are the muscles that are connected to the bone.

Skeletal muscle structure

Muscles are effector organs that bring about movement following nerve stimulation. There are three types of muscles in our body: skeletal, cardiac, and smooth muscle. Unlike the other two, skeletal muscles act under voluntary control and are the most common type of muscle found in humans.

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.

Myofibers are bundled and wrapped together in sheaths of connective tissue called endomysium. Muscle fibres bundle further into fascicles surrounded by perimysium (another layer of connective tissue). The collection of these fascicles forms the muscle, and each muscle is surrounded by yet another connective tissue sheath called the epimysium. Tendons are continuous with epimysium and connect skeletal muscles to bones.

This is beyond A-level specifications. You do not need to know the terms at this stage, but it does add more context to our topic.


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.

Skeletal Muscle The organisation within a skeletal muscle StudySmarterFigure 1. The organisation in a skeletal muscle. Source: Raul654, via Wikimedia Commons.

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).

Figure 2. The ultrastructure of myofiber.

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.

Myofibril section

Composition

I band

Only actin filaments.

A band

The section containing only myosin filaments + areas where myosin and actin filaments overlap.

H band

Only myosin filaments.

M line

The line where myosin filaments attach (centre of A band).

Z line

The line where actin filaments attach (marks the beginning and end of sarcomeres).

Sarcomere

The section of myofibril between two Z lines.

Figure 3. The organisation of myofilaments within a sarcomere.

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. Stabilising 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.

Skeletal Muscle - Key takeaways

  • Skeletal muscles are composed of thousands of cylindrical muscle cells called muscle fibres, or myofibers.
  • 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.
  • Skeletal muscle fibres contain a large number of contractile protein fibres called myofibrils. Myofibrils mostly consist of two types of protein filaments: thin actin and thick myosin myofilaments. The actin and myosin proteins are arranged in small contractile units called sarcomeres.
  • There are three types of skeletal muscle fibres:
    • Slow-twitch fibre (type I)
    • Fast-oxidative-twitch fibre (type IIa)
    • Fast-glycolytic-twitch fibres (IIb)

Skeletal Muscle

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

Skeletal muscle is one of the three significant types of muscle tissues in the human body. Each skeletal muscle is composed of thousands of muscle fibres (aka myofibers) wrapped together by layers of connective tissue. Skeletal muscle fibres contain thin actin and thick myosin filaments, which are arranged in a particular way, giving the muscle fibre a striated appearance.

Skeletal muscles grow by increasing the contractile content in the myofibers. This process is called hypertrophy and requires training that pushes muscles beyond their limits and causes micro-injuries in the muscle fibres, followed by rest. As these micro-injuries heal, the contractile proteins in the muscle fibres increase during recovery. Proper nutrition and a high protein diet are other factors needed for muscle growth.

Skeletal muscles are located throughout the body attached to bones, either directly or via tendons. They allow voluntary locomotion and movement in our body. Skeletal muscles are also found at the openings of internal tracts. These muscles allow voluntary control over functions, such as swallowing, urination, and defecation.

Slow-twitch fibre (type I), Fast-oxidative-twitch fibre (type IIa), Fast-glycolytic-twitch fibres (IIb).

Final Skeletal Muscle Quiz

Question

What is the definition of muscle?

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Answer

It refers to a number of muscle fibres bundled together in layers of connective tissue.

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Question

What is a myofiber?

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Answer

An individual muscle cell.

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Question

A muscle fibre contains multiple _______?


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Answer

Nuclei.

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Question

Can an injured muscle fibre be replaced by dividing existing skeletal muscle fibre? 


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Answer

No. Muscle fibres cannot divide.

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Question

How do muscle fibres get bigger? 


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Answer

By hypertrophy.

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Question

_____ filament is composed of 2 intertwined helical chains. Each of which contains a binding site for _____.


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Answer

Actin, myosin.

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Question

There are two I bands per sarcomere, one at each end. One end of the I bands is free. To what is the other end bound?

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Answer

Z line.

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Question

The portion of the thin filaments that does NOT overlap the thick filaments is called _____.


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Answer

I band.

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Question

What do we call a narrow dark band in the centre of the H zone that corresponds to proteins linking together the central region of adjacent myosin filaments?


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Answer

M line.

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Question

What is the section between two Z lines called?


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Answer

A sarcomere.

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Question

Name a series of tubular sleeve-like segments surrounding each individual myofibril. Their main role is to store calcium ions.


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Answer

Sarcoplasmic reticulum.

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Question

Name three common causes of muscle spasms.


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Answer

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

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