Skeletal Muscle

Skeletal Muscle

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Skeletal muscle is what helps you move your body. These muscles are connected to your bones and allow you to do many different types of movements. Skeletal muscle is an important part of your body that helps you stay active and physically fit. So, if you want to stay healthy and strong, it's important to take care of your skeletal muscles.

Skeletal muscle structure

Our muscles are responsible for movement in our body and they are activated by nerves. There are three types of muscles in our body: skeletal, cardiac, and smooth muscle. Out of the three, skeletal muscles are the most common in humans and they are the only type of muscle that are under our voluntary control. This means that we can control when and how our skeletal muscles move.

The organisation of skeletal muscles

Skeletal muscles come in different shapes and sizes. Some are broad, while others are narrow. The fibres in some muscles are arranged parallel to the muscle, while others are arranged diagonally or in more complex patterns. Skeletal muscles are a specialized type of tissue that can respond to nerve stimulation and contract. They are made up of thousands of muscle cells called muscle fibres. These fibres are multinucleated, meaning they have more than one nucleus. They are bundled together in sheaths of connective tissue called endomysium, and these bundles form fascicles surrounded by perimysium. The collection of these fascicles forms the muscle, which is surrounded by another connective tissue sheath called the epimysium. Tendons are continuous with the epimysium and connect skeletal muscles to bones. A muscle typically spans a joint and is attached to bones by tendons at both ends. When the muscle contracts, one of the bones remains stable while the other moves.

The organisation in a skeletal muscle


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).

The ultrastructure of myofiber

Within the muscle fibre, there are extensions of the sarcolemma that penetrate the centre. These are known as T tubules and they are tube-like projections. The sarcoplasm of myofibers contains mitochondria, myofibrils, and sarcasmic reticulum. The mitochondria are responsible for carrying out aerobic respiration to generate ATP, which is needed for muscle contraction. The sarcoplasmic reticulum contains protein pumps on its membrane that transport Ca2+ ions into its lumen. The sarcoplasmic reticulum plays a crucial role in muscle contraction by storing Ca2+ and releasing it when the myofiber is stimulated.


Myofibrils are primarily composed of two types of protein filaments: thin actin and thick myosin myofilaments. Actin is a family of globular proteins that combine to form two long thin filaments that are twisted around each other. Myosin molecules, on the other hand, are molecular motors shaped like long rod-like tails with bulbous heads that resemble golf clubs. These two filaments are arranged in a specific order, which gives them a striated appearance and creates different types of bands and lines, as seen in Figure 3.

Bands and lines in the myofibrils

The composition of a myofibril section can be broken down into several distinct components. The I band, for instance, contains only actin filaments. The A band, on the other hand, contains only myosin filaments, as well as areas where myosin and actin filaments overlap. The H band contains only myosin filaments, and the M line marks the point where these filaments attach to the myofibril (located at the centre of the A band). The Z line, which marks the beginning and end of sarcomeres, is the line where actin filaments attach. Finally, the sarcomere is the section of the myofibril that lies between two Z lines.


The organisation of myofilaments within a sarcomere
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: Maintaining posture. Stabilising bones and joints. Control of internal movements and locomotion. Heat generation (by shivering).

How do skeletal muscles get bigger?

The number of muscle fibres, or myofibers, present in the body remains constant from infancy to adulthood. Unlike in other tissues, myofibers do not divide. Instead, skeletal muscles grow by increasing the number of contractile proteins contain. This process, known as hypertrophy, is triggered when muscles are exercised and pushed beyond their limit, causing minor injuries in the muscle fibres. During the recovery period, the muscle fibres begin to heal, and the contractile content within them is increased.

However, there are other essential factors that contribute to muscle growth, such as adequate nutrition (including vitamins and essential minerals), rest and recovery, and a high protein diet. Continued exercise is also necessary to maintain the size and strength of our muscles. If a muscle is not used, it will atrophy and become smaller and weaker. In short, it is important to use our muscles regularly to prevent them from wasting away.

Types of skeletal muscle fibres

Skeletal muscles are composed of three types of fibres: slow-twitch fibre (type I), fast-oxidative-twitch fibre (type IIa), and fast-glycolytic-twitch fibres (IIb). Each type of fibre has its properties and is effective at a specific type of activity. Slow-twitch fibres are highly resistant to fatigue and are responsible for maintaining posture, making up the majority of muscle fibres in these muscles. They are also efficient at using oxygen to produce energy and are ideal for endurance activities such as long-distance running or cycling. Fast-twitch fibres, however, become fatigued quickly but generate greater force during contraction. Fast-oxidative-twitch fibres (type IIa) have a moderate resistance to fatigue and are useful in activities such as sprinting, where high-intensity bursts of energy are required. Fast-glycolytic-twitch fibres (IIb) have the least resistance to fatigue but generate the most force and are used for short, explosive movements such as weightlifting or jumping. Understanding the properties of each muscle fibre type can help in designing an effective exercise program to target specific muscle groups and achieve desired results.

Skeletal muscle spasm

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

Overuse and muscle fatigue. Dehydration. 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

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

What is skeletal muscle?

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. 

How to increase skeletal muscle? 

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.

What is the location of skeletal muscle?

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.

What are the three types of skeletal muscle?

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

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