Skeletal Muscles: What Is It, Function, Location, Anatomy

Definition of Skeletal Muscles

Skeletal muscles are striated, voluntary tissues of muscles attached to bones by tendons and responsible for initiating or controlling body movements. These muscles have long, cylindrical fibres and striations due to actin and myosin filaments realizing activities right from locomotion to the maintenance of posture or heat production.

Apart from the skeletal, there are other major divisions of muscles: cardiac muscle, which is involuntary and found in the heart and smooth muscle, which is involuntary, located within the walls of internal organs. All three types of muscles combine to perform very vital functions necessary for the overall operations of the body.

Structure of Skeletal Muscles

The structure of the skeletal muscles is defined as:

Muscle Fibre (Cell)

Each fibre of the external, striated skeletal muscle is, in reality, a rather long, cylindrical cell having several nuclei at its periphery. Such fibres are truly multi-nucleated and are covered by a plasma membrane known as sarcolemma. The latter forms a wrapper for the cytoplasm called the sarcoplasm.

Myofibrils & Myofilaments

These muscle fibres are covered under myofibrils, which in turn are composed of filaments called myofilaments. Myofilaments consist of actin and myosin, which interact to bring about muscle contraction. The pattern of these filaments inside the muscle gives it a striated appearance.

Sarcomere – Functional Unit

It is the functional unit of muscle contraction.It is technically the segment between two Z adjacent discs. Where overlapping in actin and myosin filaments occurs, which contracts and relaxes by extending and shortening to make a muscle move.

Connective Tissue Layers

The components of the connective tissue are:

• Endomysium: This is the thin connective tissue surrounding every individual fibre. These also provide structural support and include capillaries and nerves collecting and supplying the muscle fibres respectively.

• Perimysium: This is the connective tissue sheath enclosing bundles of muscle fibres the fascicles. It also gives some mechanical support, and it includes larger blood vessels and nerves than the endomysium does.

• Epimysium: It refers to the external connective tissue to the whole muscle. It is continuous with the tendon, and thus forms a protective sheath for the muscle as such and renders some structural integrity.

Blood Supply & Innervation

• The blood supply to the skeletal muscles is very rich, with an extensive network of capillaries furnishing the oxygen and nutrient requirements for proper muscle functioning.

• Muscle fibres are also innervated by nerves, especially motor neurons, that aid in the voluntary control and coordination of each muscle contraction.

Function of Skeletal Muscles

The skeletal muscle performs the following functions:

Movement

• Skeletal muscles of the body originate and control most of their voluntary movements.

• This is achieved through the simple act of muscle contraction, thus causing the bone to be pulled, which acts as leverage leading to movement around the joints.

• These can be simple actions like walking or picking up something too highly intricate from the playing of musical instruments to sports.

Posture & Support

• The other major functions of the skeletal muscles, in conjunction with the provision for movement, include posture and support.

• These muscles have a continuous action in counteracting the force of gravity in holding up the body in an upright position and balance.

Heat Production

• Skeletal muscle contractions produce heat through metabolic by-products of muscle contractions and play a role in the maintenance of core body temperature.

• When exercising or exposed to low temperatures, thermogenesis increases the metabolic rate to maintain the inner temperature through enhanced production of heat as a result of muscle contractions.

Protection of Organs

• The skeletal muscles also protect the internal organs through protective mechanisms and additional structural support.

• The abdominal wall and thoracic cavity muscles safeguard vital organs like the intestines, heart, and lungs from shock and other forms of physical trauma.

• This protective function assumes paramount importance in the preservation of the integrity and safety of these internal systems.

Types of Skeletal Muscle Fibers

The different types of skeletal muscle fibres are:

Red (Slow-Twitch, Type I)

• The red muscle fibres or Type I or slow-twitch fibres, are rich in myoglobin, generally giving them a red colour.

• They have a high density of mitochondria and capillaries.

• Thus, having a high amount of mitochondria inside the cell, they can efficiently produce energy in the form of aerobic metabolism.

• For that reason, the fibres are made for endurance as well as prolonged activities, such as running a long distance or simply maintaining posture by continued contraction without developing fatigue.

White (Fast-Twitch, Type II)

• White muscle fibres contain few myoglobins and a few mitochondria, hence their pale colour.

• They depend more on anaerobic metabolism for the production of energy, so they can produce rapid and powerful contractions.

• Hence, they would do better under brief, intense activities like sprinting or weightlifting.

• They are capable of applying great forces but tire more quickly than red muscles.

Comparison of Skeletal, Smooth & Cardiac Muscles

The difference between skeletal, smooth and cardiac muscle is discussed in the table below:

Disorders Related to Skeletal Muscles

Several disorders and diseases can compromise muscle and thus function and quality of life.

Muscular Dystrophy

• Genetic mutations that alter muscle proteins.

• Treatment is symptomatic and aims to retard the progression.

• Duchenne muscular dystrophy: Progressive muscle weakness

• Becker muscular dystrophy: Similar but milder

Myasthenia Gravis

• Muscle weakness, fatigue

• Autoimmune disorder at the neuromuscular junction.

• Medications aimed at improving nerve-muscle communication.

• Immunosuppressive therapies.

Muscle Cramps and Strains

• It is caused by dehydration, overuse, and electrolyte imbalance.

• This can be prevented by regular stretching and proper hydration.

• Rest, ice application, compression, elevation (RICE)

• Gentle stretch and rehydrate

Skeletal Muscles NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Structure & function of skeletal muscles

• Skeletal vs Smooth vs Cardiac muscles

• Disorders related to muscles

Practice Questions for NEET

Q1. The dark band present on myofibril is

1. Isotropic band

2. Anisotropic band

3. Hensen's zone

4. M-line

Correct answer: 2) Anisotropic band

Explanation:

One of the darker bands in a muscle fiber, known as the A-band, consists primarily of myosin filaments. In this region, one can observe the arrangement of myosin proteins since they are placed in rod-like structures that run parallel to each other and along the longitudinal axis of the myofibrils. This A-band is very important in the contraction process of muscles because it makes contact with actin filaments during the sliding filament mechanism which may contribute to increased muscle strength and functionality.

Hence, the correct answer is option 2) Anisotropic band.

Q2. The plasma membrane of the muscle fiber is called

1. Sarcoplasma

2. Sarcolemma

3. Sarcoplasmic Reticulum

4. Syncytial

Correct answer: 2) Sarcolemma

The plasma membrane in a muscle fibre, or muscle cell, is termed the sarcolemma. This crucial biological barrier performs dual functions: preserving the muscle fibre's integrity and facilitating the conduction of electrical signals. The sarcolemma's primary role lies in the transmission of action potentials, which are essential electrical impulses that initiate muscle contraction. It features specialized components, like t-tubules, which are responsible for efficiently conducting these impulses throughout the muscle fibre. T-tubules are transverse tubules that extend from the surface to the interior, allowing the action potential to reach the necessary depth within the muscle fibre. The sarcolemma's integrity is vital for the correct operation of muscle contraction mechanisms and for maintaining communication between the cell's exterior and its internal components.

Hence, the correct answer is option 2) Sarcolemma.

Q3. The muscle fibers that contract slowly are

1. Red muscle fibres

2. White muscle fibres

3. Both a and b

4. None of these

Correct answer: 1) Red muscle fibres

Explanation:

Red muscle fibers, also known as slow-twitch fibers, contract slowly but are highly resistant to fatigue. They are rich in myoglobin, mitochondria, and blood supply, which enable sustained aerobic energy production. These fibers are well-suited for endurance activities like walking, running long distances, or maintaining posture, as they rely on oxidative metabolism for energy.

Hence, the correct answer is option 1) Red muscle fibers.

Also Read:

• MCQs on Skeletal Muscles

• Vertebral Column

• Pectoral Girdle

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