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

Skeletal muscle is a type of tissue found in the body that can be used to perform movements. Its unique properties include excitability, contractility, extensibility, and elasticity. The structure of skeletal muscle is highly organized and is made up of several layers of connective tissue surrounding bundles of muscle fibers, which allows for an effective build-up of force. At the microscopic level, the ultrastructure of skeletal muscle reveals repeating units known as sarcomeres that are functional units of contraction. There are different types of skeletal muscle, which include slow-twitch and fast-twitch fibers suited to endurance or rapid movement. This is a topic from Locomotion and Movement Chapter of Biology.

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.

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Structure of Skeletal Muscles

The structure of the skeletal muscles is defined as:

Microscopic Anatomy of Skeletal Muscles

The anatomical structure of skeletal muscles includes:

Muscle Fiber (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 Components

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 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 and 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:

Role in 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 and 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 Internal 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 Muscles

• The red muscle fibres, otherwise known as 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 Muscles

• 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.

Smooth Muscle

• Involuntary Control: Smooth muscle functions under involuntary control, meaning it is worked without any conscious effort. The autonomic nervous system and various hormones control it.

• Location: Smooth muscle resides in the walls of hollow constructions that make up the digestive tract, blood vessels, bladder, and airway passages. It regulates the movement of substances through these systems.

• Cell Structure: Smooth muscle cells are spindle-shaped and contain one central nucleus. There are no striations in smooth muscles like there are in skeletal muscles, so it appears smooth under the microscope.

• Function: The smooth muscles perform involuntary movements for example, the peristalsis in the intestines, contraction, and dilatation of blood vessels, and regulating the flow of air in the passage of the respiratory tract. The muscle contractions are slower and longer compared to the skeletal muscle.

Cardiac Muscle

• Location: Found only in the heart, cardiac muscle forms the bulk of the heart's wall and is responsible for pumping blood throughout the body.

• Structure: Composed of short, granular, and slightly branched cells each having a single central nucleus. Junctions between cells are called intercalated disks, containing gap junctions and desmosomes that enable synchronised contraction.

• Striations: Like the skeletal muscles, cardiac muscle is striated due to the regular arrangement of actin and myosin filaments.

• Contraction: It is involuntary and undergoes rhythmic, continuous contraction independent of any conscious control it is medicated by the self-conducting cells or pacemaker cells of the heart—the sinoatrial node—and the autonomic nervous system.

• Function: The heart chambers contraction are rhythmic and they pump blood out into the lungs and the rest of the body. They, therefore, maintain circulation and blood pressure.

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