Skeletal Muscle Explained With Diagram

Skeletal Muscle

  • One of the three major muscle tissues in the human body is skeletal muscle. Thousands of muscle fibres are bound together by connective tissue sheaths in each skeletal muscle. Muscle is one type of tissue among four different tissues. Muscle is a specialized type of tissue and it originating from mesoderm. Muscles alone contribute around 40 to 50 percent body weight of the human. Excitability, contractility, elasticity and extensibility are the special characteristic nature of the muscle cells. Based on their location, muscles are classified into three categories:
  • Skeletal Muscles: Muscles which are associated with skeletal components are known as skeletal muscles. Microscopically their appearance is look like striped. Hence they are also called as striated muscles. These muscle activities are regulated voluntarily by the nervous system. The principle action of skeletal muscle is locomotion and it can modulate body shape.
  • Visceral muscles: Visceral muscle present in inner walls of hollow visceral organs of the body. Example includes, alimentary canal, reproductive tract, etc. Unlike skeletal muscle, they are smooth in appearance. They are involuntary in nature so nervous system cannot control voluntarily. Visceral muscle involves in food transport in digestive tract and gamete transport through the genital tract.
  • Cardiac muscles: Heart consists of cardiac muscles in which cells assemble in a branching pattern. Like skeletal muscles, they are also striated. Activity wise, they are involuntary in nature thus nervous system cannot control the cardiac muscles directly.

Structure of Skeletal Muscle

  • Muscle cells are also termed as muscle fibres. They are long, cylindrical and multinucleated cells. Muscle fibres were organized in three levels. They are epimysium, endomysium and perimysium. Muscles are covered by thick and tough connective tissues called as epimysium. 
  • Epimysium separates one muscle from another. Collagen fibres of the epimysium are wavy in appearance and it has connection with the perimysium. Each perimysium covers 100 to 150 muscle fibres and forms fascicles. Interstitial space between muscle fibres is around 1 µm which allow development of the tunnel in the perimysium thus arteries, veins and nerves can pass through it. 
  • In perimysium, collagen fibres are arranged as wavy forms with cross links which helps to improve the strength and stability of the muscle fibres. Endomysium consists of loose connective tissues and they additionally add up the strength of the muscle fibres. Endomysium also connects with the perimysium for the stability.
Skeletal Muscle

Organization of Skeletal Muscle Fibre, Image Source: open.oregonstate.education

  • Muscle fibres are lined by the plasma membrane (sarcolemma) enclosing sarcoplasm. Muscle fibres contains multiple nucleus (syncytium) and the sarcoplasm contains relatively higher amount of glycogen granules, myoglobin. Moreover high amount of calcium stored in endoplasmic reticulum. 
  • Parallel arrangement of myofilaments in sarcoplasm is one of the characteristic nature of the muscle fibre. Two types of myofilaments, known as myofibril present in the muscle fibres. Based on the thickness, myofibril termed as thin and thick myofilament which is made up of actin and myosin filaments respectively. 
  • Actin filament otherwise termed as ‘I’ band or isoelectric band whereas myosin filament termed as ‘A’ band or anisotrophic band. Actin and myosin fibrils are arranged alternatively and run across the muscle fibres longitudinally. 
  • Z – line and M – line, which are fibres with elastic in nature present in between I band and A band respectively. The portion in between to A band is termed as sarcomere which is the functional unit of the muscles which is responsible for contraction.

Skeletal Muscle – A Thin Myofilament

  • Actin: Actin is one of the major muscle proteins, which is a polymeric in nature. G-actin is the monomer unit of the actin filament and it is globular in nature. Its molecular weight is around 43 kda. Number of actin monomer units polymerize and form multimeric fibrous F-actin. Two F-actins coil around each other to form α-helix. Thickness of F-actin filament is around 6-7 nm and it consists of 14 G-actin molecules per turn. Association with tropomyosin and troponin, F-actin forms I band of the myofibril. Troponin masks the active binding sites for myosin on actin filaments.
Skeletal Muscle

Skeletal Muscle – Actin Fibre

Skeletal Muscle – Myosin Fibre

  • Myosin: Myosin is another major muscle protein. It is a hexameric protein. It consists of two heavy chains and four light chains. Molecular weight of the heavy chains and light chains are around 200 kda and 15 to 27 kda respectively. Heavy chain forms dimeric filaments by coiling α-helically each other except N terminal. Unwound N terminal of myosin is globular in nature and it serves the binding sites for other light chains as well as F-actin. N terminal of the myosin has ATPase activity. Carboxy terminal of myosin filament meet together at H zone whereas N terminals meet together at the margins of A band.
  • Tropomyosin: Tropomyosin is a rod like fibrous molecule. Its molecular weight is around 66 kda. It made up of two different α and β peptides and both are α-helical in nature. Double stranded tropomyosin run parallel with thin filaments in grooves of the F-actin strands.
  • Troponin: Troponin is a globular protein and exit as three interconnected protein system. They are (a) TpT (tropomyosin binding protein), (b) TpC (calcium binding protein) and (c) TpI (actin and TpC binding protein). Troponin lie on thin filaments with an interval of 38.5 nm, but the rest attached with F-actin and tropomyosin. Polymerization of desmin results intermediate filaments which is predominantly present in Z line. Α-α-actinin is a homodimeric protein and often anchors the ends of f-actin molecule to the Z line.

Neuromuscular junction in skeletal Muscle

Skeletal Muscle

Neuromuscular junction in skeletal Muscle

  • Neuromuscular junction is a connective region of efferent nerve fibre of nerve system and muscle fibre of muscle system through synapses. The entire junction is covered by a cell, which known as Schwann cell, for insulation purpose. An invaginated membrane appearance present in this junction is called as synaptic gutter where axon neuronal end. Sub-neuronal clefts surround the axon terminal and it increases the surface area for synaptic transmitters.

Functions

  • Through sustained contraction or alternating contraction and relaxation, muscle tissue has three key functions: producing motion, providing stabilization, and generating heat.
  1. Motion: Motion is obvious in movements such as walking and running, and in localized movements, such as grasping a pencil or nodding the head. These movements rely on the integrated functioning of bones, joints, and skeletal muscles.
  2. Stabilizing body: Besides producing movements, skeletal muscle contractions maintain the body in stable positions, such as standing or sitting. Postural muscles display sustained contractions when a person is awake, for example, partially contracted neck muscles hold the head upright. In addition, the volumes of the body cavities are regulated through the contractions of skeletal muscles. For example muscles of respiration regulate the volume of the thoracic cavity during the process of breathing.
  3. Thermo genesis: As skeletal muscle contracts to perform work, a by-product is heat. Much of the heat released by muscle is used to maintain normal body temperature. Muscle contractions are thought to generate as much as 85% of all body heat.

Further Readings