Sliding Filament Theory

“Sliding filament theory is the mechanism by which the muscle is thought to contract at the cellular level.”

What is Sliding Filament Theory?

Muscles are fibres which cause movement in our body. They also enable the functioning of our internal organs. Specialists claim that a human body has around 650 muscles, skeletal muscles to be precise.

Muscles are specialized tissues having the property of elasticity, where each muscle has innumerable muscle fibres. Muscle fibres successively have thin and tiny strands called myofibrils. For movement, muscles need to contract. It contracts when tension-generating sites within the muscle fibres are activated. This mechanism is explained by the sliding filament theory.

The sliding filament theory is a suggested mechanism of contraction of striated muscles, actin and myosin filaments to be precise, which overlap each other resulting in the shortening of the muscle fibre length. Actin(thin) filaments combined with myosin(thick filaments) conduct cellular movements.

Myosin is a protein that converts ATP (chemical energy) into mechanical energy thus creating thrust and movement. This movement generates muscular contraction and movement of non-muscle cells, such as mitosis and meiosis(cell division).

Also, actin polymerization and actin-myosin interaction are responsible for movements of a cell across a surface. Actin filaments have myosin-binding sites which are revealed when troponin molecules bind to calcium ions in filaments, facilitating bridge formation between actin and myosin. This process is fueled by ATP which acts as an energy source. ATP is hydrolysed in the heads of molecules of myosin causing a change in the shape of the head and binding to actin filaments.

See Also: Muscular System

Sarcomere

A series of basic structural units forming striations (striped pattern) in muscle cells that make up the skeletal muscles are called sarcomeres. They are organized in stacks throughout the muscle tissue. Single muscle cells exhibit thousands of sarcomeres and are replicated throughout the cell. The length of the muscle is subject to change as the proteins within modify in length, resulting in the overall change.

A single sarcomere has a bundle of many myofibrils – actin and myosin filaments. Skeletal muscles bring about voluntary movements. Sarcomeres in the skeletal muscles initiate this movement through contraction which is attributed to its structure.

  • The A-band, a zone of repeated sarcomeres maintain a constant length during contraction. This band is present in the centre of the sarcomere where filaments overlap.
  • It consists of the H-zone, composed of thick myosin.
  • The two I-bands contain a thin filament, while the thick filaments are not too far away.
  • The Z-lines are responsible for the striped nature.
  • The M-line is located in the mid of Z-lines containing myomesin.

Key Points For Sliding Filament Theory

  • The sliding filament contraction occurs in the sarcomere region.
  • The myosin filaments ratchet over actin filaments contracting the sarcomere.
  • The I and H bands within the sarcomere compress and expand to facilitate the movement.
  • The myofilaments do not expand and contract on their own.

Related Links: Muscle Contraction And Contractile Proteins

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