Question

Describe the molecular mechanism of skeletal muscle contraction in detail.

Answer 1

Mechanism of Skeletal muscle contraction

• Neuromuscular junction is the site where the motor nerve fiber and the skeletal muscle fiber meet.
• It is also known as Synapse or Synaptic cleft.
• In order to undergo the contraction of skeletal muscle requires skeletal muscle fibers to be first stimulated by a motor neuron.
• Motor unit is one motor neuron and many skeletal muscle fibers.
• Motor end plate is the specific part of a skeletal muscle fiber's sarcolemma directly beneath the neuromuscular junction (NMU).
• Neurotransmitter is a chemical substance released from the motor end fiber causing stimulation of the sarcolemma of the muscle fiber.
• Here the neurotransmitter is Acetylcholine.

Excitation contraction coupling:

• It is the sequence of events by which the transmission of an action potential along the sarcolemma leads to the sliding if myofilaments.
• The events at the NMU set the stage for coupling excitation-contraction (E-C) by providing excitation.
• Released acetylcholine binds to the receptor proteins on the sarcolemma and triggers an action potential in a muscle fiber.
• Action potential (AP) is propagated along the sarcolemma and down the T-tubules.
• Transmission of AP along the T-tubules of the triad causes the voltage sensitive tubule proteins to change the shape.
• This shape change opens the Ca²⁺ release channels in the terminal cisternae of the sarcoplasmic reticulum (SR),allowing the massive amounts of Ca²⁺ to flow into the cytosol within 1 millisecond.
• Calcium binds to troponin and removes the blocking action of tropomyosin.
• When Ca²⁺ binds, troponin changes shape, exposing binding sites for myosin on the thin filaments.
• Contraction begins - Myosin binding to actin forms cross bridges and contraction begins.
• At this point , E-C coupling is over.

Sliding filament theory:

• This is the most popular theory for contraction of muscle, was first proposed by Hugh Huxley in 1954.
• This theory states that the muscle contraction involves sliding movement of the the thin filaments past the thick filaments.
•  The sliding continues until the overlapping is complete.
• The changes in muscle during contraction are the distance between Z-lines of the sarcomeres decrease, I-bands shorten, the A-bands move closer together.

Muscular system

• The muscular system is an organ system consisting of skeletal, smooth and cardiac muscle.
• It permits movement of the body, circulates blood throughout the body and maintains posture.
• It is composed of muscle fibers for contraction.
• Muscles are attached to bones, or internal organs and blood vessels and are responsible for movement.
• All the movements in the body are the result of the muscle contraction.

Skeletal Muscles:

• Skeletal or voluntary or striped muscles are the which are under conscious control.
• They are attached to the bones.
• They are responsible for the movement of body parts and the locomotion.
• Ex: Biceps, triceps, deltoid, quadricep, hamstring.
• The main functions of these muscles are support, movement, protection, production of blood cells, storage of ions and endocrine regulation.