Molecular Mechanisms of Movement

Structure of Muscle

• 1 muscle = several fibre bundles
• 1 bundle = several muscle fibres
• 1 muscle fibre = several myofibrils
• 1 myofibril has several actin & myosin filaments arranged in end-to-end sarcomeres

Actin

• actin is the thin myofibril. It contains two main divisions:
• G-actin: the orange balls that actually do the binding to the myosin myofilaments.
• Tropomyosin: the rope structure that covers and uncovers the myosin binding sites. It is held in the "covered" position using troponin. If Ca+ is present, it inhibits troponin binding, which leaves the G-actin available for myosin binding.
• REMEMBER: the difference between G-actin and tropomyosin - Trop is a rope.

Myosin

• Myosin is the thick myofibril. It is made up of many myosin molecules.
• myosin molecules have heads that will bind to the actin and then move to contract the muscle.
• ANALOGY: just light flexing your bicep, the myosin head acts the same when doing a power stroke.
• REQUIRES ATP TO FUNCTION

Muscle Physiology

Sliding-Filament Theory

• thick filaments (myosin) move past thin filaments (actin)
• sarcomeres shorten while filaments remain same length
• requires actin-myosin binding sites, ATP and Ca2+

Role of Ca2+ in muscle contraction

• allows for myosin binding on actin, by shifting the troponin-tropomyosin complex

The actin-myosin ATP cycle

1. Pi is released (now, strong bond between actin and myosin is able to form - this is called a cross-bridge)
2. Power stroke happens
3. ADP is released

• Now, muscle is in contracted state
• ATP binds myosin again, causing release of actin ––> back to the relaxed state
• no ATP available? No relaxation. This is what happens during rigor mortis.