PHYSIOLOGY OF MUSCLE CONTRACTION
When scientists first examined the electron micrographs of skeletal muscle in the mid-1950s, they noticed that the lengths of thick and thin filaments remained the same in both relaxed and contracted muscle. This finding disproved the earlier belief that contraction involved folding of muscle fibers like an accordion. Instead, it was discovered that muscle contraction occurs because thick and thin filaments slide past one another — a process described by the Sliding Filament Mechanism.
SLIDING FILAMENT THEORY OF MUSCULAR CONTRACTION:
The sliding filament theory explains how muscles contract to produce force. Within the sarcomere, actin (thin) and myosin (thick) filaments form cross-bridges and slide past each other, resulting in muscle shortening and tension.
- Contraction begins with a stimulation in the form of an action potential from a motor neuron.
- Each motor neuron stimulates a specific number of muscle fibers, forming a motor unit.
- The junction between the axon terminal of a motor neuron and a muscle fiber is called the neuromuscular junction or motor end plate.
When an impulse reaches the muscle fibers of a motor unit, it triggers a reaction between actin and myosin filaments within each sarcomere. The myosin heads attach to actin and pull it toward the center of the sarcomere, resulting in muscle contraction. This occurs simultaneously in all sarcomeres, causing the entire muscle to shorten.
ROLE OF CALCIUM AND TROPONIN:
- Troponin is a complex of three proteins located on the actin filament and attached to tropomyosin.
- When the muscle is relaxed, tropomyosin blocks the binding sites on actin, preventing myosin from attaching.
- When a nerve impulse reaches the muscle, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm.
- Calcium binds to troponin, causing a change that moves tropomyosin away from the binding sites, allowing myosin heads to attach and initiate contraction.
STAGES OF THE SLIDING FILAMENT THEORY:
The process of muscle contraction can be broken down into four main stages:
1) Muscle Activation:
The motor neuron sends an action potential to the neuromuscular junction, stimulating the sarcoplasmic reticulum to release calcium ions into the muscle cell.
2) Muscle Contraction:
Calcium binds to troponin, enabling the actin and myosin cross-bridges to form. Myosin heads pull actin filaments using energy from ATP, causing the muscle to contract.
3) Recharging:
ATP is re-synthesized to allow the myosin heads to detach, re-cock, and reattach for another power stroke, maintaining contraction strength.
4) Relaxation:
When nerve stimulation stops, calcium ions are pumped back into the sarcoplasmic reticulum. This breaks the link between actin and myosin, causing the muscle to relax. If ATP is depleted, relaxation also fails, leading to stiffness (as in rigor mortis).
REQUIREMENTS FOR MUSCLE CONTRACTION:
For skeletal muscle contraction to occur, three conditions must be met:
- Presence of a neural stimulus
- Availability of calcium ions in the muscle cells
- Sufficient supply of ATP for energy
FACTORS THAT CAN STOP MUSCLE CONTRACTION
- Energy System Fatigue: Lack of ATP prevents the continuation of contraction.
- Nervous System Fatigue: The nervous system cannot generate impulses quickly enough to sustain contraction.
- Voluntary Control: The brain stops sending signals, halting calcium release and ending contraction.
- Sensory Feedback: Pain or excessive strain triggers sensory neurons to stop the contraction to prevent injury.
Detailed Notes:
For PDF style full-color notes, open the complete study material below: