Muscle Fiber Structure and Function

Muscle Fiber

• A muscle fiber is a single muscle cell
• A muscle fiber is encased by a membrane called the sarcolemma
• The cytoplasm of a muscle fiber is called sarcoplasm
• Muscle fiber contraction is triggered by a nerve impulse
• Nerve impulses trigger the release of calcium from the sarcoplasmic reticulum
• The sarcoplasmic reticulum is a network of tubules within the muscle fiber that stores calcium
• Calcium is required for muscle contraction
• Calcium binds to troponin, which is a protein in the muscle cell
• Troponin shifts the position of tropomyosin
• Tropomyosin is another protein that blocks the myosin binding sites on the actin filament
• Myosin is a motor protein that binds to actin
• Actin and myosin are proteins that form the contractile filaments in muscles
• Actin filaments are thin filaments in the muscle cell
• Myosin filaments are thick filaments in the muscle cell
• The binding of myosin to actin initiates a power stroke, which causes the muscle fiber to shorten and contract

The H Zone

• The H zone is the central section of a sarcomere that contains only myosin filaments, no actin filaments
• The sarcomere is the basic unit of muscle contraction  
• The H zone disappears during muscle contraction

Muscle Contraction

• A single muscle contraction is called a twitch
• Muscle contraction is initiated by a nerve impulse
• The nerve impulse is transmitted to the muscle fiber at the neuromuscular junction
• The neuromuscular junction is the site where a motor neuron synapses with a muscle fiber
• Motor neurons are nerve cells that control muscle movement

Muscle Types

• Muscle fibers are classified into three types: fast-twitch, slow-twitch, and intermediate fibers
• Type 1 muscle fibers (Slow-twitch fibers) are adapted for endurance exercise
• Type 2 muscle fibers (Fast-twitch fibers) are better suited for short bursts of high-intensity exercise
• Intermediate fibers are a hybrid of slow-twitch and fast-twitch fibers.

Nerve Impulse Transmission

• The nerve impulse is transmitted across the neuromuscular junction by chemical messengers called neurotransmitters
• The primary neurotransmitter at the neuromuscular junction is acetylcholine
• Acetylcholine binds to receptors on the muscle fiber, triggering the release of calcium from the sarcoplasmic reticulum
• Calcium binding to troponin enables the formation of cross-bridges between actin and myosin
• The release of calcium from the sarcoplasmic reticulum is essential for muscle contraction