Biophysics Lecture 8: Muscle Contraction Mechanism

Lecture 8: The Mechanism of Muscle Contraction

• The lecture covered the mechanism of muscle contraction.
• Biophysics (PHY261) lecture on 2/12/2024.

Neuromuscular Junction

• A motor unit is a group of muscle fibers controlled by one motor neuron.
• Small motor units are found in muscles needing precise control, like the eyes (<10 fibers).
• Large motor units are in larger muscles, like arms and legs (1000-2000 fibers).

Synapses Between Neurons and Muscle Fibers

• A synapse is the space between a neuron terminal and a muscle fiber.
• Neurotransmitters diffuse across this space.
• The neuromuscular junction is a specialized synapse between neurons and muscle fibers.
• Key parts of the synapse include synaptic vesicles, synaptic cleft, motor end plate, and acetylcholine.

Types of Muscles in the Body

• Skeletal muscles: Attached to bones, voluntary movement (striated).
• Smooth muscles: Found in organs, involuntary movement.
• Cardiac muscles: Found in the heart, involuntary movement.

Skeletal Muscles (Striated Muscles)

• Skeletal muscles are attached to bones for movement.
• They are voluntary muscles – under conscious control.
• The basic unit is a muscle fiber.
• Muscle fibers are striated (streaked) and multinucleated.
• Individual muscle fibers act independently.

Terminology

• Sarco-: Prefix meaning "muscle" (in Greek).
• Myo-: Prefix meaning "muscle" (in Latin).
• Sarcoplasmic Reticulum: Endoplasmic reticulum in skeletal muscles.
• Sarcolemma: The membrane of a skeletal muscle cell.
• Sarcoplasm: The cytoplasm of a skeletal muscle cell.
• Myocyte: A muscle cell.

Structure of the Muscle

• The muscle comprises fascicles.
• The epimysium surrounds the entire muscle.
• The perimysium surrounds bundles of muscle fibers (fascicles).
• The endomysium surrounds individual muscle fibers.
• Other components including blood vessels and motor neurons are also present.

Structure of the Myofibril

• Sarcomere: The basic functional unit of a myofibril, the region between two Z-lines.
• Z-line: A dense protein structure lining the sarcomere.
• M-line: A region in the center of the sarcomere.
• I-band (light band): contains only thin filaments.
• A-band(dark band): Contains both thin and thick filaments.
• Thin filaments: Composed primarily of actin.
• Thick filaments: Composed primarily of myosin.

The Sliding Filament Mechanism of Muscle Contraction

• The mechanism of contraction involves the sliding of actin and myosin filaments.
• In a relaxed sarcomere, the filaments overlap partially; in a contracted sarcomere the filaments slide past each other fully overlapping.

Structure of the Myofibril (detailed)

• The myofibril itself comprises many sarcomeres arranged in series.
• Key components include actin, myosin, titin, and other regulatory proteins.

Muscle fiber Relaxation

• Relaxation occurs when acetylcholine is broken down by acetylcholinesterase.
• Ligand-gated ion channels close reducing ion flow.
• Calcium ions are released from troponin.
• Tropomyosin blocks myosin binding sites.

Questions

• Where do the Ca2+ ions come from?
• How is muscle contraction initiated?

Explore the intricate details of muscle contraction mechanisms in this Biophysics lecture. Understand the roles of motor units, neuromuscular junctions, and different muscle types in the body. Ideal for students studying PHY261.