Overview
This lecture explains how muscle movement occurs at the cellular level, focusing on the structure of muscle tissue and the sliding filament model involving actin and myosin.
Types of Muscle Tissue
- There are three muscle tissue types: smooth, cardiac, and skeletal.
- Smooth muscle is found in organs and moves materials involuntarily.
- Cardiac muscle forms the heart, is striated, and contracts involuntarily.
- Skeletal muscle is striated, mostly voluntary, and attaches to bones to create movement.
Structure of Skeletal Muscle
- Skeletal muscles are made of bundles: myofibrils combine into muscle fibers, which bundle into fascicles, forming the muscle.
- Muscle fibers are cells with multiple nuclei, mitochondria, and a sarcolemma (cell membrane).
- Muscles have connective tissue sheaths for support and protection.
- Each muscle is supplied by nerves and blood vessels for contraction and energy.
Sarcomeres and Myofilaments
- Myofibrils are divided into sarcomeres, the basic contractile units.
- Sarcomeres contain thin filaments (actin) and thick filaments (myosin).
- Z lines mark the ends of each sarcomere.
- Muscle contraction is the shortening of sarcomeres by bringing Z lines closer.
Sliding Filament Model of Contraction
- At rest, actin and myosin are kept apart by proteins tropomyosin and troponin.
- Calcium and ATP are needed for contraction.
- Action potentials from nerves trigger calcium release from sarcoplasmic reticulum.
- Calcium binds troponin, moving tropomyosin and exposing actin’s binding sites.
- Myosin (with ADP and phosphate) binds to actin, changes shape, and pulls actin, shortening the sarcomere.
- After the power stroke, myosin releases ADP, binds new ATP, and detaches from actin.
- ATP is hydrolyzed, re-cocking myosin for another cycle.
Muscle Relaxation
- Calcium is pumped back into the sarcoplasmic reticulum.
- Troponin and tropomyosin return to blocking actin, ending contraction.
Key Terms & Definitions
- Actin — Thin filament protein in sarcomeres involved in contraction.
- Myosin — Thick filament protein with heads that pull actin to contract muscle.
- Sarcomere — Basic unit of muscle contraction, spanning from Z line to Z line.
- Sliding Filament Model — Explains muscle contraction by sliding actin and myosin filaments.
- Sarcoplasmic Reticulum — Organelle in muscle cells storing and releasing calcium ions.
- Tropomyosin — Protein blocking myosin binding sites on actin.
- Troponin — Protein that binds calcium and moves tropomyosin to allow contraction.
- ATP (Adenosine Triphosphate) — Main energy molecule used by cells.
Action Items / Next Steps
- Review the steps of the sliding filament model.
- Recall muscle tissue types and their characteristics.
- Learn the structure and function of sarcomeres and associated proteins.