Overview
This lecture explains the structure and function of muscle tissue, focusing on how actin and myosin interact to create muscle contraction through the sliding filament model.
Types of Muscle Tissue
- There are three types of muscle tissue: smooth, cardiac, and skeletal.
- Smooth muscle is found in walls of hollow organs and acts involuntarily.
- Cardiac muscle is found only in the heart, is striated, and contracts involuntarily.
- Skeletal muscle attaches to bones, is striated, and is mostly under voluntary control.
Structure of Skeletal Muscle
- Skeletal muscle is composed of bundles within bundles: myofibrils make up muscle fibers, which form fascicles, bundled to make the whole muscle.
- Each muscle is an organ containing muscle tissue, connective tissue, blood vessels, and nerves.
- Muscle fibers have mitochondria, multiple nuclei, and a sarcolemma (cell membrane).
- Connective tissue sheaths reinforce and protect muscles.
The Sliding Filament Model of Muscle Contraction
- Myofibrils are divided into sarcomeres, the functional unit of contraction.
- Sarcomeres contain thin (actin) and thick (myosin) myofilaments.
- At rest, actin and myosin are separated by tropomyosin and troponin proteins.
- Muscle contraction is triggered when calcium ions bind to troponin, causing tropomyosin to move and expose binding sites on actin.
- Myosin heads bind to actin, pull the actin filament, and cause the sarcomere to contract.
- ATP is required for myosin to release actin and reset for another contraction cycle.
- Calcium is actively pumped back into the sarcoplasmic reticulum to reset the system.
Muscle Contraction Mechanism
- An action potential travels from the brain to the muscle cell via motor neurons.
- Acetylcholine is released, opening sodium channels in the sarcolemma, generating a muscle action potential.
- The action potential travels down T-tubules, triggering calcium release from the sarcoplasmic reticulum.
- The cycle of binding, contraction, and release repeats as long as calcium and ATP are available.
Key Terms & Definitions
- Actin — Thin protein filament involved in muscle contraction.
- Myosin — Thick protein filament with heads that bind to actin and cause contraction.
- Sarcomere — Functional contractile unit of muscle fiber.
- Sliding Filament Model — Theory explaining how muscles contract via actin and myosin sliding past each other.
- Sarcoplasmic Reticulum — Specialized endoplasmic reticulum in muscle cells that stores and releases calcium ions.
- T-tubules — Tubes in the muscle cell membrane that transmit action potentials into the cell.
- ATP (Adenosine Triphosphate) — Molecule providing energy for muscle contraction.
- Troponin/Tropomyosin — Proteins regulating binding of myosin to actin.
Action Items / Next Steps
- Review the steps of the sliding filament model for muscle contraction.
- Study the structure of skeletal muscle from smallest to largest components.
- Learn the roles of calcium and ATP in muscle contraction and relaxation.