Overview
This lecture covers the structure and function of muscle tissue, emphasizing the process of skeletal muscle contraction based on the sliding filament theory, as well as the anatomy of muscle cells.
Muscle Contraction: Sliding Filament Theory
- Muscle contraction is explained by the sliding filament theory: actin (thin filament) slides past myosin (thick filament).
- The four main steps are excitation, excitation-contraction coupling, contraction, and relaxation.
- Excitation begins when a nerve impulse triggers voltage-gated calcium channels, causing calcium to enter the axon terminal.
- Calcium influx causes the release of acetylcholine (ACH), a neurotransmitter.
- Acetylcholine binds to receptors on the sarcolemma (muscle cell membrane), opening chemically gated ion channels.
- Sodium enters and potassium exits the muscle cell, creating an end plate potential (localized change in charge).
- This change opens adjacent voltage-gated ion channels, generating an action potential that propagates along the sarcolemma and into T-tubules.
- Action potential triggers calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin, causing tropomyosin to uncover binding sites on actin.
- Myosin heads, activated by ATP hydrolysis (via ATPase), bind to actin’s active sites (cross-bridge formation).
- The power stroke (myosin pulling actin) contracts the muscle; ATP is needed for both contraction and release of the myosin head.
- Not all myosin heads attach simultaneously to prevent filaments from slipping back.
Muscle Relaxation
- Acetylcholinesterase degrades acetylcholine, closing ion channels and stopping muscle contraction.
- ATP is required to actively transport calcium back into the sarcoplasmic reticulum.
- Tropomyosin covers actin’s binding sites, and muscle returns to resting length.
Types and Structure of Muscle Tissue
- Three types: skeletal (voluntary, striated, fast, fatigues quickly), cardiac (involuntary, striated, intercalated discs), smooth (involuntary, non-striated).
- Prefixes: “myo-”, “mys-”, “sarco-” relate to muscles.
- Connective tissue layers: epimysium (whole muscle), perimysium (fascicle), endomysium (muscle fiber/cell).
- Fascicle = bundle of muscle fibers (cells).
Anatomy of a Muscle Cell
- Sarcomere: the contractile unit, from one Z line/disc to the next.
- Actin (thin filament) and myosin (thick filament) partially overlap; contraction pulls Z lines closer together.
- Sarcolemma: muscle cell membrane; sarcoplasm: cytoplasm; sarcoplasmic reticulum: stores calcium; T-tubules: conduct action potentials.
- Myosin heads have ATPase for ATP breakdown and binding sites for actin and ATP.
- Troponin/tropomyosin complex regulates actin’s binding sites for myosin.
Neuromuscular Junction & Transmission
- Neuromuscular junction: site where nerve and muscle meet.
- Nerve impulse releases acetylcholine, triggering muscle action potential and subsequent contraction.
Functions & Properties of Muscle
- Properties: excitability (responds to stimuli), contractility (shortens), extensibility (stretches), elasticity (returns to original length).
- Functions: movement, posture, joint stabilization, heat production, organ protection, controlling pupil size, and causing goosebumps.
Clinical Correlations
- Acetylcholinesterase inhibitors (pesticides) prevent muscle relaxation, causing spastic paralysis.
- Tetanus (bacterial toxin) causes continuous muscle contraction; flaccid paralysis is constant muscle relaxation.
- Curare (muscle relaxant) blocks acetylcholine receptors, preventing contraction.
Key Terms & Definitions
- Actin — thin filament involved in contraction.
- Myosin — thick filament with heads that bind actin.
- Sarcolemma — muscle cell membrane.
- Sarcoplasmic reticulum — organelle storing calcium in muscle cells.
- T-tubules — channels for action potential transmission into muscle fiber.
- Troponin — protein that binds calcium and regulates muscle contraction.
- Tropomyosin — protein that blocks actin’s binding sites.
- Acetylcholine (ACH) — neurotransmitter for muscle contraction.
- Acetylcholinesterase — enzyme breaking down acetylcholine.
- Sarcomere — contractile unit of muscle cell.
- Neuromuscular junction — site where motor neuron and muscle fiber meet.
- End plate potential — localized change in membrane charge at neuromuscular junction.
- Action potential — propagating electrical signal leading to contraction.
Action Items / Next Steps
- Watch the provided videos on muscle contraction.
- Review notes and textbook/lab manual sections on muscle anatomy and physiology.
- Prepare for exam 3 by mastering the muscle contraction process and associated terminology.