Overview
This lecture introduces skeletal muscle anatomy, functions, microscopic structure, types of muscle tissues, and the mechanisms by which skeletal muscles contract.
Functions of Skeletal Muscle
- Produces skeletal movement by moving bones.
- Maintains body position and posture.
- Supports soft tissues, especially where bones are absent (e.g., abdomen).
- Guards body openings (mouth, anus, vagina).
- Maintains body temperature.
- Stores nutrients, primarily as glycogen.
Properties & Types of Muscle Tissue
- All muscle types (skeletal, cardiac, smooth) are contractile, extensible, excitable, and elastic.
- Skeletal muscle: long, cylindrical, multinucleated, voluntary, striated, attached to bones by tendons.
- Smooth muscle: spindle-shaped, uninucleated, non-striated, involuntary, found in organs and blood vessels.
- Cardiac muscle: branched, striated, intercalated disks (gap junctions), multinucleated, involuntary, found only in the heart.
Skeletal Muscle Anatomy
- Skeletal muscle wrapped in epimysium, made of fascicles covered by perimysium.
- Fascicles consist of muscle fibers (cells), each covered by endomysium.
- Muscle fiber is made up of myofibrils, which contain repeating units called sarcomeres.
Microscopic Muscle Structure
- Sarcolemma: muscle cell membrane; transmits voltage across and into the cell.
- Sarcoplasm: cytoplasm of muscle cell.
- T-tubules: invaginations of the sarcolemma; carry voltage into the cell’s interior.
- Sarcoplasmic reticulum & terminal cisternae: store and release calcium for contraction.
- Triad: two terminal cisternae plus one T-tubule.
Sarcomere & Myofilaments
- Sarcomere: functional unit, from Z line to Z line.
- Key proteins: actin (thin filament), myosin (thick filament).
- Regions: A band (length of myosin), I band (between myosin ends), H zone (between actin filaments), M line (middle of sarcomere).
- Titan: spring-like protein; connects myosin to Z line, prevents overstretching, maintains elasticity.
Muscle Contraction Mechanism
- Myosin heads pull actin filaments toward the M line (sliding filament theory).
- During contraction: Z lines move closer, I bands shorten, H zone disappears, A band length stays constant.
Neuromuscular Junction & Contraction Initiation
- Motor neuron’s terminal bouton releases acetylcholine (ACh) at the neuromuscular junction.
- ACh triggers voltage in sarcolemma, travels via T-tubules to terminal cisternae, causing calcium release and contraction.
Types of Skeletal Muscle Fibers
- Based on ATP production and contraction speed:
- Slow oxidative: slow contraction, aerobic (oxygen-using).
- Fast glycolytic: fast contraction, anaerobic (no oxygen).
- Fast oxidative: fast contraction, aerobic.
Key Terms & Definitions
- Epimysium — connective tissue covering the entire muscle.
- Perimysium — surrounds muscle fascicles.
- Endomysium — covers individual muscle fibers.
- Sarcolemma — muscle cell membrane.
- Sarcoplasm — cytoplasm of a muscle cell.
- T-tubule — pathway for voltage transmission into cell.
- Sarcoplasmic reticulum — stores calcium.
- Triad — T-tubule and two terminal cisternae.
- Sarcomere — contractile unit from Z line to Z line.
- Actin — thin myofilament.
- Myosin — thick myofilament.
- Titan — spring-like protein for elasticity.
- Neuromuscular junction — site where nerve meets muscle.
- Acetylcholine (ACh) — neurotransmitter for muscle contraction.
Action Items / Next Steps
- Read the last three slides covering detailed descriptions of slow oxidative, fast glycolytic, and fast oxidative fibers.
- Prepare for a matching section on muscle fiber types on the next exam.