Muscle I Spring 2010 Harvard University

Lecture Outline Functions of Skeletal Muscle Structural Hierarchy of Muscle Sarcomere Structure – Thick Filaments – myosin – Thin filaments – actin, troponin and tropomyosin Sliding Filaments and Cross-bridge Cycling Neuromuscular Junction and Causes of Muscle Contraction Force Production in Skeletal Muscle – Single Muscle Fibers – Whole Muscles

Functions of Skeletal Muscles Produce skeletal movement Maintain body position Support soft tissues Guard openings Maintain body temperature Store nutrient reserves

Sarcomere Structure

A cross section through a sarcomere shows that: each myosin can interact with 6 actin filaments, and each actin can interact with 3 myosin filaments.

Myosin & Thick Filaments

Thin Filament Structure

Sliding Filament Theory

Thought Question

Neuromuscular Junction

Thought Questions

Single Fiber Force Production The all–or–none principle Tension of a Single Muscle Fiber – Depends on Number Length Frequency

Motor Unit Effects on Force Production

Length Affects on Force Production Figure 10–14 The Effect of Sarcomere Length on Active Tension.

Frequency Effects on Force Production

Whole Muscle Force Production Whole Muscle Force depends on: – Contraction Type – Velocity – Muscle Fiber Type – Muscle Architecture

Types of Muscle Contraction Four Types of Skeletal Muscle Contraction – Isotonic contraction – Isometric contraction – Eccentric Contraction – Concentric Contraction

Isotonic Contraction

Isometric Contraction

Velocity Affects on Force Production

Muscle Fiber Types Three Types of Skeletal Muscle Fibers – Fast Glycolytic (FG) – Slow Oxidative (SO) – Fast Oxidative Glycolytic (FOG)

Slow-oxidative skeletal muscle responds well to repetitive stimulation without becoming fatigued; muscles of body posture are examples. Fast-oxidative skeletal muscle responds quickly and to repetitive stimulation without becoming fatigued; muscles used in walking are examples. Fast-glycolytic skeletal muscle is used for quick bursts of strong activation, such as muscles used to jump or to run a short sprint. Most skeletal muscles include all three types.

Muscle Fiber Types

Slow-oxidative Fast-oxidative Fast- glycolytic Size Principle of Muscle Recruitment

Muscle Fiber Types Muscles and Fiber Types – White muscle – Red muscle – Most human muscles

Muscle Fiber Types Muscle Hypertrophy – Muscle growth from heavy training Muscle Atrophy – Lack of muscle activity

Muscle Fiber Types Physical Conditioning – Improves both power and endurance Anaerobic activities ( e.g., 50-meter dash, weightlifting): Aerobic activities (prolonged activity):

Muscle Fiber Types What you don’t use, you lose Muscle fibers break down proteins, become smaller and weaker With prolonged inactivity, fibrous tissue may replace muscle fibers

Muscle Properties that Affect Force Cross Sectional Area Muscle Force is proportional to muscle cross- sectional area The greater a muscles cross-sectional area the greater the force

Muscle Architecture

Muscle Architecture – Parallel or Fusiform Long muscles made up of in-series, parallel fibers Design for muscles that undergo shortening contractions that produce work Muscles usually contract over long distances

Muscle Architecture – Convergent Muscle fibers converge on a single insertion.

Muscle Architecture - Pennate Designed to have large cross-sectional area Designed to produce force Three types – Unipennate – Bipennate – Multipennate

Summary Functions of Skeletal Muscle Structural Hierarchy of Muscle Sarcomere Structure – Thick Filaments – myosin – Thin filaments – actin, troponin and tropomyosin Sliding Filaments and Cross-bridge Cycling Neuromuscular Junction and Causes of Muscle Contraction Force Production in Skeletal Muscle – Single Muscle Fibers – Whole Muscles

Thought Questions