Muscle Physiology. Bio 219 Dr. Adam Ross Napa Valley College

Transcription

1 Muscle Physiology Bio 219 Dr. Adam Ross Napa Valley College

2 Muscle tissue Muscle is an excitable tissue capable of force production Three types Skeletal- striated, voluntary Cardiac- non-striated, involuntary Smooth- striated, involuntary

3 Structure of Skeletal Muscle

4 Neuromuscular Junction

5 NMJ- Micrograph

6 Nicotinic Acetylcholine Receptor

7 nachr

8 Events at NMJ Action potential causes Ach release Ach binds to nachr Allows Na+ to enter muscle cell (EPSP) Depolarization from Na+ causes opening of voltage-gated Na+ channels Leads to AP in muscle which spreads via T-Tubules* *(will come back to this shortly) ACh is brokendown by acetylcholinesterase (AChE) Also pumped back into presynaptic neuron

9 Structure of Skeletal Muscle Epimysium surrounds entire muscle Fascia surrounds individual fascicles Fasicle is made up of individual muscle fibers

10 Structure of skeletal muscle fiber

11 The Sarcomere Functional unit of skeletal muscle Smallest unit that retains all the functional properties Easy way to understand what the whole muscle is doing is to focus on one sarcomere In most cases (all in this class) all sarcomeres are doing the same thing within a single muscle

12 The sarcomere

13 The Sarcomere Major Functional Proteins: Actin: Thin filament Myosin: Thick filament Troponin + Tropomyosin : Block Actin from interacting with Myosin Other important structures: Sarcoplasmic reticulum Contains calcium T-Tubule Carries depolarization (from motor neuron) across entire muscle fiber

14 Skeletal Muscle Fiber

15 T-Tubule/ Sarcoplasmic Reticulum

16 T-Tubule/ Sarcoplasmic Reticulum * T-Tubule is depolarized by AP from NMJ (from earlier) That depolarization spreads across the muscle fiber and depolarizes the S.R. Causes calcium efflux from S.R. into the sarcoplasm Conformational change of troponin/tropomyosin complex caused by Ca 2+ binding to Troponin Opens the myosin binding site on actin *(more on this soon)

17 Molecular Basis for Contraction

18 Sliding Filament Theory

19 Sarcomere micrograph

20 Basic Events of Muscle Contraction Depolarization spreads across T-Tubules Causes calcium release from S.R. Calcium binds to troponin Causes conformational change in troponin-tropomyosin complex Myosin binds to actin ATP is used Myosin head pulls Z-lines closer together

21 Actin and Myosin (Thin and Thick Filaments)

22 Skeletal Muscle Contraction

23 Length-Tension Relationship (Sarcomere)

24 Muscle-Bone-Tendon Systems Muscle is attached to bone by tendons. Force produced by muscle pulls on tendon, which pulls on bone Creates movement around a fixed point (joint) Muscles can only pull (not push) Muscles are in antagonistic pairs Ie biceps and triceps OR Quadriceps and hamstring

25 Types of Contraction Isometric No change in length of muscle Eccentric Muscle is lengthening Concentric Muscle is shortening

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27 Motor Units Single motor neuron and all fibers it innervates All fibers activated at same time Different size motor units: Small: precise, delicate movement- fingertips Large: less delicate, for posture, movement- muscles in trunk, lower back Motor units are recruited by size: Smallest to largest Picking up keys vs. picking up 50lb bag of groceries Idea is to use as little energy as possible Recruitment results in additional force production (more fibers to contract)

28 Motor Unit

29 Muscle Twitch Smallest single contraction possible from a single motor unit If muscle is not allowed enough time to relax, twitches can summate

30 Summation in Skeletal Muscle Spatial Recruitment of motor units Temporal APs fire at faster rate Tetanic contraction Results from summation of a single fiber/ motor unit

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32 Muscle Fiber Types Glycolytic Rely on glycolysis for energy (Glycolysis) Oxidative Rely on aerobic metabolism and need oxygen Twitch Fast Slow Tonic

33 Human Twitch Muscle Fiber Types

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35 Regulation of Muscle Size Balance of hypertrophy and atrophy Gene expression plays a large role Steroids cross membrane and bind to receptors to alter gene expression Presence or absence of protein in diet can affect muscle growth Aging causes muscle loss Sarcopenia- 0.5%-1% loss per year after age 50 Muscles that are used grow Increase fiber size, not number of fibers Muscles that are not used will atrophy Muscle atrophy can exacerbate symptoms in elderly bed ridden patients

36 Disorders of Skeletal Muscle Amyotrophic Lateral Sclerosis (ALS or Lou Gehrig s Disease) Lack of muscle growth due to removal of myelin on motor neurons Myasthenia Gravis Grave muscle weakness Caused by overactive acetylcholine esterase