Transcript The Muscular System - College of the Canyons

Chapter 8
Muscle Physiology
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Objectives
• Know the functions and anatomy of a
muscle
• Understand what causes a muscle
contraction and the sliding filament
mechanism
• Know the two types of muscular
contractions
• Be able to interpret a myogram
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• Understand how the different muscle
fibers operate
Functions
• Produces movement
• Maintains posture
• Stabilizes joints
• Generates heat
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Structure
• Macro to micro
– Epimysium
– Fascicles
• Wrapped by perimysium
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– Muscle fibers (cells)
• Wrapped by endomysium
– Myofibrils
• Contractile element
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Microanatomy of Myofibril
• Dark band
– A band
• Light band
– I band
• H zone
• Z disc
– Sarcomere
• Titin
– Largest protein in body
– Stabilizes thick fillaments
– Gives muscle elasticity
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The Sarcomere
• Functional unit
• Microfilaments
– Actin
• Troponin
• Tropomyosin
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– Myosin
• Heads contain two
binding sites
• Actin and ATPase
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Muscular Contraction
• What needs to happen in order for a muscular
contraction to begin?
Sliding Filament Theory
• Proposed by Hugh Huxley in 1954
• Mechanism by which sarcomeres contract
• Involves formation of cross-bridges between actin
and myosin
• Thin filaments slide over thick filaments
– Greater overlap between filaments
Sliding Filament Mechanism
• Calcium released from sarcoplasmic reticulum
– Foot proteins stimulated by dihydropyridine receptors of T tubules
• Calcium binds to troponin
• Troponin–tropomysin complex shifts position
• Myosin binding site exposed
• Myosin heads form cross-bridges with actin
• Actin filaments pulled toward center of sarcomere
Calcium Initiates the Sliding Filament
Mechanism (cont.)
Figure 6.7
Role of ATP
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Muscle Relaxation
• Nerve activation ends, contraction ends
• Calcium pumped back into sarcoplasmic reticulum
– Ca2+ - ATPase pump
• Calcium removed from troponin
• Myosin-binding site covered
• No calcium = no cross-bridges
Energy Required for Muscle Activity
• Principle source of energy: ATP
• ATP replenished by variety of means:
– Creatine phosphate
– Stored glycogen
– Aerobic metabolism of glucose, fatty acids, and other
high-energy molecules
Activity of Muscles Can Vary
• Isotonic contractions: muscle tension remains constant as
muscle changes length
– Concentric – muscle shortens
– Eccentric – muscle stretched, but still contracting
• Isometric contractions: muscle prevented from shortening,
tension develops at constant muscle length
• Degree of nerve activation influences force
• Terms to know:
– Motor unit
– Muscle tension
– All-or-none principle
Muscle Contraction: Myogram
• Latent period
• Contraction
• Relaxation
• Summation vs. tetanus
Types of Muscle Fibers
Slow-twitch
• Slow oxidative (type I)
– Red fibers
• Low myosin ATPase activity
• Slow twitch: endurance,
long duration contraction :
– Jogging, swimming, biking
Fast-twitch
• Fast-oxidative (type IIa)
– Red fibers
– High myosin ATPase activity
• Fast-glycolytic (typeIIx)
– White fibers
• Fast twitch: strength, white
muscle, short duration
contraction:
– Sprinting, weight lifting
Features of Cardiac and Smooth
Muscles
• Activation of cardiac and smooth muscles:
– Involuntary
– Specialized adaptations in each
• Speed and sustainability of contractions
• Arrangement of myosin and actin filaments