muscular tissue chapter 11 ehow.com. objectives know the microanatomy of skeletal muscle and what a...
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Muscular Tissue
Chapter 11
ehow.com
Objectives• Know the microanatomy of skeletal muscle and what a
sarcomere is
• Understand the relationship between the somatic nervous system and muscle cells
• Understand how the sliding filament mechanism works
• Know how to interpret a myogram
• Know some characteristics of cardiac and smooth muscles
Structure• Macro to micro– Epimysium
– Fascicles• Wrapped by perimysium
– 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
– Myosin• Heads contain two
binding sites• Actin and ATPase
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Somatic Nervous System
• Motor neurons supply skeletal muscle
• Single neuron with cell bodies in ventral horn of spinal cord
• Release acetylcholine– Only excitatory
Neuromuscular Junction• Junction of motor axon with a muscle cell
– Only one junction per muscle cell
• Motor end plate– Specialized area on muscle cell for synapse– High concentration of cholinergic receptors– Open nonspecific ion channels
• Na+ rushes in producing an end-plate potential
• Action potential on muscle cell– EPP opens ion channels in adjacent membrane– Threshold is reached and an action potential
propagates away from the motor end plate
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Acetylcholinesterase
• Enzyme that inactivate Ach– Ensures that muscles only contract when needed
• Please look at chemicals and disease associated with the neuromuscular junction
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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Sliding Filament Mechanism
• http://youtu.be/WRxsOMenNQM
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– Intermediate
• Fast-glycolytic (typeIIb)– 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
• Cardiac pacemaker cells• Dense bodies• Calcium binds to calmodulin
– Activates myosin light-chain kinase
• Speed and sustainability of contractions
• Arrangement of myosin and actin filaments