Muscular Tissue Chapter 11 ehow.com. Objectives Know the microanatomy of skeletal muscle and what a...
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Transcript of Muscular Tissue Chapter 11 ehow.com. Objectives Know the microanatomy of skeletal muscle and what a...
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