Muscular Tissue

Chapter 11

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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