Muscular system

Function

• Movement– Contraction of the muscle

• Posture– Anti-gravity

• Respiration• Heat generation• Communication

• Functional characteristics– Structures for contraction– Metabolism• Energy generation

– Muscle contraction– Ion concentration gradient maintenance– Heat

Physical property

• Contractility– Movement of structure– Increased pressure

• Vessels and hollow organs

• Excitability– Responsiveness to stimuli

• Extensibility– Ability to contract after being stretched

• Elasticity/recoil– Diaphragm

Types of muscles

• Skeletal muscles– Locomotion– Facial expression– Posture– Respiration– Voluntary contraction• Exclusive control by nervous system

• Smooth muscle– Most widely distributed– Walls of hollow organs– Blood vessels– Some autorhythmic• Spontaneous excitation and contraction• Independent of stimulation• Contraction regulated by various factors

• Connective tissues– Attachment

• Tendons (extension of fascicles)

– Covering• Endomysium (fiber)• Perimysium (fascicli)• Epimysium (muscle)• Muscular fascia

– Separates/compartmentalizes muscles

• Nerves– Motor neurons

• Blood vessels

Muscle fibers

• Myofibril– Actin (thin)– Myosin (thick)– Actin and myosin are called myofilaments and

form sacromeres

Sacromeres

• Function of myosin head– Interaction with actin• Cross-bridge formation

– Contraction of muscle• Bending and recoiling

– ATP metabolism

Which band/zone corresponds to each figure?

Sliding filament model

• Actin filament sliding over the myosin– Shortening of sacromere– Responsible for muscle contraction• Shortening of myofibril

• Relaxation– Increased sacromere length• Contraction of antagonist muscle• Gravity

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Membrane potential

• Axon from motor nerve– Excitation of muscle fiber• Generation of action potential

• Differences in electrical charges between inside and outside of the plasma membrane– Differences in amount of Na and K ions– Membrane permeability• K>Na

• Membrane permeability– Channels• Voltage-gated• Chemical/ligand-gated

Action potential

Stimulation

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Na Channels openK channel closed

Na Channels closingK channel opening

Na Channels closedK channels open

• All or nothing principle• Propagation of action potential (animation)– Movement from one spot to the next

• Frequency– # AP/time period– Increased AP frequency, increased strength of

stimuli

• Source of muscular action potential– Signals from nerve fibers• Communicated via the neuromuscular junction

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• Clearance of acetylchorine– Rapid• Acetylchorinesterase• 1 presynaptic AP=1postsynaptic AP

– Recycled within the axon after metabolism

Excitation-Contraction coupling

• Conversion of neural signals into physical process of contraction– Generation of muscular action potential• Neural action potential

– Contraction• Movement of intracellular Ca in response to muscular

action potential

• Ca ions– Stored in sacroplasmic reticulum• Modified ER

– Released in response to muscular action potential• Transverse/t-tubule

General mechanism of contraction

• Eight steps– Action potential from the nerve reaches the nerve

ending on the muscle fiber– Release of acetylchorine (neurotransmitter)– Opening of multiple acetulchorine-gated channels– Increased flow of Na ions (muscular action

potential)– Propagation of action potential across muscle fiber

General mechanism of contraction

• Eight steps– Depolarization of membrane and release of Ca

ions from SR • Traveling of action potential into the inside of the fiber

– Attraction of actin and myosin fiber by increased Ca • Sliding of filaments

– Removal of Ca into SR• Restoration of filaments

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Cross-bridge movement

• Repeated interaction of myosin head and actin myofilament– Sliding of actin along the myosin surface– Repeated attachment and detachment of myosin

and actin– Requirement• ATP hydrolysis• Ca ion

• Power stroke– Movement of myosin

while attached to actin

• Recovery stroke– Restoration of myosin

head to its original position

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Muscle relaxation

• Factor that is essential for muscle contraction– Ca• Access to actin active sites

– Removal of Ca into SR• Covering of actin active site with troponin-tropomyosin

complex

– Energy-dependent process• Active Ca pumpus• Restoration of membrane potential

– Na-K pumps