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
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Endomysium (fiber)
Perimysium (fascicli)
Epimysium (muscle)
Muscular fascia
– Separates/compartmen
talizes 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 closing
K channel opening
Na Channels closed
K channels open
Na Channels open
K channel closed
• 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