Transcript Muscle Physiology

Muscle
Physiology
Properties
• Muscles have 4 main properties. The
ability to
–Respond to stimuli = excitability
–Shorten = contractility
–Stretch = extensibility
–Return to original shape = elasticity
3 types
Contraction Type
• Isotonic – constant force
–Ex: body movements
• Concentric: shortening
• Eccentric: lengthening
• Isometric – tension < resistance
–Ex: attempting to pick up a box that’s too
heavy
–Maintain posture
Muscle Tone
• Even when they appear at rest, an
amount of sustained contraction is
present in many muscles
–Maintain posture
–Hold your head up
Skeletal Muscle
• Fiber = single cell
–Sarcolemma (membrane)
–Sarcoplasm (cytoplasm)
–Nuclei
–Mitochondria
–Sarcoplasmic reticulum
–Transverse tubules (connect SRs)
–Myofibrils (parallel, thread-like structures)
Myofibrils
• Made of 2 protein filaments
–Thick = myosin
–Thin = actin
Myofibrils
• Held together by “Z lines”
–Actin attaches directly
–Myosin attaches using a protein called
titin
• Form a repeating pattern
–Z line to Z line = 1 sarcomere
Myosin
• 2 twisted protein strands
• Outward projections called heads
Actin
• 2 protein strands twisted into a helix
• Tropomyosin – rod-shaped protein that
wraps around the actin strands
• Troponin – connects tropomyosin to
actin
• The T-T complex blocks & allows access
to binding sites that allow actin to
connect to myosin
Muscle Contraction
• Muscle contraction is a complex interaction
–Leads to movement within the myofibrils
• Actin & myosin slide past each other
• Sarcomere shortens
• Fiber shortens
• Pulls on attachments which causes
movement
So How Does it Work?
• When a muscle fiber is “at rest” the
troponin-tropomyosin complex blocks
the binding sites of the actin
• When stimulated, the complex exposes
the binding site allowing links between
actin and myosin
The sliding filament theory of
muscle contraction
Contraction
• The force that shortens the sarcomere
comes from the cross-bridges pulling on
the thin filaments
• The head binds, bends a bit, lets go,
straightens, then binds again starting the
process over
• This is called a power stroke
Power
• The power for this comes from ATP
–The released energy “cocks” the cross
bridges, readying them for binding
Heads binding to actin = no ATP
Heads pulling in = no ATP
Heads popping off and flipping back into original position = ATP!
So Where Does the Stimulus Come From?
• Each fiber is connected to the
axon of a motor neuron
• Usually, contraction will only
happen with stimulation from
here
The Neuromuscular Junction
• At the connection site, the sarcolemma is
extensively folded creating a motor end
plate
–Nuclei and mitochondria are abundant
just underneath
Motor Units
• A muscle fiber usually only has 1 motor
end plate
• The axon has many branches
–Creates a motor unit so all connected
fibers are stimulated at once
Motor Units
• Large motor units stimulate numerous
muscle fibers at once
–Generate large force at the cost of
control
• Small motor units stimulate few fibers at
once
–Generate smaller forces but gain
intricate control
Muscle Contraction Stimulation
• The membrane of the neuron is
separated from the membrane of the
muscle cell by a space called the synaptic
cleft
• On the neuron side, a batch of
neurotransmitters is stored for release
upon the electrical impulse
Muscle Contraction Stimulation
• In motor neurons, that neurotransmitter
is acetylcholine (ACh)
• ACh diffuses
quickly across the
synaptic cleft
Muscle Contraction Stimulation
• When ACh binds to the receptors of the
sarcolemma it creates a muscle impulse
• This impulse, called an action potential,
is spread in all directions around the fiber
and into the T-tubules
Muscle Contraction Stimulation
• This impulse triggers the release of
Calcium ions from the SR
• The Ca2+ then binds with troponin
• This causes the tropomyosin to slide over
– exposing the binding sites
Relaxation
• When nerve impulses stop:
1. remaining ACh in synaptic cleft breaks
down preventing fiber stimulation
2. Ca2+ moved back into SR breaking the
cross links between actin & myosin
• Requires ATP