9-2. Muscle Contraction Flashcards
sliding filament theory
in a relaxed muscle fiber, the thin and thick filaments overlap only partially, but during contraction, the thin filaments slide past the thick ones towards the center of the sarcomere, which shortens
What are the steps of the sliding filament theory?
- cross bridge attachment - myosin heads (cross bridges) attach to active (myosin binding) sites on the actin > high energy position (“cocked”)
- power stroke - myosin head swings forward, pulling the actin filament toward center of the sarcomere
- cross bridge detachment - ATP binds to myosin head, enabling to to release the hold on actin and swing back to cocked position as ATP is hydrolized
- cross bridge attaches to another active site further along the actin, and sequence repeats
What is the result of the cross bridge attachment, power stroke, and detachment?
thin filaments pulled to center of sarcomere > Z lines pulled closer together > shortens > effect multiplied by thousands of sarcomeres in a single fiber
A strongly contracted muscle may shorten up to ___ it’s resting length.
1/2
energy for contraction
comes from ATP, which enables the cross bridges to detach and swing back to cocked position
rigor
in the absence of ATP, cross bridges don’t detach and muscles become rigid
rigor mortis
following death, ATP production stops along with all other metabolic activities. Cross bridges remain locked in place and all skeletal muscles become rigid
Why don’t cross bridges form in a relaxed muscle?
myosin-binding sites on the actin are covered by the troponin-tropomyosin complex, preventing the cross bridges from binding.
covered because there is a low concentration of calcium ions in the sarcoplasm
Where is calcium stored in a muscle?
sarcoplasmic reticulum
What signals contraction?
stimulation by a nerve
motor neuron
nerve cell that stimulates muscle tissue - as neuron enters muscle, it divides into many branches so that each muscle fiber gets its own branch
neuromuscular junction
connection between a motor neuron and a muscle fiber
synaptic cleft
tiny space between end of neuron and surface of muscle cell (region of sarcolemma called motor end plate)
synapic vescile
when a nerve impulse reaches the end of the neuron, these small sacs in the end of the neuron release a chemical into the cleft by exocytosis
What neurotransmitter is released by motor neurons?
acetylcholine (ACh)
receptors
binding sites on the sarcolemma - when ACh binds, an electrical current travels along the surface of the muscle cell
motor unit
a motor neuron and all the muscle fibers it stimulates. The average number of fibers per motor unit is 150
Summarize the events in a muscle contraction
- nerve impulse travels to the end of a motor neuron, causing the synaptic vesicles to release ACh, which diffuses across the synaptic cleft and binds to receptors on the sarcolemma. This generates a muscle impulse that travels along the sarcolemma and down the transverse tubules
- impulse causes sarcoplasmic reticulum to release stored calcium ions
- calcium ions bind to troponin, pulling the troponin-tropomyosin complex out of the blocking position on the actin and exposing myosin binding sites
- cross bridges form as myosin heads attach to the actin on the binding sites
- myosin heads swing forward, causing the thin filaments to slide past the thick filaments towards the center of the sarcomere
- ATP causes cross bridges to detach and swing back into their cocked position as ATP is hydrolyzed. They then reattach to another binding site, and the process repeats.
- Z lines are drawn closer together and sarcomere shortens. As the thousands of sarcomeres in a fiber shorten, the muscle contracts
What enzyme breaks down ACh?
acetylcholinesterase - released by sarcolemma into cleft
Summarize the steps of a contraction stopping
- after the nerve impulse stops, the ACh in the cleft must be destroyed or it will continue to exite the muscle fiber
- sarcolemma releases acetylcholinesterase into the cleft to break down the ACh
- muscle impulse (electricity on sarcolemma) stops
- calcium ions are actively transported back into the sarcoplasmic reticulum for storage
- troponin resumes its original shape, and the T-T complex moves back to cover the myosin binding sites on the actin
- with the cross bridges broken, the filaments slide back to their relaxed position and the sarcomeres return to their resting length (and the muscle relaxes)
twitch
rapid, jerky response to a single threshold stimulus - contracts quickly, then relaxes
What are the three phases of a twitch?
- latent period (flat line - brief time between stiumulus and beginning of contraction)
- contraction period (upward curve - as cross bridges are pulling thin filaments past thick and muscle is shortening)
- relaxation period (downward curve - as muscle relaxes, Ca2+ is pumped back into the sarcoplasmic reticulum)
tetanus
smooth, sustained contraction produced by a series of rapid stimuli - the second stimulus is applied before the muscle has relaxed from the first
Most voluntary actions invovle short-tern ____ contractions
tetanic (resulting in smooth, coordinated movements)
treppe
“staircase effect” - the same strength stimulus is repeatedly applied to a muscle, it produces a slightly stronger contraction each time (enzymes are speeding up with the heat)
The principle behind warming up in athletics is based on ___ contractions
treppe
isotonic contraction
movement without tension or resistance - thense ion in muscle remains constant as it shortens
isometric contraction
tension increases during contraction, but the muscle neither shortens nor lengthens
Too much resistance causes ____ contraction; no resistance causes ____ contraction.
isometric; isotonic
muscle tone
slightly contracted state of skeletal muscles - doesn’t produce active movement, but keeps the muscles firm, healthy and ready to respond to stimulation
