U5 - Sliding Filament Theory

contractions

for these to happen, the muscles need to be TOLD to do these (re: these things create movements)

neuromuscular junction

where the nerve cells and muscle fibers meet

motor unit

a name that groups the muscle fiber and the motor neuron that work together to cause contractions

-> really a group of muscle fibers controlled by a single motor neuron

sliding filament theory (model)

the theory that describes contractions of muscle as when the thin filaments slide past the thick filaments to shorten the H-zone/sarcomere length

acetylcholine (ACh)

a neurotransmitter that the axon terminal of motor neurons releases to trigger a muscle action potential (nerve impulse) that travels across the sarcolemma/its T-tubules -- Na+ channels create this; binds to receptors in the sarcolemma of a muscle fiber

synaptic cleft

aka the synapse

acetylcholine diffuses across this space to bind to receptors in the sarcolemma

sarcoplasmic reticulum

for the purposes of the sliding filament theory, this organelle stores calcium ions that it releases through release channels when the action potential reaches the T-tubules

this action floods the sarcoplasm with calcium ion

thin filament structure

these are made of troponin, tropomyosin, and actin

troponin binds to tropomyosin, both work to block the binding sites on actin where myosin heads bind (troponin sort of 'eggs' tropomyosin on to block it)

thick filament structure

made of intertwined myosin proteins that lead to the formation of myosin heads; these want to bind with actin molecules

calcium ions

released by the sarcoplasmic reticulum, these will attach to troponin to release from tropomyosin when in high concentration, which then releases from actin as well

will lead to myosin and actin binding together

cross-bridges

the names for the connections between the heads of myosin filaments and receptor sites on the actin filaments

power stroke

action of myosin pulling actin inward (toward the M line); this occurs when cross-bridges form; the molecular action behind muscle contractions

ATP

from cellular respiration in the mitochondria; creatine phosphate increases its regeneration

Three Functions in the Sliding Filament Theory:

1. when it binds to the myosin head and is hydrolyzed, it provides energy for the power stroke

2. when a new one of this molecule binds to the myosin head, it will release the myosin head from actin

3. its hydrolysis also actively transports calcium ions in and out of the sarcoplasmic reticulum

muscle relaxation

Occurs when calcium is pumped back into the sarcoplasmic reticulum by ATP and the release channels close, leading to troponin and tropomyosin sliding back onto the actin molecule

acetylcholinesterase

the enzyme that breaks down acetylcholine in the synaptic cleft so another muscle action potential does not arise without more ACh

rigor mortis

the condition in which muscles become rigid after death

due to the lack of ATP that leads to the inability of cells to actively pump calcium back into the sarcoplasmic reticulum

-> myosin heads cannot detach from the actin molecule