lecture 17- skeletal muscle Flashcards
what are the two key molecules for cross bridge formation and cycling
Ca2+
ATP
how does Ca2+ help with cross bridge cycling?
enables myosin heads to attach to actin by revealing myosin binding sites on actin molecules
–> high force cross bridge formation
how does ATP help with cross bridge cycling?
ATP binds to myosin and is hydrolyzed to ADP+Pi
ATP hydrolysis provides energy for the myosin “powerstroke”
ATP binding to myosin allows it to detach from actin
–> cross-bridge cycling
what does tropomyosin do when Ca2+ binds to troponin?
tropomyosin shifts away from the myosin binding site on actin
explain the 4 steps of the contraction cycle
- ATP binds to myosin
- myosin hydrolyzes ATP
- power stroke
- myosin releases ADP at the end of the power stroke
do thick and thin filaments change in length?
NO
what is true about lengthening and shortening with contraction
A band length maintained
I and H band shorten
sarcomere shortens (Z lines closer)
Muscle tension
force created by muscle
Load
weight or force opposing contraction
contraction
creation of tension in muscle
relaxation
release of tension
what are the 4 stages of excitation-contraction in skeletal muscle cells
- events at the NMJ lead to muscle cell depolarization
- muscle cell depolarization triggers Ca2+ release from the SR
- increased [Ca2+] triggers cross bridge cycling and contraction
- Ca2+ is removed from the sarcoplasm
how is an action potential initiated at the neuromuscular junction?
somatic motor neuron releases ACh at a neuromuscular junction
net Na+ influx through nAChR (nicotinic acetylcholine receptor) causes end plate potential (EPP) which is a graded potential
the graded potential (EPP) is always suprathreshold
how does muscle cell depolarization trigger Ca2+ release from the SR?
action potentials in t-tubule alters conformation of DHP (dihydropyridine) receptor
DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm
Ca2+ binds to troponin allowing actin-myosin binding
The DHP receptor is what type of channel?
Ca2+ channel
BUT Ca2+ entry is NOT required for opening of the RyR
how does increased [Ca2+] trigger cross-bridge cycling and contraction?
myosin heads execute power stroke
actin filament slides toward the center of the sarcomere
how is Ca2+ removed from the sarcoplasm?
(Skeletal muscle relaxation)
sarcoplasmic Ca2+-ATPase pumps Ca2+ back into SR
decrease in free cytosolic [Ca2+] causes Ca2+ to unbind from troponin
tropomyosin covers the binding site again.
myosin heads release, making elastic elements pull filaments back into their relaxed position
what pumps Ca2+ back into the SR?
(skeletal muscle relaxation)
the SR-Ca2+ ATPase (SERCA)
a muscle twitch is a single …… cycle
contraction-relaxation
what does summation allow for in a muscle fiber in terms of contraction?
allows for increased force of contraction
What would low force of contraction be due to?
low glycogen?
excitation-contraction failure such as SR Ca2+ leak?
lactate of acid accumulation?
NMJ failure?
is low muscle contraction force due to low ATP?
no
what is a motor unit?
one motor neuron and the muscle fiber it innervates
size of motor units is related to…
the need for refined movement
more refined movement= fewer fibers per motor unit
number and order of motor units recruited is related to…
the power needed to generate movement
smaller motor units are always recruited first!!!!
larger motor units contain more muscle fibers and thus…
generate more force/tension when activated
tension can be increased by recruitment of..
multiple motor units within a muscle
larger muscles have many….
motors units, some of which may quite large
smaller motor units have…
smaller motor neurons
-lower threshold for activation
-activated at lower frequency of stimulation from CNS
as frequency of stimulation from from CNS increases
- AP frequency and tension in motor unit increases
- larger motor units brought to threshold and contribute to total muscle tension
total muscle tension reflects the
sum of motor unit activation in a motor neuron pool
length-tension relationships in skeletal muscle
sarcomeres contract with maximal force when they are at their optimal resting length just prior to contraction
- provides optimal number of cross bridges
- normal resting length of muscle usually ensures this happens
tendon
attaches skeletal muscle to bone
ligaments
attach bone to bone
