Lecture 31. Muscle 2 Flashcards
When does muscle develop maximum tension?
when all myosin is aligned with actin filament and every myosin head can attach to actin
when does muscle develop no tension?
when the muscle is stretched out so that myosin and actin filaments do not overlap and myosin heads cannot bind to actin
OR
When the muscle is shortened so that all myosin and actin filaments are jammed together and it cannot generate tension( myosin pulls actin in opposite direction)
Why when the muscle is shortened it is harder to develop tension?
Myosin and actin overlap to the point, where myosin starts interacting with the wrong actin filament.
Cross-bridge cycle is happening but it is pulling in the wrong direction-> ineffective
Length-Tension relationship
During isometric muscle contraction
At the level of the sarcomere the maximum active force (tension
developed) is dependent on the degree of actin and myosin overlap
What is the optimum range for sarcomeres?
The maximal force between 2.0 – 2.2 µm
• A lengths >2.2 µm active forces decline as the extent of overlap between
filaments reduce, reducing the number of cross-bridges
• At lengths <2.0 µm filaments collide and interfere with each other reducing
force developed
What is the optimum range for sarcomeres?
The maximal force between 2.0 – 2.2 µm
• A lengths >2.2 µm active forces decline as the extent of overlap between
filaments reduce, reducing the number of cross-bridges
• At lengths <2.0 µm filaments collide and interfere with each other reducing
force developed
Elastic components of muscle
- Internal( eg titin)
- Externals( elastin in CT)
Passive vs active force in muscle
As muscle is stretched the connective
elements resist the stretch = passive force. No energy required
Active force- due to muscle contraction
Total tension is:
Total tension is the sum of the active tension
dependent on the sarcomere length and the passive tension
As the muscle is getting stretched passive tension contributes more to the total tension than active tension
When the muscle is stretched out what type of tension is it?
passive
Excitation contraction coupling
-Alpha motor neuron transmits a signal to the muscle fiber
Motor unit
A motor unit consists of a motor neuron and all the
muscle fibers it innervates.
One motor neuron can innervate multiple muscle cells at the same time
-> One AP in a nerve will cause contraction in more than 1 muscle fiber
Steps of muscle cell excitation
- Ach released into the neuromuscular junction
AP travels down the motoneuron
The AP at the synapse causes the opening of V-gated Ca2+ channels
Ca2+ causes the neurotransmitter vesicles to merge with the membrane, causing them to release Acetylcholine in the synaptic cleft - Activation of Ach receptors
Binding of Ach to the receptors in the muscle causes opening of ligand-gated ion (Na+) channels-> Na+ moves into the cell-> depolarization of end-plate potential - A muscle AP is triggered. If the threshold is reached -> V-gated Na+ channels open and AP is triggered. AP is propagated along sarcolemma into T-tubule
What is an area of high concentration of V-gate Ca2+ channels
In the synapse of a pre-synaptic neuron
When does the change from electrical signal to chemical signal happen?
when the neurotransmitter is released into the synapse
What channels propagate AP?
V-gated Na+ channels
why are the effects of Ach short-lasting?
It is quickly broken down
What breaks down Acetylcholine?
enzyme acetylcholine esterase
AP in skeletal muscle
- Slow depolarization at the start mostly due to the ligand-gated Na+ channels
- Once the threshold is reached-V-gated Na+ channels open-AP
- V-gated Na+ channels close, V-gated K+ channels open- repolarization
How quick is AP in muscle cell?
1-2 msec
Steps of muscle contraction
- AP spreads along the sarcolemma and down T-tubule, triggering the opening of V-gated Ca2+ channels in SR→ release Ca2+
- Ca2+ binds to troponin, moving tropomyosin of the binding site
- Cross-bridge cycle
- Ca2+ removed→ contraction ends
Where are Calcium channels and their receptors located?
Ca2+ channels- Sarcoplasmic reticulum SR
Receptors for channels(Voltage sensors)- t-tubules
How is Ca2+ removed after contraction?
Calcium is actively pumped back into the sarcoplasmic reticulum via Ca2+-ATPase pumps
Sources of ATP for muscle metabolism
- Creatine phosphate
- Anaerobic glucolysis
- Aerobic metabolism
