Muscle Unit Flashcards
To do well on this exam
In the context for the big themes of physiology, what role does muscle often have?
It often is an effector
What is the general function of skeletal muscle?
It is generally involved in voluntary movement, it is connected via tendons, in contractions the bones can be pulled closer together.
Muscle cells are represented through long _____, ____ extend throughout the cell which helps withsupplying the cell with all the materials it needs
long fibers, nuclei
Skeletal muscle fibers only contract in response to a __________ The axon of an individual motor neuron branches and makes synapses on multiple muscle fibers.
Motor neuron
Although the axon of an individual motor neuron branches and makes synapses on multiple muscle fibers, each muscle fiber is only directly controlled by ____ motor neuron.
one
What is a motor unit?
A motor neuron, and the muscle fibers it controls together
Outline the general structure of the physical contractile units within a muscle
Which physical feature(s) are set up to allow for contraction/shortening
Both the H zone and the I band can shorten
The thick filaments contain the ____
myosin cross bridges (actin binding sites, ATP binding sites)
The thin filaments contain the ____
Troponin, tropomyosin, actin, myosin binding sites
_____ at rest is preventing myosin from binding to actin
tropomyosin
myofibrils
are cylindrical structures in muscle fibers containing contractile proteins; they themselves are composed of repeating units called sarcomeres
what are t tubules
t tubules are extensions of the plasma membrane that dive into the muscle fibers themselves
what is the sarcoplasmic reticulum
specialized endoplasmic reticulum
what is the “first step” in getting a muscle to contract?
The motor neuron needs to fire an action potential
Describe the “steps” of the action potential
1) motor neuron action potential occurs
2) Ca 2+ enters voltage gated channels
3) Acetylcholine release
4) Acetylcholine binding opens ion channels
5) Na+ entry and K+ exit
6) propagated action potential in muscle plasma membrane
describe the relationship between the muscle fiber action potential and muscle contraction
there is a slight delay between the action potential and the muscle contraction
Describe the steps involved in a muscle contraction from the end of the action potential to the beginning of the contraction
Action potential propagates along muscle fiber membrane and into T-tubules
DHP receptor is a voltage-gated Ca 2+ channel that physically interacts with a Ca 2+ channel in sarcoplasmic reticulum called a ryanodine receptor.
When action potential arrives, the DHP receptor senses voltage change, undergoing its own conformational change that opens the ryanodine receptor.
Ca2+ enters cytoplasm from sarcoplasmic reticulum
Ca2+ binds to troponin, which causes tropomyosin to shift and uncover myosin binding sites on actin
1) Cross bridge binds to actin
2) Cross-bridge moves
3) ATP binds to myosin, causing cross-bridge to detach
4) Hydrolysis of ATP energizes cross-bridge
Ca 2+ ATPase pumps Ca2+ back into sarcoplasmic reticulum, removing free Ca2+ available to bind troponin
Tropomyosin blocks myosin binding sites on actin again
now, describe the steps involved in a muscle contraction from the initiation of the action potential
1) motor neuron action potential occurs
2) Ca 2+ enters voltage gated channels
3) Acetylcholine release
4) Acetylcholine binding opens ion channels
5) Na+ entry and K+ exit
6) propagated action potential in muscle plasma membrane
then ….
Action potential propagates along muscle fiber membrane and into T-tubules
DHP receptor is a voltage-gated Ca 2+ channel that physically interacts with a Ca 2+ channel in sarcoplasmic reticulum called a ryanodine receptor.
When action potential arrives, the DHP receptor senses voltage change, undergoing its own conformational change that opens the ryanodine receptor.
Ca2+ enters cytoplasm from sarcoplasmic reticulum
Ca2+ binds to troponin, which causes tropomyosin to shift and uncover myosin binding sites on actin
1) Cross bridge binds to actin
2) Cross-bridge moves
3) ATP binds to myosin, causing cross-bridge to detach
4) Hydrolysis of ATP energizes cross-bridge
Ca 2+ ATPase pumps Ca2+ back into sarcoplasmic reticulum, removing free Ca2+ available to bind troponin
Tropomyosin blocks myosin binding sites on actin again
Trace the movement of ATP through steps 1-4 of the figure 9.12 cycle
Initially, ADP + Pi are bound to the energized cross-bridge. After the cross bridge binds to the actin, the ADP + Pi leave the cross bridge head. ATP rebinding to Myosin catalyzes ATP hydrolysis, which re-energized the cross-bridge.
What maintains the smoothness of the contraction
All of the myosin heads do not bind to the actin at once
Describe the role of ATP in skeletal muscle function
- ATP detaches cross bridges
Provided energy via ATPases:
Ca 2+ ATPase
Myosin ATPase
NaK+ ATPase
What makes the most sense as an explanation for why increasing frequency of stimulation leads to an increased force generated by a muscle fiber/motor unit?
A higher proportion of myosin cross bridges can bind actin simultaneously.
All motor units (and muscle fibers…)
Are distinguishable by type (slow fibers, fast fatigue-resistant fibers, fast fatiguable fibers), such that a single motor unit will share the same type of fiber.
