Contraction Flashcards
1
Q
What are the 3 parts of skeletal muscle contraction?
A
- neuromuscular junction
- excitation-contraction
- cross-bridge cycling
2
Q
What is contraction of a muscle fiber?
A
- requires interaction from several chemical and cellular components
- results from a movement within the myofibrils, in which the actin and myosin filaments slide past one another, shortening the sarcomeres
- muscle fiber shortens and pulls on attachment points
3
Q
What is the sliding filament model?
A
- when sarcomeres shorten, thick and thin filaments slide past one another
- H zones and I bands narrow
- Z lines move closer together
- Thin and thick filaments do not change length
- overlap between filaments increases
4
Q
What is a neuromuscular junction?
A
- a type of synapse
- site where an axon of motor neuron and skeletal muscle fiber interact
- skeletal muscle fibers contract only when stimulated by a motor neuron
5
Q
What are the parts of a Neuromuscular Junction (NMJ)
A
- motor neuron
- motor end plate
- synaptic cleft
- synaptic vesicles
- neurotransmitters
6
Q
What is the stimulus that causes contraction?
A
- Acetylcholine (ACh) is the NT
- Nerve impulse causes release of ACh from synaptic vesicles
- ACh binds to ACh receptors on motor end plate
- ACh causes changes in membrane permeability to Na+ and K+ ions, which generates a muscle impulse (action potential)
- impulse causes release of Ca+ from SR, which leads to muscle contraction
7
Q
What is excitation-contraction coupling?
A
- connection between muscle fiber stimulation and muscle contraction
upon stimulation:
- muscle impulses cause SR to release Ca+2 ions into cytosol
- CA+2 ion binds to troponin to change its shape
- the position of tropmysosin is altered
- binding sites on actin are now exposed
- myosin heads bind to actin, forming cross-bridges
8
Q
What is the synapse where a motor neuron axon and a skeletal muscle fiber meet called?
A
- a neuromuscular junction
- this is where the muscle fiber membrane (sarcolemma) is specialized to form a motor end plate
- this is where the nuclei and mitochondria are abundant and the sarcolemma is extensively folded
9
Q
What is the first step in the NMJ?
A
- an action potential arrives and causes the voltage-gated calcium channels to open
- Calcium enters the channel and causes the NT acetylcholine to be released from their vesicles into the pre-synaptic cleft
- the diffusion of ACh across the synaptic cleft to ACh receptors on the postsynaptic cleft caused an increase of permeability in the ligand-gated sodium ion channels
- The movement of sodium ions into the muscle cell results in depolarization of the post synaptic membrane
- once the threshold has been reached, an action potential is propagated over the muscle cell membrane (sarcolemma)
- this impulse spreads into the transverse tubules and triggers the release of calcium ions from the sarcoplasmic reticulum
- this leads to muscle contraction
10
Q
What is acetylcholine (ACh)?
A
- is a NT that a motor neuron uses to control skeletal muscle contraction
- is synthesized in the cytoplasm of the motor neuron and is stored in synaptic vesicles near that distal end of its axon
- when an action potential reaches the end of the axon, some of these vesicles release acetylcholine into the synaptic cleft
- ACh diffuses rapidly across the synaptic cleft and binds to specific protein molecules (receptors) in the muscle fiber membrane, increasing the membrane permeability to sodium and potassium ions
11
Q
What is excitation-contraction coupling?
A
- is the connection between stimulation of a muscle fiber and contraction
- involves an increase in calcium ions in the cytosol
12
Q
How does calcium affect the contraction cycle?
A
- active transport of calcium into sarcoplasmic reticulum makes myosin binding sites unavailable
- upon muscle contraction, calcium is released from sarcoplasmic reticulum and exposes binding sites on thin filament (actin)
- calcium binds to troponin
- tropomyosin pulls aside
- binding sites on thin filament are exposed - exposed binding sites on actin allow the muscle contraction to occur
- myosin heads bind to action and form cross-bridges that connect myosin to actin
- ADP and P release from myosin and cross-bridge pulls thin filament
- new ATP binds to myosin and breaks the connection to actin
- ATP splits and provides power to myosin head and stores energy for the next reaction
13
Q
What is the sliding filament model?
A
- muscles shorten when the actin and myosin filaments slide past each other, and pull on the muscle ends
- as this occurs, the H zones and the I bands narrow; the regions of overlap widen and the Z lines move closer together, shortening the sarcomere
14
Q
What is cross-bridge cycling?
A
- the force that shortens the sarcomeres by pulling on thin filaments
- A myosin head can attach to an actin binding site forming a cross-bridge and bends slightly to pull on the actin filaments
- then the head can release, straight and combine with another binding site further down the actin filament, and pull again
15
Q
What type of energy do myosin heads contain?
A
- ATPase, which catalyzes the breakdown of ATP to ADP and phosphate
- this reaction transfers energy that provides the force for muscle contraction
- breakdown of ATP puts the myosin head in a “cocked” position
- when the muscle is stimulated to contract, the cocked myosin head attaches to action
- this forms a cross-bridge that pulls the actin filament toward the center of the sarcomere
- this movement causes a greater overlap of the actin and myosin filaments and shortens that sarcomere and thus shortens the muscle
