Podcast
Questions and Answers
What begins the process of skeletal muscle contraction?
What begins the process of skeletal muscle contraction?
- Release of Ca²⁺ from the sarcoplasmic reticulum
- Arrival of an action potential at the axon terminal (correct)
- Hydrolysis of ATP to ADP + Pi
- Binding of ACh to muscle cell receptors
What role do sodium ions play during muscle contraction?
What role do sodium ions play during muscle contraction?
- They attach to myosin heads to initiate cross-bridge formation.
- They depolarize the muscle fiber membrane. (correct)
- They release Ca²⁺ from the sarcoplasmic reticulum.
- They cause the rigor state of cross bridge cycling.
Which event occurs immediately after the binding of ACh to its receptors?
Which event occurs immediately after the binding of ACh to its receptors?
- Sodium channels open and depolarization occurs. (correct)
- Myosin heads form cross-bridges with actin.
- The action potential travels down T-tubules.
- Calcium is released from the sarcoplasmic reticulum.
What initiates the release of Ca²⁺ ions from the sarcoplasmic reticulum?
What initiates the release of Ca²⁺ ions from the sarcoplasmic reticulum?
What is the effect of troponin binding with Ca²⁺ ions?
What is the effect of troponin binding with Ca²⁺ ions?
During the power stroke, what occurs to the myosin head?
During the power stroke, what occurs to the myosin head?
What is the role of acetylcholinesterase in muscle contraction?
What is the role of acetylcholinesterase in muscle contraction?
What happens during the repolarization phase of muscle relaxation?
What happens during the repolarization phase of muscle relaxation?
What occurs as calcium ions are removed from troponin during muscle contraction?
What occurs as calcium ions are removed from troponin during muscle contraction?
What initiates an influx of calcium ions in smooth muscle contraction?
What initiates an influx of calcium ions in smooth muscle contraction?
Which component is responsible for the eventual phosphorylation of myosin in smooth muscle contraction?
Which component is responsible for the eventual phosphorylation of myosin in smooth muscle contraction?
What is the role of Rho-kinase in smooth muscle contraction?
What is the role of Rho-kinase in smooth muscle contraction?
What happens during muscle relaxation after the removal of calcium ions?
What happens during muscle relaxation after the removal of calcium ions?
What initiates the increase of intracellular calcium in response to smooth muscle stimulation?
What initiates the increase of intracellular calcium in response to smooth muscle stimulation?
What is the function of the Ca²⁺-calmodulin complex in smooth muscle cells?
What is the function of the Ca²⁺-calmodulin complex in smooth muscle cells?
How do calcium ions interact at the level of the sarcoplasmic reticulum during muscle contraction?
How do calcium ions interact at the level of the sarcoplasmic reticulum during muscle contraction?
Study Notes
Skeletal Muscle Contraction
- Muscle contraction is triggered by a nerve impulse arriving at the neuromuscular junction (NMJ).
- The nerve releases acetylcholine (Ach) into the synaptic cleft.
- Ach binds to receptors on the muscle cell's sarcolemma, opening sodium channels.
- Sodium ions flow into the muscle cell, causing depolarization and an action potential.
- The action potential travels along the sarcolemma and down the T-tubules.
- This activates voltage-sensitive L-type calcium channels (DHPR) on the T-tubules.
- DHPR is mechanically coupled to the ryanodine receptor (RyR) on the sarcoplasmic reticulum (SR).
- RyR activation releases calcium from the SR into the sarcoplasm.
- Calcium binds to troponin, causing a conformational change that moves tropomyosin away from myosin-binding sites on actin filaments.
- Myosin heads bind to the exposed sites on actin, forming cross-bridges.
- ATP hydrolysis powers the myosin head to pivot, pulling the actin filament towards the center of the sarcomere, shortening the muscle and generating tension.
Skeletal Muscle Relaxation
- The motor neuron stops releasing Ach.
- Acetylcholinesterase breaks down Ach, ending muscle stimulation.
- Potassium ions exit the muscle cell, repolarizing the sarcolemma.
- Active transport pumps in the SR membrane pump calcium back into the SR.
- Calcium is removed from troponin, allowing tropomyosin to cover the myosin-binding sites on actin.
- Myosin heads detach from actin, and cross-bridges are broken.
- The muscle fiber returns to its resting state.
Smooth Muscle Contraction
- Smooth muscle can be stimulated by neurotransmitters, hormones, or other factors.
- Stimulation activates G-protein coupled receptors (GPCRs) triggering multiple signaling cascades.
- The cascades involve:
- PLC and Rho-kinase activation
- PLC activation generates IP3 and DAG
- IP3 binds to the IP3 gated calcium channel on the SR, increasing intracellular calcium.
- DAG activates PKC, activating CPI-17, which inhibits MLCP.
- Rho-kinase activation also inhibits MLCP.
- The stimulus also leads to influx of extracellular calcium through voltage-dependent or receptor-operated calcium channels.
- Increased intracellular calcium binds to calmodulin, forming a Ca²⁺-calmodulin complex.
- The complex activates myosin light chain kinase (MLCK).
- MLCK phosphorylates the light chain of myosin molecules, allowing myosin heads to interact with actin filaments.
- Simultaneously, Ca²⁺-CaM inhibits calponin and caldesmon, the two actin bound myosin ATPase inhibitors.
- The inhibition of calponin and caldesmon allows for the myosin head to experience appropriate ATP hydrolysis, promoting cross-bridge cycling.
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Description
This quiz delves into the intricate process of skeletal muscle contraction. It covers the role of nerve impulses, the neuromuscular junction, and the steps leading to muscle fiber activation including calcium dynamics and cross-bridge cycling. Test your understanding of this fundamental physiological mechanism.
