MPP: BLOCK 2: HUM'S AI-GENERATED SKELETAL MUSCLE VS. SMOOTH MUSCLE CONTRACTION CHEAT-SHEET

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Questions and Answers

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?

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?

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?

Activation of dihydropyridine receptors on T-tubules Signup and view all the answers

What is the effect of troponin binding with Ca²⁺ ions?

It moves tropomyosin away from myosin-binding sites. Signup and view all the answers

During the power stroke, what occurs to the myosin head?

It pivots and pulls actin towards the center of the sarcomere. Signup and view all the answers

What is the role of acetylcholinesterase in muscle contraction?

It breaks down ACh in the synaptic cleft. Signup and view all the answers

What happens during the repolarization phase of muscle relaxation?

Potassium ions exit the muscle cell. Signup and view all the answers

What occurs as calcium ions are removed from troponin during muscle contraction?

Tropomyosin returns to cover the myosin-binding sites on actin. Signup and view all the answers

What initiates an influx of calcium ions in smooth muscle contraction?

Stimulation by neurotransmitters or hormones. Signup and view all the answers

Which component is responsible for the eventual phosphorylation of myosin in smooth muscle contraction?

Myosin light chain kinase (MLCK). Signup and view all the answers

What is the role of Rho-kinase in smooth muscle contraction?

To inhibit myosin light chain phosphatase (MLCP). Signup and view all the answers

What happens during muscle relaxation after the removal of calcium ions?

The muscle fiber returns to its resting state. Signup and view all the answers

What initiates the increase of intracellular calcium in response to smooth muscle stimulation?

G protein coupled receptor signaling. Signup and view all the answers

What is the function of the Ca²⁺-calmodulin complex in smooth muscle cells?

Activate myosin light chain kinase. Signup and view all the answers

How do calcium ions interact at the level of the sarcoplasmic reticulum during muscle contraction?

They are released through IP3 gated channels. Signup and view all the answers

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.