Muscle Physiology: Contraction and Relaxation

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the primary energy source produced through aerobic respiration during muscle contraction?

ATP (correct)

Oxygen

Glucose

Lactic acid

Which type of motor unit is typically associated with fine motor control?

Small motor units (correct)

Large motor units

Variable motor units

Intermediate motor units

How does the recruitment of motor units affect muscle contraction strength?

Recruitment is irrelevant to contraction strength

Recruitment decreases contraction strength

Only a single motor unit can be recruited for maximal contraction

Recruiting more motor units increases contraction strength (correct)

What characterizes larger motor units in skeletal muscles?

They are involved in gross movements and innervate many muscle fibers Signup and view all the answers

During muscle contraction, the troponin-tropomyosin complex plays a crucial role in what process?

Facilitating the cross-bridge cycling Signup and view all the answers

What role does Ca++ play in muscle contraction?

It exposes the actin-binding sites for myosin crossbridges. Signup and view all the answers

What immediate energy source is available for muscle contraction?

Creatine phosphate Signup and view all the answers

During muscle contraction, which process is responsible for the movement of thin filaments?

Cross-bridge cycling Signup and view all the answers

What happens to a muscle fiber when ATP is not available?

Myosin heads remain attached to actin, causing rigor mortis. Signup and view all the answers

What triggers the relaxation of a muscle fiber?

Stopping of nerve signals and reuptake of Ca++ into the SR. Signup and view all the answers

Which statement about troponin-tropomyosin complex is true during muscle relaxation?

Tropomyosin covers myosin-binding sites to stop contraction. Signup and view all the answers

What defines the sliding filament model of muscle contraction?

Thin filaments slide over thick filaments, resulting in shortening of the sarcomere. Signup and view all the answers

What is the primary source of ATP during prolonged muscle activity?

Aerobic metabolism of glucose Signup and view all the answers

What role does troponin play in muscle contraction?

It exposes myosin-binding sites by moving tropomyosin. Signup and view all the answers

During cross-bridge cycling, what occurs immediately after the myosin head pivots toward the M-line?

ADP and inorganic phosphate are released. Signup and view all the answers

What happens to the troponin-tropomyosin complex when calcium ions diffuse into the sarcoplasm?

It moves to expose myosin-binding sites on actin. Signup and view all the answers

What initiates the action potential in the muscle fiber?

Binding of acetylcholine to sodium channels. Signup and view all the answers

What is the immediate consequence of action potential reaching the sarcoplasmic reticulum?

Calcium ions are released into the sarcoplasm. Signup and view all the answers

Which component is responsible for re-cocking the myosin head during muscle contraction?

Hydrolysis of ATP to ADP and inorganic phosphate. Signup and view all the answers

What is the function of acetylcholine at the neuromuscular junction?

To initiate the action potential in muscle fibers. Signup and view all the answers

In the sliding filament model, what does the interaction of actin and myosin result in?

Sliding of filaments past each other without changing their lengths. Signup and view all the answers

How does the action potential in the muscle fiber travel into the cell?

Via T-tubules, extensions of the sarcolemma. Signup and view all the answers

What is the primary trigger for the power stroke in cross-bridge cycling?

Release of ADP and inorganic phosphate. Signup and view all the answers

What does the term 'cross-bridge' refer to during muscle contraction?

The attachment of myosin to actin. Signup and view all the answers

What is the effect of ATP on the myosin head after the power stroke?

It causes the myosin head to detach from actin. Signup and view all the answers

What ions are specifically involved in triggering muscle contraction through the troponin mechanism?

Calcium ions Signup and view all the answers

Study Notes

Muscle Fiber Contraction

Thin filaments are pulled and slide past thick filaments within the fiber’s sarcomeres
Requires calcium ions (Ca++) and ATP
Ca++ initiates contraction by exposing actin-binding sites for myosin cross-bridges
ATP sustains contraction: each cycle in cross-bridge cycling requires energy provided by ATP hydrolysis
Without ATP, the myosin head remains attached to actin, causing rigor mortis
Myosin is in a high-energy configuration when myosin head is cocked, this energy is used during the power stroke

Muscle Fiber Relaxation

Muscle contraction stops when:
- Nerve signal stops
- Muscle runs out of ATP and becomes fatigued
When the nerve signal stops, ACh release stops, ligand-gated Na+ channels close, and the sarcolemma and T-tubules repolarize
This causes voltage-gated Ca++ channels in the sarcoplasmic reticulum (SR) to close
Ca++ ions are actively pumped back into the SR using ATP
Tropomyosin moves to cover myosin-binding sites
Thick and thin filament interaction relaxes

ATP Sources for Muscle Contraction

Skeletal muscle has a small amount of ATP stored
To sustain contraction, ATP must be replaced quickly
Creatine phosphate: excess ATP transfers energy by producing ADP and creatine phosphate. When energy is needed, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. Can only provide 15 seconds worth of energy
Glycolysis: anaerobic breakdown of glucose to produce ATP, at a slower rate than creatinine phosphate. Provides a minute burst of energy
Aerobic respiration: aerobic breakdown of glucose or other nutrients in the presence of oxygen (O2) to produce carbon dioxide, water, and ATP. More efficient, produces 95% of ATP

Action Potential & Membrane Potential

All living cells have membrane potentials (electrical gradients across their membranes): -60 to -90 mV
When the membrane potential becomes less negative, depolarization occurs and an action potential can start
Membrane potentials change when ions either enter or leave the cell through ion channels, which can open and close depending on the stimuli. This change generates electrical signals (action potential) which travel quickly over long distances
An action potential in a nerve at the NMJ releases a neurotransmitter that leads to the start of an action potential in the muscle. This action potential in the muscle causes muscle contraction (Excitation-contraction coupling)
Every skeletal muscle fiber is innervated by a motor neuron at the NMJ. A signal from the motor neuron can cause the contraction of skeletal muscle fibers

Contraction of Skeletal Muscle

Step 1: Action potential (AP) reaches the end of the motor neuron
Step 2: Neurotransmitter (acetylcholine or ACh) is released into the NMJ
Step 3: ACh binds to specific receptors on ligand-gated ion channels for sodium on the skeletal muscle fiber, opening the sodium channels and allowing sodium to enter the sarcoplasm of the muscle fiber
Step 4: Membrane potential of muscle fiber changes, starting an action potential along the sarcolemma of muscle fiber, which travels into the interior of the cell via T-tubules (extensions of the sarcolemma)
Step 5: Action potential depolarizes the cell membrane, causing voltage-gated Ca++ channels in the SR to open, and Ca++ diffuses out of SR into the sarcoplasm
Step 6: Ca++ binds to troponin on the thin filament
Step 7: The troponin-tropomyosin complex moves to expose myosin-binding sites on the thin filament, allowing myosin to bind to actin at its myosin-binding site, forming a cross-bridge
Step 8: Adenosine diphosphate (ADP) and inorganic phosphate (Pi) generated in the previous contraction cycle are released, causing the myosin head to pivot toward the M-line at the center of the sarcomere - power stroke
Step 9: New ATP attaches to the myosin head, detaching the cross-bridge
Step 10: ATPase in the myosin head hydrolyzes ATP to ADP and Pi, releasing energy and causing the angle of the myosin head to move into a cocked position (re-cock), ready to form another cross-bridge with the next myosin-binding site. This repeated process is called cross-bridge cycling

Motor Units

Each skeletal muscle fiber is innervated by only one motor neuron. Each motor neuron innervates more than one muscle fiber, the number depends on the nature of the muscle
Motor unit: group of muscle fibers innervated by a single motor neuron
- Small motor units can innervate less than 10 muscle fibers and permit very fine motor control of the muscle. They have smaller, lower-threshold motor neurons that are more excitable
- Larger motor units can supply 1000s of muscle fibers in a muscle and are concerned with simple, or “gross,” movements, eg thigh muscles. They have bigger, higher-threshold motor neurons
Recruitment process: smaller motor units tend to be recruited first before larger ones, increasing the muscle contraction. Recruitment of more motor units will increase the strength of muscle contraction: allows for variation in picking up a feather vs a heavy weight.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Explore the intricate processes of muscle fiber contraction and relaxation in this quiz. Learn how ATP and calcium ions play crucial roles in these mechanisms, as well as the effects of nerve signals on muscle activity. Test your knowledge on key concepts related to muscle physiology.