Skeletal Muscle Physiology Quiz

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Questions and Answers

What is the resting membrane potential (RMP) of skeletal muscle?

-90 mV (correct)

-100 mV

-70 mV

-85 mV

How long does the action potential (AP) spike last in skeletal muscle?

2-4 ms (correct)

1-2 ms

4-6 ms

6-8 ms

During which process does calcium (Ca2+) bind to troponin C?

Generation of tension

Propagation of AP

Activation of muscle proteins (correct)

Release of Ca2+

What happens to excitability during the action potential (AP) in skeletal muscle?

It becomes refractory during the AP. Signup and view all the answers

How does tension develop when a muscle contracts?

By the cycling of cross-bridges with the thin filament. Signup and view all the answers

What initiates the contractile process in skeletal muscle?

Release of calcium from terminal cisternae. Signup and view all the answers

What is the speed of action potential (AP) conduction along the muscle fiber?

5 m/sec Signup and view all the answers

What is the threshold concentration of Ca2+ needed to activate muscle proteins?

0.1 μmol/L Signup and view all the answers

What initiates the first step in the cross-bridge cycle?

Binding of actin and high-energy myosin (M*-ADP-Pi) Signup and view all the answers

What is required for the detachment of the cross-bridge from the thin filament?

A new molecule of ATP Signup and view all the answers

During muscle contraction, what happens to troponin when Ca2+ is removed from the cytoplasm?

It returns to its original conformational state Signup and view all the answers

In the context of muscle fibers, what does the All or None Law imply?

Muscle fibers contract maximally or not at all after a threshold stimulus Signup and view all the answers

What is the role of ATP hydrolysis in the final step of the cross-bridge cycle?

It drives myosin back to its high-energy state Signup and view all the answers

How does the force developed by cross-bridge bending affect the muscle?

It transmits through the actin filament to the Z disk Signup and view all the answers

What occurs during a muscle twitch in response to a single action potential?

Brief contraction followed by relaxation Signup and view all the answers

What happens during the second step of the cross-bridge cycle?

Cross-bridges bend and slide the thin filament Signup and view all the answers

What is a key characteristic of isotonic contractions?

Muscle length changes during the contraction Signup and view all the answers

Which statement best describes isometric contractions?

They last longer than isotonic contractions Signup and view all the answers

What is the primary immediate energy source for muscle contraction?

ATP Signup and view all the answers

How much of the ATP present in muscles is typically sufficient for maximal contraction?

5 to 6 seconds Signup and view all the answers

Among the following, which metabolic mechanism uses phosphocreatine to regenerate ATP?

Phosphogen energy system Signup and view all the answers

What is a primary function of skeletal muscles during rest?

Maintenance of resting membrane potential Signup and view all the answers

What occurs during the isometric phase when lifting a heavy weight?

Tension increases with no change in length Signup and view all the answers

Which metabolic system is primarily used for immediate power surges in muscle activity?

Phosphogen system Signup and view all the answers

What percentage of mechanical efficiency is associated with isotonic contractions?

20 – 25 % Signup and view all the answers

During which time frame does the glycogen-lactic acid system primarily operate?

30 - 40 seconds Signup and view all the answers

What is the main purpose of the elevated oxygen consumption after exertion?

To replenish ATP and creatine phosphate stores Signup and view all the answers

What is the term used for the extra oxygen consumed after exercise to aid recovery?

Oxygen debt Signup and view all the answers

What is the primary limitation of the glycogen-lactic acid system during muscle activity?

Accumulation of lactic acid Signup and view all the answers

Which of the following statements is true regarding muscle contraction types?

Muscles can contract in isometric, isokinetic, or isotonic ways. Signup and view all the answers

Which of the following processes is responsible for restoring creatine phosphate during muscle relaxation?

Reverse reaction Signup and view all the answers

What is the maximum duration of muscle activity that the phosphogen system can support?

10-15 seconds Signup and view all the answers

Which body mechanism predominantly helps in the removal of lactic acid from body fluids?

Conversion to pyruvic acid Signup and view all the answers

What is the primary energy source for the aerobic system during prolonged exercise?

Oxidation of foodstuffs Signup and view all the answers

How is ATP produced by the aerobic system?

By oxidative processes in the mitochondria Signup and view all the answers

Which statement describes the relationship between oxygen input and energy output during exercise?

They are directly proportional up to a point Signup and view all the answers

What occurs when muscular exertion is very high in relation to ATP synthesis?

Anaerobic pathways supplement ATP synthesis Signup and view all the answers

Study Notes

Electrical and Excitability Changes

The electrical events in skeletal muscle and nerve are similar, with slight timing and magnitude differences
The resting membrane potential (RMP) of skeletal muscle is about -90mV
The action potential (AP) spike lasts 2-4 milliseconds and travels along the muscle fiber at about 5 meters per second
The AP precedes muscle contraction by 2 milliseconds
During the AP, skeletal muscle fibers are refractory to stimulation, similar to nerve fibers
The latency period of the mechanical response coincides with the absolute refractory period (ARP) of the AP
As the muscle begins to contract, it regains its excitability

Mechanical Changes

An action potential (AP) triggers the contractile process in skeletal muscle
There are four steps involved in the mechanical changes:
- Release of Ca2+:
  - The AP propagates into the T tubules
  - Dihydropyridine receptors (DHP) pull the feet of the ryanodine receptors (RyR) open
  - Ca2+ channels on the terminal cisternae open
  - Ca2+ flows from the terminal cisternae (TC) into the sarcoplasm
  - This influx of Ca2+ initiates muscle contraction
- Activation of muscle proteins:
  - When sarcoplasmic Ca2+ concentration reaches at least 0.1µmol/L, Ca2+ binds to troponin C on the thin filament
  - This binding causes a conformational change in troponin
  - Tropomyosin moves away from its position, uncovering the myosin-binding site on actin
  - The exposed myosin-binding site on actin then binds with the cross-bridges (from the thick filament)
  - This initiates the contraction
- Generation of tension:
  - Tension, the force developed during muscle contraction, is generated by the cycling of cross-bridges
  - Myosin in a rested muscle is in a high-energy conformational state (M*) and has ATP attached to it
  - The energy from ATP hydrolysis drives myosin from a low-energy state (M) to the high-energy state (M*)
  - The cross-bridge cycle involves four steps:
    - Binding of high-energy myosin (M*-ADP-Pi) to actin
    - Bending of the cross-bridges, sliding of the thin filament across the thick filament, and conversion of myosin to a low-energy state (M-A)
    - Detachment of the cross-bridge from the thin filament (requires new ATP)
    - Return of the cross-bridges to their original position through ATP hydrolysis
- Relaxation:
  - Ca2+ is removed from the cytoplasm by the Ca2+/ATPase pump located on the sarcoplasmic reticulum (SR) membrane
  - When intracellular Ca2+ concentration falls below 0.1µmol/L, troponin returns to its original state
  - Tropomyosin moves back to cover the myosin binding site on actin
  - Cross-bridge cycling stops

The All or None Law

A single skeletal muscle fiber follows the all-or-none law, meaning it contracts maximally or not at all
A threshold stimulus produces maximal contraction under constant experimental conditions

The Muscle Twitch

A single action potential causes a brief contraction followed by relaxation
This response is called a simple muscle twitch

Types of Muscle Contractions

Isotonic contraction:
- Tension changes
- Muscle shortens (sliding of filaments causes length change)
- Duration is longer and consumes more energy (to overcome inertia)
- External work is present (load is moved)
- Mechanical efficiency is 20-25%
- Importance is for moving body parts
Isometric contraction:
- Tension increases, but no change in length
- No filament sliding (no change in length)
- Duration is shorter and consumes less energy (no inertia)
- No external work (load is not moved)
- Mechanical efficiency is zero
- Importance is in tensing body parts

Metabolic Changes

Energy Sources and Muscle Metabolism:
- During Rest:
  - Skeletal muscle consumes energy for:
    - Maintenance of resting membrane potential (RMP) - 5%
    - Synthesis of chemicals like glycogen
    - Production of muscle tone
- During Contraction:
  - Energy consumption increases significantly
  - ATP is the immediate energy source for muscle contraction
  - ATP is hydrolyzed anaerobically into ADP by the muscle protein myosin (acting as ATPase)
  - There is not enough ATP for sustained contractions (5-6 seconds)
  - ATP is continuously reformed by three metabolic mechanisms:
    - Phosphocreatine System:
      - Muscle cells contain 2-3 times more phosphocreatine than ATP
      - Energy is quickly transfered from phosphocreatine to ATP
      - The phosphogen energy system (phosphocreatine and ATP together) provides maximal muscle power for 10-15 seconds (enough for a 100m run)
      - Phosphocreatine is restored during relaxation through the reverse reaction
    - Glycogen Lactic Acid System:
      - This system provides extra muscle activity for 30-40 seconds in addition to the phosphogen system
      - Glucose (or glycogen) is anaerobically broken down to lactic acid and 4 ATP via glycolysis
      - Lactic acid production causes fatigue, limiting the duration of this system
      - Lactic acid removal takes 1 hour or more and occurs in three ways:
        
        Conversion to pyruvic acid and oxidative metabolism (slow mechanism)
        
        Reconversion to glucose in the liver
        
        Use as fuel by the heart
    - Aerobic System:
      - Oxidation of glucose, fatty acids, and amino acids in the mitochondria for energy production
      - Oxygen consumption
      - Provides unlimited energy as long as nutrients and oxygen are available
      - During exercise, muscle blood vessels dilate, increasing blood flow and oxygen supply
      - Up to a point, oxygen consumption is proportional to energy expenditure
      - During strenuous exertion, the anaerobic pathway is used when ATP, creatine phosphate, and myoglobin oxygen stores are depleted
** During Recovery (Oxygen Debt):**
- After exercise ends, ventilation rate remains high for a period to:
  - Remove excess lactate
  - Replenish ATP and creatine phosphate stores
  - Replace myoglobin oxygen
- This extra oxygen consumption after exercise is called "oxygen debt"

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Description

Test your knowledge on skeletal muscle physiology, including resting membrane potential, action potential duration, calcium binding, and the contractile process. This quiz covers essential concepts related to muscle contraction and the underlying biochemical mechanisms. Perfect for students studying muscle biology and related fields.