Skeletal Muscle Structure and Function

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Questions and Answers

What initiates an action potential in the muscle fibre membrane?

Voltage-gated sodium channels opening

Depolarization of the local area of the membrane (correct)

Release of calcium from the sarcoplasmic reticulum

Binding of troponin to tropomyosin

What role does the dihydropyridine receptor (DHPR) play in muscle contraction?

It releases calcium into the extracellular space.

It binds to ATP for energy.

It acts as a Ca²⁺ ion channel voltage sensor. (correct)

It directly initiates the power stroke.

How does troponin C contribute to muscle contraction?

It hydrolyzes ATP to release energy.

It exposes myosin-binding sites on actin filaments. (correct)

It directly binds to myosin heads.

It stabilizes the tropomyosin molecule.

What occurs during the power stroke in muscle contraction?

ADP and inorganic phosphate (iP) are released from myosin. Signup and view all the answers

What does the Sliding Filament Theory describe?

How myosin filaments 'walk' along actin filaments during contraction. Signup and view all the answers

What does 'excitation-contraction coupling' refer to in skeletal muscle physiology?

The mechanism by which action potential triggers muscle contraction Signup and view all the answers

Which component of a muscle fibre is responsible for storing calcium?

Sarcoplasmic reticulum Signup and view all the answers

What initiates the opening of ACh-gated cation channels in the muscle fibre membrane?

Secretion of acetylcholine Signup and view all the answers

Which of the following structures is described as forming deep invaginations in muscle fibres?

Transverse tubules Signup and view all the answers

What is the role of the sodium ions (Na⁺) entering the muscle fibre?

To cause depolarization of the muscle membrane Signup and view all the answers

Which structure within the muscle fibre is responsible for the interaction that leads to contraction?

Cross bridges between thick and thin filaments Signup and view all the answers

Which sequence accurately describes the initial event leading to muscle contraction?

ACh opens sodium channels Signup and view all the answers

Which feature is NOT characteristic of skeletal muscle fibres?

Involuntary contraction Signup and view all the answers

What causes the relaxation of muscle fibers after contraction?

Binding of ATP to myosin head Signup and view all the answers

What is rigor mortis primarily caused by?

Lack of ATP needed for muscle relaxation Signup and view all the answers

Which statement best describes the length-tension relationship in muscles?

Maximum tension occurs at an optimal length of overlap Signup and view all the answers

What happens when a muscle reaches its maximum load?

Contraction ceases despite muscle activation Signup and view all the answers

What characterizes isometric contraction?

Muscle remains the same length while developing tension Signup and view all the answers

How does active tension behave as a muscle is stretched beyond its normal length?

Active tension decreases Signup and view all the answers

What is muscle tone?

Continuous and passive partial contraction of muscles Signup and view all the answers

What effect does a higher temperature have on the onset of rigor mortis?

It causes a faster onset of rigor mortis Signup and view all the answers

What is one mechanism by which force summation can occur in muscle contractions?

Increasing the number of motor unit contractions Signup and view all the answers

What causes muscle fatigue during prolonged contraction?

Depletion of muscle glycogen Signup and view all the answers

What condition results when intracellular calcium concentration remains high due to repeated stimulation of the muscle?

Tetanic contraction Signup and view all the answers

How does interruption of blood flow affect muscle function during intense activity?

Causes complete muscle fatigue within minutes Signup and view all the answers

What is the primary effect of frequency summation in muscle contractions?

Enhancing the force of contraction through rapid stimulation Signup and view all the answers

Which factor is least likely to contribute to muscle fatigue during extended physical activity?

High levels of muscle glycogen Signup and view all the answers

What is the term for the all muscle fibers innervated by a single nerve fiber?

Motor unit Signup and view all the answers

What happens during muscle contractions in terms of work applied?

Work is defined as force x distance Signup and view all the answers

Study Notes

Muscle Contractile Responses

Excitation-contraction coupling: the mechanism by which an action potential causes muscle fiber contraction.

Skeletal Muscle

Each muscle fiber is a single unit, multinucleated, and contains myofibrils.
Myofibrils are surrounded by the sarcolemma.
The sarcolemma forms deep invaginations called transverse tubules (T tubules) within myofibrils.
Each myofibril contains interdigitating thick and thin filaments arranged longitudinally and cross-sectionally in sarcomeres.

Skeletal Muscle Microstructure

Sarcolemma: plasma membrane of the muscle fiber.
Myofibrils: threadlike strands within muscle fibers.
- Actin (thin filament): contains troponin and tropomyosin.
- Myosin (thick filaments): involved in muscle contraction.
Sarcoplasm: cytoplasm of the muscle fiber.
Sarcoplasmic reticulum: storage sites for calcium, surrounding myofibrils.
Transverse tubules: invaginations of the sarcolemma that allow the action potential to travel deep into the muscle fiber.

The Sliding Filament Theory

The most widely accepted theory explaining muscle fiber contraction.
Myosin filaments use energy from ATP to "walk" along actin filaments with their cross-bridges.
This pulls the actin filaments closer together, causing muscle contraction.

Sequence of Steps Leading to Muscle Contraction

An action potential travels along a motor nerve to its terminal end.
The nerve releases acetylcholine (ACh) at the terminal end.
ACh binds to receptors on the muscle fiber membrane, opening cation channels.
Sodium ions (Na⁺) flow into the muscle fiber, causing local depolarization.
This depolarization opens voltage-gated Na⁺ channels, initiating an action potential that spreads throughout the fiber and into the T-tubules.

How Depolarisation in the T-tubule Membrane Opens Ca²⁺ Channels on the Sarcoplasmic Reticulum (SR) Membrane?

Dihydropyridine Receptor (DHPR): a voltage sensor located in the T-tubule, closely associated with the foot of the sarcoplasmic reticulum.
Ryanodine Receptor (RyR): a receptor located in the membranes of SR terminal cisternae, triggered by the impulse from DHPR.
The impulse causes RyRs to open and release Ca²⁺ into the intracellular space of the muscle fiber.

The Contraction Sequence

Free Ca²⁺ binds to troponin C, a protein component on the actin filaments.
The troponin complex (troponin T, troponin I, troponin C) undergoes conformational change, moving the tropomyosin molecule.
This uncovers the myosin-binding sites on the actin filaments.
Myosin heads bind to the actin filament, releasing ADP+Pi and initiating the power stroke.
This results in shortening of the sarcomere.

Relaxation of the Muscle Fiber After Contraction

ATP binds to myosin heads, releasing them from actin.
Ca²⁺ is reaccumulated in the sarcoplasmic reticulum by the sarcoendoplasmic reticulum Ca²⁺ ATPase (SERCA) pump.

Rigor Mortis

After death, muscles contract and become rigid due to a lack of ATP needed to detach the cross-bridges from the actin filaments.
This rigidity lasts until muscle proteins deteriorate, approximately 15-25 hours after death.
The process is faster at higher temperatures.

Characteristics of Whole Muscle Contraction

Length-Tension Relationship: the amount of tension generated depends on the length of the muscle before stimulation.
- Overly contracted: thick filaments are too close to Z discs and can't slide, resulting in weak contraction.
- Too stretched: little overlap between thin and thick filaments, limiting the formation of cross bridges and causing weak contraction.
- Optimal resting length: produces the greatest force when the muscle contracts.
Muscle tone: continuous and passive partial contraction of muscles during resting state.

Effect of Muscle Length on the Force of Contraction

Active tension decreases as the muscle is stretched beyond its normal length.

Relationship of Contraction Velocity to the Load

As the load increases, the velocity of contraction decreases.
When the load equals the maximum force the muscle can exert, the velocity of contraction becomes zero, and no contraction occurs.

Types of Muscle Contraction

Isometric: muscle does not shorten during contraction but develops maximal tension (e.g. gripping).
Isotonic: muscle shortens while the tension remains constant throughout the contraction (e.g. lifting a load).

Force Summation

Adding together individual twitch contractions to increase the intensity of overall muscle contraction.
Multiple fiber summation: increasing the number of motor unit contractions.
Frequency summation: increasing the frequency of contractions.

Motor Unit

All muscle fibers innervated by a single nerve fiber.

Mechanism of Tetanus

Continuous stimulation of the muscle results in sustained contraction called tetanus.
The intracellular Ca²⁺ concentration remains high, leading to continuous binding of Ca²⁺ to troponin C and continuous cross-bridge cycling.

Muscle Fatigue

Prolonged and strong muscle contraction leads to muscle fatigue.
Muscle fatigue is directly proportional to the rate of depletion of muscle glycogen.
It results in:
- Failure of contractile and metabolic processes to sustain work output.
- Diminished transmission of nerve signals through the neuromuscular junction.
- Interruption of blood flow, leading to loss of nutrient supply and oxygen.
- Fatigue diminishes muscle contraction.

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Description

This quiz explores the microstructure of skeletal muscle, including the components such as myofibrils, sarcolemma, and the role of actin and myosin in contraction. Understanding excitation-contraction coupling is key to grasping how muscle fibers operate. Test your knowledge on these essential aspects of muscle physiology.