Muscle Contraction Fundamentals

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Questions and Answers

What is primarily responsible for the activation of myosin heads during the contraction cycle?

Acetylcholine

ATP hydrolysis (correct)

Calcium ions

Tropomyosin

Which step occurs directly after ATP binds to the myosin head during muscle contraction?

Cross bridge formation

Release of calcium ions

Detachment of myosin from actin (correct)

The power stroke

What role does acetylcholinesterase (AChE) play in muscle contraction?

Activates myosin heads

Transport calcium back to the sarcoplasmic reticulum

Stimulates the release of calcium

Breaks down acetylcholine (correct)

Which characteristic distinguishes skeletal muscle from cardiac and smooth muscle?

Striated appearance Signup and view all the answers

What condition must be met for the contraction cycle to continuously repeat?

Presence of ATP and high calcium levels Signup and view all the answers

What primarily triggers muscle contraction during the excitation-contraction coupling process?

Influx of sodium ions into the muscle cell Signup and view all the answers

Which protein is classified as a regulatory protein involved in muscle contraction?

Troponin Signup and view all the answers

What is the primary role of structural proteins in myofibrils?

To maintain proper alignment and extensibility Signup and view all the answers

What happens to muscle cells after acetylcholine is broken down by acetylcholinesterase?

Muscle cells return to a relaxed state Signup and view all the answers

Which structure is primarily involved in holding thick filaments together within a sarcomere?

M line Signup and view all the answers

Which statement best describes the difference between skeletal muscle and cardiac muscle?

Skeletal muscle fibers are usually multinucleated, unlike cardiac muscle fibers Signup and view all the answers

What is the composition of the I band in a sarcomere?

It contains only thin filaments Signup and view all the answers

Which of the following statements best describes the role of myofibrils in muscle tissue?

They are the contractile elements responsible for muscle contraction Signup and view all the answers

What is a key difference between cardiac and smooth muscle?

Cardiac muscle is striated, while smooth muscle is non-striated. Signup and view all the answers

Which of the following best describes the process by which muscle contractions occur?

Muscle contraction relies on the sliding filament mechanism. Signup and view all the answers

What is the primary structural unit of skeletal muscle tissue?

Sarcomere Signup and view all the answers

How does the extensibility of a muscle fiber benefit its function?

It permits stretching without being damaged. Signup and view all the answers

What is the role of the epimysium in skeletal muscle?

It surrounds the entire muscle. Signup and view all the answers

Which muscle type is primarily responsible for generating heat in the body?

Skeletal muscle Signup and view all the answers

Which characteristic of muscle allows it to react to stimuli?

Excitability Signup and view all the answers

What distinguishes skeletal muscle fibers from cardiac muscle fibers?

Skeletal fibers are multinucleate. Signup and view all the answers

What is the function of the sarcoplasmic reticulum in muscle contraction?

It stores and releases calcium ions. Signup and view all the answers

Which muscle type is under involuntary control and has striated appearance?

Cardiac muscle Signup and view all the answers

What role do T tubules play in skeletal muscle physiology?

They facilitate calcium ion transport within the muscle cell. Signup and view all the answers

Which structure is primarily responsible for the generation of force in muscle contraction?

Myofibrils Signup and view all the answers

What is the primary function of smooth muscle?

Squeezing fluids through hollow organs. Signup and view all the answers

Which of the following best describes the appearance of smooth muscle cells?

Short, spindle-shaped, and non-striated. Signup and view all the answers

Study Notes

Muscle Tissue Overview

Muscle tissue is responsible for movement, posture maintenance, respiration, communication, joint stabilization, and heat generation.
Skeletal muscle is attached to bones and moves the body by moving bones.
Smooth muscle squeezes fluids and substances through hollow organs.

Functional Characteristics of Muscle

Contractility: Ability of a muscle to shorten and generate pulling force.
Excitability (Irritability): Capacity of muscle to respond to a stimulus. Nerve fibers cause electrical impulses to travel.
Extensibility: Muscle can be stretched back to its original length.
Elasticity: Ability of muscle to recoil to its original resting length after being stretched.

Types of Muscle Tissue

Skeletal Muscle:
- Makes up 40% of body weight.
- Attaches to bone, skin, or fascia.
- Long, cylindrical cells with striations (light and dark bands).
- Multinucleated (embryonic cells fuse).
- Voluntary control of contraction and relaxation.
Cardiac Muscle:
- Found only in the heart wall.
- Function is to pump blood (involuntary control).
- Short, branched cells with striations.
- Cells connected by intercalated discs.
- One nucleus per cell.
Smooth Muscle:
- Attached to hair follicles in skin, walls of hollow organs (blood vessels and GI).
- Short, spindle-shaped cells.
- Non-striated (no visible bands).
- One nucleus per cell.
- Involuntary control.

Anatomy of Skeletal Muscles

Epimysium: Dense regular connective tissue surrounding the entire muscle.
Perimysium: Fibrous connective tissue surrounding fascicles (groups of muscle fibers).
Endomysium: Fine areolar connective tissue surrounding each muscle fiber.
Fascicle: Groups of muscle fibers bundled together.
Muscle fiber: Individual muscle cell.
Blood vessels: Wrapped by the perimysium; blood vessels and endomysium located between individual muscle fibers.
Blood vessels are in middle of the fascicle.

Microscopic Anatomy of Skeletal Muscle

Skeletal muscle fibers: Long, cylindrical. Formed by fusion of embryonic cells. Multinucleated. Nuclei peripherally located.
Striations: Result from the internal structure of myofibrils.
Myofibrils: Long rods within the cytoplasm, making up 80% of cytoplasm. Specialized contractile organelles in muscle tissue. Composed of repeating segments called sarcomeres. Sarcomere is the function unit of skeletal muscle.

Muscle Fibers Overview (detailed)

Nucleus: Located peripherally.
Myofibrils: Contain the contractile proteins (actin and myosin).
Sarcoplasm: Cytoplasm of muscle cell filled with myofibrils and myoglobin (red-colored oxygen-binding protein). Mitochondria located in rows throughout the cell.
Sarcolemma: Muscle cell membrane.
Sarcoplasmic reticulum (SR): Specialized smooth ER interconnecting tubules surrounding each myofibril, forming cross-channels called terminal cisternae. Cisternae occur in pairs on either side of a T-tubule.
T-tubules: Invaginations of the sarcolemma into the center of the cell, filled with extracellular fluid. Carry muscle action potentials down into the cell.

Sarcomere

Basic unit of contraction:
Z disc (Z line): Boundaries of each sarcomere.
Thin (actin) filaments: Extend from Z disc toward the center of the sarcomere.
Thick (myosin) filaments: Located in the center of the sarcomere, overlapping inner ends of thin filaments.
A bands: Full length of thick filament, includes inner end of thin filaments.
H zone: Part of A band with no thin filaments.
M line: In center of H zone, holds thick filaments together.
I band: Region with only thin filaments, lies within two adjacent sarcomeres.

Muscle Proteins

Myofibrils: Composed of three kinds of protein.
Contractile proteins: Myosin and actin.
Regulatory proteins: Troponin and tropomyosin (turn contraction on and off).
Structural proteins: Titin, myomesin, nebulin, and dystrophin (proper alignment, elasticity, and extensibility).

Actin and Myosin Filaments

Actin and myosin filaments are the contractile proteins within the sarcomere.
Actin filaments are thin; myosin filaments are thick.

Innervation of Skeletal Muscle

Each skeletal muscle is supplied by a nerve, artery, and two veins.
Each motor neuron supplies multiple muscle cells (neuromuscular junction).
Neuromuscular junction is where the nerve ending and muscle fiber meet.
Each muscle cell is supplied by one motor neuron terminal branch.
Nerve fibers and capillaries are found in the endomysium between individual cells.
Motor Unit: The motor neuron and all the muscle fibers it innervates.

Events Occurring After Nerve Signal

Arrival of nerve impulse at nerve terminal causes release of ACh from synaptic vesicles.
ACh binds to receptors, opening gated ion channels allowing Na+ to rush into muscle cell.
Inside of muscle cell becomes positively charged, triggering action potential.
Action potential travels over sarcolemma and down transverse tubules (T-tubules), triggering the muscle to shorten and generate force.
Release of Ca2+ from SR into sarcoplasm is triggered.
Ca2+ binds to troponin, causing troponin-tropomyosin complex to move and reveal myosin binding sites on actin.
Acetylcholinesterase breaks down ACh, causing the muscle action potential to cease and the muscle cell to relax.

Contraction Cycle

Repeating sequence of events that cause filaments to move past each other.
4 steps:
- ATP hydrolysis (ATP is broken down, releasing energy) activates myosin heads.
- Attachment of activated myosin heads to actin (forms cross bridges). Myosin pulls.
- Power stroke: Thin filaments sliding past thick filaments.
- Detachment: ATP binds to myosin head, detaching from actin.

Relaxation

Termination of nerve impulses & Acetylcholinesterase (AChE) breaks down ACh within synaptic cleft initiates relaxation.
Muscle action potential ceases.
Active transport pumps Ca2+ back into SR (sarcoplasmic reticulum).
Calcium-binding protein (calsequestrin) helps hold Ca2+ in the SR.
Tropomyosin-troponin complex recovers binding sites on actin.

Tendons and Ligaments

Tendon: Connects muscle to bone (transmit muscle strength to bone for movement). Regular dense fibrous connective tissue.
Ligament: Connects bone to bone (hold structures stable). Irregular dense fibrous connective tissue.

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Description

Test your knowledge on the mechanisms of muscle contraction, including the roles of myosin heads, ATP, and acetylcholinesterase (AChE). This quiz will cover key differences between skeletal, cardiac, and smooth muscle, as well as the conditions necessary for muscle contraction cycles to repeat. Dive in to solidify your understanding of this essential biological process!