Muscle Biology Fundamentals

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Questions and Answers

Which subunit of troponin binds to tropomyosin?

TnT (correct)

TnI

Actin

TnC

What is the functional unit of muscle contraction?

Myofibril

Actin filament

Z disc

Sarcomere (correct)

What is the function of the M line in the sarcomere?

Site of calcium release

Site of ATP hydrolysis

Anchoring point for myosin tails (correct)

Anchoring point for actin filaments

Which protein is responsible for connecting the cytoskeleton of a muscle fiber to the extracellular matrix?

Dystrophin (B) Signup and view all the answers

Which statement about myosin and actin is correct?

Myosin filaments are thicker and less transparent to polarized light than actin filaments. (A) Signup and view all the answers

What is the main function of the T tubules in muscle cells?

Rapid transmission of action potential (C) Signup and view all the answers

What happens to the sarcomere during muscle contraction?

The Z discs move closer together. (D) Signup and view all the answers

Which of the following proteins help maintain the elastic structure of muscle and transfer tension during contraction?

Titin, nebulin, and actinin (D) Signup and view all the answers

What is the typical diameter range for adult skeletal muscle fibers?

10-100 µm (C) Signup and view all the answers

Which of the following is a primary function of muscle tissue?

Conversion of chemical energy to mechanical energy (A) Signup and view all the answers

What are the undifferentiated, mononucleated cells that fuse to form muscle fibers during development?

Myoblasts (C) Signup and view all the answers

Which of these properties enables a muscle to recoil to its original resting length after being stretched?

Elasticity (D) Signup and view all the answers

When is skeletal muscle differentiation typically considered complete?

Around the time of birth (B) Signup and view all the answers

How do skeletal muscle fibers typically repair after being damaged after birth due to injury?

Through the action of satellite cells (B) Signup and view all the answers

Skeletal muscle is characterized by which of the following features?

Multiple, peripherally located nuclei, voluntary control, striated (D) Signup and view all the answers

Which type of muscle tissue is found exclusively in the heart?

Cardiac muscle (A) Signup and view all the answers

What is the primary method through which compensation for loss of muscle tissue occurs?

Increase in the size of remaining muscle fibers (hypertrophy) (B) Signup and view all the answers

What is required for contracting skeletal muscle fibers?

Continuous delivery of oxygen and nutrients (B) Signup and view all the answers

What is a key characteristic of smooth muscle?

Non-striated, involuntary, single nucleus (A) Signup and view all the answers

Which statement is true regarding skeletal muscle's nerve and blood supply?

Each muscle is generally served by one nerve, artery and one or more veins that enter near the central part (D) Signup and view all the answers

Which muscle type is primarily responsible for peristalsis?

Smooth muscle (C) Signup and view all the answers

Which feature is unique to cardiac muscle tissue?

Intercalated disks (B) Signup and view all the answers

How do satellite cells aid in muscle repair?

They differentiate into new muscle fibers, undergoing a process similar to embryonic myoblasts. (A) Signup and view all the answers

What is the term used to describe a single skeletal muscle cell?

Muscle fiber (A) Signup and view all the answers

What layer of connective tissue surrounds each individual muscle fiber?

Endomysium (D) Signup and view all the answers

What is the primary function of myofibrils within a muscle fiber?

Facilitate muscle contraction (D) Signup and view all the answers

Which connective tissue layer surrounds groups of muscle fibers, also known as fascicles?

Perimysium (D) Signup and view all the answers

What are the two types of myofilaments that make up myofibrils?

Thick and Thin (D) Signup and view all the answers

Which protein primarily composes thick filaments in muscle fibers?

Myosin (D) Signup and view all the answers

What role does tropomyosin play in muscle contraction?

Blocks myosin binding sites on actin (B) Signup and view all the answers

What structure is formed when collagen fibers of the connective tissue layers come together at each end of a muscle?

Tendon (A) Signup and view all the answers

Which part of the myosin molecule serves as an enzyme for hydrolyzing ATP?

Myosin head (C) Signup and view all the answers

What is the primary role of the sarcoplasmic reticulum (SR) in skeletal muscle cells?

Uptake, storage, and release of calcium ions (A) Signup and view all the answers

How do T tubules relate to terminal cisternae in skeletal muscle?

They form muscle triads with terminal cisternae (C) Signup and view all the answers

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

-90 mV (D) Signup and view all the answers

What is the final stimulus for contraction in skeletal muscles?

Rise in intracellular calcium ion (Ca2+) levels (C) Signup and view all the answers

Which of the following events is NOT part of the process for skeletal muscle contraction?

Release of neurotransmitters into the bloodstream (A) Signup and view all the answers

Which type of muscle action potential duration is longer?

Skeletal muscle (2-4 ms) (A) Signup and view all the answers

What is the primary function of calsequestrin within terminal cisternae?

Binding calcium ions weakly (C) Signup and view all the answers

What is the approximate conduction speed of action potential along the skeletal muscle membrane?

5 m/s (A) Signup and view all the answers

What is the primary function of the nicotinic ACh receptors located at the postsynaptic membrane?

To allow free passage of sodium and potassium ions (A) Signup and view all the answers

Which structure is found between the nerve terminal and the muscle cell?

Synaptic cleft (D) Signup and view all the answers

What initiates the release of acetylcholine from the vesicles in the nerve terminal?

Influx of calcium ions (B) Signup and view all the answers

What is the result of the endplate potential generated at the postsynaptic membrane?

The postsynaptic membrane is depolarized (C) Signup and view all the answers

Which substance in the synaptic cleft is responsible for breaking down acetylcholine?

Acetylcholinesterase (C) Signup and view all the answers

What change occurs to the postsynaptic membrane when acetylcholine binds to its receptors?

It experiences a configurational change to open ionic channels (C) Signup and view all the answers

What role do mitochondria play in the nerve terminal of the neuromuscular junction?

They provide metabolic energy to support neurotransmission (A) Signup and view all the answers

How does the muscle cell membrane respond to the increase in permeability caused by ACh binding?

It produces an action potential leading to muscle contraction (C) Signup and view all the answers

Study Notes

Muscle Physiology Overview

Muscle tissue constitutes 50% of the body weight.
Muscle cells are specialized for converting chemical energy (ATP) into mechanical energy.
Several types of muscle exist, each adapted for specific functions (e.g., locomotion, blood pumping, peristalsis).

Muscular System Functions

Body movement.
Maintenance of posture.
Respiration.
Production of body heat.
Communication (speech, writing, expression).
Constricting organs and vessels.
Heart beat.

Properties of Muscle

Contractility: Ability to shorten with force.
Excitability: Ability to respond to a stimulus.
Extensibility: Ability to be stretched.
Elasticity: Ability to recoil to original resting length after stretching.

Muscle Tissue Types

Skeletal Muscle:
- Attached to bones.
- Multinucleated and striated.
- Voluntary control (conscious action) and involuntary control (reflexes).
Cardiac Muscle:
- Found only in the heart.
- Striated, but branching pattern and intercalated discs.
- Involuntary and automatic.
- Usually single nucleus.
Smooth Muscle:
- Found in walls of hollow organs, blood vessels, etc.
- Non-striated.
- Involuntary.
- Single nucleus.

Skeletal Muscle (Striated Muscle)

Attaches to the skeleton.
Microscopically displays striations.
Is voluntary, controlled consciously.
Multinucleated cells.
Cells have diameters between 10 and 100 μm and lengths extending up to 25 cm.

Skeletal Muscle Fibers Repair

Skeletal muscle fibers damaged after birth cannot be replaced by the division of existing muscle fibers.
However, new fibers can be formed from undifferentiated satellite cells.
Satellite cells are located adjacent to the muscle fibers and differentiate similar to embryonic myoblasts.

Skeletal Muscle: Nerve and Blood Supply

Each muscle is typically served by a nerve, an artery, and one or more veins.
Nerves, arteries, and veins enter near the central part of the muscle and branch throughout it.
Each muscle fiber (cell) is supplied with a nerve ending that controls contraction.
Contracting fibers require a continuous supply of oxygen and nutrients via arteries.
Muscle cells produce metabolic wastes that are removed via veins.

Skeletal Muscle – CT Sheaths

Skeletal muscles are surrounded by three connective tissue sheaths.
Endomysium: Delicate connective tissue surrounding each muscle fiber (cell).
Perimysium: Connective tissue surrounding groups of muscle fibers.
Epimysium: Dense regular connective tissue surrounding the entire muscle.
Tendon or aponeurosis formation at the end of the muscle is formed by the collagen fibers of these 3 sheaths uniting.
They provide support, strength, flexibility, and electrical insulation.

Microscopic Anatomy – Skeletal Muscle Fiber

Sarcoplasm: Muscle cell cytoplasm.
Sarcolemma: Muscle cell membrane.
Within the muscle fiber (cell), there are also myofibrils, sarcoplasmic reticulum, and T-tubules.
Sarcoplasm contains glycosomes (granules of glycogen) and myoglobin (oxygen-binding protein).

Myofibrils and Striations

Each muscle fiber is made from many myofibrils containing the contractile elements of skeletal muscle cells.
Myofibrils compose about 80% of the muscle volume.
Myofibrils are made up of myofilaments (thick and thin).

Myofilaments: Thick and Thin

Thick filaments: Composed of myosin protein.
Thin filaments: Composed of actin protein, and regulatory proteins tropomyosin and troponin.
Myosin consists of two heads and one tail.
Actin is a major component of the thin filament; there are myosin binding sites on actin in the thin filament.
Tropomyosin molecules are long filaments that spiral around actin and block active sites in a relaxed muscle fiber.
Troponin molecules bind to tropomyosin and actin, and bind calcium ions, triggering contraction.
A band in myofibrils/muscle contains the thick filaments, and is made up of parts of thick and thin filaments.

### Structure of Actin and Myosin

Describe the structure of actin and myosin filaments, including their components (heavy and light chains, hinge regions, etc.), and their role in sarcomere structure.

Myofibrils - Striations - Banding

The arrangement of myofibrils creates a repeating series of dark (A bands) and light (I bands) bands.
Each segment is a sarcomere, the smallest functional contractile unit of a muscle.
Specific regions within a sarcomere include the Z line (or disc), H zone or band, and M line.
There are thick filaments in the A band, and thick and thin filaments in both the A and I bands.

Sarcomere Shortening

Actin filaments move towards the center of the sarcomere.
The sarcomere shortens when the Z disks move closer together.
A and I bands change in appearance as the muscle contracts and shortens.

Ca2+ and the Contraction Mechanism

Calcium ions and regulatory proteins (troponin, tropomyosin) are essential for muscle contraction.
At low intracellular calcium levels, tropomyosin blocks the binding sites of actin, and myosin cannot bind.
Increasing intracellular calcium causes a conformational change in troponin, causing tropomyosin to move away from the actin binding sites, allowing the myosin head to attach and detach and initiating the power stroke cycle.

Neuromuscular Junction

Junction where the motor neuron stimulates the muscle fiber.
The axon of the motor neuron branches into several terminals forming the presynaptic part of the neuromuscular junction.
The postsynaptic part of the junction is the muscle cell membrane, with folds containing nicotinic ACh receptors.
The pre- and postsynaptic membranes are separated by the synaptic cleft, containing acetylcholinesterase (AChE).

Electrical Events at the Neuromuscular Junction

Depolarization of the nerve terminal causes the release of acetylcholine (ACh) into the synaptic cleft.
ACh binds to receptors on the muscle fiber, leading to depolarization (end-plate potential).
Depolarization triggers an action potential in the muscle fiber, initiating muscle contraction.

Propagation of Action Potential on Muscle Membrane

The action potential spreads through the T tubules.
This stimulates the release of calcium ions from the SR, causing muscle contraction through the cross-bridge cycle.

Dihydropyridine and Ryanodine Receptors

Action potentials in T-tubules activate DHP receptors, causing the release of calcium ions from the sarcoplasmic reticulum terminal cisternae.
Calcium ions initiate muscle contraction.

Neuromuscular Transmission (Toxins and Drugs)

Presynaptic blockade (e.g., Botulinum toxin) interferes with ACh release.
Postsynaptic blockade (e.g., Curare-Tubocurarine) prevents ACh from binding and triggering a muscle contraction.

Sliding Filament Model of Contraction

The sliding filament model explains how muscle fibers shorten during contraction.
Myosin heads bind to actin filaments, causing them to slide past each other.
The cross bridge cycle describes the individual steps involved in the overlapping, sliding filament mechanism of the contraction process.

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Description

Test your knowledge on muscle biology with questions about muscle contraction, the components of sarcomeres, and the functions of various proteins involved. Dive into the intricate world of skeletal muscle fibers and their characteristics. This quiz is perfect for students studying muscle anatomy and physiology.