Myofibril Structure Quiz

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Questions and Answers

What is the functional unit of contraction in a myofibril?

Sarcomere (correct)

Z line

A band

H zone

What is the composition of thin filaments in muscle fibers?

Tropomyosin only

Polymer of G actins

Single F actin

Two helically wound F actins (correct)

Which structure bisects the I band in a muscle fiber?

M line

H zone

A band

Z line (correct)

Which protein is responsible for preventing the binding of actin and myosin in a relaxed muscle state?

Tropomyosin Signup and view all the answers

What describes the H zone in the sarcomere?

Central part of thick filaments not overlapped by thin filaments Signup and view all the answers

What is the arrangement of A and I bands in myofibrils?

They alternate throughout the length Signup and view all the answers

What characterizes the thick filaments in muscle fibers?

They are held together by the M line Signup and view all the answers

Which of the following statements about G actins is true?

They are monomeric and polymerize to form F actins Signup and view all the answers

Which statement is true regarding the structure of a muscle fibre?

Muscle fibres are lined by a membrane called sarcolemma. Signup and view all the answers

What is the role of fascia in skeletal muscle organization?

It holds together muscle bundles or fascicles. Signup and view all the answers

What is the primary function of the sarcoplasmic reticulum in muscle fibres?

To store calcium ions necessary for contraction. Signup and view all the answers

What characterizes a muscle fibre as a syncitium?

Presence of multiple nuclei in its sarcoplasm. Signup and view all the answers

Which proteins contribute to the striated appearance of myofibrils?

Actin and Myosin. Signup and view all the answers

What distinguishes the I-band from the A-band in myofibrils?

The I-band is characterized by the presence of actin only. Signup and view all the answers

In terms of muscle contraction, what role do myofilaments play?

They are responsible for the sliding mechanism during contraction. Signup and view all the answers

What is the function of muscle fibre's plasma membrane, or sarcolemma?

To generate electrical impulses called action potentials. Signup and view all the answers

What is the function of the heavy meromyosin (HMM) in the myosin filament?

It facilitates the cross-bridge formation with actin. Signup and view all the answers

Which component of the muscle contraction mechanism directly initiates the process?

The central nervous system (CNS) sends a signal. Signup and view all the answers

According to the sliding filament theory, which filaments slide over each other during muscle contraction?

Thin filaments over thick filaments. Signup and view all the answers

What is the structure that connects a motor neuron to the muscle fiber?

Neuromuscular junction. Signup and view all the answers

Which part of the myosin filament is responsible for binding ATP?

Heavy meromyosin (HMM). Signup and view all the answers

Which aspect of the muscular structure helps reveal the sliding mechanism during contraction?

The polymerized structure of myosin filaments. Signup and view all the answers

What is the role of the light meromyosin (LMM) in myosin filaments?

It forms the tail region of the thick filament. Signup and view all the answers

What is the primary purpose of the cross arms in thick filaments?

To form bridges for interaction with actin. Signup and view all the answers

Study Notes

Myofibril Structure

Myofibrils consist of rod-like structures, with actin (thin) and myosin (thick) filaments arranged parallel to their longitudinal axis.
Actin filaments are thinner and are referred to as thin filaments; myosin filaments are thicker, known as thick filaments.
The 'Z' line bisects the 'I' band, serving as an elastic fiber attachment point for thin filaments.
The 'A' band contains thick filaments, held together by the 'M' line, a thin fibrous membrane.
Sarcomeres, the functional units of contraction, are defined as the segment between two successive 'Z' lines.
The 'H' zone is the central region of the thick filament not overlapped by thin filaments in a relaxed state.

Contractile Proteins

Each actin filament is composed of two filamentous (F) actins wound into a helix, formed from monomeric (G) actins.
Tropomyosin runs alongside the F actins, and troponin is interspersed on the tropomyosin at regular intervals.
Muscle fibers are bundled into fascicles, all held together by connective tissue called fascia.
Muscle fibers, lined by the sarcolemma, contain numerous nuclei and are surrounded by sarcoplasm, which includes the sarcoplasmic reticulum that stores calcium ions.
Myofibrils in muscle fibers display a striated pattern due to alternating dark (A band, myosin) and light (I band, actin) bands.

Myosin Structure

Each thick filament consists of polymerized proteins called meromyosins, which include heavy meromyosin (HMM) and light meromyosin (LMM).
The HMM part features a globular head and a short arm, known as a cross arm, projecting outward from the filament surface.
The globular head acts as an ATPase enzyme and contains binding sites for ATP and active sites for actin.

Muscle Contraction Mechanism

The sliding filament theory explains muscle contraction as the sliding of thin filaments over thick filaments.
Muscle contraction starts with a signal from the central nervous system (CNS) via motor neurons.
A motor unit consists of a motor neuron and the associated muscle fibers.
The neuromuscular junction, or motor-end plate, is the interface between the motor neuron and the sarcolemma of a muscle fiber.

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Description

Test your knowledge on the structure of myofibrils! This quiz covers key concepts such as the arrangement of actin and myosin filaments, the significance of the Z line, A band, and M line. Ideal for students studying muscle physiology and cellular structure.