MNB.6 Musculoskeletal & Nervous Systems

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Questions and Answers

What is the basic repeating unit of muscle structure called?

Sarcoplasm

Sarcomere (correct)

Myofibril

T-tubule

Which protein filaments are primarily responsible for muscular contraction?

Tropomyosin and titin

Actin and myosin (correct)

Collagen and elastin

Keratin and fibrin

What is the primary role of the sarcoplasmic reticulum in muscle fibers?

To collect calcium (correct)

To store glycogen

To transmit electric signals

To produce ATP

Which structure in muscle fibers transmits electrical signals?

T-tubules Signup and view all the answers

The light band in muscle fibers is referred to as which of the following?

I-band Signup and view all the answers

What is the cytoplasmic matrix of muscle fibers known as?

Sarcoplasm Signup and view all the answers

The dark line in the middle of the I-band is known as what?

Z-line Signup and view all the answers

What substance is NOT typically found in the sarcoplasm of muscle fibers?

Chlorophyll Signup and view all the answers

What structural feature do the C-terminal parts of myosin heavy chains form?

Coiled-coil α-helical tail Signup and view all the answers

Which molecule do myosin heads bind to when initiating muscle contraction?

ATP Signup and view all the answers

What is the primary role of calcium during muscle contraction?

To expose binding sites on actin Signup and view all the answers

What triggers the power stroke in muscle contraction?

Release of ADP from myosin Signup and view all the answers

What happens to the actin-myosin cross-bridge when a new ATP molecule binds to the myosin head?

It separates, ceasing the contraction Signup and view all the answers

What is the result of ATP hydrolysis in the context of muscle contraction?

It resets the myosin head to a cocked position Signup and view all the answers

Which concept explains the mechanism behind muscle contraction?

Sliding filament theory Signup and view all the answers

What initiates the movement of the cross-bridges during muscle contraction?

Energy from ATP hydrolysis Signup and view all the answers

What happens to the A-band during muscle contraction?

The A-band remains the same length Signup and view all the answers

Which of the following correctly describes the role of ATP in muscle contraction?

ATP is converted into ADP and inorganic phosphate, providing energy Signup and view all the answers

What occurs during the power-stroke phase of muscle contraction?

Myosin heads change conformation and pull actin filaments Signup and view all the answers

What role does calcium play in muscle contraction?

Calcium allows myosin binding sites on actin to be uncovered Signup and view all the answers

How much distance does actin move towards the M-line during a powerstroke?

5-12 nm Signup and view all the answers

In the resting muscle, most myosin is found in which form?

Myosin-ADP + Pi form Signup and view all the answers

What is the main reason for the slow release of ADP + Pi in muscle fibers?

Presence of troponin and tropomyosin Signup and view all the answers

What is formed when ATP replaces ADP in the myosin head during muscle contraction?

A-M-ATP complex Signup and view all the answers

What triggers the release of Ca2+ ions from the sarcoplasmic reticulum?

The depolarization of the sarcolemma Signup and view all the answers

Which of the following describes the function of Troponin-I (TnI) in muscle contraction?

Prevents actin from binding to myosin Signup and view all the answers

What is the primary role of acetylcholine (ACh) in muscle contraction?

To initiate muscle fiber depolarization Signup and view all the answers

How do depolarizing muscle relaxants work?

They cause a single contraction then promote paralysis Signup and view all the answers

What happens to Ca2+ ions during muscle relaxation?

They are pumped back into the sarcoplasmic reticulum Signup and view all the answers

Which type of muscle relaxant prevents muscle contractions entirely?

Tubocurarine Signup and view all the answers

What results from the hydrolysis of ATP during muscle contraction?

Restoration of the myosin head to resting conformation Signup and view all the answers

What gives skeletal muscle its striated appearance under the light microscope?

The overlapping proteins in the myofibril Signup and view all the answers

Which of the following are the primary components of thin filaments?

F-actin, tropomyosin, and troponin Signup and view all the answers

What is the role of calcium ions in muscle contraction?

They bind to TnC, leading to the movement of tropomyosin Signup and view all the answers

Which structure is formed by the thick filaments in a muscle fiber?

A Band Signup and view all the answers

Which component of the thin filament covers the myosin binding sites in the resting state?

Tropomyosin Signup and view all the answers

What is the composition of myosin molecules?

Two heavy chains and four light chains Signup and view all the answers

During muscle contraction, which process occurs after calcium binds to TnC?

Tropomyosin moves away from binding sites on actin Signup and view all the answers

What structures form the cross-bridges between thick and thin filaments during muscle contraction?

Myosin heads Signup and view all the answers

The heads of myosin filaments are primarily composed of which type of meromyosin?

Heavy meromyosin Signup and view all the answers

What structural feature characterizes the arrangement of the thin filament's F-actin protein?

Double stranded helix Signup and view all the answers

Study Notes

Musculoskeletal System, Nervous System & Bioelectricity

This lecture covers the musculoskeletal system, nervous system, and bioelectricity, specifically focusing on muscle ultrastructure and biochemistry of movement.
The course is MNB.6
The presenter is Prof Warren Thomas
The date of the lecture is October 31, 2024.

Learning Outcomes

Learners will be able to discuss the ultrastructure of skeletal muscle, including myofibrils and myofilaments.
Learners will be able to describe myosin and actin molecular structure.
Learners will be able to describe the sliding filament theory of Huxley and Huxley.
Learners will be able to outline the biochemistry of the ratchet mechanism of muscle action.

Muscle Ultrastructure

Muscle ultrastructure is a complex arrangement of protein-protein interactions.
The sarcomere is the basic repeating unit of muscle.
Muscle fibers contain myofibrils which are, in turn, made up of smaller sarcomeres.
Myofibrils are embedded in the sarcoplasm (cytoplasm of the muscle fiber).
Each muscle fiber is enclosed in a cell membrane called the sarcolemma which surrounds the muscle fiber.
The muscle fiber has smaller myofibrils embedded in the cytoplasmic matrix (called sarcoplasm).
Muscle fibers contain blood vessels.
Myofilaments, such as actin and myosin, are important components within the sarcomere.

Main Components Muscle Fiber

The cytosol within the muscle fiber is called sarcoplasm and rich in glycogen, ATP, Creatine Phosphate, and glycolytic enzymes.
Transverse tubules (T-tubules) are membranous folds extending from the plasma membrane that transmit electrical signals.
Sarcoplasmic Reticulum (SR) is a network of flattened vesicles surrounding myofibrils that sequesters calcium.

Muscle Sarcomere

A sarcomere is the basic unit of contraction.
It extends between two Z-lines/Z-discs.
Myofibrils have a striated appearance due to the arrangement of light and dark bands.
The I-band (isotropic) is light and the A-band (anisotropic) is dark.
The Z-line is a dark line located in the middle of the I-band.
The sarcomere contains two main filamentous components: Actin (thin filaments) and Myosin (thick filaments).

Molecular Structure of Thin Filament (Actin)

Thin filaments are composed of three components:
- A double-stranded F-actin protein molecule.
- A double-stranded α-helical protein molecule called tropomyosin.
- Troponin, consisting of three globular proteins (TnC, TnI, TnT).
These are periodically attached to the tropomyosin strand.

Thin Filament Arrangement

Tropomyosin and troponin regulate actin by covering the myosin binding sites on actin in the resting state.
One troponin protein (TnT) binds to tropomyosin; another (TnI) binds to F-actin, and the third (TnC) binds to calcium ions.

Myosin Filaments (Thick Filaments)

A thick filament is about 400 myosin polypeptides approximately 200 each side of the M-line.
Each myosin filament is composed of:
- a compact head region.
- a long tail region composed of two α-helices.
Myosin's tails form the thick central portion of the filament.
Myosin heads form cross-bridges with actin filaments.
Most of the tail is light meromyosin, and head and some tail is heavy meromyosin.
Myosin forms the A-band.

Molecular Structure of Myosin

Myosin molecules comprise two heavy chains and four light chains.
The C-terminal parts of myosin heavy chains (MHC) twist together to form a coiled-coil α-helical rod-shaped tail domain.
The N-terminal parts of heavy chains form two myosin heads.
Each head acts as an enzyme binding to ATP and then hydrolyzing it.
The head stores energy in the form of ADP and inorganic phosphate (Pi).

Sliding Filament Theory

Myosin filaments use ATP energy to move along actin filaments, forming cross-bridges.
Contraction happens as actin and myosin filaments slide past each other.
The movement of cross-bridges shortens the actin filaments.

Ratchet Mechanism of Contraction

Describes the biochemical and biophysical events during contraction.
Converts ATP energy into the physical work of actin and myosin movement.
Forms cross-bridges between myosin head and myosin binding site on the actin filament.

Power Stroke

Myosin head attaches to actin.
Myosin head contracts, moving actin.
Myosin detaches from actin.
Myosin reattaches further along the actin filament.

One ATP Molecule

Used per myosin head for each powerstroke.
Release of ADP + Pi is slow without actin.
The events in the myosin head occur cyclically.
Resting muscle, mostly Myosin-ADP-Pi form and the myosin binding site on actin is inhibited by troponin and tropomyosin.

Excitation-Contraction Coupling (ECC)

Explains how electrical signals cause muscle contraction.
Depolarization of sarcolemma and wave of depolarization passes inward via T-tubules.
Sarcoplasmic reticulum next to the T-tubules release Ca2+ ions into sarcoplasm.
Initial signal for muscle contraction is the release of acetylcholine (ACh) at neuromuscular junction (NMJ).
Ca 2+ binds to Troponin-C (TnC) which affects Troponin-I (TnI), bound to F-actin.
Tnl releases actin, causing conformational change in tropomyosin and uncovering active sites on F-actin, resulting in muscle contraction.
Ca2+ is pumped back into the sarcoplasmic reticulum, muscle relaxes.

Muscle Relaxants

Depolarizing muscle relaxants cause muscle contraction, preventing further contractions.
Non-depolarizing muscle relaxants prevent muscle contraction.
Examples of drugs of these types:
- Suxamethonium
- Tubocurarine

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Description

This quiz explores the musculoskeletal system, nervous system, and bioelectricity, focusing on muscle ultrastructure and biochemistry of movement. Learners will gain insights into skeletal muscle structure, the sliding filament theory, and muscle action mechanisms.