Muscle Fiber Structure and Functions

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Questions and Answers

What role do T-tubules play in skeletal muscle fibers?

They synthesize contractile proteins.

They provide structural support to the myofibrils.

They store calcium ions exclusively.

They facilitate the conduction of action potentials to the sarcoplasmic reticulum. (correct)

Which of the following statements about myofibrils is correct?

Myofibrils contain primarily myofilaments surrounded by plasma membrane.

Each myofibril is composed of bundles of contractile proteins. (correct)

Myofibrils are only present in cardiac muscle fibers.

Myofibrils are isolated structures not connecting to the sarcoplasmic reticulum.

What is primarily housed in the membranes of the sarcolemma and T-tubules?

Chemical signals that induce muscle contraction.

Proteins responsible for muscle fiber repair.

Mitochondria that generate ATP.

Voltage-gated channels for ion transportation. (correct)

Which statement best describes the structural importance of the sarcoplasmic reticulum?

It encloses myofibrils and is responsible for calcium storage. Signup and view all the answers

How do the diameter and quantity of myofibrils in a skeletal muscle fiber vary?

Diameter ranges from 1 to 2 micrometers, with hundreds to thousands present. Signup and view all the answers

What structure occurs when two terminal cisternae and a T-tubule come together?

Triad Signup and view all the answers

What role do Ca2+ release channels in the terminal cisternae serve during muscle contraction?

They release calcium ions in response to electrical signals Signup and view all the answers

What do Ca2+ pumps do in the sarcoplasmic reticulum?

Move Ca2+ from cytosol into the SR Signup and view all the answers

Which protein binds calcium ions for storage within the terminal cisternae lumen?

Calsequestrin Signup and view all the answers

What type of channels are found in the T-tubule membrane that respond to electrical signals?

Voltage-sensitive Ca2+ channels Signup and view all the answers

What is the primary protein composition of thick filaments?

Myosin Signup and view all the answers

Which combination of proteins is found in thin filaments?

Actin, tropomyosin, and troponin Signup and view all the answers

How does the length of myofilaments compare to that of myofibrils?

Myofilaments are not as long as myofibrils. Signup and view all the answers

Which of the following correctly describes the number of myosin molecules in thick filaments?

200 to 500 myosin molecules Signup and view all the answers

What structural role do myofilaments play within myofibrils?

They serve as the contractile proteins. Signup and view all the answers

Which of the following statements is true regarding the origin of myofilaments?

Myofilaments are bundled within myofibrils. Signup and view all the answers

What happens to the H-zone during maximal muscle shortening?

It disappears as thin filaments move past thick filaments. Signup and view all the answers

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

It helps align thick filaments during contraction and relaxation. Signup and view all the answers

What contributes to the visible striations in skeletal muscle under a microscope?

The differences in size and density between thin and thick filaments. Signup and view all the answers

Which mnemonic can be used to memorize the appearance of the A and I bands in muscle tissue?

The word 'dark' for A bands, and 'light' for I bands. Signup and view all the answers

In a cross section of an A band, how many thick filaments does each thin filament associate with?

Three thick filaments per thin filament. Signup and view all the answers

What is the primary function of the myosin head in thick filaments?

To catalyze the breakdown of ATP Signup and view all the answers

How are myosin proteins arranged in thick filaments?

Tails are directed toward the center of the filament Signup and view all the answers

Which characteristic distinguishes thin filaments from thick filaments?

Thin filaments are composed primarily of actin Signup and view all the answers

What structural analogy is used to describe the arrangement of myosin proteins in thick filaments?

Two intertwined golf clubs Signup and view all the answers

What is the role of the myosin binding site on G-actin molecules?

To allow binding with myosin heads Signup and view all the answers

What type of structure does F-actin resemble?

Two beaded necklaces intertwined Signup and view all the answers

What is the primary structural component of thin filaments?

Actin protein strands Signup and view all the answers

What is the main difference in size between thick and thin filaments?

Thick filaments are about double the diameter of thin filaments Signup and view all the answers

What is the role of the myosin-binding site on G-actin?

To allow attachment during contraction Signup and view all the answers

How does tropomyosin interact with actin strands?

It covers the myosin-binding sites on actin. Signup and view all the answers

What is a key component of the myosin protein's function?

It acts as an ATPase enzyme Signup and view all the answers

What is the arrangement of myofilaments in a sarcomere?

Thick and thin filaments overlap. Signup and view all the answers

What is the length of a single sarcomere?

2 micrometers Signup and view all the answers

Which statement is true about troponin's structure?

It contains a binding site for Ca²⁺. Signup and view all the answers

What structural form does F-actin resemble?

Two twisted, beaded necklaces Signup and view all the answers

The troponin-tropomyosin complex primarily functions to:

Regulate muscle contraction Signup and view all the answers

In what way do globular actin molecules (G-actin) form filamentous actin (F-actin)?

By polymerization into long strands Signup and view all the answers

What feature characterizes the structural organization of myofibrils in skeletal muscle?

They have repeating units called sarcomeres. Signup and view all the answers

What are Z discs primarily responsible for within a sarcomere?

Anchoring thin filaments Signup and view all the answers

What occurs to the I bands during maximal muscle shortening?

They disappear entirely Signup and view all the answers

Which part of the sarcomere contains all of the thick filaments?

A band Signup and view all the answers

What is the characteristic appearance of the A band under a microscope?

Dark Signup and view all the answers

What is unique about the H zone in a resting sarcomere?

It lacks any filament overlap Signup and view all the answers

During muscle contraction, which of the following regions disappears as the muscle shortens?

I band Signup and view all the answers

What structural role does the M line serve in the sarcomere?

Anchors thick filaments Signup and view all the answers

Which statement best describes the H zone during the process of muscle contraction?

It disappears completely Signup and view all the answers

Which of the following statements about the I band is correct?

It appears light under a microscope Signup and view all the answers

How does the length of the A band change during muscle contraction?

It remains the same Signup and view all the answers

Study Notes

Sarcolemma and T-Tubules

The sarcolemma is the plasma membrane of a skeletal muscle fiber.
T-tubules are deep invaginations of the sarcolemma that form a network of narrow, membranous tubules extending into the skeletal muscle fiber.
T-tubules connect to the sarcoplasmic reticulum, the ER of muscle.
The sarcolemma and T-tubules contain voltage-gated Na+ and K+ channels involved in conducting electrical signals.

Myofibrils

Myofibrils are long, cylindrical structures composing approximately 80% of the volume of a skeletal muscle fiber.
A skeletal muscle fiber contains hundreds to thousands of myofibrils.
Each myofibril extends the entire length of the skeletal muscle fiber and is made up of bundles of contractile proteins called myofilaments.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is the internal membrane complex of muscle tissue.
It wraps around myofibrils and has blind sacs called terminal cisternae at the ends of each segment.
Two terminal cisternae and a T-tubule form a triad.
The T-tubule membrane contains voltage-sensitive Ca2+ channels, which respond to electrical signals.
The terminal cisternae membrane contains Ca2+ release channels, and Ca2+ ions are stored within the terminal cisternae lumen.
Ca2+ pumps within the SR membrane actively transport Ca2+ from the cytosol into the SR using primary active transport.
Ca2+ is stored within the SR bound to proteins like calmodulin and calsequestrin.

Myofilaments

Myofilaments are contractile proteins bundled within myofibrils.
There are two types of myofilaments: thick filaments and thin filaments.

Thick Filaments

Composed of bundles of 200 to 500 myosin protein molecules.
Each myosin protein has two strands, each with a globular head and an elongated tail.
The head contains a binding site for actin and an ATPase site where ATP is split into ADP and phosphate.
The tails of two myosin strands are intertwined.
Myosin molecules in a thick filament are oriented with their tails toward the center and heads toward the ends.

Thin Filaments

Approximately half the diameter of thick filaments.
Primarily composed of two strands of actin protein twisted into a helical shape.
Each strand of actin contains many small, spherical G-actin molecules connected to form a fibrous strand (F-actin).
Each G-actin molecule has a myosin-binding site.

Tropomyosin and Troponin

Tropomyosin and troponin are regulatory proteins associated with thin filaments.
Tropomyosin is a short, thin, twisted filament that covers regions of actin strands, including myosin-binding sites.
Troponin is a globular protein attached to tropomyosin containing a binding site for Ca²⁺.

Sarcomere Organization

Myofilaments within myofibrils are arranged in repeating, microscopic, cylindrical units called sarcomeres.
Each sarcomere is composed of overlapping thick filaments and thin filaments.
Sarcomeres are delineated by Z discs, which anchor thin filaments.
The Z disc is visible as a flat disk in cross-section and as a zigzag line in a longitudinal view.

Muscle Contraction: Muscle Anatomy

The A band includes the entire thick filament and partially overlapping thin filaments.
The H zone is the most central portion of the A band containing only thick filaments.
The M line is a thin transverse protein meshwork structure that helps align thick filaments.

Maximal Muscle Shortening

During maximal muscle shortening, the H zone disappears as thin filaments are pulled past thick filaments.

Striations

Repeating light and dark bands (striations) in myofilaments create a striped pattern in skeletal muscle.
These striations are visible under a light microscope or an electron microscope.

Learning Strategy for Bands

The word "dark" contains the letter "A", and A bands appear dark under a microscope.
The word "light" contains the letter "I", and I bands appear light under a microscope.

Figure 10.5c

Figure 10.5c shows cross-sections through a sarcomere.
It illustrates the arrangement of thick and thin filaments in different locations within the sarcomere.
In a cross section of an A band, each thin filament is associated with three thick filaments.

Learning Strategy 10.3

To understand muscle shortening, imagine your fingers representing thin filaments and cylinders representing thick filaments.
Your thumbs represent Z discs.
Sliding your fingers together simulates muscle shortening: The sarcomere (distance between thumbs) shortens, while the thin filaments (fingers) and thick filaments (cylinders) stay the same length.

Other Structural and Functional Proteins

Connectin (titin) is a cable-like protein that extends from the Z discs to the M line.
It stabilizes thick filament position and maintains thick filament alignment.
Portions of connectin are coiled and spring-like, compressing during sarcomere shortening to create passive tension.
Dystrophin anchors myofibrils adjacent to the sarcolemma to proteins within the sarcolemma, linking internal myofilament proteins to external proteins.
Muscular dystrophy is a genetic disorder caused by abnormal structure or amounts of dystrophin protein.

Mitochondria and Other Structures Associated with Energy Production

Skeletal muscle fibers contain abundant mitochondria for aerobic cellular respiration.
They also contain glycogen stores for immediate fuel.
Myoglobin binds oxygen during rest and releases it for use during muscle contraction, enhancing aerobic cellular respiration and ATP production.
Skeletal muscle fibers contain creatine phosphate, providing a rapid means of supplying ATP.

Clinical View 10.1: Muscular Dystrophy

Muscular dystrophy is a collective term for several hereditary diseases that affect skeletal muscle.

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Description

Explore the intricate structures and functions of skeletal muscle fibers, including the sarcolemma, T-tubules, myofibrils, and sarcoplasmic reticulum. This quiz will test your knowledge on their roles in muscle contraction and electrical signaling. Perfect for students studying human anatomy or physiology.