Muscle Anatomy and Physiology Quiz

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Questions and Answers

What are the small, modified muscle cells within a muscle spindle called?

Intrafusal fibers (correct)

Efferent fibers

Somatic fibers

Extrafusal fibers

How many intrafusal fibers typically compose a muscle spindle?

5 to 7

12 to 15

8 to 10 (correct)

10 to 12

What structure surrounds the intrafusal fibers in a muscle spindle?

Perimysium

Periaxial space (correct)

Basal lamina

Endomysium

Which component encloses the periaxial space of muscle spindles?

Capsule Signup and view all the answers

What is the purpose of the fluid in the periaxial space?

To cushion the muscle spindles Signup and view all the answers

What is the main component that is NOT part of a myoneural junction?

Neurotransmitter Signup and view all the answers

Which structure is responsible for transmitting signals at the myoneural junction?

Axon terminal Signup and view all the answers

The space between the axon terminal and sarcolemma at a myoneural junction is known as what?

Synaptic cleft Signup and view all the answers

Which statement accurately describes components of a myoneural junction?

It includes an axon terminal, synaptic cleft, and sarcolemma. Signup and view all the answers

What role does the sarcolemma play at the myoneural junction?

Reception of signals from the axon terminal Signup and view all the answers

What feature do G-actin molecules possess that contributes to filament formation?

Asymmetry in structure Signup and view all the answers

What role does myosin have in relation to G-actin monomers?

Myosin binds to the sites on G-actin monomers Signup and view all the answers

What characteristic of filament formation is attributed to the polarity of G-actin?

Filaments have directional properties Signup and view all the answers

Which of the following is true about the polymerization of G-actin?

G-actin polymerization results in a filament with polarity Signup and view all the answers

What is the significance of the binding site on G-actin monomers?

It enables binding with myosin Signup and view all the answers

What structures are visible in the TEM cross-section of a muscle spindle?

Capsule, sensory myelinated axons, and intrafusal muscle fibers Signup and view all the answers

What distinguishes intrafusal muscle fibers from ordinary skeletal muscle fibers?

They contain essentially no myofibrils Signup and view all the answers

Which of the following correctly describes sensory myelinated axons?

They are involved in sensory signal transmission Signup and view all the answers

What role does the capsule play in a muscle spindle?

It surrounds and protects the intrafusal fibers Signup and view all the answers

Which statement about muscle spindles is true?

They detect changes in muscle length Signup and view all the answers

What type of information does the tendon organ primarily collect?

The degree of tension among tendons Signup and view all the answers

How does the central nervous system utilize information from the tendon organ?

To coordinate fine muscular contractions Signup and view all the answers

With which other type of sensory receptor does the tendon organ work to protect myotendinous junctions?

Muscle spindles Signup and view all the answers

What is a primary function of the tendon organ?

To collect information about tendon tension Signup and view all the answers

Which of the following best describes the role of the tendon organ in muscular coordination?

It aids in fine muscular coordination by relaying tension data Signup and view all the answers

What are the thin filaments primarily composed of?

Actin Signup and view all the answers

Which component is absent in thin filaments?

Troponin Signup and view all the answers

What are the thick filaments primarily made of?

Myosin Signup and view all the answers

Which statement about thin and thick filaments is correct?

Thin filaments are made of actin and tropomyosin, thick filaments are made of myosin. Signup and view all the answers

Which of the following best describes the structure of thick filaments?

Composed of myosin Signup and view all the answers

Study Notes

Islamic Invocation

This text contains an Islamic invocation, beginning with "Bismillah ar-Rahman ar-Rahim" (In the name of God, the Most Gracious, the Most Merciful).
Arabic script is used for the invocation.
The text then states "Subhanaka la 'ilma lana illa ma 'allamtana innaka anta al-'alimu al-hakimu" (Glory be to You, we have no knowledge except what You have taught us. Indeed, You are the All-Knowing, the Wise).

Muscle Tissue

Muscle tissue is one of the four basic tissues in the body, specialized for contraction and relaxation.
It is supported by connective tissue (C.T).
Connective tissue connects muscle cells; carries nerve fibers and blood capillaries to them.
Muscle cells are also called myocytes.
Due to their elongated and narrow shape, muscle cells are called muscle fibers.

Types of Muscle Tissue

There are three types of muscle tissue:
- Skeletal
- Cardiac
- Smooth

Skeletal Muscle

Skeletal muscle is striated (shows alternating dark and light bands) and voluntary (controlled consciously).
Its primary function is for moving the skeleton.
Found in limbs and body wall.
The shape is elongated, cylindrical, and multinucleated.
Length varies from a few millimeters to a few centimeters.
Originates mainly from somatic mesoderm.
Its basic structural unit is a long cylinderical fiber

Cardiac Muscle

Cardiac muscle is striated and involuntary (controlled unconsciously).
Its location is in the heart and basis of blood vessels joining the heart.
The shape is branched and forms a network.
The diameter is ~ 15 µm and the length is ~ 80 µm
The cytoplasm displays cross striations
The nucleus is single, oval, and centrally located.

Smooth Muscle

Smooth muscle is unstriated and involuntary.
Its location includes the walls of hollow viscera (e.g., the gastrointestinal tract, reproductive system, and urinary tracts), blood vessels, ducts of compound glands, respiratory passages, and bundles within the dermis of the skin.
The shape is fusiform, elongated, and tapered at both ends. The diameter is ~15µm and the length is ~ 0.2 mm.
The cytoplasm lacks cross-striations.
The nucleus is single and oval, with a corkscrew appearance during contraction.
It's primarily composed of thin and thick filaments.
Dense bodies adhere to the sarcolemma.
Frequently arranged in two layers, which permits peristalsis (wave-like contractions)

Muscle Fiber Components

Plasma membrane: Sarcolemma
Cytoplasm: Sarcoplasm
Smooth endoplasmic reticulum: Sarcoplasmic reticulum
Mitochondria: Sarcosomes
Muscle cells are of mesodermal origin
Muscle fibers (cells) or myocytes

Myofibrils and Sarcomeres

Myofibrils are cylinder-shaped structures that extend the length of the muscle fiber, responsible for cross-striations
The sarcomere is the contractile unit of striated muscle fibers. This is the region of the myofibril between two successive Z disks (~2.5 µm).
A band (dark) is mainly composed of thick filaments and overlapping portions of thin filaments.
I band (light) contains thin filaments only
H zone (narrow region in the center) contains thick filaments only
Z disk (a dense protein disc) anchors the thin filaments
M line (in the center of the H zone) anchors the thick filaments.

Thick Filaments (Myosin)

Thick filaments (15 nm in diameter and 1.5 µm long) are composed of 200-300 myosin molecules
Each myosin molecule has two identical heavy chains & four light chains. The rod-like polypeptide chains of heavy chains are wrapped around each other to form a tail and a globular head.
Heavy chains are cleaved into a rod-like tail, light meromyosin (LMM) and a globular head, heavy meromyosin (HMM). The globular heads of the heavy chains are active in ATP binding.

Thin Filaments

Thin filaments are composed of F-actin, tropomyosin, and troponin.
F-actin is formed from a double helix of G-actin (globular actin) molecules that have an active site for myosin binding.
Tropomyosin is a fibrous protein that blocks the active sites on actin preventing muscle contraction when there is no calcium.
Troponin is a globular protein complex with three subunits (TnT, TnC, TnI) which modulate contraction.

Sarcoplasmic Reticulum (SR)

The SR is a specialized type of smooth endoplasmic reticulum.
Sarcotubules are associated with the SR, which form a network around myofibrils.
The SR stores calcium ions within its interior.
The terminal cisternae are widened areas of the SR that closely associate with T tubules to form a triad grouping

T-tubules

T-tubules are deep invaginations of the sarcolemma that extend into the muscle fiber
T tubules are closely associated with sarcotubules to form a triad.
T tubules transmit electrical impulses deep into the muscle fiber to stimulate the release of calcium from SR.

Muscle Contraction

Contraction of muscle occurs by the interaction of actin and myosin.
Calcium ions are released from the SR during muscle stimulation.
Calcium ions bind to troponin, causing a conformational change in tropomyosin to expose the myosin-binding sites on actin.
Myosin heads bind to actin, forming cross-bridges, and the filaments slide past each other, shortening the sarcomeres.
ATP (adenosine triphosphate) supplies the energy for the sliding process.

Muscle Relaxation

Relaxation occurs when calcium ions are actively transported back into the SR
Regulatory proteins (troponin and tropomyosin) cover the myosin binding sites.
Cross-bridges between actin and myosin break. This returns the sarcomeres to their resting length.

Innervation

Skeletal muscles are innervated by motor neurons that end at the neuromuscular junction. The axon terminal (covered by Schwann cells) houses mitochondria, smooth ER, and as many as 300,000 synaptic vesicles containing the neurotransmitter acetylcholine (ACh). The stimulus from the neurons is transmitted chemically to the sarcolemma and T-tubules.

Myotendinous Junction

Tendons connect muscle to bones. The collagen fibers in tendons are continuous with those in the muscle connective tissue.
This structure provides a strong unit that helps to move the skeleton when the muscle contracts.

Structure of Muscle Spindles

Muscle spindles are proprioceptors in muscles, sensing changes in muscle length to trigger a reflex. They are encapsulated and located among voluntary muscle fibers.
The structure includes elongated, modified muscle cells (intrafusal fibers) surrounded by the periaxial space inside a capsule.
They detect changes in muscle length and trigger a protective response. Intrafusal fibers have two types: nuclear bag fibers and nuclear chain fibers.
Their nuclei lie in the center of the fiber or in a single row.

Intercalated Disks

Cardiac muscle cells are connected end-to-end by intercalated disks.
These disks contain fasciae adherentes and desmosomes in the transverse portions, and many gap junctions in the lateral portions.
Gap junctions facilitate rapid intercellular communication to coordinate contraction.

Types of Muscle Summary

This table is a summary of skeletal, cardiac, and smooth muscles: | Feature | Skeletal |Cardiac| Smooth | |---|---|---|---| | Site | Attached to skeleton | In the heart | In blood vessels, hollow organs | | Action | Voluntary contraction | Involuntary contraction | Involuntary contraction | | Striations | Present | Present | Absent | | Sarcomeres | Present | Present | Absent | | Myofibrils | Present | Present | Present | | Nuclei | Multinucleated | Single | Single | | T tubules | Present at A-I junction | Present at Z lines | Absent | | Troponin| Present | Present | Absent |

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Description

Test your knowledge on muscle spindles, myoneural junctions, and the roles of various muscle cells. This quiz covers key concepts in muscle anatomy and physiology, focusing on the structures and functions that govern muscle contraction and coordination.