Muscle Anatomy and Physiology Quiz

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 |