Histology: Smooth Muscle Tissue Flashcards

Smooth Muscle Tissue Overview

• Located in the walls of internal organs
• Characterized by the absence of cross striations

Characteristics of Smooth Muscle Tissue

• Each cell contains a single nucleus, centrally placed
• Lacks branches and sarcomeres
• Exhibits slow, rhythmic, and prolonged contractions
• Functions involuntarily

Smooth Muscle Cell Structure

• Fusiform in shape, tapering at both ends
• Sarcoplasm contains myofilaments and numerous mitochondria along the tapered ends of nuclei

Myofilaments in Smooth Muscle Cells

• Randomly distributed and poorly organized
• Thin filaments anchored to dense bodies, while thick filaments are dispersed within the sarcoplasm

Dense Bodies

• Composed of α-Actinin
• Serve a similar function as Z-lines in skeletal muscle, anchoring intermediate filaments (desmin)

Sarcolemma Structure

• Sarcolemma has multiple invaginations forming cavolae
• Close proximity of sarcolemmal vesicles and smooth endoplasmic reticulum, functioning similar to skeletal muscle’s membrane triad for calcium delivery

Intercellular Communication

• Gap junctions provide connections between smooth muscle cells, facilitating movement of molecules and ions for coordinated contractions

Connective Tissue Surrounding Smooth Muscle

• Smooth muscle cells are enclosed by external lamina and reticular fibers

Thin Filaments

• Resemble skeletal muscle thin filaments but lack troponin
• Composed of G-actin, F-actin, tropomyosin, and caldesmon

Actin Dynamics

• G-actin (globular) polymerizes into F-actin (filamentous), forming a double-stranded helical structure
• Tropomyosin covers myosin-binding sites on G-actin during rest

Caldesmon Function

• A unique smooth muscle actin-binding protein that masks myosin-binding sites on actin

Thick Filaments Composition

• Made of Myosin II, which consists of heavy and light polypeptide chains
• Myosin only binds to actin when phosphorylated, remaining inactive (folded) when dephosphorylated

Smooth Muscle Fiber Stimulation

• Neural stimulation occurs via postganglionic fibers of the autonomic nervous system
• Chemical stimulation is triggered by hormones like angiotensin II or vasopressin
• Mechanical stimulation (e.g., passive stretching) can also induce contractions

Smooth Muscle Contraction Mechanism

• Calcium ions are released into the sarcoplasm and bind to calmodulin
• The calcium-calmodulin complex activates myosin light chain kinase (MLCK), leading to myosin phosphorylation and actin binding
• Filaments slide, resulting in cell shortening and a corkscrew appearance of the nucleus

Smooth Muscle Relaxation Process

• Calcium is sequestered back into the sarcoplasmic reticulum
• Calmodulin releases from myosin light chain kinase, leading to myosin dephosphorylation
• Caldesmon rebinds to the myosin-binding site on actin, facilitating relaxation

Latch State of Smooth Muscle Cells

• Enables sustained contractions with minimal energy expenditure, allowing for prolonged muscle tone

This quiz consists of flashcards detailing smooth muscle tissue, its characteristics, and cell structure. It's designed to enhance your understanding of histology and the unique features of smooth muscle. Ideal for students studying anatomy and physiology.