HISTOLOGY: Smooth Muscle tissue

Smooth muscle relaxation

Ca++ is pumped back into sarcoplasmic reticulum Calmodulin dissociates from light chain kinase Myosin is dephosphorylated Caldesmon binds to myosin-binding site of actin filament PROLONGED EFFECT, BUT REQUIRES VERY LITTLE ENERGY

Regeneration of smooth muscles

Capable of active regenerative response -go through mitosis and replace damaged cells

Nuclei of smooth muscle cells

Centrally placed Long with tapered ends

Sarcoplasm of smooth muscle cells

Contains myofilaments and organelles Numerous mitochondria along the tapered ends of the nuclei

Dense bodies

Formed by α-Actinin Anchored into network of intermediate filaments (desmin) ANALOGOUS TO THE Z-LINE OF SKELETAL MUSCLE FIBERS

Smooth muscle tissue

Found in the walls of internal organs DO NOT SHOW CROSS STRIATIONS

Smooth muscle cell structure

Fusiform cells (spindle shaped with tapered ends)

G-actin--> F-actin

G- actin is a globular protein which exists as a free monomer Polymerization of G-actin creates a polymer microfilament called F-actin Two strands of F-actin form a double stranded helical filament to create the actin backbone of the thin filament

3 major proteins of thin filaments

G-actin--> F-actin Tropomyosin Caldesmon (smooth muscle specific actin-binding protein) (NO TROPONIN)

Myofilaments of smooth muscle cells

Randomly distributed throughout the cytoplasm and poorly organized THIN FILAMENTS -Attached to dense bodies (anchoring proteins) THICK FILAMENTS - Scattered throughout sarcoplasm

Thin filaments of smooth muscle cells

Resemble thin filaments of skeletal muscle fibers NO TROPONIN COMPLEX

Caldesmon

Smooth muscle specific actin-binding protein Masks the myosin-binding site on the actin filament

Smooth muscle contraction

1. Excitation of smooth muscle causes release of Ca++ into sarcoplasm 2. Ca++ binds to calmodulin 3. Ca-calmodulin binds to caldesmon, releasing it from actin and opening the myosin binding site on G-actin monomers 4. Ca-calmodulin activates myosin light chain kinase (MLCK) 5. MLCK phosphorylates regulatory light chain of myosin head 6. Myosin unfolds, opening the actin binding site of myosin head, and binds to actin 7. Filaments slide and cell shortens (nucleus folds and becomes corkscrew shaped) SLOW ACTION AND TAKES OVER A SECOND

Characteristics of smooth muscle tissue

1. Single nucleus per cell 2. No branches 3. No cross striations (no sarcomeres) 4. Slow, rhythmic, prolonged contractions of visceral organs 5. Involuntary movement

Latch state of smooth muscle cells

Allows smooth muscles to maintain long term contractions with minimum energy spent Caused by decrease of ATP activity while myosin head is attached to actin Used to sustain the tone of blood vessels Resembles rigor mortis of skeletal tissue

Connective tissue surrounding smooth muscle cells

External lamina and reticular fibers enclose smooth muscle cells

Tropomyosin

Filaments that in the grooves b/w the two actin polymers IN RESTING MUSCLE, tropomyosin masks the myosin binding sites on the G-actin monomers

Blood supply of smooth muscles

Moderate blood supply through capillaries in the connective tissue surrounding the muscle cells

Thick filaments of smooth muscle cells

Myosin II composed of 2 heavy polypeptide chains and 4 light chains (2 essential and 2 regulatory) Myosin binds to actin ONLY when phosphorylated Myosin is folded (inactive) when dephosphorylated

Gap junctions interconnecting smooth muscles cells

Numerous gap junction between smooth muscle cells allows for movement of molecules and ions for contraction regulation of entire fascicle

Neural stimulation of smooth muscle fibers

Occurs through postganglionic fibers of autonomic nervous system Neurotransmitter is released close to the cell and diffuses through connective tissue surrounding the muscle cell

Mechanical stimulation of smooth muscle fibers

Passive stretching of organ can lead to initiation of muscular contraction

Sarcolemma of smooth muscle cells

Sarcolemma invaginates numerous times to form cavolae Beneath the sarcolemma and in close proximity of SER are sarcolemmal vesciles CAVOLAE, SARCOLEMMAL VESICLES, AND SER FUNCTION IN ANALOGOUS MANNER TO THE MEMBRANE TRIAD OF SKELETAL MUSCLES (deliver Ca++ to the cytoplasm)

Chemical stimulation of smooth muscle fibers

Smooth muscle contraction is elicited by various hormones, angiotensin II, vasopressin, and other agenst

Location of smooth muscle cells

Walls of veins, arteries, and lymphatic vessels Extramural part of large ducts of glands GI tract (distal esophagus --> anal canal) Male and female reproductive tract (ductus deferens, uterus, and vagina) Urinary system (bladder and ureter) Iris and ciliary body (constriction and dilation of pupil) Arrectores pilorum muscles (dermis attached to hair follicle)