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Smooth Muscle Notes

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Smooth muscle Unlike cardiac and skeletal muscles, smooth muscle cells typically maintain continuous, relatively low-energy contractions, which can be modulated and coordinated to produce rhythmic or wave-like contractions throughout a tissue. The strength of contraction can be controlled by the autonomic nervous system, hormones and local metabolites. In a few cases, the autonomic nervous system directly controls contraction (eg., in the iris), but more usually it merely modulates spontaneous contractions in the muscle. In some muscles (eg., the respiratory tract), there is no autonomic innervation
-non striated muscle- responsible for contraction of hollow organsWalls of the blood vessels Respiratory system Gastrointestinal tract Urinary tract- ureters,bladder, urethra Reproductive tracts Piloerector muscles associated with hairs in the skin Iris of the eye

Structure -difference between skeletal and cardiac non striated Contractile machinery
-smooth muscle thin filament is longer than in cardiac muscle allowing greater shortening
-Lacks troponin, cardiac calcium sensitive regulatory protein, replaced by proteins caldesmon and calponin on vascular smooth muscle thin filaments
-myosin differs from the heart and participates in contraction when phosphorylated
-actin filaments are rooted in dense bands on the inner cell membrane and dense bodies in cytoplasm. Alpha actinin. Dense bodies are not aligned so lacks the striated appearance of cardiac and skeletal muscle
-intermediate filament- ctyoskeletal element links dense bodies and dense bandsso the cell contracts as a whole- proteins desmin and vitamin SR
-Sarcoplasmic reticulum, releaseable store of calcium but poorly developed in smooth muscle, calcium store is not very big
-In order to maintain tonic contraction, extracellular influx if required- so calcium channel blockers such as nifidipine are good resistance vessel dilators

-The SR is close to the sarcolemma so facilitates store release by agonists such as noradrenaline- IP3 binds to IP3 receptors
-Calcium release channel- ryanodine receptors- basal state- ryanodine receptors spontaneously release brief bursts of calcium called calcium sparks- raises the calcium concentrations locally in the subsarcolemmal region- activates nearby calcium dependent K channels- hyperpolarisation- so spark no contraction Gap junctions
-vascular myocytes are connected to each other by homocellular gap junctionsmade up of protein connexins- 6 connexins- make hemi tube- connexon- the connexons in adjacent cells join end to end to connect the cytoplasm of the cells Caveolae
-cell surface highly invaginated- increase the total surface area of the membrane
-cytoplasmic surface has a striated coat of protein- caveolin 1, caveolar membrane is enriched in cholesterol, sphingomyelin
-they contain high concentrations of B adrenergic receptors, G proteins, L type calcium channels, K atp channels Types of smooth muscle
-Visceral/ unitary smooth muscle-

Large sheets of cells with common innervation connected by gap junctions which function as low resistance electrical connections and permit coordinated contraction Found in walls of hollow visceral organs- gastrointestinal tract,uterus,, blood vessels, airways Spontaneous contractions can oocur- stretch increases tone Contractions are mostly due to circulating hormones but can be modulated by nervous stimuli There can be enpassant junctions on a few cells and the excitation spreads to other cells

Multi-unit smooth muscle-

Fibres receive individual innervations and act independently- multiunit smooth muscles are capable of finer control- found in the iris, cilary body of the eye, piloerector muscles of the skin Spontaneous contractions don't occur Contractions mostly caused by nervous triggers and is modulated by hormones Functions more like those of skeletal muscle

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