Smooth Muscle Prof K Sivapalan Smooth Muscle Found

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Smooth Muscle Prof. K. Sivapalan

Smooth Muscle Prof. K. Sivapalan

Smooth Muscle • Found in the internal organs • Usually 1 to 5 µ

Smooth Muscle • Found in the internal organs • Usually 1 to 5 µ in diameter and only 20 to 500 µ in length • Myosin and actin filaments cause contraction but the arrangement is different June 2013

Smooth Muscle Types • The smooth muscle of each organ is distinctive in several

Smooth Muscle Types • The smooth muscle of each organ is distinctive in several ways: – (1) physical dimensions, – (2) organization into bundles or sheets, – (3) response to different types of stimuli, – (4) characteristics of innervation, and – (5) function. June 2013

Major Types • Multi Unit Smooth Muscle: – Discrete fibers – Each operate independently

Major Types • Multi Unit Smooth Muscle: – Discrete fibers – Each operate independently – Main control is nervous – Examples: ciliary muscle of the eye, the iris muscle of the eye, and the piloerector muscles June 2013

Major Types • Unitary Smooth Muscle [Visceral Smooth Muscle]: – All act as single

Major Types • Unitary Smooth Muscle [Visceral Smooth Muscle]: – All act as single unit – Arranged in bundles or sheets – Fibers are attached so that the force of contraction is transmitted from to the next – Syncytial- communicate through gap junctions. – examples: the gut, bile ducts, ureters, uterus, and blood vessels June 2013

Innervation • Smooth muscles are innervated by autonomic fibers. • The nerves end in

Innervation • Smooth muscles are innervated by autonomic fibers. • The nerves end in varicosities which release the transmitter: – Sympathetic- noradrenaline [norepinephrine] – Parasympathetic- acetyl choline June 2013

Arrangement of Filaments • Actin filaments are attached to dense bodies • Dense bodies

Arrangement of Filaments • Actin filaments are attached to dense bodies • Dense bodies are distributed in the cytoplasm and the cell membrane. • Dense bodies in the membrane of adjuscent cells are bound to transmit the force. • Myosin filaments are interspersed among actin • 80 % shortening is possible June 2013

Electrical Property • RMP is -50 to -60 m. V • When stimulated spike

Electrical Property • RMP is -50 to -60 m. V • When stimulated spike potentials occur, in some with plateau [uterus]. • In some unitary muscles, slow wave rhythm is seen. • When this spontaneous change reaches the threshold level, spike potential occurs. [pace makers in GIT] June 2013

Action Potentials • Duration of spike potentials- 10 -50 ms • Where there is

Action Potentials • Duration of spike potentials- 10 -50 ms • Where there is a plateau, it may last up to 1000 ms • The depolarization is mostly due to voltage gated calcium channels [which are slow to open] than sodium channels. June 2013

Stimulation • Stimulation may or may not result in action potential. The factors react

Stimulation • Stimulation may or may not result in action potential. The factors react with receptors to cause contraction or relaxation. • Local chemical factors: – Hypoxia, hypercapnoea, acidity, lactic acid, adenosine, potassium ions, NO and temperature • Hormones: – Epinephrine, norepinephrine, acetyl choline, angiotensin, endothelin, oxytocin, serotonin, histamine • The response depends on the receptors. • Stretch June 2013

Excitation-Contraction Coupling • Calcium binds to calmodulin [no troponin] • It activates myosin kinase

Excitation-Contraction Coupling • Calcium binds to calmodulin [no troponin] • It activates myosin kinase • Regulatory chain of myosin head is phosphorilated • Binding with actin repeatedly • When calcium is removed, myosin phosphatase is activated. • It splits the phosphate from myosin head and contraction ceases June 2013

Mechanical Property • The oncet of contraction is slow and the contraction time is

Mechanical Property • The oncet of contraction is slow and the contraction time is prolonged. • Energy required is also very much less • The force of contraction per unit surface area is much more- 4 -6 kg/cm 2 [3 -4 in skeletal muscles] June 2013