) the force of contraction The force of contraclion is a big issue and has been subdivided into, 1

1 factors influencing the force of contraction. Starling's law and contractility 2 the concept OfVmax 3 effects of Call, noradrenalm 4. staircase phenomenon 5. all or none law 6 heart lung preparation of Starling Before going for details, the student should be familiar with the following Iwo terms (T) isometnc contraction, and (ii) isolonic contraction Isometric contraction In this case, the muscle contracts but does nol shorten For example, when the left ventricle is contracting isometrically, the ventricle shows all other features ol contraction but does not shorten ThusL the venlncle becomes hard generates heat, generates force, consumes extra energy, shows biochemical changes of contract tan withm ns muscles but does noi shorten, i.e . it performs no (exTemal) work and does not eject blood into the aorta Nevertheless, as it is generating force, the isometrically contracting ventncle exerts great squeezing pressun on Ihe blood in its canty Isolonic contraction In addition 10 other features of contraction. the venlncle shortens, does external work and! ejects blood Mechanism of shortening Basic anatomical facts need recapitulation, presuming, the reader is already aware of the facts, staled under, forking myocardial cells', in chap 1 sec V They are. 1 Working myocardial cells (WMC contracl but Ihe pace maker and the juncliona) tissues, although they belong to myocardium, do not The gross histology. Er. pictures and mechanism of contraction of WMC are remarkably similar to those of skeletal muscle cells (SMC) 2 Each WMl (or SMC) shows under LM (light micro scope), alternate dark and light bands called A and 1 bands (fig 9 1 I) respectively 3. fi sarcomere consist of 1/2 I band + one A band + 1/21 band (figs 5.1 5&5 1 9) 4 In a sarcomere, the thick (rnyosin) filament is covered by 6 Thm (actm) filamanisfligS.I 6 and 5.1.5), 5. Each actm consists of a double helix (fig 5 1 7) Actually s smgl thin filament may consists of some 300-400 actm molecules (a single thick filament, similarly contains roughly 4Q molecules) Thus, a thin filament is s polymer of many actin molecules called G achn, the polymer being called F actm S milarly. a thick filament is a polymer of many myosin molecules Two other pfoteins. the regulatory proteins, viz. tropomyosi and troponm are associated with the aclm helin (fig 5 1.7) The actm molecules contain sites for attachmeni with myo&i (fig 5 1 7) bul the Iropomyosm is so allachad that mosl of these sites are covered by the tropomyosm band This is Ihe picture, when the muscle fiber is at rest The Iropcmin (actually there ace several types oftroponiri, viz. troponm 1r troponm C etc) molecules are attached with Ihe tropomyosm Trooonm C .can bind with ionic Ca++ The myosin molecule has; a globular head and an elongated tail (fig 5 I 8) The head has sties by which it can bind wilh the actm molecule, provided the actin molecular sites are not covered by tropomyosin The myosin molecular head has also binding