University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

402

Pages:

3
Author:

Mariam A. A.

Cells of the heart Myocytes (25%) – mechanical pump • 90% volume • Arranged circumferential/spiral orientation around left ventricle • Atrial myocytes tend to be smaller/less than ventricular myocytes (have less ventricular force) • Terminally differentiated o ~1-10% pa new/stem cell derived turnover • Balance of myocardium o Endothelial/vascular cells o Occasional fibroblast o Inflammatory cells rare o Collagen is sparse in normal myocardium Secretory cells • Atrial: A-type electron-dense granules o Storage of atrial natriuretic peptide (ANP) o Secreted in response to atrial distension o Effects: vasodilation, natriuresis, diuresis • Ventricle: B-type (initially called brain natriuretic peptide) o Secreted in response to ↑ ventricular pressure/volume → way of controlling BP • Endothelial: C-type NO secreted in response to ↑ vascular/endothelial shear stress (coupling blood flow to need) Electrical (rate and rhythym) • Excitatory (nodes) • Conducting • Sinoatrial (SA) node = pacemaker of the heart • Atrioventricular (AV) node • Bundle of His - right and left bundle branches Difference to striated muscle • Organisation of contractile proteins is the same as found in skeletal muscle (cytomembranes aren’t) • • • • T tubules are found at the level of the Z disk, and are substantially larger than those of skeletal muscle found at the A-I junction Sarcoplasmic reticulum is not as extensive as that of skeletal muscle Diads rather than the triads seen in skeletal muscle are typical in cardiocytes o Diad = T tubule interacting with one SR cisterna (instead of 2, as in skeletal muscle) Mitochondria are more abundant in cardiac muscle than in skeletal muscle + have numerous cristae Motor protein assemblies • Sliding Filaments: Z-lines fixed, with actin filaments attached extending into neighbouring sarcomeres (actin filament polarised with opposing symmetry) • Myosin filaments attached at M-line extending with opposite polarisation into the same sarcomere (two half-sarcomeres). Myosin heads are orientated into all angular directions from the filament (filament drill). Torque of myosin head pulls actin filament towards Mline (stepsize ~10nm). • Remember: uncoordinated cross-bridge cycle of all myosin heads (no synchronization) Calcium induced calcium release 1. Depolarisation 2. Opening of L-type Ca channels releases Ca from external stores through the T-tubule membrane (20% of Ca supply) 3. Opening of Ryanodine receptor releases Ca from internal stores in the SR (80% of Ca supply) 4. Increased Ca (0.1 → 100μM) 5. Cross-bridge cycling (contractile threshold >0.1 μM) Relaxation 1. Repolarisation 2. Closure of L-type Ca channels (voltage-gated ion channel) 3. Ca is sequestered a. Taken back into the SR via Ca/H ATPase (SERCA2 channel) → 80% b. Extrusion into ECF → 20% i. Sarcolemmal Na/Ca exchanger 4. 5. 6. 7. ii. Sarcolemmal Ca2+ pump Decreasded calcium Calcium dissociates from TnC TnC blocks actin-myosin interactions Relaxation → 2H+ and 2Ca2+/ATP hydrolysed Contraction and relaxation • Energy/ATP dependent • Basal Metabolic Rate (at rest) = 25% ATP usage due to ion pumping • During muscle contraction/relaxation = 75% ATP usage o ~50% Iso-volumetic contraction o ~25% Ca2+ sequestration • Cardiac muscle cells contain more mitochondria between myofibrils than do skeletal muscle o 23% vs 2% of cell volume o Complete dependence on aerobic metabolism