L4.2 How Muscles Contract

Question 1

describe the organisation of muscle cells in smooth muscles

Answer 1

• muscle cells within smooth muscles do not run from one side to the other, they’re actually quite short
• actin and myosin is differently organised
• they start at dense body = around the cell you have little dense bodies
• thin filament (actin) attaches to the dense body and then the next thin filament attaches to a different dense body and in between we have thick filament (myosin)
• so when we get cross bridge building, it actually slides in both direction and because it is organised in a bit of a web it means when the smooth muscle cell contracts itself (bunches up from all directions)

Question 2

describe the characteristics of cardiac muscle

Answer 2

• shared characteristics with skeletal muscle: striated, troponin tropomyosin, length-tension relationship, t-tubules
• shared characteristics with smooth muscle: calcium enters from both ECF and SPR (sarcoplasmic reticulum)
• unique characteristics: syncytium, long AP (action potential)

Question 3

describe what the pacemaking cells in the heart does

Answer 3

• trigger the heart contraction
• once the pacemaking potential occurs, it depolarises all the neighbouring cardiac muscle cells and the cardiac muscle action potentials travel along specialised conduction pathways across both atria (pathways are not nerve cells, they are specialised cardiac muscle cells)
• spreads rapidly across the 2 atria which in turn will depolarise all the cardiac muscle cells in both atria = atria contracts in response and push the blood from both atria to both ventricle
• once depolarisation reaches the AV node it delays the depolarisation to travel to the ventricle
• once the AV node lets the depolarisation through, it travels along the special conducting pathways along the bundle of His, into the ventricle, into the purkinje fibres= in response ventricles start to contract and squeeze lots of blood out

Question 4

skeletal muscles work at optimum resting length, what does cardiac muscles work at?

Answer 4

cardiac muscles work at much lower length so you don’t have an optimum overlap of actin and myosin