Flashcards in Cardiac and smooth muscle Deck (16):
How would one recognise cardiac muscle?
- Centrally-positioned nuclei (1 or 2 per cell)
- Intercalated discs
How are adjacent cardiac cells electrically and mechanically coupled?
Via intercalated discs which have:
- gap junctions - for electrical coupling
- adherens-type junctions - to anchor cells and provide anchorage for actin filaments
How is actin/myosin arranged in cardiac muscle?
Distinct myofibrils are absent - actin and myosin form continuous masses in the cytoplasm.
What is the difference between the arrangement of t tubules in skeletal and cardiac muscle?
- In cardiac muscle, t tubules lie in register with Z bands.
- In skeletal muscles, they lie within A-I band junction.
What is a diad and what does it allow?
- Close associate of 1 t-tubule and 1 terminal cisternae of sarcoplasmic reticulum (at Z line of cardiac myocyte).
- Allows release of ionic calcium into sarcoplasm and subsequent cardiac muscle contraction upon membrane depolarisation.
What are Purkinje fibres?
Modified myocytes that transmit AP from the AV node to the ventricles.
Describe the characteristics of Purkinje fibre cells.
1. Abundant glycogen
2. Sparse myofilaments
3. Extensive gap junction sites
Which hormones can be used as diagnostic markers for heart failure? Why?
ANP and BNP - released by atrial and ventricular myocytes in response to distension, e.g. During LV hypertrophy, mitral valve disease and hypervolmic states.
What is the effect of ANP and BNP release?
Serve as counter regulatory system for the RAAS.
1. Vasodilation... decreases BP.
2. Increases glomerular filtration rate and decreases renin...
- decreases angiotensin II production... vasodilation.
- decreases aldosterone production... natriuresis and diuresis.
How would one recognise smooth muscle?
- Spindle-shaped cells with central nucleus
- Not striated
- No sarcomeres or t-tubules
Where is smooth muscle found?
Forms contractile walls of passageways/cavities.
E.g. Of vascular structures
E.g. In gut, respiratory tract and genitourinary tract
How is smooth muscle contraction controlled?
Is involuntary but responds to stimuli, e.g. Nerve signals (mainly parasympathetic), hormones, drugs or local concentrations of blood gases.
When do modified smooth muscles cells occur?
1. Myoepithelial cells
- Stellate cells forming basketwork around secretory units of some exocrine glands (e.g. Sweat, salivary and mammary glands) to assist secretion.
- In ocular iris to contract/dilate pupil.
- Produce collagenous matrix at sites of wound healing
- Contract (abundant actin and myosin) to promote wound closure
How are smooth muscle actin and myosin filaments arranged in the cell?
- Diagonally, spiralling down the long axis
- So smooth muscle contracts in a twisting way
Describe the limited nature of repair possible in the different types of muscle cell.
1. Skeletal muscle
- Can't divide but tissue can regenerate after muscle injury by mitotic activity of satellite cells - hyperplasia.
- Satellite cells can fuse with existing cells to increase mass - hypertrophy.
2. Cardiac muscle
- Incapable of regeneration.
- Damage... fibroblasts invade, divide and lay down scar tissue.
3. Smooth muscle
- Retain their mitotic activity and can form new muscle cells.
- E.g. Hypertrophy and hyperplasia of uterine muscle cell wall during pregnancy.