Lecture 6- Microscopic structures of cardiac muscle

1. Components of heart wall

- 1 : endocardium ( consist of endothelium and connective tissue)- 2: myocardium ( 75% are myocytes, cardiac fibroblasts, connective tissue[extracellular

matrix, collagens], 5: papillary muscle]- 3: epicardium ( epithelium, connective tissue and 4: vessels]

2. The myocardium

A1 and B: purkinje fibresA2 : endocardiumA3: myocardium

3. Properties of Cadiac myocytes

- Cardiac myocytes are striated and contain sarcomers (like skeletal muscle).- Cardiac myocytes contain a single nucleus (like smooth muscle).- Cardiac myocytes are connected by gap junctions (like smooth muscle).- Excitation-contraction coupling involves Ca2+ binding to troponin (as in skeletal muscle).- Cardiac myocytes are rich in mitochondria.- Cardiac myocytes are joined by intercalated discs.

4. Excitation-contraction coupling in cardiac muscle- Cardiac myocytes contain a transverse tubule system that facilitates excitation

contraction coupling ( as in skeletal muscle)- Cardiac myocytes contain L-type calcium channels(DHP receptors) that trigger calcium

release from the sarcoplasmic reticulum by increasing calcium concentration - Excitation (depolarization of plasma membrane)- Cause opening of plasma membrane L-type Ca2+ channels in T-tubules- Flow of Ca2+ into cytosol (extracellular Ca2+)- The L-type calcium channels are voltage-gated- Other route of Ca2+ outflow is when Ca2+ binds to Ca2+ receptors (ryanodine receptor)

on the external surface of the sarcoplasmic reticulum [intracellular calcium levels]- Opening of Ca2+ channels intrinsic to these receptors- Flow of Ca2+ into cytosol- Increased ca2+ cytosol concentration- Cause contraction

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5. How cardiac myocytes are connected to each other?

- Through intercalated disc!

- Desmosomes[which are located in intercalated disc] are protein complexes that connect cardiac myocytes

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- Intercalated disc mediate resistance of cell junctions to mechanical stress by Desmosomes (desmoplakin and other proteins, connected to the intracellular

intermediate filament system) Adherens junction (N-cadherin and other proteins connected to intracellular

actin filament network)

Monomers of actin polymerise to form long, thin fibres. It functions to provide mechanical strength to the cell, generate locomotion within the cells like when the WBC move and interact with myosin filament in skeletal muscle fibres to provide force of muscular contraction.

Intermediate filaments play roles in providng supporting framework within the cell. For example the nucleus in epithelial cells is held by int filaments made of keratins.

- Intercalated discs contribute to the conductance of electrical signals to neigbouring cell via

Gap junctions : connexions and other proteins forming pores between cells. With the pore size of about 1.5nm, it is permeable for ions but not for most proteins.