Station 4: Histology of Cardiac Muscle Flashcards
Features that differentiate cardiac from skeletal muscle under LM, or in general
Branching arrangement (rather than long cells in skel)
Single centrally located nucleus (as opposed to multiple peripherally located nuclei in skel)
Less distinct striations, due to more mitochondria, and presence of lipid and glycogen droplets between myofibrils (compared with skel muscle)
Adjacent cardiac muscle cells join via intercalated discs (which occur at z lines - where the next z line would be)
In general: cardiac is innervated and controlled by ANS, there exist pace-making cells which generate action potentials which propagate throughout myocardium, cell-to-cell, via gap junctions located at intercalated discs.
(whereas skel muscle is primarily voluntarily controlled, and must be excited by ACh at motor end plate)
Features that differentiate cardiac from skeletal muscle with EM
More mitochondria (than skel)
Presence of lipid and glycogen droplets
mitochondria and lipid and glycogen droplets occur between myofibrils, resulting in less organised striations
Intercalated discs
Gap junctions
T-tubule / SR Diads (as opposed to triads in skel)
Diads located at z-line (as opposed to the triads occurring at A-I junction in cardiac muscle)
SR less organised (compared with skel muscle)
From what type of cell to purkinje fibres develop?
From cardiac cell - they are modified cardiac cells
What do Purkinje fibres look like?
They are enlarged, modified cardiac muscle cells
They stain more pale eosinpohilic than cardiac muscle cells
They contain lots and lots of glycogen (for rapid transmission of electrical signals)
Thus, the contractile fibres are pushed to the periphery
Purkinje fibers are striated, though not clearly
You can see intercalated discs between them, which have numerous gap junctions for proparagation of electrical current
Nucleus is not always present
What is the first part of the ventricle to be stimulated to contract?
The papillary muscles, to ensure that the AV valves are tightly shut during ventricular systole
Then, the signal begins at the apex-end of the ventricles, and spreads towards the base of the heart
Describe the composition of a heart valve
Valves are outgrowths of endothelium.
On atrial side, there is the lamina spongiosa with overlying LCT and endothelial epithelium. Have a fibro-elastic ‘skeleton’ which is continuous with the fibrous ‘skeleton’ of the heart.
Sub-endothelial compartment which is high in elastin (or smooth muscle on the ventricle side)
How does the endocardium differ between the atria and ventricles?
Atria have thicker endocardium
What separates the muscle tissue of the atria from the ventricles?
What is the functional significance?
The fibrous skeleton of the heart
The skeleton provides structural support: surrounds the valves, provides attachment points for cardiac muscle.
Also serves as insulation. By separating the atria from the ventricles, the fibrous skeleton aids the electrical conduction of the heart by ensuring that the atria contract before the ventricles. *In a normal heart, the only electrical continuation between the atria and the ventricles is a the AV node and branch.
Function of papillary muscles
Papillary muscles contract, and hold the AV valve leaflets closed, via chordae tendinae, during ventricular systole
What structures and tissues lie in the coronary groove?
Fibrous skeleton of the heart
CT
Cardiac muscle
Coronary vessels:
On the left, the circumflex artery and great cardiac vein
Posteriorly: the coronary sinus
To the right: the RCA
What are the components of the pericardium?
Fibrous pericardium (outer layer)
Serous parietal pericardium
Visceral serous pericardium (also the epicardium of the heart)
