Histology of cardiac muscle

1. Histology of Cardiac Muscle 1 BY: Jehad Abdullah

2. Introduction Cardiac muscle, the myocardium, consists of cross-striated muscle cells, cardiomyocytes, with one centrally placed nucleus. Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µm wide. Cardiac muscle cells excitation is mediated by rythmically active modified cardiac muscle cells. Cardiac muscle is innervated by the autonomic nervous system (involuntary), which adjusts the force generated by the muscle cells and the frequency of the heart beat. Cardiac muscle does not contain cells equivalent to the satellite cells of skeletal muscle 2

3. Exhibit branching Adjacent cardiac cells are joined end to end by specialized structures known as intercalated discs Within intercalated discs there are two types of junctions Desmosomes Gap junctions ..allow action potential to spread from one cell to adjacent cells. Heart function as syncytium when one cardiac cell undergoes an action potential, the electrical impulse spreads to all other cells that are joined by gap junctions so they become excited and contract as a single functional syncytium. Atrial syncytium and ventricular syncytium Histological Properties of Cardiac Muscle Fibers 3

4. Cardiac Muscle Cross section Irregular profiles Centrally located nucleus (white spots)

5. Cardiac muscle in cross section The nuclei ( ) are located in the center of each fiber. Note the variable size of the muscle fibers and nuclei in cross-section.

6. Purkinje Fibres:- modified cardiac muscle cells. Compared to ordinary cardiac muscle cells: contain large amounts of glycogen. fewer myofibrils. thicker cells. extend from the atrioventricular node, pierces the fibrous body, divides into left and right bundles, and travels, beneath the endocardium, towards the apex of the heart. conduct stimuli faster than ordinary cardiac muscle cells (2-3 m/s vs. 0.6 m/s). discovered in 1839 by Jan Evangelista Purkyně) 6

7. Intercalated Discs seen in longitudinal sections. connect the individual muscle cells. permit the conduction of electrical impulses between the cells. 7

8. Junctional Components Fascia adherens  – major portion of transverse component. Anchoring sites for actin, and connect to the closest sarcomere. - The contractile unit of a  skeletal muscle  fiber. Sarcomeres are divided into bands of filaments made of actin or myosin. During muscle contraction, the filaments slide over each other to cause shortening of the sarcomere.  8

9. cont, Macula adherens  – (desmosomes) transverse and lateral components. Bind individual myocytes to one another. stop separation during contraction by binding intermediate filaments, joining the cells together. Macula adherens junctions are also called desmosomes. 9

10. Cont,  Gap junctions  - lateral component. Allow action potentials to spread between cardiac cells by passage of ions between cells, producing depolarization of the heart muscle. Allows muscle to act as syncytium. 10

11. Cardiac muscle in longitudinal section ID=intercalated discs ID ID

12. Cardiac muscle in longitudinal section CT=connective tissue ID=intercalated discs branching fibers ( ) Note: Cardiac muscle has striations, branching fibers, and intercalated discs

13. Image of primate heart stained with Alizarin blue.Red Blood Cells (orange cells) Cardiac Muscle Cells (blue) Follow the course of individual cardiac muscle cells and note fine, dark blue lines which seem to cross (traverse) the fibres. Intercalated Discs that connect the individual muscle cells and permit the conduction of electrical impulses between the cells. 13

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