Histology of Cardiac

Muscle

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BY: Jehad Abdullah

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

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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 syncytiumwhen 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

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Cardiac Muscle

Cross section

Irregular

profiles

Centrally

located nucleus

(white spots)

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.

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)

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Intercalated Discs

seen in longitudinal sections.

connect the individual muscle cells.

permit the conduction of electrical impulses

between the cells.

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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 b

ands of filaments made of actin or myosin.

During muscle contraction, the filaments sli

de over each other to cause shortening of t

he sarcomere.8

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.

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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.

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Cardiac muscle in longitudinal section

• ID=intercalated

discs

ID

ID

Cardiac muscle in longitudinal

section

CT=connective tissue

ID=intercalated discs

branching fibers ( )

Note:

Cardiac muscle has striations, branching fibers, and intercalated discs

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.

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