CARDIAC MUSCLE TISSUE

• Cardiac Muscle Tissue, or Heart muscle as it is sometimes called, is found in only one place in the body – the walls of the heart. The actual muscular wall of the heart is called the myocardium. Cardiac muscle has a very unique structure and could be described as intermediate or “in between” skeletal and smooth muscle tissue.

• Cardiac muscle fibers seem to branch, weave, and merge with each other, forming what appears to be one continuous, multinucleated mass (called a syncytium). However, high magnification shows that each cardiac muscle fiber is completely bounded by its own sarcolemma. So that even though the fibers do branch, they are not continuous with adjacent fibers.

• Schematic drawings of such a branching and weaving structure are as seen to the right and show how the muscle fibers are shaped.

• A consequence of this branching structure brings about a very prominent and identifying characteristic of cardiac muscle tissue. This is the presence of dark bands which cross the fibers at intervals. These are called intercalated discs and they are formed when the sarcolemma of two (or more) adjacent fibers or branches come together and overlap in an end to end manner.

Another example of intercalated discs.

• Cardiac muscle fibers have striations when viewed under the microscope; however, they are not as dark as those found in skeletal muscle. This is due to the myofibrils in cardiac muscle being arranged in an orderly manner as with skeletal muscle, but the number, and therefore the density of the myofibrils is less.

**(Note all the intercalated discs!)

• With respect to another key identifying factor, each cardiac muscle fiber has one nucleus located deep within the cell.

**(Note the faint striations

and the intercalated disc)

“Centrally” located, single nucleus

General Characteristics

1. Location -

Muscular wall of the heart called the myocardium

2. Cell Type or description -

Short fibers (0.05-0.1 mm), branching, weaving network with overlapping sarcolemma forming intercalated discs

3. Myofibrils -- Fill cell from end to

end but less dense than skeletal

- Orderly; with light striations visible

4. Location of nucleus or nuclei -

One oval nucleus located in the center of the cell.

5. Vascular Supply and relative rank -

- Dense blood

capillary network with continuous supply

- Ranks 1st among muscle tissues

6. Description of Contraction and Control Factor –

- Rhythmic (inherent), rapid, contractions of fibers coordinated.

- Involuntary contraction

7. Alternative names-

Involuntary, striated or Heart muscle

Microscopic Views of Cardiac Muscle (Schematic)

Longitudinal View (long Axis)

**(Note the light striations, centrally located nucleus, and intercalated discs)

Another Longitudinal View

Note the branching, weaving network (black arrow), single, centered nucleus (green arrow), and the intercalated discs (yellow arrow).

Cross-sectional View (short axis)

Note the single, centrally located nucleus

Another Cross-sectional View

Additional Pictures of Cardiac Muscle

1. What “view” is this?2. Arrow #1 is what?3. Arrow #2 is what?4. Are striations visible?

#1

#2

A comparison between cardiac (top) and skeletal (bottom).

What tissue is this?

How did you know?

How did you know what it was?

• 1. Multiple peripheral nuclei

• 2. Parallel fibers

• 3. Strongly visible striations

What tissue is this?

How did you know?

How did you know what it was?

• 1. Intercalated discs

• 2. Branching, weaving network of cells

• 3. Single nucleus in the center of fiber

• 4. Lightly visible striations