Cardiac Muscle

The cardiac-muscle cells of the myocardium are arranged in layers that are tightly bound together and completely encircle the blood-filled chambers. When the walls of a chamber contract, they come together like a squeezing fist and exert pressure on the blood they enclose.

Cardiac muscle combines properties of both skeletal and smooth muscle (Chapter 11). The cells are striated (Figure 14-15) as the result of an arrangement of thick myosin and thin actin filaments similar to that of skeletal muscle. Cardiac-muscle cells are considerably shorter than skeletal-muscle fibers, however, and have several branching processes. Adjacent cells are joined end to end at structures called intercalated disks, within which are desmosomes that hold the cells together and to which the myofibrils are attached. Adjacent to the intercalated disks are gap junctions, similar to those in many smooth muscles.

Approximately 1 percent of the cardiac-muscle cells do not function in contraction, but have specialized features that are essential for normal heart excitation. These cells constitute a network known as the conducting system of the heart and are in contact with the other cardiac-muscle cells via gap junctions. The conducting system initiates the heartbeat and helps spread the impulse rapidly throughout the heart.

One final point about the cardiac-muscle cells is that certain cells in the atria secrete the family of pep-tide hormones collectively called atrial natriuretic factor, described in Chapter 16.

Innervation The heart receives a rich supply of sympathetic and parasympathetic nerve fibers, the latter contained in the vagus nerves. The sympathetic post-ganglionic fibers release primarily norepinephrine, and the parasympathetics release primarily acetyl-choline. The receptors for norepinephrine on cardiac muscle are mainly beta-adrenergic. The hormone epi-nephrine, from the adrenal medulla, combines with the same receptors as norepinephrine and exerts the same actions on the heart. The receptors for acetylcholine are of the muscarinic type.

Blood Supply The blood being pumped through the heart chambers does not exchange nutrients and metabolic end products with the myocardial cells. They, like the cells of all other organs, receive their blood supply via arteries that branch from the aorta. The arteries

Vander et al.: Human I III. Coordinated Body I 14. Circulation I I © The McGraw-Hill

Physiology: The Functions Companies, 2001 Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Sarcomere -

Transverse tubule

Z line

Mitochondria

Sarcomere -

Transverse tubule

Z line

Mitochondria

Red Blood Cell Capillary

Red blood cell in capillary

Capillary endothelium

Gap junctions

Intercalated disk

Sarcoplasmic reticulum

Red blood cell in capillary

Capillary endothelium

Gap junctions

Intercalated disk

Sarcoplasmic reticulum

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • Leon
    What is z line in cardiac muscle?
    8 years ago

Post a comment