Page 420 - Atlas of Histology with Functional Correlations

stimulation throughout the entire ventricular musculature.

The pacemaker activities of the heart are influenced by the axons from

the autonomic nervous system (ANS) and by hormones. Axons from both
the parasympathetic division and the sympathetic division innervate the heart
and form a wide plexus at its base. Although these axons innervate the heart
myocardium, they do not affect the initiation of rhythmic activity of the

nodes. Instead, they affect the heart rate. Stimulation by the sympathetic
nerves accelerates the heart rate, whereas stimulation by the parasympathetic
nerves produces the opposite effect and decreases the heart rate.

Purkinje Fibers

Purkinje fibers are thicker and larger than cardiac muscle fibers and
contain a greater amount of glycogen and fewer contractile filaments.
Purkinje fibers are part of the conduction system of the heart. These fibers
are located beneath the endocardium on either side of the interventricular
septum and are recognized as separate tracts. Because Purkinje fibers branch

throughout the myocardium, they deliver continuous waves of stimulation
from the atrial nodes (SA and AV) to the rest of the heart musculature via the
gap junctions. This stimulation produces ventricular contractions (systole)

and the ejection of blood from both ventricular chambers.

Atrial Natriuretic Hormone

Certain cardiac muscle fibers in the atria exhibit dense granules in their
cytoplasm. These granules contain atrial natriuretic hormone (ANH) that

is released in response to atrial distention or stretching. The main function of
this factor is to decrease blood pressure by regulating blood volume. The
ANH inhibits the release of renin by kidney cells and aldosterone from the
adrenal gland cortex that induce the kidneys to excrete more sodium ions and
water (diuresis). As a result, the blood volume and blood pressure are

reduced decreasing the distention of the atrial wall and further release of the
ANH.

Summary

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