Page 315 - Atlas of Histology with Functional Correlations

very rapid spread of stimuli throughout the entire cardiac muscle mass.

Conduction of excitatory impulses to the cardiac sarcomeres is through the T
tubules and the sarcoplasmic reticulum. Diffusion of ions through the pores
in gap junctions between individual cardiac muscle fibers coordinates heart
function and allows the cardiac muscle to act as a functional syncytium,
allowing the stimuli for contraction to pass through the entire cardiac

musculature.

As in the skeletal muscle, calcium is essential for cardiac muscle
contractions. In cardiac muscles, however, the sarcoplasmic reticulum is less
well developed and does not store sufficient amounts of calcium for

uninterrupted contractions. As a result, during muscle stimulation and
contraction, calcium is imported from outside the cardiac muscle cells into
the sarcoplasm as well as from the sparse sarcoplasmic reticulum. At the end
of the stimulus, this calcium movement is reversed.

Cardiac muscle fibers exhibit autorhythmicity, an ability to

spontaneously generate stimuli. Both the parasympathetic and sympathetic
divisions of the autonomic nervous system innervate the heart. Nerve fibers
from the parasympathetic division, by way of the vagus nerve, slow the heart

and decrease blood pressure. Nerve fibers from the sympathetic division
produce the opposite effect and increase heart rate and blood pressure.

Additional information on cardiac muscle histology, the heart pacemaker,
Purkinje fibers, and heart hormones is presented in more detail in Chapter 10.

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