The Conduction System of the Swine Heart (7)
The cells showed no T tubular systems. The cell coat consisted of glycoprotein and mucosubstances. The nodal cells showed the contractile elements to stain poorly. The sarcoplasm did not have the degree of organization seen in working cardiocytes. There was a random distribution of mitochondria that were lesser in volume. The amount of glycogen varied from cell to cell, and the lipid content was less in amount as compared with regular cardiocytes. There was a paucity of the intercellular junctions consisting of fasciae adherentes, small desmosomes, and sparse nexus. The nerve supply was abundant, as seen at the light level. The myofibers were both adrenergic and cholinergic. The Golgi complexes were small. The electrophysiology of the pig differs from that of the human in several ways: (1) the sinus heart rate is markedly increased, (2) the PR interval is shortened, and (3) the SACT is shortened. We will try to provide possible explanations for each of these differences by considering the physiology of man. buy cipro
In man, sinus heart rates reflect an interplay of three factors: (1) intrinsic heart rate, (2) sympathetic tone, and (3) vagal tone. Intrinsic heart rate is defined as the rate of the sinus node when isolated from the influences of the autonomic nervous system. Jose and Collison measured intrinsic heart rate in a series of volunteers by administering large intravenous doses of both propranolol and atropine.