High threshold aortic baroreceptors afferents in the sympathetic nerve of monkey.

A total of 25 afferent units were recorded in the thoracic sympathetic rami (T3-T4) localised at the base of the bracheocephalic trunk and the descending aorta. Two types of receptors, Type I and Type II, were found. Type I receptors, localised at the base of the bracheocaphalic trunk, were fast adapting and exhibited synchronous discharge with each systolic height of blood pressure and behaved like baroreceptors. There was no discharge at the diastolic phase of the cardiac cycle. These receptors also gave occasional single spike at each systolic height of pressure of about 90-120 mmHg. When the systemic pressure was increased from 120 to 180 mmHg due to occlusion of the descending aorta or intravenous administration of adrenaline, the frequency of discharge of Type I receptors increased and they behaved like the typical sino-aortic baroreceptors. Type II receptors, localised over the wall of the descending aorta, fired irregularly and they did not have any relation with heart beat and did not behave like baroreceptors. On the basis of the present observation it may be suggested that Type I receptors are high threshold baroreceptors which play a role in the homeostatic control during high blood pressure and that Type II receptors do not play such a role.