High threshold aortic baroreceptor afferents in the sympathetic nerve

In anaesthetized cats, 40 sympathetic sensory units in the bracheocephalic artery (30 units) and the descending aorta (10 units) were recorded by means of single-unit preparation. Direct evidence is available for the mechanosensitive nature of the receptors in the sympathetic afferents at the level of T3 and T4. Two distinct types of receptors were found (Type I and Type II). Type I receptors, which were fast adapting, gave a spike discharge at each systolic height of pressure (70-110 mmHg). However, they sometimes failed to appear even at such systolic pressure. When the systemic pressure was increased by occluding the descending aorta or by infusing adrenaline solution intravenously, the frequency of discharge of Type I receptors increased and they behaved much the same as the typical sinoaortic baroreceptors. Type II receptors were activated by mechanical probing and at high systemic pressure, though they did not fire always synchronously with heart beat. On the basis of the study it may be suggested that Type I receptors are high threshold baroreceptors and like other systemic baroreceptors, play a role in homeostatic control presumably in a state of high blood pressure, but on the other hand Type II receptors do not play such a role.