Dynamic Performance of Vasomotor Responses of the Resistance Vessels of the Carotid Vascular Bed in the Rabbit

Abstract

Dynamic performance of vasomotor responses of the resistance vessels was studied in the vascular bed innervated by vasoconstrictor fibres of the cervical sympathetic nerve in rabbits. This nerve was stimulated by frequency-modulated pulses and the resulting changes of blood flow in the common carotid artery were recorded. Both the sinusoidal and the rectangular modulation of the frequency of stimulating pulses was used and thus the frequency response as well as the transient response of the examined vasomotor area were estimated; the frequency response characteristics were plotted as Bode and Nyquist diagrams. In sinusoidal changes of vasomotor stimulation the response was attenuated by 7.7 dB per oct. when the frequency increased above 0.045 cps.; in a part of experiments a certain resonant peak of the response occured at the frequency of 0.031-0.045 cps. The phase lag was relatively high exceeding 180[degree] (inversion of phase) at the frequency of 0.2 cps. (period of 5 sec.). This is due to the presence of a "dead time" identical with the latent period; after subtracting this the "net" phase lag attained only 90[degree] at the highest frequency used. The transient response had a variable shape sometimes showing superimposed flat waves of damped oscillation. The estimation of the dynamic performance of vasomotor responses is another contribution to the present ideas of the dynamics of the complicated feedback control system of the circulation.