The effect on nerve impulse frequency of an electrical stimulation of the cervical sympathetic trunk was explored in four units, by using increasing stimulation frequencies (5, 10, 15, 20, and 25 Hz;
Fig. 6A ). Electrical stimulation at different frequencies produced a transient increase in ocular blood flow and subscleral temperature, accompanied by a silent period in cold receptor activity
(Fig. 7) that was followed by a significant reduction in blood flow (−72% ± 21%,
n = 6) and temperature (−0.47 ± 0.08°C,
n = 3;
Figs. 6B 6C ) that persisted up to 200 seconds after the end of the stimulation
(Figs. 6B 6C) . Blood flow and temperature decreases were roughly proportional to the frequency of stimulation (
r 2 = 0.52 and 0.47 for blood flow and temperature, respectively). In all cases, the spontaneous firing frequency increased significantly (mean for the four units: 9.7 ± 1.5 imp/sec basal frequency to 19.3 ± 2.4 imp/sec during sympathetic stimulation,
n = 4,
P < 0.05, one-way ANOVA on ranks;
Fig. 6A ) and in a manner proportional to the frequency of stimulation (
r 2 = 0.74). In the sclera covered by the lids, electrical stimulation of the sympathetic nerve also induced a significant temperature reduction (−0.13 ± 0.04°C,
P < 0.05, paired
t-test,
n = 7) that showed similar time course and lower magnitude than in the exposed sclera
(Fig. 8A) .