Characteristics of ongoing and reflex discharge of single splenic and renal sympathetic postganglionic fibres in cats.

Abstract

SUMMARY1. Electrical discharge of thirty-nine single splenic and renal postganglionic nerve fibres was recorded in artificially respired, chloralose-anaesthetized cats.2. Ongoing discharge rates, averaged over 10 s periods, did not differ between renal and splenic fibres. All neurones of both groups had irregular discharge frequencies.3. Half of the splenic population and all renal fibres had cardiac-related discharge patterns. Of those tested for respiratory-related firing, 30 % of the splenic fibres and 69% of the renal fibres exhibited this pattern.4. Firing of splenic fibres was less inhibited than that of renal fibres by stimulation of pressoreceptors with phenylephrine-induced increases in blood pressure. Firing of splenic fibres also was less excited than that of renal fibres by unloading pressoreceptors with depressor doses of sodium nitroprusside.5. Chemical stimulation of splenic afferent nerves with bradykinin consistently elicited greater increases in splenic than renal nerve discharge by causing large increases in firing of all splenic fibres and smaller excitatory responses in 75 % of the renal fibres.6. Application of bradykinin to the intestinal serosa produced greater increases in renal than splenic nerve discharge by consistently causing increased firing of renal fibres and by causing excitation, inhibition, or no change in splenic fibre discharge.7. Responses of splenic and renal fibres to stimulation of splenic and intestinal afferent nerves after spinal cord transection were similar to those responses elicited when the neuraxis was intact.8. In conclusion, the differerntial reflex responses of splenic and renal neuronal populations can be due to the heterogeneity or to the intensity of responses within a neuronal population.

The discharge behaviour of single vasoconstrictor motoneurones in human muscle nerves.

“…This conclusion could not have been reached in microneurographic recordings of multiunit sympathetic bursts, which have been the standard approach to recording from postganglionic sympathetic fibres in human peripheral nerves. The maximum number observed within a burst was seven, which is also what Hallin & Torebjork (1974) (0 5-3 Hz;Jainig, 1985), as well as from renal, splenic and mesenteric nerves in the cat (0-03-6-4 Hz, median 0 9 Hz; Meckler & Weaver, 1988;Stein & Weaver, 1988). It should be noted, however, that our subjects had low levels of spontaneous multiunit activity (range 9-17 bursts min-').…”

“…In the present material short interspike intervals, with instantaneous frequencies greater than 50 Hz, were produced by nine units, but only 2-4% of all intervals were so short. Single vasoconstrictor motoneurones in the renal, splenic and mesenteric nerves of the cat also generate high instantaneous firing rates, up to 59 Hz, and the frequency distribution is similarly broad (Meckler & Weaver, 1988;Stein & Weaver, 1988). Such erratic instantaneous frequencies may be physiologically appropriate; electrical stimulation of sympathetic nerves has shown that irregular interstimulus intervals are more effective than regular stimuli for contracting arterioles in the muscle vascular bed of the cat, the optimal pattern being a 32 Hz stimulus applied at 4 s intervals (Andersson, 1983).…”

Resting discharge of human muscle spindles is not modulated by increases in sympathetic drive

“…More importantly, sympathetic neurones do not fire at such high rates. For instance, the median firing rate of single sympathetic neurones in the renal, splenic and mesenteric nerves in the anaesthetized cat is 0.9 Hz (Meckler & Weaver, 1988;Stein & Weaver, 1988), and in awake humans, individual cutaneous vasoconstrictor and sudomotor neurones discharge very irregularly with an average frequency of 0.5-0.6 Hz during the specific increases in sympathetic drive associated with cold-induced vasoconstriction (Macefield & Wallin, 1999b) or heat-induced sweating (Macefield & Wallin, 1996). Moreover, human muscle vasoconstrictor neurones There is evidence in experimental animals that, in addition to receiving fusimotor drive, muscle spindles are subject to modulation by the sympathetic nervous system.…”

Evidence for descending tonic inhibition specifically affecting sympathetic pathways to the kidney in rats.

“…The origin of this sympathetic firing is unknown. Sympathetic reflexes can be elicited by activation of a variety of visceral afferent nerves in spinal animals (Meckler & Weaver, 1988;. In preliminary experiments in our laboratory, blockade of afferent nerve activity by cooling the intestine produced decreases in mesenteric nerve activity before (35%) and after (16%) spinal cord transection in rats, suggesting that tonic excitatory afferent inputs may provide some on-going activity of these nerves.…”