Reflex patterns in postganglionic neurons supplying skin and skeletal muscle of the rat hindlimb

1. Reflex patterns were analyzed in spontaneously active postganglionic vasoconstrictor neurons supplying skeletal muscle [muscle vasoconstrictor (MVC) neurons] and hairy skin [cutaneous vasoconstrictor (CVC) neurons] of the rat hindlimb. Postganglionic activity was recorded from single units and from filaments containing the axons of several spontaneously active neurons (multiunit preparations). The animals were freely breathing or artificially ventilated and maintained, in different experiments, under three different types of anesthesia (pentobarbital, chloralose, urethan). Reflexes were elicited by stimulation of arterial baroreceptors, chemoreceptors, cutaneous nociceptors, and cold receptors and visceral receptors from urinary bladder and colon. 2. Spontaneous activity of single postganglionic neurons ranged from 0.3 to 3.6 imp/s (median 1.15 imp/s and 1.0 imp/s in MVC and CVC neurons, respectively). Postganglionic axons conducted at 0.56 ± 0.15 m/s (mean ± SD, MVC neurons) and 0.53 ± 0.11 m/s (CVC neurons). There was almost no difference in the rate of spontaneous activity under the three anesthetics used and whether the animals were artificially ventilated or breathing freely. 3. Stimulation of arterial baroreceptors by increasing arterial blood pressure by >30 mmHg with intravenous injections of phenylephrine or angiotensin led to a depression of the activity in almost all vasoconstrictor neurons. In simultaneous recordings, with an identical increase of blood pressure, the magnitude of inhibition was greater in MVC neurons than in CVC neurons. Phasic stimulation of the arterial baroreceptors by the pulse pressure wave evoked a pronounced cardiac rhythmicity in the activity of the majority of MVC neurons (78%), but in only a small fraction of CVC neurons (18%). In most CVC neurons the cardiac rhythmicity was weak (33%) or absent (49%). When quantified the difference in the degree of cardiac rhythmicity between simultaneously recorded MVC and CVC neurons was highly significant (P < 0.001). 4. Noxious mechanical stimulation of skin of the ipsilateral hindpaw activated 20/35 MVC preparations (57%) and inhibited 25/47 CVC preparations (53%). Some CVC neurons (19%) were also activated, whereas the remainder of neurons were not affected. The quality of responses to noxious stimulation was correlated with the degree of cardiac rhythmicity that the sympathetic neurons displayed in their activity. A similar reciprocal response pattern in CVC and MVC neurons, albeit less pronounced, was observed to intense cold stimuli (chlor-ethyl spray) applied to the hindlimb. 5. This reciprocal pattern of the responses of MVC and CVC neurons was not observed when nociceptors from the contralateral hindlimb were stimulated and when cold stimuli were applied to the abdominal skin. With the latter stimulus in some cases, the reverse to the reciprocal response pattern seen to noxious stimulation of the ipsilateral hindlimb was elicited (depression of the activity in MVC neurons and activation of CVC neurons). Reflexes elicited from the hindpaws and abdominal skin were observed under all three anesthetics used. 6. Stimulation of visceral afferents from the urinary bladder (by passive distension and by isovolumetric contractions of the organ) elicited almost only excitatory reflexes in either type of vasoconstrictor neurons. Distension of the colon had no effect on the vasoconstrictor neurons. Reflexes to stimulation of visceral afferents were only investigated with the use of urethan anesthesia. 7. Systemic hypercapnia excited most MVC neurons (81%) and 50% of the CVC neurons and depressed the ongoing activity in 35% of the CVC preparations. 8. The results show that the reflex patterns in MVC neurons and CVC neurons supplying the rat hindlimb in principle show similar differentiation as in the cat. However, the differentiation is less pronounced. These reflex patterns are probably an expression of the central organization of both vasoconstrictor pathways in spinal cord, brain stem, and hypothalamus