Smith, Julia Elizabeth; (1999) Organisation of the nervous control of the rat tail circulation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Ongoing discharges of sympathetic postganglionic neurones (PGNs) innervating the tail vasculature of anaesthetised rats are characteristically rhythmical (T-rhythm). The T-rhythm is generated independently of central respiratory drive and activation of baro- and lung stretch receptors. The purpose of the present study is to determine; i) if the T-rhythm arises centrally, ii) if the T-rhythm is a robust feature of sympathetic outflows to thermoregulatory circulations, and iii) the locations of central neurones influencing the sympathetic outflow to the rat tail. In anaesthetised rats, sympathetic discharges were recorded from ventral collector nerves (VCN) which innervate the tail. When subjected to frequency domain analysis, such discharges formed a characteristic "T-peak" in autospectra. T-peak frequency (0.63 - 0.94 Hz; 4 animals, 16 data sets) was similar (P> 0.05; Mann-Whitney 2-tailed test) to T-rhythm frequency recorded focally from PGNs innervating the caudal ventral artery (cf. 0.63 - 0.95 Hz; 5 animals, 18 data sets). During central apnoea, T-peak frequency recorded from the left VCN (0.73 - 0.94 Hz; 4 animals) was similar to T-peak frequency recorded simultaneously from the right VCN (0.78 - 0.92 Hz). Coherence displayed between paired VCN recordings was significant at left VCN T-peak frequency, consistent with the view that the T-rhythm arises centrally and not within ganglia. Frequency domain analysis of sympathetic discharges recorded from saphenous nerves (supplies innervation to the hind foot and ankle; 11 animals), dorsal collector nerves (innervates the tail; 9 animals) and renal nerves (5 animals), during central apnoea, revealed that rhythmical discharges at T-rhythm frequency are a robust feature of sympathetic activity supplying thermoregulatory circulations, but not the kidney. The region containing the intermediolateral cell column (IML) of the spinal cord was stimulated whilst recording the evoked response in the ipsilateral VCN. The size of such responses was used as an index of sympathetic preganglionic neurone (SPN) contribution to the response. The majority of SPNs controlling tail sympathetic outflow were found in spinal segments L1 and L2. CNS cell groups that may regulate tail sympathetic outflow were identified using a transneuronal viral tracing technique. Neurones that projected to SPNs controlling tail sympathetic outflow were located near the ventral medullary surface, in the rostral ventrolateral medulla, caudal raphe and A5 region of the pons.

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