Effect of the rubrospinal and the midbrain reticulospinal neurons in the cold-induced muscle tone and shivering. An experimental study.

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Unit activities of 239 neurons in the nucleus ruber (Rb) and 184 neurons in the midbrain reticular formation (mRf) were investigated during thermal stimulation of midbrain, preoptic hypothalamus and skin in the urethaneanesthetized rat. Of these, 36 and 17 neurons were identified by antidromic stimulation of spinal cord as rubrospinal (RbS) and reticulospinal (RfS) neurons, respectively. Responsiveness to midbrain temperature (Tmb) was found in about 35% of RbS neurons and 43% of mRfS neurons. While Rb warm-sensitive neurons outnumbered slightly Rb cold-sensitive neurons, cold-sensitive neurons were more frequently (36%) found in the mRf than warm-sensitive neurons (7%). There was high degree of convergence of cold signals from hypothalamus and skin on the mRf Tmb cold-sensitive neurons, particularly in RfS neurons having Tmb cold sensitivity. On the other hand, RbS neurons showed various combinations of responsiveness to temperatures of different sites. Micro-injection of procain (1μg) into the mRf in lightly anesthetized rats decreased or completely abolished the cold-induced increase in muscle tone and shivering without changes in cardiovascular and respiratory parameters, but the injection into the Rb had no effect. The results suggest that mRfS neurons are involved in the control of thermoregulatory muscle tone and shivering.