Reflex activation of rat fusimotor neurons by body surface cooling, and its dependence on the medullary raphé

Abstract

The nature of muscle efferent fibre activation during whole body cooling was investigated in urethane-anaesthetized rats. Multiunit efferent activity to the gastrocnemius muscle was detected when the trunk skin was cooled by a water-perfused jacket to below 36.0 ± 0.7• C. That efferent activity was not blocked by hexamethonium (50 mg kg −1 , I.V.) and was not associated with movement or electromyographic activity. Cold-induced efferent activity enhanced the discharge of afferent filaments from the isotonically stretched gastrocnemius muscle, demonstrating that it was fusimotor. Fusimotor neurons were activated by falls in trunk skin temperature, but that activity ceased when the skin was rewarmed, regardless of how low core temperature had fallen. While low core temperature alone was ineffective, a high core temperature could inhibit the fusimotor response to skin cooling. Fusimotor activation by skin cooling was often accompanied by desynchronization of the frontal electroencephalogram (EEG), but was not a simple consequence of cortical arousal, in that warming the scrotum desynchronized the EEG without activating fusimotor fibres. Inhibition of neurons in the rostral medullary raphé by microinjections of glycine (0.5 M, 120-180 nl) reduced the fusimotor response to skin cooling by 95 ± 3%, but did not prevent the EEG response. These results are interpreted as showing a novel thermoregulatory reflex that is triggered by cold exposure. It may underlie the increased muscle tone that precedes overt shivering, and could also serve to amplify shivering. Like several other cold-defence responses, this reflex depends upon neurons in the rostral medullary raphé.

Activation of 5-HT1Areceptors in medullary raphé disrupts sleep and decreases shivering during cooling in the conscious piglet

“…However, we cannot rule out the possibility that the effects of 8-OH-DPAT on shivering that we observed may have resulted from the activation of postsynaptic receptors on other spinally projecting neurons involved in shivering thermogenesis. Our data are consistent with recent studies showing that fusimotor activity is attenuated after "nonspecific" neuronal inhibition in the raphé with glycine (87). Our data further suggest that neurons expressing the 5-HT 1A receptor in the raphé, as well as those in the more lateral PGCL, modulate cold shivering activity, but neurons in the raphé are more involved in sympathetically driven mechanisms than those in the PGCL.…”

“…In recent experiments in conscious piglets, we showed that activation of 5-HT 1A receptors in the juxtafacial paragigantocellularis lateralis (PGCL), a region lateral to the raphé within the rostrocaudal dimension of the facial nucleus containing substantial numbers of 5-HT neurons (17,61), significantly reduced shivering during cooling, but had little or no effect on peripheral vasoconstriction (34). A role for medullary raphé neurons in modulating shivering is also suggested by recent evidence that the activity of fusimotor fibers innervating skeletal muscle during skin cooling can be attenuated by the microinjection of glycine into the medullary raphé (87). In another report, microinjection of 8-OH-DPAT or lidocaine into the raphé magnus attenuated shivering activity during cooling in conscious rats (7).…”

Serotonergic Neurons Activate Chemosensitive Retrotrapezoid Nucleus Neurons by a pH-Independent Mechanism

“…B2, Effect of SP on the discharge of RTN neurons in the absence of synaptic transmission blockers (control, n ϭ 5) or in the presence of these blockers (plus antagonists, n ϭ 6). *p Ͻ 0.001. pyramidal serotonergic neurons contribute to the regulation of thermogenesis by activating brown fat and by causing cutaneous vasoconstriction and shivering (Morrison, 2004;Nakamura et al, 2004;Tanaka et al, 2006). The upregulation of RTN by such serotonergic neurons may serve to activate breathing when CO 2 production increases as a result of shivering muscles or brown fat thermogenesis.…”

Seasonal Variations in Outdoor Activities in Adults With Spasticity

“…Although the exact mechanism of increased spasticity in cold temperatures remains untested, an increase in sensitivity of muscle spindles in response to cold weather reported in animal studies [9] may offer a possible explanation. Another possible mechanism may be related to hyperexcitability following UMN lesions.…”

“…Sachindri Wijekoon, Kim-Minh Tran-Nguyen, Farooq Ismail, Chris Boulias, Chetan P. Phadke increase in reflex excitability may also amplify the thermoregulatory reflex pathways [9] , underlying the shivering response that generates heat and restores body temperature [10] . Spasticity along with other signs of UMN lesions diminishes the individual's capacity to engage in self-care, productivity, and leisure occupations [11] .…”