K�ltezittern bei narkotisierten spinalen Hunden

Shivering in the cold: from mechanisms of fuel selection to survival

In cold-exposed adult humans, significant or lethal decreases in body temperature are delayed by reducing heat loss via peripheral vasoconstriction and by increasing rates of heat production via shivering thermogenesis. This brief review focuses on the mechanisms of fuel selection responsible for sustaining long-term shivering thermogenesis. It provides evidence to explain large discrepancies in fuel selection measurements among shivering studies, and it proposes links between choices in fuel selection mechanism and human survival in the cold. Over the last decades, a number of studies have quantified the contributions of carbohydrate (CHO) and lipid to total heat generation. However, the exact contributions of these fuels still remain unclear because of large differences in fuel selection measurements even at the same metabolic rate. Recent advances on the mechanisms of fuel selection during shivering provide some plausible explanations for these discrepancies between shivering studies. This new evidence indicates that muscles can sustain shivering over several hours using a variety of fuel mixtures achieved by modifying diet (changing the size of CHO reserves) or by changing muscle fiber recruitment (increasing or decreasing the recruitment of type II fibers). From a practical perspective, how does the choice of fuel selection mechanism affect human survival in the cold? Based on a glycogen-depletion model, estimates of shivering endurance show that, whereas the oxidation of widely different fuel mixtures does not improve survival time, the selective recruitment of fuel-specific muscle fibers provides a substantial advantage for cold survival. By combining fundamental research on fuel metabolism and applied strategies to improve shivering endurance, future research in this area promises to yield important new information on what limits human survival in the cold.

Fuel selection in shivering humans

Heat exchange has been thoroughly studied in cold-exposed humans, but the metabolic substrates used for thermogenesis have received less attention. This review deals with oxidative fuel selection in shivering humans. Lipids provide most of the heat during low-intensity shivering, whereas carbohydrates become dominant under more extreme cold conditions. The contribution from plasma glucose always remains minor, but muscle glycogen plays an important role during intense shivering. Whether the size of muscle glycogen stores influences endurance in the cold remains to be demonstrated. The fuel selection patterns of shivering and exercise are different, but the mechanisms underlying this difference have not been investigated. The simultaneous measurement of metabolic substrate oxidation and muscle fibre recruitment has allowed to characterize two different mechanisms of fuel selection in shivering humans: the recruitment of different pathways within the same fibres and of different fuel-specific fibres within the same muscles. This suggests that muscle fibre composition of each individual may affect survival. Future research promises to provide a combination of theoretical advances on fundamental principles of fuel selection and applied strategies to manipulate fibre composition (through training) or fuel metabolism (through diet) to prolong human survival in cold environments.

Fuel selection during intense shivering in humans: EMG pattern reflects carbohydrate oxidation

The thermogenic response of humans depends critically on the coordination of muscle fibre recruitment and oxidative fuel metabolism. The primary goal of this study was to determine whether the electromyographic (EMG) pattern of muscle recruitment could provide metabolic information on oxidative fuel selection during high-intensity shivering. EMG activity (of 8 large muscles) and fuel metabolism were monitored simultaneously in non-acclimatized adult men during high-intensity shivering. Even though acute cold exposure elicited similar changes in metabolic rate among subjects, lipid and carbohydrate use was very different. Depending on the subject, the cold-induced increase in carbohydrate (CHO) oxidation ranged between 2- and 8-fold, with CHO accounting for 33-78% of total heat production, and lipids for 14-60%. This high variability in fuel selection was primarily explained by differences in 'burst shivering' rate, indicating that the recruitment of type II fibres plays a key role in orchestrating fuel selection. This study is the first to show that the pattern of muscle recruitment can provide quantitative information on energy metabolism. Future work should focus on the study of shivering bursts that may provide essential clues on what limits human survival in the cold.

Neuronal circuitries involved in thermoregulation

The body temperature of homeothermic animals is regulated by systems that utilize multiple behavioral and autonomic effector responses. In the last few years, new approaches have brought us new information and new ideas about neuronal interconnections in the thermoregulatory network. Studies utilizing chemical stimulation of the preoptic area revealed both heat loss and production responses are controlled by warm-sensitive neurons. These neurons send excitatory efferent signals for the heat loss and inhibitory efferent signals for the heat production. The warm-sensitive neurons are separated and work independently to control these two opposing responses. Recent electrophysiological analysis have identified some neurons sending axons directly to the spinal cord for thermoregulatory effector control. Included are midbrain reticulospinal neurons for shivering and premotor neurons in the medulla oblongata for skin vasomotor control. As for the afferent side of the thermoregulatory network, the vagus nerve is recently paid much attention, which would convey signals for peripheral infection to the brain and be responsible for the induction of fever. The vagus nerve may also participate in thermoregulation in afebrile conditions, because some substances such as cholecyctokinin and leptin activate the vagus nerve. Although the functional role for this response is still obscure, the vagus may transfer nutritional and/or metabolic signals to the brain, affecting metabolism and body temperature.

Hypothermia due to an ascending impairment of shivering in hyperacute experimental allergic encephalomyelitis in the Lewis rat

Severe hypothermia and an ascending impairment of shivering are previously undescribed clinical signs in hyperacute experimental allergic encephalomyelitis (EAE) in the Lewis rat. These occurred in hyperacute EAE induced by inoculation with guinea pig spinal cord homogenate and heat-killed Bordetella pertussis. Hypothermia was first detected on day 6-7 post-inoculation, within 12-24 h of the onset of neurological signs, and became more severe as the disease progressed. Rectal temperatures less than or equal to 30 degrees C were common at ambient temperatures of 19-22 degrees C. Shivering was assessed by palpation and by cold tremor electromyography. Shivering was absent in the tail by day 6-7 post-inoculation. The impairment then progressed to affect the hindlimbs, thorax and occasionally the forelimbs. Shivering was absent in hindlimbs with only mild or moderate weakness. Histological studies revealed perivascular inflammation with polymorphonuclear and mononuclear cells, oedema, fibrin deposition, haemorrhage, primary demyelination and axonal degeneration in the spinal cord, dorsal root ganglia and spinal roots. The brainstem was also involved but the cerebral hemispheres, including the hypothalamus, were spared. The close relationship between the severity of hypothermia and the extent of shivering impairment indicates that reduced shivering is an important cause of hypothermia in hyperacute EAE. It is concluded that this impairment of shivering is due not to hypothalamic damage but to lesions elsewhere in the central and peripheral nervous systems.

Spectral analysis of tremorine and cold tremor electromyograms in animal species of different size

In 14 mice (36.2 +/- 5.4 g), 17 rats (425 +/- 46.4 g), and 11 rabbits (3,200 +/- 340 g) a comparative electromyographic analysis of tremorine tremor and cold tremor was performed in gastrocnemius and tibialis anterior muscles using spectral analysis. The mean frequency of cold tremor decreased with increasing body weight (mice: 40.2 +/- 4.5 Hz; rats: 31.3 +/- 4.9 Hz; rabbits: 16.4 +/- 3.2 Hz). With tremorine tremor no such allometric correlation was found for tremor frequency and body weight (mice: 17.7 +/- 3.6 Hz; rats: 19.6 +/- 5.1 Hz; rabbits: 15.9 +/- 2.1 Hz). Cross spectral analysis revealed that during cold tremor the flexor muscle (tibialis anterior) and the extensor muscle (gastrocnemius) of rabbits are activated alternately. The mean phase shift between the activation of flexor and extensor muscle was -155.5 degrees. Stronger activation was observed in the flexor muscle. Tremorine tremor was characterized by synchronous activation of flexor and extensor muscles with a mean phase angle of 3.0 degrees and a predominance of the extensor muscle. The results suggest that the nervous mechanisms for the generation of tremorine tremor and cold tremor are different.

Initiation of muscle activity in spinalized pigeons during spinal cord cooling and warming

1. The effect of spinal cord temperature changes on muscle activity was investigated in unanaesthetized intact and chronically spinalized pigeons and in acutely spinalized pigeons which were artifically respirated and lightly anaesthetized with ether. 2. Spinal cord cooling regularly produced an increase in muscle activity and visible muscle tremor in intact and spinalized pigeons. This motor cold defence reaction was less intensive in spinalized animals, but was qualitatively identical in all groups with regard to spindle shaped firing patterns and grouped discharges. 3. Intravenous injection of 60-100 mg/kg L-Dopa enhanced the motor response to spinal cord cooling in acutely spinalized pigeons. It is suggested that L-Dopa may act on dopaminergic or noradrenergic neurones in the spinal cord. 4. The results demonstrate that the generation of the motor cold defence response to cooling of the spinal cord in pigeons is basically independent from supra-spinal nervous mechanisms. The decreased intensity of cold induced muscle activity in spinalized animals may be attributed to the loss of excitatory or disinhibitory descending inputs to the spinal cord. 5. Spinal cord warming above normal body temperature (41.5 degrees C) produced an increase in muscle activity and slow muscle movements. The pattern was qualitatively different from that of the cold induced tremor.

Neural organization and evolution of thermal regulation in mammals

This article proposes a modification of the currently accepted view of the central neural integration of body temperature. In place of a single integrator with multiple inputs and outputs, the new model includes as many integrators as there are thermoregulatory responses. Futhermore, these integrators are postulated to be represented at many levels of the nervous system, with each level facilitated or inhibited by levels above and below. The purpose of such a complicated arrangement is to achieve finer and finer control over body temperature. A consideration of how endothermy might have evolved, with originally nonthermally related responses gradually coming under thermal control, makes such a brain organization highly reasonable.

Patterns of differentiation in various sympathetic efferents induced by hypoxic and by central thermal stimulation in decerebrated rabbits

The patterns of regional changes of sympathetic efferent activity evoked by thermal stimulation of the spinal cord and by arterial and primary tissue hypoxia were investigated in decerebrated, anesthetized and immobilized rabbits. Decerebration was performed either at the mid- or infracollicular level. The responses of the decerebrated rabbits evoked by spinal thermal stimulation were the same as those of intact rabbits, i.e., splanchnic and cardiac sympathetic activity increased and cutaneous sympathetic activity decreased during warming, while the reverse response was elicited by cooling. It is concluded that the typical thermoregulatory response pattern of the sympathetic nervous system can be produced also after the loss of hypothalamic integration, i.e., by integrative mechanisms in the lower brain stem and the spinal cord. In contrast, the responses of decerebrated rabbits to arterial and primary tissue hypoxia differed from those of intact rabbits in that they consisted in an overall activation in all investigated sympathetic branches. It is confirmed by this result that suprabulbar integration is essential for the generation of the inhibitory components in the differential sympathetic responses to hypoxia, which typically consist in cutaneous and cardiac sympathetic inhibition with splanchnic activation during arterial hypoxia and in cutaneous sympathetic inhibition with cardiac and splanchnic sympathetic activation during primary tissue hypoxia.

Effect of spinal deafferentation on temperature regulation and spinal thermosensitivity in pigeons

1. To study the effect of spinal deafferentation on temperature regulation and spinal thermosensitivity in acute experiments, the spinal cord of pigeons was transected at Th4 and the dorsal roots cut carefully on both sides from Th4 to C6 or C4 (13 or 15 segments); only afferent signals from the upper neck and the head could reach the CNS. Selective changes of the spinal cord temperature in the deafferented region were performed by a thermode in the vertebral canal. 2. At thermoneutral ambient conditions (Ta 23-30 degrees C) the deafferented pigeons were able to maintain a normal body temperature (about 41 degrees C). During ambient cooling (Ta 1-10 degrees C) the core temperature was regulated at a lower level of about 38 degrees C, strong shivering occurred, and heat production was increased. 3. If the decreased spinal cord temperature at low Ta was adjusted experimentally to normal values (about 41 degrees C) then shivering stopped and oxygen consumption decreased. This decrease in heat production was followed by a continuous fall in rectal temperature to values as low as 33-34 degrees C without any initiation of shivering or extra heat production. This means that shivering in the deafferented pigeons must be elicited by cold sensors in the spinal cord alone and that there are no important cold sensors in the non-deafferented region including the brain. 4. Selective spinal cooling of the deafferented region at thermoneutral Ta was followed by an immediate onset of shivering and an increase in heat production. Spinal heating resulted in an increase in wing temperature which served as an indication of vasodilatation, i.e., an activation of a heat loss mechanism. This is a confirmation of the assumption that the spinal temperature sensors are indeed located in the spinal cord and that the responses to experimental changes of spinal canal temperature are not mediated by extraspinal thermoreceptors. The results show clearly that the regulation of body temperature in pigeons at moderate thermal loads can be mediated by these spinal sensors alone. 5. Continued spinal cooling resulted in an increase in body temperature by about 2 degrees C and a subsequent regulation at this high level. This means that there must exist warm sensors in the non-deafferented cranial region.