ii84 



HANDBOOK OF PH\SIOLOGV 



NKUROPHVSIOLOGV II 



FIG. 13. Upper part: Increase of respiratory rate (upper curve) 

 and rectal temperature [lower curve) in response to external body 

 warming in intact cat. Filled circles: Normally deep breaths. 

 Open circles: Shallow breaths (polypnea). Croises: Shallow 

 breaths with open mouth, moving jaws and salivation (panting). 

 Room temperature was initially I3°C, within squared field 

 34 to 38°. Lower part: Effect of similar body warming in an 

 unanesthetized cat i ' 2 mo- after bilateral electrolytic hypotha- 

 lamic destruction. Drowsiness at start of warming is more 

 conspicuous than in intact cats; eventual restlessness at end ol 

 warming is less marked. [From Hess & StoU (109).] 



Others none (68, 187). In this situation, a positive 

 result (a definite change) would weigh more than a 

 negative one but it should be obtained in a cross- 

 circulated region or in a baroceptor-denervated ani- 



mal to be fully significant. \Vhate\"er the answer may 

 be to this particular problem, the vasomotor re- 

 sponse to local hypothalamic warming appears to 

 imitate well the reaction of the intact animal to heat 

 stress. 



Mechanical stimulation of the hypothalamus may 

 also influence cutaneous blood flow. Chronic hypo- 

 thalamic lesions (129), as well as acute mesencephalic 

 transection (189), produce cutaneous vasodilatation, 

 an effect indicating that the thermoregulatory arterio- 

 venous anastomoses in the skin are normally under 

 tonic constrictor influence from the hypothalamus. 



Electrical stimulation in certain cortical areas and 

 notably in the hypothalamus influences cutaneous 

 i)lood flow. Both vasoconstriction and vasodilatation 

 may result, the effect depending on the one hand on 

 the pre-existing vasoconstrictor tone, on the other 

 hand on the frequency of stimulation (189). Reversal 

 of effector response with change of cerebral stimula- 

 tion frequency has been noted both for the circula- 

 tory and the gastrointestinal systems. One probable 

 explanation is suggested by the observations of Pitts 

 et al. (160) that electrical stimulation suppresses local 

 spontaneous activity, probably by its synchronizing 

 effect, so that low-frequency stimulation of spon- 

 taneously active neurons would result in a net de- 

 crease of output, while high-frequency stimulation 

 of relatively inactive neurons would result in a net 

 increase of output. 



Electrical stimulation of tlie anterior hypothalamus, 

 within the location of the thermodetectors, can thus 

 evoke cutaneous vasoconstriction (189), but it can 

 also evoke a coordinated heat-loss response in the 

 unanesthetized animal, as shown in figure 6, in- 

 cluding cutaneous vasodilatation, polypnea and sup- 

 pression of shivering (4). Another more specific re- 

 sponse can also be obtained, namely, a vasodilatation 

 in skeletal muscles due to sympathetic vasodilator 

 activity (62, 136), usually accompanied by cutaneous 

 vasoconstriction. A variety of vasomotor mechanisms 

 are tlierefore represented \\ithin that small region. 



Peripheral thermoregulatory mechanisms, in addi- 

 tion to their local effect, can also influence the re- 

 activity of the effector systems (i88); thus, local 

 cooling contracts blood vessels but also renders them 

 unreactive to the central command. 



Emotional elicitation of cutaneous vasodilatation 

 mav occur in man (blushing) which may interfere 

 with thermoregulatory measures, although to a 

 quantitatively insignificant degree. An analogous but 

 more prominent reaction may occur in animals. The 

 rabliit mav react to stress bv cutaneous vasodilatation 



