Analysis of switching neurons within the thermoafferent system.

A total of 740 midbrain, thalamic, and hypothalamic neurons were tested as to their distribution and response/stimulus characteristics by means of thermal stimulation of the scrotal skin. The most frequent response type exhibited a basic discharge rate up to a threshold temperature and then switched to a maximum firing rate. The inverse behaviour was found in a low proportion. The temperature range in which the neurons switch from the 'low' to the 'high' state has been called the 'operating range' of the neuron. This study reveals, in contrast to former studies, that the 'operating range' is extremely narrow. This holds both for the individual neuron and for the populations studied. It is confirmed by simultaneous recordings and by cross correlation analysis that there is only one temperature threshold for all warm responsive neurons involved. It is concluded that according to the mean firing rate, temperature may only be discriminated as to below or above threshold. This means that a binary information is transmitted. For continuous temperature sensation and regulation either further neuronal types disposing of a continuous temperature/frequency characteristic or additional coding mechanisms have to be assumed. The major task of the switching neurons, integrating information from all over the body should be the generation of a trigger signal for warm defence.