We analyze the phase-space structure of a model for thermoreceptors in fish
and mammals. As a function of the temperature we identify a period doublin
g scenario at low temperatures, a regime where an unstable stationary fixed
point collides with the attractor and blocks the thermoreceptor, and a tra
nsition from period n+1 to period n as the temperature is further increased
. The period reduction phenomenon is due to an autoresonance between fast a
nd slow ion channels and shows the features typical for mode locking.