These latter neurons project to the ciliary ganglia behind the eyes where they activate postganglionic neurons. The postganglionic neurons travel in cranial nerve III and activate the circular smooth muscle of the iris, which produces pupillary constriction. The receptive field of a touch sensory neuron is defined by the borders of its dendrites. This in turn, is influenced by the density of receptors in the skin. The greater the density of receptors, the smaller the receptive fields, the more sensitive that area of the skin is.
Sensory adaptation is the phenomenon of decreasing perception of a continual stimulus.
It is due to a stereotypical pattern of activation observed in phasic receptors, in which the frequency of APs to a particular stimulus declines over time. The brain integrates the information received from both hot and cold receptors. Equal activation of both receptor types is perceived as ambient temperature.