Information about the conditions of the external and internal environments, like temperature, pressure, touch, spatial position, pH, metabolite levels (oxygen, carbon dioxide, etc.), light, sounds, etc., are collected by specific neural structures (each for each type of information) called sensory receptors.
Sensory receptors are distributed throughout the tissues according to their specific roles. The receptors get that information and transmit them through their own axons or through dendrites of neurons that connect to them. The information reaches the central nervous system that interprets and uses it to control and regulate the body.