Q. How does the nervous system get information about the external environment, the organs and the tissues?
Information about the conditions of the internal and external environments, like spatial position, touch, temperature, pressure, pH, metabolite levels (oxygen, carbon dioxide, and so on.), light, sounds, and so on., 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 dendrites of neurons or through their own axons that connect to them. The information reaches the central nervous system that interprets and uses it to regulate and control the body.