Sensory, or afferent, systems are the means by which the nervous system receives information about the external environment (exteroception), the internal environment (Lnteroception), and the position and movement of the body (proprioception).
The body uses sensory information to generate reflex movements (e.g., a blink of the eye when it is touched, withdrawal of a limb from a hot surface, contraction of the bowel when it is stretched) without the participation of the conscious parts of the brain.
Much (but not all) sensory information is also directed to the cerebral cortex for conscious perception.“Sensation” is the conscious perception of sensory stimuli. It is impossible to know exactly what an animal (or another person, for that matter) sees, feels, hears, or smells. We infer what sensations an animal may experience by observing its reaction to various stimuli, by identifying homologies between human and animal sensory systems, and by imagining what we might feel in similar situations.
The experience of a given sensation as it is perceived at the cortical level has qualities that make it distinct from other types of sensations. This perceptual distinction defines the sensory modality. For example, the stimulus for the photoreceptors of the retina is light; the sensory modality that is experienced when photoreceptors are stimulated is vision. Somatic sensation or somatosensation describes modalities that arise primarily from innervation of body surfaces and musculoskeletal elements; it includes pain, touch, temperature, and position sense (proprioception). Special senses include smell (olfaction), vision, taste (gustation), hearing (audition), and equilibrium (vestibular sensations). Conscious sensory experiences arising from the viscera are limited primarily to pain, stretch, and pressure.
Certain animals have sensory systems that have no homology in human beings. For instance, migratory birds and some insects are able to sense geomagnetism of the earth, information they use for navigation. A number of species of fish can detect and generate electric fields. Other animals exploit the traditional sense modalities for highly specific (and expressly nonhuman) purposes. For example, cetaceans and bats both use highly modified auditory circuits for sonar navigation. in spite of growing anecdotal support, however, there remains no reliable evidence that domestic animals can sense an impending earthquake through a sensory experience unavailable to humans.