Читать книгу The Behavior of Animals - Группа авторов - Страница 29

Sensory information to be processed comes from outside an organism’s CNS and must get first in contact with the nervous system via its receptors. This information concerns basic sensory modalities:

 Photoreception: response to radiant energy in the visible wavelength range of the electromagnetic spectrum (photons).

 Thermoreception: response to radiant thermal energy in the nonvisible wavelength range of the electromagnetic spectrum.

 Mechanoreception: response to kinetic energy, including hearing, vibration, touch, balance, etc.

 Chemoreception: response to chemical energy, including smell and taste.

Particular perceptual capabilities include electroreception (response to electrical energy) and magnetoreception (response to energy of a magnetic field). Nociception, the reception of pain, involves specific cell physiological responses to severe tissue damage caused by thermal, kinetic, and/or chemical energy. The form of energy to which the receptor cell responds determines the sensory modality. Within a sensory modality (e.g., vision), different stimulus qualities (color) and stimulus quantities (brightness) can be distinguished.

A stimulus, sensed by a receptor cell, is transduced by intracellular chemical processes (see Chapter 5). These lead to a change in the (receptor) membrane potential, which—depending on cell type—can generate nerve impulses. To respond to weak stimuli, receptors may have their own amplifying system. In rod photoreceptor cells of the vertebrate retina, for example, one photon absorbed by one molecule of rhodopsin gives rise to a signaling cascade of intracellular biochemical events that activate 6×10⁶ molecules of cyclic guanosine monophosphate, cGMP. This intracellular messenger influences the ion channels of the cell membrane leading to the receptor potential. Scent receptors in mammals have similar properties.