Chemoreception is the biological recognition of chemical stimuli, by which living organisms collect information about the chemistry of their internal and external environments. Chemoreception has three sequential stages: detection, amplification, and signaling.
In detection, a molecule typically binds to a chemoreceptor protein on the surface of a cell, changing the shape of the chemoreceptor. All chemoreceptors therefore have some degree of specificity, in that they only bind to specific molecules or specific classes of molecules.
In amplification, the cell uses energy to transform the shape change of the chemoreceptor into biochemical or electrical signals within the cell. In many cases, amplification is mediated by formation of cAMP (cyclic adenosine monophosphate), which increases the cell's permeability to sodium ions and alters the electrical potential of the cell membrane.
In signaling, the amplified signal is transformed into a physiological or behavioral response. In higher animals the nervous system does the signaling, while in single-celled organisms signaling is intracellular, which may be manifested as chemotaxis, a directional movement in response to a chemical stimulus.
Detection, amplification, and signaling are often connected by feedback pathways. Feedback pathways allow adjustment of the sensitivity of the chemoreceptive system to different concentration ranges of the elicitor molecule. Thus, the sensitivity decreases as the background concentration of the molecule increases; the sensitivity increases as the background concentration of the molecule decreases.
Chemoreceptive systems detect chemical changes within an organism (interoreception) or outside an organism (exteroreception). The most familiar examples of exteroreception in humans are the senses of taste and smell.
Humans have chemoreceptor cells for taste in taste buds, most of which are on the upper surfaces of the tongue. Each human has about 10,000 taste buds and each taste bud consists of about 50 cells. An individual taste bud is specialized for detection of a sweet, sour, salty, or bitter taste. The sense of smell is important in discriminating among more subtle differences in taste.
Human chemoreceptors in the nasal cavity can discriminate thousands of different odors. One theory of odor perception in humans proposes that each chemoreceptive cell is connected to a single neuron and that an odorant molecule binds to many different chemoreceptors with different affinities. Thus, the neural signals from many different neurons can be integrated in many different ways to yield a rich panoply of odor sensations.
Many chemoreception systems also collect information about the internal environment of multicellular organisms. For example, the carotid body in the carotid artery of humans has chemoreceptive cells which respond to changes in the pH and oxygen levels in the blood. As the amount of dissolved oxygen in the blood decreases, chemoreceptive cells in the carotid body emit an electrical discharge, which stimulates specific neurons in the hind brain respiratory centers to increase the rate of breathing. The hypothalamus in the human brain has chemoreceptive cells which respond to changes in blood glucose levels. When blood glucose levels fall, the chemoreceptive system causes a person to feel hungry; when blood glucose levels rise, this chemoreceptive system causes a person to feel satiated. The endocrine and nervous systems also have many other chemoreceptive cells which signal different organs within the body to change their activity.