A chemoreceptor, also known as chemosensor, is a sensory receptor that
transduces a chemical signal into an action potential. In more general terms,
a chemosensor detects certain chemical stimuli in the environment.
Classes There are two main classes of the
chemosensor: direct and distance. Examples of distance chemoreceptors are:
olfactory receptor neurons in the olfactory system: Olfaction involves the
ability to detect chemicals in the gaseous state. In vertebrates, the
olfactory system detects odorants and pheromones in the nasal cavity. Within
the olfactory system there are two anatomically distinct organs: the main
olfactory epithelium and the vomeronasal organ(VNO). It was initially thought
that the MOE is responsible for the detection of odorants, while the VNO
detects pheromones. The current view, however, is that both systems can detect
odorants and pheromones. Olfaction in invertebrates differs from olfaction in
vertebrates. For example, in insects, olfactory sensilla are present on their
antennae. Examples of direct chemoreceptors
include: Taste buds in the gustatory system: The
primary use of gustation as a type of chemoreception is for the detection of
tasteants. Aqueous chemical compounds come into contact with chemoreceptors in
the mouth, such as taste buds on the tongue, and trigger responses. These
chemical compounds can either trigger an appetitive response for nutrients, or a
defensive response against toxins depending on which receptors fire. Fish
and crustaceans, who are constantly in an aqueous environment, use their
gustatory system to identify certain chemicals in the mixture for the purpose
of localization and ingestion of food. Insects use contact chemoreception to
recognize certain chemicals such as cuticular hydrocarbons and chemicals
specific to host plants. Contact chemoreception is more commonly seen in
insects but is also involved in the mating behavior of some vertebrates. The
contact chemoreceptor is specific to one type of chemical.
Sensory Organs Olfaction: In vertebrates, olfaction
occurs in the Nose. Volatile chemical stimuli enter the nose and eventually
reach the olfactory cleft. Embedded in the olfactory epithelium are three types
of cells: supporting, basal, and olfactory sensory neurons. While all
three types of cells are integral to the olfactory pathway, only OSN cells make
contact with the stimulus. After the stimulus is received by the cilia
protruding from the OSNs, the signal activates an action potential which then
travels through the bony cribiform plate to the glomeruli within the olfactory
bulb. While the proximity of the olfactory cleft to the brain is close,
the signals are often slow to illicit response and often require numerous
molecules to trigger the corresponding action potential. In insects, antennae
act as distance chemoreceptors. For example, antennae on moths are made up
of long feathery hairs that increase sensory surface area. Each long hair
from the main antenna also has smaller sensilla that are used for volatile
olfaction. Since moths are mainly nocturnal animals, the development of
greater olfaction aids them in navigating the night.
Gustation: In many vertebrates, the tongue serves as the primary gustatory
sensory organ. As a muscle located in the mouth, it acts to manipulate and
discern the composition of food in the initial stages of digestion. The tongue
is rich in vasculature, allowing the chemoreceptors located on the top
surface of the organ to transmit sensory information to the brain. Salivary
glands in the mouth allow for molecules to reach chemoreceptors in an aqueous
solution. The chemoreceptors of the tongue fall into two distinct
superfamilies of G protein-coupled receptors. GPCR’s are intramembrane
proteins than bind to an extracellular ligand- in this case chemicals from
food- and begin a diverse array of signaling cascades that can result in an
action potential registering as input in an organism’s brain. Large quantities of
chemoreceptors with discrete ligand-binding domains provide for the
five basic tastes: sour, salty, bitter, sweet, and savory. The salty and sour
tastes work directly through the ion channels, the sweet and bitter taste
work through G protein-coupled receptors, and the savory sensation is
activated by glutamate. Contact Chemoreception: Contact
chemoreception is dependent on the physical contact of the receptor with
the stimulus. The receptors are short hairs or cones that have a single pore
at, or close to the tip of the projection. They are known as uniporous
receptors. Some receptors are flexible, while others are rigid and do not bend
with contact. They are mostly found in the mouthparts, but can also occur on
the antennae or legs of some insects. There is a collection of dendrites
located near the pores of the receptors, yet the distribution of these dendrites
changes depending on the organism being examined. The method of transduction of
the signal from the dendrites differs depending on the organism and the
chemical it is responding to. Cellular antennae: Within the biological
and medical disciplines, recent discoveries have noted that primary
cilia in many types of cells within eukaryotes serve as cellular antennae.
These cilia play important roles in chemosensation. The current scientific
understanding of primary cilia organelles views them as "sensory
cellular antennae that coordinate a large number of cellular signaling
pathways, sometimes coupling the signaling to ciliary motility or
alternatively to cell division and differentiation."
When inputs from the environment are significant to the survival of the
organism, the input must be detected. As all life processes are ultimately based
on chemistry it is natural that detection and passing on of the external
input will involve chemical events. The chemistry of the environment is, of
course, relevant to survival, and detection of chemical input from the
outside may well articulate directly with cell chemicals.
Chemoreception is important for the detection of food, habitat, conspecifics
including mates, and predators. For example, the emissions of a predator's
food source, such as odors or pheromones, may be in the air or on a
surface where the food source has been. Cells in the head, usually the air
passages or mouth, have chemical receptors on their surface that change
when in contact with the emissions. It passes in either chemical or
electrochemical form to the central processor, the brain or spinal cord. The
resulting output from the CNS makes body actions that will engage the food and
enhance survival. In Physiology
Carotid bodies and aortic bodies detect changes primarily in oxygen. They also
sense increases in CO2 partial pressure and decreases in arterial pH, but to a
lesser degree than for O2. The chemoreceptor trigger zone is an
area of the medulla in the brain that receives inputs from blood-borne drugs
or hormones, and communicates with the vomiting center to induce vomiting.
= Control of Breathing= Particular chemoreceptors, called ASICs,
detect the levels of carbon dioxide in the blood. To do this, they monitor the
concentration of hydrogen ions in the blood, which decrease the pH of the
blood. This can be a direct consequence of an increase in carbon dioxide
concentration, because aqueous carbon dioxide in the presence of carbonic
anhydrase reacts to form a proton and a bicarbonate ion.
The response is that the respiratory centre, sends nervous impulses to the
external intercostal muscles and the diaphragm, via the intercostal nerve and
the phrenic nerve, respectively, to increase breathing rate and the volume
of the lungs during inhalation. Chemoreceptors that regulate the depth
and rhythm of breathing are broken down into two categories.
central chemoreceptors are located on the ventrolateral surface of medulla
oblongata and detect changes in pH of cerebrospinal fluid. They have also been
shown experimentally to respond to hypercapnic hypoxia, and eventually
desensitize . These are sensitive to pH and CO2.
peripheral chemoreceptors: consists of aortic and carotid bodies. Aortic body
detects changes in blood oxygen and carbon dioxide, but not pH, while
carotid body detects all three. They do not desensitize. Their effect on
breathing rate is less than that of the central chemoreceptors.
= Heart rate= The response to stimulation of
chemoreceptors on the heart rate is complicated. Stimulation of peripheral
chemoreceptors directly activates the medullary vagal center and slows the
heart rate. However, a number of other factors are usually at play in this
situation which obscure this response. These factors include activation of
stretch receptors due to increased ventilation and the release of
circulating catecholamines. Hence, although the stimulation of peripheral
chemoreceptors causes bradycardia, this may not be the net result.
See also Cell surface receptor
Chemosensory clusters Chemoreceptor trigger zone
Diffuse chemosensory system Haptic technology
Molecular sensor Odor
Olfaction Solitary chemosensory cells
Sensory receptor Taste
Vomeronasal organ References
External links Chemoreceptors at the US National
Library of Medicine Medical Subject Headings