External chemoreceptors sense a wide spectrum of molecules that figure in an animal's life. External chemoreceptors are examined first.
External chemoreceptors are sensory neurons that respond to specific molecules in the air (as gases, liquids, or solids in vapor phase, or solids suspended as aerosols). Aquatic and marine animals also have chemoreceptors that respond to specific molecules dissolved or suspended in water. The chemical senses include olfaction (smell) of airborne substances and gustation (detection of waterborne substances). Flavor is a complex sensation involving both gustation and olfaction when food is eaten. Substances sensed by olfaction generally affect animal behavior.
Many animals are known to release pheromones, which act as sexual attractants and stimulate courting and mating behavior. The best-known pheromones are certain insect attractants. Synthetic, female gypsy moth pheromone, for example, is used to bait traps that attract and kill the male moths, who follow the scent gradient upwind to the source. Most other insects make pheromones to attract mates, as do crustaceans and vertebrates. Chemical signaling to attract mates and stimulate mating is universally distributed in the animal kingdom.
The sensing and identification of complex odorants involves an animal's entire olfactory system, which is generally composed of many thousands of olfactory cells. Each olfactory cell has surface protein receptors that bind specifically to a unique odorant molecule. The binding generally triggers a complex internal sequence of biochemical events that lead to depolarization of the olfactory cell, and spike generation on its axon. The spike frequency increases monotonically with the odorant concentration.
The olfactory systems of vertebrates can generally be considered to be massively parallel arrays of chemosensory neurons. Their axons project into the olfactory bulb in the brain, which in turn sends interneurons to the amygdala. Comparative anatomy teaches that behavior governed by the sense of smell evolved long before mammals evolved a cortex and the ability to reason. Air- and waterborne molecules evoke complex behavioral modes, ranging from "keep out" (animals mark territory with urine, feces, scent glands), to sexual receptivity (pheromones), and in directions a food source is near (blood, amniotic fluid, etc.) Scent can also alter mood (infant smells mother), leading to nursing behavior and the release of endorphins, or elicit fear (animal smells predator), leading to the release of adrenaline. Many animals have specialized glands they use for territorial marking, usually in conjunction with urine. Odorants have been extracted from the feces of predators such as African lions and marketed for use in keeping deer out of gardens. The study of odorant molecules that affect animal behavior is called semiochemistry (Albone, 1997).
Much research today is being directed toward understanding how certain animal olfactory systems can detect (as evidenced by behavior) nanomole concentrations of certain odorants. The ability of bloodhounds to track a specific person through a crowd or to follow a days-old trail when searching for lost persons or fugitives is legendary. The noses of trained dogs are also the most effective system for locating buried land mines, explosives, or drugs in luggage and automobiles. What is not known exactly is to what molecules the dogs are responding. Is it the vapor phase of the analyte (e.g., C4, TNT, cocaine, etc), or is it some less obvious scent, such as the case or envelope containing the analyte, or is it human scent, or some combination? Because of the commercial importance of odors in the food and cosmetic industries, and in detecting drugs and explosives, much research is being invested in "artificial noses," using a variety of physical, immunochemical, electronic, and photonic means. It appears that humans have a long way to go to make a sensor with the threshold sensitivity of certain insect pheromonal chemoreceptors, or the bloodhound's nose.
Vertebrate chemoreceptor neurons are unique in that they undergo apoptosis (programmed self-destruction) about every 40 to 60 days and are replaced by new cells. The new cells send their axons into the olfactory bulb (first synaptic relay point for olfactory information), where they synapse with appropriate target mitral cells. The mitral cells, like all other neurons in the CNS, do not divide. Figure 2.2-1 illustrates a schematic section through a mammalian olfactory epithelium. Each olfactory cell has from 8 to 20 cilia at its end immersed in mucus. The cilia are from 30 to 200 |im in length, and their surfaces contain the odorant receptor molecules that initiate receptor spike generation. A mouse may have 500 to 1000 different odorant receptor proteins (Leffingwell, 1999). It is estimated that about 1% of the total genes specifying an entire mammal (mouse) are used to code its olfactory receptor proteins.
Airborne odorant molecules must dissolve in the mucus before they can reach receptor molecules on the olfactory cilia. Also floating in the mucus are odorant binding proteins (OBPs), which have been postulated to act as carriers that transfer odorants to their specific receptors on the cilia. The OBPs may act as cofactors that facilitate odorant binding, and they may also participate in the destruction of a bound odorant, freeing the receptor to bind again. OBPs are made by the lateral nasal gland at the tip of the nasal cavity (Kandel et al., 1991, Ch. 34). The exact role of the OBPs is unknown.
The mechanism of olfactory transduction is as follows. An odorant molecule dissolves in the 60-| m-thick mucus layer overlying the receptor cells and their cilia. It combines with a specific OBP and then collides with and binds to a surface membrane receptor protein on a cilium that has a high affinity for that odorant. The presence of the OBP may be necessary to trigger the next step, which is the activation of a second messenger biochemical pathway that leads to a specific ionic current and receptor depolarization. Odorant binding to the receptor protein activates a G-protein on the intracellular surface of the cilium. The a-subunit of the G-protein activates a molecule of the enzyme, adenylate cyclase, which in turn catalyzes the formation of a cyclic nucleic acid, cyclic-3',5'-adenosyl monophosphate (cAMP) from ATP. cAMP then binds to and opens a cAMP-gated Ca++ channel, causing an inward JCa++. The increase in intracellular Ca++ appears to activate an outward chloride current that depolarizes the receptor (Leffingwell, 1999). The more of a given odorant that binds to a specific chemoreceptor cell, the larger the total JCa++ and JCl-, and the greater the depolarization, and the higher the spike frequency on the axon of the receptor. cAMP then degrades by hydrolysis to AMP; the G-protein also returns to its resting state.
Receptor cell axons /
To olfactory bulb
Receptor cell axons /
FIGURE 2.2-1 Schematic section through a vertebrate olfactory epithelium. Note that there are receptors, and basal and supporting cells. (There is a total of about 105 olfactory receptor neurons in the rabbit.) (From Kandel, E.R. et al., Principles of Neural Science, 3rd ed., Appleton & Lange, Norwalk, CT, 1991. With permission from the McGraw-Hill Companies.)
The receptors on a given chemoreceptor cilia can have affinity to more than one odorant molecule. Also, more than one receptor can be activated (in varying degrees) by a given odorant. Hence, the very large chemoreceptor array must have a mechanism for eliminating odor ambiguity, since several receptors may respond to a given odorant in different degrees. A further complication to understanding the physiology of olfaction is the recent evidence that there may be a second, second-messenger pathway in certain olfactory chemoreceptors. A different odorant binds to its receptor, leading to the production of the messenger, inositol-1,4,5-triphosphate (IP3). IP3 opens calcium ion channels, allowing a JCa++ to flow inward, hyperpolarizing the cell (Breer, 1997). In lobster olfactory neurons, apparently the internal role of the IP3 and cAMP messengers is reversed; odorants that release the cAMP messenger cause hyperpolarization, while IP3 causes depolarization (Breer, 1997). Olfaction is a complex process!
How are specific odorant receptors recycled? A problem in any sensory communication system is to preserve sensitivity to a new stimulus. It is known that when an odorant binds to its receptor, the second messenger cascade is initiated; and has been recently discovered that the second messenger cascade is inhibited by the second messengers activating a protein kinase, which phosphorylates the receptor protein, interrupting the transduction process (Breer, 1997). It is also reasonable to assume that the odorants are slowly broken down enzymatically. Olfactory systems are extremely sensitive, but (perhaps mercifully) adapt profoundly to a sustained, external odorant concentration to the point where one is not conscious of the odor.
Bundles of 10 to 100 axons from the ciliated olfactory chemoreceptors project through the pores in the ethmoidal cribiform plate on the roof of the nasal cavity into the olfactory bulbs of the brain, where the first level of signal processing takes place. In structure, the olfactory bulb has several layers of interneurons, not unlike a retina. Figure 2.2-2 illustrates schematically the basic, five-layered structure of a mammalian olfactory lobe. Not shown is the tremendous convergence of information in the olfactory bulb. For example, in the rabbit, about 26,000 receptors converge into ~200 glomeruli which then converge at 25:1 on each mitral cell (Leffingwell, 1999). Humans have about 5 million olfactory receptors; those having one specific receptor appear to be distributed randomly in the nasal epithelium. However, their axons sort themselves out and converge on the same area of the optic bulb. Thus, there appears to be order out of chaos and a basis for some sort of fuzzy combinatorial logic to identify complex scents containing several odorants (Berrie, 1997).
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