5.1 Natural Chemo-sensory Systems
Natural chemo-sensory systems provide information from four groups of senses:
General chemical sense: All organisms display this sense. For humans, this sense is mediated by free neurons in the skin.
Olfaction: The sense of smell, generally regarded as a distance sense.
Gustation: The sense of taste, generally regarded as a contact sense. Separating olfaction and gustation is difficult as the cellular and molecular mechanisms can be the same. We could try to separate the two as either atmospheric or fluid medium, but this breaks down in describing the two senses for underwater life forms.
Solitary chemo-receptor cells (SCCs): Best developed in a few species of fish. The receptors are scattered in the fin surfaces and provide information on the presence of food or predators.
5.1.1 Chemo-sensory capability in simple life-forms
The earliest life-forms on earth were the prokaryotes, which are cellular organisms with no nuclei, and the eukaryotes, which do have nuclei. It is believed that these organisms had the world to themselves for about two billion years. Much of our understanding of the molecular biology of chemo-sensitivity comes from experiments with contemporary bacteria called Escherichia coli, or e. coli.
Moving bacteria are propelled by flagella, which are long cilia- or hair-like protrusions that twist or turn in response to chemical stimuli. Some will rotate at around 100 Hz, energized by a transmembrane hydrogen ion concentration gradient. E. coli has 5-10 flagellum that are on either side. When all rotate counter-clockwise, the bacterium moves forward toward a chemical attractant, while when they all rotate clockwise, the motion is a random tumbling motion. With no chemical attractants, the movement is sporadic and random; with attractant present, the motion is the same except there is less tumbling when moving toward the source. The overall motion is a migration toward the source of chemical attractant.
Deep study into certain internal chemosensory system mechanisms will quickly merge into endocrinology (internal secretions) and biochemistry. The olfaction and gustation systems, however, are driven by chemical information external to the organism. Our interest is more on these exteroceptor sensory systems than on the interoreceptor sensory driven systems [Smith08].
5.1.2 Gustation in insects
Chemo-sensory receptors in insects are frequently multi-modal, serving as both a mechano-sensory receptor and a chemo-sensory receptor. The multi-modal sensilla (hairs) protrude from the outer cuticle (shell) with a terminal pore at the tips of the sensilla. Chemicals can enter the pores and travel to the nearby dendritic inputs. Bio-chemical chain reactions result from the combinations of certain chemicals with the nerve endings. These same sensilla would also have another neuron sensitive to the mechanical distortions on the sensilla caused by fluid movement or direct contact or pressure [Smith08].
5.1.3 Gustation in mammals
There are six basic taste qualities [Smith08]:
The first four are in the order of taste receptor cells (TRCs) in the human tongue from the tip and working back. Umami is a Japanese word for the taste of monosodium glutamate, a crystalline salt used for seasoning foods (C5H8O4NaN). Gustatory receptors in mammals are grouped into taste buds, which are located on projections called papillae. Four types of papillae include
filiform – contains no taste buds; serves to give tongue abrasive character (as in cats)
fungiform – resemble mushrooms; located on front and edges of the tongue; visible red spots sensitive to sweetness and saltiness; buried in the surface epithelium
foliate – located in folds at the rear of the tongue; sensitive to sourness or acidity
circumvallate – sunken in moat or trench; sensitive to sourness or bitterness
From the tip of the tongue to the back, the primary qualities that stimulate the taste buds are in this order:
1) sweetness, 2) saltiness, 3) sourness, and 4) bitterness. Taste Receptor Cells (TRCs) typically have dendrites to multiple taste buds. Similarly, each taste bud may provide input to multiple TRCs. New nerve endings “search out” new synaptic contacts as taste buds are turned over. Thus, there is a complex connection scheme of taste buds to associated TRCs. There is ongoing debate as to whether the brain recognizes different tastes by specific fiber activity or by a pattern of activity across the population of fibers [Smith08].
5.1.4 Olfaction in insects
Insect hygro-receptors, which detect humidity, are classed as olfactory (distant receptors) as there is no opening for direct contact to the environment. These sensilla are typically short pegs within a cuticular cavity. Humidity causes sufficient mechanical distortion for receptor signaling, which would explain why they are set within a cuticular cavity: normal contact with the environment will not falsely send a humidity signal.
Hygro-receptors have been detected on the antennas of all insects that have been carefully examined. Although present in all these species, they are typically very sparse among other sensilla. For example, on the cockroach, about one in every 500 sensilla is a hygro-receptor. Hygro-receptor neurons share the same sensilla with other hygro-receptor neurons and with thermo-receptor neurons.
Insect olfactory sensilla are typically multi-porous, allowing extra opportunity for the detection of a semiochemical, which is a chemical stimulant, or pheromone, with carries a specific meaning, such as a mating opportunity, danger, trail, aggregation, or dispersal. Social insects rely on trails and patches of semiochemicals. The detection of the sex pheromone is the most effective, which makes sense considering the importance of reproduction to survival. A male silkworm moth can detect a single molecule of the female pheromone. A single antenna consists of many branches, each having many sensilla. Each antenna has about 17,000 sensilla that are each 100 microns long and 2 microns in diameter. The large number of sensilla effectively amplify the detection of faint odors in windy conditions.
Olfaction begins with a chemical binding of the attractant molecule to an odorant-binding or pheromone-binding protein. There is increasing evidence that the subsequent biochemistry involving G-Protein membrane signaling is the same as found in vertebrate olfactory systems. This suggests a common process that has been developed throughout the animal kingdom [Smith08].
Rheotaxis and Anemotaxis
Insects such as moths use odor-gated anemotaxis, which means the insect moves in response to odorants present in the air currents. The moth’s flight path is modulated by odor concentrations. One simple strategy for anemotaxis is demonstrated by the male moth moving toward an attractant released by the female moth. When an attractant is detected, male moth flies upwind, and when the odorant plume is lost, it zig-zags across wind, increasing distances. If the male moth detects the attractant again, it simply flies upwind.
Lobsters move there antennas back and forth to detect a source of food underwater. However, lobsters do not use rheotaxis, which is basically underwater anemotaxis, since the underwater currents are far too turbulent for that to work. Their irregular and variable tracks to source and increased speed in middle of track suggests lobsters (and other marine animals) are steered toward plume sources by odor patches, not odor-stimulated up-current movements like the moth. An interesting description is provided by [Consi94]: “Lobsters smell via paired antennules, small antennae positioned medially to the large antennae. Each antennule contains an array of thousands of sensory cells arranged in a tuft of hairs. The antennules can act as discrete time sampling sensors: under conditions of low flow they periodically ‘flick’, ejecting a parcel of water and allowing a new packet to enter the tuft of sensory hairs for a new measurement.”
5.1.5 Olfaction in mammals and other vertebrates
Fish make incredible use of the sense of olfaction. Sharks and dogfish can detect blood and other body fluids from long distances. Salmon can use their olfactory sense to return to their spawning ground by tracing faint chemicals unique to their place of birth.
Receptor field mapping is obvious in the visual, auditory, somatosensory, and (to a lesser extent) the gustatory systems. Olfactory systems do not exhibit a receptive-field mapping corresponding to spatial location of the external environment. It does appear that there are three or four expression zones, where each zone represents each of the various types of molecular stimulants.
Individual olfactory receptor cells (ORC’s) are tightly embedded between supporting olfactory epithelium cells, with up to 20 cilia that detect stimulants and transmitting action potentials to the next layer of cells, the mitral cells. Photoreceptors in the vision system are also embedded between epithelium cells, but photoreceptors transform photonic flux into graded (analog) signals for processing by the next layers in the retina instead of action potentials. There is a convergence of about 1000 ORC’s to one mitral cell, and about 25 mitral cells to form one glomerulus. All 25000 or so ORC’s (in the rabbit) that converge to a glomerulus are specialized to detect one (or similar) odorant molecule, so that each glomerulus responds to one specific odor type [Smith 00]
5.1.6 Similarities in vision and olfactory systems: the retina and the olfactory bulb
The following table is a summary of some similarities between the preprocessing stages in the vertebrate vision and olfactory systems. In both the retinal and the olfactory bulb there are two layers of cells connected orthogonal to the direction of information flow that mediate or inhibit the forward flow. This mediation serves to accent the locations of stimuli within the receptor layer and minimize the signal energy propagated to the deeper neuronal processing layers in the brain.
|Retina||Olfactory Bulb||Preprocess Information|
|Photoreceptors||Olfactory Receptor||Receive stimulus|
|(Rods and Cones)||(ORCs)|
|(graded output)||(spiked output)|
|Horizontal||Periglomerular||Mediate (Inhibit) nearby response|
|Bipolar||Mitral/Tufted||Pass on mediated signal|
|Ganglion||Mitral/Tufted||Pass on mediated signal|
|100:1||1000:1 or 25000:1||Receptor signal compression|
|(1:2 fovea)||(ORC:Glom) (ORC to OT)|
5.1.7 Coarse-coding in vision and chemo-sensory systems
There are a relatively few specialized ORC types from which we can discern many different smells. Specific odors cause specific patterns of responses to ORC types, so odors are analyzed by spatial maps in the central nervous system like the way other distant senses are mapped [Smith08]. A ‘model nose’ [Persaud82] is discussed later in this chapter where the authors searched for unique patterns of many odorants (over 20) using the responses of only three commercially-available sensors. This demonstrates coarse coding, previously defined as the transformation of raw data using a small number of broadly overlapping filters. The power of coarse coding is that detailed resolution can be achieved with relatively few broadly overlapping sensor responses. A handful of broadly-overlapping sensors can provide raw data for identifying thousands of different categories (smells, tastes, etc.).
Gustation (taste) sensory systems are like olfactory ones in that there are relatively few types of receptor cells whose responses overlap significantly across numerous input types (tastes). A lot of work has gone into understanding psychophysics, or documenting behavioral responses to organism inputs, as well as microbiology for understanding neuronal and other cellular responses to environmental inputs. A significant gap in knowledge exists for explaining how the individual cellular responses are combined and biologically processed to give the overall behavior.
As previously mentioned, in vision systems coarse coding exists in time, space, color, spatial frequency and temporal frequency domains, and here we find in olfactory and gustatory domains as well. With relatively simple (or low-order) filters or sensory responses biology offers a high degree of acuity in these sensory information domains. We saw that in vision systems there are essentially only four broadly-overlapping chromatic detector types, three basic temporal channels, and three basic spatial channels. Neurons receiving broadly-overlapping photoreceptor responses provide higher brain processing areas the ability to discern details in color, time, and spatial domains. Similarly, broadly-overlapping olfactory and gustatory receptor responses provide higher brain processing areas the ability to discern many distinct odors and tastes.