Chemical Stimuli

The speed with which taste stimulation occurs, coupled with the fact that stimulation with toxic substances does no damage to the receptors, led Beidler to suggest that taste stimulus need not enter the interior of the taste cell in order to initiate excitation. Because a taste cell has been shown to be sensitive to a number of taste qualities, and to a large number of chemical stimuli, he and his coworkers concluded that a number of different sites of adsorption must exist on the surface of the cell. Therefore, they assumed that taste response results from adsorption of chemical stimuli to the surface of the receptor at given receptor sites. This adsorption is described by a monomolecular reaction similar to that assumed by Renqvist, Lasareff, and Hahn, but with a difference. From the fact that each type of chemical-stimulus compound has a unique level of saturation of the taste receptor, it was concluded that the magnitude of the response is dependent on the initial reaction with the receptor, and not on other, subsequent receptor-reactions that are common to all types of receptor stimulation. Therefore, it was assumed that the magnitude of neural response is directly proportional to the number of sites filled, the maximum response occurring when all of the sites are filled. Beidler derived a fundamental... 

According to Fig. 6.17 the nerve cell is linked to other excitable, both nerve and muscle, cells by structures called, in the case of other nerve cells, as partners, synapses, and in the case of striated muscle cells, motor end-plates neuromuscular junctions). The impulse, which is originally electric, is transformed into a chemical stimulus and again into an electrical impulse. The opening and closing of ion-selective channels present in these junctions depend on either electric or chemical actions. The substances that are active in the latter case are called neurotransmitters. A very important member of this family is acetylcholine which is transferred to the cell that receives the signal across the postsynaptic membrane or motor endplate through a... 

Chemoreceptor A sensory receptor responding to a chemical stimulus (e.g., smell or taste) or change in the concentration of a chemical (e.g., H+ ions in the blood or PH). 

Several molecules participate in chemical stimulus detection 930 Sodium channels are key in conveying nociceptive information from the periphery to the CNS 930... 

Chemical cues hold considerable promise for manipulating behavior in vertebrates, provided we understand an animal s natural history, biology, and behavior well. However, the development of chemical attractants, stimulants, inhibitors, and repellents for vertebrates has progressed rather slowly for several reasons. First, chemical stimulus and behavior are not connected as rigidly as in insects, for example. Second, the same stimulus may elicit different behaviors, depending on the state of the recipient and the context. Third, chemical cues often are rather complex mixtures of compounds. Fourth, learning, especially early experience plays a major role in vertebrate, notably mammalian behavior. Finally, many behaviors are modulated by several sensory modalities so that chemical stimuli alone trigger only incomplete responses at best. 

The sensitivity and selectivity of olfaction and contact chemosensation are due (1) in the brain, to the existence of a neuronal network of neurons tuned to a specific chemical stimulus, and (2) in the periphery, to the existence of olfactory/ chemosensory receptor neurons housed in sensory microorgans called sensilla. The sensilla can best be viewed as simple cuticular porous extrusions that increase the surface that captures airborne odorants or chemicals dissolved in water droplets. They contain the receptive olfactory or chemosensory structures (Schneider, 1969). The olfactory sensilla are most numerous on the antennae and mediate the reception of sex pheromones and plant volatiles, as well as other odorants. Low volatility pheromones may also be detected by contact chemoreceptors on... 

In the biomechanical approach taken here it is assumed that any molecules that will eventually be interpreted as a chemical stimulus must first make physical... 

Molecules in air are in constant motion, regularly striking each other as well as any surfaces extending into the air. Therefore, an antenna in air will be regularly struck by molecules in the air (including any potential chemical stimulus molecules) at a rate that follows directly from physical laws. Ideal gas laws predict that the... 

There is no term in equation (21.3) for air flow. External air flow around organisms is sufficiently slow (subsonic) that it may usually be treated as incompressible (Vogel, 1994). This incompressibility means that the concentration of chemical stimulus molecules (n) will not be increased noticeably by the pressures that, develop adjacent to insect sensory hairs or antennae due to moving air (or moving antennae). The replacement of any captured molecules by the arrival of fresh odorant-laden air is the primary reason why air flow has such a dramatic effect on interception rate. One way of considering the influence of air flow is that at best the air flow could bring the interception rate closer to the limit predicted by equation (21.3). In order to discuss approaches more complex than that provided by equation (21.3), we have to consider the physical bases for molecular movements diffusion and convection. 

If a chemical stimulus consists of multiple chemical components which differ in the magnitude of their diffusion coefficients, it is of interest to consider whether or not these components will be intercepted by the sensory hairs at the ratios in which they are available in the air. This question may be addressed using the equations supplied above. 

An important issue associated with molecular machines is the detection of actuations on the nanoscale level. When a chemical stimulus induces movement in a machine, several spectroscopic techniques, such as nuclear magnetic resonance (NMR) spectroscopy, UV-Vis spectroscopy, emission spectroscopy and X-ray photoelectron spectroscopy (XPS) can be used to detect their outputs. More intri-guingly, electrochemical and photochemical inputs often provide [6, 8g] a two-fold advantage by inducing the mechanical movements and detecting them. Additionally, the dual actions of the these two types of stimuli can be exploited when the time-scale of the molecular actuations, which ranges from picoseconds to seconds, falls within the detection time-scale of the apparatus. 

The softness kernels are relevant to the remaining cases of two or more interacting systems. However, they do not by themselves provide sufficient information to constitute a basis for a theory of chemical reactivity. Clearly, the chemical stimulus to one molecule in a bimolecular reaction is provided by the other. That being the case, an eighth issue arises. Both the perturbing system and the responding system have internal dynamics, yet the softness kernel is a static response function. Dynamic reactivities need to be defined. 

Nalewajski [19-21] formulated a theory of static reactivity kernels prior to but closely related to that of Berkowitz and Parr [15] which started out from a second-order perturbative treatment of the total energy. He identified an external static potential of unspecified origin with the chemical stimulus and made the first connection between softness kernels and the total energy. His result is, in our notation,... 

Here, V(r) is the unspecified external potential. The static isoelectronic softness kernel s- fr, r ) entering Nalewajski s result is the same as that defined in Eq. (71). The chemical stimulus is thus the nonlocal quantity V(r) V(r ) in Nalewajski s theory, but still unspecified. 

Most chemists grow up with the notion that a pheromone is a chemical compound. As we have discussed above, the more broadly held view is that a pheromone is a meaningful chemical stimulus, which means that it can comprise more than one compound. Thus, we have attempted to draw clear distinction here between pheromone and pheromonal component(s) (or constituents), the latter being the individual compound(s) that are the causative agent(s) of the behavioral response associated with the pheromone s activity. To be clear, a... 

The main criteria used to classify the units in Table I were stimulus response measures i.e., the units discharged or were inhibited by different chemical compounds. In addition, other criteria were used to supplement the chemical stimulus response differentiation. Thus, the two main groups in the cat (acid units and amino acid units) can also be differentiated by spontaneous activity measures, latency to electrical stimulation, area of tongue innervated, and differential response to solution temperature (3-5). This comparative work has led to a modular view of peripheral taste systems in which the different neural groups are seen to have distinct receptors responding to distinct types of chemical signals (e.g., Br nsted acids and Br nsted bases), with either excitation or inhibition. The stimulus chemistry of these groups will be briefly described. 

If I can ascertain what another organism detects via olfaction, then I can perform experiments upon it, which cannot be performed on human subjects. The objective of such experiments—to find out how odor is coded—has yet to be achieved. Suppose the olfactory code were unraveled. Reproducing an odor would become a matter of replicating the pattern of neural responses without having to duplicate the chemical stimulus (much as cinematography appears to reproduce color without necessarily matching the complete spectroscopic profile of the original scene) (Robertson, 1992). 

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