Chemically dependent stimuli

Stimuli that occur internally are classified as chemical or biological. The pH as well as the ionic strength, redox and solvent are defined chemically-dependent 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... 

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. 

At the end of each neuron are stores of chemicals called neurotransmitters that can be released to stimulate adjacent neurons (Figure 2.2). There are many different neurotransmitters, dependent on location and specific function in the nervous system. Generally, once a neuron is stimulated, the stimulus travels along the neuronal axon until it reaches the end of the neuron from which a neurotransmitter is released. The neurotrans-... 

Efficient gene delivery can also be mediated by chemically programmed polymers responding to artificial physical stimuli like heat. Depending on the stimulus, they have the ability to alter their conformation in so far as transfection enhancing effects are initiated. 

Synthesis of this enzyme is triggered by external stimuli, such as cytokines, released by cancer cells. Once synthesized, the enzyme produces large quantities of NO, which then diffuses into the tumor cells, disrupting DNA synthesis and inhibiting cell growth. The other NO synthases are present at all times, but are activated in a sequence of steps dependent on Ca concentration. An activated neuron releases a chemical messenger that opens calcium channels in the next neuron. As Ca enters the nerve cell, it binds with calmodulin and the NO synthase to activate it. The reactions described earlier for formation of NO take place, and the NO then activates another enzyme, guanylyl cyclase. From this point on, the effects are uncertain, but may include diffusion back to the first cell and reinforcement of the stimulus. One of the end results seems to be relaxation of smooth muscle, related to the effect seen in blood vessels. 

Animal studies show the olfactory and limbic pathways are particularly susceptible to kindling, the ability of a stimulus previously unable to induce a seizure to later induce one. Animal studies also show that acute administration of a high dose or intermittent repeated low-dose exposures to chemicals cause limbic kindling , and that this response is amplified depending on the time between stimuli. Kindling without a seizure has been shown to cause affective behavior changes in animals. Kindling could amplify reactivity and lower the threshold response to low levels of chemicals. 

---