Chemical neurons

Models for biochemical switches, logic gates, and information-processing devices that are also based on enzymic reactions but do not use the cyclic enzyme system were also introduced [76,115,117-122]. Examples of these models are presented in Table 1.3. It should also be mentioned that in other studies [108,112-114,116], models of chemical neurons and chemical neural networks based on nonenzymic chemical reactions were also introduced. 

Assumptions 1 to 5 as in model 13. The state of the chemical neuron is allowed to change only at discrete times, dictated by an autonomously oscillating catalyst e. The concentration of e is very small except during short intervals, e interacts with Aj or of each neuron, and rapid equilibrium occurs when its... 

A neuron is either on or off depending on the signals it has received. A chemical neuron [10-13] is a similar device. Consider the hypothetical reaction mechanism (fig. 4.1)with the following elementary reaction steps [1] ... 

There is constant inflow of h and a constant outflow of X2t. The stationary states of this reaction system, obtained by setting the time derivatives of the intermediates equal to zero, are plotted in fig. 4.2. For small values of the catalyst concentration C, the concentration of A is small and at C = 1 the value of A increases abruptly and remains large thereafter. (The numbers are arbitrary. We use roman letters for species and italic letters for concentrations.) The reaction mechanism represents a chemical neuron, either on or off, as some parameter changes. 

Next we consider the construction of logic gates by coupling of chemical neurons [2], For each neuron in the network there is one copy of the reaction mechanism (fig. 4.1) the neurons are mechanistically similar but chemically distinct. In fig. 4.3 we show a possible coupling of neuron j to neuron i. We choose an enzyme mechanism in which the concentration of either Aj or Bj becomes an activator or an inhibitor of an enzyme... 

Consider the biochemical reaction mechanism shown in fig. 4.8, which occurs in glycolysis [7] note the similarity in the structure of this mechanism to that of a model of a chemical neuron shown in fig. 4.1. Calculations of the stable stationary states of the biochemical system of fig. 4.8 are plotted in fig. 4.9. The change from high to low concentration of either compound is not as abrupt as that in fig. 4.2 for the chemical model, but is clearly present. We have identified a computational element, a fuzzy chemical neuron, in a biochemical reaction. 

The left-hand side gives the Kohonen network, which can be investigated by clicking on the neuron. The contents of the neuron, here the chemical structures, are shown in an additional window plotted on the right-hand side of the figure. 

A data set can be split into a training set and a test set randomly or according to a specific rule. The 1293 compounds were divided into a training set of 741 compounds and a test set ot 552 compounds, based on their distribution in a K.NN map. From each occupied neuron, one compound was selected and taken into the training set, and the other compounds were put into the test set. This selection ensured that both the training set and the test set contained as much information as possible, and covered the chemical space as widely as possible. 

Next wc turned our attention to the question of whether wc could still sec the separation of the two sets of molecules when they were buried in a large data set of diverse structures. For this purpose we added this data set of 172 molecules to the entire catalog of 8223 compounds available from a chemical supplier (janssen Chimica). Now, having a larger data set one also has to increase the size of the network a network of 40 X 30 neurons was chosen. Training this network with the same 49-dimcnsional structure representation as previously described, but now for all 8395 structures, provided the map shown in Figure 10,4-9. 

Even in this fiiirly diverse data set of structures, the dopamine and benzodiazepine agonists could be separated quite well only two neurons had collisions between these two types ol compounds. Even more importantly, however, we now know in which chemical space one would have to search For new lead structures for dopamine or for benzodiazepine agonists. 

The concept of discrete neurotransmitter recognition sites or receptors on nerve cells was based on work on systems physiology and dmg action (1). It was not until 1921 however, that it was shown that information could be transferred between neurons via a chemical, in this instance acetylcholine [51-84-3] (ACh), C H gN02 (1). 

Leukotrienes and Prostanoids. Arachidonic acid (AA) (213) and its metabohtes are iavolved ia cellular regulatory processes ia all three principal chemical signaling systems endocrine (see Hormones), immune, and neuronal (62). FoUowiag receptor activation or iacreased iatraceUular... 

The human brain is comprised of many millions of interconnected units, known individually as biological neurons. Each neuron consists of a cell to which is attached several dendrites (inputs) and a single axon (output). The axon connects to many other neurons via connection points called synapses. A synapse produces a chemical reaction in response to an input. The biological neuron fires if the sum of the synaptic reactions is sufficiently large. The brain is a complex network of sensory and motor neurons that provide a human being with the capacity to remember, think, learn and reason. 

Maintenance of electrical potential between the cell membrane exterior and interior is a necessity for the proper functioning of excitable neuronal and muscle cells. Chemical compounds can disturb ion fluxes that are essential for the maintenance of the membrane potentials. Fluxes of ions into the cells or out of the cells can be blocked by ion channel blockers (for example, some marine tox-... 

Chemical neurotransmission is the way in which neurons communicate by releasing chemical substances that are received by the receptors in the next neuron (or the target) and excite or inhibit it. About 50% or more of dtug mechanisms are based on modification of chemical neurotransmission. 

Diuretics This indicates the unique property of capsaicin-sensitive primary afferent neurons to release mediators (neuropeptides and others) from both peripheral and central nervous system terminals upon adequate stimulation. Capsaicin and other chemical (protons) or physical (heat) stimuli release mediators from both peripheral and... 

The epithelium covering the nasal cavity. This epithelium contains numerous cell types including the specialized olfactory sensory neurons which detect the chemical stimuli derived from smells by a specific family of G protein-coupled receptors known as olfactory receptors. 

Jones BE (2005) From waking to sleeping neuronal and chemical substrates. Trends Pharmacol Sci 26 578-586... 

A synapse is a contact site between two neurones, where information is communicated from the axon of one neurone (the presynaptic) to the cell body, the dendrites or the axon of the second neurone (the postsynaptic). In most synapses, the information is communicated chemically ... 

There are numerous transmitter substances. They include the amino acids glutamate, GABA and glycine acetylcholine the monoamines dopamine, noradrenaline and serotonin the neuropeptides ATP and NO. Many neurones use not a single transmitter but two or even more, a phenomenon called cotransmission. Chemical synaptic transmission hence is diversified. The basic steps, however, are similar across all neurones, irrespective of their transmitter, with the exception of NO transmitter production and vesicular storage transmitter release postsynaptic receptor activation and transmitter inactivation. Figure 1 shows an overview. Nitrergic transmission, i.e. transmission by NO, differs from transmission by other transmitters and is not covered in this essay. 

The nervous impulse can be observed at the interface between the axon of a neuron and the dendrite of the next neuron. The ionic (Na K+, Ca2+) and chemical (neurotransmitters) nature of the nervous impulse has been stated and clarified during the past decades. Most of the neurotransmitters have an ionic nature. So the nervous impulse contains both ionic (electrical) and chemical information, and most of the carriers have been modulated in a different way. 

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