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Synapses
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
Synapses can be divided into two broad categories: electrical and chemical. Electrical synapses have conductive pathways between the presynaptic and postsynaptic cells so that electric charge is transferred directly through the synapse, as discussed in Section 6.6, thereby changing the membrane voltage of the postsynaptic cell. The majority of synapses, however, are chemical synapses in which the presynaptic and postsynaptic cell membranes are separated by a narrow cleft, 20–40 nm wide. The operation of chemical synapses is similar to that of the neuromuscular junction (NMJ) but differs from it in many important respects, as will be clarified in this and the following sections. Overall, an action potential (AP) in the transmitting, or presynaptic neuron, produces in the postsynaptic neuron a change in membrane voltage from the resting value, termed the postsynaptic potential (psp), as illustrated diagrammatically in Figure 6.1. Operationally, the depolarization due to the AP in the presynaptic neuron triggers the release of a chemical substance, the neurotransmitter, from the presynaptic membrane. The neurotransmitter diffuses across the synaptic cleft and binds to specialized receptors in the postsynaptic membrane. In the case of ionotropic receptors, the binding of the neurotransmitter to the receptor opens an ion channel that allows ions to flow between the cytoplasm of the postsynaptic cell and the extracellular medium, thereby changing the membrane voltage of the postsynaptic cell. In metabotropic receptors, the binding of the neurotransmitter to the receptor may trigger changes in cell metabolism via intracellular second-messengers, eventually leading to the gating of ion channels, as discussed in Section 6.3. As to be expected, the direct channel-gating by ionotropic receptors is faster than the indirect gating by metabotropic receptors, producing a change in the voltage of the postsynaptic membrane within a few milliseconds or less of the depolarization of the presynaptic terminal. Consequently, synapses having ionotropic receptors are termed fast chemical synapses. However, second-messenger systems are of great diversity and can be highly complex, with far-reaching effects on many aspects of cell function.
Network dilution and asymmetry in an efficient brain
Published in Philosophical Magazine, 2020
Marco Leonetti, Viola Folli, Edoardo Milanetti, Giancarlo Ruocco, Giorgio Gosti
In this paper, we assume a discrete-time RNN composed of McCulloch–Pitts neurons [3,26] which is one of the most synthetic and minimal models capable of capturing the essential properties of a real cortical neuron network. Monte et al. [27] and Carnevale et al. [28] showed how RNNs can be used to model real cognitive processes, respectively, Monte et al. [27] presented a model that discusses how the prefrontal cortex integrates context information, and Carnevale et al. [28] presented a model that describes the response modulation of the premotor cortex. This simple model incorporates the notion of multiple inputs (postsynaptic potentials), a threshold, and a single output (action potential). In such model, the state of the ith neuron is described mathematically by a discrete, two valued variable (0, 1), and the dynamic of the system is given by the discrete-time difference equationwhere is the total postsynaptic potential of the ith neuron at time t. is the state of the jth neuron at time t, 0 (resting state) or 1 (firing state).Θ = 1 if h and 0 otherwise, η can be viewed as a threshold. is the connectivity matrix and represents the stored connections.
Cholinesterase activity as a potential biomarker for neurotoxicity induced by pesticides in vivo exposed Oreochromis niloticus (Nile tilapia): assessment tool for organophosphates and synthetic pyrethroids
Published in Environmental Technology, 2023
Muhammad Amin, Masarrat Yousuf, Mohammad Attaullah, Naveed Ahmad, Mohamad Nor Azra, Mehreen Lateef, Islam Dad Buneri, Ivar Zekker, Gaber El-Saber Batiha, Salma Mostafa Aboelenin, Muhammad Zahoor, Muhammad Ikram, Muhammad Naeem
Pesticides are highly harmful chemicals when discharged into the environment in an untreated manner. Though pesticides are used for the benefit of human beings, they also cause stress to non-targeted organisms. The important sources of environmental pollution by pesticides are from agriculture practices, in the use by the public health and release by industrial discharges [1]. Aquatic pollution caused by pesticides needs immense attention because of their adverse effects on the aquatic organisms including fish mortality. Pesticides reach into the water resources by surface leaching from the agricultural lands and enter the organisms either through food chains or by contact water [2]. Even chronic exposure, to sub-lethal dose of pesticides has shown mortality [3]. Organophosphate (OP) pesticides were first introduced in Germany during 1930s [4]. These pesticides are one of the utmost importantly used groups of pesticides utilized till present in agriculture as they are effective and highly biodegradable and not persistent in the environment [5]. However, there is a lack of accurate dosing developed for them and it was reported that they are also extremely toxic to non-target aquatic organisms. OP pesticides are extremely neurotoxic as they inhibit acetylcholinesterase (AChE) enzymes to block nerve impulses in both – central and peripheral nervous systems [6]. Sub-lethal exposures to this chemical can cause damage to several functions, including respiration, reproduction and behaviour [7–9]. The measurement of the AChE activity is used as a biomarker for organophosphate pesticides in the fish tissue [10–12]. Biomarkers are most significant as they contribute information collection regarding the effects of pollutants on the biological systems [13]. Several reports of the organophosphate pesticides toxicity on fish have been reported [14–16]. Malathion (OP) blocks active sites of the AChE enzyme in the nerve endings leading to trembling, convulsion and finally death of vertebrates [17,18]. A reduction in the total protein content and an alteration in the activities of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) enzymes were reported in the brain, gills and muscle tissues of Oreochromis niloticus as a result of malathion, chlorpyripos and lambda-cyhalothrin toxicity [19,20]. Lambda-cyhalothrin is extremely toxic to fish having a great capability to accumulate in the fish muscles [21,22]. Since fish has a weak competency to metabolize such xenobiotics, these pesticides are more toxic to fish as compared to other vertebrates [23–25]. Lambda-cyhalothrin interacts with acetylcholine, a neurotransmitter and causes neurotoxicity due to the inhibition of the AChE enzymes that result in a prolonging excitatory postsynaptic potential terminating [26–29]. Overstimulation of the muscle fibres due to excited neurone, generate severe problems, like paralysis and eventually, death [30]. Thus, the AChE activity could be used as a toxicity biomarker in aquatic ecotoxicology [29,31].