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What evolutionary process has provided humans with the ability of feeling electric current? Besides lightning and electric eel, what natural hazards include electricity that poses a threat to humans?

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Electrical currents stimulate neurons aspecifically. For example, the BrainPort artificial vision device conveys visual information through electrical stimulation of mechanoreceptors in the tongue. Similarly, cochlear implants and retinal implants convey acoustic and visual information through electrical stimulation of the inner ear and retina, respectively. The list goes on. Bottom line is, none of these organs (tongue, ear or eye) evolved to process electrical stimuli. And in fact, there are no specific current or voltage sensors in man. Electric currents aspecifically stimulate neurons, causing them to fire. When sensory afferents are activated in this way, sensory perceptions are generated.

In case of lightning (electrical shock to the skin) it is mainly pain and heat receptors that mediate the sensation. In the case of the BrainPort, it is the tactile receptors on the tongue that are mainly stimulated (taste perceptions are rare). In case of cochlear implants it is auditory nerve fibers and in retinal implants it is optic nerve fibers that are stimulated. Hence, various, and in fact all, neurons can be stimulated by electric current.

Note that sharks feature Ampullae of Lorenzini. These sensory organs are electroreceptors, which allow sharks to sense the bio-electrical fields of prey under water.

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Electricity directly stimulate neurons. You have to understand that neurons basically work via electric currents , which occur due to a difference in membrane charge. So when you give electricity to a neuron, you basically give electrons to the extracellular membrane, which makes it much more negative compared to the intracellular or the stuff that is inside the membrane. This offsets the concentration gradient that kept the careful balance until now, and then neuron fires. Anyway, i hope you got that. Note that you can't directly "feel" electricity, or electrostatic force. That is impossible, nature has not given us the mechanism to do so. What you feel is a direct consequence of electricity interfering with the functioning of your muscles and nerves.

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Electricity is essential in order to control and coordinate neurons and skeletal muscles activities. You should imagine a neuron as a very complicated logic port of an electronic circuit, that receives information through its dendrits from other neuronal cells and then integrate this information in its body. If the spatial and temporal summation of inhibiting and stimulating signals is overthreshold, then at level of the first part of the axon, there is the creation of the all-or-null response of the neuronal cell: the action potential. In general, there is the simultaneous activation of the voltage-gated sodium channels that elicit spikes spreading them along the whole axon that is connected to other neuronal dendrits or skeletal muscle cells. These ones, receive and integrate in the same manner the stimuli sent to them. As you can easily imagine, external electric stimuli, like electrostimulators or instrument used by neurologist to analyze neuronal and muscle activities in diagnostic tasks are able to alterate the natural electric potential present across the cell membranes of these excitable cell types and response to this electric field. Indeed, cells possess and actively keep an electric potential among the two sides of the phospholipidic bilayer of the cell membrane of about -70mV. This one is due to the asimmetric distribution of ionic species (sodium, potassium and chloride mainly) sustained using chemical energy obtained from exogenous sources. Thus, in general, our cells are not able to feel electricity directly but every phenomena, either phisical or chemical, able to alterate the electrical equilibium (homeostasis) of the cell membranes induce a response at inner cellular level leading, for example, to neuronal cell response or skeletal muscle contraction. You can think to the use of defibrilator to treat cardiac arithmias that send to the hearth strong current pulse-wave, that in case of fibrillation or ventricular tachycardia is frequently capable to restore normal cardiac activity and save life. Also pacemaker acts in a similar manner. Some pathologies like ALS, Alzheimer's, Parkinson, Myotonia involve also alteration of this electric mechanism leading to modified capacity of the cells to handle and interpret these electric message. In electrophysiology research, external stimulation of cells by electrodes (see patch clamp technique) is used to test the effect of drugs to treat these pathologic conditions. Finally, I can say that evolution didn't give us the ability to feel electricity, instead it has been adapted as a cellular-level instrument handled by cells to communicate and integrate (or store in brain) stimuli and information.

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