Although animal neuroscience is an established and accepted fact, the neurobiology of plants remains controversial despite the fact that electrical signaling in plants was described by M.L. Berthelon in De l'Electricité des Végétaux (Aylon, Paris) 1783, eight years before the first reference of animal electrical signaling by L. Galvani in 1791. This is likely because plant responses to environmental stimuli are significantly (1000 to 100,000 times based on measured refractory periods for action potentials (APs) in Lupinus shoots by Adam Paszewski and Tadeusz Zawadzki, Action Potentials in Lupinus angustifolius L. Shoots (Maria Curie-Sklodowska University, Lublin, Poland 1976)) slower than those in animals (with the exception of a few - the touch-sensitive mimosa (Mimosa pudica) and Venus flytrap (Dionaea muscipula) that require speed to close their leaves and shut their traps since in general, plants do not require the speed of animals to escape predators or capture prey) and because of flawed views that persisted until recently that plants are helpless, passive organisms at the mercy of their environment with little need for rapid signaling.
In reality, plants possess neurobiology analogous to cnidarian nerv
e nets, in which the existence of a brain or central nervous system is not a prerequisite. This should not be surprising when considering the identical nature between plants and animals as pointed out by Frantisek Baluska, Dieter Volkmann, Andrej Hlavacka, Stefano Mancuso and Peter W. Barlow in Neurobiological View of Plants and Their Body Plan (Communication in Plants, Springer-Verlag Berlin Heidelberg 2006) in that both rely on identical sexual processes utilizing fusion between sperm cells and oocytes (female egg cells), both develop immunity when attacked by pathogens, and both use the same methods and means to drive their circadian rhythms (patterns of biological activity synchronized to day-night cycles). In addition, plants and animals transmit electrical signals over both short and long distances and rely on the same pathways and molecules to control their physiological responses (e.g. movement in animals, growth in plants).
Cnidarians and Plants: Convergent Neurobiology
Plants and cnidarians (e.g. anemones, hydra, jellyfish) have analogous nervous systems, in which stimuli is communicated via a nerve network or web of interconnecting neurons. Neither have a brain (though some theories postulate that root apices may serve as a brain in plants) or central nervous system in the context of advanced animal life. Consistent with plant neurobiology, in which a network of electrical and chemical signaling is used to detect and respond to environmental stimuli (biotic and abiotic), cnidarians do not feel pain per se; they merely react to stimuli.
Cnidaria (a phylum of over 9000 simple aquatic animals) rely on decentralized nerve nets consisting of sensory neurons that generate signals in response to stimuli, motor neurons that instruct muscles to contract and "cobwebs" of intermediate neurons. Hydras rely on a structurally simple nerve net to bridge sensory photoreceptors and touch-sensitive nerve cells located on their body wall and tentacles. Jellyfish also depend on a loose network of nerves located within their epidermal and gastrodermal tissue (outer and inner body walls, respectively) to detect touch and a circular ring throughout the rhopalial lappet located at the rim of their body. Intercellular communication occurs in cnidaria through electronic signaling via synapses or small gaps across which electro-chemicals (called neurotransmitters) flow.
Cnidarian nerves (unlike those in advanced species) rely on neurotransmitters on both sides of their synapses enabling bi-directional action potential (AP) transmission. Cnidarian neurons communicate with all other neurons wherever they cross with such communication utilizing at least three specific pathways without preference. Basically, in cnidaria, stimuli at any point results in an impulse that radiates away in every direction providing optimal intercellular communication throughout the organism.
In both plants and cnidaria, electrical signals are transmitted through non-nerve tissues, from cell to cell through utilization of gap junctions. These gap junctions in a plant's cell wall are called plasmodesmata.
Consistent with cnidaria, plants rely on action potentials (AP) and synaptic intercellular communication utilizing auxin as their primary neurotransmitter with vascular strands representing nerves. Like cnidarians, plants rely on electrical signaling and developed pathways (phloem and sieve tubes in vascular plants; non-phloem tissue in non-vascular plants such as algae and liverworts) analogous to a nerve net "to respond rapidly to environmental stress factors (e.g. insect herbivory, pathogens, mechanical damage, etc.)" and environmental conditions (e.g. changes in temperature, light intensity, water availability, osmotic pressure, and the presence of chemical compounds). Through electrical signaling, plants "are able to rapidly transmit information over long distances... at the tissue and whole plant levels from leaves to roots and shoots and vice versa through utilization of ion channels."
VPs are slower non-self-propagating electrical signals that are elicited by stimuli that trigger a change in potential (depolarization and subsequent repolarization) at the plasma membrane of parenchyma cells that reside adjacent to xylem vessels due to a rapid loss of turgor. They are characterized (based on Eric Davies, Electrical Signals in Plants: Facts and Hypotheses (Plant Electrophysiology) by "a sharp rise followed by a lingering decline, often with spikes." Unlike APs, membrane repolarization is delayed and the signals are graded and travel at varying amplitudes (which are reduced as the distance increases) based on intensity of the stimulus. VPs also utilize the sieve element plasma membrane and plasmodesmata. VPs travel at a speed of between Electrical Signals in Plants: Facts and Hypotheses APs are generally caused by non-damaging stimuli such as "electrical stimulation, light/dark transitions, brief cooling, pollination [and sometimes] excision" and VPs are caused by damaging stimuli such as "severe wounding, [organ excision, and flaming]." SPs are generally caused by cutting and cations.5. Per Carol Kaesuk Yoon, Plants Found to Send Nerve-Like Messages (The New York Times, November 17, 1992) when electrical signals were permitted to flow freely from a caterpillar damaged tomato plant leaf, unaffected leaves initiated chemical defense mechanisms; when such electrical signals were blocked, no such defense response was initiated; and when movement of hormones was blocked, unaffected leaves still initiated defense mechanisms proving that electrical rather than chemical signals activate a plant's defens
Plants, through the use of neurobiology analogous to that of cnidaria, in which the presence of a brain and central nervous system are not prerequisite for interpretation and response, have proven their ability to respond intelligently and rapidly to complex environmental stimuli. Although plants and cnidaria do not feel pain per se as advanced animal life, both, because of their respective neurobiology are not "unfeeling" and can sense, perceive and respond accordingly. Electrical signals in plants and cnidaria provide an efficient means to rapidly transmit information systemically, detect and respond to danger (whether predators - insect herbivory with regard to plants, or pathogens) and other environmental stresses. In short, with their capacity to sense and perceive, plants are not passive, unfeeling organisms as defined by flawed paradigms
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