Sodium action potentials in placozoa: Insights into behavioral integration and evolution of nerveless animals

Neural versus alternative integrative systems: molecular insights into origins of neurotransmitters

“…It was recently shown that placozoans exhibited allor-none sodium-dependent action potentials, perhaps to support the rapid propagation and integration of electrical and chemical signals across the animal and cell layers [254]. A specialized meshwork of fibre cells (figure 9d ), located in the middle layer of cells, was considered to be an analogue of the neural and muscular systems [255].…”

Divergent Ca2+/calmodulin feedback regulation of CaV1 and CaV2 voltage-gated calcium channels evolved in the common ancestor of Placozoa and Bilateria

“…Trichoplax adhaerens and Hoilungia hongkongensis cells were recently shown to exhibit action potentials (21). Extracellular recordings of Hoilungia crystal cells, which are identified as gravity sensors in the placozoa (70), revealed action potentials ranging in duration from 1 to 3 ms, and with a frequency of ~130 Hz (21).…”

Selective Advantages of Synapses in Evolution

“…Black arrows indicate specialized contact sites (Dumoux and Hayward, 2016 ) to the nucleus with encapsulated bacteria within ER-type structures with ribosomes (dotted areas). (C) The left image shows the organization disk-shaped nerveless placozoan, Trichoplax , which contains no recognized neurons, muscles, or sensory organs but displays coordinated behaviors and action potentials (Smith et al, 2015 , 2019 ; Senatore et al, 2017 ; Armon et al, 2018 ; Varoqueaux et al, 2018 ; Fortunato and Aktipis, 2019 ; Romanova et al, 2020b ). The middle image shows the schematic reconstruction of a placozoan fiber cell with prominent sites (blue dots) of putative secretory/paracrine (non-synaptic) regions (modified from scanning and transmission electrom microscopy datasets (Romanova et al, 2021 ).…”

“…This hypothetical Urmetazoan likely used electrical and chemical communications for behavioral control as in the present-day placozoans or sponges. In placozoans, non-synaptic signaling is mediated by small secretory peptides and low molecular weight transmitters, including NO, ATP, glutamate, glycine, GABA (Nikitin, 2015 ; Varoqueaux et al, 2018 ; Moroz et al, 2020a , b , 2021b ; Romanova et al, 2020a , b ).…”