Nerve cells invented electricity hundreds of millions of years ago
A neuron is an excitable cell that fires action potentials and talks across synapses. Plants and fungi lack them; sponges and placozoans skip neural tissue too. Molecular clues push electric signaling’s debut to roughly 700–800 million years ago in the Tonian.
A neuron is a cell built to be excitable. It keeps a voltage difference across its membrane, and when that voltage shifts far enough, fast enough, it fires an all-or-nothing pulse called an action potential. The pulse races down the axon to synapses, where tiny amounts of neurotransmitter usually carry the message across a gap to the next cell. Incoming signals can be excitatory or inhibitory, nudging the voltage that reaches the cell body up or down.
Inside, a neuron has the usual kit of nucleus, mitochondria and Golgi bodies, plus two special extensions. Dendrites branch heavily, reach a few hundred micrometres from the cell body and form fractal trees where most input arrives through dendritic spines. The axon leaves at a swelling called the axon hillock and can run as far as a metre in humans, longer in some other species. Cell bodies range from 4 to 100 micrometres across, and some neurons lack dendrites or an axon altogether.
Jobs fall into three groups. Sensory neurons react to touch, sound or light and report to the spinal cord and brain. Motor neurons drive muscle contraction and gland output. Interneurons link neurons within one region, and groups wired together form circuits. Most vertebrate neurons sit in the brain and spinal cord, but some live in peripheral ganglia and many sensory ones are embedded in organs such as the retina and cochlea.
Every animal except sponges and placozoans builds nervous tissue from neurons; plants and fungi have none. That molecular dating places the origin of electrical signalling in the Tonian, some 700 to 800 million years back. The ancestors of neurons were peptide-secreting cells that later picked up gene modules for ion channels and postsynaptic scaffolds. Most neurons are made by neural stem cells during development and childhood, and in most brain regions new ones largely stop appearing in adulthood.
Source: Neuron
