![]() Neurons are typically classified into three types based on their function. The ability to generate electric signals was a key innovation in the evolution of the nervous system. ![]() They eventually gained new gene modules which enabled cells to create post-synaptic scaffolds and ion channels that generate fast electrical signals. Predecessors of neurons were the peptidergic secretory cells. Molecular evidence suggests that the ability to generate electric signals first appeared in evolution some 700 to 800 million years ago, during the Tonian period. Non-animals like plants and fungi do not have nerve cells. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. Neurons communicate with other cells via synapses, which are specialized connections that commonly use minute amounts of chemical neurotransmitters to pass the electric signal from the presynaptic neuron to the target cell through the synaptic gap. Within a nervous system, a neuron, neurone, or nerve cell is an electrically excitable cell that fires electric signals called action potentials across a neural network.
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