Short answers, pulled from the story.
A neuron is an electrically excitable cell that fires signals called action potentials across neural networks in the nervous system. Neurons communicate with each other via synapses, releasing chemical neurotransmitters across a synaptic gap to transmit signals to target cells. They are the primary components of nervous tissue in all animals except sponges and placozoans.
The human brain contains approximately 86 billion neurons. The cerebral cortex alone holds an estimated 10 to 20 billion neurons, while the cerebellum contains 55 to 70 billion. Each neuron has on average 7,000 synaptic connections to other neurons.
The German anatomist Heinrich Wilhelm Waldeyer introduced the word neuron in 1891, drawing from the ancient Greek for sinew or cord. The neuron's role as the basic functional unit of the nervous system was established through the work of the Spanish anatomist Santiago Ramon y Cajal, who published his neuron doctrine based on observations of the bird cerebellum in 1888.
The myelin sheath insulates the axon, allowing action potentials to travel faster and using less energy than in unmyelinated axons of the same diameter. In the central nervous system, myelin is formed by oligodendrocytes; in the peripheral nervous system, Schwann cells form the sheath. The sheath runs in segments roughly 1 millimeter long, interrupted by nodes of Ranvier where the electrical signal is refreshed.
Glutamate and GABA together account for more than 90 percent of neurotransmitter activity in the brain. Glutamate is the primary excitatory transmitter, while GABA is inhibitory, driving chloride ions into the postsynaptic neuron to reduce firing probability. Dopamine modulates mood and behavior, and loss of dopamine neurons in the substantia nigra is linked to Parkinson's disease.
Molecular evidence suggests the ability to generate electric signals first appeared in evolution approximately 700 to 800 million years ago, during the Tonian period. The predecessors of neurons were peptidergic secretory cells, which eventually gained gene modules enabling the creation of post-synaptic scaffolds and ion channels capable of producing fast electrical signals.