Synapse, a junction across which a nerve impulse travels. Firing in collective bursts, neurons are designed to send and receive electrical messages along a variable number of filaments. The longest of these (some reach up to three feet) carry messages away from the cell body, and are called axons. To speed up transmission, axons are padded with packets of fatty insulation, then bundled into gangs, wrapped, and rebundled into cables that can relay a signal at speeds over 200 miles per hour. Knoblike feet at the end of the axon contain sacs of chemicals (neurotransmitters) that further speed the process along.