Synapse

A SYNAPSE is the point of contact between the terminal branches of the AXON of one neuron and the DENDRITES of the next, the two being separated by a fine gap called the synaptic cleft.

The essential point, and the one that earns the mark, is that the neurons DO NOT TOUCH.

HOW THE IMPULSE CROSSES: it travels ELECTRICALLY along a neuron but cannot jump the gap. At the synapse it is converted into a CHEMICAL signal carried by the neurotransmitter ACETYLCHOLINE, and turned back into an electrical impulse in the next neuron.

WHY THIS ARRANGEMENT MATTERS: because transmission across the gap works in ONE DIRECTION only, the synapse ensures that impulses travel one way through the nervous system. If neurons were joined directly, impulses could run backwards and the system would be chaotic.