You are looking at a neuron under a microscope. you find many structures coming off the cell body. some of these are relatively thick and branch many times. one of the structures, however, is very thin and very long. this latter structure is most likely the neuron's which carries the

Axon; action potential

Explanation:

The neurons are the cells that are responsible for carrying the information from the brain to other effector organs of the body and vice versa.The basic structure fo a neuron consists of. a cell body, the dendrites, and the axons. The diagram has been shown to highlight these parts.The axon is the longest part of the neuron along which the action potential propagates and reaches the axon terminals fro where the neurotransmitters are released to pass on the signal to another neuron or effector organ. The dendrites are branch-like structures that come off the cell body and these are at the receiving end. Therefore, when observed under the microscope the thin and long structure of a neuron is an axon.