The ways in which the words can be meaningfully combined is defined by the language's syntax and grammar. The actual meaning of words and combinations of words is defined by the language's semantics.
A nerve impulse is an electrical signal that travels along an axon. There is an electrical difference between the inside of the axon and its surroundings, like a tiny battery. When the nerve is activated, there is a sudden change in the voltage across the wall of the axon, caused by the movement of ions in and out of the neuron.
Definition of nerve for English Language Learners. : 1  one of the many thin parts that control movement and feeling by carrying messages between the brain and other parts of the body. : 2  courage that allows you to do something that is dangerous, difficult, or frightening.