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.
An action potential can also be called a nerve impulse which is known to be stimulated by an external stimuli or upon internal excitation. This action potential travels through a neuron and involves charged ions (the key ones are sodium ions and potassium ions) that cross the membrane barrier of the neuron.