When neurons in a pathway are stimulated over a long period of time (such as practicing a repeated action), a phenomenon called "long-term potentiation" (LTP) occurs, in which the neurons in that pathway are changed such that they stimulate each other more efficiently. This can range from having more excitatory neurotransmitters being released at the presynaptic neuron, more receptors for excitatory neurotransmitters on the postsynaptic neuron, or general neuronal growth. LTP is probably related to neuronal atrophy, in which unused connections between neurons are severed, but most neuronal atrophy takes place as the brain is developed.
As a side note, the length that the signal has to travel down a neuron doesn't really have that much effect. The most important factors are the amount of excitatory neurotransmitters, the amount of inhibitory neurotransmitters, and the amount of receptors for either type of neurotransmitter.
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