How a nerve impulse reaches the correct destination?
OK, I think I can answer at the right level now.

Firstly, a nerve is a bundle of neurons, each neuron is like a fibre. So the sciatic nerve in the leg will carry fibres from the spinal cord to the muscles in the leg, like a data cable might connect many computers in an office to a server on another level.

Each neuron will form connections with a specific area on a specific muscle, at a specialised synapse called the "neuromuscular junction". When a signal travels down that particular neuron, it will release neurotransmitters that activate that particular muscle region. So by sending signals to the right neurons, the muscle to be activated and how much to activate it can be selected by the CNS.

A nerve impulse is a slightly misleading term as it refers to the collective activity of signals within the many neurons of the nerve, so an individual neuron won't register on an impulse. The signals in neurons are called "action potentials".

There is some information coding in the frequency of action potentials within neurons, but it is used to control the response of the muscle, rather than locate the correct muscle.

Wikipedia is pretty good on the subject if you know where to look:

neuron

withdrawal reflex

sciatic nerve