A neuron by itself can’t do much. It is the starting point. It is the connection between neurons as well as between neurons, muscles, and other effector organs that makes our brain work. It is the connection of many simpler neurons onto a higher-order neuron and the combining of this information that enables the higher-order cell to process more complex information. And it is the multiple, unique connections between certain sets of neurons that create a functional neuronal circuit or pathway. Broadly, the brain contains both sensory and motor pathways.
As Shepherd said, “it is synaptic units, organized into multineuronal circuits and assemblies, that provide the basis for nervous organization.”
During development, neurons in the brain are “over-wired.” Numerous connections are made between cells. As the infant and child learn through experience, the connections are pruned. Some of these connections are strengthened; others die.
A very simple behavior such as a reflex may involve a single circuit, while more complex behaviors may involve multiple circuits. A given neuron may be part of multiple and overlapping circuits. The neuroendocrine system, comprised of areas both inside and outside the brain, modulates certain functional circuits through the release of neurohormones into the bloodstream.
At current levels of understanding, there are only a few, simple neuronal pathways that neuroscientists fully understand. One of the challenges in understanding neuronal circuits is that the connections between brain regions are not unidirectional. Some information goes down to lower brain regions; some goes up. Sometimes, behavioral experiments can be conducted to understand the order of processing. For example, if a subject’s task is to identify the meaning of a sentence, the experimenter can study how long it takes to process different components of the sentence. In this way, the scientist can determine if the stimulus is processed in serial or parallel pathways.
Learning through experience can change both the wiring (by changing the connection between neurons) and the efficacy of the wiring (by changing the efficacy of the synapse).
 Shepherd GM (1972) The neuron doctrine: a revision of functional concepts. Yale J Boil Med 45:584-599.