The basic functional component of the nervous system is the Reflex Arc. This is the smallest, simplest portion of the system capable of receiving a stimulus and yielding a response. Its physical components and actions are:
1) a sensory receptor to initiate the signal
2) a sensory (afferent) neuron running in a peripheral nerve to transmit the
impulse to the Central Nervous System (CNS)
3) an interneuron in the CNS to pass the signal from afferent to efferent neuron.
These may also be connected to nerves in the spine and brain.
4) an Efferent or Motor Neuron, which passes thru peripheral nerves to bring
the signal to the effector organ.
5) an effector organ (muscle, gland etc.), which can respond to (correct for) the
Reflex Arcs are basically rapid acting homeostatic controls.
In a sense, a nervous system might be considered to arise from, or be composed of, a complex series of multiple interacting reflex arcs.
Technically the structural basis of a Reflex arc could be a single neuron, which both senses a stimulus and induces a response. Some arcs have only two neurons, a sensor and a motor neuron, but most have at least three neurons, with the central"interneuron" functioning as a primitive "control center".