Unquestionably the simplest neural circuits in the body are the ones responsible for our reflexes.
Reflexes are fast, automatic behaviours that are very old in evolutionary terms and that do not require any conscious action on our part.
However, every reflex still comprises the same three steps involved in many other neural circuits: sensory input, information processing, and motor output.
The example diagrammed here is the reflex that controls the degree of stretching in the leg muscles to help maintain an upright posture.
This circuit involves only two neurons: the sensory neuron that senses the stretching of the muscle, and the motor neuron that keeps the muscle partly contracted.
Between these two neurons, there is just one synapse that does all the information processing, which in this case comes down to stimulating the following neuron.
A reflex of this kind, which involves only one synapse, is called a monosynaptic reflex. But polysynaptic reflexes exist as well.