Reflex action is an involuntary physiological response to a stimulus, as the withdrawal of a body part from burning heat. An unlearned or instinctive response to a stimulus.
In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain.
When a reflex arc in an animal consists of only one sensory neuron and one motor neuron, it is defined as monosynaptic, referring to the presence of a single chemical synapse.