What is a two neuron arc?
A two neuron arc refers to reflex arc. It defines the pathway by which a reflex travels from the stimulus to sensory neuron to motor neuron to reflex muscle movement.
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.
For example when a person accidentally touches a hot object, they automatically jerk their hand away without thinking. The brain receives the sensory input while the reflex is being carried out and the analysis of the signal takes place after the reflex action. A reflex does not require any thought input.
There are two types of reflex arc, autonomic reflex arc ( affecting inner organs ) and somatic reflex arc ( affecting muscles ) .