A Reflex Arc describes the neural pathway by which sensory information is relayed to the brain, initiating an action response (Wikipedia). This is the method by which the brain receives and acts on sensory information such as pain automatically, without the direct involvement of the conscious mind. The most commonly-known type of reflex arc is the "knee-jerk response," where the lower half of the leg jerks upwards in response to a tap on the patellar tendon, just below the knee. A reflex arc comprises five basic steps:
- An external force stimulates a sensory receptor, such as a thorn poking the skin.
- The information from this sensory receptor is transmitted along a sensory neuron to the central nervous system.
- The information is transmitted inside the central nervous system by an interneuron, causing the information to be analyzed.
- The interneuron communicates with motor neurons, which respond with an instruction for response. The motor neurons then transmit the response information to the extremity where the stimulus was felt.
- The instruction from the motor neurons is transmitted to an effector, such as a muscle, and causes an action in direct response to the original stimulus.
There may be other areas of communication, but these basic five steps correspond with the majority of reflex arcs. Suppression of the reflex arc may be caused by drugs or physical injury, in which case the stimulus information may be transmitted slowly or even blocked entirely, in which case the response instruction would never be sent.