Actions performed automatically in response to a stimulus without conscious thought or decision are called reflex actions or reflexes.
Simple reflex actions are handled by the spinal cord without involvement of the brain.
Other reflexes are carried out in various relay centers in the lower brain or brainstem , but no conscious thought is involved.
Reflex actions carried out by the spinal cord occur automatically, but at the same time as the reflex occurs, sometimes a signal is sent to the brain by a connecting interneuron. When this happens, a stimulus is processed by the spinal cord as an arc and a response is sent back to the periphery, but a signal is also sent to the brain for interpretation and additional reaction .
Examples of each of these are: a. Knee jerk response (simple/spinal cord only)
b. Sneezing – one of the reflex centers in the medulla (involves brainstem activity only)
c. Hand on a hot stove – a simple reflex pulls the hand away (spinal cord arc only, no brain involvement) followed by a reaction due to an interneuron connection to the brain from the spinal cord.
The systems described above represent the simplest units of the nervous system and are called reflex arcs . There are 5 parts to these:
1. Sensory receptor(s) is/are stimulated.
Sensory neuron(s) send(s) message to the spinal cord.
** Interneuron(s) in CNS transmit the message to one or more motor neurons.
Motor neurons transmit the message to the effector.