Actions performed automatically in response to a stimulus without conscious thought or decision are called reflex actions or reflexes. <ul><li>Simple reflex actions are handled by the spinal cord without involvement of the brain. </li></ul><ul><li>Other reflexes are carried out in various relay centers in the lower brain or brainstem , but no conscious thought is involved. </li></ul><ul><li>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 . </li></ul>
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.
**optional <ul><li>The systems described above represent the simplest units of the nervous system and are called reflex arcs . There are 5 parts to these: </li></ul><ul><li> 1. Sensory receptor(s) is/are stimulated. </li></ul><ul><li>Sensory neuron(s) send(s) message to the spinal cord. </li></ul><ul><li>** Interneuron(s) in CNS transmit the message to one or more motor neurons. </li></ul><ul><li>Motor neurons transmit the message to the effector. </li></ul><ul><li>Effector (muscle) is stimulated. </li></ul>
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