Reflex
A reflex is an involuntary response to a specific stimulus (a stimulus is any act or agent which causes a reaction in a particular tissue). A reflex is considered a "stereotypical" response, in that it is a mechanical response brought about by the stimulation of a particular neuron, resulting in a particular action which is not varied through the voluntary control of the individual. An example of a reflex occurs when the correct location in the knee is tapped with a physician's hammer, resulting in the contraction of the thigh muscle. This causes the lower leg to kick out. Although one normally considers such movement to be completely at the will of the individual to initiate or prevent, this reflex occurs without the involvement of the individual's brain. It is always surprising to see one's body perform such a seemingly voluntary act, with no actual effort or desire on the part of the individual.
The reflex involved in this knee jerk (called the patellar reflex) occurs when a neuron is stimulated by a quick tap on the tendon of the knee. The sudden stretching of this tendon is transmitted quickly to the spinal cord. This neuronal impulse stimulates another neuron within the spinal cord which returns to the leg, causing the thigh muscle contraction and the straightening (kicking motion) of the lower leg. The neuronal impulses stay within neurons traveling to and from the spinal cord, never traveling to the level of the brain. Because this reflex is totally spinal, no voluntary control is possible over it.
Other reflexes do include the brain, yet they still remain at a level outside of voluntary control. For example, the pupillary reflex is a response of the muscles of the pupil to light. The presence of light causes the pupil to constrict (grow smaller), in order to protect the structures at the back of the eye from damage due to exposure to too much light. These pupillary reflexes should be brisk and equal between the two eyes. The pupillary reflex is tested when an individual is unconscious, to check on whether brain injury has occurred.
Anther reflex is called the withdrawal reflex. The withdrawal reflex occurs when someone touches something extremely hot. Before the individual can even consciously acknowledge the pain of the burn, the arm and hand will reflexively withdraw from the hot item. The neuron which senses the dangerously hot temperature passes this information on through the spinal cord to neurons which then command the muscles of the arm and hand to contract, quickly removing the hand from danger. The pathways which allow the brain to sense the pain of the burn are much slower, and the knowledge of burning pain comes after the hand has already pulled back from the burning item.
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