All of the following drugs create pupil dilation except?
Answer: (D.) Morphine
The pupil is constricted or dilated by the contraction of smooth muscle in the iris. Increased release of acetylcholine by the PARASYMPATHETIC nervous system causes contraction of circular smooth muscle. This makes the pupil (the dark hole) smaller. Thus, we say PARASYMPATHETIC CONSTRICTS THE PUPIL(MYOSIS).
Increased release of norepinephrine by the SYMPATHETIC NERVOUS system causes contraction of radial smooth muscle. This makes the pupil (the dark hole) larger. Thus, we say SYMPATHETIC DILATES THE PUPIL(MYDRYASIS).
Some drugs that cause pupillary dilation:
– CNS stimulants: cocaine, amphetamines, methamphetamine
– Hallucinogens: LSD, ecstasy
– Cannabis: marijuana, hashish, marinol
Some drugs that cause pupillary constriction:
– Narcotics: heroin, morphine, codeine, fentanyl, demerol, methadone, oxycontin
In order to understand how the pupil size will change, it is most important to understand the sympathetic and parasympathetic nervous system.
The sympathetic system is also called “flight and fight reaction.” The parasympathetic system refers to the “rest and digest reaction.”
I remember PARASYMPATHETIC using this image when I am relaxing because I ate too much and I feel “PARALYZED” and need to “rest.” Then I know that the sympathetic system is when I am alert. Now the classification of the drugs are also important to know. To simplify, if the drug is “stimulating” the sympathetic system, the pupil will grow larger. On the contrary if the parasympathetic system is “stimulated”, the pupil will become smaller.
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