<P> Emergency room physicians routinely assess the pupillary reflex because it is useful for assessing brain stem function . Normally, pupils react (i.e., constrict) equally . Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain stem death and depressant drugs, such as barbiturates . </P> <P> For normal pupillary light reflex, both pupils constrict simultaneously when light is shone into either eye . For example, if light is shone into left eye only, left pupil constriction is a direct pupillary light reflex, and simultaneous right pupil constriction is a consensual pupillary light reflex . Therefore, light shone into one eye causes ipsilateral direct pupillary light reflex and contralateral consensual pupillary light reflex . On testing light reflex for each eye, several patterns are possible . </P> <Ul> <Li> Optic nerve damage on one side: (Example in parens.: Left optic nerve, CN II, is completely transected somewhere in its course between retina and optic chiasma, therefore the left afferent limb is damaged . The rest of the pupillary light reflex neural pathway on both sides are otherwise intact .) <Ul> <Li> The ipsilateral direct reflex is lost . (Example: When the left eye is stimulated by light, neither pupils constrict . Afferent signals from the left eye cannot pass through the transected left optic nerve to reach the intact efferent limb on the left .) </Li> <Li> The contralateral consensual reflex is lost . (Example: When the left eye is stimulated by light, neither pupils constrict . Afferent signals from the left eye cannot pass through the transected left optic nerve to reach the intact efferent limb on the right .) </Li> <Li> The contralateral direct reflex is intact . (Example: Direct light reflex of right pupil involves the right optic nerve and right oculomotor nerve, which are both intact .) </Li> <Li> The ipsilateral consensual reflex is intact . (Example: Consensual light reflex of left pupil involves the right optic nerve and left oculomotor nerve, which are both undamaged .) </Li> </Ul> </Li> <Li> Oculomotor nerve damage on one side: (Example in parens: Left oculomotor nerve, CN III, is transected, therefore the left efferent limb is damaged .) <Ul> <Li> The ipsilateral direct reflex is lost . (Example: When the left eye is stimulated by light, left pupil does not constrict, because the efferent signals cannot pass from midbrain to the left pupillary sphincter .) </Li> <Li> The contralateral consensual reflex is intact . (Example: When the left eye is stimulated by light, the right pupil constricts, because the afferent limb on the left and the efferent limb on the right are both intact) </Li> <Li> The contralateral direct reflex is intact . (Example: When light is shone into right eye, right pupil constricts . Direct reflex of the right pupil is unaffected, The right afferent limb, right CN II, and the right efferent limb, right CN III, are both intact .) </Li> <Li> The ipsilateral consensual reflex is lost . (Example: When the right eye is stimulated by light, left pupil does not constrict consensually . Right afferent limb is intact, but left efferent limb, left CN III, is damaged .) </Li> </Ul> </Li> </Ul> <Li> Optic nerve damage on one side: (Example in parens.: Left optic nerve, CN II, is completely transected somewhere in its course between retina and optic chiasma, therefore the left afferent limb is damaged . The rest of the pupillary light reflex neural pathway on both sides are otherwise intact .) <Ul> <Li> The ipsilateral direct reflex is lost . (Example: When the left eye is stimulated by light, neither pupils constrict . Afferent signals from the left eye cannot pass through the transected left optic nerve to reach the intact efferent limb on the left .) </Li> <Li> The contralateral consensual reflex is lost . (Example: When the left eye is stimulated by light, neither pupils constrict . Afferent signals from the left eye cannot pass through the transected left optic nerve to reach the intact efferent limb on the right .) </Li> <Li> The contralateral direct reflex is intact . (Example: Direct light reflex of right pupil involves the right optic nerve and right oculomotor nerve, which are both intact .) </Li> <Li> The ipsilateral consensual reflex is intact . (Example: Consensual light reflex of left pupil involves the right optic nerve and left oculomotor nerve, which are both undamaged .) </Li> </Ul> </Li>

What 3 types of neurons are involved in pupil dilation