LIGHT REFLEXES It was mentioned earlier that a few fibers of the optic tract bypassed the lateral geniculate body to be involved in certain reflexes. These fibers, along with a few that originate in the lateral geniculate body pass through the brachium of the superior colliculus to terminate in the superior colliculus to terminate in the superior colliculus and pretectal region. It is this portion of the brain which modulates all light reflexes. 1. What makes up the optic tract? axons of bipolar neurons axons of ganglion cells optic nerve That's right. That's wrong. The answer is B. 2. Most fibers of the optic tract terminate in the lateral geniculate body. true false That's right. That's wrong. The answer is A. 3. The lateral geniculate body sends a few fibers through the optic radiations to be involved in light reflexes That's right. That's wrong. The answer is B. 4. Light reflexes are modulated through the superior colliculus and pretectal region That's right. That's wrong. The answer is A. A. PUPILLARY CONSTRICTION When too much light strikes the retina, a message is sent to the superior colliculus and pretectal region. From there it is sent along fibers to the Edinger-Westphal nucleus (the parasympathetic portion of the oculomotor nerve nucleus). The stimulated parasympathetic nerves cause the pupils to constrict through contraction of the circular muscles of the iris,thus reducing the amount of light reaching the retina. 5. Which nerve fibers cause miosis (pupillary constriction)? sympathetic parasympathetic abducens SVE fibers That's right. That's wrong. The answer is B. 6. Which nerve fibers cause mydriasis (pupillary dilation)? sympathetic parasympathetic oculomotor That's right. No. The answer is A. 7. Which muscle causes mydriasis? iris circular iris radial ciliary Good job. No. The answer is B. 8. In this reflex, the superior colliculus/pretectal region stimulates a nucleus located in what region? reticular formation upper thoracic spinal cord cervical spinal cord medial longitudinal fasciculus Good job. No. The answer is B. C. DIRECT VS. CONSENSUAL REFLEXES The direct light reflex refers to miosis of the eye in which a light is shined. The consensual light reflex is the simultaneous contraction of the other eye, and is usually less than the direct reflex. 9. Which nerve fibers make up the efferent limb of the direct and consensual light reflexes? sympathetic parasympathetic optic none of the above correct No. The answer is B. 10. Which nerve fibers make up the afferent limb of the direct and consensual light reflexes? sympathetic parasympathetic oculomotor optic Very good. No. The answer is D. D. ACCOMMODATION REFLEXES When the eye focuses on a near object, the lens must change its shape to accommodate for the change in image distance. As it does so, the pupil constricts reflexively this is the accommodation reflex. It begins in the superior colliculus and pretectal region,traces a path to the cerebral cortex and back to the Edinger-Westphal nucleus. The parasympathetic nerves are stimulated resulting in contraction of both the ciliary muscles and the iris circular muscles. Contraction of the ciliary muscles allows the lens to thicken,thereby increasing the refractive power. Contraction of the iris circular muscles causes pupillary constriction; This is thought to compensate for the increased amount of light focused on the retina by the shorter focal length of the lens. This reflex can be elicited by asking the patient to look at a distant object and then focus on a finger held several inches away from their face. The pupils should constrict slightly. 11. Which nerve fibers are involved in the accommodation reflex? sympathetic parasympathetic GSA SVE Correct. No. The answer is B. 12. Which muscles are involved? iris circular and radial iris circular and ciliary iris radial and ciliary Very Good. No. The answer is B. 13. In order to focus on a near object, the lens must do what? thicken thin Very good. No. The answer is A.