this lecture covered a few last concepts about the iris, the lens, and the sympathetic / parasympathetic innervation of the muscles that mediate the aperture of the pupil. the iris is the colored part of the eye, the specific color of which results from the particular distribution of melanin pigments scattered throughout the iris-- if the melanin is deep in the iris, this results in a blue color, and a more general distribution results in a brown or green color.
the iris can be dilated or constricted, allowing varying levels of light into the pupil, depending on the contraction of the dilator pupillae vs. the sphincter pupillae. the more the pupil is constricted by the sphincter pupillae, the less light is let into the eyeball, and the greater the "range of focus" is. this constriction is mediated by a parasympathetic pathway. it can be initiated by light hitting melanopsin ganglion cells in the back of the eye, which innervate the pretectal nuclei, which innervates the edinger westfall nuclei of both eyes, which projects to the ciliary ganglion behind the eyeball, which then projects post ganglionics to the dilator pupillae, constricting the pupil. it also projects to the ciliary muscles, increasing the convexity of the lens as well.
the dilation of the pupil is mediated by a sympathetic pathway which can be initiated by local reflexes or descending influences from the limbic system or hypothalamus. the sympathetic preganglionics originate from the T1-T2 level and synapse at the superior cervical ganglion, which then project post ganglionics via the arterial system to the dilator pupillae, as well as to the tarsal muscle- which connects to the tarsal plate and raises the eyelid when constricted.
questions
1. the iris contains...
2. where does eye color come from?
3. where is the melanin mainly distributed in blue eyes?
4. where is the melanin mainly distributed in brown or green eyes?
5. what does the iris control?
6. what is the relationship between pupil aperture size and the range of focus?
7. describe the action and innervation of the sphincter pupillae.
8. describe the action and innervation of the dilator pupillae.
9. describe the parasympathetic pathway of autonomic innervation of the lens muscles.
10. describe the sympathetic pathway of autonomic innervation of the lens muscles.
11. what is the edinger-westfall nucleus?
12. describe the net effect of parasympathetic innervation on the eye muscles.
13. describe the pathway by which incoming light can cause an autonomic constriction of the pupil.
14. sympathetic innervation of the iris and upper eyelid muscles is initated by...
15. describe the pathway by which sympathetics can cause dilation of the pupil.
16. what is the tarsal muscle? what does it do? where does it insert?
17. what is "ptosis"? what syndrome is this phenomenon seen in?
answers
1. pigmented striations of connective tissue, blood vessels, smooth muscle.
2. the refraction of light from the different distributions of melanin pigment in the iris.
3. on the deep surface of the iris.
4. evenly distributed throughout the iris.
5. the aperture of the pupil.
6. smaller aperture increases range of focus.
7. constricts the pupil aperture, increasing focal length. innervated by parasympathetics.
8. opens the pupil aperture, decreases focal length. innervated by sympathetics.
9. follows the oculomotor nerve CN (III) and synapses at the ciliary ganglion which is posterior to the eyeball.
10. originates from the thoracic spine, synapses on the superior cervical ganglion, and follows the arterial system to the eye.
11. the nucleus of the oculomotor nerve from which the lens muscle preganglionics originate.
12. the sphincter pupillae contracts and the ciliary bodies contract lens.
13. incoming light into at least one eye hits the melanopsin ganglion cells, which innervate the pretectal nuclei, which innervates the EW nuclei of both eyes, which projects to the ciliary ganglion, which projects to the eye muscles and contracts the sphincter pupillae and ciliary bodies.
14. local reflexes, descending influences from limbic system and hypothalamus during emotional state.
15. sympathetic influences described in question 14 can cause the innervation of preganglionics from the T1-T2 level which synapse on the superior cervical ganglion, which then sends axons out to the dilator pupillae (but not the ciliary muscle) and the tarsal muscle, causing dilation of the pupil and lifting of the eyelid.
16. the muscle that lies deep to the levator palpabrae and attaches to the tarsal plate.
17. a drooping eyelid caused by damage to the sympathetic pathway which innervates the tarsal muscle.
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