Innervated by
Sympathetic activation causes pupillary dilatation
- Origin: posterior hypothalamus.
- 1st order neuron:
- Posterior hypothalamus → lateral tegmentum of the brainstem → intermediolateral gray matter of the C8-T2 segments of the spinal cord.
- 2nd order neuron:
- Intermediolateral gray matter → through the sympathetic chain → superior cervical ganglion
- 3rd order neuron
- Pupillodilator muscle
- Superior cervical ganglion → joins the internal carotid artery → enter the cavernous sinus → joins the ophthalmic branch of the trigeminal nerve to enter the orbit → reaching the pupillodilator muscles via the long ciliary nerve .
- Other targets
- Sweat glands of face (sudomotor and vasomotor fibers to the face)
- External carotid artery, rather than the internal carotid artery → sweat glands of the face
- Superior tarsal muscles
- Internal carotid artery past the cavernous sinus → ophthalmic artery to the orbit → Superior tarsal muscles
- Responsible for elevation of the upper eyelid.
Parasympathetic activation causes pupillary constriction.
- Originate in the Preoptic and anterior (Caudal parts of the hypothalamus) → Edinger-Westphal nucleus of the oculomotor complex.
- Edinger-Westphal nucleus → accompany the oculomotor nerve through the subarachnoid space, cavernous sinus , and superior orbital fissure and into the orbit. → inferior division of the oculomotor nerve until they terminate in the ciliary ganglion .
- The ciliary ganglion is located on the temporal side of the ophthalmic artery between the optic nerve and the lateral rectus muscle.
- Postganglionic neurons in the ciliary ganglion reach the pupilloconstrictor muscles via the short ciliary nerves.
Pupillary reflexes
Light reflex
- Convergence centre: pretectal area (mesencephalic reticular formation just Dorsal to the CN3)
- Inputs from bilateral cerebral hemisphere → pretectal area → bilateral CN3
- Pathway:
- Afferent pupillary fibers start at the retinal ganglion cell layer → optic nerve → optic chiasm → optic tract → superior brachium of the superior colliculus → Posterior commissure → pretectal area of the midbrain → bilaterally to the efferent Edinger-Westphal → nuclei of the oculomotor complex → efferent pupillary parasympathetic preganglionic fibers travel on the oculomotor nerve to synapse in the ciliary ganglion → which sends parasympathetic postganglionic axons in the short ciliary nerve to innervate the iris sphincter smooth muscle via M3 muscarinic receptors
- Due to innervation of the bilateral E-W nuclei, a direct and consensual pupillary response is produced.
- Afferent fibers of the pupillary light reflex cross to the contralateral Edinger-Westfall nucleus via which structure?
- The posterior commissure
- Vertical up wards eye movements also pass through the posterior commissure
Near reflex (Accommodation reflex)
- Consists of
- Convergence of the eyes
- Accommodation of the lens
- Contraction of the ciliary muscle, which reduces the tension on the suspensory ligaments of the lens → lens to become more spherical to accommodate near vision.
- Pupillary constriction.
- Vergences
- Function: align the visual axes to maintain bifoveal fixation so that an object seen by both eyes is perceived as a single object.
- If two images of an object fall on noncorresponding retinal areas in each eye, one of two possibilities may occur:
- Diplopia: Either the object is perceived to exist in two separate locations simultaneously
- Confusion: the perception is created that two objects are located in the same position in space
- Latency: 160ms
- Driven by
- Retinal disparity
- Disparity between the location of images on the two retinas
- Associated with fusional vergence (in which the two retinal images are perceived as one)
- Retinal blur
- Defocused images
- Associated with accommodation-linked vergence
- Response with
- Pupillary constriction
- Lens accommodation
- 3 component reflex consists of
- Pupillary accommodation reflex
- Constriction of the pupil → reduce light rays onto retina → produce a sharper image
- Afferent input from the retina is sent to the lateral geniculate nucleus via the optic tract
- Fibers from the LGN then project to the visual cortex.
- Efferent parasympathetic fibers from the E-W nucleus project via the oculomotor nerve to the ciliary ganglion and then short ciliary nerves to innervate the iris sphincter muscle to cause pupillary constriction
- Lens accommodation reflex,
- Efferent pathway for lens accommodation: Efferent parasympathetic fibers from the E-W nucleus project via the oculomotor nerve to the ciliary ganglion and then short ciliary nerves to innervate the ciliary muscle to cause contraction.
- Contraction of the ciliary muscle allows the lens zonular fibers to relax and the lens to become more round, increasing its refractive power.
- Convergence reflex.
- Efferent pathway for convergence: Efferent fibers from the medial rectus subnucleus of the oculomotor complex in the midbrain innervate the bilateral medial rectus muscles to cause convergence
Pupillary Abnormalities
Disease | Pupil |
Horner | Small |
Adie | Large |
Medical CN3 palsy | Normal |
Surgical CN3 palsy | Large |
Hutchinson | Large |
Marcus Gunn pupil
- Is associated with an afferent pupillary defect.
- A dysfunction in the afferent nerve (optic nerve).
- Seen in conditions such as optic atrophy.
- Marcus Gunn pupil is one of the most sensitive indicators of optic nerve dysfunction.
- Swinging flashlight test,
- Normally, when a flashlight is shone on one eye and then moved quickly to the other, the degree of pupillary constriction that is elicited with each manoeuvre is equal (i.e., pupillary size remains constant after the flashlight is swung).
- By contrast, when a flashlight is shone on a good eye and then a bad one (i.e., one with an optic nerve lesion), a transient dilatation of the pupils will be seen as the flashlight is swung to the bad eye.
- This is because a decreased amount of light is conveyed to the system when a flashlight is shone on the affected eye, due to the affected optic nerve (afferent signal).
- Different from Marcus Gunn Phenomenon (Marcus Gunn Jaw winking syndrome
Pharmacological Mydriasis
- Atropine
- Muscarinic antagonist → pupillary dilatation (mydriasis or cycloplegia) unresponsive to light.
- Pilocarpine
- Direct acetylcholine agonist → pupillary contraction
- The diagnosis of pharmacological blockade requires a high index of suspicion.
- Pharmacological test
- Instil 1% pilocarpine (acetylcholine) into the affected eye or eyes.
- In the normal person or the patient with pharmacological blockade at the neuromuscular junction, however, no pupillary constriction will be observed.
Traumatic Mydriasis
- Ocular trauma may result in a fixed and dilated pupil
- Mechanisms of injury.
- A transient loss of parasympathetic tone may accompany ocular trauma in the same manner that loss of sympathetic tone occurs in patients with spinal shock.
- Weak miotics may cause constriction
- Direct injury to the pupillary sphincter may result in pupillary dilatation.
- Weak miotics will not cause constriction.
Adie's Tonic Pupil
- Lesion of the ciliary ganglion → parasympathetic dysfunction → unilateral fixed and dilated pupil
- Test
- Administration of a very weak para-sympathetic (0.125% pilocarpine).
- Although this solution is ineffective as a pupillary constrictor in normal persons, patients with a tonic pupil exhibit parasympathetic supersensitivity, which leads to pupillary constriction after the instillation of the weak miotic agent.
Hutchinson's Pupil (“Blown Pupil”)
- Expanding supratentorial mass → uncal herniation → CN3 compressed between uncus and tentorial edge.
- Because the parasympathetic fibers of the oculomotor nerve are located peripherally, pupillary dilatation may develop in the absence of other signs of a third nerve palsy.
Argyll Robertson Pupil
- Due to neurosyphilis.
- Accommodates but does not react
- Pupil fails to constrict to light but does react during convergence.
- This has been referred to as light-near dissociation.
- Like parinauds
- The Argyll Robertson pupil tends to be miotic and irregular. Most commonly, there is bilateral involvement.
Midbrain Pupillary Abnormalities
- Lesions in the region of the sylvian aqueduct are associated with light-near dissociation.
- These abnormalities are typically seen as part of a Parinaud's syndrome.
Pinpoint Pupils
- The classic structural lesion associated with pinpoint pupils is a pontine lesion.
- Parasympathetic originate in the midbrain and never go caudal to this so any lesion below midbrain will cause sympathetic shut down leading to small pupils
- Other causes of pinpoint pupils are eyedrops and narcotics.
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