Lecture 20 4/22/25 Flashcards
What is the pathway of the PLR reflex?
-light is stimulus
-receptors are photoreceptors
-afferent neuron is #2
-interneuron is subcortical
-efferent neuron is #3
-effector is iris sphincter
-response is pupil constriction
What is the pathway of the menace reflex?
-hand motion is stimulus
-receptors are photoreceptors
-afferent neuron is #2
-interneuron is cortical cerebellum
-efferent neurons are #6 and #7
-effectors are orbicularis oculi and retractor bulbi muscles
-response is blinking and globe retraction
What is the pathway of the dazzle reflex?
-bright light is stimulus
-receptors are photoreceptors
-afferent neuron is #2
-interneuron is subcortical
-efferent neuron is #7
-effector is orbicularis oculi muscle
-response is blinking
What is the pathway of the palpebral reflex?
-stimulus is touching lids
-receptors are touch receptors and skin
-afferent neuron is #5
-interneuron is subcortical
-efferent neuron is #7
-effector is orbicularis oculi muscle
-response is blinking
What is the pathway of the corneal reflex?
-stimulus is touching cornea
-receptors are touch receptors and cornea
-afferent neuron is #5
-interneuron is subcortical
-efferent neurons are #6 and #7
-effector is retractor bulbi muscle
-response is globe retraction
What is the pathway of the doll’s eye reflex?
-stimulus is head motion
-receptor is semicircular canals
-afferent neuron is #8
-interneuron is subcortical cerebellum
-efferent neurons are #3, #4, and #6
-effectors are extraocular muscles
-response is maintenance of line of sight
What are the most common ophthalmic manifestations of neurologic dz?
-abnormal pupil sizes
-abnormal pupillary light responses
-blindness
What is the pathway of impulses for pupillary responses and vision?
-rods and cones accept light
-electrical impulses pass to bipolar cells
-impulses continue to retinal ganglion cells, whose axons form optic nerve
-nerve fibers partially decussate from optic nerve at optic chiasm
-fibers continue from optic chiasm as optic tracts and differentiate into visual and pupillary fiber pathways
Where do the visual fibers continue to?
-synapse in lateral geniculate bodies
-travel via optic radiations to occipital cortex
Where do pupillary fibers continue to?
-continue to midbrain
-synapse in pretectal nuclei
-project and synapse in oculomotor nuclei
Where do preganglionic parasympathetic axons of the oculomotor nerve synapse?
both ciliary ganglia
What do postganglionic parasympathetic fibers innervate?
-iris sphincter
-ciliary muscles
How does sympathetic innervation of the iris control pupil size?
by providing constant antagonism to the sphincter via the dilator muscle
What is the pathway for sympathetic impulses controlling pupil size?
-originate in hypothalamus and rostral midbrain
-pass to and synapse in intermediate gray column of first 3 segments of spinal cord
-pass to thoracic sympathetic trunk
-go cranially to cervical sympathetic trunk
-pass to dilator muscle of iris
What are non-neurologic causes of abnormal pupil size and/or PLRs?
-iris atrophy
-synechia
What are the characteristics of the palpebral reflex?
-blink reflex checked by touching skin in the medial and lateral canthi
-tests sensation of eyelids (CN5) and ability to blink (CN7)
-facial nerve paralysis will lead to facial asymmetry
-parasympathetic innervation of lacrimal gland originates in parasympathetic nucleus of facial nerve; lesions can lead to neurogenic KCS
-especially important to test in animals with absent menace response, dry eye, and/or corneal ulcer
What are the characteristics of the menace response?
response, not a reflex; animal can override it
-present within 1 week in horses and other prey animals; learned response in predator animals by 7 to 8 weeks
-want to avoid air movement and touching the face
-adult cats are difficult to menace; birds and most exotics do not menace
-pathway travels through cerebellum; cerebellar dz can cause lack of menace
-lak of menace can also be due to facial nerve paralysis
What are the characteristics of pupil size at rest?
-look for symmetry and a normal size for environment
-evaluate via retroillumination in light and dark if concerned
What are the characteristics of the dazzle reflex?
-“involuntary” avoidance response to light shone into the eye
-leads to blinking and possible retraction of globe with protrusion of third eyelid
-may move head away from light
-positive response is considered evidence of continuity between eyes and brain and suggests ability for vision
-cataracts and corneal scarring are not expected to prevent response; helpful when opacity prevents menace response
What are the characteristics of pupillary light response?
-direct response occurs when pupil constricts as light is shone into that eye
-consensual response occurs when pupil constricts as light is shone into opposite eye
-consensual PLR is most important when you cannot see the pupil of one eye or when the direct PLR of one eye is abnormal
What are additional tests for vision that can be done in an exam room?
-cotton ball test/tracking
-maze test
-visual placement test/move animal towards wall/table
What are the characteristics of binocular overlap?
-overlap region in which the visual field stimulates both eyes
-improves visual acuity at the expense of peripheral vision
What are the characteristics of decussation?
-decussation is the crossing of fibers from one optic nerve to the other at the optic chiasm
-animals with 100% decussation do not have indirect PLRs; lesion in visual cortex on one side leads to complete blindness of contralateral eye
What are the characteristics of optic nerve blindness?
-fixed, dilated pupils with no PLR
-commonly caused by optic neuritis or glaucoma optic neuropathy
-optic neuritis can cause inflammation of optic nerve and sudden blindness
-glaucoma optic neuropathy can lead to acute or chronic degeneration of ganglion cells
What are the characteristics of cortical blindness?
-inability to see due to lesion of visual cortex
-will often have other neurologic signs of cortical dz
-PLRs and rest of ocular exam are normal
-diagnosed by identifying blindness with normal PLRs and ruling out SARDs via ERG
-treatment via treating underlying brain disorder
What are the etiologies of cortical blindess?
-infectious
-neoplastic
-vascular
-hydrocephalus
-inflammatory/idiopathic
What are the expected findings for a left retina lesion?
-visual in right eye, blind in left
-direct PLR only in right eye
-indirect PLR only R -> L
-retinal abnormalities seen on left fundic exam
What are the expected findings for bilateral retina lesions?
-complete blindness
-no direct or indirect PLRs
-retinal abnormalities in both eyes on fundic exam
What are the expected findings for bilateral optic nerve lesions (like bilateral optic neuritis)?
-complete blindness
-no direct or indirect PLRs
-normal retinas on fundic exam
-possible optic nerve abnormalities on fundic exam
What are the expected findings for an optic chiasm lesion?
-complete blindness
-no PLRs
-normal fundic exam
-must be identified via MRI
What are the expected findings for a right preganglionic parasympathetic neuron lesions?
-vision intact
-left direct PLR and R -> L indirect PLR normal
-right direct PLR and L -> R indirect PLR absent
What is horner’s syndrome?
lack of sympathetic innervation to eye and orbit
What are the signs of horner’s syndrome?
-often unilateral
-miosis but with normal PLRs
-enophthalmia
-protrusion of third eyelid
-ptosis/narrowing of palpebral fissure
-peripheral vasodilation and hyperemia
-sweating on ipsilateral side of face and neck in horses
What is the pathway that is affected in horner’s syndrome?
-first order neuron/central neuron from hypothalamus to spinal cord
-second order neuron/preganglionic neuron from spinal cord to cranial cervical ganglion
-third order neuron/postganglionic neuron from ganglion to eye
What are the etiologies of horner’s syndrome?
-any lesion of sympathetic pathway
-otitis media
-neck trauma
-thoracic tumors
-idiopathic
How is horner’s syndrome diagnosed?
-clinical signs
-pharmacologic localization of post-ganglionic lesions with phenylephrine
-deep cervical palpation and flexion
-otic exam
-MRI
-chest films
What are the steps to the phenylephrine test for horner’s syndrome?
-apply one drop of 1% phenylephrine to each eye
-observe pupils every 10 minutes
-phenylephrine will dilate a normal pupil in 1 to 2 hours; it will dilate a pupil with postganglionic horner’s syndrome in about 20 minutes
What is the treatment for horner’s syndrome?
-treat otitis if discovered
-monitor; idiopathic cases will typically resolve in 6 weeks to 6 months
