Pupillary Pathway Flashcards
Hippus
balance between sympathetic and parasympathetic muscle tone
constant rhythmic contractions and dilations of the pupil
results from low tonic firing of autonomic nervous system
Parasympathetic
Sphincter pupillae muscle contraction decreases pupil size
Sympathetic:
dilator pupillae muscle contraction increases pupil size
Iris
sphincter pupillae
dilator pupillae
Sphincter
smooth muscle fibres that circulate the pupillary border of the iris
Iris sphincter innervated by the parasympathetic nervous system
Dilator
smooth muscle fibres that run radially out from the sphincter throughout the iris
Iris dilator innervated by the sympathetic nervous system
tonus
result of muscle tension from the nerve input
Pupil size dependent on tonus of both dilator and sphincter muscles
Resultant balance = pupil size
Sympathetic Nervous System evokes Survival Responses
tonus increases pupil dilates. Dilator pupillae muscle contraction increases pupil size.
Release adrenaline, stop digestion, mucous secretion
Sympathetic tonus increases = pupil dilates
Parasympathetic nervous system evokes rest, relax & digest responses.
Increase in GIT digestion/ motility, decrease in heart rate and respiration rate
Parasympathetic system tonus is higher than the sympathetic the pupil constricts
Sphincter pupillae muscle contraction decreases pupil size
Parasympathetic defect
pupil gets larger
sympathetic pathway
pupil gets smaller
Sympathetic Efferent Pupil Pathway
1, 2, 3 order neurons
1st order neuron
- Contained fully in the CNS
- Descends down spinal column from hypothalamus to the ciliospinal centre of budge
- Ciliospinal centre budge is where 1st order neurons synapse with 2nd order neurons
Ciliospinal centre budge is located in the intermedial lateral columns of the spinal cord at C8 T2 level
2nd order neuron
- Leave spinal cord known as preganglionic neurons
- Targets are dendrites of the 3rd neuron pathway located in the superior cervical ganglion (in jaw)
- Ciliopsinal centre of budge –> apex of lungs –> under subclavian
- Course around apex of lung under subclavian to the superior cervical ganglion
Pupil abnormalities serious marker of life threatening disease
3rd order neuron
- Emerge from superior cervical ganglion neurons from plexus that surrounds external carotid artery
- Fibres that innervate face follow external carotid artery, other sympathetic fibre, including those innervate the pupil dilator enter through the internal carotid canal
- These fibres travel through the middle cranial fossa and into cavernous sinus where they join 6 th cranial nerve prior to joining ophthalmic division with the trigeminal nerve entering the orbit
- Then branch off with the naos ciliary branch off CN6 and enter orbit through superior orbital fissure
The pupillary fibres pass through the ciliary ganglion without synapses and terminate on the dilator muscle, mullers muscle and lacrimal glands
Horners Syndrome
Inability to dilate pupil means there is a disruption of normal sympathetic innervations of eye. In dim light conditions, pupil affected will look smaller than other eye as it cannot dilate
Patient can still open eyelid as lavador is innervated by ocular motor nerve which is not included in sympathetic pathway
Parasympathetic Pathway
- Edinger - Westphal nucleus
- Surface of oculomotor nerve (CNIII)
- Cavernous sinus
- Inferior division of oculomotor nerve
- Ciliary ganglion
- Post-ganglionic short ciliary nerves supply ciliary muscle & iris sphincter
Detailed parasympathetic pathway
Efferent parasympathetic signal starts in Edinger Westphal nucleus in pretectal midbrain. From here preganglionic fibres of the ciliary muscle and pupil travel along surface of oculomotor nerve (CNIII) because they run along outside of nerve, they are more vulnerable to compressions/ damage.
Parasympathetic pupil fibres travel with CNIII through cavernous sinus and enter the orbit through the superior orbital fissure. At the anterior end of the cavernous sinus, CNIII divides into superior and inferior divisions and the pupillary motor fibres travel through the inferior division of CNIII. They then branch to the inferior oblique muscle and synapse at the ciliary ganglion
Ciliary ganglion is located between optic nerve and lateral rectus muscle. From here approx 6-10 postganglionic short ciliary nerves originate and penetrate the posterior sclera and supply the ciliary muscle and iris sphincter.
As the parasympathetic pathway innervates the sphincter muscle, its most pronounced action is the constriction of the pupil in light reflex process
Inability to constrict the pupil in response to light stimulus may mean interruption to the normal efferent parasympathetic innervation to the eye. Could also be due to afferent pathway
Separating Afferent from parasympathetic efferent
Afferent fibres desiccate before synapsing with the Edinger Westphal nucleus. This means that an afferent signal from a light stimulus applied to only eye is communicated from the pretectal nucleus to the Edinger Westphal nucleus in both hemispheres
Direct response
- When light is shone into one eye it constricts
Shows afferent and parasympathetic efferent pathway for that eye on the side are functional
Direct response
- When light is shone into one eye it constricts
Shows afferent and parasympathetic efferent pathway for that eye on the side are functional
Consensual response
- When light shone into one eye, the other eye constricts
- This means fibres that dissecate from pretectal nucleus to the contralateral Edinger Westphal nucleus are in tact and functioning
Follows other eye that has had light reflex shone in it
- This means fibres that dissecate from pretectal nucleus to the contralateral Edinger Westphal nucleus are in tact and functioning
Afferent Defect
If light is shone into right eye and it doesn’t react and left eye also then doesn’t react
Parasympathetic Efferent defect
If light is shone into right eye and it doesn’t react but light into the left eye does
Pupil constriction and near response
Pupil constricts when focusing on near objects despite light remaining the same
Near triad response:
- Convergence - Accommodation - Pupil constriction
Thought to originate in the occipital association cortex
Pathway for near response located more ventrally in the midbrain than the afferent portion of the light reflex so innervation signal added to light signal resulting in greater pupil constriction
If someone has parasympathetic defect testing to see if the near response is preserved can help localise where the problem is occurring
Argyll Robertson Pupil
Where both pupil don’t restrict to a light stimulus but both constrict to a near stimulus.
This results from tertiary syphilis caused by Treponema Pallidum
No focal lesions in autonomic pathways.
Damage to the efferent pupillary fibres on dorsal side of Edinger Westphal Nuclei occurs causing lack of light response. Ventral side escapes damage and as nerve fibres responsible for accommodation response travel through the ventral midbrain constriction to a near target is preserved.
Conditions that present similar to Neuro-syphilis:
- Sarcoidosis
- Encephalitis
Multiple sclerosis lesions
