Pupil
is a round hole in the center of the iris.
*is the aperture of the eye's optical system formed by
the iris.
Optical functions of the pupil
1) regulates the amount of light reaching the retina.
2) increases the depth of field
3) reduces crystalline lens chromatic and spherical aberrations.
4) produces the phenomenon of apparent accommodation.
5) coveys social information.
The size of the pupil varies because?
a) at different ages
b) from person to person
c) with different emotional states
d) levels of alertness.
e) degrees of accommodation
f) ambient room light.
_________ contribute to the pupillary light reflex
Both rods & cones
Dark adaptation condition
low intensity light stimuli, below cone threshold, produce small-amplitude pupil reactions that are only
contributed to by rods.
Pupillary threshold sensitivity vs. pupillary amplitude of response
Rod- cone difference
Under dark conditions the ____ are far more sensitivie
Rods
The amount of light it takes to elicit a small threshold pupillary response if over _______ units than what it would take to elicit a threshold pupil response from cone activation
2 log units
Rods and cones relay the light signal to the?
retinal ganglion cells via
Synapses with bipolar cells
Ganglion cells that serve the pupillomotor afferent input are denset in?
The central retina
Pupil size does not change with?
Increasing levels of luminance that stimulate only retinal rods
3 main classes of ganglion cells
Alpha, beta, and gamma
Alpha ganglion cells
1 - 2 % of the cells in central retina
� 10% in the periphery
� Densest in the parafoveal region
� Respond to fast-moving stimuli
� Project to the LGN
� 50% also project to the midbrain
? Ganglion cells
� Most numerous of all classes (50 - 60%)
� Densest in central retina
� Fire tonicly
� Most responsive to contrast of small areas
� Project to LGN: tectum & pretectum
? (Gamma) ganglion cells
� The major contribution to the pretectal olivary neurons in the midbrain serving the
classic pupillary light reflex pathway comes from the ? cells.
� Identified as the melanopsin-containing cells.
� Small cell bodies & thin
� Slowly conducting axons
� La
Normal pupil diameter is ?
Approximately 3-4 mm
Pupil diameter is smaller in?
Infancy, larger in childhood, and progressively smaller with age
Pupillary size relates to varying interactions between the?
a) Sympathetically innervated iris dilator.
b) Supranuclear control from the frontal lobe (alertness).
c) Occipital lobes (accommodation)
d) Responds to respirations (i.e. hippus)
Human pupil characteristics
Circular
*Equal in diameter - isocoric
*Unequal pupil diameter - anisocoric (up to 1mm is not
considered pathological)
*Physiological anisocoria - 20% - 40% have a slight
difference in pupil size, approximately 0.5mm
Sympathetic activtiy
mobilizes mechanisms of the body to meet conditions of stress. Pupils dilate.
Parasympathetic activity
Conservation and restoration of bodily resources. Pupils construct.
The ____ is the anterior extension of the ciliary body
IRIS
Iris root/ color
Approximately 12mm
*Margin diameter: - 0.8 - 1.0 mm
*Color - darker with age
Iris at birth
The iris consists of ectodermal tissue.
*Subsequently, mesodermal tissue, the chromatophores, invade the ectodermal tissue, giving it a darker color.
Iris muscles
Smooth muscles
a) sphincter pupillae
b) dilator iridis
Sphincter pupillae
Circular band of muscle that surrounds the pupillary margin and constricts it when activated.
** (parasympathic activation)
Dilator iridis
Consists of radially oriented muscle fibers that draw the pupil open
from the periphery.
*Sympatheric activation
Light reflex
Testing
*Bright light into one eye after the patient has been in a dimly illuminated room for about 10 sec.
*Both pupils constrict and dilate by equal amounts.
*Direct light reflex - Consensual light reflex
The light reflex is entirely ______
Subcortical
Light reflex afferent fibers
In optic nerve
*Exit optic tract prior to lateral geniculate
*Enter midbrain via brachium of superior colliculus and synapse into pretectal nucleus.
*
Each pretectal nuclues ________
Decussates neurons to the cerebral aqueduct to the ipsilateral and contralateral Edinger-Westphal nucleus of the oculomotor III nerve.
Tectotermental tract
The fibers project from the pretectal nucleus to the ipsilateral and contralateral Edinger Westphal (EW) nuclei, forming the tectotegmental tract.
Damage to the tectotegmental tract can lead to?
an Argyll-
Robertson pupil, which is characterized by light-near response
dissociation.
* Argyll-Robertson pupil is associated with neurosyphilis.
Structures involved in light reflex
Optic Nerve
* Chiasm
o 53% cross (nasal fibers) o 47% uncrossed (temporal fibers)
*Synapse in pretectal nucleus
* Hemi-decussation
- Posterior commissure
- Equal input to both Edinger-Westphal nuclei
Efferent arc of the light reflex
All neutral impulses travel via a preganglionic parasympathetic
pathway from the rostral midbrain to the ciliary ganglion in the orbit.
*Via the CN III to the ciliary ganglion in the lateral orbit.
*The postganglionic fibers go via the short ciliary nerve
Efferent parasympathetic fibers are responsible for?
miosis and accommodation begin in the EW nucleus.
* Anisocoria is ALWAYS a result of an efferent pathology.
The near triad
1) Vergence
2) Accommodation
3) Pupillary constriction.
Clinical examination: Near Reflex
*Patient views a dimly lighted fixation target on a far wall.
*Physician places his/her finger approximately 40cm in the primary
position.
* Patient looks at finger and exhibits the components of the near triad.
*Patient looks at light - Pupils dilate, ey
The near response
In this case, pupillary constriction is mediated by supranuclear input from the frontal eye fields (FEF) instead of the pretectal nucleus.
*The FEF activate the EW nucleus, which projects fibers to the ciliary ganglion and then on to the sphincter muscle
Summary of normal pupillary light reflex
PERRLA
Pupils are Equal Round Reactive to Light and Accommodative stimulation
Relative Afferent Pupillary Defect (RAPD)
If an optic nerve lesion is present, the direct light response in the involved eye is less intense than the consensual response evoked when the normal eye is stimulated.
Causes of unilateral decreased vision without an afferent pupillary defect
1. Refractive error
2. Cloudy media (cataract)
3. amblyopia
4. hysteria or malingering
5. a macular lesion
6. chiasmatic problems
*Know these
Amaurotic pupillary defect
An eye that does not even see light owing to severe unilateral retinal or optic nerve disease.
Argyll Robertson pupil
The pupil is less than 3mm in diameter.
- Does NOT respond to light stimulation.
-Does accommodate (bilateral)
-Some degree of Argyll Robertson pupil is present in over 50% of patients with CNS syphilis.
Argyll Robertson pupils are commonly
1. Irregular
2. Eccentric
3. Dilates poorly with mydriatics as a consequency of concomitant iris atrophy
An incomplete Argyll Roberston pupil can be caused by?
These are infrequent causes
- diabetes
- chronic alcoholism
- encephalitis
- multiple sclerosis
- CNS degeneration disease
- tumors of the midbrain
Irregular pupil
The patient's right eye displayed areas of posterior synechiae, leading to an irregular, "clover-leaf" pupil.
Tonic pupil is due to ?
Due to an abnormal pupillary constrictor mechanism in which all or a portion of the sphincter muscle contracts slowly (tonically) to near stimulation and relaxes even more, but either response is better than the light response.
Adie's syndrome
Associated with loss of deep tendon reflexes (Adie's syndrome).
*Results from damage to the ciliary ganglion.
*A weak (0.1%) solution of pilocarpine instilled into the
conjunctival sac causes a tonic pupil to constrict; normal
pupils are not affected.
*Bi
Horner's syndrome
Caused by a lesion affecting the sympathetic nerve supply at any point along its pathway from the hypothalamus.
Horner's syndrome signs
1. Slight unilateral ptosis
2. Miosis of the affected eye
3. Ipsilateral anhydrosis
4. Heterochromia iridis (happens with congenital Horner's)
5. No associated ocular motor defects
Monocular miosis=
Horner's syndrome
*Interruption of the sympathetic fibers results in a small pupil which retains its light response.