L11 Motor Control of the Eye

Question 1

Answer 1

Question 2

How would a Sensory Pupil Defect differ from a Motor Pupil Defect?

Answer 2

Sensory Defect: SAME Response in both eyes, one just not responsive to light

Motor Defect: Different Responses in each eye

Question 3

Pathway of Sympathetic Dilation of Pupil?

Pathway regulated by _________________
=> Projects axons through Lateral Brainstem => Sympathetic preganglionic neurons in____________________________ => Project to __________________=> Postganglionic neurons travel along _________________ to orbit => Innervate Pupilodilator muscles

Answer 3

Pathway regulated by HYPOTHALAMUS
=> Projects axons through Lateral Brainstem => Sympathetic preganglionic neurons in first 3 segments of thoracic intermediolateral cell column => Project to SUPERIOR CERVICAL GANGLION => Postganglionic neurons travel along INTERNAL COROTID to orbit => Innervate Pupilodilator muscles

Question 4

3 Elements that are Adjusted to focus on a closer object?

Answer 4

Focus on CLOSER Objects: Focal Length Reduced by CONTRACTION of ciliary muscles

=> Relaxation of Suspensory Ligaments

=>Increase in lens curvature (Due to tension in lens capsule)

=> Pupil Constriction

Question 5

Lens Accommodation vs. Pupillary Light Reflex?

Answer 5

Accommodation Reflex requires processes image information as an input, rather than a light signal from the retina (like with pupillary light reflex) so must involve Visual Cortex

Question 6

Motor Control Required for Accommodation?

Answer 6

1. Ciliary Muscles (Lens Accomodation)
2. Pupillary Constriction Muscles
3. Intraocular Muscles (Enable Vergence on near object)

Question 7

Ciliary muscles are innervated the same as __________________ MUSLCES:

• Parasympathetic fibers (_____________) from ________________________________________
• Sympathetic fibers (____________) from ________________________________________

Answer 7

Ciliary muscles are innervated the same a PUPILLARY MUSCLES

• Parasympathetic fibers (Contraction: Near Focusing) from ENDINGER WESTPHAL NUCLEUS
• Sympathetic fibers (Relaxation: Far Focusing) from SUPERIOR CERVICAL GANGLION

Question 8

Ciliary muscles are innervated the same as __________________ MUSLCES:

• Parasympathetic fibers (_____________) from ________________________________________
• Sympathetic fibers (____________) from ________________________________________

Answer 8

Ciliary muscles are innervated the same as PUPILLARY MUSCLES

• Parasympathetic fibers (Contraction: Near Focusing) from ENDINGER WESTPHAL NUCLEUS
• Sympathetic fibers (Relaxation: Far Focusing) from SUPERIOR CERVICAL GANGLION

Question 9

Key differences between Pupillary Light Reflex & Accommodation?

Answer 9

Key differences are that Accommodation involves:

• Cortex (Peristriate Area 19): Inclusion of cortical networks allows more complex reflex pathway than pupillary light reflex
• Neurons controlling medial rectus muscle

Question 10

What condition is characterized by Dissociation of Pupillary Light Reflex & Accommodation?

How Does this Manifest?

Answer 10

Argyll-Robertson Pupil

Characterized by Bilateral, SMALL PUPILS that:

1) Exhibit NO constriction in response to light

2) Constrict during normal accommodation

Associated with NEROSYPHILLIS: Rare Presently, sometimes seen w diabetes. Underlying lesion not identified

Question 11

Name these Extraocular Muscles

Answer 11

Question 12

Angle of the orbit is slightly _______________

Answer 12

Angle of the orbit is slightly LATERAL

Question 13

Innervation of Extraocular Muscles?

Answer 13

• LATERAL RECTUS is innervated by Abducens Nerve (CN VI)
• SUPERIOR OBLIQUE is innervated by the Trochlear Nerve (CN IV)
• MEDIAL, INFERIOR, SUPERIOR RECTI and the INFERIOR OBLIQUE Muscles are innervated by the Oculomotor Nerve (CNIII)

Question 14

Why Does the Eye Need to Change Position?

Answer 14

Changing Object Focused on Fovea (4 Distinct Movements):

• Vergence Movements (Horizontal Rectus muscle ONLY)
• Saccadic Movements
• Smooth Pursuit Movements
• Sustained Gaze

Compensation for Head Movements (2 Systems)

• Vestibulo-Occular Movements: Stabilizes image during BRIEF head movement
• Optokinetic Movements: Stabilizes image during SUSTAINED head movement (driven by visual input)

Question 15

Eye movements for changing object focused on Fovea?

Answer 15

Vergence Movements (Horizontal Rectus muscle ONLY)

Saccadic Movements

Smooth Pursuit Movements

Sustained Gaze

Question 16

Eye movements compensating for Head movements?

Answer 16

Vestibulo-Occular Movements: Stabilizes image during BRIEF head movement

Optokinetic Movements: Stabilizes image during SUSTAINED head movement (driven by visual input)

• Note: Optokinetic Reflex ≠ Optokinetic Movements: optokinetic reflex: combined saccade and smooth pursuit seen when tracking objects moving rapidly across the visual field

Question 17

Two Components to Motor Control of a Saccade?

Pattern of neuronal firing?

Answer 17

RATE of position change + FINAL POSITION of the eye encoded by the Extraocular Motor neuron

D1-D3:

• Burst encodes rapid change of position
• Change in rate of tonic discharge between bursts holds eye in new position

D4 (Cessation of Firing)

• Allows return of eye to baseline
• Held in baseline by new, lower rate of tonic discharge

Question 18

Pathway Responsible for Saccadic Eye Movement?

Saccadic Movements are initiated by _____________ => ________________ (Directly OR Indirectly via ____________________)=> _____________________________ ACTIVATES circuit=> Activation of yoked pair of motor neurons and inhibition of corresponding contralateral pair

Answer 18

Saccadic Movements are initiated by Cortex => Superior Colliculus DIRECTLY or via BASAL GANGLIA (Caudate Nucleus/Substantia Nigra)=> Paramedian Pontine Reticular Formation => Activation of yoked pair of motor neurons and inhibition of corresponding contralateral pair

Question 19

What Prevents Image Loss due to Retinal Adaptation?

Answer 19

Microsaccades prevent Image lost through Retinal Adaptation

• The visual system is designed to deal with novelty. Therefore, images presented continuously to same region of the retina rapidly disappear due to Retinal Adaptation
• Microsaccades are smaller movements than regular saccades. A combination of microsaccades and drift result in the stimulus not being presented to the same photoreceptors for an extended period of time.

Question 20

As an object begins to move, the first movement is _______________ initiated by the ______________. Then_________________ of the object monitored and maintained by the _______________

Answer 20

As an object begins to move, the first movement is SACCADE initiated by the BASAL GANLGIA. Then SMOOTH TRACKING of the object monitored and maintained by the CEREBELLUM

Unlike the saccade, the smooth pursuit is not a two-component movement (i.e. the eyes do not move once then fix in one position)

Question 21

Brain Regions Responsible for Smooth Pursuit

Like saccadic movements, smooth pursuit is under control of the Cortex. However, different cortical regions play a role in smooth pursuit movements:

• __________________________________
• What does this region interpret?

Answer 21

Brain Regions Responsible for Smooth Pursuit

Like saccadic movements, smooth pursuit is under control of the Cortex. However, different cortical regions play a role in smooth pursuit movements:

• Middle Temporal/Medial Superior Temporal Areas
• This region interprets movements in the visual field. Therefore, as smooth pursuit involves tracking a moving object, it makes sense that information from this region flows into the control of the smooth pursuit.

Question 22

Describe the process of Gaze Tracking

Answer 22

Gaze required when the eyes move to a target, followed by the head moving

• Target moves slightly to the right => Saccade allows eye to track the movement of the target
• After the saccade, head begins to turn towards the target
• Eyes undergo another Saccade to counteract the movement of the head
• Final component of the eye movement is a Vestibulo-ocular Reflex, which fine-tunes the eye position relative to the head movement (remember, saccades cannot adapt after initiation)