21.7 Eye movement control and Pupillary Reflexes Flashcards
What are the different CNs involved in eye movement control and pupillary reflexes?
CN3, CN4, and CN6
What are the main types of eye movement you need to know about?
To bring the image of a target onto the fovea:
- Saccades
- Vergence
To keep the image on the fovea:
- Smooth pursuit (during target movement)
- Vestibulo-ocular (during head movement)
What are saccadic eye movements (a.k.a. saccades)?
[IMPORTANT]
Rapid, ballistic movements where eyes abruptly change point of fixation
e.g. scanning faces
What are the five types of saccades?
Reflexive (prosaccades) - e.g. following jumping dot on screen
Voluntary - e.g. scanning around a room, predicting movement of a target
Spontaneous - gaze shift away from a fixed point (unconscious)
Microsaccades - unconscious tiny movements of eyes
Quick phases - e.g. in nystagmus
What is vergence?
[IMPORTANT]
Alignment of the fovea with objects at different distances (cross eyed/eyes apart)
What is smooth pursuit?
[IMPORTANT]
When the eyes remain fixed on a moving object
What is the vestibulo-ocular eye movement?
[IMPORTANT]
When the eyes are stabilised on an object despite the head moving
What are the signs of lesion of the oculomotor nerve?
- Down and out position of eye at rest
- Ptosis (usually innervates levator palpabrae superioris - raises upper eyelid)
- Dilated pupil (usually innervates sphincter pupillae - constricts pupil)
What are the signs of lesion of the trochlear nerve?
- Diplopia when looking down and inwards
- Head tilt away from affected side (compensation)
What are the signs of lesion of the abducens nerve?
- Diplopia when looking to side
- Affected eye resting in adduction (looking inwards)
- Inability to abduct eye
- Rotate head to allow eye to look sideways (compensation)
What are the three brainstem nuclei involved in eye movements?
Oculomotor nucleus
Trochlear nucleus
Abducens nucleus
Where is the oculomotor nucleus located?
Midbrain, at level of mesencephalic reticular formation
Where is the trochlear nucleus located?
In midbrain, slightly caudal to oculomotor nucleus
Where is the abducens nucleus located?
In pons, at level of paramedian pontine reticular formation. Also adjacent to fasciculus of facial nerve (VII)
Where do the neuronal signals for saccades originate in the brain?
Reticular formation
Vertical saccades = mesencephalic reticular formation (MRF)
Horizontal saccades = paramedian pontine reticular formation (PPRF)
What are the MRF and PPRF also known as?
Gaze control centers
What is the connective role of the medial longitudinal fasciculus?
Links superior colliculus, oculomotor nuclei and vestibular nuclei with gaze control centres (in reticular formation)
What is the key structure through which the cerebral cortex controls saccades?
Superior colliculus
What are the three cortical inputs that the superior colliculus receives? Are they excitatory or inhibitory?
Excitatory from posterior parietal cortex
Excitatory from frontal eye fields
Inhibitory from substantia nigra
What is the process in which the inhibitory substantia nigra can be itself inhibited? (so that its inhibition is released)
Frontal eye field excites caudate nucleus
Caudate nucleus inhibits substantia nigra
What is the role of the vestibular nuclei in eye movement?
They help coordinate antagonist muscles involved in eye movement of smooth pursuit and vestibulo-ocular reflexes
What does the vestibular nuclei provide its input to?
Abducens, trochlear, oculomotor nuclei
What are the higher cortical areas that signal to the vestibular nuclei called?
Medial superior temporal gyrus/middle temporal gyrus (MST/MT)
Frontal eye field
What is the function of the pupillary light reflex?
To adjust the size of the pupil at different light levels
The pupillary light reflex is consensual - what does this mean?
Light directed in one eye produces pupil constriction in both eyes
Which two antagonistic muscle fiber types control pupil size?
Sphincter, dilator
Which branches of the autonomic system are the sphincter and dilator muscle fibres each controlled by?
Sphincter: parasympathetic
Dilator: sympathetic (think - need to see more for dangerous situations)
Which muscle fibre usually dominates in the pupillary light reflex?
Sphincter
Describe the first step of the pupillary light reflex when light hits an eye/both eyes.
Light signal sent through optic nerve(s) to pretectal nuclei of midbrain
Describe the second step in the pupillary light reflex after signal is sent to pretectal nuclei.
Pretectal nuclei BILATERALLY innervate the Edinger-Westphal nuclei of the oculomotor complex, through posterior commissure
Describe the third step in the pupillary light reflex after the signal is sent to the Edinger-Westphal nuclei.
Edinger-Westphal nuclei send oculomotor preganglionic parasympathetic fibres to ciliary ganglion
Describe the fourth (final) step in the pupillary light reflex after the signal is sent to the ciliary ganglion.
Ciliary ganglion sends signal via short ciliary nerve to iris sphincter; pupil constricts.
What is the definition of the accommodation reflex?
The changes in the eye when viewing a near object to bring it into focus
Which 3 motor responses occur in the accommodation (near) reflex?
- Eye gaze moves to be more medial/’cross-eyed’ (vergence) as medial rectus contracts
- Lens gets fatter as ciliary muscles contract
- Pupil constricts as iris sphincter contracts
What is the pathway of the afferent limb of the accommodation reflex?
- Retina
- LGN
- Visual cortex
What are the two components of the efferent limb of the accommodation reflex?
- Edinger-Westphal nucleus (parasympathetic) –> ciliary ganglion –> iris sphincter, ciliary muscle
- Oculomotor neurons –> medial rectus
What is the cause of Horner’s syndrome?
Lesion of sympathetic supply to eye/face
What is the mechanism for partial ptosis occurring in Horner’s syndrome?
There is a lesion to the sympathetic supply to levator palpebrae superioris so the eyelid cannot be held up.
Does Horner’s syndrome make the affected pupil constrict or dilate?
Constrict
(as cannot do normal sympathetic dilation)
