Oculomotor system

flowchart LR
    A["Versions = gaze movements,<br />conjugate eye movements,<br />associated ocular movements"]
    B["Fast conjugate eye movements (saccades)"]
    C["Slow conjugate eye movements"]
    D["Volitional, targeted,<br />spontaneous saccades"]
    E["Fast phases of optokinetic<br />and vestibular nystagmus"]
    F["Smooth pursuit and slow phases<br />of optokinetic nystagmus"]

    G["Vergences = disjugate or<br />disjunctive eye movements"]
    H["Accommodative vergence"]
    I["Fusional vergence"]
    J["Vestibulo-ocular reflex and<br />slow phases of vestibular nystagmus"]

    %% Connections from main categories
    A --> B
    A --> C
    G --> H
    G --> I

    %% Detailed subtypes
    B --> D
    B --> E
    C --> F
    C --> J

Vestibular-optokinetics

• Function: maintain eye position with respect to changes in head and body position.

• Stimulus: change in head and body posture

• Latency: 10 msec

• Velocity: as high as 300 degrees/sec

• Neural connections
• Vestibulo-ocular reflex
• Input
• Labyrinthine semicircular canals
• Otoliths organs
• Sensitivity to linear acceleration, generate compensatory eye movements during translational movements of the head.
• Vision
• Using smooth muscle pursuits to supplement vestibular-optokinetic
• Somatosensors (muscle spindles and joint receptors)
• Processing
• Vestibular nuclei

• Examination
• Bedside caloric testing
• Can determining the side of a peripheral vestibular lesion.
• Check to ensure tympanic membranes is intact
• Supine + head flexed 30 degrees.
• Flexion of the head places the lateral semi-circular canals in a vertical position.
• Irrigate the external auditory canals on each side with a small amount of ice water
• A normal response can be elicited with as little as 0.2 mL
• Normal: the ear that is irrigated exhibits nystagmus with its quick phase in the opposite direction.
• Abnormal: Failure to elicit this response indicates loss of unilateral vestibular function.

• Disease
• Parietal or temporal lobe lesion: abnormal optokinetic towards the side of lesion

Smooth pursuits

• Function: holds target image steady during linear movement of object or self

• Eg when walking or item moving

• To keep target that is on the fovea to be always on the fovea

• Has similar neural pathway as saccades
• Retinal information on the speed and direction of a moving target is projected on the striate visual cortex of the occipital lobes. → This information is relayed to the ipsilateral middle temporal visual area, the frontal eye fields, and the pontine nuclei in a retinotopic fashion → modification of the visual impulses in the cerebellum and vestibular nuclei → impulse projected to the ocular motor nuclei.
• Smooth pursuit movements are identified with the ipsilateral occipital lobes, in contrast to saccadic movements, which are identified with the contralateral frontal lobes

• Stimulus for smooth pursuit
• Visual stimuli: the target moving across the retina, usually within or near the fovea OR
• Non visual stimuli: proprioceptive information related to the movement of one's own limbs, may be sufficient to generate smooth tracking movements.

• Latency: 125ms
• Feedback substrate is usually continuous and slow; objects that move fast may require a combination of pursuit and saccadic movements.

• Cortical centres
• Middle temporal and medial superior temporal gyrus

• Ipsilateral cortical control



 

• Examination
• Ask the patient to hold the head still while tracking a small target, such as the tip of a pencil, held 1m away, Move the target at a slow, uniform speed, and observe whether the eye movements match the velocity and direction of the target.
• If smooth pursuit is too slow (gain is low) then catch-up saccades will be noted
• If smooth pursuit is too fast (gain is high) then back-up saccades area noted
• Pursuit movements naturally deteriorate with age.
• Smooth pursuit movements are also particularly sensitive to drugs, which may affect the results of testing. Such drugs include phenytoin, barbiturates, diazepam, chloral hydrate, methadone, alcohol, and marijuana.

• Disease
• Low gain → saccadic pursuit
• Poor initiation → frontal/parietal lobe lesion
• Deficits found usually in both vertical and horizontal plane

Saccades

• Function: rapidly bring object of interest to focus on fovea

• Can be
• Volitional (voluntary refixations) OR
• Neural connection
• Originate:
• Frontal eye fields
• Aka
• Brodmann's area 8
• Posterior middle frontal gyrus
• Initiating saccades in the contralateral direction
• Via: descending pathways to superior colliculi
• Superior colliculi has topographically arranged visual fields
• When the target image arrives to the fovea a negative feedback system is initiated to cause the eye movement to end.
• Involuntary (quick phases of vestibular and optokinetic nystagmus)

• A ballistic process: once started cannot be stopped

• Stimulus
• Visual reflexive: parietal lobe contralateral
• An object in the peripheral vision
• Latency: 200ms
• Memory guided or volitional: frontal lobe contralateral

• Centre
• Horizontal saccades: PPRF → Pons

• Vertical saccades:
• Rostral interstitial nucleus of medial longitudinal fasciculus (riMLF)+Posterior commissure (PC) → midbrain
• riMLF projects to
• Ipsilateral: Motor neurons of depressor muscle
• Bilateral: Motor neurons of elevator muscle
• Interstitial nucleus of Cajal (INC) projects
• Via posterior commissure
• To
• Motor neurons of contralateral nuclei of the 3rd and 4th CN AND
• Contralateral INC


• Definitions in describing saccades
• Latency
• Duration of stimulus to movement
• Disease: Prolonged latency: degenerative disorders
• Accuracy
• Arrival of eyes on target
• Velocity and conjugation
• Degree to which 2 eyes move together
• Disease
• Slow saccades in horizontal plane
• Pons
• Slow saccades in vertical plane
• Midbrain
• Hypometric saccades:
• Saccade that falls short of intended target
• Disease
• PPRF lesion
• Hypermetric saccades: overshoots the target
• Disease
• Cerebellar lesions