Eye Movement

 

 

INTRODUCTION

 

A.  Resource:  http://cim.ucdavis.edu/eyes/eyesim.htm (requires Adobe Flash)

 

B.  Purposes:

 

1.   position the eye so that the image of the area of interest falls on the fovea (retinal region of greatest acuity and color vision)

 

2.   stabilize the image when disturbed external or head movement

 

3.   track moving objects

 

4.   compensate for visual sensory adaptation when viewing static objects

 

5.   prevent eye movement-induced blurring when scanning

 

C.  Anatomical Basis of Eye Position Control

 

1.   Extraocular muscles (named for their position relative to the eyeball and the direction in which they move the eye upon contraction – more-or-less)

 

a.   medial rectus

b.   lateral rectus

c.   superior rectus

d.   inferior rectus

e.   superior oblique

f.    inferior oblique

 

 

Note:  extraocular muscle test:  subject asked to fixate on ends and intersections of “H” figure

 

2.   Characteristics

 

a.   very fast

b.   subject to discrete, precise control (innervation ratio 3:1, three muscle fibers per motor axon)

 


 

INTRODUCTION

 

C.  Anatomical Basis of Eye Position Control

 

3.   Innervation

 
a.   C.N. III (Oculomotor)

 

1)   oculomotor nucleus supplies the ipsilateral levator palpebrae superioris and all the internal and external muscles of the ipsilateral eye, except the lateral rectus, superior oblique, and dilator papillae

 

2)   contains preganglionic parasympathetic fibers innervating the iris and accommodation muscles;  these fibers originate from the Edinger-Westphal nucleus and synapse in the ciliary ganglion

 

 
3)   lower motoneuron lesion

 

a)   strabismus (lack of coordinated eye positioning)

b)   ptosis (drooping eyelid)

c)   pupil dilation (mydriasis)

 

b.   Trochlear (C.N. IV)

 

1)   only dorsal cranial nerve

2)   only cranial nerve that decussates in the CNS

3)   trochlear nucleus supplies the contralateral cranial nerve and the contralateral superior oblique

4)   lower motor neuron lesion

a)   lesion of the nucleus or tract within the CNS before it has crossed the midline:  paralysis of the contralateral superior oblique and strabismus

b)   lesion of the nerve (outside the CNS):  paralysis of the ipsilateral superior oblique and strabismus

 

Note:  patient will often rotate his head in an attempt to resolve the strabismus

 

c.   Abducens (C.N. VI)

1)   innervates the ipsilateral lateral rectus muscle

2)   lower motor neuron lesion:  paralysis of the ipsilateral lateral rectus and strabismus

 


 

EYE MOVEMENTS

 

A.  Saccades

 

1.   Rapid (about 0.2 sec), abrupt eye movement followed by brief pause; duration of the rapid jump phase relatively constant independent of distance moved (the saccade proper is the rapid jump, not the pause)

 

2.   Based on CNS computed distance between the current position of the eye and the desired new position

3.   Ballistic:  preprogrammed movement executed without feedback

4.   Intentional saccades:  voluntary saccades, characteristic of vision when gazing at a fixed object; frequency of several saccades per second as eye peruses points of interest

 

Bust of Nefertiti          Saccade Map                           Saccades when reading text

 
 

 

 

 

 

 

 

 

 

 

 

 

 


                        Saccades when reading text

 

5.   Reflex saccades:  eye movement in response to an external object (the saccade proper is the rapid jump, not the pause)

 

6.   Note blanking of visual consciousness during rapid jump phase of saccade

 

Note:  if the image on the retina remains invariant, its sensory effect becomes progressively less and if the scan was smooth the image on the retina would be blurred

 

B.  Smooth Pursuit Movement

 
 


1.   Eye movement when attempting to follow a moving object

 

2.   Cannot be initiated voluntarily in the absence of a moving target

 

3.   When following a train of moving objects, eye movement pattern is termed Optokinetic Nystagmus

 

Note 1:  Nystagmus:  slow movement of eyes in one direction followed by rapid movement in the opposite direction; the direction of the nystagmus is the direction of the rapid phase (e.g. slow movement of the eyes to the left followed by a jump to the right is call “right nystagmus”)

 

Note 2:  Nystagmus can be normal, as in smooth pursuit movements, or abnormal when it occurs in the absence of normal stimuli.  When abnormal, it causes inappropriate movement of the visual field on the retina and confusion in visual orientation and can result in postural defects and nausea

 

4.   Note blanking of visual consciousness during the nystagmus rapid phase

 

C.  Vestibular-Ocular Movement (Vestibular-Ocular Reflex)

 

 
1.   Movement of the eyes to compensate for head movement

 

2.   Initiated by stimulation of vestibular sensory organs associated with head movement

 

3.   Continued rotation results in nystagmus (“Rotary Nystagmus” or Physiological Nystagmus”)

 


 

 

EYE MOVEMENTS (continued)

 

D.  Vergence Movements

 

1.   Divergent movement of the two eyes to focus the same point of interest on the two foveas

 

2.   Vergence movements occur when the gaze shifts from objects at one distance to objects at another distance

 

3.   Vergence movements are disjunctive (or disconjunctive) – eyes move in different directions

 

Note 1:  the other three types of eye movements are conjunctive – eyes move in the same direction

 

Note 2:  the inability of the ocular motor system to focus the two eyes on the same visual field results in diplopia (“double vision”)

 

Note 3:  if diplopia persists for an extended  period of time, particularly in a child, it may result in amblyopia (“lazy eye”):  suppression of vision from the non-dominant eye

 

4.   Vergence movements are important in estimation of the distance of an object of interest

 

Note:  the other sources of information which aid in estimating distance (1) the apparent size of the object viewed (based upon perspective and prior knowledge of actual size) and (2) upon information derived from optical accommodation

 

NERVOUS SYSTEM CONTROL OF EYE MOVEMENTS

 

A.  Gaze Centers and Medial Longitudinal Fasciculus

 

1.   Horizontal Gaze Center

a.   located in the reticular formation of the pons (also known as the Paramedian Pontine Reticular Formation, PPRF)

b.   output to nuclei of motor nerves controlling horizontal eye movement

 

2.   Vertical Gaze Center

a.   located in the rostral part of the midbrain reticular formation (also known as the Rostral Interstitial Nucleus)

b.   output to nuclei of motor nerves controlling vertical eye movement

 

3.   Medial Longitudinal Fasciculus

a.   midline tract in the medial midbrain and pons

b.   connects the nuclei of the three eye movement motor nuclei

 

4.   Movement in arbitrary directions determined by the relative output of the two gaze centers and coordination by the medial longitudinal fasciculus

 

Example: shift gaze to the right

 


 

NERVOUS SYSTEM CONTROL OF EYE MOVEMENTS

 

B.  Superior Colliculus

 

1.   pair of nuclei located on the upper surface of the midbrain

2.   output to the gaze centers

3.   controls the direction and magnitude of saccades

 

4.   input from

a.   retina

b.   auditory system

c.   somatosensory system

d.   frontal eye field

 

C.  Frontal Eye Fields

 

1.   Located in the frontal lobe of the cerebral cortex (Brodmann’s area 8)

2.   Output to the superior colliculus and the gaze centers

3.   Input from many cortical and subcortical areas

4.   Important for

a.   eye movements not guided by a visual target

b.   scanning the visual field

 


 

 

SUMMARY DIAGRAM

 

 

 

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