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Conjugate movements (yoked movements) of the eyeballs are when both eyeballs move on one side. In a normal person only these eye ball movements are seen. The side may be right, left upwards or downwards. They are of four types.

 

TYPES OF CONJUGATE MOVEMENTS:

 

A. searching movements: When light from the retina falls on the visual cortex, axons are sent to the frontal eyefields ipsilaterally. Axons from the frontal eyefields move up and medially downwards to cross and synapse with the PONTINE UNIT (Parapontine reticular formation and 6^th nerve nucleus). The right frontal eyefield synapse with the left pontine unit and vice versa. The right pontine unit is connected to the left 3^rd nerve nucleus via the left medial longitudinal fasiculus (MLF) and vice versa.

The 3^rd nerve nuclei are connected to both medial rectus muscles while the 6^th nerve nucleus are connected to the lateral medial rectus muscles such that the frontal eyefields act as PUSH BUTTONS (when the right frontal eyefield fire, the eyeballs move towards the left and vice versa) for FAST SEARCHING MOVEMENTS.   

Note: When person watches straight both right and left frontal eyefields are firing.

 

B. Tracking movements: the occipital eyefields (located in the secondary visual cortex) are also connected with the pontine unit. The right occipital eyefield is connected with the left pontine unit and vice versa. They too act as PUSH BUTTONS, but for GRADUAL TRACKING MOVEMENTS. {When your eyes follow something slowly}.

 

REMEMBER: both searching and tracking movement are movements of the eye horizontally. THE RECTUS MUSCLES CONTRACT TO PULL EYE TOWARDS THEIR SIDE. THE PONTINE UNITS ACT AS PULL BUTTONS; THEY FIRE TO PULL EYE TOWARDS THEIR SIDE. THE EYEFIELDS ACT AS PUSH BUTTONS; THEY FIRE TO PUSH EYE TOWARDS THE OPPOSITE SIDE.

 

LESIONS:

1. If one MLF damaged due to some reason like multiple sclerosis (eg: right).

The right eye can not move left.

2. Both MLF damaged.

Right eye cannot move left and left eye cannot move right.

3. One and a half syndrome: both MLF damaged and one pontine unit damaged (eg: right).

Right eye cannot move left and left eye cannot move right. Since each pontine unit act as a pull button, the right cannot move right.

OPTHALMOPLEGIA:

Defined as the paralysis of muscles involved in eye movements.

Types:

External opthalmoplegia: paralysis of external extra-occular muscles.

Internucleaur opthalmoplegia: paralysis of medial rectus muscles due to damage to both MLF.

Internal opthalmoplegia: paralysis of the internal extra-occular muscles (dilator pupillae, constrictor papillae and ciliary muscle). Cycloplegia is a type of internal opthalmoplegia concerning ciliary muscle paralysis.

 

IF THE EYEFIELDS GET IRRITATED (overstimulation due to epilepsy or tumour), EYEBALLS MOVE AWAY FROM THE SITE OF LESION. IF THE EYEFIELDS EVENTUALLY GET DAMAGED, EYEBALLS MOVE TOWARDS THE SITE OF LESION.

Overstimulation of eyefields result in NYSTAGMUS: involuntary eyeball movements.

 

 

http://media-2.web.britannica.com/eb-media/47/63347-004-610F94B5.gif -open the link to view relevant diagram. 

Of these external eye muscles, the rectus muscles contract to pull the eye towards itself. The lateral rectus moves it in the lateral direction, the medial rectus in the medial direction, the superior rectus move it upwards while the inferior rectus moves it downwards. The oblique muscles however contract to push the eye towards the opposite. The superior oblique moves the eye downwards and the inferior oblique upwards. The levator palpebrae superioris muscle contracts to pull the eyelid upwards.

Now remember: occulomoter nerve (3^rd) is connected to the medial rectus muscles, the superior and inferior rectus muscles, the inferior oblique muscle (pushes the eye upwards) and the levator palpebrae superioris. It is thus responsibe for moving the eye medially upwards, downwards and also responsible for elevating the eyelid.

Abducent nerve (6^th) is only connected to the lateral rectus muscles. It is responsibe for moving the eyes laterally.

Trochlear nerve (4^th) is only connected to the remaining superior oblique, that pushes the eye downwards.

 

VERTICAL GAZE: the movement of eye up and down along the vertical axis.

 

At the posterior side of the brainstem the vestibular, 3^rd 4^th and 6^th nerve nuclei are connected to eachother via the right and left MLFs. The two MLFs move upwards and join with the posterior commisure (joining the pre-tectal nuclei). At the point of junction, the nucleus of Cajal (rostral or interstitial nucleus of MLF/posterior commisure) is present. Axons from the nucleus of cajal move up to synapse with the 3^rd nerve nucleus and down to synapse with the 4^th nerve nucleus. These nuclei are then connected to the respected external muscles of eye that move the eye upwards and downwards.  

 

  

 

 

Parinaud’s syndrome: Tumour in the pineal gland (located above the nucleus of Cajal) may damage the nucleus of cajal, affecting vertical gaze and the 3^rd nerve nucleus and edhinger westphal nucleus, affecting convergence.

 

Marcus Gunn Pupil test: it is a test to determine whether optic neuritis or multiple sclerosis affecting an optic nerve is completely recovered or no.

If it is not recovered, it will be weak.

E.g the right optic nerve: it light is shone on the right eye, both eye pupils constrict slightly since light stimulus cannot penetrate the optic nerve properly. However if light is shone on the left eye, both pupils constrict.

If light source is made to swing from left to right eye, it is observed that the pupils constrict and dilate. The pupils don’t actually dilate but when they constrict lightly from constricting completely, it seems they dilate.

 

3. Vestibular movements: when the head or body is moving but the eyes are fixed at one point.   

 

4. Convergence movements.