University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

390

Pages:

4
Author:

Mariam A.

Eye movements Introduction Types of eye movement  Saccadic system: shifts the fovea rapidly to bring the image of an object of interest onto the fovea.  Smooth pursuit: holds the image of a moving target on the fovea.  Vergence movements: moves the eyes in opposite directions so that images of a single object are held on the fovea of both eyes.  Vestibulo-ocular movements: hold images on the retina during brief head movements.  Optokinetic movements: hold images on the retina during sustained head rotation. Extra-ocular muscles    There are 6 extra-ocular muscles that are all involved in eye movement. Tendon insertion is on the tough white membrane on the exterior of the eye. If an individual muscle is contracted, the direction of eye movement is the cardinal direction for that muscle.  However, in reality, many muscles partake in producing movement. Movement Adduction. Main muscle Medial rectus. Others contributing Superior and inferior rectus. Abduction. Lateral rectus. Superior and inferior oblique. Elevation. Superior rectus. Inferior oblique. Depression. Inferior rectus. Superior oblique. Extorsion. Inferior oblique. Inferior rectus. Intorsion. Superior oblique. Superior rectus.   o Note: extorsion = top of eye moves away from nose; intorsion = top of eye moves towards nose. The eye does not sit straight in the orbit, but rather is on an angle. Thus, the contraction of a muscle may not be in an axis that is normal to the orbit. The muscles have different functions depending on the eye’s position (e.g. superior oblique has different role when eye is looking medial vs. lateral). Neuron control of extra-ocular muscles  Motor unit within the cranial nerve nuclei are very small (1 motor unit = 2 – 6 muscle fibres). This allows for precise control over muscle groups. Clinical correlation: damage to oculomotor nucleus  Presents with a “down and out” eye movement. o Unable to move the eyes medially when tested with a wand.  Other characteristics of this damage include slight ptosis (due to involvement of the lavatae) and pupil dilation (parasympathetic nerve runs along with CNIII).  Clinical importance of parasympathetic nerves running along with CNIII is that symptoms provide clues to what type of damage has occurred. o Damage to nerves resulting from hypertension and diabetes usually don’t present as pupil diation due to sparing of the parasympathetic nerve. o However, aneurysms or tumours, which press on to both nerves, present with pupil dilation.