The Extraocular Muscles And The Semicircular Canals Flashcards
Double vision (Diplopia)
Occurs if any of the EOMs is paralysed/weak
What is the importance of the vestibular apparatus?
For balance and detecting movement of the head
Complex pathways in the brainstem coordinate head and eye movements and are tested as part of brain-death assessment.
Extra-ocular muscles
7 muscles, control the movement of the eyes
Inside the orbit, attached to the outer surface of the eyeball
Allow us to move our eyes without moving our head
Coordinated of movement of both eyes (conjugate movements) allow us to form one image; paralysis of muscles results in diplopia (double vision).
Extraocular muscles
1 lifts the upper eyelid = levator palpabrae superioris (LPS)
6 move the eyeball
4 recti = medial rectus (MR)
lateral rectus (LR)
superior rectus (SR)
inferior rectus (IR)
2 obliques = superior oblique (SO)
inferior oblique (IO)
Rectus= straight
All of these muscles insert onto the sclera
Named according to the positions relating to the eyeball
Nerves involved
3 cranial nerves involved: CN III – Oculomotor
CN IV – Trochlear
CN VI – Abducens
Trochlear = superior oblique
Abducens = lateral rectus
Oculomotor = all the others (levator palpabrae superioris, medial rectus, superior rectus, inferior rectus, inferior oblique)
LR6 SO43
Origins of the muscles
Attached to the orbital bones
Insert into the sclera (except LPS)
LPS inserts into upper eyelid
How the muscles move the eye depends on:
Where the muscle originates from (which part of the orbit)
Where the muscle inserts on the eyeball
> Determines the direction that the muscle will ‘pull’ the eyeball when it contracts
Movements possible
Some muscles only move the eye in one direction
Some move the eye in more than one direction
Overall the movements of the eyes involve several muscles acting together as synergists or ear antagonists
Movements: additional considerations
Because the apex of the orbit lies medially, the optical axis – (the eye looking straight ahead) does not coincide with the orbital axis.
Therefore the direction of the muscle fibres does not coincide with the optical axis (as they arise medially).
Levator palpebrae superioris
Inserts unto the upper eyelid, elevating the upper lid when this muscle contracts
Innervation by the oculomotor nerve (parasympathetic) and sympathetic fibres
The sympathetic fibres allow an individual to keep their eye open
Medial rectus
Medial rectus- moves the eye medially
Adducts
Oculomotor nerve CN III
Lateral rectus
Moves the eye laterally
Abducts
Adbucens nerve CN VI
Look on slides for conditions
Superior rectus- inserts on anterior eyeball
Primary elevates the eye
Secondary adducts and medially rotates
Innervation by Oculomotor nerve
Inferior rectus- inserts on anterior eyeball
Primary depresses the eye
Secondary adducts and laterally rotates
Innervation by the oculomotor
Superior oblique- insert on posterior of eyeball
Primary medially rotates the eye
Secondary depresses and abducts
Innervation by trochlear nerve
Inferior oblique- inserts on posterior eyeball
Primary laterally rotates the eye
Secondary elevates and abducts
Innervation by oculomotor nerve
When looking straight down- depression
Coordinated actions of SO and IR
Both depress the eye- act as synergists
SO medially rotates and abducts
IR laterally rotates and adducts- act as antagonists
Examinations
On ppt
The inner ear
Contains:
Cochlea- sound
Vestibular apparatus- balance
Vestibular structures
utricle and saccule- for balance
3 semicircular ducts - contain fluid.
The semicircular ducts
Orientated at right angles to each other.
Contain fluid (endolymph).
Empty into a sac called (the utricle).
The semicircular ducts and utricle the contain sense organs for balance
Ampulla is part of the semicircular ducts
Ampulla contains the capulla
Function of the semicircular ducts
Detect movement of the head (head moves in one direction)
Endolymph, cupula and hair cells in the ampulla bend in the opposite direction
Info is sent centrally from right and left SC ducts
Via 8th cranial nerve
To nuclei in the medulla
Vestibular nuclei
Signals reach nuclei in the brainstem. Connections control:
Posture
Balance
Conscious awareness of position
The nuclei make connections with the nuclei of CNs III, IV and VI
Coordinated head and eye movements
Oculocephalic reflex
This maintains fixed gaze when the head is moving
Normally, when the head rotates:
-the eyes move in the opposite direction
-the gaze remains fixed.
Reflex can be tested to assess the vestibular apparatus and brainstem.
Absent reflex:
-eyes move in the same direction as the head
-indicates a brainstem lesion
