The Extraocular Muscles And The Semicircular Canals

Question 1

Double vision (Diplopia)

Answer 1

Occurs if any of the EOMs is paralysed/weak

Question 2

What is the importance of the vestibular apparatus?

Answer 2

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.

Question 3

Extra-ocular muscles

Answer 3

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).

Question 4

Extraocular muscles

Answer 4

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

Question 5

Nerves involved

Answer 5

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

Question 6

Origins of the muscles

Answer 6

Attached to the orbital bones
Insert into the sclera (except LPS)
LPS inserts into upper eyelid

Question 7

How the muscles move the eye depends on:

Answer 7

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

Question 8

Movements possible

Answer 8

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

Question 9

Movements: additional considerations

Answer 9

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).

Question 10

Levator palpebrae superioris

Answer 10

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

Question 11

Medial rectus

Answer 11

Medial rectus- moves the eye medially
Adducts

Oculomotor nerve CN III

Question 12

Lateral rectus

Answer 12

Moves the eye laterally

Abducts

Adbucens nerve CN VI

Look on slides for conditions

Question 13

Superior rectus- inserts on anterior eyeball

Answer 13

Primary elevates the eye
Secondary adducts and medially rotates

Innervation by Oculomotor nerve

Question 14

Inferior rectus- inserts on anterior eyeball

Answer 14

Primary depresses the eye

Secondary adducts and laterally rotates

Innervation by the oculomotor

Question 15

Superior oblique- insert on posterior of eyeball

Answer 15

Primary medially rotates the eye

Secondary depresses and abducts

Innervation by trochlear nerve

Question 16

Inferior oblique- inserts on posterior eyeball

Answer 16

Primary laterally rotates the eye

Secondary elevates and abducts

Innervation by oculomotor nerve

Question 17

When looking straight down- depression

Answer 17

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

Question 18

Examinations

Answer 18

On ppt

Question 19

The inner ear

Answer 19

Contains:
Cochlea- sound
Vestibular apparatus- balance

Question 20

Vestibular structures

Answer 20

utricle and saccule- for balance

3 semicircular ducts - contain fluid.

Question 21

The semicircular ducts

Answer 21

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

Question 22

Function of the semicircular ducts

Answer 22

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

Question 23

Vestibular nuclei

Answer 23

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

Question 24

Oculocephalic reflex

Answer 24

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