What are the components that control gaze and stabilise an image on the retina?
Oculomotor system: moves eyes in orbit whilst head is still
Head-movement system: moves eye sockets as a whole whilst head moves
Describe the different types of eye movements?
Saccadic: shift fovea rapidly to new visual target
Smooth pusruits: keep image of a moving target on fovea
Vergence: moves eyes in opposite directions
Vesticular ocular: holds image still on retina during brief head movements
Optokinetic: holds image stationery during sustained head rotation or translation
Describe the extraocular muscles?
4 rectus muscles: superior, inferior, lateral, medial
2 obliques: inferior, superior
Describe the movements that can be performed by the eye?
What determines the actions of the rectus muscles?
Their insertion onto the eyeball
Describe the primary actions of the rectus muscles?
Describe the insertions of the oblique muscles?
Insert a long way down the back of the eyeball
SO: well behind equator at an oblique angle close to LR (tendon runs through trochlea)
IO: behind equator close to LR
Describe the primary actions of the oblique muscles?
SO: torsion (also involved in depression)
IO: torsion (also involved in elevation)
How can each of the six eye muscles be tested in isolation?
Which lower motor neurons are involved in the neural control of eye movements?
Oculomotor nerve (CN III): SR, IR, MR, IO
Abducens nerve (CN VI): LR
Trochlear nerve (CN IV): SO
Describe the neural control of eye movements?
Hierarchical control of eye movement involving:
Brainstem centres (reticular formation)
Higher cortical areas
How does the neural supply of CN IV to the SO muscle differ to the innervation of the other extraocular muscles?
CN IV innervates contralateral SO
Others all have ipsilateral innervation
Which structures are responsible for coordinating the movements from one eye with the other?
Medial longitudinal fasiculus
Reticular formation: PPRF (pontine paramedian reticular formation) and MPRF (mesencephalic paramedian reticular formation)
What is the MLF?
Medial longitudinal fasiculus
White matter tract that connects the various cranial nerve nuclei
What is the function of the pontine paramedian reticular formation?
Horizontal gaze centre
Coordination of MR and LR of each eye (CN III and CN VI)
What is the function of the mesencephalic paramedian reticular formation?
Vertical gaze centre
Coordination of SO and SR of each eye (CN IV and CN III)
Where is a patient's lesion if they cannot coordinate horizontal movement of the eyes?
Where is a patient's lesion if they cannot coordinate vertical movement of the eyes?
Describe the neural control of horizontal saccades?
Requires simultaneous excitation of burst neurons and inhibition of omnipause neurons
Message from cortex > excites burst neurons > stimulates ipsilateral abducens > contraction of ipsilateral LR
At same time, activates neuron in MLF > stimulates part of contralateral oculomotor nucleus > contraction of contralateral MR
Communication from PPRF inhibits contralateral abducens nucleus > contralateral LR inhbited
At same time, via MLF, inhibitory signal from contralteral abducens nucleus to ipsilateral oculomotor nucleus > ipsilateral MR inhibited
Describe the function of burst neurons in the PPRF?
Fire at high frequency just before movement
Two types: excitatory and inhibitory
Excitatory: provide excitatory connections with ipsilateral abducens
Inhibitory: suppress activity of contralateral abducens
Describe the function of omnipause neurons in the MPRF?
Fire continuously during the saccade, excpet when the eyes move (tell eyeballs when to stop moving)
Project to contralteral abducens nucleus
Which neurotransmitter do omnipause neurons in the MPRF use?
What drives saccadic eye movements?
Higher cortical processing
Frontal eye fields, superior colliculus and basal ganglia play important roles
How is the abducens nucleus connected to the contralteral oculomotor nucleus?
How are we able to maintain focus on a stationary object while moving our heads, without loss of visual focus or dizziness?
Input from vestibular system
How does the vestibular system help us to maintain focus on a stationary object when our head is moving?
Three semiciruclar canals provide information about the head in space (fluid movement across hair cells > neural signal)
In addition, otolith organs give information about linear acceleration
How does neural signalling within semicicrcular canals alter eye movements?
When fluid goes in one direction through the semicircular canal > increase in AP firing
At the same time, fluid going through the oppsoite ear will cause a decrease in AP firing
This signalling goes to the vestibular nucleus in the brainstem > integrates with eye movement system (CN III, CN VI nuclei)
Describe the vestibular-ocular reflex?