PHTY209 Lecture Notes - Lecture 12: Superior Colliculus, Extraocular Muscles, Special Senses
Introduction to the vestibular system
• Describe the structures of the outer, middle and inner ear and explain their functions
o Role of the vestibular system
• Vestibular information is essential for postural control and for the control of
eye movements
• Vestibular apparatus contains sensory receptors that respond to the position
of the head relative to gravity and to head movements
• Information is converted into neural signals that are conveyed by the
vestibular nerve to the vestibular nuclei
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• Describe the parts of the inner ear and receptor responses that underpin postural and
ocular stability
o Peripheral vestibular apparatus has two different types of sensory receptors
• Otoliths (utricle and saccule) - respond to linear acceleration and gravity
▪ Both are called otolithic organs; membranous sacs within the vestibule
▪ Respond to head position relative to gravity and to linear acceleration
and deceleration
▪ Inside each utricle and saccule is a sensory receptor called the macula
▪ Macula consists of hair cells embedded in a gelatinous material
▪ On top of gelatinous material are otoconia (calcium carbonate crystals)
▪ Hair cells synapse with neurons of vestibular nerve (branch CN 8)
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▪ When macula is moved into different positions, weight of otoconia
bends the hair cells
▪ Bending hairs stimulates or inhibits the hair cells, depending on the
direction of the bend, which determines the frequency of firing of
neurons in the vestibular nerve
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Document Summary
Synapse with neurons of vestibular nerve (branch of cn 8) Is always a baseline level of firing, even when the head is stationary: when the head turns, inertia causes fluid (endolymph) in canal to lag behind. Flow of fluid in canal, moves cupula and bends the hairs of hair cells: bending of hair cells changes pattern of firing in neurons of vestibular nerve, work in pairs. L horizontal sc works with r horizontal sc (green: r anterior sc works with l posterior sc (blue) L anterior sc works with r posterior sc (red: each canal pair produces reciprocal signals. Suppression of vor required: done by cerebellum, dysfunction of vor, can be assessed clinically. Senses/perceives position/motion head: all planes of movement. Signals relationship to the support surface: controls the com over bos, orientates body/eyes to the vertical. Stabilises eyes in relation to head position and movement: environment is mobile/visually complex, e. g. shopping centres/exercise classes/aquatic environment, supports other senses - resolves conflict.
