FUNCTIONAL ANATOMY - Vestibular System Flashcards
What is the vestibular system?
The primary sensory system which maintains balance
What are the three structures within the inner ear which contribute to the vestibular apparatus?
Semicircular ducts
Utriculus
Sacculus
What is the function of the semicircular ducts in the vestibular system?
Sensory detection of rotary acceleration and declaration of the head
What is the name of the neuroepithelium of the semicircular ducts?
Crista ampullaris
Where in the semicircular ducts is the crista ampullaris located?
The crista ampullaris is located in the ampullae
Describe the structure of the crista ampullaries
The crista ampullaris has mechanoreceptors known as hair cells which have sterocilia projecting into the overlaying gelatinous cupula. These hair cells are innervated by the vestibular branch of CN VIII
Describe briefly how the semicircular ducts detect rotary acceleration and deceleration of the head
Rotation of the head will cause the endolymph within the semicircular ducts to move in the opposite direction to the initial movement, causing deflection of the cupula and subsequent deflection of the stereocilia of the hair cells, triggering depolarisation of the vestibular branch of CN VIII to increase action potential frequency on that side of the head and hyperpolarisation of the vestibular branch of CN VIII on the other side of the head to decrease action potential frequency
How does the arrangement of the semicircular ducts allow for the detection of head rotation in any direction?
The semicircular ducts are orientated roughly 90° from each other to maximise the ability to detect rotation of the head
What is the function of the utriculus and the sacculus in the vestibular system?
Sensory detection of horizontal and vertical linear acceleration and deceleration of the head
What is the name of the neuroepithelium in the utriculus and the sacculus?
Macula
Describe the structure of the macula
The macula has mechanoreceptors known as hair cells with sterocilia which project into the overlying gelatinous otolithic membrane with otolith crystals on the surface. These hair cells are innervated by the vestibular branch of CN VIII
How do the hair cells within the utriculus and sacculus exhibit topographical organisation?
Groups of hair cells within the utriculus and sacculus are sensitive to movement in specific directions
Describe how the utriculus and sacculus detect linear acceleration and deceleration of the head
When the head undergoes linear acceleration or deceleration, the otolith crystals will pull the otolithic membrane and subsequently the stereocilia in the opposite direction of the initial movement, stimulating depolarisation and increase action potential frequency in the vestibular neurones innervating the hair cells sensitive to that specific direction, and hyperpolarisation to decrease action potential frequency of the vestibular neurones innervating the hair cells that are not sensitive to that specific direction
Why are the hair cells in the utriculus more sensitive to horizontal linear movement?
The macula is orientated horizontally in the utriculus so the hair cells are more sensitive to horizontal linear movement
Why are the hair cells in the sacculus more sensitive to vertical linear movement?
The macula is orientated vertically in the sacculus so the hair cells are more sensitive to vertical linear movement
Which structures receive sensory information from the vestibular apparatus via the vestibular neurones of cranial nerve VIII?
Vestibular nuclei in the brainstem
What are the four vestibular nuclei?
Rostral vestibular nucleus
Caudal vestibular nucleus
Medial vestibular nucleus
Lateral vestibular nucleus
Which three structures receive efferent fibres from the vestibular nuclei?
Brainstem
Cerebellum
Spinal cord
Which two vestibular nuclei send efferent fibres to the brainstem?
Lateral vestibular nucleus
Medial vestibular nucleus
Which cranial nerve nuclei in the brainstem receive projections from the vestibular nuclei?
CN III (oculomotor) nucleus
CN VI (abducens) nucleus
What is the vestibulo-ocular reflex?
The vestibulo-ocular reflex is the compensatory movement of the eye in the opposite direction to initial head rotation to maintain the visual field in the centre of the retina
Describe the vestibulo-ocular reflex
Rotation of the head will cause depolarisation of the ipsilateral vestibular branch of CN VIII which will project to the ipsilateral vestibular nuclei at the brainstem. The vestibular nuclei will project to the contralateral CN VI (abducens) nucleus, stimulating the contraction of the contralateral lateral rectus extraocular muscle. The vestibular nuclei will also project to the ipsilateral CN III (oculomotor) nucleus, stimulating contraction of the ipsilateral medial rectus muscle
What is the function of the cerebellum in the vestibular system?
The cerebellum coordinates the position of the trunk, limbs and neck in relation to the position of the head
Which two vestibular nuclei send efferent fibres to the spinal cord?
Lateral vestibular nucleus
Medial vestibular nucleus
What is the function of the efferent fibres of the lateral vestibular nucleus projecting to the spinal cord?
Lateral vestibular nuclei have efferent projections to the cervical and lumbar regions of the spinal cord to adjust body posture
What is the function of the efferent fibres of the medial vestibular nucleus in relation to the spinal cord?
Medial vestibular nuclei have efferent projections to the cervical region of the spinal cord to adjust head and neck posture
What is postural equilibrium?
Regulation of posture with respect to gravity
What is static equilibrium?
Maintaining balance whilst the body is in a stationary position
What is dynamic equilibrium?
Maintaining balance whilst the body is in motion
What are the two forms of control involved in maintaining posture?
Feedback control
Feedforward control
What is feedback control?
Continuous monitoring and adjusting of the body based on sensory information
What is feedforward control?
Anticipating and preparing for movements or postural changes before they happen
What stimulates the vestibular reflexes?
Vestibular reflexes are stimulated by the movement of the head relative to the vertical axis
What stimulates the tonic neck reflexes?
Tonic neck reflexes are stimulated by the movement of the head relative to the rest of the body
What is the labyrinthine righting reflex?
The labyrinthine righting reflex restores normal posture in an animal that has fallen or is unbalanced
Which four components are required for the labyrinthine righting reflex?
Utriculus
Saculus
Semicircular ducts
Muscle spindles in the neck
Describe briefly the labyrinthine righting reflex in cats
The head is initially adjusted relative to the field of gravity followed by the extension of the legs to prepare the cat for landing
