Neurology 5: Vestibular System Flashcards
Briefly describe the main components of the vestibular apparatus and explain how this apparatus detects movements of the head, specifying which structures detect angular acceleration and which structures detect linear acceleration
Major components:
- Peripheral receptor apparatus- transduction of head position and motion
- Vestibular nuclei in brain stem- recieve, integrate and distribute information controlling eye and head movements, postural reflexes and spatial orientation.
- vestibulo-ocular network- control eye movements
- vestibulo-spinal network- coordinates head movements, axial musculaturem and postural reflexes
- vestibulo-thalamao-cortical network- conscious perception of movement and spatial orientation
(Angular acceleration) ➡Function of the cupula (in the semicircular canals) - slide 10
-when head is rotated in plane of canal, inertia of ends lymph creates forces that displaces cupula, causing bending of hair cells (stereocilia)
Linear acceleration: Utricle and Saccule slide 14
- detect displacements and linear accelerations of head (eg tilting)
- both contain macula, which has stereocilia and assosiated structures
- utriclar macula orientated horizontally
- saccular macula orientated vertically
-as with cupula, bending towards and away from tallest stereocilium causes depolarisation and hyper polarisation respectively.
Give me an overview of the vestibular system
- special proprioception system that enables person to maintain orientation in its environment with respect to gravity.
- helps to maintain position of eyes, trunk and limbs in relationship to movements and positioning of head.
- specialised receptor organ located in inner ear
- signals carried via vestibular nerve (CN 8) to vestibular nuclei on medulla and pons
- projections from vestibular nuclei to spinal cord, brain stem and cerebellum
Explain how movements detected by the various components of the vestibular apparatus are converted into electrical signals
-as with cupula, bending towards and away from tallest stereocilium causes depolarisation and hyper polarisation respectively.
List the 3 main neural networks to which signals from the vestibular nuclei project and describe the general function of each
Projections to cerebellum:
- input from vestibular branch of CN 8, via the caudal peduncle
- major input from vestibular nuclei, also through caudal peduncle
- cerebellum has a role in vestibulo-ocular adaption
Projections to spinal cord: MVST
MVST arises from medial, rostral and caudal vestibular nuclei
-passes causally in MLF
-both sides, but mainly ipsilateral
-innervated cervical and cranial thoracic segments
-mediates vestibulo-cervical reflex
Projections to spinal cord: LVST
- axons from LVN descend in LVST to entire ipsilateral spinal cord
- innervated axial and proximal limb muscles
- mediates vestibulo-spinal reflex
- coordinates limb, trunk and head movements in response to vestibular signals
For a normal individual, explain how signals from the medial vestibular nucleus mediate the vestibulo-ocular reflex that coordinates eye movements with head movements
Vestibulo-cervical Reflex:
- adjusts position of head
- flex activity of neck muscles in response to vestibular signals
- during a fall forwards, for example, MVST will recieve signals on:
- downward linear acceleration from saccule
- changing head position relative to gravity from both utricle and saccule
- forward movement from vertical semicircular canals
Reflex contraction of neck mud les pulls head up
-protects head from impact
Dorsal flexion of head initiates other reflexes➡extension of arms and flexion of legs to “break” fall.
Explain the sequence of events that occurs in physiologic nystagmus
Nystagmus (rotatory)
- when semicircular canals sense head rotation in one direction, eyes rotate slowly in opposite directions
- followed by rapid “resetting” of eyes in direction in which head has rotated
- fixes eyes on a new field of vision
- this normal pattern of slow and quick eye movements in response to rotation is physiologic nystagmus
- skis phase controlled by vestibular nuclei- involves MLF
- fast phase controlled by pontine gaze centre (pontine reticular formation)
Nystagmus (postrotatory) -slide 24
Explain the principles underlying the caloric testing of the vestibulo-ocular reflex in unconscious patients, how the test is conducted, and how the results are interpreted
Nystagmus caloric test
Evaluation of:
-vestibular-ocular pathways in conscious patients
-brain stem integrity in unconscious patients
Head is paces at 30 degrees above horizontal ➡horizontal canals vertical
Irrigation of external ear with warm or cold water causes endolymph to flow in semicircular ducts
-cold water elicits nystagmus with fast phase away from side been tested
-opposite response with warm water
COWS (Cold Opposite, Warm Same)
Explain the pathway and function of the vestibulo-cervical reflex and the vestibulo spinal reflex
Slide 29, 31
List the principal clinical signs that may be spent in a patient with unilateral vestibular disease and for each clinical sign on the list, explain the underlying mechanism
Varying degrees of loss of equilibrium/balance, causing postural imbalance or ataxia, with preservation of normal strength
- positive Romberg test
- abnormal nystagmus
- dizziness
- vertigo
- nausea
Anatomy of the vestibular apparatus
6, 7, 8, 9
Macula structure of a utricle and Saccule
Differs to structure of cupula
- gelatinous layer overlies stereocilia
- above this is fibrous structure called otolithic membrane, in which are embedded calcium carbonate crystals (called otoconia or otoloiths)
- otoconia provide weight, so that when head tilts, gravity causes otolithic membrane to shift relative to stereocilia
- as with cupula, bending towards and away from tallest stereocilium causes depolarisation and hyper polarisation respectively.
Righting reflex
Righting reflexes:
- Position of head adjusted relative to gravity via vestibular input
- Position of rest of body adjusted relative to head via muscle spindles in neck
- Otolith organs detect acceleration towards ground, eliciting reflex extension of legs ready for landing
- Visual system also used to adjust head orientation and gauge distance to landing
Thalmacortical Projections
-As well as receiving input from superior and lateral vestibular nuclei, thalamus also receives inputs from GSA receptors in muscles and skin
-Cortical pathways involved in perception of body orientation
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What are the general signs of vestibular disease?
-Varying degrees of loss of equilibrium/balance, causing postural imbalance or ataxia, with preservation of normal strength
-Positive Romberg test
-Abnormal nystagmus
-Dizziness
-Vertigo
-Nausea
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What is abnormal nystagmus?
-Results from disturbed vestibular input to neurons that innervate extraocular muscles
-With peripheral receptor disease, have horizontal or rotatory nystagmus always with fast phase away from the abnormal side
-With disease of vestibular nuclei or of vestibular pathways in cerebellum, nystagmus may be horizontal, rotatory, or vertical, and may change in direction with changes in the position of the head
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