Vestibular Apparatus Flashcards
Vestibular system components
1.vestibular apparatus (peripheral sensory organ)
2.CN 8- vestibulocochlear nerve
3.Vestibular nuclei (medulla)
4.VVestibulocerebellum (flocculonodular lobe)
Vestibular apparatus parts
-paired structure (right and left)
-membranous labyrinth (inner ear)
>semi-circular canals
>utricle
>saccule
-sensory structure (hair cells)
What does vestibular apparatus detect?
-static position
-linear and rotational motion of the head
Where is the vestibular apparatus?
-located inside the petrous part of the temporal bone
-near the brain
Vestibular apparatus embryological origin
-From the otic vesicle (vestibular apparatus and cochlea)
Lymph of vestibular apparatus
-Perilymph= CSF near brain and surrounding endolymph
-Endolymph= sitting within semi-circular canals
Hair cells
-the sensory structures inside the vestibular apparatus and cochlea which convert peripheral sensations to nerve impulses (transmitted via CN VIII)
Membranous labyrinth
-system of soft tissues and receptors
Cristae ampullares (x3)
-located at base of each semicircular duct and are responsible for detecting rotational changes
>each of the 3 are positioned slightly different so they can detect change
>Cupula (protein -rich structure that is neutrally buoyant) surrounds the cristae ampullares
Maculae (x2)
-paired structures
-otoliths (stones) floating inside endolymph in the walls of the utricle and saccule
-detect static and linear movement and pass on to hair cells and eventually CN VIII
Cranial Nerve VIII
-vestibulocochlear nerve
-inputs information into medulla
-pathway is very short because vestibular apparatus is near the brainstem
Where does Cranial nerve VIII travel?
-sensory axons enter medulla, and synapse mostly on vestibular nuclei within the medulla
-sometimes axons synapse directly within the cerebellum (vestibulocerebellum/ flocculonodular lobe)
Where does vestibular nuclei go?
1.LMN of neck/limbs/trunk
2.LMN of extrinsic ocular muscles (CN III, IV, VI)
3.Cortex (via thalamus)=conscious awareness of head movement
4.Medulla at emetic centre= nausea/motion sickness
Vestibular nuclei neurons and tract
-UMN running along vestibulospinal tract
Vestibular nuclei effect on head/limbs/trunk
-input to LMN of head/limbs/trunk resulting in:
>increased ipsilateral extensors
>decreased ipsilateral flexors
>decreased contralateral extensors
Vestibular nuclei effect on ocular muscle
-through CN III, IV, VI
-ocular muscles need to work together in synchrony where left and right eye movements are coordinated by opposite muscle activation/inhibition
Nystagmus
-rapid, involuntary movements of the eyes (horizontal, rotational, vertical)
>coordinated movements between left and right eyes (will include inhibition of some parts of eye to allow movement with the head)
>eyes don’t move smoothly with re-positioning of the head (step-wise movements)
Slow-phase eye movements
-eyes move away from the direction of the head movements
Saccade
-fast-phase coordinated movement of eyes moving towards the direction of the head movement
>delay in eye movement compared to head movement
Vestibular disease
-can affect anywhere within the vestibular system
>vestibular apparatus
>vestibular nuclei
>CN VIII
>flocculonodular region of cerebellum
-Lesions will result in reduced activity of the vestibular system on the ipsilateral side
Lesions of vestibular Disease
1.Asymmetric head/body position (towards the side of the lesion) = head tilt, body leaning/falling, rolling
2.Ataxia= Vestibular ataxia= circling or falling towards the side of the lesion (staggering)
3.Pathological nystagmus (spontaneous)- occurs at rest; horizontal, vertical (if central lesion), rotary. Peripheral lesion would result in same type of nystagmus no matter the body position
>imbalance between L and R vestibular systems, and reduced input to medial longitudinal fasciculus
>fast phase away from the lesion (towards the more active side of the body)
4.Positional strabismus
>abnormal eye position when the head is moved/rotated
>ipsilateral ventral or ventrolateral strabismus when he is moved dorsally
Resolution of vestibular disease
-lots of compensation mechanisms, so most clinical signs will resolve over time
>nystagmus and strabismus resolves most rapidly
-sometimes if you suspect vestibular disease but there is no nystagmus, it could be that it has resolved already. Can try re-positioning them and then spontaneous nystagmus may return
Peripheral Vestibular Disease
1.head tilt towards side of the lesion
2. nystagmus (horizontal or rotary; but no change in direction with head position)
3. normal postural reactions
4.concurrent facial nerve (VII) damage or Horner’s syndrome (ipsilateral)
Central vestibular disease
1.head tilt towards vs away (paradoxical)
2.Nystagmus (horizontal, rotary, or vertical; direction of nystagmus can change with head position)
3. Postural deficits
4. Multiple Cranial nerve deficits (ipsilateral or bilateral; can also see Horner’s syndrome)
Paradoxical Vestibular disease
-lesions that effect connections at cerebellar peduncles (connection between vestibular nuclei and cerebellum)
-head will tilt away from lesion side because inhibitory neuron at cerebellar peduncles will be turned off resulting in that side being the most excitatory so animal will tilt away from lesion
