Vestibular System And Cerebellum Flashcards
The vestibular enters the brainstem at pontomedullary sulcus and courses beneath (anterior to) restiform body (inferior cerebellar peduncle). Where do the axons terminate?
Vestibular nuclei
Fastigial nuclei (bilaterally)
Flocculo-nodular lobe
Some enter cerebellum via juxtarestiform body
Fibers from the vestibular nuclei and paramedian pontine reticular formation (PPRF) decussate in pons and bifurcate into ascending and descending components. What do these form?
Ascending component forms medial longitudinal fasciculus (MLF)
Descending forms medial vestibulospinal tract
Describe the functions of the medial longitudinal fasciculus (MLF)
Terminates in oculomotor, trochlear, and abducens nerve
Critical link in oculomotor system and has a role in coordinated, synchronized eye movements, especially horizontal gaze
Horizontal gaze involves coordinated contractions of lateral rectus of one eye and medial rectus of other
MLF conveys reciprocal connections between the abducens nucleus and contralateral oculomotor nucleus
Describe the paramedian pontine reticular formation (PPRF)
Critical center for horizontal gaze
It is an enhancement of RF immediately adjacent to abducens nucleus
It is the staging and coordinating area for oculomotor system
Receives info from frontal and occipital eye fields, superior colliculus, vestibular nuclei, auditory nuclei, cerebellum, spinal cord
It sends fibers to abducens nucleus of same side for inducing ipsilateral lateral rectus. Also projects fibers through contralateral MLF to contralateral oculomotor nucleus that innervates medial rectus
Describe vestibulocerebellar fibers
Direct input from vestibular nuclei and RF to cortex flocculonodular lobe
Describe cerebellovestibular fibers, direct fastigiobulbar fibers, and indirect fastigibulbar (descending limb and ascending limb)
Cerebellovestibular fibers from cortex and fastigii to vestibular nucleus
Direct fastigiobulbar fibers terminate in ipsilateral vestibular nuclei and RF
Indirect fastigiobulbar tract (uncinate fasciculus):
Descending limb terminates in vestibular nuclei and RF
Ascending limb exits cerebellum via superior cerebellar pednuncle. Terminates in midbrain tegmentum and ventral lateral nucleus of thalamus. Important vestibular link to extrapyramidal system
Where might the primary vestibular cortex be located?
In the lower portion of the postcentral gyrus or posterior portion of superior temporal gyrus
Vestibular nuclei do not receive direct projections from cerebral cortex or superior colliculus
Describe Doll’s eyes maneuver
In unconscious patient without cervical injury, side-to-side movement of the head results in horizontal movement of eyes in opposite direction
If head turns right:
Horizontal gaze to left -> normal finding
Right adducts, left neutral -> left VI palsy
Right neutral, left abducts -> right III palsy
No response (neutral gaze) -> possible midbrain damage
Describe oculocaloric testing for vestibuloocular response
In unconscious patient, injection of cold water into external auditory meatus results in horizontal gaze toward side of stimulus
Test right ear:
Right eye abducts, left eye adducts -> normal finding (horizontal gaze to right)
Right eye abducts, left eye no response -> left III nerve palsy
No response right eye, left eye adducts -> right VI nerve palsy
No response in either -> right VIII nerve palsy; midbrain damage (deep coma state)
Describe signs and symptoms of unilateral lesions of vestibular system
Postural impairment
Eyes, head, and body tend to turn toward side of lesion
Patient tends to fall toward side of lesion
Nystagmus is described according to fast component and is directed toward side of lesion
Vertigo: generally, dizziness is in same direction as slow component of nystagmus (opposite the lesion)
Describe internuclear ophthalmoplegeia (INO, syndrome of MLF)
Patient has an abnormal response to horizontal gaze in direction opposite the side of lesion
Unilateral lesion of MLF superior to abducens nucleus results in impairment or loss of adduction (MR) of ipsilateral and nystagmus of abducting eye
Interruption of crossed fibers from abducens area to oculomotor nucleus results in loss of adduction of ipsilateral medial rectus
Destruction of descending (reciprocal) fibers from oculomotor nucleus to abducens area results in nystagmus of abducting contralateral eye
Although horizontal gaze contralateral to MLF lesion is abnormal, INO is named according to side of oculomotor impairment. Examples?
If horizontal to right is normal and disconjugate to left (right eye does not adduct; left eye exhibits nsytagmus), this is a right INO due to lesion of right MLF
If horizontal to left is normal and disconjugate to right (left eye does not adduct; right eye exhibits nystgamus), this is a left INO with a lesion of left MLF
Describe lesions of the paramedian pontine reticular formation (PPRF)
PPRF includes a lateral gaze center
Unilateral lesion of PPRF results in paresis of paralysis of horizontal gaze toward same side of lesion
Destruction of fibers that course from PPRF to ipsilateral abducens nucleus and contralateral oculomotor nucleus
Example: lesion of right PPRF would result in patient’s inability to move both eyes in conjugate horizontal gaze to right
Describe fastigial nucleus
Medial-most nucleus of four nuclei.
Has vestibular connections and function
Describe globose nucleus
Medial and lateral parts
Cerebellovestibular efferents from medial globose and fastigial nuclei project to vestibular system
Efferents from lateral globose course with fibers from emboliform and dentate nuclei
Describe emboliform nucleus
Small, worm-shaped nucleus
Sends efferents through superior cerebellar peduncle along with fibers from dentate nucleus
Describe dentate nucleus
Large convoluted cup-shaped nucleus that gives rise to vast majority of efferents from neocerebellum
These, along with globose and emboliform, project to red nucleus, ventral lateral nucleus of thalamus, brainstem tegmentum,and reticular formation
Summarize deep cerebellar nuclei
Fastigial nucleus and medial globose
To vestibular system via ICP
Lateral globose, emboliform nucleus, dentate nucleus
To cerebellar efferents vis SCP
What is the mnemonic for cerebellar afferents coursing through inferior cerebellar peduncle?
DDT Always Ruins Olives
Dorsal spinocerebellar tract (DSCT) Direct arcuate fibers (cuneocerebellar tract) Trigeminocerebellar tract Arcuocerebellar fibers Reticulocerebellar fibers Olivocerebellar fibers
What does the dorsal spinocerebellar tract (DSCT) convey?
Unconscious, precise propriorceptive information from lower 1/2 of body and lower extremities to cerebellum
What do the direct arcuate fibers (cuneocerebellar tract) convey?
Unconscious, precise proprioceptive information from upper 1/2 of body and upper extremities
What does the trigeminocerebellar tract convey?
It is one of two unconscious sensory tracts from face
Convey general proprioceptive and exteroceptive information from head
Describe the olivocerebellar fibers
Large bundle of afferent fibers that originate in inferior olivary nucleus (ION) and terminate as climbing fibers in contralateral cerebellar hemisphere
Important processing and relay center for sensory information from spinal cord and motor information from reticular formation and extrapyramidal system
Describe central tegmental fasciculus (CTF)
Originates in red nucleus, central gray, and midbrain tegmentum
This tract is a critical link between extrapyramidal system and cerebellum
What fibers form all of the middle cerebellar peduncle?
Pontocerebellar fibers
What does the ventral spinocerebellar tract (VSCT) convey?
Unconscious, general proprioceptive information from lumbosacral levels to cerebellum
Describe trigeminocerebellar tract
Derived from fibers from main sensory nucleus of V
Projects to anterior vermis of cerebellum via superior cerebellar peduncle
Conveys precise tactile and proprioceptive information from head to cerebellum
Fastigial and medial portion of globose send efferents to where?
Into vestibular system via direct and indirect fastigiobulbar fibers
Lateral globose, emboliform, and dentate nuclei project efferents to where?
Through superior cerebellar peduncle
Fibers decussate in upper pons and lower midbrain and bifurcate into ascending and descending limbs
Where do all information entering cerebellar cortex eventually converge?
Upon Purkinje cell
Purkinje cell axon is the only efferent from the cerebellar cortex
Most terminate in deep cerebellar nuclei
Describe signs and symptoms of cerebellar dysfunctions
Ataxia: broad-based, staggering gait
Dysmetria: literally missing the mark like touching finger to nose with eyes closed
Dysdiadochokinesia: inability to perform rapidly alternating movements like pronation and supination
Intention tremor: present during voluntary movements such as reaching for pencil on table and absent at rest
Decomposition of movement: movements tend to be performed one joints at a time and take on a robotic appearance
Slurred or scanning speech: ability to coordinate complex motor speech patterns involved in vocalization
Hypotonia and hyporeflexia
Asthenia: demonstrate weakness of limb or axial musculature
Nystagmus: lesions of vestibulocerebellum
Describe Friedreich’s ataxia
Initially develops in lower limbs and progresses to upper limbs over course of a few years
Dysdiadochokinesia, dysmetria, nystagmus, and intention tremor are present bilaterally
Describe vestibular (Scarpa’s) ganglion
Found near vestibular portion of membranous labyrinth and consists of bipolar neurons
Axons form vestibular nerve
