Vestibular and cerebellum Flashcards
Development of the ear structures?
Inner ear: receptors for thr auditory and vestibular sustem are derived from ectoderm but are contained in a mesodermally derived structure (from the otic placode > otic pit > otic vesicle adjacant to the rhombencephalon.
The membranous and bony labyrinths are formed adjacent to the first and second branchial arches and their corresponding first pharyngeal pouch and first branchial groove. The first branchial groove gives rise to the external ear canal. The first pharyngeal pouch forms the auditory tube and the mucosa of the middle ear cavity. The intervening tissue forms the tympanum. The ear ossicles are derived from the neural crest of branchial arches 1 (malleus and incus) and 2 (stapes). These ossicles become components of the middle ear associated laterally with the tympanum (malleus) and medially with the vestibular window of the bony labyrinth of the inner ear (stapes).
What are perilymph and endolymph derived from?
Perilymph -> CSF
Endolymph -> blood vessels along the wall of the cochlear duct.
Name the 3 semicircular canals
anterior (vertical),
posterior (vertical), and lateral (horizontal).
Each semicircular duct is oriented at right angles to the others.
Layers of the cochlea?
1) Scala vestibuli in outer bony labyrinth
(perilymph, oval window)
2) Scala media/cochlear duct (enodlymph) in inner membranous labyrinth (hair cells and spiral organ of Corti)
3) Scala tympani (perilymph, round window) in the outer bony labyrinth
What is the end of the cochlear duct called?
Helicotrema!
Scala tympani and scala vestibuli perilymph meet and mix
Which structures are located within the vesibule of the inner ear?
Vestibule (contains perilymph, is in the outer bony labyrinth)
1) Utricle
2) Saccule
both contain endolymph and are in the inner membranous labyrinth, both contain hair cells, both have maculae (static equilibrium)
Where are the hair cells with cristae ampularis located?
Ampullae of the semicircular canals
The cristae ampularis are important for dynamic equilibrium.
Anatomy of the crista ampularis (dynamic equilibrium)
Amuplla - dilation in each membranous semicircular canal
Crista - proliferation of connective tissue on one side of the membranous ampula
Cupula - gelatinous structure on the surface of the crista (protein-polysaccharide material)
Hair cells (connected to the dendritic zones of the neurons of the vestibular portion of the vestibulocochlear nerve) have
1) stereocillia (40-80 hairs)
2) kinocilium (only 1)
which project into the cupula
What happens to the activity of the vestibular neurons with movement, for example, when you move the head to the right?
The vestibular neurons are tonically active, and their activity is excited or inhibited by deflection of the cupula in different directions. Each semicircular duct on one side is paired with a semicircular duct on the opposite side by their common position in a parallel plane. These synergistic pairs are the left and right lateral ducts, the left anterior and right posterior ducts, and the left posterior and right anterior ducts. When movement in the direction of one of these three planes stimulates the vestibular neurons of the crista of one duct, they are inhibited in the opposite duct of the synergistic pair. For example, rotation of the head to the right causes the endolymph to flow in the right lateral duct such that the cupula is deflected toward the utriculus and the cupula of the left lateral duct is deflected away from the utriculus. This action causes increased activity of vestibular neurons on the right side and decreased activity on the left side, resulting in a jerk nystagmus to the right side, which is an involuntary rhythmic oscillation of the eyes.
What are maculae?
Structures found in the saccule and utriculus, important in static equilibrium.
The surface has columnar neuroepithelial cells (hair cells) covered by a gelatinous material - statoconiorum (otolithic) membrane which contains calcareous crystalline bodies known as statoconia (otoliths).
The hair cells also have 1) stereocilia and 2) kinocilia.
The hair cells are synapsing with the dendritic zones of the vestibular neurons of CN VIII.
The macula in the saccule is oriented vertically (sagittal plane), whereas the macula of the utriculus is oriented horizontally (dorsal plane).
Name the vestibular nuclei
Rostral
medial
lateral
caudal
Located on either side of the dorsal part of the pons and medulla adjacent to the lateral wall of the fourth ventricle
Which tracts exit the vestibular nuclei?
1) Lateral vestibulospinal tract - arises from the cell bodies of the lateral vest. nucleus -> ipsilateral ventral funiculus to interneurons of the ventral grey columns of entire spinal cord > + to ipsilateral extensor muscles (- to ispilateral flexor and - to contralateral extensor muscles)
2) Medial vestibulospinal tract- arises from the cell bodies of the rostral, medial and caudal vest. nuclei > ipsilateral ventral funiculus of cervical and cranial thoracic SC!!! > interneurons of ventral grey column > innervation of neck muscles
3) Axons coursing in the medial longitudinal faciculus (MLF) to terminate in motor nuclei of CN III, IV, VI
4) Axons to reticular formation (vomiting center)
5) Axons to contralateral medial geniculate nucleus of the thalamus (synapse) > internal capsule > corona radiata > temporal cortex
6) Axons to cerebellum thru caudal cerebellar peduncle > cortex of flocconodular lobe and fastigial nucleus
When does idiopathic bening vestibular disease occur most commonly in cats?
Summer (still unknown why)
Afferent and efferent arm of the physiologic nystagmus?
Afferent: CN VIII
Medial longitudinal fasciculus
Efferent: CN VI (abducens) for the lat. Rectus muscle (abduction of eye) and CN III (oculomotorius) for the med. Rectus muscle (adduction of eye)
Why do some dogs have rythmic movements of the eyelids along with the pathologic nystagmus?
A simultaneous eyelid movement may be seen concomitant with the nystagmus that presumably is a reflex action. We presume that this occurs because of a direct pathway from the vestibular nuclei to the facial nuclei. The movement of the superior eyelid is likely through the action of the levator anguli oculi medialis muscle.
Why do some dogs have rythmic movements of the eyelids along with the pathologic nystagmus?
A simultaneous eyelid movement may be seen concomitant with the nystagmus that presumably is a reflex action. We presume that this occurs because of a direct pathway from the vestibular nuclei to the facial nuclei. The movement of the superior eyelid is likely through the action of the levator anguli oculi medialis muscle.
Neuro signs in bilateral vestibular disease? (Peripheral)
1) crouched posture closer to the ground surface
2) may often walk well but are often slow and cautious to avoid falling, especially when they move their heads suddenly
3) wide head excursions
4) loss of physiologic or pathologic nystagmus
5) head rebound phenomenon (If the head and neck are extended in these patients and the support is suddenly withdrawn, the head may rapidly descend ventrally beyond the normal neutral position) -> DD cerebellar disease
How long does the recovery in cats with diopathic bening vestibular disease last?
Usually slower than in dogs,
7-10 days for ataxia,
2-4 weeks for head tilt (may persist)
Name some breeds of dogs and cats with congential peripheral vestibular disease
German shepherd dog, doberman pinscher, Akita, beagle, English cocker spaniel
Burmese and Siamese cats.
Typical clinical picture of thiamin deficiency in small animals?
Short phase of vestibular ataxia followed by:
mydriasis
seizures
(common in cats that eat a fish diet, as it contains thianimase)
MRI signs of thiamin deficiency encephalopathy?
bilateral symmetric lesions in the:
1) vestibular nuclei,
2) caudal colliculi,
3) nuclei of the oculomotor nerve,
4)lateral geniculate nuclei
Which disease entity commonly follows otitis media/interna in horses?
temporohyoid osteopathy, with temporal bone fracture
Infection with which agent is common in horses in the central vestibular system?
Sarcocystis neurona
(rabies and EEV usualy also have prosencephalic signs, EHV-1 and WNV usually doesnt affect the central vestibular system)
The most common caudal brainstem disorder of adult cattle?
Listeriosis
(ddx: rabies, abscess, suppurative meningitis, thrombotic meningoencephalitis)
Hoe old are most cattle that develop listeriosis?
> 1 year of age
Typical clinical signs of listeriosis in cattle?
-obtundation,
-facial paralysis,
-tongue paresis,
-dysphagia,
-jaw paresis,
-dysfunction of the central components of the vestibular system,
-UMN and GP system dysfunction,
-loss of their normal sensorium,
-circling
Route of entry of Listeria monocytogenes in cattle?
through abrasions and lacerations of the oral mucosa and gains access to the dendritic zones of general somatic afferent neurons in the branches of the trigeminal nerve. The bacterium travels retrograde over these axons through the trigeminal ganglion and into the pons, where the fifth cranial nerve attaches to the brainstem. Inflammation occurs in these trigeminal nerve branches and in the caudal brainstem.
Prognosis in listeriosis?
Rigorous treatment with penicillin will usually improve or resolve the clinical signs in cattle that are still standing.
The prognosis is less favorable in small ruminants.
Recumbent animals have a poor prognosis regardless of the species.
Which agent causes thrombotic meningoencephalitis of cattle?
bacterium Histophilus somni (Hemophilus somnus)
DDx for a goat with central vestibular signs?
Listeriosis,
caprine arthritis encephalitis (CAE),
viral encephalitis, Parelaphostrongylus tenuis myiasis,
abscess,
neoplasm
Which pathology is associated with congenital pendular nystagmus of belgian shepherd dogs?
agenesis of optic chiasm - the optic fibers continued into the ipsilateral optic tract uninterrupted and with no deccusation.
Which pathology is associated with cingenital pendular nystagmus in vats?
1) ocular albinism,
2) an abnormality in the retinogeniculate projections and the neuronal organization of the lateral geniculate nucleus has been observed in the Siamese cat and the white Persian tiger. 3) More retinal ganglion neurons project their axons contralaterally in Siamese cats than the normally pigmented feline breeds. No obvious impairment of vision is noted. Many of these cats also have a mild strabismus.
4) This congenital pendular nystagmus occurs in some cats and cattle with the Chédiak-Higashi syndrome, in which pigmentation and melanin granules are abnormal.
We have also studied a 13-week-old mixed-breed dog with bilateral retinal dysplasia that had pendular nystagmus without visible structural abnormalities of the brain on MRI.
Which structure does the cerebellum develop from?
the alar plate of the metencephalon
Its first appearance is a dorsal bulge of the alar plate, which extends the alar plate tissue dorsally and medially in the roof plate, where the growths from each side eventually join each other. The first growth of each alar plate is called the rhombic lip, which arises from the side of the rhomboid fossa of the fourth ventricle. This rhombic lip consists of proliferating cells from the germinal layer adjacent to the fourth ventricle.
Name the parts of the cerebellum
1) vermis (central median region)
2) 2 hemispheres
1) body (vermis and hemispheres)
2) small floconodular lobe (vestibular cerebellum)
Name the cerebellar peduncles and which fibers pass thru them?
1) Rostral peduncle - cerebellum to mesencephalon (mostly efferents)
2) Middle peduncle - transverse fibers of the pons with cerebellum (mafferents from pons)
3) Caudal peduncle - medulla and SC to cerebellum (mostly afferents)
from lateral to medial: Middle Caudal Rostral (MCR)
Two mayor types of afferents to the cerebellum based on the morphology of their telodendrons?
1) mossy fibers
2) climbing fibers
1) more abundant, origin in the brainstem and SC, synapse on neurons of the cerebllar nuclei
2) axons of the olivary neurons (enter thru the caudal peduncle) and synapse on the neurons of the cerebellar nuclei
Name the types of neurons in the cerebellar cortex, their role
Molecular layer:
1) stellate neurons (outer)
2) basket neurons)
both inhibitory to the Purkinje neurons. are scattered through the granular layer.
Granule cell layer
1) granular neurons - facilitatory to the Purkinje neurons - glutamate
2) large stellate neurons (Golgi) - inhibitory to granule cell neurons
Schema of the cerebro-cerebellar circuit
The motor cortex of the frontoparietal lobe > corona radiata > centrum semiovale > internal capsule > crus cerebri > longitudinal fibers of the pons > pontine nucleus cross in the transverse fibers of the pons > middle cerebellar peduncle > cerebellar medulla > folial white lamina > granular layer neurons > Purkinje neurons > folial white lamina > cerebellar medulla > lateral cerebellar nucleus > rostral cerebellar peduncle > cross in ventral tegmental decussation > ventral lateral nucleus of thalamus> thalamocortical fibers> internal capsule> centrum semiovale> corona radiata, > motor cortex of frontoparietal lobe
Which lobules are usually affected in Kerry Blue terriers with cerebellar abiotrophy?
The dorsal lobules (unknown why)
Main pathological findings in FPV cerebellar hypoplasia?
This parvovirus has a predilection for rapidly dividing cells, and therefore exposure to this virus at the time of birth puts the cerebellar external germinal layer at risk. Destruction of this layer prevents the formation of the granular layer, hence the name granuloprival hypoplasia.
In some kittens extensive destruction to the parenchyma occurs, with loss of all layers of the cerebellar cortex and much of the cerebellar medulla and nuclei. These severe lesions may be the result of an earlier in utero infection with this virus. The necrosis of neuronal tissue that has completed its development results in atrophy.
Main pathological findings in FPV cerebellar hypoplasia?
This parvovirus has a predilection for rapidly dividing cells, and therefore exposure to this virus at the time of birth puts the cerebellar external germinal layer at risk. Destruction of this layer prevents the formation of the granular layer, hence the name granuloprival hypoplasia.
In some kittens extensive destruction to the parenchyma occurs, with loss of all layers of the cerebellar cortex and much of the cerebellar medulla and nuclei. These severe lesions may be the result of an earlier in utero infection with this virus. The necrosis of neuronal tissue that has completed its development results in atrophy.
Causative agent for calves with cerebellar hypoplasia/atrophy and patho-mechanism?
BVDV
This malformation should not be summarily diagnosed as cerebellar hypoplasia. Only the absence of granular layer neurons represents hypoplasia because BVDV infection destroyed the external germinal layer, which is the origin of this neuronal layer. The massive destruction of the already differentiated Purkinje neurons, the folial white matter laminae and, in some calves, the cerebellar medulla and nuclei is a degeneration of parenchyma caused by the necrotizing action of the virus and the resulting inflammation. This degeneration results in atrophy. Thus the malformation observed at birth is a combination of hypoplasia and atrophy. The degree of cerebellar lesion in newborn calves varies from a slight gross deformity to almost complete absence of any cerebellar tissue. In the latter case, only a small band of smooth parenchyma may remain over the fourth ventricle, where the cerebellum should normally be located. Where folia are present, the cortex is often depleted of varying amounts of its neuronal layers, and the area of the folial white matter lamina is replaced by a cavity. Mild astrogliosis may be the only indication of where a previous cortex existed.
Two forms of cererbellar cotical degeneration (CCD)?
1) primary Purkinje cell degeneration (most common)
2) primary granule cell or “granuloprival” degeneration.
Name te breeds with identified genetic variants and regions of interest with canine cerebellar cortical degeneration as well as the gene
Genetic variants have so far only been associated with Purkinje cell degenerations:
Beagle,
Finnish Hound,
Gordon Setter,
Old English Sheepdog,
Viszla.
Regions of interest have been identified in the Scottish Terrier (primary Purkinje cell degeneration) and
Australian Kelpie (granuloprival degeneration)
Name the breeds for different spinocerebellar degenerations
SAMS = spinocerebellar ataxia with (or without) myokymia, seizures, or both;
SDCA = spongy degeneration and cerebellar ataxia
Breeds without a known genetic variant in cerebellar cortical degeneration (abiotrophy)
Breeds with spinocerebellar degeneration without known gene variant
What is neuromyotonia?
Myokymia and neuromyotonia are often discussed together as most dogs suffering from myokymia develop neuromyotonic episodes (about 80% of dogs).
Neuromyotonia is characterized by muscle stiffness leading to a collapse into lateral recumbency without loss of consciousness, lasting from several minutes to several hours. Potentially lifethreatening hyperthermia (>43C) and tachypnea are frequent during those neuromyotonic episodes, causing spontaneous death of some dogs. In about half of the dogs, myokymia, muscle stiffness, and collapse are preceded by intense facial rubbing.
In a minority of dogs, neuromyotonia can precede myokymia.
Electromyography of muscles clinically affected by myokymia shows myokymic or neuromyotonic discharges.
BAER changes in dogs with hereditary spinocerebellar ataxia?
Brainstem auditory evoked recordings of these dogs show loss of waves III, IV, and V, as well as in most cases mildly increased latencies for waves I and II
Pathohystological changes in dogs with hereditary spinocerebellar ataxia?
Histopathology reveals a bilateral symmetrical myelopathy and severe degenerative changes in the
CNS auditory pathways. The myelopathy is characterized by a predominant axonopathy with myelin swelling, vacuolization, and astrogliosis (most severe in the dorsolateral and ventromedial funiculi of the cervical spinal cord). A common feature is loss of myelinated axons and diffuse gliosis in the central auditory pathways most
severely affecting the dorsal nucleus of the trapezoid body, but also involving the trapezoid body, cochlear nuclei, and lateral lemniscus.
These degenerative CNS auditory pathway changes are not described in Smooth Haired Fox Terrier, where the lesions appear limited to the spinal cord.
