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
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.
