D23 - Internal ear, acoustic and vestibular pathways

Question 1

Internal ear

What is the inner ear composed of?

Answer 1

• Membranous labyrinth
  
  • ​A series om membranous fluid-filled ducts and chambers
  • Surrounded by osseus labyrinth
• Osseous labyrinth
  
  • ​Within the pars petrosa of os temporale
• The membranous layrinth consists of:
  
  • Vestibular labyrinth (with receptor organs for balance)
  • Cochlear labyrinth (with organ of hearing)
• The osseous labyrinth consists of:
  
  • Vestibulum
  • 3 semicircular canals
  • Cochlea
• Membranous labyrinth is filled with endolymph (K⁺)
• Osseous labyrinth is filled with perilymph (Na⁺)
• Spatia perilympahtica:
  
  • ​Between the membranous labyrinth and the osseous labyrinth​
  • Fluid-filled clefts
  • Connected to subarachnoid space by the vestibular and cochlear aqueduts
  • Filled with perilymph (Na⁺)

Question 2

Internal ear

Vestibule

Answer 2

• Central chamber of the osseuous labyrinth
• Communicates with:
  
  • Rostrally: cochlea
  • Caudally: semicircular canals
• The lateral wall of the vestibule has 2 windows:
  
  • Vestibular window
  • Cochlear window

Question 3

Inner ear

Vestibular labyrinth

Answer 3

• Sense and conduct impulses of balance bia n. vestibularis
• Composed of:
  
  • Saccule
  • Utricle
  • Semicircular ducts
• Semicircular canals sense physical movement of the head
• Otolitic organ sense linear acceleration (gravity)
  
  • Otolitic organ are located within the saccule and utricle
• Innervation:
  
  • Sensory (afferent)
  • N. vestibulocochlearis → n. vestibularis

Question 4

Internal ear

Vestibular labyrinth: saccule & utricle

Answer 4

• 2 enlargements within the osseous vestibule
• Otolitic organ is located within the utricle and saccul
• Semicircular ducts: arise from the utricle (for balance)​
• Spiral cochlear duct: arise from the saccule (for hearing)
• Macule:
  
  • Medial wall
  • Macula sacculi
  • Macula utriculi

Question 5

Internal ear

Vestibular labytinth: semicircular canals

Answer 5

• Arise from the utricle
• House the semicircular duct of the vestibular labyrinth
• 3 channels:
  
  • Anterior channel
  • Lateral channel
  • Posterior channel
• Amullae membranae:
  
  • ​Enlargement of the semicircular ducts near their junction of with the utriclulus
  • One for each duct
• ​​​Crista ampullaris:
  
  • ​Projecting into the ampullae memebranae
• Cupula:
  
  • ​Gelatinous layer on crista ampullaris
• Crura membrancea:
  
  • ​End of semicircular ducts, where they join the utriculus
• Semicircular ducts are filled with endolymph
• When the head moves, endolymph flows to the ampulla
• Endolymph pushes the cupula with hair cells that transduce the mechanical movement to electrical signal

Question 6

Inner ear

Cochleae labyrinth

Answer 6

• Location: wall of membranous cochlear labyrinth
• Receptor organ: ggl. spiralis
• Cochlea is a duct forming a spiral around a bone column, divided into three chambers (scala) by:
  
  • Basilar membrane
  • Lamina spiralis ossea
  • Ressiner’s membrane / vestibular membrane
• Scala vestibuli
  
  • ​Perilymph (Na⁺)
  • Begins at the vestibular window (oval window)
• Scala tympani
  
  • ​Perilymph (Na⁺)
• Scala media / cochlear duct
  
  • Endolymph (K⁺)
  • Ends at the cochlear window (round window)
  • Contains:
    
    • Organ of corti (organ of hearing)
    • Tympanic membrane (floor of the cochlear duct)
    • Vestibular membrane (roof of cochlear duct)
    • Lateral membrane (formed by lig. spirale)
• Scala vestibuli and scala tympani communicate at the apex of the cochlea
  
  • The apex is called helicotrema
• Turns of the spiral:
  
  • Car: 3
  • Eq: 2.5
  • Su: 4
  • Ru: 3.5

Question 7

Inner ear

Cochlear labyrinth: organ of Corti

Answer 7

• The hearing organ
• Within the cochlear duct
• Organum spirale
• Organ of Corti includes two types of cells
  
  • Sensory cells called hair cells
  • Supporting cells
• The hair cells are resting on the basilar membrane and reaching into the space of the cochlear duct
• Role of supporting cells: during displacement of the basilar membrane, the hair cells touch the tectorial membrane → excitation

Question 8

Innervation

Answer 8

• N. vestibulocochlearis (VIII) (afferent)
  
  • N. vestibularis (from utricle and saccule
  • N. cochlearis (from base of cochlea)
• Special sensory:
  
  • Nucl. cochlearis
  • Nucl. vestibularis
• Corpus trapezodium (location: pons)→ porus acousticus internus

Question 9

Vestibular pathway

Answer 9

Question 10

Acoustic pathway

Answer 10