1. INNER EAR, AUDITORY PATHWAYS
Lab #8:
INNER EAR, AUDITORY PATHWAYS

2. OVERVIEW OF AUDITORY SYSTEM
The auditory system allows for conscious perception of sound. Impulses from hair cells of the Organ of Corti traverse the peripheral processes of the cochlear nerve (of VIII). These fibers have cell bodies in the spiral ganglion (modiolus of the cochlea) and central processes that end in the dorsal and ventral cochlear nuclei. From these nuclei, fibers decussate in the auditory stria and impulses travel up the brain stem bilaterally as the lateral lemniscus, with synapses in the superior olivary nuclei, trapezoid nuclei, and nuclei of the lateral lemniscus. Most ascending fibers terminate in the nucleus of the inferior colliculus, from which fibers traverse the brachium of the inferior colliculus to reach the medial geniculate nucleus of the thalamus. From the thalamus, impulses traverse the auditory radiations (sublenticular fibers) to reach the primary auditory cortex in the transverse temporal (Heschl’s) gyri.

3. Blackboard 17 Modiolus – - bone in core of cochlea Scala Vestibuli
Scala Media
Scala Tympani
Part of spiral ganglion
Auditory part of CN8
- fibers in central modiolus

4. scala vestibuli - upper space (bony labyrinth, perilymph)
scala tympani - lower space (bony labyrinth, perilymph)
scala media ( cochlear duct ) - middle space (membranous labyrinth, endolymph)
- contains receptor cells for hearing
vestibular membrane - separates scala media & vestibuli
basilar membrane - separates scala media & tympani

5. Blackboard 18 scala vestibuli Scala media cochlear duct scala tympani
spiral lamina (on left) - shelf-like projection of modiolus to basilar membrane
auditory nerve fibers - dark fibers in spiral lamina
vestibular membrane - delicate membrane below scala vestibuli
Structures of the cochlear duct:
stria vascularis - highly vascularized epithelium that lines the lateral wall of the scala media from the vestibular membrane to the spiral ligament
scala vestibuli
Scala media
cochlear duct
inner hair cell - 1 row
outer hair cells - 3 rows
tectorial membrane (TM) - plastic-looking, rigid membrane
- receives cilia of hair cells
basilar membrane - membranous floor of cochlear duct
Spiral Ligament
scala tympani

6. Blackboard 19 inner hair cell - 1 row outer hair cells - 3 rows
tectorial membrane (TM) - plastic-looking, rigid membrane
- receives cilia of hair cells

7. TEMPORAL BONE IMAGE BANK
The following images are labeled views of the temporal bone model used in the middle ear lab in Gross Anatomy of the Head and Neck course and the auditory system lab in the Brain and Behavior I course

8. Auditory tube Chorda tympani nerve Tensor tympani muscle tendon
LATERAL WALL OF MIDDLE EAR

9. Epitympanic recess Incus Malleus: head handle Tegmen tympani Aditus
LATERAL WALL OF MIDDLE EAR

10. Mastoid antrum and air cells
LATERAL WALL OF MIDDLE EAR

11. Aditus (hallway) leading from the epitypmanic recess to the mastoid antrum (cave) and air cells
LATERAL WALL OF MIDDLE EAR

12. Tympanic membrane
LATERAL WALL OF MIDDLE EAR

13. Stapedius muscle
Stapes (on the oval window)
MEDIAL WALL OF MIDDLE EAR

14. Facial canal
Facial nerve exiting facial canal through the stylomastoid foramen
Promontory (basal turn of cochlea) - tympanic plexus containing fibers of CN IX covers the promontory during life
MEDIAL WALL OF MIDDLE EAR

15. Petrotympanic fissure – is the space between the petrous part and tympanic part of the temporal bone
Chorda tympani nerve – is a branch of facial in the cheek fascia that ascends into the petrous temporal bone
Mastoid process
Facial nerve exiting the stylomastoid foramen
Styloid process
LATERAL ASPECT OF TEMPORAL BONE MODEL

16. Genu of the facial nerve
SUPERIOR VIEW OF TEMPORAL BONE MODEL
Semicircular canal
Cochlea
Genu of the facial nerve

17. Facial nerve entering the internal acoustic meatus
Semicircular canals
Facial nerve entering the internal acoustic meatus
Jugular bulb
MEDIAL ASPECT OF THE TEMPORAL BONE MODEL

18. Blackboard 21

19. Haines 2-20

20. 2-20

21. 2-34

22. Blackboard 22
note the relations of cranial nerves 7 and 8 as they pass through the internal auditory canal. Note their relation to the posterior cranial fossa, pons, and cerebellum.

23. Haines 3-1
Wernicke's Area
auditory association cortex
Brocha’s Area

24. Haines 5-12 dorsal and ventral cochlear nuclei Rostral Medulla Level
- located lateral & dorsal to the inferior cerebellar peduncle (restiform body)
- contain cell bodies of second order neurons in auditory path
- receive fibers from cochlear nerve
- will send axons into the acoustic stria
Rostral Medulla Level

25. Haines 5-18
superior olivary nuclei
trapezoid
Rostral Pons Level

26. Haines 5-19
lateral lemniscus -
Rostral Pons Level

27. Haines 5-20 lateral lemniscus and nuclei of lateral lemniscis
Rostral Pons Level

28. Haines 5-22 Lateral Lemniscis Inferior Colliculus
Snake eating egg
Inferior Colliculus
Pons/midbrain jctn Level

29. Haines 5-25 Brachium of Superior Colliculus
Medial Geniculate Nucleus – receives fibers from brachium of inferior colliculus
Brachium of Inferior Colliculus
Rostral Midbrain Level

30. Haines 5-26
MGN
Rostral Midbrain Level