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

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.

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

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

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

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

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

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

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

Mastoid antrum and air cells LATERAL WALL OF MIDDLE EAR

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

Tympanic membrane LATERAL WALL OF MIDDLE EAR

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

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

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

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

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

Blackboard 21

Haines 2-20

2-20

2-34

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.

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

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

Haines 5-18 superior olivary nuclei trapezoid Rostral Pons Level

Haines 5-19 lateral lemniscus - Rostral Pons Level

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

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

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

Haines 5-26 MGN Rostral Midbrain Level