Histology of the Ear notes PDF

Preview

Summary

Summary of section concerning the ear, covers all expected material for the exam and topics are repeated in lecture....

Description

Histology of the Ear – Notes April 13, 2016 Learning Objectives  Understand gross anatomical divisions of the ear and their general functions in hearing and balance.  Observe the histological features of the external, middle, and inner ears.  Correlate the histological features of Macula and the Spiral Organ of Corti to their functions in the transduction of mechanical stimuli to neural signals.  Identify structures of the inner ear on diagrams and H&E sections. Anatomical Overview  The ear is divided into three anatomical parts based on relative location and function.  External ear  Auricle/pinna  External acoustic meatus  Collects, amplifies, and conducts sound  Middle ear  Ossicles (malleus, incus, stapes)  Two skeletal muscles (Stapedius and Tensor tympani)  Openings to mastoid air cells and nasopharynx  Oval and round windows  Sound conduction  Inner ear  Bony and membranous labyrinths  Branches of CN VII and CN VIII  Transduction of mechanical stimuli by sensory organs to neural signals

External ear  Auricle is elastic cartilage covered by skin with hair, eccrine sweat glands, and sebaceous glands. It funnels sound waves into external auditory meatus.  External acoustic meatus is a tube formed by elastic cartilage (outer one third) and bone (inner two thirds), lined by keratinized stratified squamous epithelium continuous with the skin of the Auricle.  The skin of the External acoustic meatus has hair follicles, sebaceous and ceruminous glands.  Ceruminous glands are modified apocrine sweat glands whose secretion combines with sebum to make cerumen. Cerumen has an antimicrobial and protection function.  Tympanic membrane: thin fibroelastic membrane that is covered externally by thin skin (epidermis and dermis) and internally by simple cuboidal epithelium that lines the middle ear.  Fibroelastic layer consists of type II and type III collagen and a little bit of type I collagen. Middle ear  Cavity is lined by simple squamous or simple cuboidal epithelium  Three ossicles that articulate with one another via synovial joints amplify vibrations of the tympanic membrane.  Tensor tympani (skeletal muscle) associated with the malleus.  Stapedius muscle (skeletal muscle) associated with stapes.  These muscles take part in the attenuation reflex which limits the movement of the ossicles in response to loud noises that could damage structures of the inner ear.  Anteriorly, the middle ear communicates with the nasopharynx via pharyngotympanic (auditory) tube.  Tube is lined by respiratory epithelium (ciliated pseudostratified columnar epithelium with goblet cells).  Supported at its proximal end inside the temporal bone, at its distal/pharyngeal end by fibrocartilage.  A short and more horizontal auditory tube in children facilitates the occurrence of bacterial and viral infections that cause inflammation of the middle ear cavity because of its communication with the upper respiratory tract.  Posteriorly, the middle ear communicates with mastoid air spaces of the temporal bone via mastoid antrum/epitympanic recess.  Malleus articulates with tympanic membrane; footplate of the Stapes sits on the membrane of the oval window of the inner ear.

Inner ear  All the structures that make up the inner ear (vestibular-cochlear apparatus) are within the petrous part of the temporal bone.  The “bony labyrinth” is divided into three spaces that house the “membranous labyrinth”. Between the bony and membranous structures, a fluid-filled space (perimembranous space) contains perilymph.  Perimembranous space is lined by vascular periosteum.  Membranous labyrinth in filled with endolymph and lined by simple squamous epithelium except in the endolymphatic sac (adjacent to subarachnoid space, simple columnar) where reabsorption of endolymph is believed to take place.  Three parts of the bony labyrinth: vestibule (central), semi-circular canals (posterior), cochlea (anterior):  Vestibule: centrally located, contains utricle and saccule  Semicircular canals: posterior to vestibule, contains semicircular ducts  Cochlea: anterior to vestibule, contains cochlear duct  Four parts of the membranous labyrinth: utricle and saccule (in the vestibule), semi-circular ducts (in the semi-circular canals), cochlear duct (cochlea)

Vestibular Apparatus  Associated with the 3 semicircular canals, utricle, and saccule.  Sections of neuroepithelium in the simple squamous-lined utricle and saccule (called maculae) detect position of the head in space relative to gravity.  Macula: supporting columnar cells, hair cells, nerve fibers (branches of CN VIII), otolithic membrane  Otolithic membrane: gelatinous matrix with rock-like structures called otoliths.  Otoliths: calcium carbonate in the form of calcite; otoliths make the otolithic membrane heavier and more sensitive to movemen t.  The kinocilia and stereocilia of hair cells are embedded in the otolithic membrane.  The macula of the utricle is horizontally oriented; the macula of the saccule is vertically oriented.  Sections of neuroepithelium in the simple squamous-lined semi-circular ducts (crista ampullaris) detect rotational movement of the head.  Cristae ampullaris are located in the ampulla of each semi-circular canal: supporting columnar cells, hair cells, nerve fibers (branches of CN VIII), cupula.  Cupula: gelatinous matrix, similar to the otolithic membrane, but lacks otoliths.  The kinocilia and stereocilia of the hair cells are embedded in the cupula.  The cupula spans the width of the ampulla and is sensitive to the movement of endolymph within the semi-circular duct. Hair cells

a. Tightly packed actin filaments cross-linked by fimbrin and espin (actinbundling proteins) form the internal core structure of a stereocilium. The high density of actin filaments and the extensive cross-linking pattern imparts rigidity and stiffness to the shaft of the stereocilium. b. Tight junctions between hair cells and adjacent supporting cells maintain separation of endolymph and extracellular fluid. c. Bending of stereocilium toward kinocilium causes depolarization of the cell membrane (potassium flows into hair cell from endolymph through mechanically gated ion channels, Ca2++ influx from extracellular space, neurotransmitter release), signal transduction. d. Bending of stereocilium away from kinocilium causes hyperpolarization (potassium channels close, reduction of neurotransmitter release), no signal. Auditory Apparatus  Associated with the Cochlear duct within the Cochlea.  The duct within the cochlea makes 2 ¾ turns, ~35 mm long.  Cochlear duct situated within cochlea forms three fluid filled chambers, called Scala media (duct), Scala vestibuli, and Scala tympani.  Scala vestibuli is continuous with vestibule (starts near oval window); Scala tympani begins at the round window. Apex of cochlea is called the helicotrema, where Scala vestibuli and Scala tympani meet.  The Scala media contains endolymph; the Scalae vestibuli and tympani contain perilymph.  Cochlear branch of CNVIII splits into multiple branches, each with a “spiral” ganglion (collection of bipolar neuron cell bodies) are situated in the spongy bone core of the spiral, called the Modiolus.  The Modiolus contains blood vessels and surrounds the cell bodies and processes of the Acoustic branch of CN VIII.

   

      

SV: Scala vestibuli (perilymph) ST: Scala tympani (perilymph) SM: Scala media (cochlear duct; endolymph) VM: vestibular membrane** Extensive tight junctions between cells of this membrane block ion diffusion between perilymph and endolymph. BM: basilar membrane Svasc: Stria vascularis (production and maintenance of endolymph) G: spiral ganglion (VA cell bodies, within Modiolus) N: nerve (branch of CN VIII) Slig: Spiral ligament SL: Spiral limbus O: Osseous spiral ligament (extension of Modiolus)

The Spiral Organ of Corti  The Spiral Organ of Corti is the sensory organ in the Scala media (Cochlear duct). It detects sound. It runs the length of the cochlear duct.  The Spiral Organ of Corti is situated on the basilar membrane, which runs between the Osseous spiral ligament and Spiral ligament.  Three rows of outer hair cells and one row of inner hair cells sit on supporting cells (called phalangeal cells). They lack kinocilia, V-shaped arrangement of stereocilia are embedded in the Tectorial membrane.  The tectorial membrane is attached to the Spiral limbus....