development of the ear

how does the inner ear form

• forms first

• ectoderm thickens into otic placodes

• roll inwards to form otic pit

• pinch off to form otic vesicle - octic vesicle will form membranous labyrinth (sensory organ)

how does the middle ear form

• forms from an outgrowth of the 1st pharyngeal pouch

• the inside of the embryo’s throat

• endoderm

how does the external ear space form

• from the 1st pharyngeal groove

• outside of the embyro’s neck

• ecotderm

where is the ear drum and ear bones formed

• also known as the tympanic membrane

• formed where the pouch and groove touch

• ear bones develop endochondrally from neural crest cells inbetween

describe the outer ear

• outside of the skull

• the pinna (auricle) - made of skin and hyaline cartilage

• auricular muscles - control position of ear

• ear canal (externa auditory meatus)

• ends at ear drum (tympanic membrane)

• direct and transmits sound to middle ear

describe the function of the pinna

• funnel shape to collect and direct sound

• highly mobile, moved by auricular muscles (locate sounds & communication function)

• domestication syndrome - weakened ear cartilage, floppy ears

• thermoregulation - surface area and blood supply

• innervated by CNVII (facial nerve)

terminology used to describe pinna

• helix - the long side

• apex - the tip

• tragus - base of ear in center (targi hairs protecting entrance to meatus)

• annular cartilage - right at the base

describe the external auditory meatus

• lined by skin - contains ceruminous glands

• cerumen (earwax) traps particles that might damage delicate tympanic membrane

• in carnivores/pigs - ear canal has a bend (vertical canal, horizontal canal)

describe the tympanic membrane

• separates external and middle ear

• stretched like drumskin over tympanic ring (incomplete dorsally)

• first ossicle (malleus) attaches to it - visible as stria malleris

• sound waves cause membrane to vibrate - vibration transmitted along the chain of ossicles

describe the middle ear

• housed inside temporal bone of skull - includes a bulbous expansion of temporal bone (tympanic bulla)

• lined by simple squamous epithelium

• middle ear communicates with pharynx via auditory tube - useful for pressure equalisation and drainage of gland secretions

describe the middle ear ossicles shapes

• malleus - hammer shape

• incus - anvil shape

• stapes - stirrup shape

functions of the middle ear ossicles

• transmit vibrations

• connect tympanic membrane to oval window of inner ear

• amplify vibrations - tympanic membrane much larger than oval window

• muscles attaching to ossicles alter sensitivity

describe the guttural pouches in horses

• airfilled cavity with structures that run through it

• outpouchings of auditory tubes - may function in brain cooling

• divided into two compartments by stylohyoid bone - medial and lateral

• blood vessels and nerves pass through this pouch

• infections/disorders common can lead to severe complications

what does the inner ear consist of

• membranous labyrinth - series of interconnected ducts and chambers, filled with endolymph

• bony labyrinth - hollow cavity in temporal bone of skull

• perilymph fills the space between osseous and membranous labyrinths

functions of the inner ear

• hearing - cochlea

• balance - vestibular system

describe the cochlea and hearing

• divided into three parts - cochlear duct, scala vestibuli, scala tympani

• two scala communicate at the tip of the cochlea’s spiral

describe the process of hearing to brain

• as the stapes vibrates in the oval window, it compresses the perilymph in the scala vestibuli

• pressure waves are transmitted to the endolymph in the cochlear duct via reissners membrane

• movement of the endolymph causes pressure on the tectorial membrane inside the cochlear duct

• tectorial membrane puts pressure on hair cells, which sends impulse along vestibulocochlear nerve to brain (as hair cells bend because tectorial membrane doesn’t move)

describe the organ of corti - auditory senses

• sits on dorsal surface of basilar membrane

• contains acoustically sensitive hair cells that have stereocilia

• support cells

• tectorial membrane - extends from spiral limbus, in contact with stereocilia of hair cells

• vibration in tectorial membrane displaces the stereocilia leading to signal

describe the inner ear and the vestibular system and balance

• vestibular part of inner ear is responsible for sensing balance and movement - 3 semicircular canals and vestibule - saccule and utricle

• information is carried by vestibular part of vestibulocochlear nerve to the brain

• usually sensory input is balanced, if there is in-balance, the brain interprets this as drift towards the side with reduced sensation

what do semicircular canals detect

• rotations of the head

what does the saccule and utricle detect

• accelerations

label histological photo of organ of corti

describe the semicircular canals

• three canals perpendicular to one another

• each detects rotation around one directional axis - pitch, roll, yaw

• rotations cause endolymph to move inside canal, moving hair cells in the ampulla of the canal

describe the saccule and utricle

• expansions of the membranous labyrinth

• detect linear acceleration

• contain hair cells in thickened patches (maculae) - covered with gel like layer contain crystals

• endolymph movement bends the hairs and information about movement is transmitted via the vestibulocochlear nerve