THE INNER EAR Two Sensory Divisions; one dedicated to hearing, the other to maintaining balance Vestibular Division - The balance organs - SC Canals - Utricle - Saccule Auditory Division - The cochlea

THE INNER EAR

Nerve supply to inner ear - VIIIth

THE INNER EAR Apex Base:Posterior (helicotrema:Anterior)

OW RW 5 months GA 0.5 mm The Cochlea The modiolus Human cochlea has 2 and ¾ turns The bony shell of the cochlea coils around a central bony canal – the modiolus At the base the diameter is approx. 9 mm; the cochlea is about 5 mm high

THE INNER EAR

The organ of Corti lies on the top of the BM TM – tectorial membrane IHC – inner hair cell OHC – outer hair cell BM – Basilar membrane TC – tunnel of Corti

The organ of Corti The tectorial membrane is a gelatinous flap covering the hair cells The TM is attached to the spiral limbus The tallest OHC stereocilia tips are firmly imbedded in the underside of the TM (the kinocilia)

THE INNER EAR

SENSORY COMPONENTS Hair Cell populations – 15,000 total, about 3x as many outer as inner The two populations of cells are structurally distinct, and appear to carry out different jobs The vast majority of the transforming of sound is carried out by IHCs OHCs seem to be responsible for increasing the sensitivity of the IHCs

THE INNER EAR

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THE INNER EAR SEM of guinea pig organ of Corti (mid-basal turn) Both the surface of hair cells and the inside of the organ of Corti (at site of fracture) are seen. Lateral to the OHCs, remnants of the marginal net of the tectorial membrane (which has been removed) are visible. Blue arrows point to OHCs bodies, the asterisk indicates the tunnel of Corti where nerve fibres are crossing scale bar: 20 µm.

THE INNER EAR Base of the cochlea Apex of the cochlea

Hair cells The two populations of cells are structurally distinct and appear to carry out different jobs The vast majority of the transforming of sound is carried out by IHCs; OHCs seem to be responsible for increasing the sensitivity of the IHCs A single row of IHCs, 3 rows of OHCs (4-5 rows reported in the apical turn) Tube-shaped Flask-shaped

THE INNER EAR SENSORY COMPONENTS - THE BASILAR MEMBRANE – Characterized by its stiffness gradient - or the varying amount of stiffness from place to place Stiffer at the base than the apex Consider the implications for resonance frequency – Set into motion by the vibration of cochlear fluids The fluids set in motion by the vibration of the stapes Therefore, the system is considered hydraulic - a system in which fluids move structure

THE INNER EAR The Basilar Membrane - tonotopic organization: Resonance frequency arises from the interplay between the mass and stiffness of an object – For the piano, it’s the mass and stiffness of the strings – For the xylophone, it’s the mass and stiffness of the keys – In the cochlea, the fluids provide the mass, while the B. Membrane provides the stiffness – Stiffness varies from place to place – Therefore, due to the membrane’s unique stiffness characteristics, the different frequencies resonate at different locations

TRAVELING WAVE

The inner ear: tonotopic organization Auditory tour: TW

The Basilar Membrane - tonotopic organization Resonance frequency along the membrane changes as a function of location

THE INNER EAR High Frequency Sound: Blue Line Low Frequency Sound: Red Line

THE INNER EAR – Mechanical Response

THE INNER EAR The Basilar Membrane – Response to travelling wave – The movement of the membranes creates a shearing motion between the stereocilia of the hair cells, and the tectorial membrane of the organ of corti – Shearing motion facilitates the flow of ions from the scala media through the hair cell – Synaptic activity ensues, which triggers the generation of nerve impulses, or action potentials Therefore, the movement of the membrane, a mechanical action, is transduced (transformed) into neural activity Therefore, the movement of the membrane, a mechanical action, is transduced (transformed) into neural activity

The Inner Ear: Active Mechanical Response Inner ear is physiologically vulnerable and metabolically active. Actively responds to sound as energy is fedback into the system, heightening the TWave’s displacement Brownell (1983) – OHC electromotility: electrical energy is converted into mechanical energy

The Inner Ear Fluids Endolymph – fluid contained in the membranous labyrinth of the inner ear. The fluid is mainly potassium, which is secreted from the stria vascularis. The high potassium content carries the electrical current in the hair cells. This is known as the mechano- electric transduction (MET) current. Endolymph has a high positive charge (from mV in the cochlea). Perilymph – extracellular fluid in the inner ear. The fluid is mainly Sodium

It seems that the stereocilia of the IHCs are not embedded in the TM at all OHCs absorb stimulus energy to operate IHCs are moved by cochlear fluid

The bodies of the HCs are surrounded by perilymph Stereocilia are surrounded by endolymph The boundary between endolymph and perilymph occurs at the reticular lamina SN OT TC

THE INNER EAR

THE INNER EAR: Nerve Supply VIIIth Nerve - In the cochlea – Efferent innervation of the cochlea – about 1,500-3,000 fibers Roughly 70% innervate OHCs – Innervation is provided at the base of the cell – therefore is pre-synaptic – More efferent innervation found at the base of the cochlea than at the apex The remaining 30% innervate IHCs – Synapse directly onto afferent nerve endings, therefore post-synaptic