Secondary Receptor Cell (Hair Cells) No axon Output in form of secretion of chemical transmitter agent

Hair Cells Inner Hair Cells Outer Hair Cells

Inner Hair Cells (IHC) There is one row of approx. 3,500 inner hair cells. These cells receive about 95% of the innervation from the afferent nerve fibers from the acoustic portion of the VIII nerve. FYI--The cuticle capped apex of an IHC is slightly concave and bears about 48 stereocilia, oriented parallel to the longitudinal axis of the cochlea. The cell body height of the IHC is fairly constant over the extent of the entire cochlea. Stereocilia vary in length and diameter within an individual hair cell and also among the individual turns of the cochlea. The number of cells also varies along the length of the basilar membrane. Bredberg (1968) found about 80 inner hair cells per mm at the basal end and 155 cells per millimeter at the apical end.

Outer Hair Cells (OHC) There are three rows of approx. 13,500 outer hair cells. Although they are much greater in number than the inner hair cells, they receive only about 5% of the innervation of the afferent nerve fibers from the acoustic portion of the VIII nerve. These cells contain muscle-like filaments that contract upon stimulation and fine tune the response of the basilar membrane to the movement of the traveling wave. The OHC cilia are in a “W” shape.

Innervation of the Organ of Corti AFFERENT ascending between cochlea to cochlear nucleus (CN) going to brain 30,000 fibers, located in the bony canal (Internal Audiotory Meatus) at junction of Modiolus & Spiral Lamina. MYELINATED central to habenula, and nonmyelinated peripheral to habenula

Classification of Neurons 1)Afferent or Efferent 2)Direction of travel in cochlea after passing thru habenula Radial – travel from center outward Spiral – travel the length of cochlea 3)Fibers that supply IHC or fibers that supply OHC

Hair Cells Direction Aff/Eff Density of Innervation Radial Inner 1 : 1 hair cell Afferent SpiralEfferent1 : many IHC OuterRadial Efferent 1 : few OHC OuterSpiralAfferent 1 : many OHC

Efferent Neurons Nerve supply comes from Olivocohlear bundle (OCB) from the Superior Olivary Complex (SOC) OCB is also called the Ramussen’s Bundle of the SOC

OCB Cochlea 80% 20% 80% 20% EFFERENT AFFERENT CN Cochlea 100% 500 fibers 30,000 fibers

HOCB COCB homolateral (same side) Inner Spiral bundle enters habenula and branches off to 3,000 nerve fibers course longitudinally along inner hair cells synapse on afferent nerve rather than on IHC itself crossed (opposite side) Outer Radial bundle enters habenula and branches off to 8,000 at Organ of Corti, and then branches to 40,000 nerve endings synapse directly on OHC

The VIII Nerve The VIII nerve is twisted like a rope with low frequencies (apical fibers) on the inside & high frequencies (basal fibers) around the outside. HI HIGHER LOWER LO