This image represents a cross-section of the basal end of the human cochlea.
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The inner hair cells of the cochlear are the primary sensory receptors of the cochlea. There is a single row of around 3000-3500 inner hair cells within the cochlea. While less in number the inner hair cells are more innervated than the outer hair cells and are responsible for 95% of the afferent information being sent to the brain.
Inner Hair Cells
The outer hair cells of the cochlear are quite atypical sensory cells. There are three rows of outer hair cells with around 12,000 outer cells within the cochlea. The outer hair cells enhance the sensitivity and frequency selectivity of sound using electro-mechanical transduction. The outer hair cells can contract and are therefore motor units that amplify the motion of the basilar membrane.
Outer Hair Cells
The Basilar Membrane
The basilar membrane is a stiff pseudo-resonant structure that varies both in width and stiffness across its length. It is located in the roof of the scala tympani and the pressure waves in the perilymph induce a unique travelling wave that experiences membrane displacement peaks correlating audio frequency to position along the basilar membrane: high frequencies at the basal end and low frequencies at the apical end.
The tectorial membrane is a relatively stiff acellular gelatinous membrane that affix the tips of the stereocilia of the sensory inner and electrically motile outer hair cells. When vibrations pass through the external membranes the corresponding motion of the hair cells towards the tectorial membrane bend the stereocilia activating the hair cells.
Reissner’s membrane (the vestibular membrane or vestibular wall) is a membrane between the cochlear duct and the scala vestibuli. Its primary function is a diffusion barrier maintaining positive endocochlear potential and allowing nutrients to travel from the perilymph to the endolymph. The vibration of Reissner’s Membrane is affected both by the pressure waves in the perilymph and the vibrations of the cochlear duct.
The scala vestibuli (or vestibular duct) is the pressure entry cavity to the cochlea which is filled with perilymph, a fluid having a similar ionic composition to extracellular fluid found elsewhere in the body. The ossicles vibrate the oval window of the cochlea, generating pressure waves to the perilymph. These pressure waves in turn transduce into vibrations of Reissner’s membrane.
The scala tympani (or tympanic duct) is the pressure exit cavity of the cochlea filled perilymph, a fluid having a similar ionic composition to extracellular fluid found elsewhere in the body. The fluid of the scala vestibuli is shared with the scala tympani such that the pressure waves subsequently travel from the apical to the basal end of the scala tympani, vibrating the basilar membrane. The pressure is eventually amortised at the round window.
The cochlear duct is a unique cavity in the cochlear filled with endolymph, a fluid rich in potassium giving the cochlear duct a positive potential. It is notable that only the stereocilia of the hair cells in the cochlear duct are bathed in endolymph and the bodies of the hair cells are bathed in perilymph creating a positive potential across the whole hair cell.
The Cochlear Duct
The spiral limbus is a bony lamina of two plates encapsulating the canals containing the nerve fibres between the hair cells and the spiral ganglion. The upper plate (limbus spiralis) touches Reissner’s membrane and the lower plate (limbus laminæ) bends inwards in the shape of the letter C.
The spiral (cochlear) ganglion is a group of nerves located in the spiral canal of the modiolus. It consists of bipolar cells that connect the cochlear hair cells to the cochlear nuclei in the brainstem.
The cochlea itself is entirely encased in bone within the temporal bone of the skull. This direct bone to bone contact makes it possible to activate the cochlea by applying externally vibrations to the skull that travel through the temporal bone to the cochlea indirectly vibrating membranes and cells activating sound transduction.
The most common form of cochlear stimulation uses an array of electrodes surgically inserted through the bone of the cochlear and positioned carefully into the scala tympani in order to electrically stimulate the spiral ganglion cells creating the experience of sound. The implant is better at producing high frequency sounds than low frequency sounds as it is difficult for the electrode to access the apical (inner) end of the cochlear.
Infrared neural stimulation (INS) has been proposed as an alternative method to electrical stimulation due to its spatial selective stimulation. Lasers mounted within a traditional cochlear implant can deliver infrared radiation directly to specific regions of the spiral ganglia. This is currently only a concept not an actual clinical technology.
Neurotrophin-3 delivered into the cochlea basal region via a traditional cochlear implant can stimulate nerve regrowth at the hair cell/cochlear nerve interface. Cochlear delivery of neurotrophins in humans is likely achievable as either an office procedure via transtympanic injection or via a traditional cochlear implant. This is currently only a concept not an actual clinical technology.