The inner ear consists of two majorfunctional parts. They are:1. The vestibular portion (balance)2. The cochlear portion (hearing)
The cochlea is one of the most complicated organs in the body, but its purpose is simple. It changes sound into electricity and electricity is the “language” the BRAIN understands.
The size of the cochlea is relatively small—about the size of the tip of your little finger. Its diameter is about that of the tympanic membrane. Its snail shape has only two and a half turns in humans, and varies among other mammals.
The cochlea is almost completely formed by the end of the first trimester of pregnancy. Hearing (the cochlea) is one of the first senses of embryonic development for electrical stimulation of the brain.
The cochlea is located within the petrous portion of the temporal bone. The bony labyrinth is the hardest bone within the human body.
The Cochlea It is a fluid filled cavity which is divided into three canals. They are:1. The scala vestibuli2. The scala media (cochlear duct)3. The scala tympani
The top chamber of the scala vestibuli begins at the oval window and ends at the helicotrema.
The scala media is the middle chamber where the hair cells are located. It is separated from the scala tympani by the basilar membrane upon which the Organ of Corti (where the hair cells are) is resting.
The scala tympani begins at the round window and meets the scala vestibuli at the helicotrema (the apex of the cochlea) The scala tympani and the scala vestibuli share the same perilymphatic fluid (the fluid is mostly potassium—less sodium)
The scala media has a more thick endolymph fluid which consists of exactly the opposite proportions of potassium and sodium relative to the perilymph fluid.
Perilymph has a viscosity similar towater. The cochlear duct (scala media)includes endolymph, hair cells, thetectorial membrane and the basilarmembrane. It is the consistency ofgelatin.There is no physical discontinuitybetween the cochlear duct (scala media)and the perilymph fluid.
Cochlear blood supply Is very complex, but primary supplies arearound the scala media area.The stria vascularis maintains the chemicalcomposition of the endolymph.There are a group of small blood vesselsjust below the basilar membrane whichsupply the organ of Corti with nutrients.
Cochlear blood supplyPlease note, there is no directcontact with the vascular system andthe structures of the cochlea.There is an “intermediate transfersystem” through the intracochlearfluids of each site.
Perilymph fluid movement Since the bony portion of the labyrinth is solid, the only release for fluid movement is the round window.If the round window was solid bone, the stapes would be unable to move the fluid from the oval window side.
Characteristics of the Basilar Membrane It is about .1mm at its base increasing in width to about .5mm at its apex. Its stiffness is about one hundred times greater at its base than its apex. These characteristics become the determinants of its frequency response patterns.
Characteristics of the Basilar Membrane There are transverse bands (side-to-side) which are located along the length of the basilar membrane. These transverse bands vary in stiffness as they are spaced along the basilar membrane. Each band is (frequency) sensitive to the various waves of energy received along the traveling wave pathway.
Characteristics of the Traveling WaveThe first portion of sound stimulus waveundulation of the basilar membrane isgenerally received close to the stapes. Thewave continues with increased undulation toa maximum point along the basilarmembrane; this maximum amplitude point isdependent upon the frequency of thestimulus.
Characteristics of the Traveling WaveThe point of maximum amplitude for highfrequencies is closest to the basal end; whilethe maximum amplitude for low frequenciesis closer to the apical end.
Characteristics of the Traveling WaveThe velocity and therefore the wavelengthdecrease as a function of distance from thestapes.This reduction of amplitude, velocity, andwavelength is commonly found with anysound transmission through a fluid.
Characteristics of the Traveling Wave This frequency dependent maximum basilar membrane displacement is a clear indication that the cochlea performs a mechanical frequency analysis. This is defined as the Place Theory of frequency resolution.