The spiral organ, also known as the Organ of Corti, functions as the sensory receptor for hearing within the auditory system. The organ acts as a transducer, converting the physical energy of sound waves into electrical impulses. This conversion process is fundamental to the sense of hearing, allowing for the precise encoding of different frequencies and intensities.
Anatomical Home Within the Inner Ear
The precise location of the spiral organ is deep within the inner ear, housed inside a fluid-filled, spiral-shaped chamber known as the cochlea. The cochlea is divided lengthwise into three parallel compartments. The spiral organ is situated within the central chamber, which is called the cochlear duct or scala media.
The organ rests upon the basilar membrane, a flexible, fibrous structure that separates the cochlear duct from the lower chamber, the scala tympani. The basilar membrane’s properties change along its length, which is fundamental to pitch discrimination.
The upper boundary of the cochlear duct is formed by Reissner’s membrane, which separates it from the scala vestibuli, the upper chamber. Both the scala vestibuli and scala tympani are filled with perilymph, while the scala media contains endolymph, a fluid with a high concentration of potassium ions. This difference in fluid composition creates an electrochemical gradient, an important energy source for the transduction process.
Essential Components of the Organ
The spiral organ is composed of several distinct cell types, primarily the sensory hair cells and various supporting cells. The hair cells are the true mechanoreceptors, organized into two main categories: inner hair cells and outer hair cells. The inner hair cells form a single row along the length of the cochlea, totaling approximately 3,500 cells in a human ear.
Hair Cells
These inner hair cells are the primary transmitters of auditory information to the brain, with nearly all auditory nerve fibers connecting to them. In contrast, the outer hair cells are arranged in three to four parallel rows and are far more numerous, with an estimated 12,000 cells. The outer hair cells play a role in amplifying and fine-tuning the sound signal, cochlear amplification.
Both types of hair cells possess dozens of tiny, hair-like projections called stereocilia on their apical surface. These stereocilia are organized into bundles that step up in height, resembling a staircase. Above the hair cells lies the tectorial membrane, an acellular, gelatinous sheet that extends over the organ.
Supporting Structures
The tallest stereocilia of the outer hair cells are embedded in the underside of this tectorial membrane. Supporting cells, such as the pillar cells, provide structural integrity by forming the rigid framework that separates the inner and outer hair cells. Other cells, like Deiters’ cells, cradle the base of the outer hair cells.
The Mechanical Process of Transduction
The process of hearing begins when sound waves cause the fluid within the cochlea to move, creating pressure waves in the perilymph. These pressure waves ultimately cause the basilar membrane, upon which the spiral organ rests, to vibrate in an undulating pattern. Different sound frequencies cause the basilar membrane to vibrate most intensely at specific locations along its length, which is how pitch is initially encoded.
As the basilar membrane moves up and down, the stereocilia of the hair cells are deflected because the tectorial membrane remains relatively stationary. This relative motion creates a shearing force that physically bends the hair bundles. This mechanical bending is the direct stimulus that converts sound into an electrical signal.
The bending of the stereocilia opens mechanically-gated ion channels located at the tips of the “hairs.” Tiny filaments called tip links connect the stereocilia and pull open these ion channels when the bundle is deflected. The opening of these channels allows a rapid influx of potassium ions from the potassium-rich endolymph fluid into the hair cell.
This inward flow of positive ions causes the hair cell to depolarize, generating an electrical potential. This potential triggers the release of neurotransmitters at the base of the hair cell, which then excites the corresponding nerve endings of the auditory nerve. The resulting electrical impulses travel along the auditory nerve to the brain, where they are finally interpreted as sound.