Cochlea hair cell
Cochlea Hair Cell is a neuron located in the cochlea of the inner ear of mammals and some nonmammals such as birds
There are two types of cochlea hair cells, outside hair cells and inside hair cells. They both assist in the process of relaying sound to the brain. These cells are found in a small area in the cochlea called the organ of Corti. Anywhere from 17,500-23,500 hair cells can be found in the human ear. Cochlear hairs cells get their name from special structures called stereocilia that cover each cell making them look hairy. Stereocilia is a primary structure used in sound transduction. With stimulation, the stereocilia bend which cause a signal that goes to the auditory nerve and eventually to the auditory cortex allowing sound to be processed by the brain.
Neuronal Type: Mechanoreceptor cells
The cochlear hair cells are located in the organ of Corti, a small organ found in the scala media within the cochlea in the ear. Hair cells sit on the basilar membrane and have an overhead cover called the reticular lamina. Two structures called the rods of Corti support the reticular lamina and the basilar membrane. Directly above the reticular lamina is the tectorial membrane. Each hair cell consists of many cilia called stereocilia. These structures sit on top of the cell and can be found touching the tectorial membrane and/or the reticular lamina. The direction of the movement of the stereocilia determines whether the cell depolarizes or hyperpolarizes. There are two types of cochlear hair cells, outer hair cells and inner hair cells. Inner hair cells are far less in number, approximately 3500, then outer hair cells which can number up to 20,000 in each ear. The inner cells are located in between the modiolus and the rods of Corti, while the outer cells are located on the other side of the rods of Corti. Both cells are innervated by the spiral ganglion which transmits signals to the auditory nerve and eventually to the auditory cortex in the brain.
- Neurotransmitter: Glutamate
When the stapes, a small bone that is part of the middle ear, is stimulated to remove from the oval window, a K+ rich fluid called endolymph rushes into the cochlea. This change in fluid amount causes a bend in the basilar membrane, which in turn, bends the reticular lamina, rods of Corti, and the tectorial membrane. This bending also causes the stereocilia to bend back and forth on the hair cells. On the tip of each stereocilia is a channel called transient receptor potential A1 channels or TRPA1, which allows for the inflow of K+ into the hair cell. Each TRPA1 channel is connected to one other by a link known as the tip link. This link can be thought of as a string that opens and closes a door. As the stereocilia bend each way, the "doors" to the channels open allowing K+ into the hair cell or close preventing K+ overflow. When K+ enters the hair cell, depolarization occurs. This unique physiological property is due to the extremely high concentration of K+ in the endolymph which results in a high equilibrium potential for K+ of 0 mV. With a surge of K+ influx into the cell, the calcium channels are activated to open, which in turn activates the release of glutamate in the synapse with the spiral ganglion. The spiral ganglion then sends the signals through the auditory pathways to the brain.
Synaptic output is to the spiral ganglion. The inner hair cells are low in numbers compared to the outer hair cells; however, the inner hair cells transmit most of the information. For this reason, several spiral ganglion synapse with the inner hair cells, and not so many synapse with the outer hair cells.
Importance of each cell
Inner hair cells are important because they innervate spiral ganglion and transmit most of the information. Outer hair cells are important because they amplify the sound coming into the cochlea. With proteins in the membrane, the length of the outer hair cells are changed when endolymph is in the cochlea. The changing of the outer hair cells result in more bending of the inner hair cells, which creates innervation with the spiral ganglion.
Although some animals such as birds can regenerate hair cells, mammals cannot regenerate cochlear hair cells. If degeneration of hair cells occurs, the mammal is deaf. In humans, technology has been introduced that allows people who are deaf to hear again by a device called cochlear implants. This system bypasses the hair cells and directly innervated the auditory nerve.
1. Kwan T., White PM, and Segil N. (2009) "Development and Regeneration of the Inner Ear", Annals of the New York Academy of Sciences vol 1170:28-33.
2. Lewis G. Tilney, David J. Derosier, and Michael J. Mulroy (1980)"The organization of actin filaments in the stereocilia of cochlear hair cells", The Journal of Cell Biology, Vol 86, 244-259.
3. Bear, M., Connors, B. and Paradiso, M. (2007) "The auditory and vestibular systems", Neuroscience: Exploring the Brain, 3rd Ed., 344-365.
4. Encyclopedia Britannica (1997) "Organ of Corti", http://www.britannica.com/EBchecked/topic-art/431888/535/Structure-of-the-organ-of-Corti
5. Stasiunas A, Verikas A, Miliauskas R, and Stasiuniene N.(2009)"An adaptive model simulating the somatic motility and the active hair bundle motion of the OHC", Computers in Biology and Medicine,39(9):800-9.
6. Shands Hospital, University of Florida Health System