Hair cell is the term used to define the sensory receptors of the vestibular system and auditory systems in vertebrate animals. Auditory cells in mammals are located within the corti organ on a skinny basilar membrane in the cochlea of inner ear. Hair cells are generally classified into two and are called as inner hair cell (IHC) and outer hair cell (OHC). Human cochlea contains around 3,500 IHCs and 12000 OHCs and are formed within the first ten weeks of fetal gestation and the final number of hair cells is decided during this time period.
INNER CELLS OF THE HAIR
Inner hair cells are the types of hair cells present in the cochlea, which are responsible for signal transduction. They are located inside the tunnel of Corti and lies in a single row along the inner side. This type of hair cell comes under type I spiral ganglion neurons and humans have around 3000 to 3500 inner hair cells.
Positively charged ions, such as potassium ions, are allowed to enter the cell through a perfunctorily gated ion channel that is opened by the deflection of inner hair cells. The action potential of hair cells are not caused by themselves but are due to the depolarization of scala media due to the arrival of positive ions from endolymph.
OUTER HAIR CELLS
Active vibrations are caused on the cell body of mammalian external hair cells due to a receptor potential and this response to electrical signals is known as somatic electro motility. These types of hair cells can be seen only in mammals even though the hearing capability of mammals is similar to that of other vertebrate animals. These types of hair cells play an important role in hearing because without these types of hair cells, the hearing sensitivity will go down by 50db. For some marine mammals, this sensitivity is increased by around 200Khz. Outer hair cells also help in distinguishing between different frequencies and humans are one specie with greater advantage of this benefit.
Outer hair cells non-linearly amplifies sounds and thus a wide range of high pressure sound can be reduced to a much smaller scale and will result in a reduced hair displacement.
Both the types of hair cells are very important in hearing because they are responsible for converting the sound signals into neural signals. Even though they are very important, these cells will not regrow if destroyed once and the number of hair cells in an individual is determined during the early stages of fetal growth. This type of hair cells can be damaged due to old age also.
However, researches are being done on replacing and re growing the hair cells and such researches are leading to medical treatments that can even bring back the hearing ability of an individual. Even though the researches are progressing, contradiction still remains on the possible success of hair cell regeneration in mature mammals. The identification of the gene, that is responsible for inhibiting the regrowth capability of hair cells, is also of great importance.