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Physiology of external auditory canal
By
Dr. T. Balasubramanian M.S. D.L.O.
The physiological functions of the
external auditory canal can be studied under two heads:
1. Auditory functions
2. Non auditory functions
Auditory functions: The external canal can be compared to a resonating
tube closed at one end and open at the other. It is approximately
2.5 cm long and 0.7 cm in diameter. In a closed cylinder,
resonance occurs at a wave length four times the length of the
cylinder. For the external canal it could correspond to a
resonating frequency of 2.6 KHz. Experiments have shown that
external auditory canal contributes a peak 10 - dB gain between 2 and 5
KHz. Although these are relatively small gains, the frequencies
that are boosted by the external canal include important speech
frequencies.
Non auditory functions:
Maintenance of patency of external canal:
A patent external canal is mandatory for
efficient conduction of sound to the ear drum. The most important
non auditory function of the external canal is to maintain itself
patent, free of debris, infection and foreign bodies.
Foreign bodies are prevented from entering the external canal due to
the presence of hairs in the cartilagenous portion of the canal.
The hairs are oriented in such a way that their tips are directed
laterally. The hairs get sparser as the bony cartilagenous
junction is reached. The bony portion of the external canal is
totally free of hairs. These hairs are thicker and longer in
males i.e. secondary sexual characteristic.
Cerumen: Commonly termed as ear wax is a mixture of the secretory
products of the two glands that are found in the cartilagenous
portion of the external canal: sebum from sebaceous glands, and
the secretory products of ceruminous glands. Desquamated
epithelial cells, shed hairs and foreign bodies are also mixed into the
cerumen. Genetic differences in the composition of wax have been
noted. There are two distinct phenotypes of ear wax, "WET" and
"DRY". Dry wax is light greyish in color and is flacky in
nature. This type of wax is common in Asians. Wet wax is
golden brown in color and is sticky in nature. Wet wax is more
common in whites. Wax phenotype is determined by a single gene
pair, the wet wax allele being dominant.
Biochemical analysis has shown lipid to be the
major component of cerumen. There is no significant difference in
cerumen production or composition with respect to age or sex.
Cerumen is known to possess significant antimicrobial properties due to
the presence of lysozyme. The cerumen of diabetic patients was
found to be less acidic than that of non diabetic individuals thereby
potentially favouring bacterial growth.
The most important protective function of
cerumen is its water proofing effect on the external canal.
Cerumen is primarily composed of lipids, which are very
hydrophobic. Water that enters the external canal do not
penetrate cerumen and stagnate. Stagnation of water can always
cause epithelial damage and maceration.
Self cleaning function and epithelial migration:
But for nail beds, the skin of the external canal is the only squamous
epithelium in the human body that migrates. The purpose of this
migration in the external canal is to keep the canal free of
debris. In all keratinizing squamous epithelium there is
detachment and desquamation of the stratum corneum. In the
external canal this occurs in an orderly fashion. The shedding of
the epithelium occurs towards the periphery of the external
canal. Mastication may have a role to play in this epithelial
migration, by alternatively compressing and expanding the cartilagenous
canal thus propelling it laterally out of the external canal.
Copyright drtbalu 2007
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