Physiology of external auditory canal

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.