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Chemosensory elements of nasal cavity


By


Dr. T. Balasubramanian


Introduction:

It is common knowledge that nasal cavity is capable of olfaction. The primary sense of olfaction is really primitive phylogenetically and is perceived by the first cranial nerve i.e. olfactory nerve. In humans this sensation plays a vital role in:

  1. Determining the flavours of food and beverages

  2. In maintaining the nutrition of the individual

  3. Plays a vital role in safety of the individual

Olfactory dysfunction could be an early pointer for a number of neurological disorders which include:

  1. Alzheimer’s disease

  2. Parkinsonism

  3. Multiple sclerosis

  4. Frontal Meningiomas

  5. Nasopharyngeal carcinoma


Types of nasal chemosensory elements:

There are 4 different types of chemosensory elements present in the nasal cavity of humans. These include:

  1. Nervous terminalis also known as terminal nerve system. It is also arbitrarily christened as cranial nerve zero as this nerve was identified after all the cranial nerves have been named.

  2. Main olfactory nerve i.e. olfactory nerve

  3. Vomeronasal or accessory olfactory system

  4. Trigeminal sensory system


The first cranial nerve (olfactory nerve) mediates the classic sense of olfaction and perception of flavour. This nerve innervates the olfactory epithelium present in the roof of the nasal cavity and the corresponding portion of nasal septum.

The Free nerve endings of fifth cranial nerve innervate the whole of the nasal cavity including the olfactory area. These nerve endings are sensitive to irritation, burning, cooling and tickling sensations. These nerves also initiate nasal reflexes like increased secretions from nasal mucosal glands, halting the inhalation of potentially noxic substances which could harm the nasal and tracheal air ways. Most odorants in high concentrations stimulate these nerve endings. It has been suggested that free nerve endings of trigeminal nerve may interact with olfactory nerves. The role played by this interaction is still being studied.

Cranial nerve 0 has been described in almost all vertebrates including humans. It is composed of loose plexus of nerve fibres within the nasal cavity. These nerve fibres can be identified by the presence of ganglia at nodal points. These nerve endings demonstrate high levels of gonadotropin releasing hormone. These nerves have been postulated to play a vital role in reproduction. Secretion of gonadotropin releasing hormone in these nerve endings are partly regulated by oestrogen levels.





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