Anatomy Of Nose Essay

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3. Physiology and Anatomy of Nose:
3.1 Nasal Anatomy:
The human nose is the most protruding part of the face which extends approximately 3-5 cm in height externally and 10 cm long nasal cavity internally (Sforza, Grandi, De Menezes, Tartaglia, & Ferrario, 2011). Human nasal cavity starts from external nares (nostrils) to the nasopharynx (the upper section of the throat) which is equally divided by the nasal septum into 2 similar non-connected halves i.e., left- and right-nasal cavity. Anatomically, the nasal cavity is the space between the lower base of the human skull and roof of the mouth which is supported by upside by ethmoid bones, laterally by ethmoid, maxillary and inferior conchae bones and downside by conchae bones (Chien & Chang,
When a nasal drug formulation is delivered deep and high enough, it reaches the brain and CSF via olfactory receptor neurons. While the precise underlying mechanism of transport of the drug from nose to the brain is not entirely understood yet, but recent developments have highlighted the possible pathways of direct transport of drugs to the brain through the nasal route in man. An accumulated evidence of literature showed that nerves emerging from the brain and spinal cord are connected to the nasal cavity, are mainly responsible for the transport of the drug molecule. In addition to this, vasculature system, CSF, and lymphatic system have been played a key role in the nose to brain tissue drug transportation. It is clearly evident that combination of one or two pathways responsible for transportation, although one mechanism may predominate depending upon the delivery device employed, characteristics of neurotherapeutics and formulation
Although olfactory epithelium consists only 3% of the nasal epithelium but this pathway serves as a most direct drug transport route to the brain and CSF, bypassing the BBB (Lisbeth Illum, 2003). It is the simplest and a slow pathway through three different pathways across the olfactory epithelium (i) transcellular pathway involving receptor-mediated endocytosis, fluid phase endocytosis or by passive diffusion. Transcellular pathway is mainly responsible for the transport of the lipophilic drugs and rate of their transport depends on their lipophilicity. (ii) paracellular pathway; this pathway is suitable for the transport of the hydrophilic drugs most probably occur by passive diffusion via tight junctions between sustentacular cells and olfactory neurons or open clefts in the membrane. The rate of drug transportation through this pathway mainly depends on the molecular weight of the drug. Drugs having a molecular weight less than 1000 Da easily transport through this mechanism and gives good bioavailability without any absorption enhancer. (iii) Olfactory nerve pathway; where the drug is mainly transported via intraneuronal (axonal transport) or extraneuronal pathway through perineural channels. The intraneuronal pathway is a slow and takes hours to days for drug transportation to the different brain region. Extraneuronal pathways allow the drug

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