तद्ध्युत्तमाङ्गमनुप्रविश्य मुञ्जादीषिकामिवासक्तां केवलं विकारकरं दोषमपकर्षति|||२२|| (Cha/Si/2/22) 

According to Caraka, the Nasya Dravya instilling into nostril will reach Uttamāṅga and expel the doṣās there just like how Iṣikā grass has been removed from its stalk. 

नासा हि शिरसो द्वारम्|तत्रावसेचितमोउषधं स्रोतःशृङ्गाटकं प्राप्य व्याप्य च मूर्धानं नेत्रश्रोत्रकण्ठादिसिरामुख़ानि च मुञ्जादिषीकामविसक्तामूर्ध्वजत्रुगतांवैकारिकीमशेषामाशु दोषसंहतिमुत्तमाङ्गादपकर्षति|  

(As San/Su/29/3) 

Ashtaṅga Sangraha gave  a detailed explanation  that nose is the entry door to śiras. Hecne the medicine given through that route will fill all related Srotases and reach the Śṛṅgāṭaka Marma and spread the whole of head  and the channels and veins in the eyes, ears and throat by its Vīrya and expels out completely  the accumulated vitiated Doṣās which are stationed in the parts  above the clavicle just like an Iṣikā is removed from the Munja grass. 

Śṛṅgāṭaka Marma  

गुदापस्तम्भविधुरशृङ्गाटानि नवादिशेत्| 
मर्माणि धमनीस्थानि—-(As Hr/Sa/4/40)  

Śṛṅgāṭaka Marma is a Dhamani Marma and Sadyo Prāṇahara Marma. 

जिह्वाक्षिनासिकाश्रोत्रखचतुष्टयसङ्गमे| 
तालुन्यास्यानि चत्वारि स्रोतसां, तेषु मर्मसु||३४|| 
                                                                                                           (As Hr/Sa/4/35) 

It’s the meeting point of tongue, eyes, nose and ears. 

The Nasya medicaments are going to reach in the Śṛṅgāṭaka area and the Doṣā residing in Mukha, Nāsa, Akṣi &  Jihva are expelled out. 

शृङ्गाटकमभिप्लाव्य निरेति वदनाद्यथा | (Su/Chi/40/29) 

In nasya vyāpat Suśruta has told excessive Vairecanika Nasya will result in Mastulunga Srāva which shows the direct connection of Nāsā Śiras. 

According to the commentator Indu, the exact Sthāna of the Śṛṅgāṭaka Marma is 

 “शिरसो अन्तर्मध्यम् मूर्धानां” which can be considered for the middle cranial fossa.  On basis of above facts, the verse i.e. “नासा हि शिरसोद्वारं” can be justified which reflexes the action  of Nasya in  head and systemic disorders. 

NASAL ANATOMY AND PHYSIOLOGY RELATED TO NASYA 

Nasal cavity is divided into three functional zones  

• Vestibular region 

• Respiratory region 

• Olfactory region 

Vestibular region: This region filters the air coming into the nasal cavity. 

Respiratory region:  This region carries out the function of respiration with maximum of its vascularity. 

Olfactory region:  Area of olfaction as well as act as connecting area between brain and vestibule 

The two nasal cavities are the uppermost part of the respiratory tract and contain the olfactory receptors. The smaller anterior region of the cavities is enclosed by the external nose whereas the large posterior region are more central within the skull. The openings of the para-nasal sinuses are the extension of the nasal cavity into the cranial cavity. The nasal cavity is related with the anterior and middle cranial fossae, orbit and paranasal sinuses. The superior aspect of sphenoidal sinus is related with the hypophysis, the optic nerves, optic chiasma and literally to the cavernous sinus and internal carotid artery. The cribriform plate is a part of the ethmoid bone, forms the portion of the roof of the nasal cavity it contains very small perforations, allowing fibers of the olfactory nerve to enter and exit. Sphenopalatine foramen located at the level of superior meatus allows communication between the nasal cavity and the pterygo-palatine fossa. 

The   pharmacodynamics   of Nasyakarma can    be    explained    in    light    of    modern  

anatomical and physiological studies as follows. 

• Vascular pathway:     The   nasal   tissue   is   highly   vascularized   making   it   an  

attractive site for rapid and efficient systemic absorption. Vascular   path   transportation   is possible through the pooling of nasal venous blood into the facial vein which occurs naturally.    It    communicates    freely    with    the    intracranial circulation. It   communicates   through   pterygoid plexus with the cavernous venous sinus. 

• Neurological pathway: Olfactory nerve is chemoreceptor in nature.  It is known  

that through olfactory pathway this nerve is connected with limbic system and hypothalamus which are having control over endocrine secretions. Moreover, hypothalamus    is    considered    to    be    responsible    for integrating the functions of the endocrine system and the nervous system. So, the drugs administrated here stimulate the higher centers of brain which shows action on regulation of endocrine and nervous system functions. 

•  Diffusion through mucosa:  In the absorption of drug from the nasal cavity  

first step is passage through the mucus. Mechanisms   for   absorption   of   drug   through   the   nasal mucosa include  

1. Paracellular route is the first mechanism which is an aqueous route of transport. This is slow and passive route. 
2. Transcellular   process   is   the   second   mechanism   of transport through a lipoidal route and is responsible for the    transport    of    lipophilic    drugs like Sneha Nasya Dravya that    show    a    rate dependency on their lipophilicity.  Drugs also cross cell membranes   by   an   active   transport   route   via   carrier mediated means or transport through the opening of tight junctions. 

These are the possible mode of action of how a Nasya Dravya works.