We now know about the special sense and the general sense. Last two blogs were about the special senses: Taste and Hearing. The other one is vision. In this blog let us discuss the special sense Olfaction or Smell.

The special organ meant for olfaction or smelling is the nose. Each individual has a single nose in the centre of the face, usually inbetween, slightly below the eyes.

• Anatomy of the nose
The external nose consists of paired nasal bones and upper and lower lateral cartilages (fig 1).

Fig 1: Anatomy of a nose.

The nasal passages are divided by the nasal septum in the midline. Each lateral nasal wall is formed by four turbinates (inferior, middle, superior, and supreme) as seen in fig 2. The turbinates within the nose act as barrier to the air flow resulting into turbulence in the air depositing the odorants within the mucus.

Fig 2: Interior structure of the nose.

• Olfactory Epithelium

The most interior part of the nasal cavity is lined with olfactory epithelium. This includes the 1/3 of the nasal septum, surrounding area of superior turbinate and roof of the nasal cavity (fig 2). This layer has a peculiar yellowish tinge and has presence of olfactory neurons, basal cells and the supportive cells (fig 3).

~Olfactory neurons
These neurons are bipolar cells and have single dendrite with non motile sensory cilia. The dendrites of these neurons extends to the epithelial surface and
their cilia lie over the epithelium in the lumen of nasal cavity (fig 3). The cilia have olfactory receptors on their surfaces, which are type of G protein-coupled receptors. The axons of these neurons transmit signals to the brain.

~Supportive cells:

Also known as sustencular cells, these cells are similar to neural glial cells, and function as metabolic and physical support for the olfactory cells. They are tall columnar cells with microvilli.

~Basal cells

These are the stem cells, which give rise to the different structural components of the olfactory epithelium, Bowman’s glands, etc (fig 3).

Also present in the epithelium and lamina propria are the Bowman’s glands (fig 3). These glands secrete mucus and keep the interior of the nose moist. The secretions also facilitates binding of odorants and dissolving the odor-containing gases.

Fig 3: The neuroepithelium and adjoining layers.

• The process of Olfaction:

As mentioned before, the olfactory receptors belong to the family of transmembrane G- protein coupled receptors (GPCR). When the odorants bind the receptor, it undergoes conformational changes and leads to the activation of the enzyme called adenylyl cyclase, which catalyzes the conversion of ATP into cyclic adenosine monophosphate (cAMP).

(Just for info: More on GPCR in this paper)

Fig 4: Position of Cribiform plate

Second messenger cAMP leads to the activation of cAMP-gated ion channels. The opening of these channels cause calcium and sodium ion influx into the neurons resulting into depolarization and eventually into action potential. The receptor neuron synapses with the next neuron and eventually the information is passed on to the brain.

• Neural pathway:

The axons of the olfactory sensory cell passes through the basal lamina to join axons from other sensory cells in the lamina propria. These axons perforate the cribiform plate to synapse with neurons known as mitral cells in the glomeruli of the olfactory bulb of the brain (see fig 5).

(Just for info: Read more about Olfactory bulb)

Fig 5: The axons of olfactory epithelium and olfactory bulb.

The olfactory bulb (fig 6) is an ovoid structure containing specialised neurons called mitral cells. The olfactory nerve fibres synapse with the mitral cells, forming collections known as synaptic glomeruli.

Fig 6: Olfactory bulb (top view).

From the glomeruli, second order nerves then pass into the olfactory tract. These nerves then send the information to the olfactory cortex. From olfactory cortex the information is sent to different parts of the brain (fig 7) namely:

Hippocampus (linking the smell with emotions and memory)

Anterior insula and amygdala (limbic-related cortex)

Frontal cortex (deals with cognitive skills such as emotional expression, problem solving, memory, language, judgment, and sexual behavior)

Hypothalamus (Autonomic Nervous System)

(Just for info: Read the paper Odor maps in the olfactory cortex)

Fig 7: Different parts of the brain involved with olfaction.

This is how the information about an odorant is passed to the brain and perceived.

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Also read other posts by The Biotech Notes:

High Pressure Liquid Chromatgraphy (HPLC).

Detection and Measurement of Radioactivity: Geiger Muller Counter and more.

Transduction..

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Image Sources:

Fig 1: Fattahi (2012) Nasal Fractures. Current Therapy In Oral and Maxillofacial Surgery.

Fig 2: Lavin et al (2017) Superior turbinate eosinophilia correlates with olfactory deficit in chronic rhinosinusitis patients. The Laryngoscope.

fig 3: Basic Neurochemistry (Eighth Edition). Principles of Molecular, Cellular, and Medical Neurobiology (2012). Chapter 52 – Molecular Basis of Olfaction and Taste/

Fig 4: Kelly and Curtin (2016) Neuroimaging Part I. Handbook of Clinical Neurology.

Fig 5: Huart et al (2013) Olfaction. In: Önerci T. (eds) Nasal Physiology and Pathophysiology of Nasal Disorders. Springer, Berlin, Heidelberg

Fig 6: Gray (1918) Anatomy of the Human Body. Philadelphia: Lea & Febiger, 1918; Bartleby.com, 2000. http://www.bartleby.com/107/. [Date of Printout].

Fig 7: Saive et al (2014). A review on the neural bases of episodic odor memory: From laboratory-based to autobiographical approaches. Frontiers in behavioral neuroscience.