- Dr. Chintan*Physiology of Olfaction

- Smell is the least understood of our senses.

- This results partly from the fact that the sense of smell is a subjective phenomenon that cannot be studied with ease in lower animals.

- Another complicating problem is that the sense of smell is poorly developed in human beings (Microsmatic) in comparison with the sense of smell in many lower animals (Macrosmatic).

- Important for pleasure and for enjoying the taste of food.

- alert us to potential dangers, e.g. smoke

*Sense of Smell

- lies in the superior part of each nostril

- In each nostril, the olfactory membrane has a surface area of about 2.4 square centimeters

- Olfactory Cells - The receptor cells for the smell sensation - bipolar nerve cells derived originally from the CNS

- about 100 million of these cells in the olfactory epithelium interspersed among sustentacular cells

*Olfactory Membrane

- The mucosal end of the olfactory cell forms a knob from which 4 to 25 olfactory hairs (also called olfactory cilia), project into the mucus that coats the inner surface of the nasal cavity

- cilia react to odors in the air and stimulate the olfactory cells

- Spaced among the olfactory cells - Bowman’s glands that secrete mucus onto the surface of the olfactory membrane.

- Olfactory cells are constantly being replaced with a half-time of a few weeks

*Olfactory Membrane

- The portion of each olfactory cell that responds to the olfactory chemical stimuli is the olfactory cilia.

- The odorant substance, on coming in contact with the olfactory membrane surface, first diffuses into the mucus that covers the cilia.

- Then it binds with receptor proteins in the membrane of each cilium - OBP

- Receptor protein – G protein – cAMP pathway – opening of Na channels – action potential – exciting the olfactory neuron - olfactory nerve – CNS

* Excitation of the Olfactory Cells

- Only volatile substances that can be sniffed into the nostrils can be smelled

- Substance must be at least slightly water soluble so that it can pass through the mucus to reach the olfactory cilia.

- substance to be at least slightly lipid soluble, presumably because lipid constituents of the cilium itself are a weak barrier to non-lipid-soluble odorants.

- Sniffing is a semi-reflex response that usually occurs when a new odor attracts attention.

* Excitation of the Olfactory Cells

- The olfactory receptors adapt about 50 per cent in the first second or so after stimulation.

- Thereafter, they adapt very little and very slowly.

- our own experience that smell sensations adapt almost to extinction within a minute or so after entering a strongly odorous atmosphere.

*Adaptation

- Because this psychological adaptation is far greater than the degree of adaptation of the receptors themselves

- most of the additional adaptation occurs within the CNS

- Large numbers of nerve fibers pass from the olfactory regions of the brain backward along the olfactory tract and terminate on special inhibitory cells in the olfactory bulb, the granule cells.

*Adaptation

- 1. Camphoraceous- 2. Musky- 3. Floral- 4. Pepperminty- 5. Ethereal- 6. Pungent- 7. Putrid

- 100 primary sensations of smell

*Primary Smell Sensations

- affective quality of either pleasantness or unpleasantness.

- smell is probably even more important than taste for the selection of food.

- a person who has previously eaten food that disagreed with him or her is often nauseated by the smell of that same food on a second occasion.

*Affective Nature of Smell

- the minute quantity of stimulating agent in the air can elicit a smell sensation.

- the substance methylmercaptan can be smelled when only one 25 trillionth of a gram is present in each milliliter of air.

- Because of this very low threshold, this substance is mixed with natural gas to give the gas an odor that can be detected when even small amounts of gas leak from a cylinder.

*Threshold for Smell

- Olfactory bulb - olfactory tract – olfactory nerve – 1st cranial nerve

- both the tract and the bulb are an anterior outgrowth of brain tissue from the base of the brain

- olfactory bulb lies over the cribriform plate, separating the brain cavity from the upper nasal cavity

- The cribriform plate has multiple small perforations through which an equal number of small nerves pass upward from the olfactory membrane in the nasal cavity to enter the olfactory bulb in the cranial cavity

*Smell Pathway

- short axons from the olfactory cells terminating in multiple globular structures within the olfactory bulb called glomeruli

- Each glomerulus is the terminus for dendrites from about 25 large mitral cells and about 60 smaller tufted cells, the cell bodies of which lie in the olfactory bulb superior to the glomeruli – granule cells - Periglomerular cells

- mitral and tufted cells send axons through the olfactory tract to transmit olfactory signals to higher levels in the CNS

- Mucus – cilia - Axons of olfactory cells – glomeruli in bulb – dendrites of mitral, tufted cells in bulb – axons of mitral, tufted cells in tract - CNS

* Smell Pathway

- Olfactory tract divides into- medially into the medial olfactory area (stria) of

the brain stem – very old olfactory system- other passing laterally into the lateral olfactory area

(stria) - a newer & less old system

- The Medial Olfactory Area (very old) – septal nuclei – hypothalamus – limbic system – removal – not much effect

- The Less Old Lateral Olfactory Area - prepyriform and pyriform cortex plus portion of the amygdaloid nuclei – limbic system (hippocampus) – learning & aversion

*Smell CNS Pathway

- lateral olfactory area - anteromedial portion of the temporal lobe (cerebral cortex)

- *This is the only area of the entire cerebral cortex where sensory signals pass directly to the cortex without passing first through the thalamus

- The Newer Pathway - passes through the thalamus, passing to the dorsomedial thalamic nucleus - orbitofrontal cortex

- conscious analysis of odor

- Granule cells, Periglomerular cells – lateral inhibition

*Smell CNS Pathway

- In rodents and various other mammals, the nasal cavity contains another patch of olfactory mucous membrane located along the nasal septum in a well-developed vomeronasal organ.

- This structure is concerned with the perception of odors that act as pheromones.

- Its receptors project to the accessory olfactory bulb and from there primarily to areas in the amygdala and hypothalamus that are concerned with reproduction

- The organ is not well developed in humans.

*Vomeronasal Organ

- Evidence for the existence of pheromones in humans ???

- close relationship between smell and sexual function - The perfume ads

- The sense of smell is said to be more acute in women than in men, and in women it is most acute at the time of ovulation.

- Smell and, to a lesser extent, taste have a unique ability to trigger long-term memories

*Vomeronasal Organ

- Pain Fibers in the Nose - Naked endings of many trigeminal pain fibers are found in the olfactory mucous membrane - stimulated by irritating substances

- peppermint, menthol and chlorine - sneezing, lacrimation, respiratory inhibition

- anosmia (absence of the sense of smell), hyposmia (diminished olfactory sensitivity), and dysosmia (distorted sense of smell).

- Olfactory thresholds increase with advancing age, and more than 75% of humans over the age of 80 have an impaired ability to identify smells.

- Anosmia + Hypogonadism (Kallmann's syndrome)

*Applied

*Thank you…