Sense of Smell Dr.Mohammed Sharique Ahmed Quadri Assistant Professor Physiology Al Maarefa College 1

Smell (Olfaction) Olfactory receptors in nose are specialized endings of renewable afferent neurons Olfactory mucosa – 3cm 2 of mucosa in ceiling of nasal cavity 2

Smell (Olfaction) Olfactory mucosa contains 3 cell types Olfactory receptor cell Supporting cells – Secrete mucus Basal cells – Precursors of new olfactory receptor cells (replaced about every two months) 3

Smell (Olfaction) Olfactory receptor cell – Afferent neuron whose receptor portion is in olfactory mucosa in nose and afferent axon traverses into brain – Receptor portion is enlarge knob like bearing several long cilia – Cilia contains binding site for attachment of odorant molecule – Axons of olfactory receptor cells collectively form olfactory nerve 4

Smell (Olfaction) Odorants – Molecules that can be smelled To be smelled, substance must be – Sufficiently volatile that some of its molecules can enter nose in inspired air – Sufficiently water soluble that it can dissolve in mucus coating the olfactory mucosa 5

Smell (Olfaction) Human nose contains approx. 5 million olfactory receptors 1000 different types of olfactory receptors During olfaction and odor is dissected into various componant.each part of odor is detected by one of the 1000 different receptors Afferent signals are sorted according to scent component by glomeruli within olfactory bulb 6

Physiology of Olfaction Odorant binds to the receptor of an olfactory hair→ G-protein activation→ activation of adenylate cyclase→ production of cAMP→ opening of Na+ channels→ inflow of Na+ →generator potential→ nerve impulse through olfactory nerves→ olfactory bulbs→ olfactory tract→ primary olfactory area of the cerebral cortex. Copyright 2009, John Wiley & Sons, Inc.

Olfactory transduction Copyright 2009, John Wiley & Sons, Inc.

Processing of Scents in Olfactory Bulb 9

Olfactory pathway Fibers originating in olfactory bulb travel in two different routes – Subcortical route, to limbic system – close coordination between smell and behavioral reactions, associated with eating, etc. – Route to primary visual cortex (piriform cortex) 10

Abnormalities Anosmia (absence of sense of smell), Hyposmia (diminished olfactory sensitivity) dysosmia (distorted sense of smell) Several dozens different anosmias have been detected in humans. They are presumably due to absence or disrupted functions of one of the many members of odorant receptor family

Thank You 12

References Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11 th edition Text book of physiology by Linda.s contanzo,third edition