بسم الله الرحمن الرحيم Sense of Smell

Smell (=Olfaction) Normal individual can discriminate more than 10.000 odors, but not highly developed in human as in some animals. Olfactory receptors in nose are specialized endings of renewable afferent neurons Olfactory mucosa: 3 cm2 of mucosa in ceiling of nasal cavity.

Significance of olfaction Olfaction is related to taste (in severe rhinitis loss of specific taste of some foods). The receptors of olfaction are chemoreceptors (moderately adapting) and they are true neurons & close to external environment. Olfaction may produce changes in GIT motility & secretion. Olfaction may produce Sexual & Psychogenic effects. Olfaction has a Protective function (dangerous odors as gases or fires).

Olfactory Mucosa Olfactory mucosa contains 3 cell types: 1- Olfactory receptor cells: true bipolar neurons derived from CNS. 2- Supporting cells: Secrete mucus 3- Stem Basal cells: Precursors of new olfactory receptor cells (replaced about every two months)

Olfactory receptor cells: Afferent neuron whose receptor portion is in olfactory mucosa in nose and afferent axon traverses into brain. Receptor portion is enlarged knob-like bearing several long cilia. Cilia contains binding sites for attachment of odorant molecule. Axons of olfactory receptor cells collectively form olfactory nerve

* Physiological information about olfaction: Odorant substances must be volatile to come in contact with olfactory epithelium and must be water soluble to be dissolved in mucous and to cross the cell membrane. The olfactory mucus contains odorant binding proteins that transmits & concentrate the odorant substances to the receptors. Human nose contains approx. 5 millions olfactory receptors

There are 1000 different odorant receptors encoded in 1000 different genes and this explains the ability of normal individual to discriminate more than 10.000 odors. During olfaction odor is dissected into various components. 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.

Mechanism of stimulation of olfactory receptors: Chemical theory: RMP of olfactory receptors is -55 mv and at this level  continuous impulses arise from these receptors at a rate of 1: 3/ sec. Stimulation of olfactory receptors by odorant substance   this potential from -55 to -30 mv (depolarization)   No. of impulses up to 20/sec. When odorant substances come in contact with the olfactory area, they diffuse into mucous layer and bind with receptor protein on cilia. Such binding  activate G protein  activate adenyl cyclase   cAMP  open Na+ channels  depolarization (receptor potential)  nerve impulse through olfactory nerves  olfactory bulb  olfactory tract  primary olfactory area of the cerebral cortex Copyright 2009, John Wiley & Sons, Inc.

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

Olfactory pathway Olfaction is the only sense which has no area in neocortex unlike other sensations but it is detected mainly in limbic system. The first order neurons: are the receptor cells in the olfactory mucosa. Their axons unit to from "Olfactory nerve" (1st cranial nerve), these axons penetrate cribriform plate of ethmoid bone to enter the "Olfactory bulb".  The second order neurons: are "mitral & tufted cells" in the olfactory bulb and they synapse with axons of olfactory cells forming "Olfactory glomeruli", each glomerulus contains axons of 25,000 receptor cells that synapse with 25 mitral and 60 tufted cells. Different glomeruli respond to different odors. The axons of tufted and mitral cells pass posteriorly to form "Olfactory tract" which enter the brain then divides into three stria.

Processing of Scents in Olfactory Bulb

Vomeronasal organ (VNO) A structure on each side of the nasal septum of human and more common in animals. It is a chemical sensory center for detection of pheromones (chemical signals that carry information between individuals of the same species). It is responsible for good or bad feeling between people. Binding of pheromones to receptors on neurons in VNO  action potential travels through non olfactory pathway to limbic system.

Smell Abnormalities Anosmia: Absence or marked loss of smell sensation Hyposmia (diminished olfactory sensitivity) Dysosmia (distorted sense of smell) Parosmia: smelling of non existing odors usually is Psychogenic. Cacosmia: receiving a bad smell due to Chronic sinusitis, foreign body in the nose or lung abscess. 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.

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