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Learning Objectives

• By the end of this lecture you should:-

• Describe the functions of the olfactory neuroepithelium

• Understand how odors stimulate the olfactory receptors

• Understand how the bipolar olfactory neurons are stimulated

Olfactory neuroepithelium

• Orderly arrangement of the :

• Bipolar olfactory receptor cells

• Micovillar cells

• Sustentacular cells

• Basal cells

Bipolar cell

• Dendrites have bulb-shaped vesicles projecting into the mucosa layer

• Each vesicle has 6-8 cilia containing the odorant

• Each bipolar cell has 56cm2 of surface area to receive olfactory stimuli

Microvillar and Sustentacular cells

• Microvillar cells located adjacent to receptor cells on the surface of neuroepithelium

• Sustentacullar cells are not specialized to secrete mucus

• Complete function off sustentacular cell is unknown but form tight barrier separating neurons from the environment

Basal cells

• Progenitors of other cells including the bipolar cells

• Regular turnover of bipolar receptor cells which serve as primary sensory neurons

• Injury to cell body or axon stimulates the differentiation of the basal cells

Primary sensory systems

• Regularly replaced

• Regenerate after injury

• Restablish central neural connections


• Unmyelinated neurons of receptor cells form fila of the olfactory nerve

• Fila pass through the cribriform plate and terminate within spherical masses of the neuropil called glomerula in the olfactory bulb

• More fibers converge on the glomeruli than those who leave

Mitral cells

• Main second order neurons

• Primary dendrite of each mitral cells extends into a single glomerulus

• Axons of mitral cells project onto axons of adjacent tufted cells to the limbic system(anterior olfactory nucleus and amygdala)

Olfactory chemosensation

• Prepiriform cortex or amygdaloid nuclear mediate the cognitive awareness of smell

• Epithelium of vomeronasal organ(a tubular structure opening into the ventral septum)

• Sensory neurons in this organ detect pheromones


• Smell sensation begins when odorants touches cilia of bipolar neurons

• Most odorants are hydrophobic

• Transported between cilia by the odorant binding proteins which reversibly bind the cilia surface

Olfactory receptor proteins

• G-protein coupled receptors that also include:-• Rhodopsins

• α- and β– adrenergic receptors

• Muscarinic Acetylcholine receptors • Dopamine receptors

• Serotonin receptors

• Substance P receptors

Human olfactory receptor genes

• 300-1000 genes belonging to 20 different families in clusters at 25 different chromosome locations

• Each olfactory neuron expresses only one receptor gene providing the basis for odor discrimination

Discrete spatial zones

• Bipolar cells with similar receptors are scattered across discrete spatial zones

• These converge on a select few glomeruli in the olfactory bulb

• Potential spatial map results on how we receive odor stimuli