Olfaction is the sense of smell.

With other senses, it forms the sense of flavor.

Occurs when odorants bind to specific sites on olfactory receptors located in the nasal cavity.

The aggregation of signals from these receptors transmit them to the olfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification, memory, and emotion.

There are at least six morphologically and biochemically different olfactory epithelium cell types.

Humans have about 10 cm2 (1.6 sq in) of olfactory epithelium.

Odorant molecules dissolve in nasal passage mucus lining in the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons.

Odorant molecules diffuse or bind the odorant to odorant-binding proteins.

The mucus overlying the epithelium contains mucopolysaccharides, salts, enzymes, and antibodies acts as a solvent for odor molecules, flows constantly, and is replaced approximately every ten minutes.

The binding of the ligand, odor molecule or odorant, to the receptor leads to an action potential in the receptor neuron.

The odorants stimulate adenylate cyclase to synthesize cAMP via a G protein called Golf.

Olfactory sensory neurons axons go to the brain within the olfactory nerve the first cranial nerve.

The olfactory sensory neuron axons lack myelin sheaths and pass to the olfactory bulb of the brain through perforations in the cribriform plate.

From the olfactory bulb the olfactory sensory neuron axons project olfactory information to the olfactory cortex and other areas.

The axons from the olfactory receptors converge in the outer layer of the olfactory bulb within 50 micrometers in diameter, structures called glomeruli.

In the inner layer of the olfactory bulb, mitral cells form synapses with the axons of the sensory neurons within glomeruli and send the information about the odor to other parts of the olfactory system, where multiple signals may be processed to form a synthesized olfactory perception.

About 25,000 axons synapse on 25 or so mitral cells, and each of these mitral cells project to multiple glomeruli.

Neuromodulators like acetylcholine, serotonin and norepinephrine all send axons to the olfactory bulb.

The mitral cells of the olfactory bulb leave in the lateral olfactory tract, which synapses on five major regions of the cerebrum: the anterior olfactory nucleus, the olfactory tubercle, the amygdala, the piriform cortex, and the entorhinal cortex.

The anterior olfactory nucleus projects in to the contralateral olfactory bulb, inhibiting it.

The piriform cortex has two major divisions: The anterior piriform cortex appears to be better at determining the chemical structure of the odorant molecules, and the posterior piriform cortex has a role in categorizing odors and assessing similarities between odors.

The piriform cortex projects to the medial dorsal nucleus of the thalamus, which then projects to the orbitofrontal cortex.

The orbitofrontal cortex mediates conscious perception of the odor.

The entorhinal cortex projects to the amygdala and is involved in emotional and autonomic responses to odor.

Odor information is stored in long-term memory and has strong connections to emotional memory.

Since any one receptor is responsive to various odorants, and there is convergence at the level of the olfactory bulb, it may seem strange that human beings are able to distinguish so many different odors.

Each individual odor gives a particular spatial map of excitation in the olfactory bulb.

Inputs from the two nostrils have separate inputs to the brain, and each person may experience perceptual rivalry in the olfactory sense akin to that of binocular rivalry.

Humans can detect individuals that are blood relatives-mothers/fathers and children but not husbands and wives from olfaction.

Similarly mothers can identify by body odor their biological children but not their stepchildren.

Pre-adolescent children can olfactorily detect their full siblings but not half-siblings.

Olfactory kinship detection process involves the frontal-temporal junction, the insula, and the dorsomedial prefrontal cortex, but not the primary or secondary olfactory cortices, or the related piriform cortex or orbitofrontal cortex.

The odorant binds to receptors that recognize only a specific functional feature of the odorant, explaining why the chemical nature of the odorant is important.

After binding the odorant, the receptor is activated and will send a signal to the glomeruli.

Signals from the glomeruli are then sent to the brain, where the combination of glomeruli activation encodes the different chemical features of the odorant.

The brain then identified and perceived the odorant, and is able to detect specific odors in mixtures of many background odors.

The average individual is capable of distinguishing over one trillion unique odors.

The ability to distinguish between smells is not analogous to being able to consistently identify them, and that people are not typically capable of identify individual odor stimulants

Different people smell different odors, and such differences in smell are caused by genetic differences.

Odorant receptor genes account for one of the largest gene families in the human genome.

Few genes have been linked conclusively to particular smells:, the odorant receptor OR5A1 and its genetic alleles are responsible for the ability to smell β-ionone, a key aroma in foods and beverages,, the odorant receptor OR2J3 is associated with the ability to detect the odor of grass, cis-3-hexen-1-ol, and thee preference, or dislike, of cilantro has been linked to the olfactory receptor OR6A2.

Flavor perception is an aggregation of information: auditory, taste, haptic, and smell sensations.

During the process of mastication, the tongue manipulates food to release odorants, which enter the nasal cavity during exhalation

The olfaction of food has the sensation of being in the mouth because of co-activation of the motor cortex and olfactory epithelium during mastication, but the retronasal smell plays the biggest role in the sensation of flavor.

The tongue can distinguish only among five distinct qualities of taste

The nose can distinguish among hundreds of substances, in minute quantities.

During exhalation olfaction contribution to flavor.

Proper smell, occurs during the inhalation phase of breathing.

The olfactory system connects directly to the forebrain.

Disorders of olfaction:

Anosmia – inability to smell

Hyperosmia – an abnormally acute sense of smell

Hyposmia – decreased ability to smell

Presbyosmia – the natural decline in the sense of smell in old age

Dysosmia – distortion in the sense of smell

Parosmia – distortion in the perception of an odor

Phantosmia – distortion in the absence of an odor

Heterosmia – inability to distinguish odors

It is estimated that dogs have an olfactory sense approximately ten thousand to a hundred thousand times more acute than a human’s.

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