Physiology Of Smell & Taste

The nasal cavity is divided into two segments: the respiratory segment and the olfactory segment.

The olfactory segment is lined with a specialized type of pseudo-stratified columnar epithelium, known as olfactory epithelium(olfactory mucous membrane), which contains receptors for the sense of the smell.

This segment is located along the dorsal roof of the nasal cavity

Each olfactory receptor is a neuron. Each neuron has a short, thick dendrite with

an expanded end called an olfactory rod. From these rods, cilia project to the surface

of the mucous. There are 10-20 cilia per receptor neuron. The axons of the olfactory receptor neurons

pierce the cribriform plate of the ethmoid bone and enter the olfactory bulbs.

The olfactory neurons are constantly replaced.

The new ones arise from the basal cells, small non-specialized cells located in the basal lamina of the olfactory epithelium.

The olfactory mucous membrane is constantly covered by mucous produced by Bowman’s gland(also located in basal lamina of the olfactory epithelium).

In the olfactory bulbs, the axons of the receptors contact the primary dendrites of the mitral cells and tufted cells to form the complex globular synapses called olfactory glomeruli.

In addition to mitral and tufted cells, the olfactory bulbs contain periglomerular cells and granule cells

Periglomerular cells are inhibitory neurons connecting one glomerulus to another.

Granule cells have no axons and make reciprocal synapes with the lateral dendrites of the mitral or tufted cells.

At these synapses granule cells are excited by the glutamate molecules released by mitral and tufted cells.Granule cells in turn inhibit the both type of cells(mitral and tufted) by releasing GABA

The axons of the mitral and tufted cells pass posteriorly through the intermediate olfactory stria and lateral olfactory stria to the olfactory cortex.

The axons end on the apical dendrites of pyramidal cells in the olfactory cortex.

Olfactory receptors respond only to substances that are in contact with the olfactory epithelium and are dissolved in the thin layer of mucus that covers it.

Sensitivity of olfactory receptors differs from one substance to another.

Olfactory discrimination is remarkable. Human can discriminate 10,000 different odours.

Abnormalities of olfaction include:

Anosmia(absence of the sense of smell) Hyposmia(diminished olfactory sensitivity) Dysosmia(distorted sense of smell) ◦Olfactory thresholds increases with age and

more than 75% of humans over the age of 80 have an impaired ability to identify smells.

Anosmia is a lack of functioning olfaction, or in other words, an inability to perceive odours.

Anosmia may be either temporary or permanent.

Some people may be anosmic for one particular odour. This is known as "specific anosmia". The absence of the sense of smell from birth is called “Congenital Anosmia”.

Hyposmia is a reduced ability to smell and to detect odours.

Based on latest scientific evidence, hyposmia might be a very early sign of Parkinson’s disease.

 Some of the causes of olfaction problems are allergies, nasal polyps, viral infections and head trauma.

Dysosmia, also known as olfactory dysfunction, is the impairment of olfactory stimuli leading to an altered sense of smell.

These dysfunctions can present in a variety of ways, such as the stimuli not activating the olfactory bulb, some odours being interpreted as other odours, or hallucination of smells.

The tongue is a muscular organ on the floors of the mouths of most vertebrates which manipulates food for mastication.

It is the primary organ of taste  (Gustation), as much of the upper surface of the tongue is covered in papillae and taste buds.

It is sensitive and kept moist by saliva, and is richly supplied with nerves and blood vessels.

 In humans a secondary function of the tongue is phonetic articulation(speech).

Taste buds are the sense organs for taste. Each taste bud is made up of 4 types if cells:

i. Basal cells.ii. Type I and II which are sustentacular cells.iii. Type III cells which are gustatory receptor cells that

make synaptic connections to sensory nerve fibers.

Type 1,2 and 3 cells have microvilli and the necks of all these cells are connected to each other by tight junction

Taste buds are constantly replaced with the half-time of about 10 days.

In humans, the taste buds are located in the mucosa of the epiglottis, palate, pharynx, and in the walls of the fungiform and vallate papillae of the tongue.

The fungiform papillae are found most numerous near the tip of the tongue.

The vallate papillae are arranged in a V on the back of the tongue.

The sensory nerve fibers from the taste buds on the anterior two-thirds of the tongue travel in the chorda tympani branch of the facial nerve.

Fibers from the posterior third of the tongue reach the brainstem via glossopharyngeal nerve.

The fibers from areas other than the tongue(epiglottis,pharynx) reach brainstem via vagus nerve.

On each side, the myelinated but relatively slowly conducting taste fibers in these three nerves unite in the nucleus of the tractus solitarius in the medulla oblongata.

There,they synapse in second-order neurons,the axons of which cross the midline join the medial leminiscus,ending with fibers for touch,pain and temperature in the specific sensory relay nuclei of the thalamus.

Impulses are relayed from there to the cerebral cortex at the foot of the postcentral gyrus.

There are 4 basic tastes in humans:A.SweetB.SourC.Salty D.Bitter An additional taste modality named

“umami” has been claimed to exist. It is said to mediate the taste of glutamate and the monosodium glutamate used extensively in Asian cuisine.

Abnormalities of taste include:

1)Ageusia(absence of the sense of taste)2)Hypogeusia(diminished taste sensitivity)3)Dysgeusia(disturbed sense of taste)

Hypogeusia is a reduced ability to taste things (to taste sweet, sour, bitter, or salty substances).

Causes of hypogeusia include the chemotherapy drug bleomycin, an antitumor antibiotic.

In addition, drugs such as captopril, penicillamine, which contain sulfhyldryl groups causes temporary loss of taste sensation.

Ageusia is the loss of taste functions of the tongue, particularly the inability to detect sweetness, sourness, bitterness, saltiness, and umami.

It may be caused due to the damage of the lingual and glassophayngeal nerve or problems with endocrine system.

Dysgeusia is the distortion of the sense of taste. The distortion in the sense of taste is the only

symptom, and diagnosis is usually complicated since the sense of taste is tied together with other sensory systems.

Common causes of dysgeusia include chemotheraphy, asthma treatment with albuterol, and zinc deficiency.

Due to the variety of causes of dysgeusia, there are many possible treatments that are effective in alleviating or terminating the symptoms of dysgeusia. These include artificial saliva, pilocarpine, zinc supplementation, alterations in drug therapy, and alpha lipoic acid.