Organ of Taste and Smell

• ORGAN OF TASTE

The organ of taste is the tongue. The posterior part of the tongue is considered as the base of the tongue. Through the base of the tongue, blood vessels and nerve fibers enter into the tongue and come out from the tongue. Taste buds are present on the tongue to receive external stimuli to detect taste.

Taste buds

Taste buds are oval made of the following three types of cells: Taste bud Cells, Supporting Cells, and Basal Cells. It detects taste. Sense of taste helps to distinguish useful, harmful, toxic, or poisonous substances.

• 1. Taste bud cells (gastato-receptor cells):

  • · Structure: Taste buds are oval with one opening called the taste pore. These taste buds have microvilli (Taste Hairs). Microvilli push food molecules in solution form into taste pores.

  • · Chemoreceptor: Taste bud cells are receptors of tastes. These receptors detect taste based on the chemical nature of food.

  • · Taste detection: Taste bud cells can detect the taste of liquid food. It cannot detect the taste of solid food. Saliva converts solid food to liquid food to detect the taste of solid food.

  • · Lifespan: The life span of taste bud cells is 10 days. After 10 days. They are replaced by new taste bud cells. The number of taste bud cells decreases with ageing.

• 2. Supporting Cells: Supporting cells are present in between taste bud cells. These supporting cells have also microvilli but no nerve endings.

• 3. Basal Cells: The periphery of taste buds cells is occupied by basal cells. They are also supporting cells.

Types of Taste buds:

• 1. The upper surface of the tongue is covered with three types of papillae. Papillae are tiny raised outgrowth present on the tongue surface.

• 2. Papillae have taste buds. These are Circumvallate Papillae, Fungiform Papillae, and Filiform Papillae.

• · Circumvallate Papillae: They are present on the base of the tongue and arranged in a “V-shape”. They are also known as vallate papillae. They are least in number but larger in size than other forms of papillae. Taste buds are present on their lateral surface.

• · Fungiform Papillae: They are fungus-like dome-shaped papillae present on the whole surface of the tongue. They are present in between filiform papillae. Most of them are present on the tip of the tongue and side of the tongue. They look reddish.

• · Filiform Papillae: They are cone-shaped present on the whole surface of the tongue and mostly present on the front two-thirds of the tongue. They do not have taste buds.

Mechanism:

• 1. Sensory nerve fibers transmit sensory impulses from taste buds to the taste center present in the parietal lobe in the cerebrum inside the brain. It detects the taste.

• 2. The human tongue can detect four basic tastes. These are sweet, salty, sour, and bitter.

• 3. The human tongue has a special separate area for each taste. Food may have an aroma (flavor) with taste. The aroma is not sensed by the tongue. The tongue only senses taste, aroma is sensed by the organ of smell.

ORGAN OF SMELL (Olfactory receptor or Organ of olfaction)

Olfaction is the chemoreception to sense smell. It helps to detect hazards, food, and pheromones. The organ of olfaction (Olfactory organ or organ of smell) is the nose. The nose has a nasal cavity. The nasal cavity roof has olfactory epithelium. The olfactory epithelium is yellow with receptors of smell. The organ of smell consists of the following four types of cells: Olfactory cells, Bowman’s Gland, Supporting cells, and basal cells.

• 1. Olfactory Receptor Cells: These are spindle-shaped, thin neurons that terminate in knob shape structures called the olfactory bulbs. Knobs have non-motile sensory cilia cells also called olfactory hair. Olfactory cells receive as well as transmit the sensory impulse of smell.

• 2. Supporting cells: They are present in between olfactory receptor cells to support olfactory receptor cells. Supporting cells are columnar with an oval nucleus. They also have endocytic vesicles to engulf toxic metabolites, chemicals, and particulates.

• 3. Bowman’s Glands: These glands secrete mucus over olfactory receptor cells and olfactory hairs to keep them moist. The mucus protects olfactory receptor cells from dust and microbes.

• 4. Basal Cells: Basal cells by cell division produce new olfactory cells. The life span of olfactory receptor cells is about two months. Olfactory receptor cells are nerve cells. It is exceptional nerve cells that are produced after birth.

(Pheromones are chemicals secreted or excreted by the body that impact another member of the same species).

MECHANISM: Chemical molecules in gaseous form enter into the nasal cavity (Internal Nostrils). It gets dissolved in mucus near olfactory hairs to bind with receptors. Here it generates sensory impulses. Sensory impulse is transmitted from the olfactory bulb to the olfactory lobe in the temporal lobe of the cerebral hemisphere inside the brain.

Smell perception in the human brain's olfactory center lasts for 2.5 minutes to 10 minutes. After that time humans cannot sense that particular smell. Females have more developed organs of smell than males. Olfactory cell numbers decrease with ageing or smoking. A reduction in the ability to sense smell is called hypoxemia.

Trigeminal Nerves (Dentist’s Nerve): These nerves are present inside the nasal chamber and tongue. Irritant odors or noxious odors like ammonia, vinegar, acetic acid, etc. stimulate these nerves. These nerves activate Bowman’s gland to secrete more mucus to wash out these irritant odorous chemicals.

Dr. Pramila Singh