Taste and Smell

Taste Buds: Approximately 4000 taste buds exist on the tongue.
  - Lingual Papillae: Wart-like bumps located under the mucous membrane of the tongue.
    - Function: Increase the surface area of the tongue several times, enhancing the perception of individual tastes.
    - Also referred to as “the magnifying effect of the tongue.”
    - Papillae contain several taste buds with sensory cells.

Filiform Papillae:
- Description: Spike-like structures without taste buds; abundant on the human tongue.
- Function: Primarily used for sensing the texture of foods (mouthfeel) and grooming fur in animals.
Foliate Papillae:
  - Description: Weakly developed papillae visible to the naked eye on the rear edges of the tongue.
  - Quantity: Approximately 20 foliate papillae, each containing several hundred taste buds.
  - Lifespan: Most degenerate by age 2 or 3.
Fungiform Papillae:
  - Description: Most common type, with 200 to 400 bumps spread across the tongue's surface, especially at the tip and edges.
  - Function: Detect taste and contain sensory cells for touch and temperature.
  - Each papilla contains 3 to 5 taste buds.
Circumvallate Papillae (Vallate):
  - Description: Very large papillae located at the base of the tongue, where the throat begins.
  - Quantity: Each person has 7 to 12 circumvallate papillae, which contain several thousand taste buds each.
  - Arrangement: Round, raised, visible to the naked eye, arranged in a V-shape at the back of the tongue.
  - Function: Surrounded by a trench containing glands that “rinse” taste-producing substances into the sensory cells.

Sweet:
  - Description: Usually caused by sugar and derivatives such as fructose or lactose.
  - Association: Generally linked with carbohydrates and high-calorie foods.
  - Other Substances: Can also be activated by protein building blocks like amino acids and alcohols in fruit juices or drinks.
Sour:
- Description: Generally associated with acidic solutions, like lemon juice or organic acids.
- Mechanism: Taste sensation caused by hydrogen ions (H+) released from acid dissolved in a watery solution.
Salty:
- Description: Primarily associated with food containing table salt (sodium chloride).
- Chemical Basis: Salt crystals consist of sodium and chloride; other salts like potassium or magnesium ions can also cause saltiness.
Bitter:
- Description: Resultant of many different substances; bitter taste presents an evolutionary context where recognition of poisonous plants was crucial.
- Substances: Includes spoiled foods and alkaloids such as nicotine, caffeine, quinine, and morphine, as well as basic solutions.
Umami (Meaty):
- Description: Elicits a taste reminiscent of meat broth, often caused by glutamic acid or aspartic acid.
- Origins: Amino acids found in many proteins in food, present in ripe tomatoes, meat, and cheese.

Cranial Nerves Involved:
  - CN VII (Facial Nerve): Innervates the anterior 2/3 of the tongue.
  - CN IX (Glossopharyngeal Nerve): Innervates the posterior 1/3 of the tongue.
  - CN X (Vagus Nerve): Taste buds on the epiglottis, palate, and pharynx.

Olfactory System:
- Olfactory Mucosa: Located in the roof of the nasal cavity, responsible for odor detection; covers the superior concha, cribriform plate, and nasal septum.
- Contains 10 to 20 million olfactory cells (neurons) along with support cells and basal stem cells.

Shape: Cells resemble a bowling pin with the widest part being the neurosoma. The neck and head are modified dendrites containing 10-20 cilia (olfactory hairs).
Basal end: Tapers to become an axon.
- Axons collect into fascicles of CN I, passing through the cribriform plate (cribriform foramina).
Lifespan: Olfactory cells have a limited life of 60 days; they are replaceable as basal cells divide into new olfactory cells.

Comparison: Human sense of smell is less sensitive than that of many animals but more sensitive than the sense of taste.
Gender Differences: Women typically have a heightened sensitivity to odors compared to men.
Distinguishing Odors: Humans can distinguish between 2000 to 4000 different odors, with some individuals identifying up to 10,000.

Process: Odorant molecules must bind to receptors on olfactory hairs. This triggers action potentials through a sequence of steps leading to brain signal transmission.
Alternative Pathway: Some odorants activate nociceptors of CN V, causing discomfort (e.g., ammonia, menthol, chlorine, capsaicin).

Synapses: Occur in structures called glomeruli, each specialized for particular odors.
Information Processing: Signals travel from glomeruli to the olfactory bulbs and then along olfactory tracts (CN I) to the primary olfactory cortex located in the temporal lobe.
- Further Transmission: Signals proceed to the insula and orbitofrontal cortex for identification and discrimination of odors.
- Integration: The orbitofrontal cortex also integrates taste and smell into flavor perception.
Memory Activation: Produced signals also reach the hippocampus, amygdala, and hypothalamus, explaining why smells can significantly trigger memories.