Human Sensory Systems: Smell and Taste

Posted on Nov 23, 2024 in Biology

Sensory Systems Overview

Human sensory systems process information for perception. These include:

  • Somatic: Touch, pressure, vibration, heat, cold, pain, etc.
  • Visceral

Sensory processing involves:

  1. Receptor stimulation
  2. Transduction: Stimulus conversion into an electrical impulse
  3. Nerve Impulse Generation: Electrical potential becomes a nerve impulse
  4. Transmission: Nerve impulse transmission to the brain
  5. Integration: Information integration and perception

Receptor Characteristics:

  1. Transduction: Capability to turn a stimulus into a nerve impulse
  2. Excitability: Ability to respond to a stimulus
  3. Specificity: Each stimulus has a specific receptor type
  4. Adaptation: Adjustment to a prolonged stimulus

Types of Receptors:

  • Mechanoreceptors: Stimulated by mechanical changes (e.g., ear)
  • Thermoreceptors: Stimulated by temperature changes (e.g., skin)
  • Nociceptors: Stimulated by pain (e.g., skin)
  • Photoreceptors: Stimulated by light (e.g., sight)
  • Chemoreceptors: Stimulated by chemical changes
  • Osmoreceptors: Stimulated by osmotic changes

Sense of Smell (Olfaction)

The olfactory area, about 5.5 cm2, contains millions of receptors located at the top of the nasal cavity, covering the cribriform plate of the ethmoid bone. These receptors are highly sensitive, capturing low-intensity stimuli immediately.

Olfactory Epithelium

Consists of three cell types:

  • Chemoreceptors: Bipolar neurons capturing chemical stimuli, with cilia dendrites. Their axons form the olfactory nerve, passing through the cribriform plate to the olfactory bulbs.
  • Basal Cells: Stem cells that divide and differentiate into receptor cells, renewing them approximately every 30 days.
  • Supporting Cells: Columnar epithelial cells providing support.

Bowman’s Glands

Produce mucus to moisten the epithelium and dissolve olfactory substances. Innervated by the facial nerve (VII cranial nerve), also supplying the lacrimal gland and the rest of the nasal mucosa.

Olfactory Pathway

  • Olfactory Nerve: Axons of receptor neurons forming the olfactory nerve (cranial nerve I), which goes to the olfactory bulbs.
  • Olfactory Bulbs: Part of the brain located below the frontal lobe, where olfactory processing begins.
  • Olfactory Tract: Extends to the olfactory cortex.
  • Olfactory Cortex: Located in the temporal lobe, responsible for conscious perception of smell and connections to other brain areas related to emotions (pleasure, arousal, rejection, disgust).

Example: Natural gas has a chemical added that smells like rotten eggs, alerting us to gas leaks.

Unlike taste, there are hundreds of primary odors, each with a specific receptor. Odorous substances must dissolve in the mucus to stimulate receptors and generate nerve impulses. Olfaction has a high adaptation capacity, losing sensitivity to prolonged stimuli.

Sense of Taste (Gustation)

The sense of taste involves the gustatory epithelium, gustatory pathway, and cortical area.

Gustatory Epithelium

This is a non-keratinized stratified epithelium forming papillae (small hills) containing taste buds (sensory receptors). It consists of three cell types:

  1. Basal Cells: Stem cells that divide and give rise to other cells.
  2. Supporting Cells: Provide structural support.
  3. Receptor Cells: Have cilia and transmit taste information via nerves.

Papillae Types:

  1. Circumvallate: Located at the V-shape back of the tongue.
  2. Foliate: Leaf-shaped, on the lateral edges of the tongue.
  3. Fungiform: Mushroom-shaped, on the anterior part of the tongue.

Five Primary Tastes:

  • Sweet
  • Salty
  • Sour
  • Bitter
  • Umami (derived from glutamate)

Chemicals stimulate taste receptors, transducing chemical stimuli into electrical impulses and then nerve impulses. The gustatory pathway involves cranial nerves and the brain’s gustatory cortex.

Cranial Nerves Involved:

  1. Facial Nerve (VII): Senses the anterior part of the tongue.
  2. Glossopharyngeal Nerve (IX): Senses the posterior 1/3 of the tongue.
  3. Vagus Nerve (X): Senses the throat and epiglottis.

The gustatory cortex, located in the parietal lobe, is responsible for taste perception.