Chapter 17: The Special Senses - Comprehensive Notes

Special Senses

• Sensation: Conscious or subconscious awareness of internal or external stimulus.
• External stimulus (in this chapter): Light rays, sound waves, air/food molecules, gravity.
• Receptors for special senses (smell, taste, vision, hearing, equilibrium) are anatomically distinct and concentrated in the head.
• Specific afferent pathways and brain translation sites exist for special sense information.

General Senses vs. Special Senses

• General Senses
  • Include somatic sensations (tactile, thermal, pain, proprioceptive) and visceral sensations.
  • Scattered throughout the body.
  • Relatively simple structures.
• Special Senses
  • Include smell, taste, vision, hearing, and equilibrium.
  • Concentrated in specific locations in the head.
  • Anatomically distinct structures.
  • Form complex neural pathways.

Olfaction and Taste

• Olfaction: Process of perceiving smells. Smell and taste interpret chemicals in the environment.
• Olfactory and gustatory (taste) impulses travel to the cerebral cortex and limbic system.
  • This causes emotional responses and triggers strong memories.
• Gustation and olfaction work together.

Olfaction

• Olfactory epithelium: Located in the superior part of the nasal cavity, covering the cribriform plate and extending along the superior nasal concha.
• Consists of three cell types:
  • Olfactory receptors: Bipolar neurons with cilia (olfactory hairs); 10-100 million receptors respond to odorant molecules.
  • Supporting cells: Provide support and nourishment.
  • Basal cells: Stem cells that replace olfactory receptors.
• The olfactory apparatus detects about 10,000 different odors at concentrations as low as 1/25 billionth of a milligram per milliliter of air.
• Odorant binding initiates intracellular events:
  • G-protein activation → cAMP production → Na+ channel opening → Na+ inflow → generator potentials.
• Nerve impulses travel through olfactory nerves → olfactory bulbs → olfactory tract → primary olfactory area in the temporal lobe.
• Olfaction has direct cortical projections without thalamus relay.
• Olfactory sensory pathways are rapidly adapting (50% decrease in activity in 1 second, complete accommodation in 1-2 minutes).
• Olfactory supporting cells and glands are innervated by the facial (VII) nerve, providing parasympathetic motor innervation to lacrimal glands and nasal mucous membranes.
  • This explains why certain odors can trigger runny noses and tear production.

Gustation

• Gustation (taste) is simpler than olfaction; only five primary tastes are distinguished: sour, sweet, bitter, salty, and umami.
• Umami is stimulated by monosodium glutamate (MSG).
• Other flavors are combinations of the five primary tastes with accompanying olfactory and tactile sensations.
• Nearly 10,000 taste buds are located on the tongue, soft palate, pharynx, and larynx (number decreases with age).
• Each taste bud contains about 50 gustatory receptor cells, surrounded by supporting cells.
• Basal cells multiply and differentiate into supporting cells and then gustatory receptor cells.
• Gustatory hair (microvillus) projects from each receptor cell through the taste pore.
• Each gustatory receptor cell has a lifespan of about 10 days.
• Taste buds are found in three types of papillae:
  • Vallate papillae: 12 large papillae form a row at the back of the tongue (each houses 100–300 taste buds).
  • Fungiform papillae: Mushroom-shaped, scattered over the tongue surface (containing about 5 taste buds each).
  • Foliate papillae: Located in small trenches on the lateral margins of the tongue (most degenerate in early childhood).
• Filiform papillae contain tactile receptors but no taste buds, increasing friction between the tongue and food.
• Three cranial nerves contain axons of first-order gustatory neurons:
  • Facial (VII) nerve: Anterior 2/3 of the tongue.
  • Glossopharyngeal (IX) nerve: Posterior 1/3 of the tongue.
  • Vagus (X) nerve: Throat and epiglottis.
• Nerve impulses propagate to the gustatory nucleus in the medulla oblongata, then to the hypothalamus, limbic system, and thalamus.
• Taste is perceived consciously as signals from the thalamus arrive at the primary gustatory area at the base of the somatosensory cortex in the parietal lobe.
• Taste threshold varies for each primary taste; we are most sensitive to bitter substances (protective function).
• Complete adaptation to a specific taste can occur in 1–5 minutes.

Vision

• Visual perception relies on the eye, accessory structures, optic tracts, and visual cortex.
• Vision is possible due to photoreceptors that