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 1/251/25 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