The Special Senses: Olfaction

The Special Senses

• Five special senses include olfaction (smell), gustation (taste), vision, equilibrium, and hearing

• Olfaction (Smell)

  • Olfactory organs provide the sense of smell
  • Located in the nasal cavity, on either side of the nasal septum
  • Made up of two layers: olfactory epithelium and lamina propria
  • Olfactory epithelium contains olfactory receptors, supporting cells, and basal (stem) cells
  • Lamina propria includes areolar tissue, blood vessels, and nerves
  • Olfactory glands secrete substances that coat the surfaces of olfactory organs
  • Olfactory receptors are specialized neurons that detect dissolved chemicals and interact with odorant-binding proteins
  • Olfactory pathways follow axons leaving the olfactory epithelium, collect into bundles, and penetrate the cribriform plate of the ethmoid. They then reach the olfactory bulbs in the cerebrum, where the first synapse occurs. Axons then leave the olfactory bulb, travel along the olfactory tract, and reach the olfactory cortex, hypothalamus, and portions of the limbic system. Arriving information reaches various centers without always first synapsing in the thalamus
  • We can distinguish thousands of chemical stimuli
  • The CNS interprets smells by the pattern of receptor activity
  • There is a considerable turnover of olfactory receptor population
  • Number of olfactory receptors declines with age

• Gustation (Taste)

  • Gustation provides information about foods and liquids
  • Taste receptors (gustatory receptors) are distributed on the tongue and portions of the pharynx and larynx
  • Clustered into taste buds
  • Taste buds contain basal (stem) cells and gustatory cells
  • Gustatory cells extend taste hairs through taste pores and survive about 10 days before replacement.
  • Dissolved chemicals contact taste hairs, bind to receptor proteins, activate ion channels, stimulation produces depolarization, and release neurotransmitters
  • Neurotransmitters generate action potentials
  • Taste sensitivity exhibits significant individual differences
  • Some conditions are inherited
  • Ex: phenylthiocarbamide (PTC) – 70% of people taste it, but 30% do not
  • Number of taste buds begins declining rapidly by age 50

• Vision (The Eye)

  • Accessory structures of the eye provide protection, lubrication, and support
  • Including: palpebrae (eyelids), superficial epithelium of the eye, and the lacrimal apparatus
  • Structures of the Eye (1 of 3): palpebrae (eyelids), are continuations of skin
  • Blinking keeps surface of eye lubricated, free of dust and debris
  • Lacrimal caruncle is a soft tissue that contains glands producing thick secretions
  • Generates "sand-like" deposits after sleeping
  • Structures of the Eye (2 of 3): conjunctiva is an epithelium covering the outer surface and inner surface of eyelids
  • Structures of the Eye (3 of 3): lacrimal apparatus produces, distributes and drains tears, lacrimal gland is the "tear gland
  • Secretions contain lysozyme – an antibacterial enzyme
  • Three layers of the eye
  • Outer fibrous tunic including sclera and cornea
  • Middle vascular tunic (also known as the uvea) includes the choroid
  • Inner neural tunic including retina

• Glaucoma

  • Over time optic nerve becomes damaged, usually due to high intraocular pressure

• Age-Related Macular Degeneration

  • Deterioration of the macula (location of fovea)

• The Eye

  • Small anterior segment: aqueous humor
  • Fluid circulates within anterior eye chamber into Canal of Schlemm, re-enters circulation
  • Intraocular pressure
  • Fluid pressure in aqueous humor helps maintain eye shape
  • The lens is the main structure involved in light refraction
  • Bending of light by cornea and lens
  • Focal point - Specific point of intersection on retina
  • Focal distance - Distance between center of lens and focal point
  • Accommodation
  • Shape of lens changes to focus image on retina

• Visual Abnormalities: myopia, hyperopia, astigmatism

• Visual Physiology: Photoreceptors

  • Rods respond to almost any photon (they don't discriminate light colors), regardless of energy content
  • Cones- Color vision is the integration of information from red, green, and blue cones. Color blindness is the inability to detect certain colors
  • Central Processing of Visual Information
  • Axons from ganglion cells converge on optic disc
  • Ganglion cells (M cells and P cells) monitor rods and cones
  • Axons penetrate the wall of the eye, proceed as optic nerves, and reach the diencephalon at the optic chiasm
  • Visual data from combined field of vision arrives at the visual cortex of the occipital lobes