Special Senses

• Unlike the general senses found throughout the body, the special senses are housed in complex sensory organs and located in very specific places
• Sense of smell


  Olfactory receptors detect chemicals, which bind to the olfactory nerve (cranial nerve 1) and send signals to the olfactory areas of the brain
• Olfactory adaptation


  • Decreasing sensitivity to a smell over time, but can reset when leaving the environment
  • Olfactory threshold is very low, only requiring a few molecules to detect a smell
• Olfactory pathway


  Olfactory receptors -> olfactory nerve -> olfactory bulb -> olfactory tract -> primary olfactory area of temporal lobe
• Sense of taste (gustation)

  Taste buds on the tongue contain chemoreceptors that detect chemicals in food and send signals via the facial and glossopharyngeal nerves to the gustatory cortex
• Basic taste sensations


  • Sweet
  • Salty
  • Sour
  • Bitter
  • Umami (savory)
• Taste bud structure


  • Contains receptor cells, supporting cells, and basal cells
  • Have a single gustatory hair protruding through the taste pore
• Taste adaptation


  Decreasing sensitivity to a taste over time, usually 1-5 minutes
• Bitter and poison tastes have the lowest threshold, while salty and sweet have the highest thresholds
• Taste mechanism


  Dissolved substances contact gustatory hairs -> receptor potential -> neurotransmitter release -> nerve impulse
• Sensitivity of primary tastes

  • Most sensitive: bitter/poisons (to avoid ingesting harmful substances)
  • Least sensitive: salty, sweet (found in high amounts in processed foods)
• Mechanism of taste


  1. Dissolved substances contact gustatory receptors

  2. Receptor potential results in neurotransmitter release

  3. Nerve impulse formed in first-order neuron
• Gustatory nerves


  • Cranial nerve 7 (facial) - anterior 2/3 tongue
  • Cranial nerve 9 (glossopharyngeal) - posterior 1/3 tongue
  • Cranial nerve 10 (vagus) - palate, epiglottis
• Taste signals


  Travel to thalamus, limbic system, hypothalamus
• Limbic system


  Involved in emotional connections and memories related to taste and smell
• Taste perception


  1. Taste fibers extend from thalamus to primary gustatory area of parietal lobe

  2. Provides conscious perception of taste
• Taste vs Flavor


  Taste is the 5 primary tastes, flavor involves both taste and olfaction (smell)
• Plugging nose reduces food flavor perception
• Visual accessory organs


  • Eyebrows
  • Eyelids
  • Eyelashes
  • Lacrimal apparatus
• Eyebrows


  Help keep sweat from rolling into eyes
• Eyelids


  Wipe dust/grit off eyes, close to protect eyes, stimulate blink reflex
• Lacrimal apparatus


  Produces tears to nourish and clean the eyes
• Lacrimal gland


  Located in upper outer quadrant of orbit, produces tears
• Extrinsic eye muscles


  Control movement of the eyeball
• Intrinsic eye muscles


  Control pupil size and lens shape
• Pupil control


  Circular muscles constrict pupil, radial muscles dilate pupil
• Visual pathway


  1. Light stimulates photoreceptors

  2. Nerve impulses travel optic nerve

  3. Optic chiasm splits visual field

  4. Signals processed in occipital lobe
• Right visual field processed in left brain, left visual field in right brain
• Nasal side


  Side of the eyeball closest to the nose
• Temporal side


  Side of the eyeball closest to the temple
• Light entering the eye

  1. Hits the nasal side of the left eye

  2. Hits the temporal side of the right eye
• Optic chiasm


  Point where the optic nerves from both eyes converge
• Information transmission through the optic chiasm

  1. Light from right visual field crosses over to left side of brain

  2. Light from left visual field goes to right side of brain
• Photoreceptors


  Receptors in the retina that respond to light
• Photoreceptor in the dark

  1. Sodium channel allows sodium ions to enter

  2. Cell becomes more positively charged (depolarized)

  3. Cell releases neurotransmitter
• Photoreceptor in the light

  1. Light closes sodium channel

  2. Cell becomes more negatively charged (hyperpolarized)

  3. Cell does not release neurotransmitter
• Rods and cones


  Two types of photoreceptors
• Parts of the ear

  • Outer ear
  • Middle ear
  • Inner ear
• Ossicles


  Three small bones in the middle ear (hammer, anvil, stirrup)
• Organ of Corti


  Main hearing receptor in the inner ear