Chapter 15 - The Special Senses

Describe the sound conduction pathway to the fluids of the internal ear

Sound Detection → Hearing is the reception of an air sound wave that is converted to a fluid wave that ultimately stimulus mechanosensitive cochlear hair cells that end impulses to the brain for interpretation

Anatomy of the Cochlea

Auditory Pathway

1. Tympanic membrane

   • Sound waves enter external acoustic meatus and strike tympanic membrane, causing it to vibrate

     • The higher the intensity, the more vibration

2. Auditory ossicles

   • Transfer vibration of eardrum to oval window

Describe the structure of the Retina

• Retina → Originates as an out-pocketing of brain

  • Contains:

    1. Millions of photoreceptors cells that transduce light energy

    2. Neurons

    3. Glial cells

  • Delicate two layered membrane

    1. Outer pigmented layer

    2. Inner neural layer

Two Membrane Layers of the Retina

Outer pigmented layer

• STRUCTURE

  • Single-cell-thick lining next to choroid

  • Extends anteriorly, covering ciliary body and iris

• FUNCTION

  • Absorbs light and prevents its scattering

  • Phagocytizes photoreceptor cell fragments

  • Stores vitamin A

Inner neural layer

• STRUCTURE

  • Transparent layer

  • Composed of three main types of neurons

    1. Photoreceptors

    2. Bipolar neurons

    3. Ganglion cells

• FUNCTION

  • Signals spread from photoreceptors to bipolar cells to ganglion cells

  • Ganglion cell axons exit eye as optic nerve

Optic Disc

• Site where optic nerve leaves eye

• Lacks photoreceptors, so referred to as blind spot

Type Types of Photoreceptors

Modified neurons

1. Rods

   • Dim light, peripheral vision receptors

   • More numerous and more sensitive to light than cones

   • No color vision or sharp images

   • Numbers greatest at periphery

2. Cones

   • Vision receptors for bright light

   • React more quickly than rods

   • High-resolution color vision

Light

• Packets of energy (photons or quanta) that travel in wavelike fashion at high speeds

• Color that eye perceives is a reflection of that wavelength

  • EX: Grass is green because it absorbs all colors EXCEPT green

  • EX: White reflects all colors, and black absorbs all colors

Refraction and Lenses

• Refraction → Bending of light rays

  • Due to change in speed of light when it passes from one transparent medium to another and path of light is at an oblique angle

    • EX: from liquid to air

• Lenses → Can also refract light because they are curved on both sides

  1. Convex: Thicker in center than at edges

     • Bend light passing through it → so that rays converge at focal point

     • Image formed at focal point is upside-down and reversed from left to right

  2. Concave: Ticker at edges than in center

     • Disperse light, preventing light from being focused

Describe the Pathway of Light though the Eye

• Pathway of light entering eye

  1. Cornea

  2. Aqueous humor

  3. Lens

  4. Vitreous humor

  5. Entire neural layer of retina

  6. Photoreceptors

• Light is refracted three times along path:

  1. Entering cornea

  2. Entering lens

  3. leaving lens

• Lens is able to adjust its curvature to allow for fine focusing

  • Can focus for distant vision and for close vision

Explain Color blindness and Astigmatism

• Color blindness → Lacks one or more cone pigments

  • Inherited as a X-linked condition, so more common in males

  • The most common type is red-green, in which either red cones or green cones are absent

    • Depending on which cone is missing, red can appear green, or vice versa

    • Relay on different shades to get cues of color

Taste

• Sensory organ for taste

• Most of 10,000 taste bunds are located in papillae, peg-like projection of tongue mucosa

Describe Structure and Function of 3 Types of Taste Buds

1. Fungiform papillae

   • Tops of these mushroom-shaped structures house most taste buds

   • Scatted across tongue

2. Foliate papillae

   • On the side walls of tongue

3. Vallate papillae

   • Largest taste bund with 8-12 forming “V” at back of tongue

Two types of Epithelial cells on each Taste Bud

1. Gustatory epithelial cells

   • Taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva

   • Sensory dendrites coiled around gustatory epithelial cells send taste signals to brain

   • Three types of gustatory epithelial cells

     • One releases serotonin

     • Others lack synaptic vesicles

     • But one releases ATP as neurotransmitter

2. Basal epithelial cells

   • Dynamic stem cells that divide every 7-10 days

List the five basic taste sensation

1. Sweet

   • Sugars

   • Saccharin

   • Alcohol

   • Some amino acids

   • Some lead salts

2. Sour

   • Hydrogen ions in solution

3. Salty

   • Metal ions (inorganic salts)

   • Sodium chloride tastes saltiest

4. Bitter

   • Alkaloids such as quinine and nicotine

   • Caffeine

   • Non-alkaloids; aspirin

5. Umani

   • Amino acids glutamate and aspartate

     • EX: beef or cheese taste

   • Monosodium glutamate

Physiology of Taste

1. Be dissolved in saliva

2. Diffuse into taste pore

3. Contact gustatory hairs

Influence of Other Sensations on Taste

• Taste is 80% smell

  • If nose is blocked, foods taste bland

• Mouth also contains thermoreceptors, mechanoreceptors, and nociceptors

  • Temperature and texture enhance or detract from taste

  • Spicy hot foods can excite pain receptors in mouth, which some people experience as pleasure

    • EX: hot chili peppers