Chapter 15 - Special Senses

CHEMICAL SENSES

• Gustation (taste)
  • Their chemoreceptors respond to chemicals in aqueous solution
  • Taste Buds
    • Most of the 10,000 or so taste buds are found on the tongue
    • Taste buds are found in papillae of the tongue mucosa
    • Papillae come in three types: filiform, fungiform, and circumvallate
    • Fungiform and circumvallate papillae contain taste buds
  • Taste Physiology -In order to be tasted, a chemical:
    • Must be dissolved in saliva
    • Must contact gustatory hairs
    • Binding of the food chemical:
    • Depolarizes the taste cell membrane, releasing neurotransmitter
    • Initiates a generator potential that elicits an action potential
  • Taste is 80% smell
  • Thermoreceptors, mechanoreceptors, nociceptors also influence tastes
• Olfaction - Sense of Smell
  • Chemoreceptors respond to chemicals dissolved in fluids of the nasal membranes
  • The organ of smell is the olfactory epithelium, which covers the superior nasal concha
  • Olfactory receptor cells are bipolar neurons with radiating olfactory cilia
  • Olfactory receptors are surrounded and cushioned by supporting cells
  • Olfactory Pathway
    • Olfactory receptor cells synapse with mitral cells
    • Glomerular mitral cells process odor signals
    • Mitral cells send impulses to:
    • The olfactory cortex
    • The hypothalamus, amygdala, and limbic system

Eye and Associated Structures

• 70% of all sensory receptors are in the eye
• Most of the eye is protected by a cushion of fat and the bony orbit
• Accessory structures include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles
• Lubricating glands associated with the eyelids
  • Meibomian glands and sebaceous glands
  • Ciliary glands lie between the hair follicles
• Conjunctiva transparent membrane that:
  • Lines the eyelids as the palpebral conjunctiva
  • Covers the whites of the eyes as the ocular conjunctiva
  • Lubricates and protects the eye
• Lacrimal Apparatus
  • Consists of the lacrimal gland and associated ducts
  • Lacrimal glands secrete tears
  • Tears
    • Contain mucus, antibodies, and lysozyme
    • Enter the eye via superolateral excretory ducts
    • Exit the eye medially via the lacrimal punctum
    • Drain into the nasolacrimal duct
• Extrinsic Eye Muscles
• Vascular Tunic: Iris
  • The colored part of the eye
  • Pupil - central opening of the iris
    • Regulates the amount of light entering the eye during:
      • Close vision and bright light - pupils constrict
      • Distant vision and dim light - pupils dilate
      • Changes in emotional state - pupils dilate when the subject matter is appealing or requires problem-solving skills
• Retina: Photoreceptors
  • Rods:
    • Respond to dim light
    • Are used for peripheral vision
  • Cones:
    • Respond to bright light
    • Have high-acuity color vision
    • Are found in the macula lutea
    • Are concentrated in the fovea centralis
• Cow's eye dissection photo's

The Ear: Hearing and Balance

• The three parts of the ear are the inner, outer, and middle ear
• The outer and middle ear are involved with hearing
• The inner ear functions in both hearing and equilibrium
• Receptors for hearing and balance:
  • Respond to separate stimuli
  • Are activated independently
• The Outer Ear
  • The auricle (pinna) is composed of the helix (rim) & the lobule (earlobe)
  • External auditory canal
    • Short, curved tube filled with ceruminous glands
  • Tympanic membrane (eardrum)
    • Thin connective tissue membrane that vibrates in response to sound
    • Transfers sound energy to the middle ear ossicles
    • Boundary between outer and middle ears
• The Middle Ear
  • A small, air-filled, mucosa-lined cavity
    • Flanked laterally by the eardrum
    • Flanked medially by the oval and round windows
  • Auditory tube - connects the middle ear to the nasopharynx
    • Equalizes pressure in middle ear cavity with the external air pressure
  • Ear Ossicles
    • Tympanic cavity contains three small bones: malleus, incus, and stapes
      • Transmit vibratory motion of the eardrum to the oval window
      • Dampened by the tensor tympani and stapedius muscles
• Inner Ear
  • Bony labyrinth
    • Tortuous channels worming their way through the temporal bone
    • Contains the vestibule, the cochlea, and the semicircular canals
    • Filled with perilymph
  • Membranous labyrinth
    • Series of membranous sacs within the bony labyrinth
    • Filled with a potassium-rich fluid
  • The Vestibule
    • The central egg-shaped cavity of the bony labyrinth
    • Suspended in its perilymph are two sacs: the saccule and utricle
      • House equilibrium receptors called maculae
      • Respond to gravity and changes in the position of the head
  • The Semicircular Canals
    • Three canals that each define 2/3's of a circle and lie in the three planes of space
    • The ampulla is the swollen end of each canal and it houses equilibrium receptors in a region called the crista ampullaris
    • These receptors respond to angular movements of the head
  • The Cochlea
    • The cochlea is divided into three chambers:
      • Scala vestibuli
      • Scala media (contains the Organ of Corti)
      • Scala tympani
• Sound and Mechanisms of Hearing
  • Sound vibrations beat against the eardrum
  • The eardrum pushes against the ossicles, which presses fluid in the inner ear against the oval and round windows
    • This movement sets up shearing forces that pull on hair cells
    • Moving hair cells stimulates the cochlear nerve that sends impulses to the brain

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