Special Senses

• Transduction is when sensory cells translate chemical, electromagnetic, and mechanical stimuli into action potentials that our nervous system can interpret
• Sensory adaptation is the process of getting used to a sense
• Olfactory sensory adaptation occurs rapidly, with a 50% decrease after the first second
• General senses include temperature, pressure, and pain, while special senses include smell, taste, sight, hearing, and equilibrium
• Receptors for different senses:

  • Vision: Photoreceptors detect light waves
  • Hearing/Balance: Mechanoreceptors detect sound waves and pressure on the skin
  • Taste/Smell: Chemoreceptors detect chemical substances
  • Nociceptors: Pain receptors
  • Thermoreceptors: Detect changes in temperature
• Smell:

  • Anosmia can result in partial or complete loss of the sense of smell
  • Causes of anosmia include head trauma, respiratory infections, and aging
  • Smell decreases with age
  • Molecules must be volatile (gaseous) to be smelled
  • Particles that don't get caught by nose hairs make their way to the olfactory epithelium
  • Once in the epithelium, particles dissolve in mucus and bind to receptors on olfactory sensory neurons
  • Action potential is fired off through the ethmoid bulb to the olfactory area of the brain
• • Olfactory tract to olfactory cortex, with pathways to the frontal lobe for identification and the limbic system for memory association
• Taste:

  • 80% of taste is influenced by smell
  • Taste buds are located between papillae on the tongue
  • Tastants must dissolve in saliva to bind to gustatory receptors
  • Different tastes are triggered by specific substances
  • Taste information is relayed through the 7th, 9th, and 10th cranial nerves
• Hearing/Balance:

  • Initiated by vibrations and sound waves
  • External ear structures include the pinna/auricle, auditory canal, and tympanic membrane
  • Middle ear contains the tympanic membrane, auditory ossicles, and the auditory/eustachian tube
  • Inner ear includes the cochlea for hearing and the vestibular apparatus for equilibrium
  • Hearing pathway: Cochlear/Auditory Nerve → Auditory Pathway → Auditory Cortex
  • Vestibular apparatus consists of three semicircular canals that read head movement via hair cells
• Vision:

  • Eye protection includes the bony orbit, fat cushions, eyelids, eyelashes/eyebrows, conjunctiva, ciliary glands, and lacrimal apparatus
  • Eye structure: Fibrous tunic (sclera and cornea), choroid layer, ciliary body, iris, pupil, sensory layer (retina), photoreceptors (rods and cones), blind spot (optic disc), lens, vitreous humor, aqueous humor
  • Visual pathway: Photoreceptors → Optic nerve to optic chiasm → Optic tracts → Thalamus → Visual Cortex in occipital lobe
• Epithelial receptor cells in taste buds are gustatory (responsible for tasting) and basal (replace gustatory cells)
• Tastants must dissolve in saliva to pass through taste pores and bind to gustatory receptors
• Different taste sensations:

  • Salty: triggered by sodium
  • Sweet: triggered by carbohydrates
  • Sour: triggered by acids
  • Bitter: triggered by many organic compounds
  • Umami: associated with deliciousness
• Taste signals are relayed through the 7th (facial), 9th (glossopharyngeal), and 10th (vagus) cranial nerves once the action potential is activated
• Hearing and Balance:

  • Initiated by vibrations and sound waves
  • External Ear Structure:
  • Pinna/auricle: made of elastic cartilage, collects sound and directs it into the ear
  • Auditory Canal: funnels sound to the tympanic membrane (eardrum)
  • Middle Ear (Tympanic Cavity):
  • Tympanic membrane: vibrates in response to sound waves, separates external and middle ear
  • Auditory ossicles (Malleus, Incus, Stirrup): enhance pressure of sound waves for transmission to the inner ear
  • Auditory/Eustachian Tube: connects middle ear to throat, maintains pressure balance
  • Inner Ear:
• • Cochlea: fluid-filled structure containing the organ of Corti and hair cells
  • Vestibular Apparatus: includes three semicircular canals for equilibrium
• Proliferation can cause holes in the eardrums
• Vision:

  • Eye Protection:
  • Bony orbit (occipital lobe) and fat cushions around the eye
  • Accessory Structures:
  • Eyelid, eyelashes/eyebrows, conjunctiva, ciliary glands, lacrimal apparatus
  • Fibrous Tunic (Outer Layer):
  • Sclera: tough connective tissue protecting the eye
  • Cornea: allows light transmission and refraction
  • Choroid Layer:
  • Ciliary Body and Iris: control lens shape and eye color
  • Pupil: regulates light entry
  • Sensory Layer (Retina):
  • Retina with photoreceptors (rods and cones) for vision
  • Rods: for dim light and peripheral vision
• • Cones: for color vision and sharp images
  • Lens: elastic structure for focusing
  • Internal Eye Chambers:
  • Vitreous Humor: maintains eye shape
  • Aqueous Humor: maintains intraocular pressure and provides nutrients
  • Visual Pathway: from photoreceptors to visual cortex in the occipital lobe