Week 7 ELM 15: Sensory Pathways II - Hearing, Chemical Senses, Touch

Smell (Olfaction)

• Periphery Similarity: The peripheral olfactory system is similar across species (Firestein, 2001).
• Convergence: Olfactory receptor neurons converge on glomeruli in the olfactory bulb.
• Mechanism:
  • Odorants bind to olfactory receptors (GPCRs) located on olfactory cilia.
  • This binding activates adenylate cyclase, leading to the opening of cyclic nucleotide-gated channels.
  • The opening of these channels results in depolarization of olfactory receptor neurons.
• Combinatorial Coding:
  • Odorants have varied chemical structures.
  • Individual olfactory neurons:
    • Each odorant binds to multiple receptors and activates many neurons.
    • Each neuron is activated by multiple odorants, forming a combinatorial code.
• Comparison with Vision:
  • Olfaction:
    • Stimulus is received by olfactory receptor neurons (ORNs) in the olfactory epithelium.
    • Information processed through glomeruli in the olfactory bulb.
    • Involves apical and secondary dendrites, periglomerular cells, mitral cells, and granule cells.
    • Signal sent to the olfactory cortex.
  • Vision:
    • Stimulus received by photoreceptor cells in the retina.
    • Information processed through bipolar cells and horizontal cells.
    • Involves retinal ganglion cells and amacrine cells.
    • Signal sent to central targets.

Taste (Gustation)

• Basic Tastes: Five basic tastes related to survival:
  • Bitter: Avoid poisons.
  • Sweet: Indicates sugar and carbohydrates.
  • Umami: Indicates l-amino acids (monosodium glutamate).
  • Salty: Indicates Na+.
  • Sour: Indicates acids/H+.
  • Fat: Potential sixth taste (Chaudhari N, and Roper S D J Cell Biol 2010;190:285-296).
• Receptor Location:
  • Taste receptors are located in taste buds, which are found on papillae.
  • Three types of papillae: circumvallate, foliate, and fungiform.
• Receptor Types:
  • Taste receptors are molecularly diverse.
  • Epithelial Na channel (ENaC): Not present in humans.
  • Receptor for fat: Not yet isolated.
  • H+ selective ion channel: OTOP1 (for sour taste).

Hearing (Audition)

• Air Pressure Waves: Sound is detected as variations in air pressure.
  • Insects use a different mechanism by detecting the speed of moving particles.
• Frequency Range: Normal human hearing ranges from 20Hz to 20,000Hz.
  • Lower frequency waves correspond to lower pitch; lower intensity corresponds to quieter sounds.
• System Components:
  • Vestibular System:
    • Semicircular canals (posterior, horizontal, anterior).
    • Otolith organs (utricle, saccule).
  • Auditory System:
    • External ear.
    • Middle ear (bones: malleus, incus, stapes).
    • Inner ear (cochlea).
• Cochlea: Hair cells transduce sound into electrical signals.
  • Outer hair cells: Provide active amplification.
  • Inner hair cells: Send signals to the brain.
• Hair Cell Mechanism: K^+ goes into hair cells, causing depolarization.
• Tonotopy:
  • Tonotopic representation of sound in the cochlea due to properties of the basilar membrane.
  • Different frequencies activate different regions of the basilar membrane (e.g., 0.5 kHz at the apex, 8 kHz at the base).
• Sound Localization:
  • Barn owls detect the position of a sound source by comparing the time of sound arrival to both ears (interaural time difference).
• Vestibular System Details:
  • Otolith organs: Use calcium carbonate (CaCO_3) crystals to detect the force of gravity and acceleration.
  • Semicircular canals: Detect head rotation.

Touch (Mechanosensation)

• Types of Touch Receptors:
  • Steady pressure (skin indentation, texture discrimination):
    • Merkel cells
    • Ruffini endings
  • Vibration:
    • Meissner’s corpuscles
    • Pacinian corpuscles
  • High-threshold, nociceptors: Detect pain.
• Piezo in Merkel Cells:
  • Piezo2 channels in Merkel cells are involved in mechanotransduction.

Comparison of Sensory Systems