Sensory Systems

• While humans have chemoreceptors, mechanoreceptors, photoreceptors, and thermoreceptors, they do NOT have electroreceptors or magnetoreceptors.
• Stimulus modality: The “labeled line” theory is a particular afferent neuron is associated with one type of receptor, following a specific pathway for integration
• Stimulus location: smaller receptive fields are more sensitive than larger ones.
• Stimulus duration is encoded by 2 kinds of receptors, each responding differently to prolonged stimulation:

  • Tonic receptors: produce continuous action potentials as long as stimulation persists
  • Phasic receptors: respond quickly to stimuli but will stop responding after continuation
• Lateral inhibition improves acuity upon the excitation of a neuron by inhibiting the stimuli responses of neighboring neurons.
• The dynamic range of a receptor is a limited range of intensities that encompasses the initial threshold intensity all the way to full receptor saturation. Outside of this range, increased stimuli will not result in a higher magnitude of response.
• There is a tradeoff between dynamic range and discrimination of a receptor:

  • A receptor with a small dynamic range is highly sensitive, as a small change in stimulus intensity causes a large change in response; this provides fine discrimination between intensities
  • A receptor with a large dynamic range has less sensitivity and cannot discriminate as well
• Range fractionation: individual receptor cells are sensitive to only a small portion of the possible range of intensities, but multiple receptors cover different parts of the range.
• Logarithmic encoding provides fine discrimination and high sensitivity when stimulus intensity is low, but has limited changes in response for higher intensities.
• Signal transduction in olfactory receptor cells 

  1. Binding of odorant to odorant receptor
  2. Conformational change causes the activation of G protein, which moves through the membrane and activates adenylate cyclase
  3. Adenylate cyclase converts ATP into cAMP
  4. cAMP opens cAMP-gated ion channels
  5. Ca2+ and Na+ enter the cell, generating a membrane potential
  6. The Ca2+ also opens gated Cl- channels, causing Cl- to leave the cell, increasing the depolarization
  7. The generator potential opens voltage-gated Na+ channels, triggering action potentials
• Effects of COVID-19 on olfaction and gustation:

  • Disruption (dysosmia) or loss (anosmia) of sense
  • Disruption (dysgeusia) or loss (ageusia) of taste
• Sweet taste: G protein-coupled receptor, activates adenylate cyclase, cAMP > closing of K+ channel, depolarization > opening of Ca2+ channels > NT release
• Bitter taste: G protein-coupled receptor, activates phospholipase C, converts PIP2 to IP3, IP3 > release of Ca2+ from intracellular stores > NT release
• Salty taste: Na+ enters channels > opens Ca2+ channels > NT release
• Sour taste: H+ ions block K+ channel > K+ cannot leave the cell > depolarization > opens Ca2+ channels > NT release
• Epithelial sodium channel (ENaC) is a Na+ channel with a mechanoreceptor attached to it.

  Upon pressure, extracellular “anchor” will move and physically open the channel, causing Na+ to enter and depolarize the membrane.
• Transient receptor potential (TRP) channel is similar to ENaC, but is non-selective (not just Na+).
• Proprioception: 2 "receptors" work in opposition to each other, but both monitor position, muscle force, and effort.

  • Muscle spindle fiber = increases force
  • Golgi tendon organ = causes relaxation of muscle if there is too much force
• Mechanoreceptors used for balance and detection of motion are located in the inner ear, corresponding to different dimensions. These include hair cells located in the utricle (horizontal movement), saccule (vertical movement), and semicircular canal (rotation).
• Hair cells perform mechanoreception via stereocilia, which have TRP channels; when moved in a specific direction (towards the taller stereocilia), ion channels are opened and K+ rushes in, causing a depolarization. Moving in the opposite direction results in hyperpolarization due to closing of the same channels.
• The middle ear transforms sound waves into basilar membrane vibrations. This membrane is the base layer of the organ of Corti, a structure that contains both outer and inner hair cells.
• The hair cells of the organ of Corti have different functions.

  • Inner hair cells recognize sound
  • Outer hair cells amplify sound

  These both move with compression and rarefaction of the basilar membrane.
• When stimulated, ciliary photoreceptors become hyperpolarized, which leads to less NT release. On the other hand, rhabdomeric photoreceptors become depolarized when stimulated.
• Opsins are covalently bound to vitamin A-derived chromophores, forming pigments. This chromophore will leave the opsin when light hits, converting from its cis to its trans form, and will either cause hyperpolarization or depolarization of the cell depending on the photoreceptor type.
• The cornea and lens are naturally convex lenses. Most of the refraction of light occurs at the cornea.
• Many rods synapse onto a single bipolar cell, meaning that not many optic nerves are used. This is why these work best in the dark.
• Cones, which are primarily located at the fovea, connect to a single bipolar cell, with no other receptors synapsing there. This is an example of range fractionation, because each photoreceptor will likely absorb a different spectrum (i.e. different stimulus intensity).
• Pancinian corpsucles are sensory nerve endings that act as receptors for pressure.
• Otoliths are involved in movement of the gelatinous layer, which indirectly causes hair cell bending.