Week 6 - Sensory Systems: Vision, Taste, Smell

Vision

  • Eye anatomy and chambers
    • Three chambers: Anterior chamber, Posterior chamber, Vitreous chamber
    • Lens changes shape via actions of the ciliary muscle and zonule fibres
    • Key structures: lens, pupil, ciliary muscle, retina, cornea, fovea, optic nerve
    • Retina contains six layers: Ganglion cell layer; Inner Plexiform Layer (IPL); Inner Nuclear Layer (INL); Outer Plexiform Layer (OPL); Outer Nuclear Layer (ONL); Photoreceptor outer segments
  • Photoreceptors
    • Rod photoreceptors: ~9.1×1079.1\times 10^7
    • Cone photoreceptors: ~4.5×1064.5\times 10^6
    • Rods: light intensity detection; Cones: colour and high acuity
    • Fovea: high acuity vision, cone-dominated
    • Distribution: low cone density throughout retina except at fovea; high rod density throughout retina except at fovea
  • Lens shape and accommodation
    • Distant objects: least refractive index; focused on retina
    • Near objects: increased curvature of lens; greatest refractive index; changes in ciliary muscle and zonule fibres
  • Refractive errors
    • Myopia: eyeball too long; image focused in front of retina; distant images blurred
    • Hyperopia: eyeball too short; refracting system too weak; near images blurred
  • Phototransduction (retina in darkness vs light)
    • In dark: high cGMP\text{cGMP}; cGMP opens Na+ channels; Na+ influx; photoreceptor depolarizes; neurotransmitter released
    • In light: rhodopsin activation leads to ↓cGMP\text{cGMP}; Na+ channels close; hyperpolarization; reduced neurotransmitter release
  • Visual pathway (light activation to central visual pathway)
    • Retina → Optic nerve → Optic chiasm → Optic tract → Lateral geniculate nucleus (LGN) → Optic radiations → Striate (visual) cortex
    • Additional targets: Pretectum (pupil and lens reflex), Superior colliculus (head/eye movements), Hypothalamus (circadian rhythms)
  • Central retina and visual features
    • Optic disk: site where ganglion cell axons exit; no photoreceptors (blind spot)
    • Cone vs Rod functions: cones for colour and high acuity; rods for motion and low light
    • Colour vision: cones contain photopigments for Short, Medium, and Long wavelengths; humans have trichromacy; dichromacy examples include protanopia and deuteranopia
  • Central projections and cortex
    • Visual information reaches cortex via the retino-geniculo-cortical pathway; some information also projects to hypothalamus and other subcortical structures
    • Pathways contribute to circadian regulation, reflexes, and orientation

Taste

  • Papillae and taste buds
    • Types of papillae: Circumvallate, Foliate, Fungiform
    • Each taste bud contains ~50–100 taste receptor cells
  • Taste pathway
    • Anterior two-thirds of tongue: CN VII (facial) via chorda tympani
    • Posterior one-third: CN IX (glossopharyngeal)
    • Epiglottis: CN X (vagus)
    • Nucleus of solitary tract (gustatory nucleus) as first relay
    • Thalamus: gustatory relay from solitary nucleus (differs by cranial nerve input: anterior 2/3 via CN VII; posterior 1/3 via CN IX; epiglottis via CN X)
    • Gustatory cortex: project to orbitofrontal cortex; integrates with olfactory information
  • Taste qualities and significance
    • Five basic tastes: Sour, Sweet, Bitter, Salty, Umami (MSG)
    • Metabolic and dietary significance; taste aversion as a behavioral response

Smell

  • Olfactory receptor neurons and epithelium
    • Olfactory receptor neurons (ORNs) are chemoreceptors; bipolar cells with cilia bearing odorant receptors
    • Olfactory epithelium located in the nasal cavity; Bowman's glands secrete mucus
    • Olfactory receptor gene family: humans ≈ 5!!!0−7.5×1025!!!0-7.5\times 10^2 genes
  • Olfactory pathway
    • ORN axons project through the cribriform plate to the olfactory bulb
    • Olfactory bulb processes odors via glomeruli (population coding)
    • Direct projection from OB to piriform cortex; the olfactory system is unique in not requiring a thalamic relay before cortex
  • Central olfactory projections and behavior
    • Olfactory tract targets include piriform cortex, orbitofrontal cortex, amygdala, entorhinal cortex, hippocampus, thalamus, and hypothalamus
    • Odorant perception involves broader networks influencing emotion, memory, and reward
    • Odorant perception and receptor numbers vary across species: humans ≈ 12×10612\times 10^6 ORNs; dogs ≈ 4×1094\times 10^9 ORNs; rats ≈ 1.5×1071.5\times 10^7 ORNs
  • Olfactory dysfunction
    • Anosmia: loss of sense of smell; increases risk from fire, toxins, gas leaks, spoiled food
    • Olfactory decline can precede motor and cognitive symptoms in neurodegenerative diseases (e.g., Parkinson's, Alzheimer's)