Ch. 2 Senses & Perception

Sense Organs, Energy, and Transduction

  • Sense organs = “windows” through which the brain accesses the external world.
    • World itself contains only physical energies (light waves, air-pressure waves, chemicals) – no inherent colors, sounds, tastes, or smells.
  • Transduction
    • Universal first step for every modality.
    • Definition: conversion of external stimulus energy or molecular signals into \text{electrical signals} inside receptor cells.
    • Electrical messages travel via nerve fibers to specialized cortical areas where they are integrated into conscious perception.

Vision

  • General facts
    • Most complex human sense; ~30\% of the cerebral cortex is visual.
    • Model organisms: phototransduction → fruit flies & mice; higher-level processing → monkeys & cats.
  • Camera analogy
    • Cornea + lens ≈ camera optics; retina ≈ photographic film.
    • Image on retina is two-dimensional and reversed: right objects → left retina, upper objects → lower retina, and vice versa.

Ocular Optics

  • Path of light
    • Cornea: rigid, performs initial focusing.
    • Pupil: opening whose diameter is regulated by the iris to control light influx.
    • Lens: flexible, changes curvature to fine-tune focus for near/far objects (accommodation).
    • Retina: light-sensitive sheet lining inner surface of eyeball.

Three-Layered Retina

  • Neuron classes
    • Photoreceptors (rods & cones) – outermost layer, surprisingly farthest from light entry.
    • Interneurons (bipolar, horizontal, amacrine cells) – middle processing layer.
    • Ganglion cells – innermost layer; their axons bundle to form optic nerve.
  • Photoreceptor statistics & functions
    • Total ≈ 1.25 \times 10^8 per eye.
    • Rods (~95\%): extremely light-sensitive, enable scotopic (dim-light) vision.
    • Cones (~5\%): high acuity & color vision; three cone types tuned to long (red), medium (green), short (blue) wavelengths.
  • Regional specialization
    • Fovea (center of retina): highest cone density (only red & green), maximal spatial resolution.
    • Macula: zone around fovea critical for tasks like reading & driving; photoreceptor death here → macular degeneration (leading cause of blindness >55\,\text{yrs} in developed nations).
  • Convergence & receptive fields
    • Central retina: 1 ganglion cell may receive input from only one cone → fine detail.
    • Peripheral retina: each ganglion cell pools from many photoreceptors → coarse vision.
    • Receptive field = portion of visual space driving a single ganglion cell.

From Eye to Brain

  • Optic nerve exits retina, creating a photoreceptor-free blind spot; brain fills gap using other eye.
  • Optic chiasm: partial crossover.
    • Left visual field info → right brain; right field → left brain, regardless of eye of origin.
  • Thalamic relay: lateral geniculate nucleus (LGN).
  • Primary visual cortex (V1) in occipital lobe
    • Layered structure; middle layer copies LGN center-surround map.
    • Superficial/deep layers code oriented edges, bars, motion direction.
  • Parallel processing streams
    • Dorsal “Where/How” stream → parietal lobe; spatial relations, motion, unconscious visually guided action.
    • Ventral “What” stream → temporal lobe; object identity, color, conscious recognition.
    • Modern view: significant crosstalk; division not absolute.
  • Binocular vision & depth
    • Overlapping visual fields + proper eye alignment → stereopsis.
    • Strabismus (crossed eyes) disrupts fusion; if untreated >!8\,\text{yrs} old, can lead to permanent blindness in one eye.

Treatments & Research

  • Early surgical/patch therapy for strabismus now done <!4\,\text{yrs}.
  • Degenerative blindness (e.g., macular degeneration): gene & stem-cell therapy aim to rescue/replace photoreceptors.
  • Retinal prosthetics: directly stimulate ganglion cells (analogous to cochlear implant).

Hearing (Audition)

  • Functions: early warning (e.g., oncoming car), social communication (speech parsing), music perception.
  • Signal qualities extracted: pitch (frequency), loudness (amplitude), duration, spatial location.

Ear Anatomy & Sound Transmission

  • Outer ear: pinna funnels air-pressure waves through auditory canal → tympanic membrane (eardrum).
  • Middle ear ossicles: malleus (hammer) → incus (anvil) → stapes (stirrup).
    • Stapes acts like piston on oval window (boundary to cochlea).
  • Inner ear: cochlea (fluid-filled, snail-shaped).
    • Oval window converts mechanical to fluid pressure waves.
    • Basilar membrane winds through cochlea; “place code” tuned along length:
    • Base (near oval window) ≈ high frequencies.
    • Apex (center) ≈ low frequencies.
    • Hair cells (sensory receptors) sit on basilar membrane; stereocilia bend against tectorial membrane → ion channels open → receptor potentials → excite auditory nerve fibers.

Auditory Pathway & Processing

  • Auditory nerve → brainstem nuclei → thalamus (medial geniculate) → primary auditory cortex (superior temporal lobe).
  • Tonotopic map (frequency map) preserved from basilar membrane to cortex.
  • Specialized cortical neurons process intensity, duration, frequency sweeps, complex sounds.
    • Higher-order regions integrate harmony, rhythm, melody; recognize voices/instruments.
  • Hemispheric specialization: left auditory cortex (incl. Wernicke’s area) critical for speech comprehension; damage → word deafness despite intact hearing.

Hearing Loss & Therapies

  • Majority due to irreversible hair-cell death (do not naturally regenerate in mammals).
  • Research directions: developmental biology of hair cells, gene/stem-cell induced hair-cell regeneration, neurogenesis.

Taste (Gustation) & Smell (Olfaction)

  • Both detect chemicals; jointly create flavor perception.
  • Receptor cells directly contact environment → high turnover; olfactory neurons continually replaced throughout life (one of few CNS sites with adult neurogenesis).

Taste

  • Taste buds (5,000–10,000), each with 50\text{–}100 gustatory cells tuned to one basic taste:
    • Sweet, sour, salty, bitter, umami (savory).
    • Myth debunked: all tastes detected across tongue, not regionalized.
  • Neural pathway: gustatory cells → cranial nerves VII (facial), IX (glossopharyngeal), X (vagus) → brainstem → thalamus → gustatory cortex (frontal lobe & insula).

Smell

  • Odorant molecules bind receptors on olfactory sensory neurons in nasal epithelium.
  • ~1,000 receptor types → combinatorial coding allows \sim 20,000 distinguishable odors.
  • Axons pass through cribriform plate → olfactory bulbs → primary olfactory cortex (anteromedial temporal lobe) WITHOUT thalamic relay (unique among senses).
  • Olfactory bulbs exhibit lifelong neurogenesis; structure & neuron organization can change with experience.

Flavor Integration

  • Convergence of taste & smell information in orbitofrontal/inferior frontal cortex produces complex flavor.
    • Example: sugar tastes sweeter when accompanied by congruent strawberry odor.
  • Nasal congestion illustrates dependence: taste seems bland when olfaction is blocked.

Aging & Prospective Therapies

  • Receptor loss with age → diminished taste & smell.
  • Stem-cell research aims to regenerate gustatory/olfactory neurons to restore chemosensation.

Touch, Itch, and Pain (Somatosensation)

  • Skin = primary organ; modalities include light touch, pressure, vibration, temperature, texture, itch, pain.
  • Receptors located at different skin depths; in hairy skin, nerve endings wrap hair follicles.
  • Sensory fibers → spinal cord → thalamus → somatosensory cortex (post-central gyrus).
    • Fast A-beta (thick, myelinated) vs. slow C (thin, unmyelinated) fibers carry different touch qualities.

Cortical Somatosensory Map

  • Body surface projected somatotopically (“homunculus”).
    • High receptor density (lips, fingertips) = larger cortical representation → higher acuity.
  • Two-point discrimination test quantifies tactile resolution.

Pain & Itch Mechanisms

  • Dual nature: sensory (tissue status) + emotional (unpleasantness).
  • Nociceptors: high-threshold receptors for thermal, mechanical, chemical damage; also respond to capsaicin (hot peppers) & other spicy compounds.
  • Itch-specific receptors exist (e.g., histamine, recently discovered non-histaminergic types).
  • Injury → inflammatory soup (prostaglandins, etc.) → receptor sensitization, hyperalgesia, allodynia.
    • Chronic neuropathic pain (e.g., diabetic neuropathy) arises from nervous system malfunction rather than ongoing damage.

Pain Pathways & Modulation

  • Peripheral A-delta (fast, sharp pain) & C fibers (slow, diffuse pain) → spinal cord → brainstem → thalamus → cortical areas that create conscious pain/itch.
  • Descending modulation: cortex → periaqueductal gray (PAG) → brainstem nuclei → spinal cord → inhibit ascending signals.
    • Endorphins (endogenous opioids) & adrenaline mediate analgesia.
  • Individual variability
    • Efficacy of descending pathways & emotional state alter pain perception.
    • Chronic pain often linked to dysfunctional modulation circuits.

Pain Management Strategies

  • Pharmacological: opioid drugs delivered directly to spinal cord peri-operatively; efforts to minimize long-term opioid risks.
  • Electrical: spinal cord stimulation under study.
  • Non-pharmacological: meditation, hypnosis, massage, cognitive behavioral therapy, targeted cannabis use – mainly act on limbic/emotional pain components.
    • Imaging shows cannabis suppresses activity mainly within limbic pain circuits.

Ethical, Clinical, and Real-World Relevance

  • Early intervention in strabismus exemplifies critical periods; delays have lifelong consequences.
  • Gene & stem-cell therapies raise ethical considerations (e.g., genetic editing, resource allocation).
  • Cochlear & retinal implants illustrate bioengineering solutions bridging damaged receptors and CNS.
  • Chronic pain treatment underscores opioid crisis; drives research into alternative modulation methods.
  • Aging population will face increased sensory deficits; advances in regeneration and prosthetics have societal importance.