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.