Sensory Processing

Sensory Receptors & Coding - The Body's Detectives

• Receptors: Specialized transducers converting stimuli to electrical signals.
• Types: (📌 Mnemonic: Many Tall Nurses Prefer Coffee)
  • Mechanoreceptors (touch, pressure)
  • Thermoreceptors (temperature)
  • Nociceptors (pain)
  • Photoreceptors (light)
  • Chemoreceptors (taste, smell, $O_2$, $CO_2$)
• Receptor Potential: Graded, local electrical response; if threshold met → AP.
• Sensory Coding:
  • Modality: Labeled Line Principle.
  • Intensity: AP frequency & # active receptors. Weber-Fechner Law: $S = K \log R$.
  • Duration: Receptor adaptation (phasic/tonic).
  • Location: Receptive fields; lateral inhibition (sharpens contrast).

⭐ Muller's Law (Law of Specific Nerve Energies): Sensation depends on pathway activated, not stimulus type.

Somatosensory Pathways - Touch, Pain & Position

• Dorsal Column-Medial Lemniscus (DCML): Fine touch, vibration, proprioception.
  • 1° Neuron: Enters SC, ascends ipsilaterally (Fasciculus Gracilis - lower limb; Cuneatus - upper limb). Synapse: N. Gracilis/Cuneatus (Medulla).
  • 2° Neuron: Decussates in medulla (internal arcuate fibers) → Medial Lemniscus. Synapse: VPL (Thalamus).
  • 3° Neuron: VPL → Primary Somatosensory Cortex.
• Anterolateral System (ALS) / Spinothalamic: Pain, temperature, crude touch.
  • 1° Neuron: Synapse in Dorsal Horn (e.g., Substantia Gelatinosa).
  • 2° Neuron: Decussates in SC (Anterior White Commissure) → Spinothalamic Tract. Synapse: VPL (Thalamus).
  • 3° Neuron: VPL → Primary Somatosensory Cortex.
• Spinocerebellar Tracts: Unconscious proprioception to cerebellum.

⭐ Brown-Séquard Syndrome: Spinal cord hemisection. Ipsilateral loss of fine touch/proprioception, contralateral loss of pain/temperature below lesion level.

Special Senses I - Vision & Hearing Hits

• Vision
  • Photoreceptors: Rods (scotopic, rhodopsin, night vision), Cones (photopic, iodopsin, color, fovea).
  • Pathway: Retina → Optic N. (CN II) → Chiasm (nasal decussation) → Optic Tract → LGN (Thalamus) → Optic Radiation → Visual Cortex (Occipital). 📌 LGN for Light.

*   Pupillary light reflex: Afferent **CN II**, Efferent **CN III**.
*   Accommodation: Ciliary muscle contraction (**CN III**), ↑lens convexity, miosis.
*   Refractive errors: Myopia (concave lens), Hyperopia (convex lens).

• Hearing
  • Organ of Corti: Hair cells (mechanoreceptors).
  • Pathway: Cochlear N. (CN VIII) → Cochlear Nuclei → Superior Olivary Complex (sound localization) → Lateral Lemniscus → Inferior Colliculus → MGN (Thalamus) → Auditory Cortex (Temporal). 📌 MGN for Music.
  • Middle Ear Ossicles: Malleus, Incus, Stapes (amplification).
  • Tests: Rinne/Weber differentiate conductive vs. sensorineural hearing loss.

  ⭐ Meyer's loop (inferior optic radiation) lesion causes contralateral superior homonymous quadrantanopia ("pie in the sky").

Special Senses II - Chemical Senses & Balance Beats

• Olfaction (Smell):
  • Receptors: Olfactory receptor cells (CN I) in olfactory epithelium; GPCRs.
  • Pathway: Olfactory bulb → piriform cortex, amygdala.
  • 📌 Unique: Neurons regenerate.
• Gustation (Taste):
  • Receptors: Taste buds (gustatory cells) on papillae.
  • Tastes: Sweet, Sour, Salty, Bitter, Umami (GPCRs for Sweet, Bitter, Umami).
  • Nerves: CN VII, IX, X → solitary nucleus → VPM thalamus → insula.
• Vestibular System (Balance):
  • Organs: Utricle & Saccule (linear acceleration; otoliths); Semicircular canals (angular acceleration; cupula).
  • Receptors: Hair cells.
  • Key: Vestibulo-ocular reflex (VOR) for gaze stability.

⭐ Olfactory receptor neurons are unique as they are replaced throughout life (neurogenesis).

High‑Yield Points - ⚡ Biggest Takeaways

• Receptor potentials are graded; summation leads to action potentials.
• Phasic receptors (e.g., Pacinian) adapt rapidly; tonic receptors (e.g., Ruffini) adapt slowly.
• DCML pathway: fine touch, vibration, proprioception. ALS pathway: pain, temperature, crude touch.
• Thalamus (VPL/VPM) is the key sensory relay (except olfaction).
• Somatosensory cortex (postcentral gyrus) has somatotopic organization (homunculus).
• Lateral inhibition sharpens sensory discrimination and contrast.
• Labeled line principle: specific pathways for specific modalities.