General & Special Senses – Skin, Receptors, Ear, and Eye

Integumentary System & General Senses

• Skin (cutaneous membrane) = largest organ system
  • 2 true layers
  • Epidermis → keratinized, stratified, squamous epithelium; avascular
  • Dermis → vascular, houses sensory nerves & blood supply for epidermis
  • Hypodermis (subcutaneous layer)
  • NOT technically part of skin
  • Highly vascular → preferred site for sub-Q injections (e.g., Ozempic)

General vs. Special Senses

• Generalized senses → mostly via skin: pressure, vibration, touch, pain, temperature, proprioception
• Specialized senses → vision, olfaction, gustation, hearing, equilibrium
  • Require dedicated organs: eye, nasal cavity, tongue, ear

Sensory Receptors – Foundational Concepts

• Begin every reflex arc; job = transduction
  • Converts environmental stimulus → neuronal signal (action potential)
  • Nervous system’s “language” = ionic movement: Na^+ influx (depolarization) / K^+ efflux (repolarization)
• No such thing as a “motor receptor” – receptors are strictly sensory

Classification by Adaptation Speed

• Fast-adapting (phasic)
  • e.g., corpuscles that sense clothing pressure; stimulus quickly becomes unnoticed
• Slow-adapting (tonic)
  • e.g., chronic pain receptors; continue firing long after initial injury

Classification by Location

• Exteroceptors – near body surface (skin, mucosa)
• Interoceptors (visceroceptors) – in viscera & blood vessels
• Proprioceptors – in muscles, tendons, joints → body-position sense

Classification by Stimulus Type (Know Cold!)

Structural Categories

• Free nerve endings – unencapsulated; more sensitive, widespread (e.g., pain, temp)
• Encapsulated nerve endings – terminal fiber wrapped in connective tissue capsule → corpuscles
  • Meissner (tactile) corpuscles – light touch, low-freq vibration
  • Pacinian (lamellated) corpuscles – deep pressure, high-freq vibration
  • Ruffini endings – deep touch, stretch

Nociception & Pain

• 3 main nociceptor types (thermal, mechanical, polymodal) → complex pain theories (gate theory, etc.)
• Pain assessment purely subjective: intensity (0–10), quality (sharp, dull), radiation, aggravating/alleviating factors
• Referred pain – perception site ≠ origin due to neural convergence (e.g., gallbladder → R shoulder)

Discriminative Touch – Two-Point Test

• Density of receptors dictates resolution
• Areas with clustered free nerve endings (fingertips, lips) discern two close points; areas with sparse endings cannot

Somatotopy

• Sensory homunculus (post-central gyrus) → cortical map proportional to receptor density
• Motor homunculus (pre-central gyrus) → proportional to fine motor control demand

Proprioception

• Only two true proprioceptors (both mechanoreceptors, always in muscle)
  • Muscle spindle – in muscle belly, senses length/stretch
  • Golgi tendon organ – in tendon, senses tension/load
• Enable smooth gait & coordinated movement

Gustation (Taste)

• Tongue papillae; clinically most relevant = circumvallate (form upside-down “V” at junction of anterior 2/3 & posterior 1/3)
• 5 taste modalities: sweet, salty, sour, bitter, umami (savory)
• Taste = ½ smell → anosmia or sinus congestion dulls flavor

The Ear & Hearing

External / Middle / Inner Regions

• External (outer): pinna → external acoustic meatus (auditory canal)
  • Canal ≠ eustachian tube
• Middle ear: tympanic membrane + 3 ossicles (lateral→medial): malleus → incus → stapes
  • Ossicles transmit vibrations efficiently via bone (sound travels faster in solids)
• Inner ear: cochlea (hearing) + vestibular apparatus (balance)

Cochlea Microanatomy

• Spiral shell; if unrolled: base → apex
• 3 fluid-filled chambers
  • Scala vestibuli (perilymph)
  • Scala media / cochlear duct (endolymph) – houses Organ of Corti (true mechanoreceptors)
  • Scala tympani (perilymph)
• Sound path: oval window vibration → endolymph movement → hair cells of Organ of Corti bend → AP via cochlear branch of CN VIII → temporal lobe auditory cortex
• Tonotopy: high-frequency detected near base; low-frequency at apex → explains why we notice scream before bass

Auditory vs. Eustachian Tube

• Auditory canal: air-way for sound waves
• Eustachian (pharyngotympanic) tube: connects middle ear ↔ nasopharynx; equalizes pressure, infection route in infants

Eye Anatomy & Vision

Supporting Structures

• Orbit formed by 9 cranial/facial bones
• Lacrimal apparatus (exocrine)
  • Gland in superolateral orbit secretes tears → sweep medially → drain via nasolacrimal duct into nasal cavity (explains “runny nose” while crying)

Three Eye Layers (Tunics)

1. Fibrous (outer) – sclera (white); anterior continuation visible = conjunctiva
2. Vascular (middle) – choroid (blood-rich), ciliary body, iris (colored portion)
3. Neural (inner) – retina → photoreceptors
   • Rods (grayscale, low-light, abundant)
   • Cones (color; c = color)
   • No photoreceptors over optic nerve head → optic disc / blind spot

Chambers & Humors

• Anterior chamber (cornea → lens) – aqueous humor (watery)
• Posterior chamber (lens → retina) – vitreous humor (gel-like)

Optical Pathway

1. Light enters cornea (avascular; common site of abrasion)
2. Passes aqueous humor → pupil (hole) within iris
3. Lens bends (accommodates) light; only structure that can vary refraction
4. Passes vitreous humor → focused on retina
5. Photoreceptors transduce → AP through optic nerve (CN II)
6. Nerves partially cross at optic chiasm → project to occipital lobe visual cortex

Accommodation Errors & Correction

• Myopia (near-sighted) – image focuses anterior to retina; need concave lenses
• Hyperopia (far-sighted) – image focuses posterior to retina; need convex lenses

Clinical & Ethical Highlights

• Pain management requires thorough subjective interview; lab/imaging cannot quantify pain
• Diabetic retinopathy: choroidal vessels visible via ophthalmoscope → essential screening
• Conjunctivitis (pink eye) = inflammation of conjunctiva
• Chronic otitis media in infants due to horizontal eustachian tube
• Subcutaneous injections exploit hypodermal vascularity
• Education: explain myopia/hyperopia, importance of regular eye/ear check-ups, safe lens/contact practices

Essential Terminology Recap

• Transduction – conversion of stimulus → AP
• Accommodation – lens curvature change to focus light
• Action potential – Na^+ in / K^+ out sequence
• Homunculus – cortical body map
• Organ of Corti – auditory mechanoreceptor site
• Golgi tendon organ / Muscle spindle – proprioceptors
• Circumvallate papillae – “V” shaped taste bud cluster
• Blind spot (optic disc) – no photoreceptors
• Eustachian tube – ear–nasal connection; pressure equalization