Sensory Reception: Hearing & Vision

Overview of Sensory Reception & Special Senses

• Sensory reception = entry point for the nervous system; converts external energy into neural signals.
• “Special senses” (cranial-nerve–based) = taste, hearing, vision, equilibrium, olfaction.
• Lecture focuses on two: hearing (auditory system) and vision (ocular system).

Hearing (Auditory System)

External Ear (Conducts air vibrations)

• Pinna (auricle)
  • Cartilaginous “satellite dish.”
  • Primary role = funnel sound waves toward auditory canal.
• External auditory canal
  • Common site for earwax & Q-tips.
  • Terminates at the tympanic membrane (TM) / eardrum.
  • Metaphors: trampoline fabric or tambourine skin – vibrates with incoming sound.

Middle Ear (Mechanical amplifier)

• Air-filled cavity housing ossicles (ear bones) – smallest bones in body.
  1. Malleus (“hammer”) – directly attached to TM.
  2. Incus (“anvil”) – hinged to malleus and stapes.
  3. Stapes (“stirrup”) – footplate rests on oval window.
• Vibration sequence: TM → malleus → incus → stapes → oval window.
• Ossicles provide impedance matching (air → fluid) & ~20× force amplification.

Inner Ear (Fluid dynamics & transduction)

Gross anatomy

• Vestibular apparatus (semicircular canals + vestibule) = equilibrium; not discussed here.
• Cochlea = spiral bony tube performing hearing.
  • Stapes strikes the oval window at cochlear base.
  • Pressure relieved at round window.

Cochlear duct system (simplified straightened view)

• Three parallel compartments:
  1. Scala vestibuli (superior) – contains perilymph.
  2. Scala media / cochlear duct (central) – contains endolymph.
  3. Scala tympani (inferior) – contains perilymph.
• Fluid path: oval window → scala vestibuli → helicotrema (apex) → scala tympani → round window.

Fluids & ionic milieu

• Perilymph ≅ cerebrospinal fluid (high $Na^+$, low $K^+$).
• Endolymph (secreted by stria vascularis)
  • Unusual: high $K^+$ (~150 mM), low $Na^+$.
  • Establishes large endocochlear potential: $(\Delta V \approx -150\ \text{mV})$ (hair-cell interior negative relative to endolymph).

Organ of Corti (sensory epithelium)

• Sits on basilar membrane between scala media & scala tympani.
• Components:
  • Hair cells (inner & outer) with stereocilia embedded in tectorial membrane (pink in diagram).
  • Mechanotransduction:
  • Fluid wave bends stereocilia → mechanically-gated $K^+$ channels open.
  • $K^+$ influx (down electrochemical gradient) → depolarization (contrasts with Na-driven depolarization elsewhere).
  • Depolarization → $Ca^{2+}$ entry → neurotransmitter release (glutamate) onto cochlear nerve fibers.

Neural pathway

• Cochlear branch of cranial nerve VIII (vestibulocochlear).
• Synapse in medulla oblongata (cochlear nuclei) → ascending projections to auditory cortex in temporal lobe.

Key Concepts & Terminology

• Mechanotransduction: mechanical energy → electrical signal.
• Impedance matching: ossicles prevent sound loss at air–fluid interface.
• Endocochlear potential: large voltage that sensitizes hair cells to minimal sounds.
• Even faint sound waves can trigger depolarization due to huge $K^+$ gradient.

Practical / Clinical Notes & Metaphors

• Trampoline / tambourine = TM analogy.
• Hearing loss etiologies: TM rupture, otosclerosis (ossicle fixation), K+-secretion defects (stria vascularis pathology).
• Q-tip warning: deep insertion can damage TM.

Vision (Ocular System)

Pupil & Iris Mechanics (Light-gate)

• Pupil: central aperture controlling light entry.
• Iris = colored muscular diaphragm.
  • Radial/dilator muscles (purple, sympathetic) – contract ⇒ pupillary dilation (mydriasis).
  • Circular/sphincter muscles (parasympathetic) – contract ⇒ pupillary constriction (miosis).
• Low light → sympathetic drive → large pupils; bright light → parasympathetic drive → small pupils.
• Field sobriety test: officer shines flashlight; expects prompt constriction.
• Sphincter analogy: wringing a wet rag (circumferential tightening reduces aperture).

Optical Pathway

• Cornea → aqueous humor → pupil → lens → vitreous body → retina (neural tissue) → optic nerve.
• Posterior retina includes pigmented epithelium (absorbs stray photons) & optic disc.

Retinal Cellular Architecture (inside-out)

1. Ganglion cells (innermost toward vitreous).
2. Bipolar cells (interneurons).
3. Photoreceptors: rods (dim light, monochrome) & cones (color, acuity).

• Light must traverse ganglion & bipolar layers before photoreceptor absorption (counter-intuitive ordering).

Phototransduction Cascade (Simplified functional logic)

Dark condition (baseline)

• Photoreceptor is depolarized (open $Na^+/Ca^{2+}$ “dark current”).
• Continuous release of glutamate onto bipolar cell.
• In this context, glutamate binds inhibitory metabotropic receptors (mGluR6) on ON-bipolar cell → hyperpolarizes bipolar cell.
• Result: bipolar & ganglion cells are silent ⇒ no action potentials along optic nerve.

Light condition (photon absorption)

• Photopigment (rhodopsin/photopsin) activates → closure of cGMP-gated channels.
• Photoreceptor hyperpolarizes ⇒ stops glutamate release.
• Lack of inhibitory glutamate dis-inhibits / depolarizes ON-bipolar cell.
• Depolarized bipolar cell releases excitatory glutamate onto ganglion cell.
• Ganglion cell fires action potentials along optic nerve (cranial nerve II) to occipital lobe (primary visual cortex).
• Same neurotransmitter (glutamate) can be inhibitory or excitatory depending on receptor subtype.

Additional Points & Connections

• Signal transduction = conversion of photon energy into neural code.
• Rods vs cones: not detailed, but remember rods dominate scotopic (low-light) vision; cones enable photopic (daylight) vision & color.
• Pupillary reflex arc: optic nerve → pretectal nuclei → Edinger-Westphal nucleus → oculomotor nerve (parasymp) → constrictor muscle.

Practical / Clinical Nuggets

• Non-reactive (fixed) pupils can indicate drug impairment, brainstem injury, or optic nerve damage.
• Heterochromia: variation in iris pigment, no change in muscle function.
• LASIK & cataract surgery preserve iris/pupil mechanics but alter cornea or lens.

Cross-Topic Themes & Comparative Insights

• Both systems transform mechanical or electromagnetic energy → electrochemical signals.
• Unique ionic exceptions:
  • Ear: $K^+$ influx causes depolarization (rare example where extracellular $K^+$ > intracellular).
  • Eye: light causes hyperpolarization, opposite to typical sensory-neuron depolarization.
• Cranial nerves: VIII (hearing/balance) vs II (vision).
• Clinical tests: tuning-fork (Rinne, Weber) for hearing; pen-light reflex for vision.