A&P CH 15

What distinguishes general from special senses?

• General senses: found throughout the body

• Special senses: localized in the head

• Most processed by sensory nuclei → thalamus → primary somatosensory cortex

• Special senses go further to association areas for interpretation
  (Exception: olfaction does not synapse in thalamus)

Where does olfaction start and what cells are involved?

• Starts at olfactory epithelium

• Involves modified bipolar olfactory neurons (chemoreceptors)

• Axons form CN I (olfactory nerve)

What is the olfaction pathway?

Olfactory nerve → synapse on mitral cells in olfactory bulb → form olfactory tract → travel to primary olfactory cortex in the temporal lobe

What additional structures are in the olfactory epithelium?

• Basal cells (blue): regenerative

• Supporting cells (purple)

• Olfactory glands

How is olfaction activated?

• Odorant binds to metabotropic receptor → Na+ and Ca²⁺ enter → local potential → summation → action potential → signal sent to mitral cells

What senses contribute to gustation?

• Olfactory chemoreceptors

• Thermoreceptors

• Nociceptors

• Gustatory chemoreceptors

What factors make up taste?

• Odorants reaching olfactory epithelium

• Temperature and texture

• Spicy foods stimulate nociceptors via trigeminal nerve

Where does gustation begin?

• In taste buds located in papillae

• Gustatory cells (epithelial receptor cells) have microvilli projecting into taste pores

• They synapse with sensory neurons

Which cranial nerves carry gustatory input?

• CN VII (Facial)

• CN IX (Glossopharyngeal)

• CN X (Vagus)

What other cells are in taste buds?

• Basal cells (blue)

• Support cells (purple)

• Note: Filiform papillae sense texture/temperature, not taste

Which chemicals activate sour and salty taste?

• Sour = H⁺ ions (acids)

• Salty = Na⁺ ions (metal ions)

What activates sweet, bitter, and umami tastes?

• Sweet = simple sugars (glucose, fructose)

• Bitter = alkaloids, plant poisons, rancid food

• Umami = glutamate, amino acids (meaty/brothy)

What is the mechanism for sweet, bitter, umami detection?

• Activate G-protein → closes K⁺ channels → depolarization → local potential

What is the gustatory pathway to the brain?

CN nerves → solitary nucleus (medulla) → thalamus → primary gustatory cortex (parietal lobe)

What structures contribute to light entry and focusing?

• Cornea: admits light

• Lens: flexible, rounded; adjusts for focus

• Ciliary body + suspensory ligaments: change lens shape

What parts of the eye help manage light scattering and focus?

• Choroid: melanocytes reduce light scatter

• Fovea centralis: detailed vision (high cone density)

• Optic disc: no photoreceptors (blind spot)

What is the pathway of light in distant and near vision?

• Distant: lens flattens via stretched suspensory ligaments

• Near: lens rounds (accommodation), pupil constricts, eyes converge

What are rods and what do they do?

• Contain rhodopsin (opsin + retinal)

• Function in low light

• Low acuity, no color vision

What are cones and what do they do?

• Contain iodopsin (retinal + photopsin)

• High acuity, color vision

• Function in bright light

• Concentrated in fovea centralis

What is the synaptic sequence in retina?

Photoreceptors (rods/cones) → bipolar cells → retinal ganglion cells

What do horizontal and amacrine cells do?

• Horizontal cells: enhance visual contrast

• Amacrine cells: detect changes in light/movement

What is the complete visual pathway?

Photoreceptors → bipolar → ganglion → optic nerve (CN II) → optic chiasma → optic tract → thalamus → primary visual cortex (occipital lobe)

What happens to photoreceptors in the dark?

• Depolarized (ON)

• Rhodopsin inactive → Na⁺ enters via cGMP → glutamate released

• Bipolar cells inhibited (IPSP)

• Ganglion cells not stimulated

What happens to photoreceptors in the light?

• Hyperpolarized (OFF)

• Light activates retinal → activates G-protein + PDE → cGMP → GMP → Na⁺ channels close

What is the result of photoreceptor hyperpolarization?

• Glutamate stops releasing → bipolar cells depolarize

• Ganglion cells stimulated → action potential sent to brain

What structures are in the outer and middle ear?

• Outer: auricle, external canal, tympanic membrane

• Middle: ossicles (malleus, incus, stapes), oval window

What are the regions and fluids of the inner ear?

• Regions: cochlea, vestibule, semicircular canals

• Fluids:

  • Perilymph: low K⁺ (ECF-like)

  • Endolymph: high K⁺ (cytosol-like)

What are the 3 ducts of the cochlea?

• Scala media (cochlear duct) – endolymph

• Scala vestibuli – perilymph

• Scala tympani – perilymph

What detects sound in the cochlea?

• Spiral organ (in cochlear duct)

• Hair cells with stereocilia

• Vibrations from basilar membrane → stereocilia bend → K⁺ channels open → depolarization

What is the full auditory pathway?

Cochlear nerve → CN VIII → cochlear nuclei (medulla-pons) → superior olivary nucleus (pons) → inferior colliculus (midbrain) → thalamus → primary auditory cortex (temporal lobe)

What is the purpose of LASIK surgery?

Reshapes the cornea to adjust refraction so light focuses on the retina’s focal point.

What vision conditions can LASIK treat?

• Hyperopia: blurry near vision

• Myopia: blurry distant vision

• Astigmatism: irregular lens or cornea curvature → blurred vision at all distances

What are some steps and effects during and after LASIK?

• Numbing drops + suction ring hold eye

• Patient stares at light; corneal tissue removed (distinct odor)

• Post-op: flap heals, eye drops given

• Possible effects: halos, dry eyes, reduced night vision, regression

What are cataracts and what causes them?

• Cloudy, irreversible lens — common cause of blindness

• Caused by trauma, UV radiation, diabetes, or aging (most common)

How do cataracts develop with age?

• Lens thickens and becomes less flexible

• Eye tissue breaks down → cloudy lens

What is the treatment for cataracts?

• Surgical lens replacement with intraocular lens (IOL)

  • Standard (monofocal): corrects distance

  • Premium (multifocal/accommodative): corrects distance + near

• Glasses/contacts may still be needed

Define anosmia and hyposmia.

• Anosmia: loss of smell

• Hyposmia: reduced smell sensitivity

What risks are associated with anosmia and hyposmia?

• Can’t detect smoke, gas leaks, or spoiled food

• Can lead to malnutrition

What health conditions can cause anosmia or hyposmia?

• Nasal polyps (non-tender)

• Deviated/fractured nasal septum

• Swollen turbinates

• Swollen respiratory epithelium

Is loss of smell caused by nasal polyps, deviated septum, swollen turbinates, or swollen respiratory epithelium usually permanent?

• Often temporary if airflow is restored

• Olfaction returns with treatment

What can cause permanent anosmia?

• Head injury (damaging olfactory nerve via cribriform plate fracture)

• Neural degeneration (Alzheimer’s, Parkinson’s)

Why does hyposmia develop with age?

• Fewer olfactory neurons replaced by basal cells

• Noticeable after age 60

What treatments do ENT specialists recommend for smell loss?

• Oral anti-inflammatory meds

• Antibiotics

• Allergy testing, antihistamines, corticosteroids

• Nasal surgery

What is tinnitus and its most common cause?

• Perception of noise without sound (ringing, buzzing, whistling, hissing)

• Often due to damaged/misshapen stereocilia → continuous neurotransmitter release

What are other causes of tinnitus?

• Fused ossicles

• Tympanic membrane inflammation

• Certain medications (e.g., high-dose aspirin)

• CNS damage or unknown reasons

• Sometimes temporary

What is conduction hearing loss?

issue in outer/middle ear prevents sound from reaching inner ear

What causes conduction hearing loss?

• Ear wax buildup

• Middle ear infection

• Perforated tympanic membrane

• Fused ossicles

• Sound may still reach inner ear via bone conduction

What is neural hearing loss?

Signal fails to travel through cochlear nerve or CNS pathways

What causes neural hearing loss?

• Strokes

• Acoustic neuroma (benign tumor at medulla/pons junction)

What is sensory hearing loss?

• Action potentials can’t be generated in cochlea

• Hair cell dysfunction

What causes sensory hearing loss?

• Loud noise exposure

• Certain drugs (e.g., gentamicin blocks K⁺ channels → no depolarization)

• Aging: lose stereocilia in proximal cochlea

What are treatments for sensorineural hearing loss?

• Hearing aids

• Cochlear implants