Special Senses

Posterior segment of the eye

Back portion of the eye

Anterior segment of the eye

Front portion of the eye

Retina location

Lines the posterior interior of the eye

Layers of the retina (light hits in order)

Axons of ganglion cells → Ganglion cell bodies → Bipolar cells → Photoreceptors (rods & cones)

Photoreceptors in the dark

Constantly depolarized, release neurotransmitter → bipolar cells hyperpolarized, no signal to ganglion cells

Photoreceptors in light

Ion channels close → photoreceptor hyperpolarizes → bipolar cells release neurotransmitter → ganglion cells activated

Rods – main features

Rhodopsin only, highly light sensitive, low acuity, numerous, located in periphery, best for night vision

Cones – main features

Three photopigments (blue, green, red), less light sensitive, high acuity, fewer, located in fovea/macula, best for color/day vision

Optic disc

Blind spot; axons & blood vessels exit; no rods or cones

Fovea centralis

Contains only cones, retinal layers pushed aside → sharpest vision

Refraction in the eye

Cornea & lens bend light to focus on the fovea

Lens accommodation for distant objects

Lens flattens; ciliary muscle relaxes; suspensory ligament pulls lens

Lens accommodation for near objects

Lens rounds; ciliary muscle contracts; suspensory ligament slackens

Myopia

Nearsightedness; distant objects blurry

Hyperopia

Farsightedness; near objects blurry

Astigmatism

All objects blurry; irregular curvature of cornea/lens

Inner ear structures for hearing and balance

Cochlea (sound), vestibule & semicircular canals (motion/orientation)

Nerves of the inner ear

Vestibular nerve and cochlear nerve

Cochlea compartments

Scala vestibuli, scala tympani, scala media

Physiology of hearing

Sound waves move basilar membrane → stereocilia bend → ion channels open/close → hair cells signal brain

Vestibule (utricle & saccule)

Contains maculae; otoliths shift with gravity/inertia → stereocilia bend → sense head position & linear acceleration (static equilibrium)

Semicircular canals

Contain cristae ampullares; endolymph flow bends cupula → stereocilia bend → sense rotation & angular acceleration (dynamic equilibrium)

Location of olfactory receptors

In cribiform plate of ethmoid bone

Activation of olfactory receptors

Odorant molecules stimulate receptor cells → communicate with olfactory bulb neurons

Human olfactory capacity

~400 receptor types; ~1 trillion odors distinguishable

Location of taste buds

In papillae on tongue and epiglottis

Gustatory receptors

Detect dissolved chemicals (tastants)

Five primary tastes

Salty, sour, sweet, bitter, umami

Taste bud regional sensitivity

Not limited to one type of taste; each region has buds of distinct shapes but can detect multiple tastes

Visual system & homeostasis

Lens accommodation (focus at different distances); pupil constriction/dilation (light control)

Auditory/vestibular system & homeostasis

Hair cell movement adjusts posture & muscle tone for balance

Olfactory system & homeostasis

“Nose blindness” filters out constant odors, allowing focus on new ones

Gustatory system & homeostasis

Negative experiences with tastants/odorants → avoidance (food aversion)

Brain as control center

Coordinates homeostatic mechanisms involving special senses