Sensory Receptors and Pathways in Human Physiology

Mechanoreceptors

Detect pressure, stretch, and vibration.

Chemoreceptors

Detect chemicals dissolved in fluid (taste, smell).

Photoreceptors

Detect light for vision.

Thermoreceptors

Detect temperature changes.

Nociceptors

Detect pain and tissue damage.

Free nerve endings

Sense pain and temperature.

Merkel cells

Sense light touch.

Meissner corpuscles

Sense fine touch and low-frequency vibration.

Pacinian corpuscles

Sense deep pressure and high-frequency vibration.

Ruffini corpuscles

Sense skin stretch.

Root hair plexus

Detect hair movement.

Proprioceptors

Sense body position, muscle stretch, and joint movement.

Taste buds

Contain chemoreceptors that detect dissolved chemicals.

Five basic tastes

Sweet, salty, sour, bitter, umami.

Filiform papillae

Provide texture; do not contain taste buds.

Taste bud turnover

Replaced every 8-12 days.

Spicy sensation

Activation of nociceptors and thermoreceptors (not a taste).

Facial nerve (VII)

Carries taste from anterior 2/3 of tongue.

Glossopharyngeal nerve (IX)

Carries taste from posterior 1/3 of tongue.

Vagus nerve (X)

Carries taste from epiglottis and pharynx.

Gustatory pathway

Taste receptors → CN VII/IX/X → medulla → thalamus → insula.

Olfactory receptors

Chemoreceptors detecting airborne chemicals.

Olfactory adaptation

Decreased sensitivity to a continuous odor.

Anosmia

Loss of smell.

Causes of anosmia

Trauma, infections, mucus, COVID, zinc deficiency, neurodegenerative disease.

Smell during sleep

Sense of smell turns off during sleep.

Olfactory receptor regeneration

Olfactory neurons can regenerate.

Olfactory hallucinations (uncinate fits)

False smell perceptions.

Lacrimal apparatus

Produces, drains, and moves tears into nasal cavity.

Lens

Biconvex structure that focuses light onto retina.

Aqueous humor

Fluid in anterior segment; maintains pressure.

Vitreous humor

Gel in posterior cavity that maintains eye shape and holds retina in place.

Accommodation

Lens changes shape to focus on near or far objects.

Pupil

Opening that regulates amount of light entering eye.

Iris

Muscle that controls pupil size.

Fovea centralis

Area with highest density of cones for sharp vision.

Optic disc

Blind spot; no photoreceptors.

Cones

Detect color and sharp vision in bright light.

Rods

Detect dim light and peripheral vision; no color.

Myopia

Nearsighted; image forms in front of retina.

Hyperopia

Farsighted; image forms behind retina.

Astigmatism

Irregular curvature causing blurred/distorted vision.

Presbyopia

Loss of lens flexibility with age.

Cataract

Clouding of the lens.

Glaucoma

High intraocular pressure damaging the optic nerve.

Primary visual pathway

Retina → optic nerve → chiasm → tract → thalamus → occipital cortex.

Color blindness

X-linked recessive trait affecting color vision.

Outer ear

Auricle and external auditory canal.

Middle ear

Tympanic membrane and ossicles.

Inner ear

Cochlea, vestibule, semicircular canals.

Ossicles

Malleus, incus, stapes.

Cochlea

Organ for hearing, converts vibrations into neural signals.

Organ of Corti

Hair cells that transduce sound.

Basilar membrane

Vibrates at different locations depending on sound frequency.

High-frequency detection

Base of cochlea (short, stiff fibers).

Low-frequency detection

Apex of cochlea (long, flexible fibers).

Conductive deafness

Sound conduction issue in outer/middle ear.

Sensorineural deafness

Damage to hair cells or auditory nerve.

Static equilibrium

Detects head position and linear movement (utricle, saccule).

Dynamic equilibrium

Detects rotational movement (semicircular canals).

Otoliths

Crystals that shift with gravity to activate hair cells.

Crista ampullaris

Receptor in semicircular canals for rotation.

Why dizziness occurs after spinning

Endolymph continues moving, bending hair cells.

Weber and Rinne tests

Assess conductive vs. sensorineural hearing loss.