Sensory Receptors and Cranial Nerve Functions in Human Anatomy

What is tactile localization? What does error distance tell you?

The ability to identify the precise location of a tactile stimulus on the body. A smaller error distance = higher receptor density = smaller receptive field (like fingertips). A larger error = lower density = larger receptive field (like the back).

What is two-point discrimination, and what does the threshold indicate?

The minimum distance at which two simultaneous touch points can be felt as separate. A small threshold = high receptor density (e.g., fingertip). A large threshold = low density (e.g., calf, back of hand).

Which body areas have the smallest vs. largest receptive fields?

Smallest (most sensitive): fingertips, lips, tongue. Largest (least sensitive): back of hand, forearm, calf, back of neck.

Define sensory (neural) adaptation. What changes?

The decrease in receptor responsiveness with prolonged or constant stimulation — the receptor fires less frequently until the sensation is no longer perceived.

Define phasic receptors and give two examples.

Receptors that adapt quickly to a stimulus. Examples: hair receptors, lamellar (Pacinian) corpuscles, tactile (Meissner's) corpuscles. They fire at stimulus onset/offset but not continuously.

Define tonic receptors and give two examples.

Receptors that adapt slowly and continue generating signals during prolonged stimulation. Examples: proprioceptors (muscle spindles, tendon organs) and nociceptors (pain receptors).

True/False: Thermoreceptors are phasic receptors.

TRUE. Thermoreceptors adapt quickly — the temperature sensation fades with prolonged exposure (like getting used to a hot tub).

What is referred pain? Give the classic clinical example.

Pain perceived at a location distant from its actual source. Classic example: myocardial infarction (heart attack) → pain felt in the chest, left arm, back, and jaw. The ulnar nerve (behind elbow) innervates the medial hand — pressing the elbow can cause referred sensation in the hand.

What receptor types are unencapsulated vs. encapsulated? Give 3 of each.

Unencapsulated: free nerve endings (pain/heat/cold), tactile discs (light touch/pressure), hair receptors (movement). Encapsulated: tactile (Meissner's) corpuscles, lamellar (Pacinian) corpuscles, bulbous (Ruffini) corpuscles.

What do lamellar (Pacinian) corpuscles detect, and where are they?

Deep pressure, stretch, tickle, and vibration. Located in dermis, joint capsules, periosteum, breasts, genitals, and some viscera.

Exteroceptors vs. interoceptors vs. proprioceptors — what does each detect?

Exteroceptors: external stimuli (skin, special senses). Interoceptors (visceroceptors): internal organ stimuli (blood CO2, organ stretch). Proprioceptors: body position and movement (muscles, tendons, joint capsules).

CN I — Olfactory. Composition, function, damage, and lab test.

Sensory only. Function: smell. Damage: anosmia. Lab: close one nostril, close eyes — identify coffee vs. spice with the other nostril.

CN II — Optic. Composition, function, damage, and lab test.

Sensory only. Function: vision. Damage: anopsia (vision loss). Lab: read a portion of a printed page with either eye.

CN III — Oculomotor. Composition, function, damage, and lab test.

Mixed (mostly motor). Function: movement of eye/eyelid, pupil constriction, focusing. Damage: strabismus, ptosis, diplopia. Lab: follow finger moving up and down without moving the head.

CN IV — Trochlear. Composition, function, damage, and lab test.

Mixed (mostly motor). Function: medial/downward oblique eye movement. Damage: strabismus, diplopia. Lab: follow finger moving laterally around the head — tests superior oblique muscle.

CN V — Trigeminal. Composition, function, damage, and lab test.

Mixed. Motor: chewing. Sensory: facial pain, temperature, touch (3 branches). Damage: trigeminal neuralgia. Lab: clench teeth (motor); cotton swab on face dry then wet for temperature (sensory).

CN VI — Abducens. Composition, function, damage, and lab test.

Mixed (mostly motor). Function: lateral eye movement (lateral rectus). Damage: crossed eyes. Lab: same as trochlear — follow finger laterally.

CN VII — Facial. Composition, function, damage, and lab test.

Mixed. Function: facial expression, tears/saliva secretion, anterior taste. Damage: Bell's palsy. Lab: wrinkle forehead, raise eyebrows, inflate cheeks (motor); NaCl then sucrose on tongue tip (sensory).

CN VIII — Vestibulocochlear. Composition, function, damage, and lab test.

Mixed (mostly sensory). Function: hearing and equilibrium. Damage: deafness, tinnitus, vertigo. Lab: whisper a sentence (cochlear); walk a straight line (vestibular).

CN IX — Glossopharyngeal. Composition, function, damage, and lab test.

Mixed. Function: salivary secretion, taste, sensation of tongue and outer ear. Damage: difficulty swallowing, reduced salivation, bitter taste loss. Lab: see Vagus procedure.

CN X — Vagus. Composition, function, damage, and lab test.

Mixed. Function: swallowing, speech, heart rate, visceral sensation/motility. Damage: vocal cord paralysis, reduced visceral motility. Lab: say 'aaah' — uvula should rise in midline symmetrically.

CN XI — Accessory. Composition, function, damage, and lab test.

Mixed (mostly motor). Function: swallowing, head/neck movement. Damage: can't raise shoulders or turn head. Lab: press down on shoulders while partner raises them (trapezius); hands on sides of head while partner turns (SCM).

CN XII — Hypoglossal. Composition, function, damage, and lab test.

Mixed (mostly motor). Function: tongue movements. Damage: difficulty chewing, speaking, swallowing. Lab: stick tongue out — deviation toward one side = injury on that side.

Which cranial nerves are purely sensory? Purely motor? Mixed?

Sensory only: I (olfactory), II (optic), VIII (vestibulocochlear). Mostly motor: III, IV, VI, XI, XII. Fully mixed: V, VII, IX, X.

Describe the path of a sound wave from outer ear to brain.

Sound → outer ear (auricle) → auditory canal → tympanic membrane → ossicles (malleus→incus→stapes) → oval window → perilymph in cochlea → vestibular/basilar membranes → organ of Corti (hair cells) → cochlear nerve → vestibulocochlear nerve (CN VIII) → brain.

What is the Rinne Test? What do the results mean?

Tests bone vs. air conduction. Stem on mastoid → when gone, tines to ear. Normal: air conduction lasts longer (2x). If bone is louder/longer → conduction deafness. In sensorineural loss, both are equally decreased.

What is the Weber Test? What does lateralization indicate?

Tuning fork stem on forehead — note which ear is louder. Conduction deafness: sound louder in the DEAF ear (more sensitive to bone vibration). Sensorineural deafness: sound louder in the GOOD ear.

Compare conduction deafness vs. sensorineural deafness.

Conduction: blockage of sound transmission to inner ear (otitis media, perforated tympanic membrane, ossicle damage). Sensorineural (nerve): damage to cochlea or CN VIII — most common cause is presbycusis (age-related deterioration of organ of Corti).

What does an audiometer measure? What is the threshold?

Frequency (Hz = pitch) and intensity (decibels = loudness) at which sounds are heard. The threshold is the last/quietest level at which a tone was heard. Normal threshold ~0 dB; audiometry tests 125-8000 Hz.

What is presbycusis and what causes it?

Age-related sensorineural hearing loss caused by gradual deterioration of the organ of Corti. Progressive and slow, typically noticed in one's sixties.

Name the 3 layers (tunics) of the eyeball and their key structures.

1) Fibrous tunic: sclera (white, tough) + cornea (transparent, light entry). 2) Vascular tunic: choroid + ciliary body + iris. 3) Nervous tunic: retina + beginning of optic nerve.

What is the fovea centralis? What type of cells are there?

A thickened area within the macula lutea containing a high concentration of cone cells — the area of greatest visual acuity (sharpest color vision).

Cones vs. rods — function and location.

Cones: color vision, concentrated in fovea centralis/macula. Rods: black-and-white/low-light perception, found in the rest of the retina. No photoreceptors at the optic disc = blind spot.

What is the blind spot and how was it tested in lab?

The optic disc — where the optic nerve exits the eye; contains no photoreceptors. Lab: close one eye, hold page at arm's length, focus on + sign, move page slowly closer until the fly disappears.

Define myopia and hyperopia (hypermetropia). Which lens corrects each?

Myopia (nearsightedness): eyeball too long, image focuses IN FRONT of retina. Corrected with concave lens. Hyperopia (farsightedness): eyeball too short, image focuses BEHIND retina. Corrected with convex lens.

What is astigmatism? How is it tested?

Abnormal curvature of the lens or cornea — only part of the image focuses on the retina. Tested with an astigmatism chart (wheel with radiating lines) — blurry or lighter lines = astigmatism present.

What is glaucoma? What does it affect first?

Increased intraocular pressure that compresses the retinal artery. Loses peripheral vision first. Tested in lab by measuring how far a colored stick can be seen from the periphery.

What is accommodation and what causes presbyopia?

Accommodation: ciliary muscles contract → suspensory ligaments loosen → lens thickens for near focus. Presbyopia: with age, the lens hardens and loses flexibility → difficulty focusing on close objects.

Explain 20/20 vs. 20/30 vs. 20/15 Snellen chart results.

20/20 = normal (you see at 20 ft what a normal person sees at 20 ft). 20/30 = worse than normal (you need to be at 20 ft to see what a normal person sees at 30 ft). 20/15 = better than normal (you see at 20 ft what most see at 15 ft).

What is protanopia vs. deuteranopia? How is each tested?

Protanopia: red cone deficiency. Deuteranopia: green cone deficiency. Both are X-linked recessive. Tested with Ishihara plates — embedded numbers visible to normal vision but not to colorblind individuals.

What is the pupillary light reflex? How was it tested?

Shining light in one eye causes both pupils to constrict (direct and consensual reflex — mediated by CN III). Lab: block light with a notebook, shine flashlight into right eye, observe left pupil for consensual reflex.

What is the near point of the eye? What does a close near point indicate?

The closest distance at which the eye can focus clearly (test: move page toward eye until letters blur, then move back to point of clarity). A close near point = good accommodation/young lens. Increasing with age = presbyopia.

What are the 4 basic tastes and where on the tongue is each perceived?

Salty: tip. Sour: lateral edges. Bitter: posterior. Sweet: tip. (Note: lab only mapped 4 — bitter was omitted in this version.)

What nerves carry taste signals, and where do they go?

Facial nerve (CN VII): anterior 2/3 of tongue. Glossopharyngeal (CN IX): posterior 1/3. Vagus (CN X): epiglottis/pharynx. Signals travel to medulla → thalamus → parietal lobe of cerebral cortex.

True/False: Taste results from contact of substances dissolved in saliva with receptors in the taste buds.

TRUE. The receptors are gustatory cells (specialized neurons) whose dendrites project to the tongue surface. Substances must dissolve in saliva to be detected.

How does smell affect taste? How was this tested in lab?

Olfaction heavily influences taste perception. Lab test: pinch nose closed, apply a solution to the tongue — the taste sensation is diminished compared to smelling freely.

Where are smell receptors located and what pathway do signals take?

Located in the epithelium of the superior nasal concha. Olfactory neurons → dendrites project into nasal cavity → axons through cribriform plate of ethmoid bone → olfactory bulbs → olfactory tract → brain (CN I).

What is olfactory fatigue? How was it demonstrated in lab?

The adaptation of olfactory receptors with prolonged exposure to an odor — the scent is no longer detected. Lab: inhale wintergreen oil through one nostril until the scent disappears; record the time.

Ear anatomy: Name the 3 ossicles and the order sound reaches them.

Malleus (hammer) → Incus (anvil) → Stapes (stirrup). They transmit vibrations from the tympanic membrane to the oval window.

Ear anatomy: What is the organ of Corti and where is it?

The sensory organ of hearing — located on the basilar membrane inside the cochlea. Contains hair cells that convert fluid vibrations into neural signals.

Ear anatomy: What fills the cochlea and what do the vestibular/basilar membranes divide?

Perilymph fills the outer chambers; endolymph fills the space between the vestibular and basilar membranes (the scala media/cochlear duct). The basilar membrane supports the organ of Corti.

Eye anatomy: What are the 3 chambers and what fills them?

Anterior chamber (between cornea and iris): aqueous humor. Posterior chamber (between iris and lens): aqueous humor. Vitreous chamber (behind lens): vitreous humor (maintains shape/pressure).

Eye anatomy: What is the choroid, ciliary body, and iris?

Choroid: thin vascular membrane, nourishes retina. Ciliary body: holds lens via suspensory ligaments + ciliary muscles for accommodation. Iris: colored ring that controls pupil size (light in = constrict).