Lecture #112: Anatomy: Special Afferents of the Head and Neck

What percentage of the brain is dedicated to visual processing?

Approximately 30–60% of the brain is dedicated to processing visual information, making vision the most significant special sense in humans

Through which structure does the optic nerve exit the endocranium?

The optic nerve exits through the optic canal, acting as an anterior extension of the forebrain

What are the three layers of the eye?

Fibrous layer (sclera and cornea), vascular layer (iris, ciliary body, choroid), and neural layer (retina)

What is the function of the sclera?

The sclera provides structural protection, blocks light, and serves as the attachment site for extraocular muscles

What is the function of the cornea?

The cornea refracts about two-thirds of incoming light, blocks UV radiation, and provides a clear entry for light. It is avascular but highly sensitive to touch

What are the three components of the vascular layer?

Iris (controls pupil size), ciliary body (controls lens shape and produces aqueous humour), and choroid (provides blood supply to retina)

Which muscles control pupil size and how are they innervated?

Sphincter pupillae (constriction, parasympathetic) and dilator pupillae (dilation, sympathetic)

What determines eye color?

Amount and distribution of melanin within the iris; more melanin results in darker eyes

What is the function of the ciliary body?

Produces aqueous humour and controls lens shape via zonular fibers (Zonule of Zinn); parasympathetically innervated

What is the function of the choroid?

Highly vascular layer that nourishes the retina and maintains attachment of the retina via vitreous pressure

What are the two layers of the retina?

Neural layer (photoreceptors and neurons) and pigmented layer (absorbs light, prevents scattering)

Why is the human eye described as “built backwards”?

Light passes through non-photoreceptive cells before reaching photoreceptors; this causes a blind spot where the optic disc is located

What is the optic disc?

The point where all retinal neurons converge to form the optic nerve; lacks photoreceptors, creating the blind spot

What is the macula lutea and fovea centralis?

Macula lutea is a yellow spot on the retina; the fovea centralis at its center provides the sharpest vision due to the absence of non-photoreceptive cells

What is accommodation?

The ability of the eye to adjust the focal point by altering lens shape via ciliary muscle contraction

How does the lens adjust for near vs distant vision?

Near vision: ciliary muscle contracts, lens becomes rounder. Distance vision: ciliary muscle relaxes, lens flattens

What causes presbyopia?

Age-related loss of lens elasticity, reducing the ability to focus on near objects

What is a cataract?

Age-related denaturation of lens proteins leading to clouding; risk factors include aging, smoking, hypertension, and poor nutrition

What causes glaucoma?

Blocked aqueous humour drainage through the canal of Schlemm increases intraocular pressure, compressing the optic nerve

What is retinal detachment?

Separation of the neural retina from the choroid due to loss of intraocular pressure; presents as flashes or floaters and is a medical emergency

What are the two components of CN VIII?

Vestibular nerve (balance) and cochlear nerve (hearing); both exit the internal acoustic meatus with CN VII

What are the two labyrinths of the inner ear?

Endosseous (bony) labyrinth filled with perilymph and membranous labyrinth filled with endolymph

What are the main vestibular structures?

Otolith organs (utricle and saccule) and semicircular canals (anterior, posterior, horizontal)

What is the function of the utricle?

Detects horizontal acceleration; sensory macula lies on its floor

What is the function of the saccule?

Detects vertical acceleration and gravity; macula lies on medial wall

What are otoliths?

Calcium carbonate crystals that give weight to the utricle and saccule, aiding gravity and motion sensing

How do semicircular canals detect head movement?

Endolymph movement bends the cupula in ampullae, altering afferent neuron firing to indicate head rotation

How are the semicircular canals paired?

Work in functional pairs: right/left horizontals, right anterior with left posterior, and left anterior with right posterior

What is the cochlea?

Spiral organ of hearing that transduces sound waves into neural impulses via the cochlear nerve

How does sound travel through the ear?

Sound waves vibrate the tympanic membrane → ossicles (malleus, incus, stapes) → oval window → cochlea → round window

What is the Organ of Corti?

Sensory epithelium in the cochlear duct containing stereocilia that depolarize in response to specific sound frequencies

How are different frequencies processed in the cochlea?

High frequencies near the base; low frequencies travel farther up the cochlear spiral

What muscles protect the inner ear from loud sounds?

Stapedius (CN VII) and tensor tympani (CN V3) dampen ossicle vibration

Differentiate conductive vs sensorineural hearing loss.

Conductive: middle ear pathology (tympanum, ossicles); Sensorineural: cochlea or nerve damage

What is tinnitus?

Perception of ringing or buzzing due to hair cell loss or neural dysfunction; affects 15–20% of adults

What causes increased sensorineural hearing loss in recent years?

Chronic earbud use at high volume causes sustained cochlear damage

What is Benign Paroxysmal Positional Vertigo (BPPV)?

Condition where loose otoliths enter semicircular canals causing vertigo and balance disturbance

How is BPPV treated?

Often self-resolving; can be corrected with canalith repositioning (Epley maneuver)

What is the vestibulo-ocular reflex?

Reflex that stabilizes gaze by coordinating head movement with eye position via CNs III, IV, and VI

What is nystagmus?

Involuntary oscillatory eye movement due to delayed vestibulo-ocular reflex; can be congenital, drug-induced, or neurologic

Which cranial nerve mediates the sense of smell?

Cranial nerve I (Olfactory nerve). It is the least studied cranial nerve and functions as an anterior extension of the forebrain that transmits olfactory information to the brain

Where do olfactory nerve fibers end?

In the mucosa of the superior part of the nasal cavity. Approximately 10 small nerves per olfactory bulb pass through the cribriform plate to reach this region

Which part of the nasal cavity is responsible for smell?

Only the superior-most portion, including the superior concha and adjacent walls, contains olfactory epithelium. The remainder of the nasal passage primarily regulates temperature and humidity

What are the three nasal conchae and their function?

Superior, middle, and inferior conchae. They increase surface area for air filtration, warming, and humidification; only the superior concha is involved in olfaction

Through what structure do olfactory nerves pass?

The cribriform plate of the ethmoid bone, through which olfactory fibers project to synapse in the olfactory bulb

What are the two olfactory striae and their function?

Lateral and medial striae, divisions of the olfactory tract that distribute olfactory information to different brain regions. CN I uniquely projects directly to the cerebrum, bypassing the thalamus

What is anosmia?

The loss or reduction of smell. Causes include chemical damage (epithelium), mechanical injury (cribriform plate fracture), congenital absence of CN I, viral infection (e.g., COVID-19), or aging. It reduces taste and safety awareness

Which cranial nerves convey taste sensation?

Facial nerve (CN VII) for anterior 2/3 of tongue, Glossopharyngeal nerve (CN IX) for posterior 1/3, and Vagus nerve (CN X) for small areas of the soft palate and pharynx

What are the four types of lingual papillae?

1) Filiform (numerous, no taste buds), 2) Foliate, 3) Fungiform, and 4) Circumvallate (8–12 large papillae with many taste buds)

How many taste receptors are in each taste bud?

Each taste bud contains 50–150 taste receptor cells. Hundreds of taste buds are located on each papilla, collectively detecting five main taste categories

What are the five primary taste categories?

Sweet, salty, sour, bitter, and umami (savory). All regions of the tongue can detect all five; there is no true “taste map” localization

Why is spice perceived as a “taste”?

Spicy sensations are not true tastes but rather pain (nociceptive) responses. Capsaicin and similar compounds activate pain receptors in mucosal linings; the brain interprets this as part of gustation

What nerve carries taste from the anterior two-thirds of the tongue?

The chorda tympani branch of CN VII (facial nerve), which exits through the petrotympanic fissure and joins the lingual nerve (CN V3) to reach the tongue

What is the relationship between the lingual nerve and chorda tympani?

Though they travel together, CN VII fibers (taste) and CN V3 fibers (general sensation) remain distinct and do not intermingle

How is the tongue innervated embryologically?

Develops from the first four pharyngeal arches, each with its own cranial nerve: CN V3 (arch 1), CN VII (arch 2), CN IX (arch 3), CN X (arch 4). Facial nerve fibers migrate forward into the anterior tongue during development

What is ageusia?

Loss of taste sensation. Causes include toxic chemical exposure, head trauma, viral infection (COVID-19), or nerve injury. It may impair nutrition and is often accompanied by anosmia

What are the main consequences of ageusia?

Reduced appetite, poor nutrition, and potential food safety risks due to inability to detect spoiled food. May cause compensatory overuse of salt or sugar

How are taste and smell integrated?

They combine in the thalamus and orbitofrontal cortex to form the perception of flavour. Approximately 80% of flavour perception arises from olfactory input

What is meant by “coupled senses”?

Two or more senses working together to produce a combined perception (e.g., smell + taste = flavour). This integration enhances sensory experience and food perception