Special Senses Anatomy and Physiology

Cranial nerve assessment, pupillary reflexes, hearing and vision changes in older patients: all of it traces back to the structures here. Learn the eye and ear cold, because these are the systems you check first in a neuro exam.

Functions of Special Senses

The five special senses:

1. Vision. The eye focuses visible light onto photoreceptors in the retina, which generate nerve impulses coding color, hue, and brightness.
2. Hearing. Audition, the perception of sound.
3. Taste. Detection of substances such as food, certain minerals, and poisons.
4. Smell. Olfaction, the second chemical sense. Odor molecules excite specific receptors to varying degrees, and that combination is what we perceive as smell.
5. Touch. Somatosensation (tactition, mechanoreception), from neural receptors in the skin and hair follicles, plus the tongue, throat, and mucosa.

The Eye and Vision

Vision is the most-studied sense. Of all the body's sensory receptors, 70% are in the eyes.

Anatomy of the Eye

Vision takes the most learning, and the eye is easily fooled. "You see what you expect to see" is often true.

External and Accessory Structures

The accessory structures are the extrinsic eye muscles, eyelids, conjunctiva, and lacrimal apparatus.

• Eyelids. Protect the eyes anteriorly and meet at the medial and lateral commissures (canthi).
• Eyelashes. Project from each eyelid border.
• Tarsal glands. Modified sebaceous glands at the eyelid edges that secrete an oily lubricant. Ciliary glands (modified sweat glands) lie between the lashes.
• Conjunctiva. A delicate membrane lining the eyelids and covering part of the eyeball, fusing with the corneal epithelium at the cornea's edge.
• Lacrimal apparatus. The lacrimal gland plus ducts that drain secretions into the nasal cavity.
• Lacrimal glands. Sit above the lateral end of each eye and release a salt solution (tears) onto the eyeball through small ducts.
• Lacrimal canaliculi. Tears flush medially into the lacrimal canaliculi, then the lacrimal sac, then the nasolacrimal duct, which empties into the nasal cavity.
• Lysozyme. Tears also carry antibodies and lysozyme, an enzyme that destroys bacteria, so they cleanse and protect while moistening the eye.
• Extrinsic eye muscle. Six external muscles attach to the eye's outer surface and produce gross movements to track objects: lateral rectus, medial rectus, superior rectus, inferior rectus, inferior oblique, and superior oblique.

Internal Structures: The Eyeball

The eyeball is a hollow sphere with a three-layer wall, filled with fluids (humors) that maintain its shape.

Layers of the Eyeball Wall

• Fibrous layer. The outermost layer: the protective sclera and the transparent cornea.
• Sclera. Thick, glistening white connective tissue, seen anteriorly as the "white of the eye."
• Cornea. The crystal-clear central anterior window through which light enters.
• Vascular layer. The middle layer, with three regions: choroid, ciliary body, and iris.
• Choroid. The most posterior region, a blood-rich nutritive tunic with dark pigment that stops light from scattering inside the eye.
• Ciliary body. Anteriorly the choroid forms two smooth muscle structures: the ciliary body, to which the lens attaches by the ciliary zonule (suspensory ligament), and then the iris.
• Pupil. The rounded opening in the pigmented iris that admits light.
• Sensory layer. The innermost two-layered retina, extending forward only to the ciliary body.
• Pigmented layer. The outer retinal layer of pigmented cells that absorb light and prevent scattering.
• Neural layer. The transparent inner retina holding millions of photoreceptors, the rods and cones, which respond to light.
• Two-neuron chain. Signals pass from photoreceptors through bipolar cells, then ganglion cells, leaving via the optic nerve to the optic cortex. The result is vision.
• Optic disc. Where the optic nerve leaves the eyeball, with no photoreceptors. The blind spot.
• Fovea centralis. A tiny pit lateral to each blind spot, containing only cones.

Lens

The lens, a flexible biconvex crystal-like structure, focuses light on the retina.

• Chambers. The lens splits the eye into two segments. The anterior (aqueous) segment holds clear watery aqueous humor; the posterior (vitreous) segment holds gel-like vitreous humor (vitreous body).
• Vitreous humor. Reinforces the eyeball internally and keeps it from collapsing inward.
• Aqueous humor. Similar to blood plasma, continually secreted by the choroid. It maintains intraocular pressure.
• Canal of Schlemm. Aqueous humor drains into venous blood through the scleral venous sinus (canal of Schlemm) at the sclera-cornea junction.

Eye Reflexes

Both external and internal eye muscles are needed for proper function.

• Photopupillary reflex. Sudden bright light makes the pupils constrict at once, protecting the photoreceptors.
• Accommodation pupillary reflex. The pupils also constrict when viewing close objects, sharpening near vision.

The Ear: Hearing and Balance

The ear handles hearing and balance.

Anatomy of the Ear

The ear has three areas: external (outer), middle, and internal (inner).

External (Outer) Ear

Made of the auricle and external acoustic meatus.

• Auricle. The pinna, the shell-shaped structure around the canal opening, what most people call the "ear."
• External acoustic meatus. A short, narrow chamber in the temporal bone. Its ceruminous glands secrete waxy yellow cerumen (earwax) that traps foreign bodies and repels insects.
• Tympanic membrane. The eardrum. Sound waves vibrate it, and it separates the external from the middle ear.

Middle Ear

The middle ear (tympanic cavity) is a small, air-filled, mucosa-lined cavity in the temporal bone.

• Openings. Bounded laterally by the eardrum and medially by a bony wall with two openings, the oval window and the membrane-covered round window.
• Pharyngotympanic tube. Runs obliquely down to link the middle ear with the throat, with continuous mucosa between them.
• Ossicles. The three smallest bones in the body span the cavity and transmit eardrum vibration to the inner-ear fluids: hammer (malleus), anvil (incus), and stirrup (stapes).

Internal (Inner) Ear

The inner ear is a maze of bony chambers, the bony (osseous) labyrinth, deep in the temporal bone.

• Subdivisions. Three parts: the pea-sized cochlea, the vestibule, and the semicircular canals.
• Perilymph. A plasma-like fluid filling the bony labyrinth.
• Membranous labyrinth. A system of membrane sacs suspended in the perilymph, following the shape of the bony labyrinth.
• Endolymph. The thicker fluid inside the membranous labyrinth.

Chemical Senses: Taste and Smell

Taste and olfaction receptors are chemoreceptors, responding to chemicals in solution.

Olfactory Receptors and Smell

The human nose still picks up small differences in odors.

• Olfactory receptors. Thousands of them occupy a postage-stamp-sized area in the roof of each nasal cavity.
• Olfactory receptor cells. Neurons with olfactory hairs (long cilia) that protrude from the nasal epithelium and are bathed in mucus.
• Olfactory filaments. When chemicals dissolved in the mucus stimulate the cilia, impulses travel along the olfactory filaments, bundled axons that make up the olfactory nerve.
• Olfactory nerve. Carries the impulses to the olfactory cortex.

Taste Buds and Taste

The word taste comes from the Latin taxare, "to touch, estimate, or judge."

• Taste buds. The receptors for taste, scattered through the oral cavity. Of the 10,000 or so we have, most are on the tongue.
• Papillae. Small peg-like projections covering the dorsal tongue.
• Circumvallate and fungiform papillae. Taste buds line the sides of the large round circumvallate papillae and the tops of the more numerous fungiform papillae.
• Gustatory cells. Epithelial cells that respond to chemicals dissolved in saliva.
• Gustatory hairs. Long microvilli that protrude through the taste pore. When stimulated, they depolarize and send impulses to the brain.
• Facial nerve. The facial nerve (VII) serves the anterior tongue.
• Glossopharyngeal and vagus nerves. Serve the other taste-bud areas.
• Basal cells. Taste bud cells are among the most dynamic in the body, replaced every 7 to 10 days by basal cells deep in the taste buds.

Physiology of the Special Senses

Pathway of Light and Refraction

When light passes between substances of different density, its speed changes and its rays bend (refract).

• Refraction. The refractive power of the cornea and humors is constant, but the lens can change shape, becoming more or less convex, to focus light on the retina.
• Lens. More convexity bends light more; a flatter lens bends it less.
• Resting eye. Set for distant vision. Light from far away arrives as parallel rays, so the lens needs no shape change.
• Light divergence. Light from a close object scatters and diverges, so the lens must bulge more. The ciliary body contracts, letting the lens become more convex.
• Accommodation. The eye's ability to focus on close objects (those less than 20 feet away).
• Real image. The retinal image is real: reversed left to right, upside down, and smaller than the object.

Visual Fields and Pathways to the Brain

Retinal axons bundle at the back of the eyeball and leave as the optic nerve.

• Optic chiasma. Fibers from the medial side of each eye cross to the opposite side of the brain.
• Optic tracts. Each tract carries fibers from the lateral side of the same-side eye and the medial side of the opposite eye.
• Optic radiation. Tract fibers synapse in the thalamus, whose axons form the optic radiation running to the occipital lobe, where visual interpretation occurs.
• Visual input. Each side of the brain gets input from both eyes: the lateral field of its own-side eye and the medial field of the other.
• Visual fields. Each eye sees a slightly different view, but the fields overlap heavily. The result is binocular vision and depth perception, as the visual cortex fuses the two images.

Mechanisms of Equilibrium

The inner ear's equilibrium receptors (the vestibular apparatus) split into two arms: static equilibrium and dynamic equilibrium.

Static Equilibrium

Receptors called maculae in the vestibule sacs drive static equilibrium.

• Maculae. Report head position relative to gravity when the body is still.
• Otolithic hair membrane. Each macula is a patch of hair cells with hairs embedded in a jelly-like mass studded with otoliths, tiny calcium-salt stones.
• Otoliths. As the head moves, otoliths roll with gravity, pulling the gel, which slides over the hair cells and bends their hairs.
• Vestibular nerve. This activates the hair cells, which signal the cerebellum via the vestibular nerve (a division of cranial nerve VIII) about head position.

Dynamic Equilibrium

The dynamic equilibrium receptors in the semicircular canals respond to angular or rotatory head movement.

• Semicircular canals. Oriented in the three planes of space, so movement in any plane is detected.
• Crista ampullaris. In the ampulla at the base of each canal, a tuft of hair cells under a gelatinous cupula.
• Head movements. When the head turns angularly, the endolymph lags behind.
• Bending of the cupula. The cupula drags against the stationary endolymph and bends like a swinging door.
• Vestibular nerve. This stimulates the hair cells, sending impulses up the vestibular nerve to the cerebellum.

Mechanism of Hearing

The route of sound through the ear and activation of the cochlear hair cells:

• Vibrations. To excite the hair cells in the organ of Corti, sound vibrations must pass through air, membranes, bone, and fluid.
• Sound transmission. The cochlea is drawn uncoiled to make the events easier to follow.
• Low frequency sound waves. Low-frequency waves below the hearing threshold travel around the cochlear duct without exciting hair cells.
• High frequency sound waves. Higher-frequency waves create pressure waves that penetrate the cochlear duct and basilar membrane to reach the scala tympani, making the basilar membrane vibrate maximally in specific areas and stimulating particular hair cells and neurons.
• Length of fibers. Fiber length tunes specific regions to specific frequencies. The highest notes, 20,000 Hertz (Hz), are detected by shorter hair cells at the base of the basilar membrane.

Age-Related Changes of the Senses

All five senses lose efficiency with age.

Vision: acuity drops and presbyopia (loss of focus and accommodation from an inflexible lens) can start as early as age 40. Loss of peripheral vision, lacrimal gland atrophy, and trouble telling apart similar colors (blues, greens, purples) are common.

Hearing: patients over age 65 develop gradual hearing loss called presbycusis, more common in men. Loss is greater at higher frequencies. Hard consonants (k, d, t) and long vowels (ay) are easier to catch, while sibilants (s, th, f) are hardest.

Taste and smell: both dull, with smell usually declining more than taste, which is why appetite changes are common in older patients.

Touch: older patients lose skin receptors gradually and become less sensitive to pain, touch, and temperature.