The Special Senses: Vision and Hearing

Detailed Notes

19.1 Overview of Special Senses

The special senses — vision, hearing (audition), equilibrium (balance), olfaction (smell), and gustation (taste) — rely on specialized receptor cells housed within complex sensory organs. Unlike general senses (touch, temperature, pain), which use widely distributed, structurally simple receptors, special sense receptors are concentrated in specific organs in the head. This topic focuses on the two most clinically prominent special senses: vision (photoreception) and hearing (mechanoreception of sound waves).

19.2 Vision: Accessory Structures of the Eye

The eye is protected and supported by several accessory structures that do not directly participate in photoreception but are essential for eye health and function.

19.2.1 Eyebrows and Eyelids

The eyebrows are thickened ridges of skin covered in short hairs that arch above the orbital sockets. They divert sweat and debris away from the eyes and play a role in facial expression. The eyelids (palpebrae) are mobile folds of skin, muscle, and connective tissue that cover the anterior surface of each eye. Their primary functions are to:

1. Protect the eye from foreign bodies and excessive light
2. Spread tears across the ocular surface during blinking, keeping the cornea moist

The orbicularis oculi muscle closes the eyelids; the levator palpebrae superioris opens them. The eyelashes at the lid margins are innervated by sensory nerve endings that trigger a blink reflex when touched.

19.2.2 The Conjunctiva

The conjunctiva is a thin, transparent mucous membrane that lines the inner surface of the eyelids (palpebral conjunctiva) and reflects onto the anterior surface of the eyeball (bulbar conjunctiva), covering the sclera but not the cornea. It contains goblet cells that secrete mucus to lubricate the eye. Conjunctivitis ("pink eye") is inflammation of this membrane.

19.2.3 The Lacrimal Apparatus

The lacrimal apparatus produces, distributes, and drains tears. It consists of:

• Lacrimal gland: Located in the superolateral orbit, secretes tears (lacrimal fluid) containing water, salts, lysozyme (antibacterial enzyme), and antibodies
• Lacrimal ducts: Convey tears from the gland to the eye surface
• Lacrimal puncta: Small openings on the medial margin of each eyelid that collect tears
• Lacrimal canaliculi: Drain tears from the puncta to the lacrimal sac
• Lacrimal sac → Nasolacrimal duct: Drains tears into the nasal cavity (explaining why crying makes your nose run)

19.2.4 Extrinsic Eye Muscles

Six extrinsic eye muscles attach to the outer surface of the eyeball and move it within the orbit:

19.3 Vision: The Three Tunics of the Eye

The eyeball is a hollow sphere approximately 2.5 cm in diameter, composed of three concentric tissue layers (tunics).

19.3.1 Fibrous Tunic (Outer Layer)

The fibrous tunic provides mechanical support, maintains the eye's shape, and serves as an attachment site for extrinsic muscles. It consists of two regions:

• Sclera: The white, opaque posterior five-sixths of the fibrous tunic. Composed of dense, irregular connective tissue rich in collagen fibers. It is the "white of the eye" and is continuous with the dura mater of the optic nerve.
• Cornea: The transparent anterior one-sixth. It is avascular and receives nutrients from tears and the aqueous humor. Its transparency depends on the precise, crystalline arrangement of collagen fibers and its relative dehydration. The cornea is the most powerful refractive structure in the eye, providing approximately two-thirds of the eye's total light-bending power. The corneal reflex (blinking in response to corneal touch) is mediated by CN V (Trigeminal, sensory) and CN VII (Facial, motor).

19.3.2 Vascular Tunic (Middle Layer) — Uvea

The vascular tunic, also called the uvea, is a highly vascularized, pigmented layer that supplies blood to the eye's tissues. It has three components:

1. Choroid: A darkly pigmented membrane lining the posterior five-sixths of the inner surface of the sclera. Its rich capillary bed supplies the outer layers of the retina. The dark melanin pigment absorbs scattered light, preventing internal reflection that would blur the image.

1. Ciliary body: A thickened, ring-shaped structure encircling the lens, composed of:
   • Ciliary muscle: A ring of smooth muscle that controls lens shape for focusing (accommodation)
   • Ciliary processes: Fingerlike projections that secrete aqueous humor
   • Suspensory ligaments (zonule fibers): Extend from the ciliary processes to the lens, holding it in place

1. Iris: The colored portion of the eye, a thin, contractile diaphragm with a central opening called the pupil. It contains two sets of smooth muscle:
   • Sphincter pupillae (circular): Constricts the pupil in bright light (parasympathetic, CN III)
   • Dilator pupillae (radial): Dilates the pupil in dim light or sympathetic stimulation

19.3.3 Neural Tunic (Inner Layer) — Retina

The retina is the innermost tunic, consisting of two layers:

• Pigmented layer (outer): Absorbs light, stores vitamin A for photoreceptor recycling
• Neural layer (inner): Houses the photoreceptors and processing neurons

The neural retina is transparent except where light-sensitive pigments are present. The retina's layered organization is counterintuitive: light must pass through several layers of neurons to reach the photoreceptors at the back, then the signal travels back forward to the ganglion cells, whose axons form the optic nerve.

19.4 Vision: Chambers of the Eye and Aqueous Humor

The internal cavity of the eye is divided into three compartments:

1. Anterior chamber: The space between the cornea and the iris, filled with aqueous humor
2. Posterior chamber: The small space between the iris and the lens, also filled with aqueous humor
3. Vitreous chamber: The large posterior cavity behind the lens, filled with vitreous body (vitreous humor), a gelatinous substance that maintains eye shape, holds the retina in place, and transmits light

Aqueous humor is a watery, plasma-derived fluid produced by the ciliary processes. Its circulation pathway:

1. Secreted by ciliary processes into the posterior chamber
2. Flows through the pupil into the anterior chamber
3. Drains through the trabecular meshwork into the canal of Schlemm (scleral venous sinus)
4. Returns to the venous circulation

Aqueous humor nourishes the avascular cornea and lens. Impaired drainage elevates intraocular pressure, causing glaucoma, which can damage the optic nerve.

19.5 Vision: The Lens and Accommodation

The lens is a transparent, biconvex, avascular structure held in place by suspensory ligaments attached to the ciliary body. It provides the remaining one-third of the eye's refractive power and, uniquely, can change shape to focus on near or far objects — a process called accommodation.

Mechanism of Accommodation

1. Distant vision (default state): The ciliary muscle is relaxed → suspensory ligaments are taut → lens is flattened (least convex) → low refractive power.
2. Near vision: The ciliary muscle contracts (parasympathetic, CN III) → the ciliary body moves forward and inward → suspensory ligaments slacken → the lens rounds up (becomes more convex) → increased refractive power → near objects focused on the retina.

The closest point an object can be placed and still seen clearly is the near point of accommodation. This point moves farther away with age as the lens loses elasticity (presbyopia).

19.6 Vision: The Retina — Layers and Photoreceptors

19.6.1 Retinal Neuron Layers

The neural retina contains three major layers of neurons (from outer to inner):

1. Photoreceptor layer: Rods and cones — the cells that transduce light into electrical signals
2. Bipolar cell layer: First-order neurons that relay signals from photoreceptors
3. Ganglion cell layer: Second-order neurons whose axons form the optic nerve (CN II)

Between these layers, horizontal cells and amacrine cells provide lateral inhibition and integration, enhancing contrast and edge detection.

19.6.2 Photoreceptors: Rods and Cones

The fovea centralis is a small depression at the center of the macula lutea, the region of highest visual acuity. It contains only cones, and the overlying neural layers are pushed aside so light reaches the cones directly.

Rhodopsin (visual purple) is the photopigment in rods. It consists of opsin (a G-protein-coupled receptor protein) and retinal (a vitamin A derivative). When a photon strikes rhodopsin, retinal isomerises from 11-cis to all-trans configuration, triggering a G-protein signaling cascade:

1. Light → retinal isomerization → opsin activation (now bleached rhodopsin)
2. Activated opsin → transducin (G-protein) activation
3. Transducin → phosphodiesterase (PDE) activation
4. PDE hydrolyses cGMP → cGMP levels fall
5. cGMP-gated Na⁺ channels close → rod hyperpolarises
6. Decreased glutamate release at the synapse → signal transmitted to bipolar cells

This pathway is a hyperpolarizing signal — rods release less neurotransmitter when stimulated by light, the opposite of most sensory receptors.

19.7 Vision: The Visual Pathway

The visual pathway carries signals from the retina to the primary visual cortex:

1. Photoreceptors (rods/cones) → Bipolar cells → Ganglion cells
2. Ganglion cell axons converge at the optic disc (blind spot) → Optic nerve (CN II)
3. At the optic chiasm, fibers from the nasal half of each retina cross to the opposite side; fibers from the temporal half remain ipsilateral. This arrangement ensures that the left visual field from both eyes is processed by the right hemisphere, and vice versa.
4. Beyond the chiasm, axons continue as the optic tract → synapse in the lateral geniculate nucleus (LGN) of the thalamus
5. LGN neurons project via the optic radiations → primary visual cortex (V1, striate cortex) in the occipital lobe

Collateral fibers from the optic tract project to the superior colliculus (visual reflexes) and the pretectal nucleus (pupillary light reflex).

Table 19.1: Visual Pathway Summary

19.8 Vision: Common Disorders

19.9 Hearing: The External Ear

The ear is divided into three anatomical regions: external, middle, and inner. The external ear collects sound waves and directs them inward.

1. Auricle (pinna): The visible, elastic-cartilage flap on the side of the head. Its curved shape funnels sound waves into the external auditory canal and helps with sound localization (determining direction of sound source).

1. External auditory canal (meatus): A 2.5 cm tube extending from the auricle to the tympanic membrane. Its outer third is cartilaginous; the inner two-thirds passes through the temporal bone. Ceruminous glands in the canal wall secrete cerumen (earwax), which traps debris and repels insects.

1. Tympanic membrane (eardrum): A thin, semitransparent membrane stretched across the medial end of the external auditory canal. It vibrates in response to sound waves and transmits these vibrations to the middle ear ossicles. It separates the external ear from the middle ear cavity.

19.10 Hearing: The Middle Ear

The middle ear (tympanic cavity) is an air-filled space within the temporal bone. It contains the auditory ossicles and connects to the nasopharynx via the auditory tube.

19.10.1 Auditory Ossicles

Three tiny bones — the smallest in the human body — form a lever system that transmits and amplifies vibrations from the tympanic membrane to the inner ear:

1. Malleus (hammer): Attached to the inner surface of the tympanic membrane at its handle. Its head articulates with the incus.
2. Incus (anvil): The middle bone; articulates with the malleus laterally and the stapes medially.
3. Stapes (stirrup): The footplate of the stapes fits into the oval window, the membrane-covered opening into the inner ear.

The ossicular chain amplifies sound by approximately 22×, overcoming the impedance mismatch between air (external/middle ear) and fluid (inner ear). Two mechanisms contribute:

• Lever action: The malleus handle is longer than the incus long process, providing mechanical advantage (~1.3×)
• Area ratio: The tympanic membrane's surface area is ~17× larger than the oval window's — the same force concentrated over a smaller area increases pressure

19.10.2 Oval Window and Round Window

• Oval window: The opening where the stapes footplate transmits vibrations into the fluid-filled cochlea
• Round window: A second, membrane-covered opening below the oval window. It bulges outward when the oval window is pushed inward, relieving pressure in the incompressible cochlear fluid

19.10.3 Auditory (Eustachian) Tube

The auditory tube connects the middle ear cavity to the nasopharynx. It is normally closed but opens during swallowing, yawning, or chewing to equalize air pressure on both sides of the tympanic membrane. Blockage of this tube (as in a cold) leads to pressure imbalance and muffled hearing.

Two small skeletal muscles in the middle ear protect against excessively loud sounds:

• Tensor tympani (CN V): Pulls the malleus medially, tensing the tympanic membrane
• Stapedius (CN VII): Pulls the stapes away from the oval window

This attenuation reflex dampens loud sounds, especially low-frequency ones, but has a latency of ~40 ms, so it cannot protect against sudden impulsive sounds like gunshots.

19.11 Hearing: The Inner Ear

The inner ear (labyrinth) is housed within the petrous part of the temporal bone. It consists of two major divisions: the bony labyrinth and the membranous labyrinth.

19.11.1 Bony vs. Membranous Labyrinth

• Bony labyrinth: A series of interconnected cavities and canals within the temporal bone, filled with perilymph (similar to extracellular fluid, high Na⁺, low K⁺)
• Membranous labyrinth: A continuous system of membranous tubes and sacs suspended within the bony labyrinth, filled with endolymph (similar to intracellular fluid, high K⁺, low Na⁺)

The membranous labyrinth is surrounded by perilymph and floats within it. This arrangement electrically isolates the receptor cells.

The labyrinth has three anatomical regions:

1. Vestibule: Central chamber containing the utricle and saccule (balance organs)
2. Semicircular canals: Three loop-shaped canals for rotational equilibrium
3. Cochlea: The hearing organ, a spiral-shaped canal

19.11.2 Cochlear Anatomy

The cochlea is a coiled tube that spirals about 2.5 turns around a bony core called the modiolus. In cross-section, the cochlear duct is divided into three parallel, fluid-filled compartments (scalae):

At the apex of the cochlea, the scala vestibuli and scala tympani communicate through a small opening called the helicotrema.

19.12 Hearing: The Organ of Corti and Sound Transduction

19.12.1 The Organ of Corti

The organ of Corti (spiral organ) is the receptor organ for hearing, resting on the basilar membrane within the scala media. Its key components:

• Hair cells: The auditory receptor cells. They are called hair cells because of the stereocilia (long microvilli) projecting from their apical surface. There are two types:
  • Inner hair cells (~3,500): Arranged in a single row; the primary sensory receptors. About 95% of afferent cochlear nerve fibers synapse on inner hair cells.
  • Outer hair cells (~12,000): Arranged in three rows; amplify and fine-tune basilar membrane vibrations (cochlear amplifier).
• Supporting cells: Provide structural support
• Tectorial membrane: A gelatinous shelf that overlies the hair cells. The stereocilia of hair cells are embedded in or touch the tectorial membrane.

19.12.2 Mechanism of Sound Transduction

Sound transduction in the cochlea is a mechanoelectrical process:

1. Sound waves → tympanic membrane vibrates → ossicles transmit vibration to the oval window
2. Stapes pushes the oval window inward → pressure wave travels through the perilymph of the scala vestibuli
3. The pressure wave propagates along the basilar membrane as a traveling wave
4. The basilar membrane is tonotopically organized: its stiffness varies along its length. High-frequency sounds produce maximal displacement near the base (stiff, narrow); low-frequency sounds produce maximal displacement near the apex (flexible, wide)
5. When the basilar membrane moves upward, stereocilia of hair cells are deflected against the tectorial membrane
6. Stereocilia deflection opens mechanically gated K⁺ channels at the stereocilia tips (the tip links)
7. K⁺ enters the hair cell from the K⁺-rich endolymph → depolarization
8. Depolarization opens voltage-gated Ca²⁺ channels → Ca²⁺ influx triggers exocytosis of glutamate from hair cell base
9. Glutamate excites the afferent cochlear nerve fibers → action potentials travel to the brainstem

The endolymph's uniquely high K⁺ concentration is maintained by the stria vascularis, a specialized epithelium in the lateral wall of the scala media.

19.13 Hearing: The Auditory Pathway

The central auditory pathway conveys signals from the cochlea to the auditory cortex:

1. Cochlear nerve (part of CN VIII, Vestibulocochlear): First-order neurons whose cell bodies are in the spiral ganglion of the cochlea. They synapse in the cochlear nuclei of the medulla oblongata.
2. Most fibers from the cochlear nuclei decussate (cross over) to the opposite side at the level of the medulla, forming the trapezoid body, and ascend in the lateral lemniscus. Some fibers remain ipsilateral.
3. Many fibers synapse in the superior olivary nucleus of the pons. This nucleus is the first site of binaural integration — it compares input from both ears, which is essential for sound localization.
4. Fibers ascend via the lateral lemniscus to the inferior colliculus of the midbrain. The inferior colliculus processes auditory information and mediates reflexive head-turning toward sounds.
5. From the inferior colliculus, fibers project to the medial geniculate nucleus (MGN) of the thalamus.
6. MGN neurons project via the auditory radiations to the primary auditory cortex (A1, transverse temporal gyri of Heschl) in the superior temporal lobe.

There is extensive bilateral representation at every level above the cochlear nuclei, so unilateral cortical damage does not cause deafness in one ear — it impairs sound localization and complex auditory processing.

Table 19.2: Auditory Pathway Summary

19.14 Hearing: Pitch and Loudness Encoding

The auditory system encodes sound frequency (pitch) and intensity (loudness) through distinct mechanisms.

19.14.1 Pitch (Frequency) Encoding

Two complementary theories explain how we perceive pitch:

1. Place theory (von Békésy): Different frequencies stimulate different regions of the basilar membrane. High frequencies maximally displace the stiff, narrow base; low frequencies maximally displace the wide, flexible apex. The brain interprets which hair cells are firing as the pitch. This is the dominant mechanism for frequencies above ~4,000 Hz.

1. Temporal (frequency) theory: For lower frequencies (below ~4,000 Hz), the cochlear nerve fires action potentials phase-locked to the sound wave. The brain interprets the firing rate as the pitch. Below ~1,000 Hz, individual fibers can fire with each cycle (volley principle); above this, groups of fibers fire in volleys.

19.14.2 Loudness (Intensity) Encoding

Loudness is encoded by:

1. Firing rate: Louder sounds produce larger amplitude basilar membrane displacement → more stereocilia deflection → more depolarization → higher action potential frequency in cochlear nerve fibers
2. Recruitment: Louder sounds recruit additional hair cells and nerve fibers beyond those at the peak displacement region
3. The dynamic range of human hearing spans approximately 120 dB (from threshold to pain)