The General and Special Senses

The Core Distinction

Key Concepts Shared by All Sensory Receptors

Receptive field: The area monitored by a single receptor cell. Smaller receptive fields = more precise localization. Fingertips and tongue have fields less than 1 mm in diameter. General body skin receptors may have fields 7 cm in diameter.

Sensation vs Perception: Sensory information arrives at the CNS as action potentials — this is a sensation. When the brain consciously interprets it, this is a perception. The CNS interprets the nature of information based entirely on which brain area is stimulated — not the stimulus itself. Rubbing your eyes produces "flashes" even though the stimulus is mechanical, because the optic pathway is activated.

Adaptation: A reduction in sensitivity in the presence of a constant stimulus. Involves changes in receptor sensitivity or inhibition along sensory pathways. Most sensory information is routed to spinal cord or brain stem; only about 1 percent of afferent fiber information reaches the cerebral cortex and conscious awareness.

1. Nociceptors (Pain)

Free nerve endings. Most common in superficial skin, joint capsules, periostea of bones, and blood vessel walls. Most visceral organs have few nociceptors. Large receptive fields — difficult to localize source of pain.

Stimuli: extremes of temperature, mechanical damage, dissolved chemicals from injured cells. All produce a similar sensation ("burning") regardless of source.

Referred pain: Sensation of pain in a body surface region that is not actually stimulated — because visceral and somatic afferents share the same spinal nerves. Example: cardiac pain perceived as left arm and upper chest.

2. Thermoreceptors (Temperature)

Free nerve endings in the dermis, skeletal muscles, liver, and hypothalamus. Cold receptors are 3–4 times more common than warm receptors. No known structural differences between warm and cold receptors. Temperature sensations travel the same pathways as pain sensations. Adapt rapidly to stable temperatures.

3. Mechanoreceptors (Touch, Pressure, Position)

Respond to physical distortion (stretching, compression, twisting) — distortion opens mechanically regulated ion channels.

A — Tactile Receptors (Touch/Pressure/Vibration)

B — Baroreceptors (Pressure) — free nerve endings branching in walls of distensible organs. Monitor blood pressure (carotid sinus, aortic sinus), lung expansion (sets respiratory pace), digestive and urinary tracts (trigger reflexes for urination, peristalsis, defecation). Adapt rapidly.

C — Proprioceptors (Position) — monitor joint position, tendon tension, and muscle length. Do NOT adapt. Most information processed subconsciously.

• Free nerve endings in joint capsules — detect pressure, tension, movement
• Golgi tendon organs — between skeletal muscle and tendon; monitor tension/strain during contraction
• Muscle spindles — monitor skeletal muscle length; trigger stretch reflexes

4. Chemoreceptors (Chemical Stimuli)

Respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid. Adapt within seconds. Send information to brain stem centers controlling respiratory and cardiovascular function.

• Respiratory centers (medulla oblongata): respond to pH and carbon dioxide concentration in cerebrospinal fluid
• Carotid bodies: near origin of internal carotid arteries; monitor blood pH, carbon dioxide, and oxygen; afferents travel via N IX
• Aortic bodies: between aortic arch branches; same monitoring role; afferents via N X

Olfactory Organ Structure

Paired olfactory organs located in the nasal cavity on either side of the nasal septum. Each organ contains an olfactory epithelium with three cell types:

• Olfactory receptor cells: Modified neurons. Free surface provides base for cilia extending into mucus. Odorant-binding proteins on cilia surfaces are the actual receptors.
• Supporting cells: Structural support and maintenance
• Basal cells (stem cells): Divide to replace worn-out receptor cells — olfactory receptors are regularly replaced throughout life

Olfactory glands in underlying areolar tissue secrete a pigmented mucus that keeps the epithelium moist and prevents buildup of overpowering stimuli. Approximately 10–20 million olfactory receptor cells packed into roughly 5 cm² of epithelium.

Mechanism of Olfactory Reception

A normal relaxed inhalation carries about 2 percent of inhaled air to olfactory organs. Sniffing increases flow across the epithelium, intensifying stimulation.

Olfactory Pathway

Taste Receptor Anatomy

Gustatory receptors distributed over the tongue and adjacent portions of pharynx and larynx. Most important receptors are on the tongue. Adult pharyngeal/laryngeal receptors decrease in importance with age.

Papillae: Epithelial projections on the tongue surface. Taste buds lie along the sides of papillae — protected from mechanical stress of chewing. Circumvallate papillae (largest) form a V pointing toward the base of the tongue and contain the greatest number of taste buds.

Each taste bud contains:

• Gustatory cells: Slender sensory receptors; extend microvilli called taste hairs through a narrow taste pore into the surrounding fluid
• Supporting cells: Structural support

Mechanism of Gustatory Reception

Dissolved chemicals contact taste hairs → change membrane potential of gustatory cell → action potentials in sensory neuron. Parallels olfaction — chemical must be dissolved to activate receptor. If you dry the tongue completely, you cannot taste anything placed on it.

The Six Primary Taste Sensations

Gustatory Pathway

Taste buds monitored by three cranial nerves: N VII (facial) — anterior 2/3 of tongue; N IX (glossopharyngeal) — posterior 1/3; N X (vagus) — pharyngeal/laryngeal taste receptors.

Pathway: Afferent fibers → synapse in nucleus of medulla oblongata → thalamus → primary sensory cortex. Trigeminal nerve (N V) provides additional information about food texture and "peppery/spicy" sensations.

Accessory Structures

Three Layers of the Eyeball

Layer 1 — Fibrous Layer (outermost):

Layer 2 — Vascular Layer (middle):

Layer 3 — Inner Layer (retina):

Chambers of the Eye

Aqueous humor: Secreted by epithelial cells of ciliary processes into posterior chamber → flows through pupil → anterior chamber → drains through scleral venous sinus (canal of Schlemm) into scleral veins. Blockage → elevated pressure → glaucoma → retinal and optic disc distortion → blindness.

The Lens and Accommodation

Held by suspensory ligaments from ciliary body. Primary function: focus image on photoreceptors by changing shape.

Light and Refraction

Visible light wavelength: 400–700 nm. ROY G BIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet). Red = longest wavelength, least energy. Violet = shortest wavelength, most energy.

Light bends (refracts) when it passes from one medium to another of different density. In the eye: greatest refraction at the cornea (air to cornea density transition). The lens provides fine-tuning refraction to focus on the retina.

Focal point: where light rays converge. Focal distance: distance from center of lens to focal point. Closer object → longer focal distance needed → rounder lens. Rounder lens → shorter focal distance.

Accommodation and Vision Problems

Note: The eye focuses by changing lens shape — not by moving the lens toward or away from the retina. This distinguishes eye accommodation from camera focusing.

Image Formation on the Retina

The image projected onto the retina is inverted (upside down) and reversed (left-right mirror). Light from the top of an object hits the bottom of the retina; light from the left side hits the right side. The brain corrects both reversals without conscious awareness.

Rods and Cones — Photoreception

Each photoreceptor has an outer segment (hundreds of flattened membranous discs containing visual pigments) and an inner segment (organelles; releases neurotransmitters).

Visual pigments derive from rhodopsin = opsin (protein) + retinal (pigment synthesized from vitamin A). Retinal is identical in rods and cones; opsin differs between the two.

Color Vision and Color Blindness

Three cone types: blue, green, and red — each sensitive to different wavelengths. Color perception results from the combination of stimulation across all three cone types.

Color blindness: one or more cone types absent or nonfunctional. Most common: red-green color blindness (red cones absent). Prevalence: 10% of males, 0.67% of females. Total color blindness (no cone pigments): 1 in 300,000. X-linked inheritance explains sex-linked prevalence pattern.

The Visual Pathway — Step by Step

Collateral Visual Processing Destinations

Visual Cortex Map

The visual cortex of each occipital lobe contains a sensory map of the entire visual field. As with the motor/sensory homunculus, the map is not proportionally accurate: the area assigned to the fovea covers about 35 times the surface it would if the map were proportional — reflecting the dense cone packing of the fovea.

The Hair Cell — Universal Internal Ear Receptor

Hair cells are the basic receptor unit of the entire internal ear (both equilibrium and hearing). Each hair cell supports 80–100 stereocilia. Hair cells do not actively move; when external forces displace stereocilia, the cell surface distorts and neurotransmitter release changes.

Displacement in one direction → stimulates the hair cell → more neurotransmitter. Displacement in the opposite direction → inhibits the hair cell → less neurotransmitter.

Dynamic Equilibrium — Semicircular Ducts

Monitors rotational movements of the head. Three semicircular ducts (anterior, posterior, lateral) — each responds to rotation in one plane:

• Lateral duct: horizontal rotation ("no" head shake)
• Anterior duct: nodding ("yes" head movement)
• Posterior duct: side-to-side tilt

Each duct contains a swollen region called the ampulla. Hair cells attached to the ampulla wall form a raised structure: the crista ampullaris. Stereocilia are embedded in a gelatinous mass called the cupula that nearly fills the ampulla.

Mechanism: Head rotates in the plane of a duct → endolymph flows along the axis of that duct → pushes against the cupula → cupula deflects → stereocilia bend → receptor stimulated. Endolymph must flow along the axis — only rotation in the duct's plane achieves this.

Static Equilibrium — Utricle and Saccule

Monitors gravity and linear acceleration. Located in the vestibule (paired membranous sacs).

• Utricle: sensitive to horizontal acceleration
• Saccule: sensitive to vertical acceleration

Hair cells cluster in oval regions called maculae. Stereocilia are embedded in a gelatinous otolithic membrane whose surface is covered by a thin layer of densely packed otoliths (calcium carbonate crystals — "ear stones").

Mechanism: Head tilts → gravity pulls the heavy otoliths to the side → distorts sensory hairs → CNS receives signal that head is no longer level. Otoliths are also responsible for sensing linear acceleration — they lag behind when the body suddenly moves, bending the stereocilia.

Equilibrium Pathway

Vestibular hair cells → sensory neurons → vestibular branch of N VIII → vestibular nuclei (boundary of pons and medulla oblongata).

The vestibular nuclei then: (1) integrate information from both sides; (2) relay to the cerebellum; (3) relay to cerebral cortex (conscious position sense); (4) send motor commands to nuclei for N III, N IV, N VI, N XI (eye, head, neck movements); (5) send descending commands via vestibulospinal tracts to adjust peripheral muscle tone.

Three Ear Regions — Overview

External Ear — Structures

Middle Ear — Structures

Internal Ear — Cochlear Anatomy

The cochlea is spiral-shaped (snail shell). In cross-section, three fluid-filled chambers run its length:

The spiral organ (organ of Corti) sits on the basilar membrane (separates cochlear duct from scala tympani). Hair cells in the spiral organ have stereocilia in contact with the overlying tectorial membrane (firmly anchored to inner cochlear wall).

Basilar membrane properties: Narrow and stiff near oval window → wide and flexible at tip. High-frequency sounds vibrate the basilar membrane near the oval window. Low-frequency sounds cause maximum distortion far from the oval window (at the apex).

The 6 Steps of Hearing

Smell and Aging

Unlike most neurons, olfactory receptor cells are regularly replaced by stem cell division in the olfactory epithelium throughout life. Despite this, the total number of receptors declines with age, and remaining receptors become less sensitive. Elderly individuals need higher concentrations to detect odors — explaining why elderly people tend to apply excessive perfume or aftershave (they need more to smell it themselves).

Taste and Aging

Tasting ability declines due to: (1) thinning of mucous membranes, (2) reduced number and sensitivity of taste buds. We begin life with more than 10,000 taste buds; numbers begin declining dramatically by age 50. Combined with declining olfactory receptor numbers, elderly individuals find food tastes bland and unappealing. Children find the same food too spicy — their higher receptor numbers mean greater sensitivity.

Vision and Aging

Hearing and Aging

Hearing is generally affected less by aging than other senses. However, the tympanic membrane loses elasticity → more difficult to hear high-pitched sounds first.

Presbycusis (presbys = old man + akousis = hearing): the progressive hearing loss that occurs with aging. Loss of neuron axons conducting sensory action potentials cannot be easily compensated for.