General and Special Senses: Sensory Receptors, Pathways, and Physiology

General and Special Senses

Overview of Sensory Receptors

Sensory receptors are specialized cells or structures that detect changes in the environment and send action potentials to the central nervous system (CNS), initiating appropriate responses. Senses are divided into two broad categories: general senses and special senses.

• General senses include pain, touch, pressure, stretching, chemical changes, cold, and heat.

• Special senses include taste, smell, vision, hearing, and equilibrium.

• Each sense depends on specific types of sensory receptors and neural pathways.

Sensation and Perception

Sensation is the conscious or subconscious awareness of a change in the internal or external environment. Perception is the conscious awareness of a sensation, determined by the area of the brain receiving the action potentials.

• For example, action potentials reaching the visual area of the occipital lobe are interpreted as visual sensations.

• A strong blow to the eye or back of the head may produce a visual sensation (flash of light), even if the stimulus is mechanical.

Types of Sensory Receptors

Sensory receptors are classified by the type of stimulus they detect:

Type	Stimulus Detected
Thermoreceptors	Temperature changes
Mechanoreceptors	Mechanical forces
Nociceptors	Tissue damage
Chemoreceptors	Chemicals
Photoreceptors	Light energy

• The greater the frequency of action potentials, the greater the intensity of the sensation.

• The frequency of action potentials sent to the brain is dependent upon the action of sensory receptors.

General Senses

Temperature

Temperature is detected by free nerve endings in the skin:

• Warm receptors: Free nerve endings in the innermost part of the dermis, sensitive to temps above 25°C (77°F).

• Cold receptors: Free nerve endings in the outermost part of the dermis, sensitive to temps below 20°C (68°F).

• Temps above 45°C (113°F) or below 10°C (50°F) result in painful sensations.

Pressure, Touch, and Stretch

These sensations are detected by mechanoreceptors:

• Free nerve endings: Found in the dermis and extended into spaces between epidermal cells; detect pain, itch, and temp.

• Tactile (Meissner) corpuscles: Located in outer dermis, especially in hairless skin (fingertips, lips, palms); detect light touch and pressure.

• Tactile (Merkel) cells: Located in the stratum basale of the epidermis; slowly adapting receptors for continuous touch and pressure.

• Lamellar (Pacinian) corpuscles: Rapidly adapting receptors for deep pressure and stretch; found in the innermost part of the dermis and in ligaments and tendons.

Baroreceptors (Pressure Sensors)

Baroreceptors are free nerve endings that detect stretching in distensible internal organs (e.g., blood vessels, stomach, bladder).

• Functions: Regulate visceral reflexes such as blood pressure, digestion, and urination.

• Example: Baroreceptors in the bladder trigger urination reflex as the bladder fills.

• Do not exhibit sensory adaptation; must remain active to regulate vital reflexes.

Proprioceptors (Body Position Sensors)

Proprioceptors are found in muscles, tendons, and joints; they detect body position and movement.

• Muscle spindles: Detect muscle stretch and trigger reflex contraction.

• Tendon organs: Detect tension during contraction to prevent overstretching.

• Functions: Maintain posture, equilibrium, and muscle tone.

• Do not exhibit sensory adaptation; continuous feedback is needed for coordination and balance.

Chemoreceptors

Chemoreceptors monitor chemical changes in body fluids and are found in areas that sense:

• Blood glucose levels

• Blood gases (O2, CO2)

• Functional: Maintain internal homeostasis

• Signals from chemoreceptors are processed below the level of the cerebral cortex and are not consciously perceived.

Nociceptors (Pain Receptors)

Nociceptors are free nerve endings found throughout most body tissues (except brain tissue).

• Especially abundant in skin

• Stimulated by tissue damage, extreme temp, or chemicals released by injured cells

Referred Pain

Referred pain occurs when pain from internal organs (visceral pain) is perceived as coming from a different body region (usually skin or limb).

• Happens because sensory neurons from organs and skin share the same nerve pathways in the spinal cord.

Examples:

• Heart attack: pain referred to the left chest, shoulder, and arm

• Gallbladder or liver pain: referred to right shoulder or right upper abdomen

• Appendix pain: starts near navel, then moves to right lower quadrant

• Ovary pain (female): referred to lower abdomen or groin

Importance: Helps in diagnosis of internal organ disorders since referred pain patterns are consistent among individuals.

Special Senses

Types of Sensory Receptors in Special Senses

• Chemoreceptors: Taste and smell (respond to chemicals)

• Mechanoreceptors: Hearing and equilibrium (respond to movement/vibration)

• Photoreceptors: Vision (respond to light energy)

Taste (Gustation)

Taste is detected by chemoreceptors located in taste buds, primarily on the tongue.

• Structure:

  • Taste buds: microscopic organs containing chemoreceptors

  • Located mainly on tongue, also palate, pharynx, and esophagus

  • Gustatory epithelial cells = taste receptor cells

  • Taste hairs (microvilli) project to detect chemicals in saliva

• Functions: Chemicals from food must dissolve in saliva to stimulate taste receptors.

• Basic tastes:

  • Sweet – sugars

  • Sour – acids (H+ ions)

  • Salty – metal ions (Na+, K+)

  • Bitter – alkaloids

  • Umami – amino acids (glutamate, savory flavor)

• Nerve pathways:

  • Facial nerve (CN VII): front 2/3 of tongue

  • Glossopharyngeal nerve (CN IX): back 1/3 of tongue

  • Vagus nerve (CN X): base of tongue and pharynx

  • Signals travel: medulla oblongata → thalamus → parietal lobe of cerebrum

Smell (Olfaction)

Smell is detected by chemoreceptors located in the olfactory epithelium at the top of the nasal cavity.

• Each receptor is a neuron with cilia that extend into the mucus layer to detect airborne chemicals.

• Odor molecules dissolve in mucus, bind to receptor, trigger action potentials sent through olfactory nerve (CN I) to olfactory bulbs, then to temporal lobe for processing.

• Humans have ~350 functional olfactory receptor types; can distinguish 2,000–4,000 odors, up to 10,000 with training.

• Women typically have a stronger sense of smell than men.

• Olfactory receptors adapt quickly to continuous odors.

• Pheromones: chemicals affecting human behavior and reproduction are detected here.

• Olfactory epithelium regenerates; receptors live ~1 month.

Hearing and Equilibrium

Hearing and equilibrium are detected by mechanoreceptors in the ear. The ear is divided into three regions:

• External ear: Funnels sound waves into the ear; includes external acoustic meatus and tympanic membrane (eardrum).

• Middle ear: Contains auditory ossicles (malleus, incus, stapes) that transmit vibrations from the eardrum to the inner ear.

• Internal ear (labyrinth): Located in the temporal bone; responsible for hearing and balance.

  • Bony labyrinth: Outer layer, filled with perilymph

  • Membranous labyrinth: Inner layer, filled with endolymph

  • Main parts: cochlear (hearing), vestibule (static equilibrium), semicircular canals (dynamic equilibrium)

Cochlea

• Coiled, snail-shaped portion of the inner ear

• Divided into three chambers: scala vestibuli (perilymph), scala tympani (perilymph), cochlear duct/scala media (endolymph)

• Separated by vestibular and basilar membranes

Spiral Organ (Organ of Corti)

• Located on the basilar membrane

• Contains cochlear hair cells (mechanoreceptors) with stereocilia that detect sound vibrations

• When sound waves move the basilar membrane, hair cells bend against the tectorial membrane, generating nerve impulses

• Signals travel via cochlear branch of CN VIII (vestibulocochlear nerve) → brainstem → thalamus → auditory cortex

Summary of Hearing Pathway

1. Sound wave → external acoustic meatus

2. Vibrate → tympanic membrane

3. Middle ear transmission: malleus → incus → stapes → inner ear (oval window)

4. Inner ear fluid movement: pressure waves in perilymph → movement of basilar and vestibular membranes

5. Hair cell activation: stereocilia bend, open ion channels, generate receptor potentials

6. Transmission to brain: action potentials travel via CN VIII to temporal lobe

Pitch and Loudness

• Pitch (frequency): Determined by which region of the basilar membrane vibrates

  • High-pitched sounds: stimulate short, stiff fibers near the base of the cochlea

• Loudness (amplitude): Determined by the intensity of vibration of the basilar membrane

  • Louder sounds = stronger vibrations = more frequent action potentials to the brain

Equilibrium (Balance)

The vestibular apparatus (in the inner ear) monitors equilibrium. Information comes from:

• Eyes – visual cues

• Proprioceptors – muscle and joint position

• Vestibular receptors – detect head position and motion

There are two types of equilibrium:

• Static equilibrium: Detects position of the head relative to gravity (stationary or in linear acceleration)

• Dynamic equilibrium: Detects rotational or angular motion (turning, spinning)

Static Equilibrium

• Structure: Maculae (sensory organs in the utricle and saccule of the vestibule); hair cells embedded in a gelatinous layer topped with otoliths (calcium carbonate crystals)

• Mechanism: When the head tilts, gravity pulls the otolithic membrane, bending the stereocilia of hair cells, stimulating the vestibular hair cells and generating action potentials

• Signals travel via vestibular branch of CN VIII to the brainstem and cerebellum

• The cerebellum helps maintain posture and subconscious balance

Dynamic Equilibrium

• Function: Detects rotational or angular motion

• Structures: Semicircular canals (anterior, posterior, lateral) positioned at right angles to each other; each canal has an enlarged base called the ampulla, which contains the crista ampullaris

• The crista ampullaris contains vestibular hair cells with stereocilia embedded in a gelatinous structure called the cupula

Additional info:

• Equations for action potential frequency and intensity:

  • $\text{Intensity} \propto \text{Frequency of Action Potentials}$

• Table of sensory receptor types and their stimuli included for comparison.