Sensory receptors are the body's gatekeepers, detecting stimuli from our environment and within us. From taste buds to photoreceptors, these specialized cells transform physical and chemical signals into electrical impulses our brains can understand.

The sensory system is a complex network that processes and integrates information from various receptors. This intricate dance of detection, transduction, and interpretation allows us to perceive the world around us and respond appropriately to our environment.

Sensory Receptors and Functions

Types and functions of sensory receptors

• Chemoreceptors detect chemical stimuli in the environment or within the body
  • Gustatory receptors in taste buds respond to dissolved chemicals (sugars, salts, acids, amino acids) enabling the sense of taste
  • Olfactory receptors in the nasal cavity bind to airborne chemicals (odorants) allowing for the perception of smell
• Photoreceptors absorb light energy and transduce it into electrical signals
  • Rods and cones in the retina of the eye enable vision by detecting different wavelengths and intensities of light
• Mechanoreceptors respond to mechanical forces such as pressure, touch, stretch, and vibration
  • Hair cells in the cochlea of the inner ear detect fluid vibrations caused by sound waves enabling the sense of hearing
  • Hair cells in the vestibular system (utricle, saccule, semicircular canals) detect head position and movement maintaining balance and equilibrium
• Thermoreceptors detect changes in temperature relative to body temperature
  • Cold receptors increase firing rate when temperature drops below body temperature
  • Warm receptors increase firing rate when temperature rises above body temperature
• Nociceptors respond to potentially damaging stimuli that may cause tissue injury
  • Mechanical nociceptors detect excessive pressure or physical deformation
  • Thermal nociceptors respond to extreme heat (>45℃) or cold (<5℃)
  • Chemical nociceptors detect toxic substances (capsaicin in chili peppers) or inflammatory mediators (prostaglandins)
• Sensory receptors exhibit adaptation, gradually reducing their response to continuous stimuli

Sensory Structures and Transduction

Structures for sensory perception

• Taste (gustation) relies on taste buds containing gustatory receptor cells
  • Taste buds are found on the tongue, soft palate, and epiglottis
  • Each taste bud has 50-100 gustatory receptor cells that are chemoreceptors sensitive to dissolved molecules
• Smell (olfaction) depends on olfactory receptor neurons in the nasal cavity
  • Olfactory receptor neurons are chemoreceptors with cilia that bind to odorant molecules
  • Olfactory epithelium lines the roof of the nasal cavity and contains millions of olfactory receptor neurons
• Hearing (audition) and balance (equilibrium) involve mechanoreceptors in the inner ear
  • Cochlea is a coiled, fluid-filled tube that transduces sound waves into electrical signals
    • Organ of Corti within the cochlea contains hair cells that detect fluid vibrations
  • Vestibular system includes the utricle, saccule, and three semicircular canals
    • Utricle and saccule contain hair cells that detect linear acceleration and head position
    • Semicircular canals have hair cells that detect rotational acceleration
• Vision relies on photoreceptors (rods and cones) in the retina of the eye
  • Rods are highly sensitive to light and enable low-light and peripheral vision
  • Cones require brighter light and are responsible for color vision and visual acuity
  • Fovea is a small region in the center of the retina that contains the highest density of cones

Transduction in taste sensations

• Five basic taste qualities are sweet, salty, sour, bitter, and umami (savory)
• Transduction mechanisms differ for each taste quality
• Sweet, umami, and bitter tastes involve G protein-coupled receptors (GPCRs)
  1. Taste molecules bind to specific GPCRs on the gustatory receptor cell membrane
  2. Binding activates a G protein (gustducin), which activates adenylyl cyclase
  3. Adenylyl cyclase increases cAMP levels, opening ion channels and depolarizing the cell
• Salty and sour tastes involve direct interaction with ion channels
  • Salty taste: sodium ions (Na+) enter through epithelial sodium channels (ENaCs) depolarizing the cell
  • Sour taste: hydrogen ions (H+) from acids block potassium channels (K2P) preventing K+ efflux and depolarizing the cell

Mechanoreceptors in hearing and balance

• Hearing (audition) transduction occurs in hair cells of the cochlea
  1. Sound waves cause vibrations in the fluid-filled cochlea
  2. Vibrations bend stereocilia (hair-like projections) on hair cells
  3. Bending opens mechanically-gated ion channels allowing K+ and Ca2+ influx
  4. Ion influx depolarizes hair cells and releases neurotransmitter onto auditory nerve fibers
  5. Auditory nerve transmits signals to the brainstem and auditory cortex for processing
• Balance (equilibrium) transduction involves hair cells in the vestibular system
  • Utricle and saccule contain otoliths (calcium carbonate crystals) that bend hair cell stereocilia in response to linear acceleration and head position
  • Semicircular canals detect rotational acceleration as fluid movement bends hair cell stereocilia
  • Hair cell depolarization and neurotransmitter release transmit signals to the vestibular nerve and cerebellum for processing

Visual System and Phototransduction

Structure of the eye

• Cornea is the transparent, dome-shaped front part of the eye that refracts light
• Iris is the colored part of the eye that controls pupil size to regulate light entry
• Lens is a transparent, biconvex structure that focuses light onto the retina
  • Ciliary muscles control lens shape allowing for accommodation (focusing on near or far objects)
• Retina is the light-sensitive layer lining the back of the eye containing photoreceptors
  • Fovea centralis is a small region in the center of the retina with the highest density of cones providing sharp central vision
• Sclera is the white, tough, outer protective layer of the eye
• Choroid is a pigmented, vascular layer between the retina and sclera that supplies oxygen and nutrients to the outer retina
• Optic nerve carries visual information from the retina to the brain for processing

Phototransduction process in vision

1. Light enters the eye and is focused onto the retina by the cornea and lens

2. Photons are absorbed by photopigments in the outer segments of rods and cones

   • Rhodopsin in rods consists of the protein opsin bound to the chromophore retinal
   • Photopsins in cones have different opsins that confer sensitivity to different wavelengths

3. Light absorption causes retinal to change from the 11-cis to all-trans configuration

4. Retinal configuration change activates a G protein called transducin

5. Activated transducin activates the enzyme phosphodiesterase (PDE)

6. PDE hydrolyzes cGMP, reducing its concentration in the photoreceptor

7. Reduced cGMP closes cGMP-gated sodium channels in the photoreceptor membrane

8. Closure of sodium channels hyperpolarizes the photoreceptor, reducing glutamate release

9. Bipolar cells and other retinal interneurons process the signal

10. Ganglion cells integrate the processed signal and transmit action potentials along the optic nerve to the brain

Sensory Processing and Integration

• Sensory processing involves the detection, transduction, and interpretation of sensory stimuli
• Sensory threshold is the minimum stimulus intensity required to produce a sensation
• Sensory integration combines information from multiple sensory modalities in the brain
• The sensory cortex in the brain is responsible for processing and interpreting sensory information
  • Primary sensory areas receive input from specific sensory modalities
  • Association areas integrate information from multiple sensory modalities