14.1 Sensory Perception

Sensory receptors detect stimuli from the environment and from within the body, then convert those stimuli into electrical signals the nervous system can use. Understanding how each receptor type works, and how transduction differs across the senses, is central to this unit.

Sensory Receptors and Functions

Types and functions of sensory receptors

Chemoreceptors detect chemical stimuli, whether dissolved in fluid or carried in air.

• Gustatory receptors in taste buds respond to dissolved chemicals (sugars, salts, acids, amino acids), producing the sense of taste.
• Olfactory receptors in the nasal cavity bind airborne chemicals called odorants, producing the sense of smell.

Photoreceptors absorb light energy and convert it into electrical signals.

• Rods and cones in the retina detect different wavelengths and intensities of light, enabling vision.

Mechanoreceptors respond to mechanical forces like pressure, touch, stretch, and vibration.

• Hair cells in the cochlea of the inner ear detect fluid vibrations caused by sound waves, enabling hearing.
• Hair cells in the vestibular system (utricle, saccule, semicircular canals) detect head position and movement, maintaining balance and equilibrium.

Thermoreceptors detect temperature changes relative to normal body temperature.

• Cold receptors increase their firing rate when temperature drops below body temperature.
• Warm receptors increase their firing rate when temperature rises above body temperature.

Nociceptors respond to potentially damaging stimuli that could cause tissue injury. There are three subtypes:

• Mechanical nociceptors detect excessive pressure or physical deformation.
• Thermal nociceptors respond to extreme heat (above ~45°C) or extreme cold (below ~5°C).
• Chemical nociceptors detect toxic substances (like capsaicin in chili peppers) or inflammatory mediators (like prostaglandins).

Sensory adaptation is the gradual reduction in receptor response during continuous stimulation. For example, you stop noticing the pressure of clothing on your skin shortly after putting it on. Not all receptors adapt equally: nociceptors adapt very slowly, which makes sense because ongoing pain signals warn you of tissue damage.

Sensory Structures and Transduction

Structures for sensory perception

Taste (gustation) relies on taste buds, which are found on the tongue, soft palate, and epiglottis. Each taste bud contains 50–100 gustatory receptor cells. These are chemoreceptors sensitive to dissolved molecules.

Smell (olfaction) depends on olfactory receptor neurons housed in the olfactory epithelium, which lines the roof of the nasal cavity. These neurons are chemoreceptors with cilia that bind odorant molecules. The olfactory epithelium contains millions of these receptor neurons.

Hearing (audition) and balance (equilibrium) both involve mechanoreceptors in the inner ear.

• The cochlea is a coiled, fluid-filled tube that converts sound waves into electrical signals. Inside it, the organ of Corti contains hair cells that detect fluid vibrations.
• The vestibular system includes the utricle, saccule, and three semicircular canals.
  • The utricle and saccule contain hair cells that detect linear acceleration and head position.
  • The semicircular canals contain hair cells that detect rotational acceleration.

Vision relies on photoreceptors (rods and cones) in the retina.

• Rods are highly sensitive to light and handle low-light and peripheral vision.
• Cones need brighter light and are responsible for color vision and sharp detail (visual acuity).
• The fovea is a small central region of the retina packed with the highest density of cones, giving you your sharpest central vision.

Transduction in taste sensations

There are five basic taste qualities: sweet, salty, sour, bitter, and umami (savory). The transduction mechanism differs depending on the taste.

Sweet, umami, and bitter tastes use G protein-coupled receptors (GPCRs):

1. Taste molecules bind to specific GPCRs on the gustatory receptor cell membrane.
2. Binding activates a G protein called gustducin, which in turn activates adenylyl cyclase.
3. Adenylyl cyclase increases cAMP levels inside the cell, opening ion channels and depolarizing the cell.

Salty and sour tastes involve direct interaction with ion channels:

• Salty taste: $Na^+$ ions enter through epithelial sodium channels (ENaCs), directly depolarizing the cell.
• Sour taste: $H^+$ ions from acids block potassium channels ($K_{2P}$), preventing $K^+$ efflux. This keeps positive charge inside the cell, causing depolarization.

Mechanoreceptors in hearing and balance

Hearing transduction occurs in the hair cells of the cochlea:

1. Sound waves cause vibrations in the fluid-filled cochlea.
2. Vibrations bend stereocilia (hair-like projections) on the hair cells.
3. Bending opens mechanically-gated ion channels, allowing $K^+$ and $Ca^{2+}$ influx.
4. Ion influx depolarizes the hair cells, triggering neurotransmitter release onto auditory nerve fibers.
5. The auditory nerve transmits signals to the brainstem and then to the auditory cortex for processing.

Balance transduction involves hair cells in the vestibular system:

• In the utricle and saccule, otoliths (small calcium carbonate crystals) sit on top of a gel layer above the hair cells. When your head tilts or you accelerate linearly, gravity shifts the otoliths, bending the stereocilia and triggering depolarization.
• In the semicircular canals, rotational head movement causes fluid (endolymph) to push against a structure called the cupula, bending the stereocilia of embedded hair cells.
• In both cases, hair cell depolarization leads to neurotransmitter release, sending signals through the vestibular nerve to the cerebellum for processing.

Visual System and Phototransduction

Structure of the eye

• Cornea: The transparent, dome-shaped front surface that performs most of the eye's light refraction.
• Iris: The colored ring of muscle that controls pupil size, regulating how much light enters.
• Lens: A transparent, biconvex structure behind the iris that fine-focuses light onto the retina. Ciliary muscles change the lens shape for accommodation (focusing on near vs. far objects).
• Retina: The light-sensitive layer lining the back of the eye, containing the photoreceptors.
  • Fovea centralis: The small central pit with the highest cone density, responsible for sharp central vision.
• Sclera: The tough, white outer layer that protects the eye.
• Choroid: A pigmented, blood vessel-rich layer between the retina and sclera that supplies oxygen and nutrients to the outer retina.
• Optic nerve: Carries visual information from retinal ganglion cells to the brain.

Phototransduction process in vision

This is a multi-step cascade worth knowing in order, because each step triggers the next:

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.

   • Rods contain rhodopsin (the protein opsin bound to the chromophore retinal).
   • Cones contain photopsins, which have different opsin variants that respond to different wavelengths (red, green, or blue).

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

4. This shape change activates a G protein called transducin.

5. Activated transducin activates the enzyme phosphodiesterase (PDE).

6. PDE breaks down cGMP, lowering its concentration in the photoreceptor.

7. With less cGMP available, cGMP-gated $Na^+$ channels close.

8. Channel closure hyperpolarizes the photoreceptor, reducing its release of the neurotransmitter glutamate.

9. Bipolar cells and other retinal interneurons detect this change in glutamate release and process the signal.

10. Retinal ganglion cells integrate the processed signal and fire action potentials along the optic nerve to the brain.

Sensory Processing and Integration

Sensory processing refers to the full sequence of detecting a stimulus, transducing it into electrical signals, and interpreting those signals in the brain.

The sensory threshold (also called the absolute threshold) is the minimum stimulus intensity needed to produce a conscious sensation. Below this threshold, the stimulus goes undetected.

Sensory integration is how the brain combines information from multiple sensory modalities to build a coherent picture of your surroundings. For example, recognizing food involves integrating taste, smell, texture, and even visual cues simultaneously.

The sensory cortex handles processing and interpretation:

• Primary sensory areas receive input from one specific modality (e.g., primary visual cortex processes vision, primary auditory cortex processes hearing).
• Association areas integrate information across multiple modalities, allowing for more complex perception and recognition.