Sensory Receptors and Perception

• Sensory receptors convert physical stimuli into electrical signals.
• The brain processes these signals with past experiences and current mood to interpret the environment.
• Perceptions might not always be accurate reflections of reality, leading to visual illusions.

Functional Anatomy

• Sensory receptors are specialized cells, usually neurons or epithelial cells.
• They transduce stimuli into graded electrical signals known as receptor potentials.
• Receptor potentials are similar to postsynaptic potentials, varying in speed and polarity.
• Receptors respond best to particular stimuli, referred to as adequate stimulus.
• Receptors are classified by their adequate stimulus, including chemoreceptors, mechanoreceptors, photoreceptors, and thermoreceptors.
• Nociceptors, or pain receptors, can respond to multiple types of tissue-damaging stimuli.
• All receptors consist of a cell body, a zone for receiving and transducing stimuli, and an area rich in mitochondria for energy.
• Also all receptors have a zone for passing information towards the central nervous system.
• Receptors like photoreceptors and auditory/vestibular possess specialized transduction zones suited to their function

Histology

• Microvilli and cilia are adapted as transduction zones in some sensory receptors.
• Auditory and vestibular receptors use actin-filled microvilli.
• Retinal rods and cones use modified cilia with photopigment.
• Olfactory receptors have chemosensitive cilia emerging from a dendrite.
• Some somatosensory and visceral receptors use free nerve endings for transduction without any specializations.

Short vs Long Receptors

• Receptor potentials decay over short distances and, do not need to produce action potentials.
• These receptors synapse on primary afferent neurons or interneurons.
• The receptor potential alters the rate of transmitter release, influencing postsynaptic potentials.
• Examples of short receptors include taste receptor cells, retinal cells, and inner ear hair cells.
• Long receptors generate action potentials to signal over longer distances.
• The receptor potential spreads to a trigger zone, initiating action potentials that propagate to the CNS.
• Somatic sensation receptors, olfactory receptors, and most visceral receptors use long axons.
• All vertebrate long receptors depolarize and increase their firing rate in response to stimuli.

Sensory Transduction

• Sensory receptor membranes are similar to postsynaptic membranes but are sensitive to physical stimuli.
• Transduction mechanisms are categorized as ionotropic (stimulus-gated ion channels) and metabotropic (G protein-coupled).
• Ionotropic mechanisms are fast and are found in auditory, vestibular, and mechanoreceptors.
• Metabotropic mechanisms amplify signals and are in olfactory receptors and photoreceptors.
• Some taste receptor cells use a simple ionotropic mechanism where extracellular Na+ increases from salty foods cause depolarization.
• Sour tastes are transduced by H+ influx through cation channels.
• Warm and cool receptors are gated by temperature changes.

Ionotropic vs Metabotropic mechanisms

• Mechanoreceptors use channels linked to the cytoskeleton and extracellular structures, gated by deformation.
• Metabotropic receptors use second-messenger machinery.
• Photoreceptors and olfactory receptors use cyclic nucleotide-gated ion channels.
• Stimulation causes changes in cyclic nucleotide concentration through G proteins.

Control of Signaling

• Sensory reception involves stimulus delivery, transduction, and signal transmission.
• These steps are adjustable.
• Receptors adapt to maintained stimulation.
• Longer-term sensitivity changes can be triggered by Ca++ entry.
• CNS projections can adjust receptor sensitivity.
• Muscle stretch receptors and auditory receptors exemplify this adjustment.

Coding

• Sensory receptors signals judge stimuli nature, timing, intensity, and location.
• Coding involves receptor type, receptor potential duration and size and location.

Stimulus Nature

• Classical sensory modalities include touch, vision, hearing, taste, and smell.
• Additional modalities exist, such as balance, pain, and temperature with associated submodalities.
• There is a mapping between receptor type and sensory modality.
• Vestibular receptors responding to head movement induce a sensation of movement.
• Visceral receptors work behind the scenes for blood chemistry.
• The CNS compares combined information from multiple receptors to understand stimuli.

Stimulus Timing

• Stimulus duration is often encoded by receptor potential duration.
• Receptors adapt to maintained stimulus.
• Some receptors provide constant output for critical variables, others adapt to some degree.
• Adaptation can partly occur in the CNS/receptors.
• Some receptors adapt slowly.
• Others adapt quickly, encoding stimulus onset and offset.
• Rapid adaptation encodes stimulus duration of a stimulus by signaling its beginning and end.
• Receptor-level adaptation depends on receptor function aspects.
• Tissue damage can increase nociceptor sensitivity.

Stimulus Amplitude

• Stimulus intensity can be encoded by larger receptor potentials or by activating less sensitive receptors.
• Grading receptor potential sizes is more important.
• Multiple receptor populations are often used (ex: retina rods vs cones).
• Sensory threshold represents lowest detectable stimulus intensity.
• Sensory thresholds are influenced by experience, fatigue, or context.

Stimulus Location

• Orderly mapping organizes the nervous system.
• Stimulus location is coded by the location of the responding receptor.
• Photoreceptors and tactile receptors possess receptive fields.
• Localizing sounds, smells, or tastes relies on central processing.