Sensory Physiology

• Afferent Nervous System


  • Receptor/Sensor
  • Visceral Afferents
  • Sensory Afferents
• Visceral Afferents


  Subconscious input, e.g. blood pressure, CO2 concentration
• Sensory Afferents


  Conscious awareness, somatic (body) sensations
• Special Senses


  • Eyes
  • Ears
  • Tongue
  • Nose
• Sensory cells


  • Have ion channels and receptor proteins with specific modalities
• Transduction


  Conversion of one form of energy (external stimulus) into electrical energy, often due to actions of receptor proteins
• Types of Gated Channels

  • Mechanically-gated
  • Chemically-gated
  • Voltage-gated
  • Thermally-gated
• Classification of Sensors or Receptors

  • Mechanoreceptors
  • Chemoreceptors
  • Thermoreceptors
  • Photoreceptors
  • Electroreceptors
  • Magnetoreceptors
  • Nocireceptors
• Roles of Sensors or Receptors

  • Interoreceptors
  • Proprioceptors
  • Exteroreceptors
• Receptor Cell Physiology


  • Receptors may be specialized endings of an afferent neuron or separate cells closely associated with peripheral ending of a neuron
  • Stimulus alters receptor's permeability which leads to graded receptor potential
  • Usually causes nonselective opening of all small ion channels
  • This change in membrane permeability can lead to the influx of sodium ions
  • The magnitude of the receptor potential represents the intensity of the stimulus
  • A receptor potential of sufficient magnitude can produce an action potential which is propagated along an afferent fiber to the CNS
  • May adapt slowly or rapidly to sustained stimulation
• Types of receptors according to their speed of adaptation


  • Tonic receptors (do not adapt at all or adapt slowly, e.g. muscle stretch receptors, joint proprioceptors)
  • Phasic receptors (rapidly adapting, e.g. tactile receptors in skin)
• Uses of Information from Receptors

  • Afferent input is essential for the control of efferent output
  • Processing of sensory input by the reticular activating system in the brain stem is critical for cortical arousal and consciousness
  • Central processing of sensory information gives rise to our perceptions of the world around us
  • Selected information delivered to the CNS may be stored for future reference
  • Sensory stimuli can have a profound impact on our emotions
• Mechanoreception


  Detection of physical forces
• Mechanoreceptors


  • Integument
  • Muscles, tendons, joints
  • Statocysts
  • Lateral line system
  • Vestibular apparatus
  • Cochlea
• Unencapsulated mechanoreceptors in the integument


  • Merkel Cells
  • Free Nerve Endings
  • Root Hair Plexuses
• Encapsulated mechanoreceptors in the integument


  • Meissner Corpuscles
  • Pacinian Corpuscles
  • Krause End Bulbs
  • Ruffini Corpuscles
• Proprioception


  Detection of motion and position
• Statocyst


  Gravity receptors, containing statoliths (grains of CaCO3)
• Lateral Line System


  Somesthetic and proprioceptive mechanoreceptor system, containing neuromast organs with hair cells
• Vestibular Apparatus


  Proprioceptive structure essential for equilibrium and posture
• Semicircular Canals


  Detect rotational or angular acceleration or deceleration of the head (equilibrium)
• Otolith Organs


  Provide position of the head relative to gravity and changes in rate of linear motion, including the utricle (detect horizontally directed linear acceleration/deceleration) and saccule (detect vertically directed linear acceleration/deceleration), containing otoliths (CaCO3)
• Chemoreception


  Detecting chemicals to generate neural signals
• Chemoreceptors


  • Exteroreceptors
  • Interoreceptors
• Gustation (taste)


  Sensilla (specialized projections from the cuticle) and taste buds (chemoreceptors)
• Tastant


  Substance that alters the cell's ionic channels to produce a depolarizing receptor potential
• Transduction Pathways for Taste

  • Salt Taste
  • Sour Taste
  • Sweet Taste
  • Bitter Taste
  • Umami Taste
• Olfaction (smell)


  Olfactory receptors are specialized endings of renewable afferent neurons
• Odorant


  Sufficiently volatile that some of its molecules can enter nose in inspired air, and sufficiently water soluble that it can dissolve in mucus coating the olfactory mucosa
• Olfactory Transduction and Processing

  • Odors are dissected into components, each detected by one of a thousand receptors
  • Spatial summation
  • G-protein coupled mechanism
  • Depolarization
  • Processing in the olfactory bulb, glomeruli, "smell files", mitral cells, olfactory tract, subcortical and thalamic-cortical routes
• Olfactory Adaptation


  • Reduced sensitivity or habituation due to intrinsic receptor adaptation and extrinsic adaptation in the CNS
  • Lingering of smell due to "odor-eating" enzymes
• Vomeronasal Organ (VNO)


  In mammals including humans, governs reproductive and social behaviors, detects pheromones
• Photoreception


  Light detection uses universal photopigments
• Types of Photoreceptors


  • Ciliary (e.g. rod cells)
  • Rhabdomeric
• Light-sensing Organs


  • Eyespots
  • Pinhole eye
  • Camera eye
  • Compound eye
• Light


  • Electromagnetic radiation with intensity (amplitude or height) and light rays (forward movement in a particular direction)
  • Refraction (bending of light rays)
• Vertebrate Eyes


  • Sensory organ for vision
  • Mechanisms that help protect eyes from injury, including eyelids, eyelashes, tears
  • Spherical, fluid-filled structure enclosed by three tissue layers: sclera/cornea, choroid/ciliary body/iris, retina
• The interior of the eye consists of two fluid-filled cavities separated by the lens, with the posterior cavity containing the vitreous humor
• Eyelashes


  • Act like shutters to protect eye from environmental hazards
• Eyelashes


  • Trap fine, airborne debris such as dust before it can fall into eye