SENSORY PHYSIOLOGY

• Sensory receptors


  Specialized cells that generate graded potentials called receptor potentials in response to a stimulus
• Major divisions of sensory receptors

  • Mechanoreceptors
  • Thermoreceptors
  • Photoreceptors
  • Chemoreceptors
  • Nociceptors
• Coding is the conversion of a stimulus into a signal that is conveyed to the central nervous system
• Information conveyed


  By both the frequency and the amplitude of the resulting signals
• Stimulus duration


  • Rapid adaptation
  • Slow adaptation
• Stimulus type

  Also known as stimulus modality. Examples: temperature, pressure, sound, light
• Stimulus intensity


  Determines the strength of the sensory signal
• Stimulus location


  Determines where the sensory signal is coming from
• Overlapping receptive fields


  • Allow the brain to localize a stimulus
• Lateral inhibition


  Enables the localization of a stimulus site for some sensory systems. Information from afferent neurons whose receptors are at the edge of a stimulus is strongly inhibited compared to information from the stimulus' center. This enhances the contrast between the center and periphery of a stimulated region, thereby increasing the brain's ability to localize a sensory input.
• Central control of afferent information

  • Sensory signals are subject to extensive modification before they reach higher levels of the central nervous system. Modification can come from inhibition, pathways descending from higher centers in the brain, synapses on the axon terminals, or via interneurons.
• Neural pathways in sensory systems

  The afferent sensory is the beginning of a chain of three or more neurons that form an ascending pathway to the central nervous system. There are specific and nonspecific pathways to transmit information.
• Sensory areas of the cortex

  • Processing of afferent information does not end in the primary cortical receiving areas, but continues from these areas to association areas in the cerebral cortex where complex integration occurs.
• Association cortex & perceptual processing

  The primary somatosensory cortex relies upon various association areas of the cortex to properly process sensory information. Further perceptual processing involves arousal, attention, learning, memory, language, and emotions, as well as comparison of information from different senses.
• Factors that affect perception

  • Receptor adaptation and afferent processing
  • Emotions and experiences
  • Not all stimuli give rise to a conscious sensation
  • Lack of receptors for certain stimuli
  • Damaged neural pathways
  • Drugs
• Somatic sensation


  Sensation from the skin, muscles, bones, tendons and joints, initiated by somatic receptors that respond to touch, pressure, sense of posture and movement, temperature, and pain.
• Pain


  Pain differs from other somatosensory modalities. After transduction, a series of changes occur in the pain pathway that alter how it responds to subsequent stimuli. Hyperalgesia is an increased sensitivity to painful stimuli.
• Pain management mechanisms


  • Electrical stimulation of specific areas of the CNS
  • Pharmacological agents (NSAIDs, opioids)
  • Endogenous opioid pathways
  • Acupuncture
  • Transcutaneous Electrical Stimulation (TEMS)
  • Massage
• Referred pain


  The brain is "confused" and you feel pain from an internal organ as another area of the body.
• The somatosensory cortex


  Processes information from the various somatic receptors.
• Vision


  The eyes are composed of an optical component that focuses the visual image, and a neural component that transforms the image into a pattern of graded and action potentials.
• Light


  The physical stimulus for vision.
• Anatomy of the human eye


  Includes the optical and neural components.
• Optics of vision: refraction


  The bending of light as it passes through the cornea and lens to focus the image on the retina.
• Photoreceptor cells


  Rods and cones that transduce light into graded potentials.
• Phototransduction


  The process by which photoreceptors convert light into electrical signals.
• Neural pathways of vision

  Light signals are converted into action potentials through the interaction of photoreceptors with bipolar and ganglion cells. There are ON and OFF pathways that improve image resolution.
• Color vision


  The colors we perceive are related to the wavelengths of light that objects reflect, absorb, or transmit.
• Neurons of the visual pathway

  • Some project to regions of the brain other than the visual cortex
• Neurons that project to regions other than visual cortex

  • Ganglion cells containing melanopsin that carry visual information to the suprachiasmatic nucleus
• Suprachiasmatic nucleus


  Lies just above the optic chiasm and functions as part of the "biological clock"
• Entrainment of the neuronal clock to a 24-hour day


  1. Information about daily cycles of light intensity from ganglion cells

  2. Used to entrain the circadian rhythm
• Other visual information


  • Passes to the brainstem and cerebellum
  • Used in coordination of eye and head movements, fixation of gaze, and change in pupil size
• Color


  Perceived based on wavelengths of light that objects reflect, absorb, or transmit
• An object appears red because it absorbs shorter wavelengths and reflects longer wavelengths
• Light perceived as white is a mixture of all wavelengths, and black is the absence of all light
• Cone photoreceptor cells


  • Contain photopigments that activate in response to different wavelengths (L, M, S cones)
• Each type of cone is excited most effectively by light of one particular wavelength, but can respond to a range of wavelengths
• Color vision in bright vs dim light

  • In bright light, differential cone response allows good color vision
  • In dim light, only rods respond and objects appear in shades of gray
• Red-green color blindness


  • Most common form, affects 1 in 12 men
  • Caused by lack or abnormality of red or green cone pigments