2014 Sensory Physiology

Critical facts iconCRITICAL FACTS for General Principles and Chemosensation
(if med school is a Minnesota forest with millions of trees, these are the red pines)

  1. Information processing involves 3 fundamental steps: TRANSDUCTION, TRANSMISSION, and PERCEPTION. The CNS continuously filters sensory information, and is capable of switching priority (i.e., paying attention).  PERCEPTION is a CONSCIOUS process; SENSATION is not.  For example, nociception is the sensation; pain is the perception.

  2. Transduction is the unique physiological process common to all sensory systems.  In this multistep process, stimulus energy (which can be electromagnetic, mechanical or chemical) is converted into electrical potentials that can be interpreted by the nervous system.

  3. A receptor potential is a graded response to a stimulus that may be DEPOLARIZING or HYPERPOLARIZING. Receptor potentials have a threshold in stimulus amplitude that must be reached before a response is generated, and their amplitude saturates in response to intense stimuli.

  4. Receptors encode stimulus modality by responding to one form of energy more than any other, and (individually) to only a narrow range of that energy.  The type of energy that a receptor responds to under NORMAL conditions is called the ADEQUATE STIMULUS.  Perception is generally localized by modality, and sensations are “mapped” in ways that maintain an orderly representation of the stimulus ("labelled line" theory).

  5. Stimulus intensity is encoded in two ways:  1) frequency coding, where the firing rate of sensory neurons increases with increased intensity and 2) population coding, where the number of primary afferents responding increases (also called RECRUITMENT).

  6. Adaptation is the process by which the response of a receptor to a CONSTANT stimulus DECLINES over time.  If the change in receptor potential occurs SLOWLY, the response is called TONIC; if it occurs RAPIDLY, it is called PHASIC.

  7. Acuity is the ability to localize a stimulus.  It is determined by receptive field size and receptor density.  There is an inherent paradox between the intensity of a stimulus and the ability to localize it.  This paradox is resolved through the CNS process of LATERAL INHIBITION whereby application of a stimulus to the center of the receptive field excites a neuron, but a stimulus applied near the edge inhibits it.

  8. The olfactory and gustatory systems both respond to chemical stimuli.  There are ~1000 olfactory receptor proteins, resulting in the detection of olfactants that can be separated by along multiple dimensions.  In contrast, the gustatory receptor cells respond to 5 basic tastants:  sweet, sour, salty, bitter, and umami.

  9. The olfactory system uses both labelled line and pattern coding of information.  There is an odourtypic map (similar to the homunculus in the somatosensory system) established at the level of the 2nd order neurons in the olfactory bulb, while the rich variety of odours is represented by the pattern of receptor responses.

  10. OLFACTORY TRANSDUCTION occurs via a 4 step process that uses a 2nd messenger cascade to amplify the signals.  In contrast, GUSTATORY TRANSDUCTION is more complicated.  Bitter, sweet and umami receptors use a similar 2nd messenger process, while salty and sour compounds cause depolarization by directly interacting with ion channels.

  11. The perception of flavour is a combination of smell, taste and texture, which results in considerable overlap between gustatory and olfactory disorders.  Patients with anosmia typically complain of ageusia as well.

Email: Dr. Janet Fitzakerley | ©2014 University of Minnesota Medical School Duluth | Last modified: 5-feb-14 0:13 AM