Senses

• Types of sensory areas of cortex

  • Primary sensory cortex
  • Secondary sensory cortex
  • Association cortex
• Primary sensory cortex


  The area of sensory cortex that receives most of its input directly from the thalamic relay nuclei of that system
• Secondary sensory cortex


  The areas of the sensory cortex that receive most of their input from the primary sensory cortex of that system or from other areas of secondary sensory cortex of the same system
• Association cortex


  Receives input from more than one sensory system, most input comes via areas of secondary sensory cortex
• Principles characterizing interactions among sensory structures

  • Hierarchical organization
  • Functional segregation
  • Parallel processing
• Hierarchical organization


  A system whose members can be assigned to specific levels or ranks in relation to one another, sensory structures are organized in a hierarchy on the basis of the specificity and complexity of their function
• Two general phases of sensory processing

  • Sensation - detecting the presence of stimuli
  • Perception - higher-order process of integrating, recognizing, and interpreting complete patterns of sensations
• Functional segregation


  Organization of sensory systems
• Parallel processing


  Evidence suggests sensory systems have multiple parallel pathways that simultaneously analyse signals in different ways, can influence behaviour with or without conscious awareness
• Many sensory neurons carry information from lower to higher levels of their respective sensory hierarchies
• Aspects of sound


  • Amplitude (loudness)
  • Frequency (pitch)
  • Complexity (timbre)
• Sound is always associated with complex patterns of vibrations
• Fourier analysis


  Mathematical procedure for breaking down complex waves into their component sine waves
• There is a close relationship between the frequency of a tone and its perceived pitch, but the relation between the frequencies that make up natural sounds and their perceived pitch is complex
• An important aspect of pitch perception is the missing fundamental
• Pathway of sound through the ear

  1. Sound waves travel down auditory canal

  2. Cause tympanic membrane to vibrate

  3. Vibrations transferred to ossicles

  4. Oval window vibrates

  5. Vibrations travel through cochlear fluid

  6. Dissipated by round window
• Organ of Corti


  The auditory receptor organ within the cochlea
• Basilar membrane


  Contains the auditory receptors and hair cells
• Tectorial membrane


  Rests on hair cells
• Tonotopic organization


  The organization of the auditory system according to frequency
• Semicircular canals


  The receptive organs of the vestibular system, carry information about head movements to help maintain balance
• Sound localization


  1. Mediated by lateral and medial superior olives

  2. Neurons in medial superior olive respond to differences in time of arrival of signals from two ears

  3. Neurons in lateral superior olive respond to differences in amplitude of sounds from two ears

  4. Both project to superior and inferior colliculi
• Superior colliculus


  Receives auditory input in deep layers, organized according to a map of auditory space, receives visual input in superficial layers organized retinotopically, functions to locate sources of sensory input in space
• Regions of auditory cortex

  • Primary auditory cortex
  • Secondary auditory cortex (belt)
  • Parabelt areas
• Many neurons in auditory cortex respond weakly to simple stimuli like pure tones
• Auditory cortex organization


  Organized in functional columns and tonotopically
• Two main cortical streams of auditory processing

  • Anterior auditory pathway - more involved in identifying sounds
  • Posterior auditory pathway - more involved in locating sounds
• Sensory interactions occur in association cortex and even in primary sensory cortex
• Five types of taste receptor cells

  • Sweet
  • Sour
  • Bitter
  • Salty
  • Umami (savory)
• Salt influence on taste receptor cells

  Salt enters through specific sodium ion channels, not via G-protein linked receptors
• Taste receptor proteins


  There appears to be only one receptor per taste receptor cell, and they are found not just in oral cavity but also in throat, esophagus, and lungs
• Gustatory afferent pathways


  1. Facial (VII), glossopharyngeal (IX), and vagus (X) cranial nerves carry taste information from front, back of tongue, and back of oral cavity respectively

  2. Secondary gustatory cortex is in orbitofrontal cortex
• Olfactory receptor cells


  Located in olfactory mucosa in upper part of nose
• Olfactory bulbs


  Where olfactory receptor axons synapse, organized topographically (chemotopic map)
• Each new olfactory receptor cell survives only a few weeks before being replaced
• Olfactory pathways


  1. Olfactory bulbs project to amygdala and piriform cortex

  2. Two major pathways leave amygdala/piriform area, one projects diffusely to limbic system, one via thalamus
• The olfactory system is the only sensory system whose major pathway reaches cerebral cortex without first passing through thalamus
• Olfactory glomeruli


  • The axons of olfactory receptors terminate in discrete clusters of neurons that lie near the surface of the olfactory bulbs
• The olfactory bulbs are organized topographically
• Chemotopic map


  The poorly understood topographic organization of the olfactory bulbs