IV Neuroanatomy and Neurophysiology

• Levels of processing of sensory information:
• Receptor level:

  • Stimulus energy must match receptor specificity
  • Stimulus must be applied within the receptive field of the receptor
  • Transduction needs to occur (conversion of stimulus energy into a graded potential)
  • Graded potentials must reach threshold in first-order sensory neuron
• Adaptation:

  • Reduction in sensitivity in the presence of a constant stimulus
  • Peripheral adaptation is at the level of the receptor and reduces information sent to the CNS
  • Central adaptation is at the level of the neural pathway to the brain
• Phasic receptors:

  • Fast adapting; e.g. lamellar & tactile corpuscles
  • Provide information on rate of change of the stimulus
• Tonic receptors:

  • Sustained response with little to no adaptation
  • Provide information about the presence and strength of a stimulus
• Processing at the Circuit Level:
• Goal is to get information to the correct area of the cortex for perception and localization of the stimulus
• Chain of 3 neurons:

  • 1st order neuron brings information to the CNS
  • Branching occurs leading to motor reflexes or synapsing with 2nd order neurons
  • 3rd order neurons take information to the correct sensory area of the cerebral cortex
• Processing at the Perceptual Level:
• Information has to get to the right place for understanding and localization
• Properties of Sensory Perception:

  • Perceptual detection
  • Magnitude estimation
  • Spatial discrimination
  • Feature abstraction
  • Quality discrimination
  • Pattern recognition
• Pain:

  • Warns of tissue damage and motivates action
  • Pain is personal and can't be measured
  • Sharp pain followed by burning or aching
  • Pain suppression through endogenous opioids like endorphins and enkephalins
• Pain Tolerance:

  • Pain threshold is the same for everyone
  • Tolerance varies and can be influenced by genetics, mental state, gender, social isolation, stress
• Some Pain Terminology:

  • Somatic pain
  • Visceral pain
  • Referred pain
  • Phantom pain
• Taste:

  • Almost all 10,000 taste buds are located on the tongue in papillae that give a bumpy surface to the tongue
  • Three types of papillae:
  • Fungiform: mushroom-shaped, over entire surface of tongue, 1-5 taste buds each
  • Vallate: largest and have many taste buds each; 8-12 vallate papillae total that make a V at the back of the tongue
  • Foliate: laterally on tongue, many taste buds in each during childhood but their numbers decrease with age
  • A small number of taste buds scattered over soft palate, cheeks, pharynx, even the epiglottis
• 5. Umami

  • Taste receptors have different thresholds, adapt rapidly
• • Gustatory epithelial cells have long microvilli called gustatory hairs that extend through a taste pore to the surface of the tongue where they are bathed by saliva containing dissolved food chemicals
  • Gustatory hairs have receptors for food chemicals (tastants) and once activated by tastant binding, they depolarize, release transmitter (serotonin, ATP), and activate the cranial nerve responsible for that taste information
  • Turnover of taste cells is 7-10 days, stem cell populations called basal epithelial cells
  • Five basic taste modalities:

  1. Sweet

  2. Sour

  3. Salty

  4. Bitter
• Mechanisms:

  • Salt: influx of Na+ through channels depolarizes gustatory epithelial cells
  • Sour: H+ can directly go in and/or block leaky K+ channels leading to depolarization
  • Sweet, Bitter, Umami: All bind to appropriate cell surface receptors coupled to the G protein gustducin and use second messengers to open channels leading to depolarization and release of the neurotransmitter ATP
  • Mouth also contains thermoreceptors, mechanoreceptors, and nociceptors
  • Actual taste is a small component of the total experience, 80% of what we attribute to taste is actually smell
• • Each receptor responds to one or more odorants and each odorant binds to several different receptor types
  • Each receptor cell expresses only one type of receptor
  • Pain and temperature receptors are also in nasal cavities
  • Olfactory tracts have two destinations: the olfactory cortex where smells are identified and interpreted, and the limbic system which links scent with memories and emotions
• Smell:

  • Olfactory receptors are chemoreceptors
  • Olfactory epithelium located in the roof of the nasal cavity
  • Olfactory sensory neurons are bipolar neurons with radiating olfactory cilia
  • Olfactory neurons have several long cilia projecting from a single apical dendrite
  • Axons gather into small fascicles to form filaments of the olfactory nerve (cranial nerve I)
  • Specificity of Olfactory Receptors:
  • Each smell may contain hundreds of different odorants
  • Humans have ~350 different odorant receptors