L04 - The Somatosensory System

Receptor Properties
1. Transduction
  1. Larger the stimuli the larger the receptor potential
    1. If past threshold then AP caused
      1. Frequency of APs (rate code) establishes strength of stimulus
2. Line Code
  1. Activation of a particular pathway
    1. Gives information about the type of stimulus
      1. Region of the brain activated
        Incoming axon identifies the incoming sensory info = line code
      2. Receptor specificity
        Maximally receptive to a particular kind of stimulus energy
Receptor Types
1. Pain and Temperature
  1. Free nerve endings
    1. A-delta fibres
      1. Cold, pricking pain
      2. Thin, myelinated
      3. Receptor
        TRPM8
        Cold
        Menthol taste
    2. C fibres
      1. Thin, no myelination
      2. Slow Conduction
      3. Pain, itch, warmth
      4. Receptor
        TRPV1
        Noxious heat
        Capsaicin - 'vanilloid receptor'
        TRPA1
        Noxious Cold
        Mustard/Cinnamon
        TRPV3/4
        Warm
2. Receptor protein types
  1. Transcient Receptor Potential family
    1. Non-specific cation channels
3. Pressure
  1. Merkel's Disk
    1. A-beta fibres
      1. Mechanically-gated channel response
        Ion channels with accessory proteins
        Degenerin/Epithelial Na+ channel
        DEG/ENaC + accessory protein MEC-2
        TRP (A1)
        KCNK (dual pore K+ channels)
        ASIC
  2. Ruffini's Ending
4. Vibration
  1. Meissner's Corpuscle
    1. Touch vibration at 30-40Hz
  2. Hair follicle receptor
    1. Light touch vibration at 30-40Hz
  3. Pacinian Corpuscle
    1. Pressure vibration at 250-300Hz (tickle)
    2. A-beta sensory axons
  4. Vibration sense depends on AP adaptation
    1. When the stimulus is constant then the number of APs decreases
    2. Slow adaptation
      1. Detects stimulus strength
        Continued activity throughout
      2. Merkel's
      3. Ruffini's
    3. Fast adaptation
      1. Detects rate of change of stimulus
        Activity at the start and the end
      2. Meissner's
      3. Pacinian
        Adaptation of Pacinian due to lamallae surrounding the receptor
        No lamellae = no sensitivity at high frequency and no sensitivity to stimulus being there or now
5. Localisation
  1. Receptor Density
    1. Top lip = high density
    2. Back = low density
  2. Receptive field size
    1. Meissner's corpuscle
      1. Very small receptive field
        Smallest at fingertips
        Superficial receptor
    2. Pacinian corpuscles
      1. Large receptive field
        Deep receptor
        Palm
  3. Ability to discriminate between two independent stimuli
6. Proprioception
  1. Detects mechanical status of muscle
    1. Joint position
    2. Muscle length
    3. Muscle movement
    4. Acceleration
    5. Tension/force
  2. Stretch relfex
    1. Muscle spindle in quadriceps
      1. Senses muscle length
        II afferents
      2. Senses rate of change of length
        Ia afferents
  3. A-alpha fibres
  4. Tendon Organ
    1. Senses muscle tension and inhibits muscles
      1. Preventing muscle damage
    2. Output modified by other receptors and descending pathways
The Somatosensory Cortex
1. Primary Somatosensory cortex
  1. Posterior to central sulcus
  2. All information from skin goes here first
  3. For areas where we need more control, we have a larger area of the brain devoted to it
2. Secondary Somatosensory cortex and posterior parietal cortex
  1. Integrate information from the primary somatosensory cortex

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L04 - The Somatosensory System

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