Cutaneous system

• Mechanoreceptor


  Sensory receptor that responds to mechanical pressure or distortion
• Classification of cutaneous mechanoreceptors by function

  • Pacinian corpuscles
  • Meissner's corpuscles
  • Merkel's discs
  • Ruffini endings
  • Free nerve endings

  
• Pacinian corpuscles


  • Rapid vibratory pressure and touch (max sensitivity at about 250 Hz)
  • Rapid adaptation
  • Deep skin location
  • Encapsulated
• Meissner's corpuscles


  • Light touch
  • Changes in texture
  • Relatively slow vibrations (up to 50 Hz)
  • Rapid adaptation
  • Superficial skin location
  • Encapsulated
• Merkel's discs


  • Touch
  • Pressure
  • Changes in texture
  • Respond from steady state to low frequencies (up to 15 Hz)
  • Slow adaptation
  • Superficial skin location
  • Not encapsulated
  • Sustained response to mechanical deflection
• Ruffini endings


  • Continuous tension
  • Slow adaptation
  • Deep skin location
  • Not encapsulated
  • Sensitive to skin stretch and joint angle change
• Free nerve endings

  • Touch
  • Pressure
  • Stretching
  • Temperature
  • Pain
  • Different adaptation rates
  • Wide distribution
  • Not encapsulated
  • Polymodal
• Mechanoreceptors are primary neurons or nerve endings that respond to mechanical stimuli by firing action potentials
• Cutaneous mechanoreceptors have different function and location
• Phasic mechanoreceptors


  Receptors that adapt quickly (i.e. quickly return to a normal pulse rate)
• Tonic mechanoreceptors


  Receptors that are slow to return to their normal firing rate
• Phasic mechanoreceptors are useful in sensing such things as texture or vibrations, whereas tonic receptors are useful for temperature and proprioception among others
• Encapsulated mechanoreceptors


  • Pacinian and Meissner's corpuscles
  • Less sensitive to continuous (steady-state) deformation
• Generator potential


  Graded response in the sensory neuron arising within the mechanoreceptor, the greater the deformation the greater the generator potential
• Mechanism of action potential generation in Pacinian corpuscle

  1. Deformation creates generator potential

  2. If generator potential reaches threshold, action potentials triggered at first node of Ranvier

  3. Magnitude of stimulus encoded in frequency of action potentials
• Loewenstein's experiments showed the transducer is located in the nerve ending of the Pacinian corpuscle
• Meissner's corpuscles

  • Responsible for sensitivity to light touch
  • Highest sensitivity at vibrations lower than 50 Hz
  • Rapidly adaptive
• Merkel's discs are sensitive to touch, pressure and texture
• Merkel's discs


  • Slowly adapting
  • Sensitive to low frequency vibrations up to 15 Hz
  • Sustained response to mechanical deflection
  • Mediate high resolution tactile discrimination
• Ruffini endings


  • Slowly adapting
  • Sensitive to skin stretch and joint angle change
  • Contribute to kinesthetic sense and grip control
• Free nerve endings


  • Unencapsulated
  • Most common type
  • Detect temperature, mechanical stimuli, and pain
  • Polymodal