Proprioception

• Proprioception


  The awareness of the sense of position of body segments in space
• Kinesthesia


  The sense of movement
• Proprioception and kinesthesia are both encompassed by the broad term proprioception
• Proprioception


  • Allows us to perform accurate movements without continuous visual control
  • Adjust motor control patterns
  • Perform motor tasks that require multi-limb coordination
• Proprioception is abnormal in many disorders, including strokes, demyelinating diseases, large-fibre neuropathies (including Guillain-Barré syndrome), cerebellar disease and many dysfunctional states of the central and peripheral nervous system
• Proprioceptors


  • Muscles
  • Joints
  • Cutaneous receptors
• Primary afferent axons


  • Various sizes
  • Diameter correlated with conduction velocity and type of sensory receptor
  • C fibers mediate pain and temperature
  • Aβ mediates touch sensations

  
• Receptor physiology


  • Receptors transduce stimulus energy into usable CNS code (i.e. action potentials)
  • For a separate receptor, the chemical messenger released initiates an action potential
  • For a specialised ending, local current flow between the depolarized receptor ending and the afferent fiber initiates an action potential

  
• Sensory units


  • A single afferent neuron with all of its receptor endings
  • Usually many receptors per neuron, each with the same preferential modality
  • Receptive field is the area over which stimulation can generate action potentials

  
• Coding of sensory stimuli


  • Modality
  • Intensity
  • Location

  
• Muscle mechanoreceptors


  • Muscle spindles arranged parallel to extrafusal fibers
  • Golgi tendon organs lie in series, between muscle fibers and attachment points

  
• When the muscle contracts


  Golgi tendon organs respond to increased tension and transmit this to the spinal cord

  
• Muscle spindles and vibration


  Vibration activates muscle spindle afferents, increasing frequency of action potentials which the brain interprets as muscle lengthening

  
• Mechanoreceptors of the skin


  • Pacinian corpuscles
  • Ruffini endings
  • Meissner's corpuscles
  • Merkels' disks
• Microneurography


  Mapping receptive fields of single sensory axons using a microelectrode

  
• Mechanoreceptors of the skin vary in their preferred stimulus frequencies, pressures and receptive field sizes

  
• Adaptation and receptive field size


  Variations in receptive field size and adaptation rate of different skin mechanoreceptors
• Nearly all Ruffini and Pacinian corpuscles with a receptive field overlying a joint respond to joint movement, as do a majority of Merkel receptors and Meissner corpuscles
• Receptor types


  • Fast Adapting (FA)
  • Slow Adapting (FA)
  • Type I Ruffini (tissue stress)
  • Type II Paciniform (compression)
  • Type III Golgi
  • Type IV Free nerve endings
• Joint receptors


  • Signal potentially damaging joint states like high capsule tension, extreme angles, and inflammation
  • Limited ability to accurately signal joint angle

  
• Proprioception is best when all mechanoreceptors can contribute, with muscle and tendon proprioceptors contributing most, particularly at slow speeds
• There is better performance with joint and cutaneous receptors than cutaneous receptors alone for discriminating distal interphalangeal joint motion

  
• Ascending pathways 1


  • Proprioceptive information carried to somatosensory cortex mainly via dorsal column-medial lemniscal pathway
  • Pathway carries fine touch, vibration, movement, proprioception, pressure

  
• Ascending pathways 2


  • Proprioceptive information enters spinal cord and branches, generating reflexes and travelling to brain
  • Spinothalamic pathway mainly responsible for thermal, pain, tickle, itch

  
• Cortical representation


  Somatotopic mapping of body surface sensations onto somatosensory cortex

  
• Sensory discrimination


  Two point discrimination varies with receptor density, receptive field size, cortical representation
• Proprioception overview


  • Signals from sensory receptors and motor commands (efference copy) involved in perception

  
• Periodontal mechanoreceptors


  Ruffini-like mechanoreceptors in periodontal ligament, important for motor control during chewing
• Detection of object size between teeth relies on proprioceptive signals
• Anaesthesia of teeth produces minimal reduction in ability to detect thin objects between teeth
• Altered sensations with dentures


  • Loss of periodontal ligament receptors
  • Impaired perception of intra-oral forces
  • Decreased appreciation of texture, size and hardness
  • Considerable motor learning required to adapt