Proprioception

Question 1

what is the definition of proprioception

Answer 1

• proprius (“ones own”, “individual”) and perception
• the sense of the relative position of neighbouring parts of the body
• (vestibular sensation sometimes included)

Question 2

what are joint capsule receptors

Answer 2

• ruffini, paciniform, golgi-type and free nerve endings contained within joint capsule
• historically thought to be the main source of joint position sense

Question 3

what is the minimal role for the joint capsule

Answer 3

• animal recordings suggest joint capsule afferents provides ambiguous information, mainly acting as limit detectors (injury prevention)
• normal sense of joint rotation after hip replacement
• knee anaesthesia barely affects sense of joint rotation
• disengaging muscle from joint motion leads to poor motion detection

Question 4

what is the muscle spindle

Answer 4

• consists of:
  
  • bundle of thin muscle fibres contained within a capsule
  • situated in parallel with main ‘extrafusal’ muscle, but generates no useful force
  • wrapped around by a pair of sensory axons that detect muscle strength
  • gamma motor neurons cause active contraction of spindle (‘alpha’ motor neurons innervate the main body of muscle)
• detects:
  
  • static length of muscle (i.e. position)
  • rate of change of muscle stretch (i.e. velocity)
• joint angles can be calculated from changes in muscle length signalled by spindles
• central part of muscle spindle is non-contractible
• stretch sensitive ion channels in sensory axons are activated when muscle is stretched
• ~400 spindles in soleus muscle
• primary endings: signal position and velocity transmitted by 1a afferents. sensitive to vibration
• secondary endings: position only, via group II afferents
• gamma fibres: contractile element of spindle. no useful force output - purely intended to keep spindle straight
• alpha-gamma coactivation: maintains sensitivity of spindle in the face of changing muscle length

Question 5

how does alpha-gamma coactivation maintain spindle sensitivity

Answer 5

• 1a spindle afferent continuously fires to signal muscle length
• contraction of main muscle (via alpha motor neurons) causes spindle to slacken - this causes a drop in firing rate and a loss of sensitivity
• gamma activation contracts the spindle to keep it taught, thus maintaining sensitivity

Question 6

what is muscle vibration

Answer 6

• ~80-100Hz vibration selectively activates 1a afferents
• produces illusions of muscle stretch (both position and velocity)
  
  • e.g. the Pinocchio illusion
• calf muscle vibration evolves a strong sway response

Question 7

what are the effects of muscle conditioning

Answer 7

• prior muscle stretch / contraction significantly affects joint position sense
• due to muscle becoming slack / taut, thus affecting spindle output

Question 8

what is the golgi tendon organ

Answer 8

• indirectly contributes to joint rotation sense - signals force and heaviness
• maybe important when muscle movement is ambiguous (e.g. concentric vs eccentric contraction)
• situated between muscle and tendon - unlike spindles, golgis are in-series with muscle
• when golgi tendon organ is stretched (usually because of muscle contraction), the 1b afferent axon is compressed by collagen fibres and its rate of firing increases

Question 9

how does sensory loss affect motor performance

Answer 9

• sequential finger movements in a patient with severe peripheral sensory neuropathy
• in the absense of vision, movement accuracy degrades overtime

Question 10

how is standing made possible with propioception

Answer 10

• equivalent body used to isolate ankle joint signals
• imperceptible perturbations used to test balance
• ankle information alone is sufficient to keep body upright

Question 11

how does vestibular interact with other body senses

Answer 11

• galvanic vestibular stimulation (GVS) - vestibular information must be interpreted in the context of neck orientation
• illusions of neck orientations cause vestibular information to be reinterpreted

Question 12

how is proprioception affected with age and falls

Answer 12

• ankle proprioception declines with age
• however, proprioception-falls link not strong
• peripheral neuropathy associated with falls

Question 13

what is the interaction between muscle sensation and strength

Answer 13

• strongest and weakest subjects studied separately
• no difference in lower limb sensory function
• sway disproportionately increased by eye closure in weaker subjects
• not due to weakness per se but an inability to utilise proprioceptive signals