Proprioception Class Notes

Proprioception   Group of sensory modalities that allow us to: o Know the positions of our limbs in space. o Detect and assess the magnitudes of imposed (passive) movements and forces without vision. o Sense effort during volitionally generated (active) movements and forces. It is not the same of kinaesthesia, which means the sense of movement. Proprioceptive ability after viral infection  Cell bodies of large-diameter axons in the dorsal root ganglia are destroyed.  Produces a selective loss of the largest myelinated sensory axons.  Patients have no loss of motor power (motoneurones are unaffected), but sensorimotor control is greatly compromised.  Patients need to rely on vision to guide their movements, though they can also obtain some clues from thermal signals. Sense of position  Capacity to know the positions of limb segments and joint angles on the basis of somatic sensory inputs alone.  Imposing very slow passive movements about a joint generates a perception of a change in position, but not a perceived movement.  More rapid imposed movements are detected as both a movement (and knowledge of the direction of movement) and change in position. Joint receptors  It was believed that mechanoreceptors in joints encoded change in joint angle. o However, microelectrode recordings showed that the majority of these joint receptors responded only towards the extremes of angular excursion.  Microstimulation of single joint afferents can be perceived as: o Local pressure. o Extreme (and unnatural) angular position.  This suggests that joint receptor act more as “limit detectors” than as encoders of angular joint position.  Joint position sense is not affected by intra-articular injection of local anaesthetic, or by replacement of the joint by an artificial joint.  This suggests that joint afferents are not essential for proprioception, but they can provide information in the absence of other proprioceptive inputs. Cutaneous receptors  Like joint receptors, most mechanoreceptors in the skin (particularly those around the joints) respond at the limits of joint rotation, and often in both directions.  Microstimulation of single cutaneous afferents can be perceived as: o Pressure (slowly-adapting type I afferents, SA I). o Flutter (fast-adapting type I, FA I). o Vibration (fast-adapting type II, FA II), but not as movement.  One class of cutaneous mechanoreceptor, the slowly adapting type II (SA II) afferent, has large receptive fields and can respond to lateral skin stretch. o Unlike the other classes of cutaneous afferent, microstimulation of SA II generally does not produce sensations. o May be because many SA II afferents needs to be activate in order to generate perceptions of joint movements. o In the absence of actual joint movement, signals in stretchsensitive cutaneous afferents can generate perceptions of joint movement. Muscle receptors  Encapsulated sensory endings distributed throughout skeletal muscles.  Muscle spindles endings are length detectors. o Extremely sensitive to small stretches of the muscle. o Respond to movements of joints in only one direction; that which causes stretch of the receptor-bearing muscle (e.g. biceps in extension and triceps in flexion),  There are two types of endings: o Primary endings (group Ia afferents): exhibit a high dynamic sensitivity to small changes in muscle length, and respond to high-frequency vibrations (>80Hz). o Secondary endings (group II afferents): more sensitive to changes in static muscle length.  Receptors maintain a constant firing rate that is linearly related to the resting length of the muscle (e.g. the greater the length, the greater the firing rate). o When a joint is flexed, muscle spindles in the antagonist (extensor) muscle are stretched and increase their firing rate, whilst those in the agonist (flexor) muscles are unloaded, resulting in a decrease in their firing rate. o Accordingly, joint angle can be calculated from firing rates of muscle spindles in the set of muscles acting about a joint.  Since muscle spindles (especially primary endings) are so sensitive to small length changes, they can be activated by vibration applied either to the tendon or muscle belly.  Goodwin, McCloskey and Matthews demonstrated that muscle spindle afferents were the primary source of sensory information for proprioception. o When 100Hz vibration was applied to the biceps tendon, subjects perceived the elbow to be more extended (induced increase in firing rate was interpreted by the brain as an increase in length, as occurs in extension). o When vibration applied to both Achilles tendons, subjects perceive that they are leaning forward (muscle spindles in triceps surae are being activated) and compensate by leaning back. o When vibration is applied to the biceps tendon while the subject touches his or her nose with a finger of the same arm, the brain has to interpret conflicting information from two sources; according to the muscle spindles in biceps, the elbow is being extended but according to cutaneous input from the finger and nose it is not  results in the Pinocchio effect, where the nose is perceived to be getting longer.  Microstimulation of single muscle spindle afferents cannot be perceived, yet intraneural electrical stimulation of many muscle spindle afferents generate perceptions of joint rotation in the direction that would occur if the muscle was being stretched. The magnitude of perceived rotation depends on frequency of stimulation (e.g. higher the frequency, greater the perceived change). Evidently, many muscle spindles need to be activated in order to generate meaningful sensory signals of changes in muscle length.   Proprioceptive performance  Optimal when all somatic afferent sources (joint, cutaneous and muscle) are intact.  When muscles acting on the distal phalanx are disengaged (by flexing the middle interphalangeal joint and extending the proximal and distal interphalngeal joints of the adjacent fingers), subjects are poor at detecting the angle and direction of imposed (passive) joint movements.  When the tendon of a muscle is surgically exposed and cut, the subject can correctly detect the direction and corresponding joint angle associated with the imposed changes in length.