Elbow proprioception is normal in patients with a congenital absence of functional muscle spindles

Sensorimotor control in the congenital absence of functional muscle spindles

Hereditary sensory and autonomic neuropathy type III (HSAN III), also known as familial dysautonomia or Riley-Day syndrome, results from an autosomal recessive genetic mutation that causes a selective loss of specific sensory neurones, leading to greatly elevated pain and temperature thresholds, poor proprioception, marked ataxia and disturbances in blood pressure control. Stretch reflexes are absent throughout the body, which can be explained by the absence of functional muscle spindle afferents - assessed by intraneural microelectrodes inserted into peripheral nerves in the upper and lower limbs. This also explains the greatly compromised proprioception at the knee joint, as assessed by passive joint-angle matching. Moreover, there is a tight correlation between loss of proprioceptive acuity at the knee and the severity of gait impairment. Surprisingly, proprioception is normal at the elbow, suggesting that participants are relying more on sensory cues from the overlying skin; microelectrode recordings have shown that myelinated tactile afferents in the upper and lower limbs appear to be normal. Nevertheless, the lack of muscle spindles does affect sensorimotor control in the upper limb: in addition to poor performance in the finger-to-nose test, manual performance in the Purdue pegboard task is much worse than in age-matched healthy controls. Unlike those rare individuals with large-fibre sensory neuropathy, in which both muscle spindle and cutaneous afferents are absent, those with HSAN III present as a means of assessing sensorimotor control following the selective loss of muscle spindle afferents.

Proprioceptors in extraocular muscles

Proprioception is the sense that lets us perceive the location, movement and action of the body parts. The proprioceptive apparatus includes specialized sense organs (proprioceptors) which are embedded in the skeletal muscles. The eyeballs are moved by six pairs of eye muscles and binocular vision depends on fine-tuned coordination of the optical axes of both eyes. Although experimental studies indicate that the brain has access to eye position information, both classical proprioceptors (muscle spindles and Golgi tendon organ) are absent in the extraocular muscles of most mammalian species. This paradox of monitoring extraocular muscle activity in the absence of typical proprioceptors seemed to be resolved when a particular nerve specialization (the palisade ending) was detected in the extraocular muscles of mammals. In fact, for decades there was consensus that palisade endings were sensory structures that provide eye position information. The sensory function was called into question when recent studies revealed the molecular phenotype and the origin of palisade endings. Today we are faced with the fact that palisade endings exhibit sensory as well as motor features. This review aims to evaluate the literature on extraocular muscle proprioceptors and palisade endings and to reconsider current knowledge of their structure and function.

A reassessment of the role of joint receptors in human position sense

In the past, the peripheral sense organs responsible for generating human position sense were thought to be the slowly adapting receptors in joints. More recently, our views have changed and the principal position sensor is now believed to be the muscle spindle. Joint receptors have been relegated to the lesser role of acting as limit detectors when movements approach the anatomical limit of a joint. In a recent experiment concerned with position sense at the elbow joint, measured in a pointing task over a range of forearm angles, we have observed falls in position errors as the forearm was moved closer to the limit of extension. We considered the possibility that as the arm approached full extension, a population of joint receptors became engaged and that they were responsible for the changes in position errors. Muscle vibration selectively engages signals of muscle spindles. Vibration of elbow muscles undergoing stretch has been reported to lead to perception of elbow angles beyond the anatomical limit of the joint. The result suggests that spindles, by themselves, cannot signal the limit of joint movement. We hypothesise that over the portion of the elbow angle range where joint receptors become active, their signals are combined with those of spindles to produce a composite that contains joint limit information. As the arm is extended, the growing influence of the joint receptor signal is evidenced by the fall in position errors.

Proprioception: A New Era Set in Motion by Emerging Genetic and Bionic Strategies?

The generation of an internal body model and its continuous update is essential in sensorimotor control. Although known to rely on proprioceptive sensory feedback, the underlying mechanism that transforms this sensory feedback into a dynamic body percept remains poorly understood. However, advances in the development of genetic tools for proprioceptive circuit elements, including the sensory receptors, are beginning to offer new and unprecedented leverage to dissect the central pathways responsible for proprioceptive encoding. Simultaneously, new data derived through emerging bionic neural machine-interface technologies reveal clues regarding the relative importance of kinesthetic sensory feedback and insights into the functional proprioceptive substrates that underlie natural motor behaviors.

Evolution of touch and proprioception of the limbs: Insights from fish and humans

The function of the hands is inextricably linked to cutaneous mechanosensation, both in touch and in how hand movement and posture (proprioception) are controlled. The structure and behavior of hands and distal forelimbs of other vertebrates have been evolutionarily shaped by these mechanosensory functions. The distal forelimb of tetrapod vertebrates is homologous to the pectoral fin rays and membrane of fishes. Fish fins demonstrate similar mechanosensory abilities to hands and other distal tetrapod forelimbs in touch and proprioception. These results indicate that vertebrates were using the core mechanosensory inputs, such as fast adapting and slow adapting nerve responses, to inform fin and limb function and behavior before their diversification in fish and tetrapod lineages.

Two senses of human limb position: methods of measurement and roles in proprioception

The sense of position of the body and its limbs is a proprioceptive sense. Proprioceptors are concerned with monitoring the body's own actions. Position sense is important because it is believed to contribute to our self-awareness. This review discusses recent developments in the debate about the sources of peripheral afferent signals contributing to position sense and describes different methods of measurement of position sense under conditions where vision does not participate. These include pointing to or verbal reporting of the perceived position of a hidden body part, alignment of one body part with the perceived position of another, or using memory-based repositioning tasks. The evidence suggests that there are at least two different mechanisms involved in the generation of position sense, mechanisms using different central processing pathways. The principal sensory receptor responsible for position sense is believed to be the muscle spindle. One criterion for identifying mechanism is whether position sense can be manipulated by controlled changes in spindle discharge rates. Position sense measured in two-limb matching is altered in a predictable way by such changes, while values for pointing and verbal reporting remain unresponsive. It is proposed that in two-limb matching the sensation generated is limb position in postural space. In pointing or verbal reporting, information is provided about limb position in extrapersonal space. Here vision is believed to play a role. The evidence suggests that we are aware, at the same time, of sensations of limb position in postural space as well as in extrapersonal space.