A&O note – INPUT – interoception & proprioception

ART and ORGANISM

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BIOLOGICAL BACKGROUND: INPUT and PERCEPTION


We see the world not as it is,
But as we are
TALMUD
(and Anais Nin, and Goethe, and Stephen Covey)
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PROPRIOCEPTION (from Hillier, Immink, and Thewlis  2015, Introduction)[i]

Ambiguity Leading to Confusion

In the early 1830s, Sir Charles Bell described the sixth sense, referring to the sense of position and action of the limbs.1 Proprioception was further discussed by Sherrington in his seminal text and lecture series some 70 years later.2 Since then the term has been used to describe a variety of senses and therefore has become somewhat ambiguous. The word proprioception comes from the Latin proprius, meaning “one’s own,” combined with the concept of perception: thus a literal translation is that of “perceiving one’s own self.” This notion of self-perception reflects one’s ability to have both a sense of body orientation and position as well as a sense of body and limb motion. Accordingly, words often used interchangeably with proprioception are (joint) position sense, kinesthesia, movement sense, body position in space, sense of effort, or sense of force. The confusing and inconsistent application of these terms, particularly in the clinical domain, reflects the slowly emerging understanding of the nature of how we sense ourselves.

Berthoz3 classically defines kinesthesia as the sense of movement, of which proprioception (sense of position and of velocity) is a part, along with cutaneous receptors, and the vestibular and visual systems. Conversely, other authors cite proprioception as having 3 submodalities: kinesthesia, joint position sense, and sensation of force (see, eg, Niessen et al4). Others again add body segment static position, displacement, velocity, acceleration, and muscular sense of force, effort, or heaviness (see Ogard5 or Proske and Gandevia6) to the list of proprioceptive constructs. In this review, we will refer to proprioception as a collective term for these “subsenses,” unless the evidence only pertains to one in which case we will refer to that subsense individually.

Finally, proprioception can be considered one of the subsystems within the somatosensory system (along with pain, touch, and thermal sensation) and has also been considered interoceptive in that the sensory information is derived from changes within internal structures. This classification is in distinction to exteroception where the stimulus originates from outside the body such as external heat for thermoreception or light stimuli for vision.

Proprioceptive Receptors

Proprioception, or proprioceptive acuity, is a complex system that involves both peripheral and central systems. The evidence for the prime proprioceptive receptor favors muscle afferent input,6in particular muscle spindles. These receptors are specialized fibers within the muscle that detect change in muscle length and also the velocity of contraction7 (or body part motion as the first derivative of length, ie, the rate of the change in length). If a passive lengthening is applied to a muscle, spindle exafferent signals are produced and interpreted as a sensation of movement, with increasing velocities causing an increasing response.6 The spindle system has the capacity to anticipate length change because it can detect velocity as well as length (which changes more quickly).3 Furthermore, motion direction can be perceived relative to which particular muscle has shortening or lengthening activity, and presumably by comparison between agonist and antagonist activity ratios, joint position can be perceived. The muscle spindle is also under fusimotor control (the gamma system), during contractions, which has the capacity to alter the calibration or sensitivity of the receptor by altering its internal length.8 This modulation allows adaptation of the receptors during action and also allows simulation in the absence of real action.3 Related to the sensitivity of the spindle is the role of thixotrophy—the phenomenon that demonstrates a relationship between a muscle’s properties and its immediate history of contracting/lengthening. It is a property of muscle (and muscle spindles) and influences proprioception—that is to say proprioception (spindle sensitivity) can be altered, dependent on whether the muscle has recently contracted or not (see Proske and Gandevia6)

Proske and Gandevia6 summarize the evidence that receptors in the skin (cutaneous receptors) also contribute to joint position and motion sense, for example, as skin strain, particularly at the digits, elbow, and knee. Receptors analogous to the cutaneous receptors also exist in joint structures. For example, the more superficial Ruffini endings in the joint capsule, ligaments, and menisci are slow adapting mechanoreceptors. This allows detection of static joint position, intra-articular pressure, and possibly joint motion in terms of amplitude and velocity.8 Pacinian corpuscles are deeper in these joint connective tissues with a lower mechanical threshold and are faster adapting—allowing them to be more responsive to changes in velocity, that is, acceleration and deceleration. Lephart and Fu8 also describe free nerve endings widely distributed in articular structures and which may play a role in detecting severe mechanical deformation or inflammatory changes. However, it is now accepted that these mostly mechanical receptors predominate at extreme angles of joint position and motion and are relatively silent midrange.6,9 Golgi tendon organ-like receptors are also found in cruciate and collateral ligaments and menisci; like the mechanoreceptors listed, it is reported that they are useful at extremes of range as limit detectors.8 This then leaves the muscle spindles to provide most information in the middle region of joint action.6

Effort-based signals, that is, the efferent commands to muscles, have a role in proprioception (particularly sense of force and/or position sense). These signals are distinct from the traditional kinesthetic senses in that they are based on motor commands and therefore not experienced in a passive limb. However, Gandevia et al10 investigated paralyzed and anesthetized upper limbs to demonstrate an alteration in position sense at the wrist joint, relative to the effort of attempting to produce a contraction in the paralyzed muscle(s), concluding there is a definitive role for “outflow” (efferent) signals in proprioception.

In summary, proprioception is based on an ensemble of sensory inputs that serve sensing, producing, predicting, and simulating joint position, joint motion (velocity and direction), and force specification.3 There is considerable specificity and sensitivity in this ensemble arrangement as well as redundancy, particularly when proprioception is converged with the visual and vestibular systems in detecting motion and spatial orientation (from visual- and gravity-referenced coordinates).3,11 The most basic of postural control tasks, such as standing in regular environments,12 is contingent on the coordination of these processes.

 

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SELECTED INPUT LINKS


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1-2009 / 2017

 

 

 


[i] Assessing Proprioception Susan Hillier, PhD, Maarten Immink, PhD, Dominic Thewlis, PhD

Neurorehabilitation and Neural Repair   Vol 29, Issue 10, pp. 933 – 949  First published date: February-23-2015

10.1177/1545968315573055