Physiological characteristics of low-threshold mechanoreceptors in joints, muscle and skin in human subjects.

Solid gjennomgang av mechanoreceptorer i forskjellige deler av kroppen, muskler, ledd og hud.

Nevner bl.a. at ruffini er viktige mechanoreceptorer i ligamenter og ledd.

Og at muskelspindler er er svært sensitive og kan reagere på trykk og vibrasjon også. Men sammen med ruffinier, så oppfatter ikke hjernen enkeltvis spindel-stimulans. De må være flere for at hjernen skal reagerer.

Nevner også en tredje type mechanoreceptoer, SAIII, som reagerer på hudstrekk. Dette må undersøkes videre.

http://www.ncbi.nlm.nih.gov/pubmed/15730450

From this work, we know that there is specificity in the sensory channels: electrical stimulation of a single Meissner or Pacinian corpuscle generates frequency dependent illusions of ‘flutter’ or ‘vibration’, whereas microstimulation of a single Merkel afferent can produce a percept of ‘pressure’ and stimulation of a single joint afferent can evoke a sensation of ‘joint rotation’. Interestingly, the input from a single Ruffini ending in the skin cannot be perceived and the same is true of muscle spindle afferents.

Meissner and Merkel endings have very small receptive fields and respond only to local forces. Pacinian corpuscles have an exquisite sensitivity to brisk mechanical events and could respond to such stimuli transmitted through the bone to a remote receptor, but would not be able to encode sustained forces. Ruffini endings also respond to forces applied remote to the receptive field and, unlike the Pacinian corpuscles, respond in a sustained fashion, but would their signals be perceived? Like muscle spindles, it is possible that the coactivation of many Ruffini endings could provide meaningful information.

The present paper will deal only with what is known about the properties of mechanosensitive endings in skin, joint and muscle in human subjects.

MECHANORECEPTORS IN JOINTS AND LIGAMENTS

Because they are located at the interfaces between bones, mechano- receptors within the joint capsule and ligaments could, conceivably, respond to forces transmitted through the bone. Mechanoreceptors in the posterior capsule of the cat knee joint, identified as Ruffini endings, maintain a sustained discharge to a constant stimulus.

Ruffini and Golgi-like receptors have also been identified in the human knee joint, as have Pacinian corpuscles.35 Microelectrode record- ings from the median and ulnar nerves of awake human subjects have shown that mechanoreceptors associated with the inter- phalangeal joints and metacarpophalangeal joints, also believed to be Ruffini endings, do not respond to forces applied to bone when there is no movement of the joint and have very high mechanical thresholds to indentation applied over the joint capsule.11,12 Although they do respond to joint movements, they respond primarily at the limits of angular excursion.

MECHANORECEPTORS IN TENDONS

Tendons contain specialized sensory endings, the encapsulated Golgi tendon organs, the long axes of which are orientated in series with the collagenous fibres of the tendon and the muscle fibres to which they are attached. Because of this in-series coupling to muscle fibres, Golgi tendon organs are ideally suited to encode the forces developed by the contracting muscle fibres. However, Golgi tendon organs are notoriously poor at encoding changes in muscle length.36,37 As such, these endings do not respond to the longi- tudinal strains associated with passive joint rotation, but can respond to punctate compressive forces applied directly to the receptive field within the musculotendinous junction or tendon proper.

MECHANORECEPTORS IN MUSCLES

Muscles contain highly specialized stretch receptors, the muscle spindles, that have been the subject of much investigation. Each muscle spindle comprises several intrafusal (‘within the spindle’) muscle fibres enclosed within a capsule. There are two types of sensory ending, the primary ending and the secondary ending, both of which adapt slowly to a maintained stretch.

Unlike the Golgi tendon organs, muscle spindles are arranged in parallel to the muscle fibres, rendering them incapable of encoding forces generated by the contracting muscle, but very sensitive to length changes within the muscle.

ving an efferent innervation: activation of fusimotor (gamma) neurons causes contraction of the intrafusal (but not extrafusal) muscle fibres, thereby recruituing a silent spindle ending, increasing its resting discarge or changing its sensitivity to imposed stretch.

These receptors can be exquisitely sensitive, respond- ing to rather light tapping, vibration or pressure applied to the skin overlying the receptive field within the muscle belly, and respond to brisk mechanical events transmitted through the tendon, as well as to sustained forces applied to the tendon. Muscle spindles are very sensitive to vibration of the muscle belly or tendon, responding to a wide range of frequencies:42 the spindle illustrated in Fig. 3 even responded to vibration over the nail of the big toe!

Muscle spindles are the sensory endings primarily responsible for our proprioceptive acuity: small-amplitude vibration applied to muscles or tendons was the first convincing demonstration that these intramuscular stretch receptors contribute to propriocep- tion.43

However, microstimulation of a single muscle spindle afferent is not perceived by the subject; apparently, the synaptic strength between spindle afferents and higher-order neurons is so weak that coactivation of many spindle afferents is required to generate perceptual responses.12

MECHANORECEPTORS IN THE SKIN

The skin contains many specialized mechanosensitive endings that subserve the broad sense of ‘touch’ and also contribute to proprioception and motor control. The majority of human micro- neurography studies have characterized the physiology of tactile afferents in the glabrous skin of the hand, but mechanoreceptors in the hairy skin of the hand, forearm, leg and face have also been examined. Microelectrode recordings from the median and ulnar nerves in human subjects have revealed the existence of four classes of low-threshold mechanosensitive afferent supplying the glabrous skin of the hand, which correspond to the four types of specialized sensory endings identified histologically:46 Meissner and Pacinian corpuscles, Merkel cell–neurite complexes and Ruffini endings.

Two classes of afferent adapt rapidly to a sustained indentation of the skin (‘fast- adapting’), types FAI and FAII, and two classes of afferent maintain their firing throughout the stimulus (‘slowly adapting’), SAI and SAII. Based on behavioural similarities with afferents recorded in the cat and monkey,47 it is believed that the FAI and FAII afferents supply the Meissner and Pacinian corpuscles, respectively, and the SAI and SAII afferents supply the Merkel cell–neurite complex and Ruffini ending, respectively.48 Type I tactile afferents have small circular or ovoid receptive fields with distinct borders, each receptive field encompassing several small zones of maximal sensitivity (‘hot-spots’) that represent the individual Meissner corpuscles supplied by a single FAI afferent and the Merkel cell–neurite complexes innervated by each SAI axon.48,49 The type II afferents have a single zone of high sensitivity and large, poorly defined borders.

Fast-adapting type I (Meissner) afferents can only be activated by discrete stimuli in a small, well-defined area. They are particularly sensitive to light stroking across the skin, responding to local shear forces and incipient or overt slips within the receptive field.

The FAII (Pacinian) afferents are exquisitively sensitive to brisk mechanical transients. Unlike the FAI afferents, FAII afferents respond vigorously to blowing over the receptive field, responding to the fricative quality of the airflow generated by the experimenter blowing through pursed lips onto the receptive field area (they do not respond when blowing through a straw, for example). This is illustrated in Fig. 4.

Slowly adapting type I afferents (Merkel) characteristically have a high dynamic sensitivity to indentation stimuli applied to a discrete area and often respond with an off-discharge during release. Although the SAII afferents do respond to forces applied normal to the skin, a unique feature of the SAII afferents is their capacity to respond also to lateral skin stretch.

Five classes of myelinated tactile afferent have been recorded from the lateral antebrachial cutaneous nerve, which supplies the hairy skin of the human forearm: two types of slowly adapting afferent (SAI and SAII) that can be classified in a similar fashion to those in the glabrous skin and three types of rapidly adapting afferent (hair units, field units and Pacinian units).51

Hair units respond specifically to movements of individual hairs and air puffs onto the receptive field, whereas field units respond to actual skin contact. Hair units in the forearm have large ovoid or irregular receptive fields composed of multiple sensitive spots that corresponded to individual hairs. On average, each afferent supplies 20 hairs.51 The field units show a similar arrangement of multiple high-sensitivity spots11–13 making up a similarly large area, although the individual spots are larger and less isolated than those of the hair units.

Unlike glabrous skin, the hairy skin is only loosely connected to the subcutaneous tissues, thereby allowing greater stretch and, hence, greater activation of stretch-sensitive cutaneous afferents.53–55 There do appear to be significant differences in the movement sensitivity of tactile afferents in the non-glabrous and glabrous skin. For instance, 92% of the afferents on the dorsum of the hand responded to finger movements,53 whereas only 68% of afferents on the palmar side of the hand responded to passive finger movements11 and 77% responded to active movements.56

The FAI afferents respond only to movements of the joint over which they are located, but both the SAI as well as SAII afferents are very sensitive to the skin stretch associated with finger movements, whether this is produced by rotation of the nearest joint or by movements of remote digital joints. The static sensitivity to stretch is high for both classes of slowly adapting afferent:54 when measured at an equivalent joint angle, the static sensitivity of the SAI and SAII afferents is similar to that of muscle spindle endings in the long extensors of the fingers, 0.2–0.5 Hz per degree of rotation of the metacarpophalangeal joint.37,54 Figure 5 shows the behaviour of an SAI afferent recorded from the common peroneal nerve, cutaneous fascicles of which innervate the hairy skin on the dorsum of the foot and the lateral aspect of the leg.

It responded in a slowly adapting fashion to punctate stimuli applied within its receptive field, but also responded to skin stretch applied remote to the receptive field. Indeed, this receptor responded to skin stretch applied 5 cm proximal to the ankle, some 25 cm away!

Recently, Edin57 has shown that there may be a third type of slowly adapting receptor in hairy skin; the so-called SAIII was found in recordings from the lateral cutaneous femoral nerve, which supplies the anterior thigh and knee area and possesses properties intermediate between the SAI and SAII afferent types. Like these two classes, the SAIII exhibits a high static and dynamic sensitivity to skin stretch, responding with high fidelity to movements of the knee joint.

Small movements may also be sensed by specialized mechanoreceptors in the skin overlying the bone: FAII (Pacinian) and SAII (Ruffini) afferents can respond to stimuli applied remotely to their receptive field, as can SAI (Merkel) afferents in the more mobile hairy skin. The other types of cutaneous mechanoreceptor respond to stimuli applied only within their small receptive fields.

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