A B Vallbo

The responses of afferent fibres from the glabrous skin of the hand during voluntary finger movements in man.

1. Afferent activity of 111 single units from the glabrous skin area was recorded percutaneously in the median nerve of human subjects, using tungsten electrodes. 2. The majority of the units (103) were classified as low‐threshold mechano‐sensitive units belonging to one of the four categories previously described: rapidly adapting with small receptive fields (RA), rapidly adapting with large receptive fields (PC, presumed Pacinian corpuscle units), slowly adapting with small fields (SA I), and slowly adapting with large fields (SA II). The size of the responses (in number of impulses) to indentation and stretching of the skin was compared with that of the responses elicited during voluntary isotonic finger movements, which avoided trivial excitation of the units by direct touch. 3. All four types of units, and 77% of the single units, were activated by isotonic movements. The decreasing order of responsiveness was PC, SA II, SA I, RA. 4. Almost all responsive units were excited during the dynamic phase of ramp and smooth oscillatory movements. Static responses, on the other hand, occurred only with 50% of the slowly adapting units, corresponding to a third of the total sample (SA II, 81%; SA I, 17%. 5. For all four types of units the dynamic responses to movements were of similar size as the responses to localized skin indentation with a von Frey hair at five times threshold. 6. The results are discussed with regard to the possible implications for kinaesthesia and motor control.

Detection of tactile stimuli. Thresholds of afferent units related to psychophysical thresholds in the human hand.

1. Psychophysical thresholds were determined at 162 points in the glabrous skin area of the human hand when slowly rising, triangular indentations of controlled amplitudes were delivered with a small probe. The method of constant stimuli was used with either the two alternative forced choice or the yes‐no procedure. It was found that the distribution of the psychophysical thresholds varied with the skin region. Thresholds from the volar aspect of the fingers and the peripheral parts of the palm were low and their distribution was unimodal with a median of 11.2 micrometers. In contrast, there was an over‐representation of high thresholds when observations from the centre of the palm, the lateral aspects of the fingers and the regions of the creases were pooled, and the distribution was slightly bimodal with a median of 36.0 micrometers. 2. Nerve impulses were recorded from single fibres in the median nerve of human subjects with percutaneously inserted tungsten needle electrodes. The thresholds of 128 mechanosensitive afferent units in the glabrous skin area of the hand were determined when stimuli were delivered to partly the same points as stimulated for the assessment of the psychophysical thresholds. Of the four types of units present in this area the Pacinian corpuscle (PC) and rapidly adapting (RA) units had the lowest thresholds with medians of 9.2 and 13.8 micrometers, followed by the slowly adapting type I and slowly adapting type II units with medians of 56.5 and 33.1 micrometers. There was no indication of a difference between thresholds of units located in different skin areas. 3. In the region of low psychophysical thresholds there was good agreement between the thresholds of the rapidly adapting and Pacinian corpuscle units and the psychophysical thresholds, particularly at the lower ends of the samples. In the skin regions of high thresholds, on the other hand, practically all psychophysical thresholds were higher than the thresholds of the most sensitive afferent units. Moreover, simultaneous recording of nerve impulses during a detection task indicated that subjects did not detect stimuli strong enough to elicit several impulses in afferent units in this region. 4. Circumstantial evidence led to the conclusion that detection was dependent on one impulse in one or a few rapidly adapting units under optimal conditons in the region of low psychophysical thresholds, whereas it seemed unlikely that activity in Pacinian corpuscle units was crucial. 5. The findings are consistent with the interpretation that human subjects are able to detect an input consisting of a single impulse in a single rapidly adapting unit.

Intensity of sensation related to activity of slowly adapting mechanoreceptive units in the human hand

1. Impulses were recorded from single afferent fibres in the median and ulnar nerves of human subjects. The response of slowly adapting mechanosensitive units with receptive fields in the glabrous skin of the hand were studied when rectangular indentations of varying amplitudes and invariant time duration were delivered. Simultaneously the subject was asked to estimate the magnitude of his sensation associated with the stimuli.

Spatial properties of the population of mechanoreceptive units in the glabrous skin of the human hand.

Impulses were recorded in low threshold mechanoreceptive afferents innervating the glabrous skin area of the human hand by means of needle electrodes percutaneously inserted in the median nerve of waking human subjects. In accordance with earlier investigations, the units were separated into 4 groups, mainly on the basis of their adaptation and receptive field properties: RA, PC, SA I and SA II units. The extent of the receptive fields of 255 units were mapped with von Frey hairs. The fields of the RA and SA I units were of similar size with medians of 12.6 sq. mm and 11.0 sq. mm. In contrast, the fields of the SA II and PC units were about 5 and 10 times larger, respectively. A difference in size between fields from different skin regions was found only for the RA fields which were smaller the more distal the skin region. The distributions of the field sizes were positively skewed for all 4 unit types. A model was constructed of the population of RA and SA I fields within the various regions of the hand. The model was based on (i) an estimate of the densities of units reported in a previous study, and (ii) the sizes of the receptive fields. The number and the spatial distribution of the fields which would be stimulated by indentations of simple and defined geometries were deduced from the model. The findings indicate that the RA and SA I units provide the information required for the psychophysical capacity of spatial analysis and discrimination in the glabrous skin of the hand.

The absence of position response in spindle afferent units from human finger muscles during accurate position holding.

1. Single unit activity of muscle spindle afferents from finger extensor muscles was recorded in the radial nerve of waking human subjects. The mean discharge rate of the afferent units was determined while the receptor related finger was held at fixed angular positions of the metacarpo‐phalangeal joint. 2. During a visual tracking task the subjects had to maintain specified angular positions against a load of constant torque which opposed finger extension. For each unit a comparison was made between the mean discharge rates at two angular positions which differed by 20 deg. Under such isotonic conditions the rates of afferent discharge at the two joint positions did not significantly differ, neither for the whole sample of primary, nor for that of secondary units. This was true, no matter whether the load was small or intermediate. Large loads were not tested. 3. For comparison, the passive position responses of a sample of spindle afferent units from the same muscles were studied when the finger was held in fixed positions while the muscles were voluntarily relaxed. Under these conditions a significant position sensitivity was found for both primary and secondary afferents. The mean values were 0.28 impulses/sec. deg (primaries), and 0.21 impulses/sec. deg (secondaries). 4. The absence of position response during active position holding was interpreted as a manifestation of changes in fusimotor outflow which depended on joint position and were large enough to compensate for changes in muscle length.

Thresholds of mechanosensitive afferents in the human hand as measured with von Frey hairs

Abstract Afferent activity in low threshold mechanoreceptive units innervating the glabrous skin area of the hand was recorded from the median nerve of human subjects, using tungsten needle electrodes. The units were classified as RA, PC, SA I and SA II units mainly on the basis of their adaptation and receptive field properties Thresholds of the units were determined by two methods, the force threshold was determined with von Frey hairs and the indentation threshold with triangular indentations of controlled amplitudes. The characteristics of the force developed by the von Frey hairs as a function of movement parameters was analyzed when the hairs were prodded against a force transducer. The rise time was 10–20 msec and the variability of the peak force was within-20% of the steady-state force during repetitive prodding with various repetition rates. The von Frey thresholds were determined for 284 units and the indentation threshold for 116 of them. The RA and PC units were the most sensitive unit types with median thresholds of 0.58 mN and 0.54 mN. The medians of the slowly adapting SA I and SA II units, were 1.3 mN and 7.5 mN. The threshold distributions for all unit types were positively skewed. No regional differences were found but the thresholds were identical in skin areas where the psychophysical thresholds have been shown to be diverse. The relation between force threshold and indentation threshold was analyzed for 116 units. There was a positive correlation between the two thresholds, although the relation appeared not to be identical for the four types of units and the scatter was considerable.

Sensations evoked from the glabrous skin of the human hand by electrical stimulation of unitary mechanosensitive afferents

A needle microelectrode in the median nerve was used to record impulses from tactile afferent units and to deliver trains of electrical pulses for microstimulation. The findings suggest that the activation of single SA I units produces a sensation of light pressure within a small area located within the receptive field of the unit whereas the activation of RA units seemed to elicit sensations of varying qualities: touch, vibration, and tickle.

Discharge in muscle spindle afferents related to direction of slow precision movements in man.

Single‐unit activity was recorded with needle electrodes in eighteen muscle spindle afferents (eleven primaries, seven secondaries) from finger extensor muscles in the radial nerve of awake human subjects. The discharge rate of the afferents was determined during precisely controlled voluntary movements. The subjects performed a standardized visual ramp‐and‐hold tracking task, which included very slow finger extension and flexion movements (2.5 deg/s) with an amplitude of 20 deg. Throughout the tracking task a constant torque load of small or intermediate size, i.e. less than 30% of maximum voluntary contraction force, opposed finger extension. Altogether, 131 trials were studied. For most units the discharge rate was lower during shortening compared with active position holding, and it was higher during lengthening contractions. Thus, the majority of units responded to phasic stretch during the active movements, although the size of the movement response varied considerably between units and was never large. A few units even exhibited a reversed stretch response pattern. Hence, when estimated from pooled data, movement responses of the unit sample as a whole were small, around 1 impulse/s. The over‐all response pattern of an individual afferent during the tracking task was very similar between successive tests. Although the discharge rate of most units increased with the load during movements as well as during position holding, the presence as well as the magnitude of movement responses depended only little on the size of the load. However, a few afferents exhibited a stretch response pattern with small loads and a reversed stretch response pattern with larger loads. In spite of the predominant increase of afferent firing during muscle lengthening there was no systematic modulation of the discharge rate in relation to the joint angle during the active movements, either in the primary or in the secondary afferents. The present findings suggest that human muscle spindles provide information about the occurrence as well as the direction of slow isotonic movements at low velocities in a precision motor task. This is in contrast to the lack of accurate position response which has previously been demonstrated.

Human muscle spindle response in a motor learning task.

1. Impulse discharge of single muscle spindle afferents from the finger extensor muscles was recorded in the radial nerve of conscious human subjects, during a motor learning task engaging the metacarpo‐phalangeal joint of a single finger, using the microneurography technique. 2. Subjects were requested first to pay attention to a complex sequence of imposed single joint movements, and immediately afterwards to reproduce actively the same sequence. No external load was added to the finger and visual control was denied altogether so that subjects relied on mechanoreceptor input exclusively for the sampling and reproduction of movement. In addition, sequences of imposed movements were delivered while subjects were not attending in order to allow analysis of the attention effect. 3. The response of the individual unit was uniform in repeated tests. There were clear differences between spindle firing rate in imposed and actively reproduced movements with most units. However, the difference was complex during the individual sequence, in that firing rate was usually higher during periods of reproduced movements when the muscle was relatively short whereas it was identical when the muscle was relatively long. 4. The hypothesis that reproduction and verification of an imposed movement may be based on simple matching between identical spindle firing in imposed and active movements, was difficult to reject altogether because identical spindle input was present during considerable sections of the movement sequence. It may be speculated that agonists and antagonists cover different ranges of joint excursion, with identical spindle firing rates in imposed and reproduced movements. 5. Attention to imposed movements was associated with a minute and inconsistent increase of spindle firing rate in some afferents and then usually with a slight increase of EMG activity of the parent muscle as well. 6. It was concluded that focusing attention on the kinaesthetic input during imposed movement was not associated with a consistent increase of fusimotor drive.

Stretch sensitization of human muscle spindles.

1. Sixty‐seven afferents from the finger extensor muscles were consecutively recorded by microneurography. 2. The units were classified as primary or secondary muscle spindle afferents or Golgi tendon organ afferents on the basis of their responses to ramp‐and‐hold stretches, sinusoidals superimposed on ramp‐and‐hold stretches, maximal twitch contractions and isometric contractions and relaxations. 3. The muscle was repeatedly stretched and then either kept short or long for a few seconds followed by a slow ramp stretch. The responses of the muscle afferents to the slow stretch were compared under the two conditions. 4. Thirty out of thirty‐eight units classified as primary spindle afferents and four out of eleven units classified as secondary afferents showed an enhanced response to the slow ramp when the muscle had been kept short compared to the response when the muscle had been kept long. 5. None of the eighteen Golgi tendon organ afferents showed any difference in this respect. 6. It is concluded that stretch sensitization does occur in human muscle spindles and, when present, constitutes firm evidence of the afferent originating from a muscle spindle rather than a Golgi tendon organ. In addition, due to differences in the response characteristics of primaries and secondaries, the test may aid in separating muscle spindle primary afferents from secondary afferents.