Ulf Norrsell

A system of unmyelinated afferents for innocuous mechanoreception in the human skin

It is generally held that tactile mechanisms in the human skin are served by fast-conducting myelinated nerve fibres, whereas touch-sensitive afferents with unmyelinated axons are lacking in man, in contrast to other mammals. In the present study we found evidence that sensitive mechanoreceptive afferents with unmyelinated fibres are quite common and widespread in the hairy skin of human subjects. Their biological role remains an enigma which might attract more attention now that their existence in man has been demonstrated.

Observations on human tactile directional sensibility

1. The ability to tell the direction of a motion across the skin deserve attention for being an easily observed function which provides a sensitive test for disturbances of the peripheral and central nervous systems. The mode of operation, on the other hand, of this tactile directional sensibility is still uncertain. 2. The dependence of directional sensibility on the contact load and distance of movement of a blunt metal tip, has now been determined for the skin of the forearm of normal subjects with the two‐alternative forced‐choice method. The testing was done under two conditions: elbow bent or straight. Straightening of the arm always reduced the accuracy of the directional sensibility. It also caused measurable changes of cutaneous mechanical properties, which presumably decreased the reliability of afferent information about lateral distension. 3. The average accuracy of the directional sensibility was found to be correlated linearly to the logarithm of the contact load, and straightening of the arm decreased the accuracy for each load by corresponding amounts. Similar relationships were found between the accuracy and the distance of movement. 4. Straightening of the arm did not cause any significant average reduction of the contact threshold for point stimulation of the same receptive field. A consistently lowered contact sensitivity, however, was observed for some of the subjects, which may have contributed to the reduction of the directional sensibility in these cases. 5. Correct directional estimations of the movement of the metal tip were obtained for a distance which was a fifth of the shortest distance for a corresponding estimation of the movement of a frictionless stimulus. The findings thus indicated that the friction between a moving object and the underlying skin, which can be mediated via stretch‐sensitive cutaneous receptors, is critical for the determination of its direction of motion. 6. The present observations and previous observations by various authors are suggested to indicate that typical tactile directional sensibility depends on parallel processing of direction‐selective data, and spatial data expressed as a function of time.

Spinal pathways projecting to the cerebral first somatosensory area in the monkey

1. Potentials evoked with short latency in the cerebral post‐central gyrus after intradermal electrical, or adequate light tactile stimulation of the contralateral body half have been studied in the monkey, before and after transection of different spinal pathways at the upper cervical level.

Directional sensibility for quantification of tactile dysfunction

Examination of tactile directional sensibility, i.e., the ability to tell the direction of an objects motion across the skin, has been recommended by several authors for examination of patients with somatosensory disorders. Recent findings about the physiological mechanisms underlying directional sensibility suggested possibilities to further improve the test. In the present investigation a test was constructed that allowed a semiquantification of the directional sensibility of six body areas within 20 min. Normal values were obtained by testing healthy subjects (n = 40), and the normal values were compared to those obtained in a group of patients with tactile symptoms (n = 20). Ten of the patients had abnormal sensory conduction in one or several nerves, and they also had abnormal directional sensibility. Hence, examination of directional sensibility, according to the present protocol, provides a semiquantitative test that appears to be as sensitive as electrophysiological measurement of conduction in detecting dysfunction in tactile nerves.

Remarkable capacity for perception of the direction of skin pull in man

We determined the ability to appreciate the direction of a skin pull caused by a moving pin that was glued to the forearm skin. A majority of the subjects were able to tell the direction of pin movements with an excursion of 0.13 mm (>/=66% correct responses, p<0.05). Local skin anaesthesia showed that stretch sensitive receptors located over 15 mm in front and behind the pin correctly signalled the direction of these minute skin pulls. It was concluded that information about patterns of skin stretch is an important component of the somatosensory system that may contribute not only to kinaesthetic, but also to cutaneous sensations.

An evoked potential study of spinal pathways projecting to the cerebral somatosensory areas in the dog.

SummaryEvoked potentials from stimulation of the hindlimbs were recorded in the cerebral somatosensory areas of dogs using Nembutal anaesthesia. Various transections of the low thoracic spinal cord were made in order to determine the pathways projecting to these areas. The shortest latency cortical activation was obtained via two pathways: the dorsal column pathway and the spinocervical tract which is located in the dorsomedial part of the lateral funiculus (DLF). Cortical activation after longer latency was obtained via ventral pathways in both the contralateral and ipsilateral spinal halves. The cortical potentials elicited by adequate (hair) stimuli depended mainly on transmission via the dorsal column and DLF pathways.

Tactile directional sensibility and diabetic neuropathy.

Five different procedures used to diagnose neuropathy were compared in a “blind” study with diabetic patients. The aim was to evaluate tests of tactile directional sensibility. Three matched groups were examined, two groups with type I diabetes, either with or without suspected neuropathy, and one of healthy controls. Testing consisted of: (1) examination by a specialist in neurology, (2) electrophysiologic measurement of nerve conduction velocity and determination of cool sensitivity, and (3) determination of directional sensibility in two stages, with categorical and quantitative techniques. Abnormal test results were obtained for both groups of diabetic patients. Quantitatively measured directional sensibility had the highest sensitivity (89%) and specificity (85%) when calculated for patients who had received a diagnosis of neuropathy from the neurologist, despite one case of abnormal directional sensibility among the healthy controls. Conduction velocity testing was almost comparably sensitive (80%) and cool sensitivity, comparably specific (85%) when calculated in the same manner.

Temperature sensitivity of the paw of the cat: a behavioural study

1. Cats were trained to respond to differences between the temperatures of the floors of two corridors of a modified T‐maze for a food reward.

A second tactile system in the human skin with unmyelinated primary afferents

In man, unmyelinated afferents from the skin have been identified exclusively with pain and temperature senses on the basis of the stimuli which excite them. In other mammals, on the other hand, many C-afferents respond to light touch. This difference has lead to the suggestion that a phylogenetically old system of unmyelinated mechanoreceptors has largely faded out in the evolutionary process from mammals to man. However, a few touch sensitive C-afferents have recently been described in the human facial skin, suggesting a vestige in a specialized skin region. In the present study we have shown, with microneurography, that touch sensitive C-afferents are present in the human forearm. Their conduction velocities, thresholds to skin indentation, response characteristics to moving stimuli, and receptive field geography have been studied. The findings suggest that man, as other mammals, is equipped with a general system for cutaneous mechanoreception subserved by C-afferents, although it seems to be lacking in the peripheral parts of the extremities.

Magnus Gustaf Blix (1849-1904); Neurophysiological, Physiological, and Engineering Virtuoso

This paper was written to honour the 150th anniversary of the birth of Magnus Gustaf Blix. Blix belongs to the small group of 19th century physiologic neuroscientists who still regularly are cited, on account of having presented fundamental results. He contributed to three fields: somatic sensation, and visual and muscular function. He was the first to publish evidence regarding modality specific receptors in the skin. He extended the work of Hermann von Helmholtz on the optical properties of the anterior ocular chamber of living humans, after having constructed the necessary apparatus. He also measured the heat production of contracting muscles. For this purpose he constructed the apparatus that provided a start for A. V. Hill’s Nobel Prize-winning work in the field. He showed for the first time that the power of muscle contractions depended on the length/extension of the muscle fibres. He worked on the possibility of muscle powered human aviation. For this purpose he constructed a bicycle dynamometer for measuring the maximal human power output. He was the vice-chancellor of Lund university when he died from an acute disease in 1904 at the age of only 55 years.