Acta physiol. scand. 1976. 97. 66-74
::
From the Department of Physiology, University of Umefi, Sweden
Prediction of Propagation Block on the Basis of Impulse Shape in Single Unit Recordings from Human Nerves BY
A. B. VALLBO Received 17 October 1975
Abstract VALLBO,A. B. Prediction of propagation block on the basis of impulse shape in single unit recordings from human nerves. Acta physiol. scand. 1976. 97. 66-74. The occurrence of impulse block was studied in single unit recordings and related to impulse shape when peripheral nerves were impaled with tungsten needle electrodes. Exposed nerves were exploited in frog and cat and non-exposed nerves in man. Three different impulse shapes were seen: I . Negative spikes of very short duiations which nearly all propagated. 2. Positive double peaked spikes which all propagated. 3. Positive single peaked spikes which were of two different natures: one which propagated and one which did not. The findings suggest thzt unitary impulses recorded with a needle electrode impaling the myelin are positive and single peaked when the injury is minimal. Gradually a propagation delay might develop at the site of impalement giving rise to a double peak. Later the propagation may be blocked which is seen as a drop out of the second peak. The collected sample of observations indicates that it was possible to predict in practically all cases whether the propagation was blocked or not from the shapes of the single unit impulses and their alterations as seen by the tungsten needle electrode. Key words: Man, single nerve fibre, propagation block
The method of percutaneous recording of nerve impulses from awake human subjects with intraneural electrodes (Vallbo and Hagbarth 1968) has allowed the functional analysis of several neuronal systems in the intact organism. Effect autonomic activity has been studied as well as proprioceptive afferent inflow from muscles during voluntary contractions and afferent impulses from sense organs in the skin (for ref. see Vallbo and Hagbarth 1973). With regard to the somatosensory system, this type of experiments provide a unique opportunity to study, in the same subject, neural afferent inflow and psychophysical responses based upon this afferent inflow. It seems that an analysis of the responses at these two levels of the somatosensory system to well controlled stimuli might contribute to the understanding of the relation between biophysical events in the nervous system and perceptive mechanisms of the mind. When considering the potentialities of this experimental approach, it is a fundamental step to assess whether the unitary impulses recorded with the needle electrode are propagated past the recording site to reach the central nervous system or if the impulses are blocked due 65
67
IMPULSE BLOCK IN HUMAN NERVES
A
B
C
D
Fig. I. Schematic diagram showing the experimental arrangements. Mechanical stimuli were applied with a glass rod o r a drawing brush (A) to the skin (9). Nerve impulses of a single fibre from a skin mechanoreceptor unit were recorded with a tungsten needle electrode (C) inserted into the nerve and the activity of the whole nerve was recorded with a hook electrode further proximal o n the same nerve (D).
to injury by the electrode tip. If propagation is unimpaired the recorded impulses might constitute a part of the very signal upon which the sensation of the subject is based when a stimulus is delivered. On the other hand, if propagation is blocked, it is obvious that the sensation must be based upon nerve impulses which are not seen by the recording electrode but carried in other fibres. The analysis must clearly follow different courses in the two cases. The aim of the present study was t o analyse to what extent impulse propagation and impulse block are predictable from the impulse shape. It is obvious from a number of earlier studies that the shapes of unitary nerve impulses are variable in percutaneous recordings from human nerves with intraneural tungsten electrodes (Knibestol and Vallbo 1970, Torebjork et al. 1970, Vallbo 1972). This is true also for recordings with electrodes of much finer tip diameters which usually are employed in animal experiments (Tasaki 1952, Woodbury 1952, Burgess and Per1 1967, Cooper, Robson and Waldron 1969). Further, it has been shown in frog nerves, that the impalement of a single fibre with a small microcapillary is followed by characteristic alterations of the impulse shape which in turn are associated with the development of propagation block (Tasaki 1952, Woodbury 1952). In the present study it was found that unimpaired propagation was predictable with a high degree of confidence in recordings from frog and cat peripheral nerves with tungsten electrodes of the type used for percutaneous impalement of human nerves. The findings constitute a firm basis for the prediction of impulse block in human experiments as the shapes of the single unit impulses and their changes with time were identical in the three species.
Methods Three different species were employed: frog, cat and man. The unitary impulse shapes and their changes during lasting elcctrode impalements were studied in the three species and the development of impulse block was studied and related to the impulse shape in frog and cat. The numher of experiments and the number of units studied are shown in Table 1. In the frog experiments the tibia1 nerve was exploited between the level of thc h n x , just distal to the branches to the triceps surae muscles and the ankle joint. In the cat, the sural nerve was exploited. Impulses were recorded from two different sites along the nerves and with two different types of electrodes as indicated in Fig. I . Single unit impulses were recorded with a tungsten needle electrode (C in Fig. 1) inserted into the nerve in the midregion between knee and ankle joint. The electrode was held in a micromanipulator and its position was adjusted until single unit impulses could be discriminated which originated from a skin receptive field on the foot. Further proximally, at about the level of t h e knee joint the activity from the w h o k nerve was recorded at the cut end with an Ag-AgCI hook electrode ( D in Fig. I ). Mechanical stimuli were applied manually to the appropriate skin area on the foot with a glass rod or a painting brush to induce afferent impulses. At the point of needle electrode impalement the nerve sheath was cut open i n the frog experiments as this was found to increase considerably the yield of the experiment in this preparation. In order to assess whether the unitary impulses propagated beyond the recording site or were blocked
68
C
A.
8. VALLBO
peaked spikes. The records to the left were obtained from exposed nerves in frog (A and C) and cat (B). The records to the right were obtained in percutaneous recordings from human nerves. An upward deflection represents a positive signal at the tungsten
by the needle electrode impalement, the initial phase of the impulses seen by this electrode was made to trigger the sweep of a dual beam oscilloscope which displayed, on the other beam, the signal at the hook electrode. This procedure resulted in a display of the type illustrated in Figs. 3 ,4 and 5 , when several sweeps were photographically superimposed. The experiments on humans have been described in detail in earlier reports (Vallbo and Hagbarth 1968, Vallbo 1972). A tungsten needle electrode was inserted percutaneously into the median nerve about 10 cm proximal to the elbow and the electrode position was adjusted manually until unitary impulses were discriminated. Tungsten electrodes of the same properties were employed in the three preparations for the recording of single unit impulses. The electrodes were insulated to the tip with Arddire A Z 15. The point diameter was 5-15pn1, the length of the uncoated tip was 2&50,um, and the electrode impedance was 100-300 kohm at 1 kHz (Vallbo 1972).
Results Shapes of single unit impulses
Basically 3 different impulse shapes were encountered when unitary potentials were recorded with tungsten needle electrodes inserted into peripheral nerves. These shapes were the same in the 3 species studied, i.e. frog, cat and man. Sample records are shown in Fig. 2. The impulses in Fig. 2 A are characterized by the most prominent phase being a negative deflection. For the other two types the initial and most prominent phase was in the positive direction. The peak was either single (Fig. 2 C) or split into two (Fig. 2 B). The positive deflection was followed by a smaller and usually more long lasting negative deflection. From recordings of many superimposed impulses it was obvious that the double peak was a regular phenomenon and a characteristic of the impulse shape and not an effect of accidental noise superimposed (Fig. 4). These three types of shapes will be denoted negative spikes, positive single peaked spikes and positive double peaked spikes respectively. There was clearly some variation in detail within these three groups, particularly for the negative spikes. The large negative deflection was often preceded and/or followed by smaller positive deflections (Fig. 2 A left) such that the shape of the impulse approached the triphasic membrane current record (Katz 1939, Hodgkin and Rushton 1946, Lorente de N6 1947). In other records the impulse shape seemed quite distored in various ways, e.g. the main negative deflection was broad and/or split into two peaks. This was seen more often in frog than in cat and man. In these two species the negative spikes usually had very short
69
IMPULSE BLOCK IN HUMAN NERVES
TABLE I. Number of experiments, units and relative frequencies of occurrence of impulse types. Frog Experiments Units Negative spikes, % Positive double peaked spikes, % Positive single peaked spikes which did not change into double peaked spikes, % Positive single peaked spikes which changed into double peaked spikes, %
Cat
Man
13
3
28
14 52 14
53
18
100 3
24
51
34
50
21
0
8
19
Total 44 221
durations and they were difficult to hear over an ordinary loudspeaker. A rough estimate of their frequency content indicated that the main energy was in the band 5-15 kHz, whereas for the other two types of impulses the main energy was around 1 kHz. The shapes of the positive single peaked spikes, on the other hand, was quite uniform although impulses of this shape were of two different natures, as will be shown below. The variation in shape was more pronounced for the positive double peaked spikes: the separation of the peaks varied considerably as well as the relative heights of the two peaks (e.g. Fig. 4 and 5). All the 3 impulse shapes were seen in the 3 species as shown in Table I where the number of experiments, units and the proportions of observations with regard to impulse shapes are given. The group of positive single peaked spikes were separated into two subgroups on the basis of the analysis to be presented in the following sections. The sample from man is unbiassed whereas there might be a slight underrepresentation of positive single peaked spikes in the data from cat and frog due to an intentional selection during the experiments. No attempts were made to analyse the factors accounting for the differences between the three samples. There was no obvious difference with regard to the impulse size and shape between recordings from nerves with intact or severed sheath. It may therefore be concluded that the nerve sheath does not constitute the major recording resistance, nor that it is in any other way essential for the recording of single unit impulses with this type of electrode. Changes of impulse shape
In many instances the shape of the single unit impulse changed from one type to another during the recording. Such transitions were seen in 29% of the 227 units studied, They were essentially of three different types. First, some of the negative spikes changed into positive single peaked spikes, sometimes via more bizarre shapes. Second, the positive double peaked spikes often changed into positive single peaked spikes providing the unit was held for a sufficiently long time. This transition often occurred quite slowly and in a characteristic way as illustrated in Fig. 4 and 5 : the second peak of the spike was gradually delayed (Fig. 4) until it began to drop out (Fig. 5 B) and shortly afterwards disappeared definitely. Finally, the third type of transition was seen in some cases when the impulse was initially positive single peaked. A notch developed close to the peak and the impulse gradually turned into a double peaked spike. This type of transition occurred mostly quite fast, the double peaked shape appearing within a minute or two from the moment when the impulse was first discriminated. In some units more than one of these transitions were seen. Of particular interest
70
A
A. B. VALLBO
-
-B
C
Fig. 3. Unitary impulse propagation and block related to impulse shape. Each pair of traces show superimposed records of unitary impulses as seen by a tungsten electrode inserted into the nerve (lower traces) and by a hook electrode further upstream on the nerve (upper traces). Electrode arrangement as in Fig. 1. Sweeps were triggered by the impulses seen by the tungsten electrode. Unimpaired propagation in A and B as seen by the deflection in the upper traces. Impulses blocked in C. Lower pair of records in A and C from frog. The other records from cat. Calibrations. Time: 2.5 ms for lower pair of records in C, 5 ms for all other records. Amplitude: 50 pV for lower pair of records in A and for the lower trace in upper pair of records in B, 100 ,uVfor all other records.
is the following series which were observed in three units: a positive single peaked spike changed into a double peaked spike which after some time of recording lost its second peak and turned into a single peaked positive spike which then remained unchanged. These findings clearly show that the positive single peaked spikes were of two different natures: one which developed into a double peaked spike within a short time and, on the other hand, one which had developed from a double peaked spike but then remained of constant shape. It was assumed that a third type which was originally positive single peaked and remained unchanged was of the same nature as the latter one of the two, as will be further discussed in a later section. On the basis of this view the single peaked positive spikes were separated into two subgroups in Table I: first, spikes which changed into double peaked positive spikes within a few minutes, and second, spikes which did not. Prediction of impulse block from impulse shape
Impulse propagation past the recording tungsten needle electrode was determined by the technique described in methods. An analysis was made for all the units studied in frog and cat of the coincidence of impulse block on the one hand and anyone of the impulse types as seen by the tungsten needle electrode on the other. Fig. 3 shows representative sample records. The lower records of the pairs display the impulses seen by the tungsten needle electrode, whereas the top records show the impulses recorded with the hook electrode further proximal on the nerve. Two negative spikes are shown in Fig. 3 A which both propagated as seen by the deflection at a constant latency in the upper records. At the proximal electrode the response was always all-or-none as illustrated in Fig. 5. The records in Fig. 3 B show above a positive single peaked spike and below a positive double peaked spike which two spikes also propagated. The two positive single peaked spikes in Fig. 3 C, on the other hand, were blocked which is obvious from the fact that there was no deflection in the upper records. In other recordings from the same nerve it was demonstrated that single unit impulses were readily discriminable at the proximal electrode.
71
IMPULSE BLOCK IN H U M A N NERVES
TABLE 11. Impulse propagation related to impulse shape in total sample of observations from frog and cat.
Negative spikes Positive double peaked spikes Positive single peaked spikes which did not change into double peaked spikes Positive single peaked spikes which changed into double peaked spikes
Number of observations
Per cent unimpaired propagation
64 31
100
70
0
6
100
89
Quantitative data on propagation related to impulse shape are collected in Table 11. It should be pointed out that the term observation as used in the table and the text refers to one type of impulse shape as seen in a single unit and hence that some units provided more than one observation since the impulse shape changed during the recording. It may be seen that most of the negative spikes propagated as did all the double peaked positive spikes. With regard to the positive single peaked spikes which were separated into two groups as in Table I it was found that unimpaired propagation was the rule in the cases when the impulse later changed into a double peaked impulse, whereas propagation block was the rule in the other group. Of the hundred units studied in man the impulses of seventeen units were positive single peaked during the whole observation period, indicating a propagation block. In addition, some spikes changed from double peaked to single peaked and then remained unchanged (cf. Table I). Changes of impulse shape related to propagation
It has been shown in two earlier studies on frog nerves that the impulse shape may be characteristically altered during a lasting impalement with a microelectrode and further that propagation is blocked at a definite phase of this process (Tasaki 1952, Woodbury 1952). In order to clarify whether the same course of events was at hand in the present experiments a more detailed study was made of the conduction during the phase when the impulse block was developing. The findings shed some light on the mechanisms involved and also provided a basis for the interpretation of the factors accounting for the variation of the impulse shapes. In Fig. 4 are shown representative records of a positive double peaked impulse as seen by the tungsten needle electrode. The 3 pairs of records were taken successively from top to bottom with about 3 min interval during the experiment. It may be seen that the second peak of the impulse was gradually delayed as was the small impulse recorded by the hook electrode located further proximal on the nerve. It is obvious that the shifts of the two phenomena run parallel as indicated by the fact that the latency from the second peak of the big impulse to the impulse at the hook electrode was constant, i.e. about 0.6 ms, although the two were shifted by about 0.25 ms from A to C in Fig. 4. It seems that these findings justify the conclusion that a shift of the second peak indicates a gradual slowing of the impulse propagation at the very site of the electrode impalement, presumably related to injury of the fibre. Further evidence for this view is provided by the records of Fig. 5 where individual traces are identifiable. It may be seen in Fig. 5 A that the variations in the delay of the second peak of the
A.
72
B. VALLBO
?%@----
Fig. 4
Fig. 5
Fig. 4. Records t o demonstrate parallel time shifts in second peak of an impulse, seen by a tungsten needle electrode, and the propagated impulse. Electrode set u p as in Fig. 1. Records arranged as in Fig. 3. All the records are from the same unit and were taken in time order, from above, with an interval of about 3 min. Frog. Calibrations. Time: 2 ms. Amplitude: 100 p V . Fig. 5. Records t o demonstrate parallel time shift and drop out of the second peak of an impulse seen by a tungsten needle electrode, and the propagated impulse. Electrode set up as in Fig. 1. Records arranged as in Fig. 3. All the records are from the same unit and were taken within 2 min while the propagation block was developing. Frog. Calibrations. Time: 2 ms. Amplitude: 100 p V .
larger spike correspond accurately with the variations in the delay of the small spikes displayed by the upper traces in the pairs. Fig. 5 B shows the same phenomena, and in addition, that a drop out of the second peak of the large spike was associated with a drop out of the small spike. It may be seen that this occurred twice in each one of the two records of Fig. 5 B. Thus a drop out of the second peak indicated a propagation block at the tungsten electrode impalement.
Discussion Three different impulse shapes were seen which were denoted negative spikes, positive single peaked spikes and positive double peaked spikes respectively. Similar 3 basic shapes of impulses have been reported in earlier studies on exposed nerves or dorsal root filaments in animal experiments (Tasaki 1952, Woodbury 1952, Thomson 1953, Frank and Fuortes 1955, Maturana et al. 1960, Miller and Weddell 1966, Burgess and Per1 1967, Cooper, Robson and Waldron 1969) as well as in percutaneous recordings from human nerves (Knutsson and Widen 1967, Vallbo and Hagbarth 1968, Knibestol and Vallbo 1970, Torebjork et al. 1970, Vallbo 1972). The main aim of the present study was to assess the coincidence of impulse block with any of the 3 impulse shapes in recordings when electrodes of fairly large tip diameters (5-15 pm) were used as in percutaneous recordings from human nerves. The fact that the findings were basically uniform in the present investigation and in many earlier ones in which different preparations and slightly different techniques were used, indicates that the impulse shape was not vitally dependent upon the electrode tip size, the nerve sheath being intact or severed,
IMPULSE BLOCK IN HUMAN NERVES
73
nor whether the nerve was exposed or not. Further, this agreement simplifies the interpretation of the recordings from nonexposed human nerves which are not readily accessible for a critical analysis of impulse propagation in single units. Many of the negative spikes were rather like the triphasic membrane current recordings (Katz 1939, Hodgkin and Rushton 1946, Lorente de Nb 1947). They probably represent extracellular recordings from intact fibres as suggested by Cooper, Robson and Waldron (1969). Most of these impulses also propagated. However, some of the units in this group had a shape which was more bizarre and which could not reasonably be interpreted in this way. They might represent extracellular recordings from fibres which were slightly injured by the electrode impalement. The other two impulse shapes were initially positive single peaked and double peaked respectively. The fact that the initial and most prominent phase was in the positive direction indicates that there was a low impedance route from the electrode tip to the interior of the axon, i.e. that the fibre was impaled by the electrode tip which saw, essentially from the inside, the inward currents produced by the neighbouring nodes. This interpretation is consonant with Tasaki’s detailed analysis under visual control of impulse shape related to fibre impalement in isolated myelinated frog fibres (Tasaki 1952). Tasaki’s findings also indicate that an impalament of just the myelin, and not necessarily the axon, might be sufficient for the recording of an impulse of this shape. In the present study alterations of the shapes of the impulses and the development of a propagation block were found which also agreed in principle with the findings of earlier studies in which electrodes of small tip diameters were used, 0.5 pm or less (Tasaki 1952, Woodbury 1952). When the fibre was first impaled the spike was in some cases positive single peaked. A notch soon developed close to the peak making the spike double peaked. The two peaks were gradually more and more separated and this separation went parallel with a delay of the conduction. Finally the second peak dropped out and this coincided exactly with a propagation block of the impulse. It seems justified to apply a similar interpretation of the mechanisms as given by Tasaki (1952). When the electrode initially impales the myelinated fibre the positive impulse is single peaked as the inward currents in the two neighbouring nodes appear with a very short interval (Huxley and Stampfli 1949, Rasminsky and Sears 1971). When the impalement progresses and gives rise to a more pronounced extra-intracellular shunt and/or a compression of the axon the conduction is delayed at this site and the inward currents produced by the two nodes on either side of the electrode are separated in time to such an extent that a splitting of the peak is seen. Thus, the first one of the two peaks originates largely from activity in the nearest node, upstream to the site of electrode impalement, and the second peak originates from the nearest node downstream. When the injury effect of the electrode increases and the fibre runs down, the nearest node downstream from the electrode fails to produce a regenerative response and the propagation is blocked. This is seen as a drop out of the second peak which results in an impulse of basically the same shape as the one seen initially before any appreciable conduction delay at the recording site has developed. With regard to experiments on human subjects involving an analysis of psychophysical responses in relation to activity in first order afferents, the present findings provide a firm basis for the prediction of single unit impulse block on the basis of impulse shape. The
74
A. B. VALLBO
experimental data were largely unequivocal and, in addition, they fitted well into a theory which is supported by even more direct experiments than those of the present study (Tasaki 1952).
The findings indicate that impulse propagation or block is predictable from the impulse shape and its alterations in 97% of the units. Some uncertainty remains for the other 3 per cent. However, in this small group the odds are only one to ten that the impulse is blocked. Further it was possible to conclude that, in approximately 80% of the units recorded percutaneously from human nerves, the impulse propagation was unimpaired during the whole observation period or part of it. This investigation was supported by grants from the Swedish Medical Research Council, project no. 04X3548.
References BURGESS, P. R. and E. R. PERL,Myelinated afferent fibres responding specifically to noxious stimulation of the skin. J. Physiol. (Lond.) 1967. 190. 541-562. The action potentials recorded from undamaged nerve COOPER,G. F., J. G. ROBSONand I. WALDRON, fibres with micro-electrodes. J. Physiol. (Lond.) 1969. 200. 9-1 1 P. Potentials recorded from the spinal cord with microelectrodes. J. FRANK,K. and M. G. F. FUORTES. Physiol. (Lond.) 1955. 130. 625-654. HODGKIN, A. L. and W. A. H. RUSHTON, The electrical constants of an crustacean nerve fibre. Proc. roy. Soc. B 1946. 133. 4 4 4 4 7 9 . HUXLEY, A. F. and R. STAMPFLI, Evidence of saltatory conduction in peripheral myelinated nerve fibres. J. Physiol. (Lond.) 1949. 108. 315-339. KATZ,B., Electric excitation of nerue. Oxford University Press, London. 1939. 151 pages. Single unit analysis of mechanoreceptor activity from the human glabrous KNIBESTOL, M. and A. B. VALLBO, skin. Acta physiol. srand. 1970. 80. 178-195. KNUTSSON, E. and L. W I D ~ NImpulses , from single nerve fibres recorded in man using microelectrodes. Nature (Lond.) 1967. 213. 606-607. LORENTE de N6, R., A study of nerve physiology. Stud. Rockefeller Inst. med. Res. 1947. 132. 384-497. MATURANA, H. R., J. Y.LETTVIN, W. S. MCCULLOCH and W. H. PiTTS, Anatomy and physiology of vision in the frog (Rana pipiens). J. gen. Physiol. 1960. 43. 125-176. Mechanoreceptors in rabbit ear skin innervated by myelinated nerve fibres. MILLER,S. and G. WEDDELL, J. Physiol. (Lond.) 1966. 187. 291-305. RASMINSKY, M. and T. A. SEARS,Internodal conduction in undissected demyelinated nerve fibres. J. Physiol. (Lond.) 1972. 227. 323-350. TASAKI, I., Properties of myelinated fibres in frog sciatic nerve and in spinal cord as examined with microelectrodes. Jap. J. Physiol. 1952. 3. 73-94. THOMSON, L. C., The localization of function in the rabbit retina. J . Physiol. (Lond.) 1953. 119. 191-209. TOREBJORK, H. E., R. G. HALLIN,A. HONGELL and K.-E. HAGBARTH, Single unit potentials with complex waveform seen in microelectrode recordings from the human median nerve. Brain Res. 1970. 24.443-450. VALLBO,A. B., Single unit recording from human peripheral nerves: muscle receptor discharge in resting muscles and during voluntary contractions. Neurophysiology studied in man. Ed.: G . G . Somjen. Excerota Medica, Amsterdam. 1972. 281-295. VALLBO,A. B. and K.-E. HAGBARTH, Activity from skin mechanoreceptors recorded percutaneoulsy in awake human subjects. Exp. Neurol. 1968. 21. 270-289. VALLBO,A. B. and K.-E. HAGBARTH, Micro-electrode recordings from human peripheral nerves. New deuelopments in electromyography and clinical neurophysiology. Ed.: J. E. Desmedt. Karger, Basel. 1973. Vol. 2. 67-84. WOODBURY, J. W., Direct membrane resting and action potentials from single myelinated nerve fibres. J. cell. romp. Physiol. 1952. 39. 323-339.
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From the Department of Physiology, University of Umefi, Sweden
Prediction of Propagation Block on the Basis of Impulse Shape in Single Unit Recordings from Human Nerves BY
A. B. VALLBO Received 17 October 1975
Abstract VALLBO,A. B. Prediction of propagation block on the basis of impulse shape in single unit recordings from human nerves. Acta physiol. scand. 1976. 97. 66-74. The occurrence of impulse block was studied in single unit recordings and related to impulse shape when peripheral nerves were impaled with tungsten needle electrodes. Exposed nerves were exploited in frog and cat and non-exposed nerves in man. Three different impulse shapes were seen: I . Negative spikes of very short duiations which nearly all propagated. 2. Positive double peaked spikes which all propagated. 3. Positive single peaked spikes which were of two different natures: one which propagated and one which did not. The findings suggest thzt unitary impulses recorded with a needle electrode impaling the myelin are positive and single peaked when the injury is minimal. Gradually a propagation delay might develop at the site of impalement giving rise to a double peak. Later the propagation may be blocked which is seen as a drop out of the second peak. The collected sample of observations indicates that it was possible to predict in practically all cases whether the propagation was blocked or not from the shapes of the single unit impulses and their alterations as seen by the tungsten needle electrode. Key words: Man, single nerve fibre, propagation block
The method of percutaneous recording of nerve impulses from awake human subjects with intraneural electrodes (Vallbo and Hagbarth 1968) has allowed the functional analysis of several neuronal systems in the intact organism. Effect autonomic activity has been studied as well as proprioceptive afferent inflow from muscles during voluntary contractions and afferent impulses from sense organs in the skin (for ref. see Vallbo and Hagbarth 1973). With regard to the somatosensory system, this type of experiments provide a unique opportunity to study, in the same subject, neural afferent inflow and psychophysical responses based upon this afferent inflow. It seems that an analysis of the responses at these two levels of the somatosensory system to well controlled stimuli might contribute to the understanding of the relation between biophysical events in the nervous system and perceptive mechanisms of the mind. When considering the potentialities of this experimental approach, it is a fundamental step to assess whether the unitary impulses recorded with the needle electrode are propagated past the recording site to reach the central nervous system or if the impulses are blocked due 65
67
IMPULSE BLOCK IN HUMAN NERVES
A
B
C
D
Fig. I. Schematic diagram showing the experimental arrangements. Mechanical stimuli were applied with a glass rod o r a drawing brush (A) to the skin (9). Nerve impulses of a single fibre from a skin mechanoreceptor unit were recorded with a tungsten needle electrode (C) inserted into the nerve and the activity of the whole nerve was recorded with a hook electrode further proximal o n the same nerve (D).
to injury by the electrode tip. If propagation is unimpaired the recorded impulses might constitute a part of the very signal upon which the sensation of the subject is based when a stimulus is delivered. On the other hand, if propagation is blocked, it is obvious that the sensation must be based upon nerve impulses which are not seen by the recording electrode but carried in other fibres. The analysis must clearly follow different courses in the two cases. The aim of the present study was t o analyse to what extent impulse propagation and impulse block are predictable from the impulse shape. It is obvious from a number of earlier studies that the shapes of unitary nerve impulses are variable in percutaneous recordings from human nerves with intraneural tungsten electrodes (Knibestol and Vallbo 1970, Torebjork et al. 1970, Vallbo 1972). This is true also for recordings with electrodes of much finer tip diameters which usually are employed in animal experiments (Tasaki 1952, Woodbury 1952, Burgess and Per1 1967, Cooper, Robson and Waldron 1969). Further, it has been shown in frog nerves, that the impalement of a single fibre with a small microcapillary is followed by characteristic alterations of the impulse shape which in turn are associated with the development of propagation block (Tasaki 1952, Woodbury 1952). In the present study it was found that unimpaired propagation was predictable with a high degree of confidence in recordings from frog and cat peripheral nerves with tungsten electrodes of the type used for percutaneous impalement of human nerves. The findings constitute a firm basis for the prediction of impulse block in human experiments as the shapes of the single unit impulses and their changes with time were identical in the three species.
Methods Three different species were employed: frog, cat and man. The unitary impulse shapes and their changes during lasting elcctrode impalements were studied in the three species and the development of impulse block was studied and related to the impulse shape in frog and cat. The numher of experiments and the number of units studied are shown in Table 1. In the frog experiments the tibia1 nerve was exploited between the level of thc h n x , just distal to the branches to the triceps surae muscles and the ankle joint. In the cat, the sural nerve was exploited. Impulses were recorded from two different sites along the nerves and with two different types of electrodes as indicated in Fig. I . Single unit impulses were recorded with a tungsten needle electrode (C in Fig. 1) inserted into the nerve in the midregion between knee and ankle joint. The electrode was held in a micromanipulator and its position was adjusted until single unit impulses could be discriminated which originated from a skin receptive field on the foot. Further proximally, at about the level of t h e knee joint the activity from the w h o k nerve was recorded at the cut end with an Ag-AgCI hook electrode ( D in Fig. I ). Mechanical stimuli were applied manually to the appropriate skin area on the foot with a glass rod or a painting brush to induce afferent impulses. At the point of needle electrode impalement the nerve sheath was cut open i n the frog experiments as this was found to increase considerably the yield of the experiment in this preparation. In order to assess whether the unitary impulses propagated beyond the recording site or were blocked
68
C
A.
8. VALLBO
peaked spikes. The records to the left were obtained from exposed nerves in frog (A and C) and cat (B). The records to the right were obtained in percutaneous recordings from human nerves. An upward deflection represents a positive signal at the tungsten
by the needle electrode impalement, the initial phase of the impulses seen by this electrode was made to trigger the sweep of a dual beam oscilloscope which displayed, on the other beam, the signal at the hook electrode. This procedure resulted in a display of the type illustrated in Figs. 3 ,4 and 5 , when several sweeps were photographically superimposed. The experiments on humans have been described in detail in earlier reports (Vallbo and Hagbarth 1968, Vallbo 1972). A tungsten needle electrode was inserted percutaneously into the median nerve about 10 cm proximal to the elbow and the electrode position was adjusted manually until unitary impulses were discriminated. Tungsten electrodes of the same properties were employed in the three preparations for the recording of single unit impulses. The electrodes were insulated to the tip with Arddire A Z 15. The point diameter was 5-15pn1, the length of the uncoated tip was 2&50,um, and the electrode impedance was 100-300 kohm at 1 kHz (Vallbo 1972).
Results Shapes of single unit impulses
Basically 3 different impulse shapes were encountered when unitary potentials were recorded with tungsten needle electrodes inserted into peripheral nerves. These shapes were the same in the 3 species studied, i.e. frog, cat and man. Sample records are shown in Fig. 2. The impulses in Fig. 2 A are characterized by the most prominent phase being a negative deflection. For the other two types the initial and most prominent phase was in the positive direction. The peak was either single (Fig. 2 C) or split into two (Fig. 2 B). The positive deflection was followed by a smaller and usually more long lasting negative deflection. From recordings of many superimposed impulses it was obvious that the double peak was a regular phenomenon and a characteristic of the impulse shape and not an effect of accidental noise superimposed (Fig. 4). These three types of shapes will be denoted negative spikes, positive single peaked spikes and positive double peaked spikes respectively. There was clearly some variation in detail within these three groups, particularly for the negative spikes. The large negative deflection was often preceded and/or followed by smaller positive deflections (Fig. 2 A left) such that the shape of the impulse approached the triphasic membrane current record (Katz 1939, Hodgkin and Rushton 1946, Lorente de N6 1947). In other records the impulse shape seemed quite distored in various ways, e.g. the main negative deflection was broad and/or split into two peaks. This was seen more often in frog than in cat and man. In these two species the negative spikes usually had very short
69
IMPULSE BLOCK IN HUMAN NERVES
TABLE I. Number of experiments, units and relative frequencies of occurrence of impulse types. Frog Experiments Units Negative spikes, % Positive double peaked spikes, % Positive single peaked spikes which did not change into double peaked spikes, % Positive single peaked spikes which changed into double peaked spikes, %
Cat
Man
13
3
28
14 52 14
53
18
100 3
24
51
34
50
21
0
8
19
Total 44 221
durations and they were difficult to hear over an ordinary loudspeaker. A rough estimate of their frequency content indicated that the main energy was in the band 5-15 kHz, whereas for the other two types of impulses the main energy was around 1 kHz. The shapes of the positive single peaked spikes, on the other hand, was quite uniform although impulses of this shape were of two different natures, as will be shown below. The variation in shape was more pronounced for the positive double peaked spikes: the separation of the peaks varied considerably as well as the relative heights of the two peaks (e.g. Fig. 4 and 5). All the 3 impulse shapes were seen in the 3 species as shown in Table I where the number of experiments, units and the proportions of observations with regard to impulse shapes are given. The group of positive single peaked spikes were separated into two subgroups on the basis of the analysis to be presented in the following sections. The sample from man is unbiassed whereas there might be a slight underrepresentation of positive single peaked spikes in the data from cat and frog due to an intentional selection during the experiments. No attempts were made to analyse the factors accounting for the differences between the three samples. There was no obvious difference with regard to the impulse size and shape between recordings from nerves with intact or severed sheath. It may therefore be concluded that the nerve sheath does not constitute the major recording resistance, nor that it is in any other way essential for the recording of single unit impulses with this type of electrode. Changes of impulse shape
In many instances the shape of the single unit impulse changed from one type to another during the recording. Such transitions were seen in 29% of the 227 units studied, They were essentially of three different types. First, some of the negative spikes changed into positive single peaked spikes, sometimes via more bizarre shapes. Second, the positive double peaked spikes often changed into positive single peaked spikes providing the unit was held for a sufficiently long time. This transition often occurred quite slowly and in a characteristic way as illustrated in Fig. 4 and 5 : the second peak of the spike was gradually delayed (Fig. 4) until it began to drop out (Fig. 5 B) and shortly afterwards disappeared definitely. Finally, the third type of transition was seen in some cases when the impulse was initially positive single peaked. A notch developed close to the peak and the impulse gradually turned into a double peaked spike. This type of transition occurred mostly quite fast, the double peaked shape appearing within a minute or two from the moment when the impulse was first discriminated. In some units more than one of these transitions were seen. Of particular interest
70
A
A. B. VALLBO
-
-B
C
Fig. 3. Unitary impulse propagation and block related to impulse shape. Each pair of traces show superimposed records of unitary impulses as seen by a tungsten electrode inserted into the nerve (lower traces) and by a hook electrode further upstream on the nerve (upper traces). Electrode arrangement as in Fig. 1. Sweeps were triggered by the impulses seen by the tungsten electrode. Unimpaired propagation in A and B as seen by the deflection in the upper traces. Impulses blocked in C. Lower pair of records in A and C from frog. The other records from cat. Calibrations. Time: 2.5 ms for lower pair of records in C, 5 ms for all other records. Amplitude: 50 pV for lower pair of records in A and for the lower trace in upper pair of records in B, 100 ,uVfor all other records.
is the following series which were observed in three units: a positive single peaked spike changed into a double peaked spike which after some time of recording lost its second peak and turned into a single peaked positive spike which then remained unchanged. These findings clearly show that the positive single peaked spikes were of two different natures: one which developed into a double peaked spike within a short time and, on the other hand, one which had developed from a double peaked spike but then remained of constant shape. It was assumed that a third type which was originally positive single peaked and remained unchanged was of the same nature as the latter one of the two, as will be further discussed in a later section. On the basis of this view the single peaked positive spikes were separated into two subgroups in Table I: first, spikes which changed into double peaked positive spikes within a few minutes, and second, spikes which did not. Prediction of impulse block from impulse shape
Impulse propagation past the recording tungsten needle electrode was determined by the technique described in methods. An analysis was made for all the units studied in frog and cat of the coincidence of impulse block on the one hand and anyone of the impulse types as seen by the tungsten needle electrode on the other. Fig. 3 shows representative sample records. The lower records of the pairs display the impulses seen by the tungsten needle electrode, whereas the top records show the impulses recorded with the hook electrode further proximal on the nerve. Two negative spikes are shown in Fig. 3 A which both propagated as seen by the deflection at a constant latency in the upper records. At the proximal electrode the response was always all-or-none as illustrated in Fig. 5. The records in Fig. 3 B show above a positive single peaked spike and below a positive double peaked spike which two spikes also propagated. The two positive single peaked spikes in Fig. 3 C, on the other hand, were blocked which is obvious from the fact that there was no deflection in the upper records. In other recordings from the same nerve it was demonstrated that single unit impulses were readily discriminable at the proximal electrode.
71
IMPULSE BLOCK IN H U M A N NERVES
TABLE 11. Impulse propagation related to impulse shape in total sample of observations from frog and cat.
Negative spikes Positive double peaked spikes Positive single peaked spikes which did not change into double peaked spikes Positive single peaked spikes which changed into double peaked spikes
Number of observations
Per cent unimpaired propagation
64 31
100
70
0
6
100
89
Quantitative data on propagation related to impulse shape are collected in Table 11. It should be pointed out that the term observation as used in the table and the text refers to one type of impulse shape as seen in a single unit and hence that some units provided more than one observation since the impulse shape changed during the recording. It may be seen that most of the negative spikes propagated as did all the double peaked positive spikes. With regard to the positive single peaked spikes which were separated into two groups as in Table I it was found that unimpaired propagation was the rule in the cases when the impulse later changed into a double peaked impulse, whereas propagation block was the rule in the other group. Of the hundred units studied in man the impulses of seventeen units were positive single peaked during the whole observation period, indicating a propagation block. In addition, some spikes changed from double peaked to single peaked and then remained unchanged (cf. Table I). Changes of impulse shape related to propagation
It has been shown in two earlier studies on frog nerves that the impulse shape may be characteristically altered during a lasting impalement with a microelectrode and further that propagation is blocked at a definite phase of this process (Tasaki 1952, Woodbury 1952). In order to clarify whether the same course of events was at hand in the present experiments a more detailed study was made of the conduction during the phase when the impulse block was developing. The findings shed some light on the mechanisms involved and also provided a basis for the interpretation of the factors accounting for the variation of the impulse shapes. In Fig. 4 are shown representative records of a positive double peaked impulse as seen by the tungsten needle electrode. The 3 pairs of records were taken successively from top to bottom with about 3 min interval during the experiment. It may be seen that the second peak of the impulse was gradually delayed as was the small impulse recorded by the hook electrode located further proximal on the nerve. It is obvious that the shifts of the two phenomena run parallel as indicated by the fact that the latency from the second peak of the big impulse to the impulse at the hook electrode was constant, i.e. about 0.6 ms, although the two were shifted by about 0.25 ms from A to C in Fig. 4. It seems that these findings justify the conclusion that a shift of the second peak indicates a gradual slowing of the impulse propagation at the very site of the electrode impalement, presumably related to injury of the fibre. Further evidence for this view is provided by the records of Fig. 5 where individual traces are identifiable. It may be seen in Fig. 5 A that the variations in the delay of the second peak of the
A.
72
B. VALLBO
?%@----
Fig. 4
Fig. 5
Fig. 4. Records t o demonstrate parallel time shifts in second peak of an impulse, seen by a tungsten needle electrode, and the propagated impulse. Electrode set u p as in Fig. 1. Records arranged as in Fig. 3. All the records are from the same unit and were taken in time order, from above, with an interval of about 3 min. Frog. Calibrations. Time: 2 ms. Amplitude: 100 p V . Fig. 5. Records t o demonstrate parallel time shift and drop out of the second peak of an impulse seen by a tungsten needle electrode, and the propagated impulse. Electrode set up as in Fig. 1. Records arranged as in Fig. 3. All the records are from the same unit and were taken within 2 min while the propagation block was developing. Frog. Calibrations. Time: 2 ms. Amplitude: 100 p V .
larger spike correspond accurately with the variations in the delay of the small spikes displayed by the upper traces in the pairs. Fig. 5 B shows the same phenomena, and in addition, that a drop out of the second peak of the large spike was associated with a drop out of the small spike. It may be seen that this occurred twice in each one of the two records of Fig. 5 B. Thus a drop out of the second peak indicated a propagation block at the tungsten electrode impalement.
Discussion Three different impulse shapes were seen which were denoted negative spikes, positive single peaked spikes and positive double peaked spikes respectively. Similar 3 basic shapes of impulses have been reported in earlier studies on exposed nerves or dorsal root filaments in animal experiments (Tasaki 1952, Woodbury 1952, Thomson 1953, Frank and Fuortes 1955, Maturana et al. 1960, Miller and Weddell 1966, Burgess and Per1 1967, Cooper, Robson and Waldron 1969) as well as in percutaneous recordings from human nerves (Knutsson and Widen 1967, Vallbo and Hagbarth 1968, Knibestol and Vallbo 1970, Torebjork et al. 1970, Vallbo 1972). The main aim of the present study was to assess the coincidence of impulse block with any of the 3 impulse shapes in recordings when electrodes of fairly large tip diameters (5-15 pm) were used as in percutaneous recordings from human nerves. The fact that the findings were basically uniform in the present investigation and in many earlier ones in which different preparations and slightly different techniques were used, indicates that the impulse shape was not vitally dependent upon the electrode tip size, the nerve sheath being intact or severed,
IMPULSE BLOCK IN HUMAN NERVES
73
nor whether the nerve was exposed or not. Further, this agreement simplifies the interpretation of the recordings from nonexposed human nerves which are not readily accessible for a critical analysis of impulse propagation in single units. Many of the negative spikes were rather like the triphasic membrane current recordings (Katz 1939, Hodgkin and Rushton 1946, Lorente de Nb 1947). They probably represent extracellular recordings from intact fibres as suggested by Cooper, Robson and Waldron (1969). Most of these impulses also propagated. However, some of the units in this group had a shape which was more bizarre and which could not reasonably be interpreted in this way. They might represent extracellular recordings from fibres which were slightly injured by the electrode impalement. The other two impulse shapes were initially positive single peaked and double peaked respectively. The fact that the initial and most prominent phase was in the positive direction indicates that there was a low impedance route from the electrode tip to the interior of the axon, i.e. that the fibre was impaled by the electrode tip which saw, essentially from the inside, the inward currents produced by the neighbouring nodes. This interpretation is consonant with Tasaki’s detailed analysis under visual control of impulse shape related to fibre impalement in isolated myelinated frog fibres (Tasaki 1952). Tasaki’s findings also indicate that an impalament of just the myelin, and not necessarily the axon, might be sufficient for the recording of an impulse of this shape. In the present study alterations of the shapes of the impulses and the development of a propagation block were found which also agreed in principle with the findings of earlier studies in which electrodes of small tip diameters were used, 0.5 pm or less (Tasaki 1952, Woodbury 1952). When the fibre was first impaled the spike was in some cases positive single peaked. A notch soon developed close to the peak making the spike double peaked. The two peaks were gradually more and more separated and this separation went parallel with a delay of the conduction. Finally the second peak dropped out and this coincided exactly with a propagation block of the impulse. It seems justified to apply a similar interpretation of the mechanisms as given by Tasaki (1952). When the electrode initially impales the myelinated fibre the positive impulse is single peaked as the inward currents in the two neighbouring nodes appear with a very short interval (Huxley and Stampfli 1949, Rasminsky and Sears 1971). When the impalement progresses and gives rise to a more pronounced extra-intracellular shunt and/or a compression of the axon the conduction is delayed at this site and the inward currents produced by the two nodes on either side of the electrode are separated in time to such an extent that a splitting of the peak is seen. Thus, the first one of the two peaks originates largely from activity in the nearest node, upstream to the site of electrode impalement, and the second peak originates from the nearest node downstream. When the injury effect of the electrode increases and the fibre runs down, the nearest node downstream from the electrode fails to produce a regenerative response and the propagation is blocked. This is seen as a drop out of the second peak which results in an impulse of basically the same shape as the one seen initially before any appreciable conduction delay at the recording site has developed. With regard to experiments on human subjects involving an analysis of psychophysical responses in relation to activity in first order afferents, the present findings provide a firm basis for the prediction of single unit impulse block on the basis of impulse shape. The
74
A. B. VALLBO
experimental data were largely unequivocal and, in addition, they fitted well into a theory which is supported by even more direct experiments than those of the present study (Tasaki 1952).
The findings indicate that impulse propagation or block is predictable from the impulse shape and its alterations in 97% of the units. Some uncertainty remains for the other 3 per cent. However, in this small group the odds are only one to ten that the impulse is blocked. Further it was possible to conclude that, in approximately 80% of the units recorded percutaneously from human nerves, the impulse propagation was unimpaired during the whole observation period or part of it. This investigation was supported by grants from the Swedish Medical Research Council, project no. 04X3548.
References BURGESS, P. R. and E. R. PERL,Myelinated afferent fibres responding specifically to noxious stimulation of the skin. J. Physiol. (Lond.) 1967. 190. 541-562. The action potentials recorded from undamaged nerve COOPER,G. F., J. G. ROBSONand I. WALDRON, fibres with micro-electrodes. J. Physiol. (Lond.) 1969. 200. 9-1 1 P. Potentials recorded from the spinal cord with microelectrodes. J. FRANK,K. and M. G. F. FUORTES. Physiol. (Lond.) 1955. 130. 625-654. HODGKIN, A. L. and W. A. H. RUSHTON, The electrical constants of an crustacean nerve fibre. Proc. roy. Soc. B 1946. 133. 4 4 4 4 7 9 . HUXLEY, A. F. and R. STAMPFLI, Evidence of saltatory conduction in peripheral myelinated nerve fibres. J. Physiol. (Lond.) 1949. 108. 315-339. KATZ,B., Electric excitation of nerue. Oxford University Press, London. 1939. 151 pages. Single unit analysis of mechanoreceptor activity from the human glabrous KNIBESTOL, M. and A. B. VALLBO, skin. Acta physiol. srand. 1970. 80. 178-195. KNUTSSON, E. and L. W I D ~ NImpulses , from single nerve fibres recorded in man using microelectrodes. Nature (Lond.) 1967. 213. 606-607. LORENTE de N6, R., A study of nerve physiology. Stud. Rockefeller Inst. med. Res. 1947. 132. 384-497. MATURANA, H. R., J. Y.LETTVIN, W. S. MCCULLOCH and W. H. PiTTS, Anatomy and physiology of vision in the frog (Rana pipiens). J. gen. Physiol. 1960. 43. 125-176. Mechanoreceptors in rabbit ear skin innervated by myelinated nerve fibres. MILLER,S. and G. WEDDELL, J. Physiol. (Lond.) 1966. 187. 291-305. RASMINSKY, M. and T. A. SEARS,Internodal conduction in undissected demyelinated nerve fibres. J. Physiol. (Lond.) 1972. 227. 323-350. TASAKI, I., Properties of myelinated fibres in frog sciatic nerve and in spinal cord as examined with microelectrodes. Jap. J. Physiol. 1952. 3. 73-94. THOMSON, L. C., The localization of function in the rabbit retina. J . Physiol. (Lond.) 1953. 119. 191-209. TOREBJORK, H. E., R. G. HALLIN,A. HONGELL and K.-E. HAGBARTH, Single unit potentials with complex waveform seen in microelectrode recordings from the human median nerve. Brain Res. 1970. 24.443-450. VALLBO,A. B., Single unit recording from human peripheral nerves: muscle receptor discharge in resting muscles and during voluntary contractions. Neurophysiology studied in man. Ed.: G . G . Somjen. Excerota Medica, Amsterdam. 1972. 281-295. VALLBO,A. B. and K.-E. HAGBARTH, Activity from skin mechanoreceptors recorded percutaneoulsy in awake human subjects. Exp. Neurol. 1968. 21. 270-289. VALLBO,A. B. and K.-E. HAGBARTH, Micro-electrode recordings from human peripheral nerves. New deuelopments in electromyography and clinical neurophysiology. Ed.: J. E. Desmedt. Karger, Basel. 1973. Vol. 2. 67-84. WOODBURY, J. W., Direct membrane resting and action potentials from single myelinated nerve fibres. J. cell. romp. Physiol. 1952. 39. 323-339.
