endings) have been found to be distributed to the different motor nuclei (Liddell & Sherrington, 1924; Lloyd, 1946b; Laporte & Lloyd, 1952; Eccles,

Transcription

1 565 J. Physiol. (I959) I47, SUPRASPINAL CONTROL OF INTERNEURONES MEDIATING SPINAL REFLEXES BY ROSAMOND M. ECCLES AND A. LUNDBERG From the Department of Physiology, Australian National University, Canberra, and the Institute of Physiology, University of Lund, Sweden (Received 16 February 1959) A number of investigations have been devoted to the functional organization of synaptic actions by impulses in muscle afferents. The excitatory and inhibitory actions by impulses in I a afferents (spindle afferents with annulospiral endings) have been found to be distributed to the different motor nuclei according to an intricate pattern suggesting that the I a system has the double function of subserving both postural reflexes and co-ordination of movements (Liddell & Sherrington, 1924; Lloyd, 1946b; Laporte & Lloyd, 1952; Eccles, Eccles & Lundberg, 1957 b; Eccles & Lundberg, 1958c). The synaptic actions by impulses in Ib afferents (i.e. from Golgi tendon organs) have also been investigated in detail and found to have a characteristic distribution (Laporte & Lloyd, 1952; Eccles, Eccles & Lundberg, 1957c). In part this pattern of action suggests that by autogenetic inhibition the Ib system provides protection against excessive tension (cf. Granit, 1950; Granit & Str6m, 1951). Widespread effects from extensor muscles and in particular from flexor digitorum longus (Eccles et al c) indicate that the Ib system may also be of importance in terminating the extension phase and initiating the flexion phase of stepping (Eccles & Lundberg, 1958a). The reflex action of Group II afferents (12-4,u) from the flower-spray endings on muscle spindles (Hunt, 1954), and also of the still smaller Group III afferents (< 41L) are those of the general flexion reflex (Lloyd, 1943; Brock, Eccles & Rall, 1951; Laporte & Lloyd, 1952; Eccles & Lundberg, 1959). The actions by impulses in Group II fibres are of particular interest. Group II fibres are activated by a slight stretch of the muscle. The threshold for excitation is only slightly higher than for the Ia fibres (i.e. from annulospiral endings) (Hunt, 1954). Likewise both type of muscle-spindle afferents may be discharged by impulses in y-efferents (Hunt, 1954). Thus there are two systems of afferents from muscle spindles which are activated similarly and yet with different central connexions to motoneurones. From this fact

2 566 ROSAMOND M. ECCLES AND A. LUNDBERG there arose the following problem: The pattern of Ia action is apparently well adapted to subserve locomotion, yet it is difficult to see how these connexions assert themselves in the reflex control of movement if superimposed on the very large non-specific actions of Group II impulses. One possible answer is that the reflex actions in the spinal preparation are not characteristic of those in the non-spinal animal. Originally it was believed that in the cat spinal transection was followed by depression of all reflexes in the regions posterior to the transection (Sherrington, 1906). Later Sherrington & Sowton (1915) and Forbes, Cobb & Cattell (1923) found that spinal transection actually increased the flexion reflex evoked by single stimuli. It has also been shown that inhibitory reflex actions may be increased considerably after spinal transection. The increased susceptibility of the knee jerk to inhibitory stimuli (Ballif, Fulton & Liddell, 1925; Fulton, Liddell & Rioch, 1930 a, b; Liddell, 1934, 1936) indicates that inhibitory reflex actions may also be increased after spinal section. Fulton (1926) suggested that the release was due to disappearance of inhibitory action on the interneurones mediating the reflexes, and decisive experimental evidence in support of this suggestion was given by Job (1953). A control of interneurones in the dorsal horn from higher centres was postulated by Austin (1952), by Lindblom & Ottosson (1955) and by Kleyntjens, Koizumi & Brooks (1955), who found that components of the cord dorsum potential and polysynaptic reflexes were decreased on stimulation of the reticular formation. Hagbarth & Kerr (1954) studied the control of transmission in ascending tracts. In the present series of experiments the synaptic actions exerted on motoneurones by the individual systems of muscle afferents have been compared in decerebrate and spinal cats (cf. Job, 1953). It will be demonstrated that supraspinal centres exert a tonic inhibitory action on interneurones that relay impulses in Group Ib, II and III afferents, whereas no evidence for such a control has been obtained for interneurones mediating I a inhibitory actions. A preliminary report of some of the results has been published (Eccles & Lundberg, 1958a). METHODS These experiments were done on cats decerebrated by intercollicular section, using pentothal or ether as the anaesthetic. The decerebration was performed when the animal had been mounted in the frame and all other dissection had been completed. In most experiments transection of the spinal cord was made in the first lumbar segment after local injection of cocaine. Special reference will be made to the experiment in which the cord was divided more rostrally. Since the anaesthetics employed were all short-acting the results may be considered to be from animals virtually unanaesthetized. In most of the experiments the synaptic actions were measured by their action on testing monosynaptic reflexes recorded in the ventral roots (Renshaw, 1940; Lloyd, 1941, 1946a). In some experiments intracellular records were taken from motoneurones, the excitatory and inhibitory post-synaptic potentials (EPSP and IPSP) giving a direct measure of the respective synaptic actions (Eccles, Eccles & Lundberg, 1957a, b, c; Eccles & Lundberg, 1959). In order to investi-

3 SUPRASPINAL CONTROL OF INTERNEURONES 567 gate the synaptic actions of the various groups of muscle afferents, volleys were evoked by stimuli of different strengths, which were expressed in multiples of the threshold strength. When recording monosynaptic reflexes two volleys were often used to evoke the monosynaptic reflex used for testing. The strength of the first volley was subliminal for reflex excitation, and preceded the second stronger volley by an interval that gave maximal facilitation of the monosynaptic reflex evoked by the second volley. With this technique larger and more stable reflexes can be elicited. An important advantage was that the size of the monosynaptic reflex could be altered simply by varying the strength of the first shock. Thus the monosynaptic reflex could be adjusted to the same size after spinal transection. Furthermore, with the double-volley technique it was often possible to evoke monosynaptic reflexes which could not be potentiated poettetanically and which therefore may have comprised the whole motoneurone pool (Beswick & Evanson, 1955). However, in order to ensure an adequate subliminal fringe, the test reflexes were usually about one third of maximal size. For experiments on monosynaptic reflexes, the time-consuming procedure was employed of obtaining interaction curves at 4-6 different conditioning strengths. At each strength different intervals were selected and superimposed records at a frequency of 1/sec were taken at each interval. The mean of these records was plotted as a percentage of the mean control reflex, with time as the abscissa. When the conditioning afferent volley displayed the separation into Ia and Ib components, it was possible to differentiate between the actions of Ia and Ib impulses (cf. Bradley & Eccles, 1953; Laporte & Bessou, 1957; Eccles et al. 1957a). This procedure was important in those cases in which the conditioning volley gave a monosynaptic reflex which would cause recurrent (Renshaw) inhibition (cf. Eccles, Fatt & Koketsu, 1954). When there was no separation into Ia and Ib components, the stimulus setting of the conditioning volley was adjusted so that it was just maximal for the monosynaptic reflex and the effects of this volley were compared with those obtained when the conditioning strength was maximal for Group I. Any difference in action can then be ascribed to Ib impulses. This method gives significant results because, even in the absence of separation in Ia and Ib components by conduction velocity, there is usually a considerable difference in threshold between these two groups of afferents (cf. Eccles et at. 1957a). Renshaw inhibition secondary to the discharge of impulses by motoneurones presents a complication in an analysis of the present type. For interpretation of the present results it is of importance to know that there is no release of Renshaw inhibitory action after spinal transection (Holmqvist & Lundberg, unpublished). Discriminative tests were also necessary in order to differentiate between actions caused by high-threshold Ib fibres and low-threshold Group II fibres. For this purpose the actions were examined at strengths just maximal and then slightly supramaximal for Group I, and, when no increase of action occurred, the effects could be ascribed to impulses in Group I b fibres. Even in unanaesthetized animals it was a constant finding that despite the overlap in threshold with Ib afferents, the actions by Group II afferents did not appear until the stimulus strength was raised markedly above the strengths needed for the maximal activation of Group I. Presumably a considerable summation is needed for activation of the interneurones mediating Group II actions. There is also a differentiation of effects caused by Group II and Ill afferents (Eccles & Lundberg, 1959). RESULTS Flexor nuclei Action of impulses in muscle afferents. The curves in Fig. 1 illustrate the effect produced by conditioning volleys in the plantaris nerve upon the monosynaptic reflex from the posterior biceps-semitendinosus (BSt) nerve. At all strengths there was no significant facilitatory action in the decerebrate state (circles). After complete section of the spinal cord at L I the facilitatory actions

4 568 ROSAMOND M. ECCLES AND A. LUNDBERG characteristic of the spinal animal appeared. In the series illustrated by the upper curve the strength('of the conditioning stimulus (1-54 times threshold) was just maximal for Group 1. Since the facilitation appeared with an approximately half-maximal Group I volley and did not increase when the highest threshold Group I fibres were added to the conditioning volley, it is concluded that the facilitation was caused by Ib impulses (cf. Eccles et al. 1957a). The increment in facilitation at 3-04 and 6-1 times threshold may be attributed to 4ooT Plantaris-BSt 120 ; 1S54xthreshold io xthreshold J.. * : ; < S E. 6-1 x threshold 1201/ 100 e 80< x 12 threshold 120 /\ 100 / ,\ 140 / /\ 120 / 24-3 x threshold Fig. 1. Effects of conditioning volleys in the nerve to plantaris on the monosynaptic reflex from the nerve to posterior biceps-semitendinosus. In each instance 10% on the ordinates represents the unconditioned amplitude of the test monosynaptic reflex. Conditioned amplitude of monosynaptic test reflex, expressed as percentage of control amplitude, is plotted as a function of time interval between incoming conditioning and testing volleys which were recorded at the dorsal root entry zone. The experiment was made on a decerebrate cat and effects examined before (0) and after spinal transection (A). The curves were obtained at different strengths of conditioning stimuli. For each curve this strength is expressed in multiples of threshold strength.

5 SUPRASPINAL CONTROL OF INTERNEURONES 569 impulses in Group II fibres (cf. Eccles & Lundberg, 1959). The late phase of facilitation that appeared with increase of stimulus strength from 12-2 to 24-3 times threshold was due to Group III impulses since a stimulus of 8-10 times threshold strength suffices to excite all Group II fibres (Eccles & Lundberg, 1959). Hence it can be concluded that the facilitatory actions by Ib, II and III impulses show a release after cord section. The synaptic actions by Quad-BSt % ~~~~~~~~~140j % ; 1W _ / l " *I xthreshold / _ i0!n \,.~~_. a 60^ 1 40 ~2-1 x threshold % / 160Ō\ tg /*---.~- *~- 80I-/ nn xthreshold 40i 90x threshold 20 \ Fig. 2. As in Fig. 1, but conditioning volleys in the nerve to quadriceps. Conventions as for Fig. 1. these afferents are mediated by interneurones and the only plausible interpretation of these findings is that, in the decerebrate preparation, these interneurones are tonically inhibited by descending impulses. In two control experiments it was found that a similar release occurred after an upper spinal (C 1) section; hence these descending impulses must originate in the brain stem. In the curves of Fig. 2 the monosynaptic reflex from posterior bicepssemitendinosus was preceded by a volley from the antagonist muscle, quadriceps. The quadriceps volley displayed complete separation into I a and Ib components (cf. Bradley & Eccles, 1953; Eccles et al a; Laporte & Bessou, 1957). The conditioning volley at 1-65 times threshold included approximately half, and, at 2-1 times threshold, the whole Ib component. From ten experiments it was noted that the magnitude of the I a inhibition does not appreciably alter after spinal transection. In the upper curves of Fig. 2 the decay of 36 PHYSIO. CXLVII &

6 570 ROSAMOND M. ECCLES AND A. LUNDBERG inhibition is faster after cord section and is succeeded by a phase of facilitation. This is the characteristic Ib facilitatory action and its absence before transection of the cord corresponds with the findings in the upper curve of Fig. 1. Lloyd (1946a) originally described a 'direct' inhibition curve that changed exponentially from its maximum with a time constant of about 4. When Laporte & Lloyd (1952) later found an initial more rapid phase of decay from the maximuw, they attributed this deviation to the superposition of an excitatory action by Ib impulses. However, Bradley & Eccles (1953) observed the initial fast decay of the inhibitory curve when using a Ia volley, and later Coombs, Eccles & 120j 1W*. ; 80-\ 60j~ 40jb Quad-BSt 0 *0 11 Sx threshold ; \./ 1-69xthreshold 20 O0-4 ^ N Fig. 3. As in Fig. 1, but conditioning volleys from the quadriceps nerve. The volleys displayed complete separation into Ia and Ib components. A stimulus of 1-15 times threshold strength (upper curve) evoked a Ia volley of less than half the maximal size. The conditioning volley evoked at 1-69 times threshold (lower curve) included about half of the Ib fibres. Conventions as for Fig. 1. Fatt (1955) concluded that the brief action was due to the increased subsynaptic currents through the IPSP elements, the later more prolonged action being attributable to the hyperpolarization of the whole post-synaptic membrane. In the present investigation a rapid non-exponential decay of the direct inhibitory curve was often observed. Figure 3 shows curves obtained on conditioning the monosynaptic reflex from BSt with volleys from the quadriceps nerve. This nerve displayed complete separation into Ia and Ib volleys and at a stimulus strength of 1-15 times threshold less than half the I a fibres were activated, whereas at 1-69 times threshold stimulation was supramaximal for Ia and included about halfthe Ib afferents. The double composition ofthese inhibitiory curves is evident also before cord section, and this is of particular interest because, since Ib excitatory actions are absent in the decerebrate state, the composite curve cannot be caused by impulses in such Ib fibres as may 'contaminate' the Ia volley (cf. Laporte & Bessou, 1957; Eccles et al. 1957a). In the lower curves of Fig. 2 a later phase of inhibition appeared at Group II strength and reversed to facilitation after section of the spinal cord. When the

7 SUPRASPINAL CONTROL OF INTERNEURONES 571 stimulus strength was raised to 23-5 times threshold in order to include Group III fibres, this late phase of inhibition increased considerably (not illustrated). This late phase of inhibition was regularly found when the conditioning stimuli were applied to the quadriceps nerve. Though it is not observed in Fig. 1, it occurred occasionally when the nerves to plantaris, gastrocnemius, soleus and flexor digitorum longus were given conditioning stimuli at Group III strengths. Possibly its regular appearance on stimulation of the quadriceps nerve is merely due to the larger number of fibres in this nerve. Since the onset of the Group II and III inhibition is so early, its pathway cannot extend beyond the lumbar region of the cord. One possible explanation is that the Group II and III volleys may evoke a flexor reflex discharge along ventral roots other than the one under observation and this discharge may give rise to the inhibition via the Renshaw feed-back (Eccles, et at. 1954) Sometimes, when a very small monosynaptic reflex was used for testing it has been possible to demonstrate that the conditioning volley may have a small excitatory action, but it failed to evoke a reflex discharge and consequently the inhibition must be regarded as a genuine action by impulses in the Group II and III afferents and not an effect secondary to Renshaw activation by a motoneuronal discharge. It should be recalled that inhibitory action on flexor motor nuclei by high-threshold muscle afferents has been found also in the spinal preparation in which the excitatory actions were lacking; but on the other hand there was no evidence from intracellular recording that the excitatory actions normally received by flexor motoneurones concealed an inhibitory component (Eccles & Lundberg, 1959). The present finding of high threshold inhibition in flexor motoneurones in the decerebrate preparation (Fig. 2 C, D) may be a parallel phenomenon, which becomes detectable when the excitatory pathway is depressed in the decrebrate animal. As regards the functional significance of this inhibition the possibility exists that the central nervous system has two pathways at its disposal by which impulses in high-threshold muscle afferents may act on flexor motoneurones. However, the fact that the inhibitory component is rarely evoked except on stimulation of the very large quadriceps nerve necessitates caution. Undoubtedly the single-volley technique has its limitations and experiments with adequate stimulation would be valuable. The action by impulses in high-threshold muscle afferents on flexor motoneurones is further illustrated in Figs. 4 and 5. Six posterior biceps-semitendinosus motoneurones were impaled by a micro-electrode before cord section and another six afterwards. In none of the cells before transection was there any significant addition to the monosynaptic EPSP when the stimulus strength was raised above Group I strength (Fig. 4, F-H). On the other hand, after spinal transection (Fig. 5, A-K) the characteristic late excitatory action appears at Group II strength (D) and grows with inclusion of more 36-2

8 572 ROSAMOND M. ECCLES AND A. LUNDBERG Group II fibres (E-G). A late EPSP is added by impulses in Group III fibres (J) leading to impulse generation (K). On stimulation of the quadriceps nerve there is no evidence of lb excitatory action (Fig. 4, J-K), whereas a lb EPSP can be clearly seen after section of the spinal cord (Fig. 5, O-P). A late inhibitory potential appears as a delayed decay of the I a IPSP (Fig. 4, record N). It is uncertain whether the large increase of the late IPSP (Fig. 4, P) is contributed by the lowest threshold Group III afferents; but with further increase of stimulus strength (Q-T) an additional IPSP appears that must be A 1-07 N I L3-8 ~TJ Uf vi if,! vi I Fig. 4. Intracellular recording, with micro-electrode filled with 0 6 m-k2s04, from a motoneurone of the posterior biceps-semitendinosus nerve. A-H, responses evoked by stimulation of the nerve to posterior biceps-semitendinosus; I-T by stimulation of the nerve to quadriceps. The stimulus strengths relative to threshold strength are indicated in the records. In the triphasic records of the dorsal root volley, negativity is unconventionally signalled by a downward deflexion. The animal was decerebrated. 4 mv due to Group III fibres. After spinal transection Group II and Group III contribute the usual late EPSP's (Fig. 5, R-Z) leading to generation of impulses in Y and Z. The findings in Figs. 4 and 5 were consistent in all the cells investigated and correspond in detail with the results obtained in the experiments in which the monosynaptic reflex was used as an index of the motoneuronal excitability. The steadier base line in Fig. 4, compared with Fig. 5, suggests that the

9 SUPRASPINAL CONTROL OF INTERNEURONES 573 flexor motoneurones in the spinal animal receive a more intense background bombardment from interneurones than in the decerebrate state. Action ofimpulses injoirt and skin afferents. Figure 6 A and B shows the effect of conditioning volleys in the posterior nerve to the knee joint and in the superficial peroneal nerve, respectively. In the decerebrate preparation there was little or no facilitatory effect even when the joint nerve was stimulated at 1-17 B -54 c E F G L 1-08 M r onow, 2-32 Q, 3-28 _9~? v r 5-4 I 7-6 J IF I v I q -'m D H e _6 B1QWh' w 328 X v54 Fig. 5. Intracellular records, with micro-electrode filled with 10-6 M-K2S04, from a motoneurone of the posterior biceps-semitendinosus group. The records were obtained in the same experiment as those of Fig. 5 but after transection of the spinal cord at L 1. A-K show responses evoked by stimulation of the posterior biceps-semitendinosus nerve, L-Z by stimulation of the quadriceps nerve. Records I-K and W-Z were obtained at the slower sweep speed. strengths 25 times threshold, but after transection of the cord a large facilitation, as in Fig. 6A, could regularly be evoked (cf. Eccles & Lundberg, 1959). On the other hand, a conditioning volley from the superficial peroneal nerve usually gave some facilitation mixed with inhibition, as in Fig. 6B, and after cord section there occurred a large increase of the facilitatory action. In some animals, particularly those anaesthetized with ether, a small

10 574 ROSAMOND M. ECCLES AND A. LUNDBERG inhibitory effect was produced by impulses in the nerve from the knee joint, and after cord section it reversed to facilitation (Fig. 6 C). This effect resembles that found when conditioning by Group II and III muscle afferents, and presumably they are related phenomena since they are always found in the same animals. 250 A Joint -BSt / A ~~~~~~~~~~~SP-BSt 150 / 150 \ 100 * A C A A Joint-BSt A 17 2x threshold 140A. /\ A A 120 A.1 ~~~~~~~~~~~~A I. * E *. -N i t i i i Fig. 6. As in Fig. 1, but conditioning volleys in the posterior nerve to the knee joint (A and C) and the superficial peroneal nerve (B). Curves A and B were obtained from the same animal. Conventions as for Fig. 1. Extensor nuclei Action of impulses in muscle afferents. In Fig. 7 are shown curves obtained when the monosynaptic reflex to the ankle extensor, gastrocnemius-soleus, was conditioned by volleys in the nerve to another extensor muscle, flexor digitorum longus (FDL). A maximal Group I volley (1-55 times threshold) produced a small inhibition of uncertain time course, which was probably a Renshaw effect caused by the monosynaptic reflex in the motor fibres to FDL, though the early phase might be lb inhibition. The important finding in

11 SUPRASPINAL CONTROL OF INTERNEURONES 575 Fig. 7 is that with increase of stimulus strength to include Group II (5.1 times threshold) and Group III (10*3-26 times threshold) there was no evidence that these impulses contributed inhibitory action in the decerebrate state. On the other hand, after spinal transection the usual inhibitory effect occurred at all strengths of stimulation. It has already been demonstrated that this inhibitory effect does occur in the spinal animal, even if the conditioning volley does not give rise to a reflex discharge in flexor nuclei with the consequent % FDL-gastrocn. 120i x threshold lo ' 120. i Athreshold ! i 120 sec m 1 nnl t threshold C \. 26 X threshold m 25 Fig. 7. As in Fig. 1, but the testing monosynaptic reflex from gastrocnemius-soleus, and the conditioning volleys in the nerve to flexor digitorum longus. Conventions as for Fig. 1. Renshaw inhibition (Eccles & Lundberg, 1959). Consequently, this experiment shows a release of inhibitory action evoked by impulses in I b, II and III fibres. There was a similar release of inhibitory action by Group II and III volleys on an extensor motor nucleus when the conditioning volley was in the nerve to an extensor muscle, operating at another joint (quadriceps-gastrocnemius) or in the nerve to a flexor muscle (BST-gastrocnemius). However, only in a

12 576 ROSAMOND M. ECCLES AND A. LUNDBERG few cases were inhibitory effects completely absent in the decerebrate preparation. If the animals had been anaesthetized with pentothal until the decerebration, there was regularly inhibitory action by Group III volleys, and sometimes a little by Group II volleys, in all the nerves investigated. If ether had been used, then the inhibitory effects in decerebrate cats were always less and sometimes absent. In a later series of experiments made on cats decerebrated under ether, it was regularly found that maximal Group III volleys in the nerves to gastrocnemius-soleus, FIDL or plantaris did not in the non-spinal state evoke inhibitory effects in extensor motor nuclei (Holmqvist & Lundberg, unpublished). 400 Lat. gastrocn.-med. gastrocn. 300 o Fig. 8. Effects of conditioning volleys in the nerve to lateral gastrocnemius-soleus on the monosynaptic reflex from medial gastrocnemius. The experiment was made on a non-spinal decerebrate cat x threshold, x 1-8 x threshold, [] 9 0 x threshold. In some of the experiments appreciable I b inhibitory actions were found in the non-spinal state. All the curves in Fig. 8 are from a non-spinal decerebrate preparation. Maximal heteronymous facilitation, and hence stimulation of all I a fibres, was obtained with a conditioning volley at 1-33 times threshold. A decay of facilitatory action faster than the expected exponential curve (Lloyd, 1946 a) may indicate that some Ib fibres were stimulated at this strength. At 1-8 times threshold (maximal for Group I) the Ib inhibitory action has increased considerably, whereas there was no further change of the curve with the addition of the Group II volley to the conditioning volley (9 times threshold). Appreciable Ib inhibitory actions were sometimes found in animals in which Group III volleys had no inhibitory action. On the other hand, I b effects could be suppressed in the non-spinal animal when Group III

13 SUPRASPINAL CONTROL OF INTERNEURONES 577 volleys had some inhibitory action. Hence, it seems reasonable to assume that the interneurones mediating these two groups of actions are independently controlled. Of considerable interest is the finding that there are different patterns of I b inhibitory action in the decerebrate and the spinal preparations. The spinal animal is characterized by very large actions from flexor digitorum longus (Eccles et al. 1957c; Eccles & Lundberg, 1959), whereas in the decerebrate preparation the lb inhibitory actions from FDL are the smallest. The curves in Fig. 9 illustrate an experiment in which Ib actions from the nerves to plantaris, FDL and quadriceps were investigated in the gastrocnemius-soleus nucleus. Of the three plantaris curves to the left, A was obtained with a 120 A Plant.-gastrocn. loo~~?' ~ * ''1.17)c threshold 0 s rT 1.65 X threshold 120 FDL-gastrocn. 120 D x threshold i E Quad-gastrocn. 100: 80 \, 2-0 x threshold 120Sn o is s 10 15s Fig. 9. As in Fig. 1, but the testing monosynaptic reflex is from gastrocnemius-soleus, and the conditioning volleys in the nerves to plantaris, (A-C) flexor digitorum longus (D), and quadriceps (E). Further description in text. Conventions as for Fig. 1. conditioning strength of 1-17 times threshold. At 1-65 times threshold (B) the Group I volley was maximal and the increase of inhibitory action in comparison with the upper curve can be attributed to I b impulses. That they were not due to low-threshold Group II fibres, which may overlap in threshold with the highest-threshold Group I fibres, is shown in C, where an increase of the strength of the conditioning stimulus to 2-7 times threshold did not bring about any increased inhibitory action. It appears from the curves that there was only a moderate increase in inhibition after section of the cord. On the other hand, with a conditioning FDL volley in Fig. 9 D, the inhibition before cord transection was smaller than with plantaris, but it was increased much more by the transection, and similar results were obtained with a conditioning quadriceps volley (Fig. 9F). This is further illustrated in Figs. 10 and 11, showing intracellular records from two plantaris motoneurones in the same animal. A very weak homonymous volley in plantaris caused the EPSP in Fig. 10P. With increased strength of stimulation an impulse was regularly generated in this motoneurone, hence

14 578 ROSAMOND M. ECCLES AND A. LUNDBERG it was not possible to analyse I b actions from the plantaris nerve. Records A-D show the effects of volleys evoked in the nerve FDL by different stimulus strengths. The heteronymous monosynaptic EPSP (cf. Eccles et al. 1957b) had its usual characteristic time course in A and did not change when highthreshold Group I fibres were included (B-D). This is in striking contrast to what is found in the spinal cat, where increase of the Group I volley causes FDL _ E 1-23 F 1-35 G 1-47 H 214 Lat. G _ I 1-26 J 1-63 K 2-94 L 6-1 Med. G" M 51 N J Oj D.P. 2.0 p N mv1 Sural a 1~~~~~~~~~AR tt _._... A..i Fig. 10. Intracellular recording, with micro-electrode filled with 0-6 m-kao,,, from a plantaris motoneurone in a decerebrate non-spinal cat. The records were obtained on stimulation of the nerves to following muscles: A-D, flexor digitorum longus (FDL); E-H, lateral gastroenemius-soleus (Lat. G); I-L, medial gastrocnemius (med. G); 0, extensor digitorum longus and tibialis anticus (D.P.); and P, plantaris. To evoke the response in M the sural nerve was stimulated and in N the posterior nerve to the knee joint. Pi. - - E_ ~1.1 f 12 G 198 FDLL ~U _.m- m J 1-29 K 1-49 Lat. G _m % H -Wa~~~~ 2 mv - < ~~~3-98 L 2-1 Fig. 11. Intracellular recording, with micro-electrode filled with 0-6 m-k2so4, from a plantaris motoneurone. The records were obtained in the same experiment as those of Fig. 10, but after transection of the spinal cord at Li. The nerves to the following muscles were stimulated: A-D, plantaris (P1); E-H, flexor digitorum longus (FDL); and I-L. lateral gastrocnemiussoleus (Lat. G).._ A

15 SUPRASPINAL CONTROL OF INTERNEURONES 579 a Ib IPSP to be superimposed on the monosynaptic EPSP, as in Fig. 11, F-G (of. Eccles et al. 1957c). On the other hand, in Fig. 10 Ib IPSPs were evoked before the cord section from the nerves to lateral gastroenemius-soleus (E-H) and to medial gastrocnemius (I-L), and they were of the same order of magnitude as those evoked by the lateral gastrocnemius-soleus volley in another motoneurone after the cord had been divided (Fig. 11, I-L). There was also a relatively large homonymous Ib IPSP from plantaris (Fig. 11, B-D). Hence both the experiments with intracellular recording and those with conditioning of monosynaptic reflexes have indicated that the Ib inhibitory actions from FDL are more effectively suppressed in the decerebrate preparation than those from the other anlle extensors. Fig Joint-gastrocn. 100_ ;- *-** * 80 - / 60./ 40 \ s 20 As in Fig. 1, but the testing monosynaptic reflex is from gastrocnemius-soleus, and the conditioning volleys in posterior nerve to knee-joint. Action by imputlses tn joint and skin afferents. The effects of conditioning volleys in joint and skin nerve afferents are also different before and after spinal section. In Fig. 12 a conditioning volley in the posterior nerve to the knee joint had no effect before section of the spinal cord but afterwards the same volley gave pronounced inhibition. Renshaw inhibition from the discharge in flexor nuclei may have contributed to this action but is not the exclusive cause of it, because strong inhibitory effects were found also in those cases in which no reflex discharge was noted in the ventral roots on stimulation of the joint nerve. For investigation of effects from cutaneous afferents we stimulated the sural or the superficial peroneal nerve. There was usually, but not always, some inhibition action also in the non-spinal state, but the effect always increased very considerably after spinal transection. In the non-spinal state there was always a close parallelism in the degree of suppression of synaptic actions from high-threshold muscle afferents, skin and joint afferents. DISCUSSION In this investigation Job's (1953) finding has been confirmed, that in decerebrate and spinal preparations there are very considerable differences in synaptic actions evoked in motoneurones by impulses in somatic afferents.

16 580 ROSAMOND M. ECCLES AND A. LUNDBERG Job used 'supramaximal' conditioning volleys in muscle nerves and found, for extensor nuclei, a reversal from excitation to inhibition after cord section. In the present series of experiments there was never any excitatory action in extensor motor nuclei on conditioning with volleys in Group II and III muscle afferents. However, after interruption of the pathways from higher centres, there was always a release of inhibitory action. Though Job's conditioning volley may have included C fibres, the onset of excitatory action was too early to be explained in this way. It must not necessarily be assumed that the excitatory action (Job, 1953) was caused by impulses in high-threshold afferents. The possibility that I a afferents under some conditions may have polysynaptic excitatory actions cannot be entirely excluded (Eccles, Eccles & Lundberg, unpublished). In flexor motor nuclei there was often complete suppression of polysynaptic excitatory action in the non-spinal state. The occasional slight inhibition in the decerebrate preparation reverted to the usual type of excitation on spinal transection. It has already been pointed out that caution must be observed when evaluating the functional significance of these inhibitory effects in flexor motoneurones (cf. also Eccles & Lundberg, 1959). However, the dominating result from section of the spinal cord is the release of excitatory action to flexor nuclei and inhibitory action to extensor nuclei. In agreement with Job (1953), it is concluded that section of the cord removes a tonic inhibitory action which suprasegmental centres exert on interneurones mediating actions from muscle afferents. What functional significance is to be attributed to the existence of two systems of muscle-spindle afferents with different patterns of synaptic actions on motoneurones? How can the detailed patterns of Ia connexions assert themselves among the large non-specific actions of Group II afferents? It is important to know the nature of the influence of the higher centres on the transmission of impulses across the Group I a and the Group II reflex arcs. It has now been demonstrated that the interneurones mediating Group II actions are very effectively inhibited from suprasegmental levels. Hence, it is possible that, under circumstances in which the I a system operates optimally in reflex control of movements, the impulses in Group II afferents are prevented from exerting any action on the motoneurones because of the inhibitory bias on their interneurones. There is no evidence that the I a pathways are under a similar control. The I a excitatory actions are exerted monosynaptically and there is no evidence of any change after spinal transection. On the other hand, it is known that there is an interneurone in the inhibitory pathway (Eccles, Fatt & Landgren, 1956; Eccles & Lundberg, 1958 b), but spinal transection has no appreciable effect on I a inhibition (Figs. 2, 3 and Job, 1953). If, as suggested above, Group II impulses are prevented from exerting any

17 SUPRASPINAL CONTROL OF INTERNEURONES 581 action during locomotion, it remains to discover their functional significance. It was noted that there was a close parallelism between the effect of spinal transection on synaptic actions exerted by Group II muscle afferents and those of Group III muscle afferents as well as skin and joint afferents. This finding supports the hypothesis (Eccles & Lundberg, 1959) that the flexor reflex action exerted by Group II afferents is part of the general flexion reflex, which presumably is of protective importance and functions mainly when the animal is at rest. In the alerted animal, on the other hand, it would be beneficial if these reflexes were prevented, since they would limit the animal's capacity for expedient action. Of considerable interest is the finding that the interneurones mediating Group lb actions, both excitatory and inhibitory, are also tonically inhibited in the decerebrate preparation. At least, with respect to inhibition, it was possible experimentally to differentiate this control from that exerted on the interneurones mediating the actions of the general flexion reflex. It is not difficult to suggest a physiological significance for this control. It has been proposed that the Ib system may be of importance in terminating the extension and initiating the flexion phase of stepping (Eccles & Lundberg, 1959). It has been seen (Figs. 9-11) that the suprasegmental control is particularly effective with those interneurones relaying the Ib impulses from flexor digitorum longus. If Ib impulses from flexor digitorum longus should be of special importance in the reflex regulation of stepping, a control from higher centres of these reflex actions would be needed in order to regulate the movements of stepping and running. A control of Ib interneurones would presumably also be of importance in voluntary movements of various strengths. The central nervous system apparently has powerful possibilities to regulate the central action of individual receptor systems. The control of the I a system is via the y-efferents which change the sensitivity of the receptor itself (Leksell, 1945; Hunt & Kuffler, 1951; Granit & Kaada, 1952; Eldred, Granit & Merton, 1953). A more complex situation prevails with the actions of Group II fibres with flower-spray endings or muscle spindles, since in this case both the receptor and the interneurones are controlled. There is no knowledge of any control of the receptor with other systems, and regulation may be entirely at the interneuronal level. With regard to the control of interneurones of the flexion reflex, it should be recalled that there are several ascending pathways which are influenced by the same system of afferents. Two pathways have been found which are polysynaptically excited by impulses in Group II and III muscle afferents, by skin and by joint afferents from very wide receptive fields (Laporte, Lundberg & Oscarsson, 1956; Holmqvist, Lundberg & Oscarsson, 1956; Oscarsson, 1958). Neurones of another pathway, the ventral spinocerebellar tract, receive powerful inhibitory actions from these afferents as well as monosynaptic

18 582 ROSAMOND M. ECCLES AND A. LUNDBERG excitatory actions from Ib afferents (Oscarsson, 1957). Since all these pathways are influenced by the same system of afferents that give rise to the flexion reflex, there is a possibility that they are informative with respect to this reflex; they may, for example, be afferent links in the suprasegmental regulation of the inhibitory bias on the interneurones mediating the flexion reflex (cf. Holmqvist, Lundberg & Oscarsson, 1959). SUMMARY 1. The synaptic actions by impulses in muscle, joint and cutaneous afferents have been compared in decerebrate cats before and after section of the spinal cord. 2. In flexor motor nuclei of the non-spinal animal, impulses in Group II and III muscle and joint afferents have little or no excitatory action, whereas large actions appear after spinal transection. Excitatory actions by impulses in cutaneous afferents also increase considerably after cord section. Correspondingly, in extensor motor nuclei inhibitory action by the same systems of afferents appears or, if already present, increases after spinal transection. It is concluded that the interneurones mediating the actions of the general flexion reflex are tonically inhibited from the brain stem in the decerebrate preparation. 3. A similar release on spinal transection is found for the excitatory and inhibitory actions evoked by Ib impulses, showing that interneurones of the Ib pathways are also tonically controlled from the brain stem. The interneurones mediating Ib inhibitory actions from flexor digitorum longus are more effectively controlled from higher centres than those mediating Ib effects from other ankle extensors. 4. Some findings suggest that the suprasegmental control of the interneurones mediating Ib inhibitory actions is functionally independent of that controlling interneurones mediating the actions of the general flexion reflex. 5. There has been no evidence of any suprasegmental control of interneurones mediating I a inhibitory actions. 6. The results are discussed mainly in relation to the significance of the different receptor systems from muscle in the reflex taxis of the animal. This work was supported by a grant from the Rockefeller foundation. Technical assistance was given by Miss Ruth Araldsson. REFERENCES Ausrnf, G. M. (1952). Suprabulbar mechanisms of facilitation and inhibition of cord reflexes. Re8. Publ. A88. nerv. ment. Di8. 30, BAT.Tr., L., FULTON, J. F. & LIDDELL, E. G. T. (1925). Observations on spinal and decerebrate knee-jerks with special reference to their inhibition by single break-shocks. Proc. Roy. Soc. B, 98, BESWICK, F. B. & EVANSON, J. M. (1955). The heterosynaptic activation motoneurones during post-tetanic potentiation. J. Phy8io. 128,

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