- Email: [email protected]
Spinocerebellar tract neurones with long descending axon collaterals
Brain Research, 142 (1978) 147-15l © Elsevier/North-Holland Biomedical Press
147
Spinocerebellar tract neurones with long descending axon collaterals
N. HIRAI*, T. HONGO and T. YAMAGUCHI Laboratory of Physiology, Institute of Basic Medical Sciences, University of Tsukuba, Niihari-gun Ibaraki 300-31 (Japan)
(Accepted September 14th, 1977)
Studies of the axonal trajectory of the spinocerebellar tract neurone have so far been focused mainly on its stem axon, projecting (directly) to the cerebellar cortex and thus characterizing the cell as spinocerebellar. There is, however, increasing evidence indicating the existence of collateral branches terminating outside the cerebellar cortex. Observations of these collaterals have been concerned mostly with supraspinal structures such as the cerebellar nuclei 9, the vestibular nuclei 6, the nucleus Z 4. Little is known regarding the collateral branchings within the spinal cord 1°. Morphologically it has been shown that some of the funicular neurones in the spinal cord send collateral branches to the spinal grey from their longitudinally running axons, or even possess both ascending and descending axonal branches bifurcated in T-shape near the level of their cell bodies3, s. There would be no reason to assume a priori that these do not apply to the spinocerebellar tract cells, and in fact we found a group of spinocerebellar tract cells in the cervical cord which issue collateral branches descending to the lumbar cord, as will be described below. Of interest in this connexion is the recent physiological finding that neurones in the upper cervical cord receiving synaptic inputs from corticospinal and rubrospinal tracts give off both descending and ascending axons, projecting to the forelimb motor nucleus and the lateral reticular nucleus respectively (Illert and Lundberg, personal communication). Experiments were performed on cats anesthetized with pentobarbital sodium (40 mg/kg) and immobilized with gallamine triethiodide. A glass capillary microelectrode (2 M potassium citrate) was inserted into the spinal cord at the seventh cervical through first thoracic segments to record from neurones of the spinocerebellar tracts. These neurones were identified as spinocerebellar by antidromic invasion from the cerebellum. The method of antidromic stimulation in the cerebellum and criteria for identifying the response as antidromic were described previously, i.e. (1) an all-or-none appearance of spikes at a fixed latency and (2) capability to follow repetitive stimuli at high frequenciesL At the lowest thoracic segment the spinal cord was split into right * Present address: The Rockefeller University, New York, N.Y. 10021, U.S.A. (On leave from Department of Physiology, Kyohrin University, School of Medicine).
irh
B
F ,
Cb
A
sll
~'
i
lOmsec
2msec
I I mV
E
D
C
Cb-coTh
coTh L
C b - i Th
4rnsec
G
iFh
,~
lOre.see
|7,~v
I
U
lmm
4~ oc
149 and left halves after removing the dorsal column, and each of the dissected spinal half was placed on bipolar electrodes for stimulation in the ascending direction. Various forelimb nerves, mainly ipsilateral to the side o f recording, were also dissected and served for stimulation. The method of determining the site o f cells recorded was the same as given previously 5. The present paper is based on 67 spinocerebllar tract neurones identified as above and further subjected to examination o f the effect of stimulating the spinal funiculus at Th13. The main outcome of this survey was that a considerable p r o p o r t i o n of neurones were activated antidromically from the lower cord. Fig. 1 exemplifies responses of such a neurone recorded first extracellularly and afterwards intracellularly. While single stimuli to the ipsilateral cerebellar white matter evoked antidromic spikes in this neurone (A), a similar response was also p r o d u c e d from the thoracic cord ipsilateral to the recording side. Thus, single spikes were invariably evoked with a fixed latency after stimuli, and the response followed each of repetitive shocks at high frequencies (150-400 Hz) as shown in Fig. 1R. The antidromic nature o f the response was further indicated by a blockade o f the thoracic-evoked spikes on a manner of collision when preceded by antidromic invasion from the cerebellum (Fig. 1C). In this case, the thoracic stimulus effectively produced the spike (C1), when the interval between the preceding cerebellar-evoked spike and the thoracic stimulus was 4.0 msec. On the other hand, it did not (CD, when the interval was 3. I msec. The critical interval for the collision was close to the latency o f the thoracic response which was 3.4 msec. The same was true, as a matter o f course, when the order o f the two stimuli was reversed, i.e. the thoracic stimulus preceding the cerebellar. Single shocks to the contralateral thoracic cord also activated this cell with latencies which were approximately constant but seen to slightly fluctuate (Fig. 1D). A collision test revealed that this spike response was not blocked by a preceding cerebellar stimulation (Fig. 1E) by contrast, indicating that the spike generation was due to synaptic activation and not antidromic. After the above observations the same cell was penetrated intracellularly. Antidromic invasion from the ipsilateral thoracic cord was confirmed by an all-or-none appearance o f m-spikes (Fig. I G, superposed on IPSPs), as was the case with cerebellar stimulation (Fig. IF). Whether the spike response evoked from the thoracic cord was antidromic or
Fig. 1. Responses of a spinocerebellar neurone having a long descending axon collateral, recorded extracellularly (A-E) and intracellularly (F-I), and locations of such neurones (J). Bottom trace in A, B, D, H and I show cord surface potentials, and other traces are microeleetrode recordings. A : an antidromic spike evoked from the ipsilateral cerebellar. B : an antidromic response evoked from the ipsilateral cord at the lowest thoracic level. C: collision test at two different stimulus intervals with thoracic stimulation preceded by cerebellar. D: responses evoked by single stimulus to contralateral thoracic cord. E: collision test for combination of cerebellar and contralateral thoracic stimulation. F-I: intracellular recordings (positivity upward) from the same cell as shown in A-E. m-Spikes are evoked in an all-or-none fashion from ipsilateral cerebellum (F) and ipsilateral thoracic cord (G). H and i show 1PSPs (top traces) evoked from the median and ulnar nerve respectively, and juxtaextracellular responses (middle traces). J : location of neurones having a descending axon, sampled in one cat. See text for further details.
150 trans-synaptic was carefully determined particularly for extracellularly recorded cell,. according to the criteria as described above, including the blockage of spikes in a manner of collision when preceded by cerebellar stimulation. As the result, 18 out ot the 67 cells examined (27,,,) exhibited antidromic activation fl'om the lowest level of the thoracic cord, indicating that these neurones send descending axon collaterals down to the lumbar cord. The descending axons were located, at the Thia level, on the side ipsilateral to the cell soma in 17 of the 18 cells and on the contralateral side m the remaining cell, as,judged from the effective side for antidromic activation. As previously reported, the spinocerebella~ tibres of these cells ascend ipsilaterally to the cell soma at the Ca level s . The conduction velocities of the descending axon as estimated by the ratio of conduction distance versus latency of the antidromic spike were 16 93 m/sec (mean 54 m/sec). The corresponding values t~r antidromic responses from the cerebellum were 24 67 m/sec (mean 44 m/sec). When the conduction velocities of the descending and the ascending axon were compared in individual cells, no general tendency could be observed that either of the descending or ascending axons was faster-conducting than the other. It has earlier been shown that spinocerebellar neurones in the cervical enlargement consist of at least two major groups one located more dorsally and provided with potent excitatory inputs from the primary afferents and the other located more ventrally and predominantly inhibited from the periphery, particularly by flexor reflex afferents (FRA) s. Among the 67 neurones examined in the present study, 25 cells were classified as belonging to the former and 42 cells to the latter group. Characteristically, all the 18 cells antidromically invaded from the low thoracic cord were shown to belong to the ventral group and none to the dorsal group. Thus. 43 o~; of the ventral, peripherally inhibited group of cells were revealed to possess descending axon collaterals. The typical response of predominant, polysynaptic IPSPs from the forelimb FRA are shown in Fig. I H and I. Fig. I J illustrates locations of 4 neurones having descending axon collaterals and sampled in the same preparation. Note that they were all ventrally located (cf. Fig. I of Hirai et al.:'). The above results indicate that a considerable number of spinocerebellar neurones in the cervical cord issue long propriospinal axons descending beyond the lowest thoracic segment and therefore should possess double functions, i.e., propriospinal and spinocerebellar. Presumably, the descending axons are connected with neuronal circuits involved in the movement of the hindlimb and thus subserve forelimb-hindlim b coordination. The cerebellum, on the other hand, can receive via the ascending axons the same information as is conveyed propriospinally down to the lumbar cord. It may be of functional importance that such descending axon-collaterals are found only in those spinocerebellar neurones located more ventrally and predominantly inhibited by flexor reflex afferents (FRA) a. The organization of their synaptic inputs in many respects resembles that of ventral spinocerebellar tract neurones especially of spinal border cell origin a,s,7 Hence two spinocerebellar tracts may be functionally equivalent. If so, it would be warranted to investigate whether the VSCT axons reciprocally send off collateral branches terminating in the cervical cord.
151 1 Burke, R., Lundberg, A. and Weight, F., Spinal border cell origin of the ventral spinocerebellar tract, Exp. Brain Res., 12 (1971) 283-297. 2 Burton, J. E., Bloedel, J. R., and Gregory, R. S., Electrophysiological evidence for an input to lateral reticular nucleus from collaterals of dorsal spinocerebellar and cuneocerebellar fibers, J. Neurophysigl., 34 (1971) 885-897. 3 Cajal, S. Ramon y, Histologie du systdme nerveux de l'homme et des vert~brds, Tome I, chap. XII1, XIV, Maloine, Paris (1909). 4 Johansson, H. and Silfvenius, H., Axon-collateral activation by dorsal spinocerebellar tract fibres of group I relay cells of nucleus Z in the cat medullar oblongata, J. Physiol. (Lond.), 265 (1977) 341-369. 5 Hirai, N., Hongo, T., Kudo, N. and Yamaguchi, T., Heterogeneous composition of the spinocerebellar tract originating from the cervical enlargement in the cat, Brain Research, 190 (1976) 387-391. 6 Lorente de N6, R., l~tudes sur le cerveau post6rieur. 1II. Sur les connexions extra-c6r6belleuses des fascicule afferents au cerveau, et sur la fonction de cet organe, Tray. Lab. Rech. Biol. Univ. Madr., 22 (1924) 51-65. 7 Lundberg, A. and Weight, F., Functional organization of connexions to the ventral spinocerebellar tract, Exp. Brain Res., 12 (1971) 295-316. 8 Mannen, H., Reconstruction of axonal trajectory of individual neurons in the spinal cord using Golgi-stained serial sections, J. comp. Neurol., 159 (1975) 357-374. 9 Matsushita, M. and lkeda, M., Spinal projections to the cerebellar nuclei in the cat, Exp. Brain Res., 10 (1970) 501-511. 10 R6thelyi, M., The Golgi architecture of Clarke's column, Acta morph. Acad. Sci. hung., 16 (1968) 311-330.
147
Spinocerebellar tract neurones with long descending axon collaterals
N. HIRAI*, T. HONGO and T. YAMAGUCHI Laboratory of Physiology, Institute of Basic Medical Sciences, University of Tsukuba, Niihari-gun Ibaraki 300-31 (Japan)
(Accepted September 14th, 1977)
Studies of the axonal trajectory of the spinocerebellar tract neurone have so far been focused mainly on its stem axon, projecting (directly) to the cerebellar cortex and thus characterizing the cell as spinocerebellar. There is, however, increasing evidence indicating the existence of collateral branches terminating outside the cerebellar cortex. Observations of these collaterals have been concerned mostly with supraspinal structures such as the cerebellar nuclei 9, the vestibular nuclei 6, the nucleus Z 4. Little is known regarding the collateral branchings within the spinal cord 1°. Morphologically it has been shown that some of the funicular neurones in the spinal cord send collateral branches to the spinal grey from their longitudinally running axons, or even possess both ascending and descending axonal branches bifurcated in T-shape near the level of their cell bodies3, s. There would be no reason to assume a priori that these do not apply to the spinocerebellar tract cells, and in fact we found a group of spinocerebellar tract cells in the cervical cord which issue collateral branches descending to the lumbar cord, as will be described below. Of interest in this connexion is the recent physiological finding that neurones in the upper cervical cord receiving synaptic inputs from corticospinal and rubrospinal tracts give off both descending and ascending axons, projecting to the forelimb motor nucleus and the lateral reticular nucleus respectively (Illert and Lundberg, personal communication). Experiments were performed on cats anesthetized with pentobarbital sodium (40 mg/kg) and immobilized with gallamine triethiodide. A glass capillary microelectrode (2 M potassium citrate) was inserted into the spinal cord at the seventh cervical through first thoracic segments to record from neurones of the spinocerebellar tracts. These neurones were identified as spinocerebellar by antidromic invasion from the cerebellum. The method of antidromic stimulation in the cerebellum and criteria for identifying the response as antidromic were described previously, i.e. (1) an all-or-none appearance of spikes at a fixed latency and (2) capability to follow repetitive stimuli at high frequenciesL At the lowest thoracic segment the spinal cord was split into right * Present address: The Rockefeller University, New York, N.Y. 10021, U.S.A. (On leave from Department of Physiology, Kyohrin University, School of Medicine).
irh
B
F ,
Cb
A
sll
~'
i
lOmsec
2msec
I I mV
E
D
C
Cb-coTh
coTh L
C b - i Th
4rnsec
G
iFh
,~
lOre.see
|7,~v
I
U
lmm
4~ oc
149 and left halves after removing the dorsal column, and each of the dissected spinal half was placed on bipolar electrodes for stimulation in the ascending direction. Various forelimb nerves, mainly ipsilateral to the side o f recording, were also dissected and served for stimulation. The method of determining the site o f cells recorded was the same as given previously 5. The present paper is based on 67 spinocerebllar tract neurones identified as above and further subjected to examination o f the effect of stimulating the spinal funiculus at Th13. The main outcome of this survey was that a considerable p r o p o r t i o n of neurones were activated antidromically from the lower cord. Fig. 1 exemplifies responses of such a neurone recorded first extracellularly and afterwards intracellularly. While single stimuli to the ipsilateral cerebellar white matter evoked antidromic spikes in this neurone (A), a similar response was also p r o d u c e d from the thoracic cord ipsilateral to the recording side. Thus, single spikes were invariably evoked with a fixed latency after stimuli, and the response followed each of repetitive shocks at high frequencies (150-400 Hz) as shown in Fig. 1R. The antidromic nature o f the response was further indicated by a blockade o f the thoracic-evoked spikes on a manner of collision when preceded by antidromic invasion from the cerebellum (Fig. 1C). In this case, the thoracic stimulus effectively produced the spike (C1), when the interval between the preceding cerebellar-evoked spike and the thoracic stimulus was 4.0 msec. On the other hand, it did not (CD, when the interval was 3. I msec. The critical interval for the collision was close to the latency o f the thoracic response which was 3.4 msec. The same was true, as a matter o f course, when the order o f the two stimuli was reversed, i.e. the thoracic stimulus preceding the cerebellar. Single shocks to the contralateral thoracic cord also activated this cell with latencies which were approximately constant but seen to slightly fluctuate (Fig. 1D). A collision test revealed that this spike response was not blocked by a preceding cerebellar stimulation (Fig. 1E) by contrast, indicating that the spike generation was due to synaptic activation and not antidromic. After the above observations the same cell was penetrated intracellularly. Antidromic invasion from the ipsilateral thoracic cord was confirmed by an all-or-none appearance o f m-spikes (Fig. I G, superposed on IPSPs), as was the case with cerebellar stimulation (Fig. IF). Whether the spike response evoked from the thoracic cord was antidromic or
Fig. 1. Responses of a spinocerebellar neurone having a long descending axon collateral, recorded extracellularly (A-E) and intracellularly (F-I), and locations of such neurones (J). Bottom trace in A, B, D, H and I show cord surface potentials, and other traces are microeleetrode recordings. A : an antidromic spike evoked from the ipsilateral cerebellar. B : an antidromic response evoked from the ipsilateral cord at the lowest thoracic level. C: collision test at two different stimulus intervals with thoracic stimulation preceded by cerebellar. D: responses evoked by single stimulus to contralateral thoracic cord. E: collision test for combination of cerebellar and contralateral thoracic stimulation. F-I: intracellular recordings (positivity upward) from the same cell as shown in A-E. m-Spikes are evoked in an all-or-none fashion from ipsilateral cerebellum (F) and ipsilateral thoracic cord (G). H and i show 1PSPs (top traces) evoked from the median and ulnar nerve respectively, and juxtaextracellular responses (middle traces). J : location of neurones having a descending axon, sampled in one cat. See text for further details.
150 trans-synaptic was carefully determined particularly for extracellularly recorded cell,. according to the criteria as described above, including the blockage of spikes in a manner of collision when preceded by cerebellar stimulation. As the result, 18 out ot the 67 cells examined (27,,,) exhibited antidromic activation fl'om the lowest level of the thoracic cord, indicating that these neurones send descending axon collaterals down to the lumbar cord. The descending axons were located, at the Thia level, on the side ipsilateral to the cell soma in 17 of the 18 cells and on the contralateral side m the remaining cell, as,judged from the effective side for antidromic activation. As previously reported, the spinocerebella~ tibres of these cells ascend ipsilaterally to the cell soma at the Ca level s . The conduction velocities of the descending axon as estimated by the ratio of conduction distance versus latency of the antidromic spike were 16 93 m/sec (mean 54 m/sec). The corresponding values t~r antidromic responses from the cerebellum were 24 67 m/sec (mean 44 m/sec). When the conduction velocities of the descending and the ascending axon were compared in individual cells, no general tendency could be observed that either of the descending or ascending axons was faster-conducting than the other. It has earlier been shown that spinocerebellar neurones in the cervical enlargement consist of at least two major groups one located more dorsally and provided with potent excitatory inputs from the primary afferents and the other located more ventrally and predominantly inhibited from the periphery, particularly by flexor reflex afferents (FRA) s. Among the 67 neurones examined in the present study, 25 cells were classified as belonging to the former and 42 cells to the latter group. Characteristically, all the 18 cells antidromically invaded from the low thoracic cord were shown to belong to the ventral group and none to the dorsal group. Thus. 43 o~; of the ventral, peripherally inhibited group of cells were revealed to possess descending axon collaterals. The typical response of predominant, polysynaptic IPSPs from the forelimb FRA are shown in Fig. I H and I. Fig. I J illustrates locations of 4 neurones having descending axon collaterals and sampled in the same preparation. Note that they were all ventrally located (cf. Fig. I of Hirai et al.:'). The above results indicate that a considerable number of spinocerebellar neurones in the cervical cord issue long propriospinal axons descending beyond the lowest thoracic segment and therefore should possess double functions, i.e., propriospinal and spinocerebellar. Presumably, the descending axons are connected with neuronal circuits involved in the movement of the hindlimb and thus subserve forelimb-hindlim b coordination. The cerebellum, on the other hand, can receive via the ascending axons the same information as is conveyed propriospinally down to the lumbar cord. It may be of functional importance that such descending axon-collaterals are found only in those spinocerebellar neurones located more ventrally and predominantly inhibited by flexor reflex afferents (FRA) a. The organization of their synaptic inputs in many respects resembles that of ventral spinocerebellar tract neurones especially of spinal border cell origin a,s,7 Hence two spinocerebellar tracts may be functionally equivalent. If so, it would be warranted to investigate whether the VSCT axons reciprocally send off collateral branches terminating in the cervical cord.
151 1 Burke, R., Lundberg, A. and Weight, F., Spinal border cell origin of the ventral spinocerebellar tract, Exp. Brain Res., 12 (1971) 283-297. 2 Burton, J. E., Bloedel, J. R., and Gregory, R. S., Electrophysiological evidence for an input to lateral reticular nucleus from collaterals of dorsal spinocerebellar and cuneocerebellar fibers, J. Neurophysigl., 34 (1971) 885-897. 3 Cajal, S. Ramon y, Histologie du systdme nerveux de l'homme et des vert~brds, Tome I, chap. XII1, XIV, Maloine, Paris (1909). 4 Johansson, H. and Silfvenius, H., Axon-collateral activation by dorsal spinocerebellar tract fibres of group I relay cells of nucleus Z in the cat medullar oblongata, J. Physiol. (Lond.), 265 (1977) 341-369. 5 Hirai, N., Hongo, T., Kudo, N. and Yamaguchi, T., Heterogeneous composition of the spinocerebellar tract originating from the cervical enlargement in the cat, Brain Research, 190 (1976) 387-391. 6 Lorente de N6, R., l~tudes sur le cerveau post6rieur. 1II. Sur les connexions extra-c6r6belleuses des fascicule afferents au cerveau, et sur la fonction de cet organe, Tray. Lab. Rech. Biol. Univ. Madr., 22 (1924) 51-65. 7 Lundberg, A. and Weight, F., Functional organization of connexions to the ventral spinocerebellar tract, Exp. Brain Res., 12 (1971) 295-316. 8 Mannen, H., Reconstruction of axonal trajectory of individual neurons in the spinal cord using Golgi-stained serial sections, J. comp. Neurol., 159 (1975) 357-374. 9 Matsushita, M. and lkeda, M., Spinal projections to the cerebellar nuclei in the cat, Exp. Brain Res., 10 (1970) 501-511. 10 R6thelyi, M., The Golgi architecture of Clarke's column, Acta morph. Acad. Sci. hung., 16 (1968) 311-330.