- Email: [email protected]
Projections of reticular neurones to dorsal regions of the spinal cord in the cat
Neuroscience Letters, 2 (1976) 7--11
7
© Elsevier/North-Holland, Amsterdam -- Printed in The Netherlands
PROJECTIONS OF R E T I C U L A R N E U R O N E S TO D O R S A L REGIONS OF THE SPINAL CORD IN THE CAT
M.J. PEACOCK* and J.H. WOLSTENCROFT
Department of Physiology, Medical School, University of Birmingham, Birmingham B15 2TJ (Great Britain) (Received November 9th, 1975) (Accepted January 6th, 1976)
SUMMARY
Reticulospinal neurones in the cat were identified b y extracellular recording and antidromic stimulation of their axons in the cord. Approximately 34% of reticulospinal neurones in the medulla, and 28% in the pons, were found to project to dorsal regions of the cord, between T9 and L2. Most of these neurones had one branch situated dorsally and another in the ventral or ventrolateral funiculus. Some branches travelled for short distances in the dorsal columns. Microstimulation techniques demonstrated the presence of branches of reticulospinal fibres in the dorsal horn. The results may provide an anatomical basis for the widespread effects of stimulation of the reticular formation on afferent transmission in the spinal cord.
A projection of reticulospinal fibres to dorsal regions of the spinal cord could provide an anatomical basis for some of the effects, such as depression of afferent transmission, which occur following stimulation of the reticular formation. Anatomical studies have given equivocal evidence in support of such a dorsal projection. From the results of experiments in which degeneration was studied in the cord after lesions in the brain stem, Nyberg-Hansen [ 15,16 ] concluded that reticulospinal fibres descend in the ventral and lateral white matter of the cord and project no more dorsally than lamina VII. Petras [20], however, using a similar technique, described a more widespread distribution of descending fibres in the white matter and a projection to the basal region of the dorsal horn. Electrophysiological studies [1,19] have also shown a more widespread distribution of reticulospinal fibres than that proposed b y Nyberg-Hansen [15]. In the present experiments dorsal projections have been investigated by the technique of antidromic stimulation in the spinal cord with extracellular recording from medial reticular neurones in the pons and medulla. ~Present address: Glaxo Research Ltd., Greenford, Middlesex, Great Britain.
Cats weighing 2.0--3.5 kg were decerebrated under halothane anaesthesia which was discontinued before recording began. The cerebellum was removed to expose the floor of the IVth ventricle and a dorsal l a m i n e c t o m y was perf o r med to expose the spinal cord at lower thoracic and upper lumbar and occasionally also at cervical levels. The cord was sectioned transversely in the upper lumbar or lower thoracic region and impaled longitudinally with an array of ten stainless steel stimulating electrodes equally spaced {about I mm apart) around the outer part of the cut section of the cord. Bipolar stimulation between adjacent electrodes was usually used, with 0.1 msec pulses. To facilitate localization of an axon the pairings were changed, or one electrode was paired with an indifferent electrode on the cat's back. Other exposed regions were stimulated using a glass micropipette containing 2% Pontamine Blue in 0.5 M sodium acetate [10] which could also be used to mark sites from which antidromic responses were obtained. Such sites were later examined histologically, sections being stained with neutral red. In some experiments the dorsal columns were dissected out for a length of several centimetres and placed on silver wire stimulating electrodes. Glass micropipettes filled with 4 M NaC1, with tip resistances of 1--4 M~2, were inserted ~erpendicular to the floor of the IVth ventricle between 2 and 12 mm rostral to the obex and between 0.4 and 2.5 mm from the midline. Conventional techniques were used to record f r om single units. Antidromic responses following stimulation in the spinal cord were recognized using the criteria of constant latency, high following rate and collision with a spontaneous or evoked action potential [ 8,24], when these occurred. A search was made for neurones in the medial reticular form at i on which gave antidromic responses to stimulation in the spinal cord, and a total of 450 reticulospinal (RS) neurones were studied. A p p r o x i m a t e l y 34% of RS neurones in the medulla (22% ipsilateral and 12% contralateral) and 28% of RS neurones in the pons (15% ipsilateral and 13% contralateral) could be activated antidromically by electrodes in the dorsal or dorsolateral cord at T9--L2. Most of these were also activated by the ventrally or ventrolaterally placed cord electrodes (i.e., low threshold responses were obtained b o t h from dorsally or dorsolaterally placed and f r om ventrally or ventrolaterally placed electrodes, but n o t f r o m electrodes in between these positions). Thus the neurones activated from b o t h dorsal and ventral electrodes had axons which divided into two divergent branches. This was confirmed by demonstrating collision between the antidromic spikes obtained from the t w o electrodes. Calculations based on the time during which collision occurred suggested t hat branches originated n o t more than 5 cm rostral t o the point of stimulation at T9--L2. Branches o f reticulospinal fibres could also be found in the dorsal columns. These were most clearly dem ons t r a t e d by stimulating a raised, dissected strip of dorsal column a b o u t 2--3 cm long. The strips were sectioned, remaining attached either rostrally or caudally. Antidromic responses could be obtained, surprisingly, from a strip of dorsal column which was attached to the cord at its caudal end, as well as f r om one attached rostrally. This indicates that a
branch from a RS fibre descending in another part of the white matter may travel rostrally or caudally through the dorsal columns. RS fibres in the dorsal columns do n o t travel any great distance in this way since section of the dorsal columns 3 cm rostral to the point of stimulation did n o t abolish antidromic responses to dorsal column stimulation. In an a t t e m p t to localize some of the dorsal branches more precisely, a microstimulation technique was used. A glass micropipette was used for stimulation and in one example it was possible to trace the course taken by a dorsal branch of a RS fibre descending ventrolaterally (Fig. 1). This RS neurone was excited antidromically from a ventrally placed steel macroelectrode at L2, and stimulation at T10 through a microelectrode, over a rostro-caudal distance of 0.4 mm, produced antidromic excitation from the points illustrated in Fig. 1. The current required to stimulate the fibre was
i Fig. 1. Diagram o f transverse s e c t i o n o f spinal c o r d at T10. T h e filled circles i n d i c a t e p o i n t s w h e r e a s t i m u l a t i n g m i c r o e l e c t r o d e gave a n t i d r o m i c responses, a t c u r r e n t s of less t h a n 16 # A , in a n e u r o n e in t h e m e d i a l r e t i c u l a r f o r m a t i o n . This n e u r o n e was also e x c i t e d a n t i d r o m i c a l l y b y a n e l e c t r o d e placed in t h e v e n t r a l w h i t e m a t t e r a t L2.
less than 8 #A at all points shown in Fig. 1 except for the three points farthest from the main axon where it was 10, 12 and 15 pA respectively. The final position in the base of the dorsal horn contralateral to the main axon was marked with Pontamine Sky Blue. It seems likely that a terminal branch was being stimulated since the extreme two points on the left gave antidromic latencies of 3.5 and 4.75 msec, while the latencies from the other points were between 3.0 and 3.45 msec, increasing gradually with distance from the ventrolateral white matter. Thus this branch travelled through the ipsilateral dorsal horn, crossed the dorsal Columns, and entered the contralateral dorsal horn and appeared to terminate in its basal region. In other experiments microstimulation yielded antidromic responses from sites in the dorsal horn (four neurones), dorsal columns (four neurones) or dorsolateral white matter (four neurones) but the terminal position was not identified. These responses were obtained from penetrations between T9 and L1 except for one at C2 which gave an antidromic response from a position in the dorsal columns, close to the border with the dorsal horn.
10
RS neurones with dorsal projections were found more frequently in dorsal regions of the reticular formation, in nucleus reticularis gigantocellularis, pontis caudalis and pontis oralis. The conduction velocity of dorsally projecting fibres, measured from the latency following stimulation of the axon in the white matter, ranged from 20 to 135 m/sec, mean 63.7 m/sec. These experiments suggest that some RS neurones project to dorsal regions of the spinal cord. These projections arise from branches of RS fibres descending ventrally or ventrolaterally, although the possibility of dorsolaterally descending fibres has not been excluded. Branches may travel rostrally or caudally in the dorsal columns for short distances, and in one example evidence was obtained for terminal branching in the basal part of the contralateral dorsal horn. The projection of reticulospinal neurones to the dorsal spinal cord is obviously well-placed to influence afferent transmission, and such effects have been widely reported in the literature. Stimulation of the brain stem reticular formation has been shown to produce dorsal root potentials and primary afferent depolarization [3,13,17], to depress transmission from l b and flexor reflex afferents (very high threshold muscle and cutaneous afferents) to motoneurones and interneurones [6,11], to produce IPSPs in some interneurones in dorsal and intermediate regions [ 7], and to depress transmission in ascending pathways including those activated by flexor reflex afferents [6,9,23]. These effects are presumably related to the suppression of reflex flexion [ 5,12,21,22] and of the static stretch reflex [ 2] and other reflexes [4] which occur in the decerebrate animal. Dorsal fibres were not observed by Nyberg-Hansen [15] in his studies of degenerating fibres in the cord after brain stem lesions. However, it is possible that his lesions did not destroy many of the dorsally projecting neurones. Our results demonstrate that many of the reticular neurones which project to dorsal regions of the cord are distributed in the dorsal part of the reticular formation, but the lesions made by Nyberg-Hansen [15] appear to have been placed mainly ventral and lateral to these regions. However, the anatomical studies of Papez [18] and Petras [20], and the observations of Niemer and Magoun [ 14], concerning the effects of dorsal and ventral cord lesions, agree with our results in providing evidence for fibres in the dorsal quadrants and the dorsal horn. Thus the present results help to provide an anatomical basis for the widespread effects of reticulospinal neurones on afferent transmission in the spinal cord. REFERENCES 1 Avanzino, G.L., Carta, M., Peacock, M., and Wolstencroft, J.H., Reticulospinal pathways, J. Physiol. (Lond.), 224 (1972) 88P. 2 Burke, D., Knowles, L., Andrews, C., and Ashby, P., Spasticity, decerebrate rigidity and the clasp-knife phenomenon: an experimental study in the cat, Brain, 95 (1972) 31--48.
11
3 Carpenter, D., Engberg, I., and Lundberg, A., Primary afferent depolarization evoked from the brain stem and the cerebellum, Arch. ital. Biol., 104 (1966) 73--85. 4 Downman, C.B.B., and Hussain, A., Spinal tracts and supraspinal centres influencing visceromotor and allied reflexes in cats, J. Physiol. (Lond.), 141 (1958) 489--499. 5 Eccles, R.M., and Lundberg, A., Supraspinal control of interneurones mediating spinal reflexes, J. Physiol. (Lond.), 147 (1959) 565--584. 6 Engberg, I., Lundberg, A., and Ryall, R.W., Reticulospinal inhibition of transmission in reflex pathways, J. Physiol. (Lond.), 194 (1968) 201--203. 7 Engberg, I., Lundberg, A., and Ryall, R.W., Reticuiospinal inhibition of interneurones, J. Physiol. (Lond.), 194 (1968) 225--236. 8 Fox, J.E., and Wolstencroft, J.H., Autocancellation, a technique for identifying antidromic neurone potentials, J. Physiol. (Lond.), 206 (1969) 7P. 9 Hagharth, K.E., and Kerr, D.I.B., Central influences on spinal afferent conduction, J. Neurophysioi., 17 (1954) 295--307. 10 Hellon, R.F., The marking of electrode tip positions in nervous tissue, J. Physiol. (Lond.), 214 (1971) 12P. 11 Jankowska, E., Lund, S., Lundberg, A., and Pompeiano, O., Inhibitory effects evoked through ventral reticulospinal pathways, Arch. ital. Biol., 106 (1968) 124--140. 12 Liddell, E.G.T., Mathes, K., Oldberg, E., and Ruch, T.C., Reflex release of flexor muscles by spinal section, Brain, 55 (1932) 239--246. 13 Lundberg, A., and Vyklick~, L., Inhibition of transmission to primary afferents by electrical stimulation of the brain stem, Arch. ital. Biol., 104 (1966) 86--97. 14 Niemer, W.T., and Magoun, H.W., Reticulospinal tracts influencing motor activity, J. comp. Neurol., 87 (1947) 367--379. 15 Nyberg-Hansen, R., Sites and mode of termination of reticulospinal fibres in the cat. An experimental study with silver impregnation methods, J. comp. Neurol., 124 (1965) 71--100. 16 Nyberg-Hansen, R., Functional organization of descending supraspinal fibre systems to the spinal cord. Anatomical observation and physiological correlations, Ergebn. Anat. Entwickl.-Gesch., 39 (1966) (2) 1--48. 17 Owen, M.P., and Hodge, C.J., Positive dorsal root potentials evoked by stimulation of the brain stem reticular formation, Brain Res., 54 (1973) 305--308. 18 Papez, J.W., Reticulo-spinal tracts in the cat. Marchi method, J. comp. Neurol., 41 (1926) 365--399. 19 Peacock, M.J., The Origin, Course and Termination of Reticulospinal Fibres, Ph.D. Thesis, University of Birmingham, 1974. 20 Petras, J.M., Cortical, tectal and tegmental fiber connections in the spinal cord of the cat, Brain Res., 6 (1967) 275--324. 21 Sherrington, C.S., Flexion reflex of the limb, crossed extension reflex, and reflex stepping and standing, J. Physiol. (Lond.), 40 (1910) 28--121. 22 Sherrington, C.S., and Sowton, S.C.M., Observations on reflexes to single shock stimulation, J. Physiol. (Lond.), 49 (1915) 331--348. 23 Taub, A., Local, segmental and supraspinal interaction with a dorsolateral spinal cutaneous afferent system, Exp. Neurol., 10 (1964) 357--374. 24 Wolstencr~ft, J.H., Reticulospinal neurones, J. Physiol. (Lond.), 174 (1964) 91--108.
7
© Elsevier/North-Holland, Amsterdam -- Printed in The Netherlands
PROJECTIONS OF R E T I C U L A R N E U R O N E S TO D O R S A L REGIONS OF THE SPINAL CORD IN THE CAT
M.J. PEACOCK* and J.H. WOLSTENCROFT
Department of Physiology, Medical School, University of Birmingham, Birmingham B15 2TJ (Great Britain) (Received November 9th, 1975) (Accepted January 6th, 1976)
SUMMARY
Reticulospinal neurones in the cat were identified b y extracellular recording and antidromic stimulation of their axons in the cord. Approximately 34% of reticulospinal neurones in the medulla, and 28% in the pons, were found to project to dorsal regions of the cord, between T9 and L2. Most of these neurones had one branch situated dorsally and another in the ventral or ventrolateral funiculus. Some branches travelled for short distances in the dorsal columns. Microstimulation techniques demonstrated the presence of branches of reticulospinal fibres in the dorsal horn. The results may provide an anatomical basis for the widespread effects of stimulation of the reticular formation on afferent transmission in the spinal cord.
A projection of reticulospinal fibres to dorsal regions of the spinal cord could provide an anatomical basis for some of the effects, such as depression of afferent transmission, which occur following stimulation of the reticular formation. Anatomical studies have given equivocal evidence in support of such a dorsal projection. From the results of experiments in which degeneration was studied in the cord after lesions in the brain stem, Nyberg-Hansen [ 15,16 ] concluded that reticulospinal fibres descend in the ventral and lateral white matter of the cord and project no more dorsally than lamina VII. Petras [20], however, using a similar technique, described a more widespread distribution of descending fibres in the white matter and a projection to the basal region of the dorsal horn. Electrophysiological studies [1,19] have also shown a more widespread distribution of reticulospinal fibres than that proposed b y Nyberg-Hansen [15]. In the present experiments dorsal projections have been investigated by the technique of antidromic stimulation in the spinal cord with extracellular recording from medial reticular neurones in the pons and medulla. ~Present address: Glaxo Research Ltd., Greenford, Middlesex, Great Britain.
Cats weighing 2.0--3.5 kg were decerebrated under halothane anaesthesia which was discontinued before recording began. The cerebellum was removed to expose the floor of the IVth ventricle and a dorsal l a m i n e c t o m y was perf o r med to expose the spinal cord at lower thoracic and upper lumbar and occasionally also at cervical levels. The cord was sectioned transversely in the upper lumbar or lower thoracic region and impaled longitudinally with an array of ten stainless steel stimulating electrodes equally spaced {about I mm apart) around the outer part of the cut section of the cord. Bipolar stimulation between adjacent electrodes was usually used, with 0.1 msec pulses. To facilitate localization of an axon the pairings were changed, or one electrode was paired with an indifferent electrode on the cat's back. Other exposed regions were stimulated using a glass micropipette containing 2% Pontamine Blue in 0.5 M sodium acetate [10] which could also be used to mark sites from which antidromic responses were obtained. Such sites were later examined histologically, sections being stained with neutral red. In some experiments the dorsal columns were dissected out for a length of several centimetres and placed on silver wire stimulating electrodes. Glass micropipettes filled with 4 M NaC1, with tip resistances of 1--4 M~2, were inserted ~erpendicular to the floor of the IVth ventricle between 2 and 12 mm rostral to the obex and between 0.4 and 2.5 mm from the midline. Conventional techniques were used to record f r om single units. Antidromic responses following stimulation in the spinal cord were recognized using the criteria of constant latency, high following rate and collision with a spontaneous or evoked action potential [ 8,24], when these occurred. A search was made for neurones in the medial reticular form at i on which gave antidromic responses to stimulation in the spinal cord, and a total of 450 reticulospinal (RS) neurones were studied. A p p r o x i m a t e l y 34% of RS neurones in the medulla (22% ipsilateral and 12% contralateral) and 28% of RS neurones in the pons (15% ipsilateral and 13% contralateral) could be activated antidromically by electrodes in the dorsal or dorsolateral cord at T9--L2. Most of these were also activated by the ventrally or ventrolaterally placed cord electrodes (i.e., low threshold responses were obtained b o t h from dorsally or dorsolaterally placed and f r om ventrally or ventrolaterally placed electrodes, but n o t f r o m electrodes in between these positions). Thus the neurones activated from b o t h dorsal and ventral electrodes had axons which divided into two divergent branches. This was confirmed by demonstrating collision between the antidromic spikes obtained from the t w o electrodes. Calculations based on the time during which collision occurred suggested t hat branches originated n o t more than 5 cm rostral t o the point of stimulation at T9--L2. Branches o f reticulospinal fibres could also be found in the dorsal columns. These were most clearly dem ons t r a t e d by stimulating a raised, dissected strip of dorsal column a b o u t 2--3 cm long. The strips were sectioned, remaining attached either rostrally or caudally. Antidromic responses could be obtained, surprisingly, from a strip of dorsal column which was attached to the cord at its caudal end, as well as f r om one attached rostrally. This indicates that a
branch from a RS fibre descending in another part of the white matter may travel rostrally or caudally through the dorsal columns. RS fibres in the dorsal columns do n o t travel any great distance in this way since section of the dorsal columns 3 cm rostral to the point of stimulation did n o t abolish antidromic responses to dorsal column stimulation. In an a t t e m p t to localize some of the dorsal branches more precisely, a microstimulation technique was used. A glass micropipette was used for stimulation and in one example it was possible to trace the course taken by a dorsal branch of a RS fibre descending ventrolaterally (Fig. 1). This RS neurone was excited antidromically from a ventrally placed steel macroelectrode at L2, and stimulation at T10 through a microelectrode, over a rostro-caudal distance of 0.4 mm, produced antidromic excitation from the points illustrated in Fig. 1. The current required to stimulate the fibre was
i Fig. 1. Diagram o f transverse s e c t i o n o f spinal c o r d at T10. T h e filled circles i n d i c a t e p o i n t s w h e r e a s t i m u l a t i n g m i c r o e l e c t r o d e gave a n t i d r o m i c responses, a t c u r r e n t s of less t h a n 16 # A , in a n e u r o n e in t h e m e d i a l r e t i c u l a r f o r m a t i o n . This n e u r o n e was also e x c i t e d a n t i d r o m i c a l l y b y a n e l e c t r o d e placed in t h e v e n t r a l w h i t e m a t t e r a t L2.
less than 8 #A at all points shown in Fig. 1 except for the three points farthest from the main axon where it was 10, 12 and 15 pA respectively. The final position in the base of the dorsal horn contralateral to the main axon was marked with Pontamine Sky Blue. It seems likely that a terminal branch was being stimulated since the extreme two points on the left gave antidromic latencies of 3.5 and 4.75 msec, while the latencies from the other points were between 3.0 and 3.45 msec, increasing gradually with distance from the ventrolateral white matter. Thus this branch travelled through the ipsilateral dorsal horn, crossed the dorsal Columns, and entered the contralateral dorsal horn and appeared to terminate in its basal region. In other experiments microstimulation yielded antidromic responses from sites in the dorsal horn (four neurones), dorsal columns (four neurones) or dorsolateral white matter (four neurones) but the terminal position was not identified. These responses were obtained from penetrations between T9 and L1 except for one at C2 which gave an antidromic response from a position in the dorsal columns, close to the border with the dorsal horn.
10
RS neurones with dorsal projections were found more frequently in dorsal regions of the reticular formation, in nucleus reticularis gigantocellularis, pontis caudalis and pontis oralis. The conduction velocity of dorsally projecting fibres, measured from the latency following stimulation of the axon in the white matter, ranged from 20 to 135 m/sec, mean 63.7 m/sec. These experiments suggest that some RS neurones project to dorsal regions of the spinal cord. These projections arise from branches of RS fibres descending ventrally or ventrolaterally, although the possibility of dorsolaterally descending fibres has not been excluded. Branches may travel rostrally or caudally in the dorsal columns for short distances, and in one example evidence was obtained for terminal branching in the basal part of the contralateral dorsal horn. The projection of reticulospinal neurones to the dorsal spinal cord is obviously well-placed to influence afferent transmission, and such effects have been widely reported in the literature. Stimulation of the brain stem reticular formation has been shown to produce dorsal root potentials and primary afferent depolarization [3,13,17], to depress transmission from l b and flexor reflex afferents (very high threshold muscle and cutaneous afferents) to motoneurones and interneurones [6,11], to produce IPSPs in some interneurones in dorsal and intermediate regions [ 7], and to depress transmission in ascending pathways including those activated by flexor reflex afferents [6,9,23]. These effects are presumably related to the suppression of reflex flexion [ 5,12,21,22] and of the static stretch reflex [ 2] and other reflexes [4] which occur in the decerebrate animal. Dorsal fibres were not observed by Nyberg-Hansen [15] in his studies of degenerating fibres in the cord after brain stem lesions. However, it is possible that his lesions did not destroy many of the dorsally projecting neurones. Our results demonstrate that many of the reticular neurones which project to dorsal regions of the cord are distributed in the dorsal part of the reticular formation, but the lesions made by Nyberg-Hansen [15] appear to have been placed mainly ventral and lateral to these regions. However, the anatomical studies of Papez [18] and Petras [20], and the observations of Niemer and Magoun [ 14], concerning the effects of dorsal and ventral cord lesions, agree with our results in providing evidence for fibres in the dorsal quadrants and the dorsal horn. Thus the present results help to provide an anatomical basis for the widespread effects of reticulospinal neurones on afferent transmission in the spinal cord. REFERENCES 1 Avanzino, G.L., Carta, M., Peacock, M., and Wolstencroft, J.H., Reticulospinal pathways, J. Physiol. (Lond.), 224 (1972) 88P. 2 Burke, D., Knowles, L., Andrews, C., and Ashby, P., Spasticity, decerebrate rigidity and the clasp-knife phenomenon: an experimental study in the cat, Brain, 95 (1972) 31--48.
11
3 Carpenter, D., Engberg, I., and Lundberg, A., Primary afferent depolarization evoked from the brain stem and the cerebellum, Arch. ital. Biol., 104 (1966) 73--85. 4 Downman, C.B.B., and Hussain, A., Spinal tracts and supraspinal centres influencing visceromotor and allied reflexes in cats, J. Physiol. (Lond.), 141 (1958) 489--499. 5 Eccles, R.M., and Lundberg, A., Supraspinal control of interneurones mediating spinal reflexes, J. Physiol. (Lond.), 147 (1959) 565--584. 6 Engberg, I., Lundberg, A., and Ryall, R.W., Reticulospinal inhibition of transmission in reflex pathways, J. Physiol. (Lond.), 194 (1968) 201--203. 7 Engberg, I., Lundberg, A., and Ryall, R.W., Reticuiospinal inhibition of interneurones, J. Physiol. (Lond.), 194 (1968) 225--236. 8 Fox, J.E., and Wolstencroft, J.H., Autocancellation, a technique for identifying antidromic neurone potentials, J. Physiol. (Lond.), 206 (1969) 7P. 9 Hagharth, K.E., and Kerr, D.I.B., Central influences on spinal afferent conduction, J. Neurophysioi., 17 (1954) 295--307. 10 Hellon, R.F., The marking of electrode tip positions in nervous tissue, J. Physiol. (Lond.), 214 (1971) 12P. 11 Jankowska, E., Lund, S., Lundberg, A., and Pompeiano, O., Inhibitory effects evoked through ventral reticulospinal pathways, Arch. ital. Biol., 106 (1968) 124--140. 12 Liddell, E.G.T., Mathes, K., Oldberg, E., and Ruch, T.C., Reflex release of flexor muscles by spinal section, Brain, 55 (1932) 239--246. 13 Lundberg, A., and Vyklick~, L., Inhibition of transmission to primary afferents by electrical stimulation of the brain stem, Arch. ital. Biol., 104 (1966) 86--97. 14 Niemer, W.T., and Magoun, H.W., Reticulospinal tracts influencing motor activity, J. comp. Neurol., 87 (1947) 367--379. 15 Nyberg-Hansen, R., Sites and mode of termination of reticulospinal fibres in the cat. An experimental study with silver impregnation methods, J. comp. Neurol., 124 (1965) 71--100. 16 Nyberg-Hansen, R., Functional organization of descending supraspinal fibre systems to the spinal cord. Anatomical observation and physiological correlations, Ergebn. Anat. Entwickl.-Gesch., 39 (1966) (2) 1--48. 17 Owen, M.P., and Hodge, C.J., Positive dorsal root potentials evoked by stimulation of the brain stem reticular formation, Brain Res., 54 (1973) 305--308. 18 Papez, J.W., Reticulo-spinal tracts in the cat. Marchi method, J. comp. Neurol., 41 (1926) 365--399. 19 Peacock, M.J., The Origin, Course and Termination of Reticulospinal Fibres, Ph.D. Thesis, University of Birmingham, 1974. 20 Petras, J.M., Cortical, tectal and tegmental fiber connections in the spinal cord of the cat, Brain Res., 6 (1967) 275--324. 21 Sherrington, C.S., Flexion reflex of the limb, crossed extension reflex, and reflex stepping and standing, J. Physiol. (Lond.), 40 (1910) 28--121. 22 Sherrington, C.S., and Sowton, S.C.M., Observations on reflexes to single shock stimulation, J. Physiol. (Lond.), 49 (1915) 331--348. 23 Taub, A., Local, segmental and supraspinal interaction with a dorsolateral spinal cutaneous afferent system, Exp. Neurol., 10 (1964) 357--374. 24 Wolstencr~ft, J.H., Reticulospinal neurones, J. Physiol. (Lond.), 174 (1964) 91--108.