Brain Research, 124 (1977) 415-426 ~5 Elsevier/North-Holland Biomedical Press, Amsterdam
41 5 Printed in The Netherlands
A C O M P A R I S O N OF T H E R E F L E X O R G A N I Z A T I O N OF T H O R A C I C A N D L U M B A R S E G M E N T S IN T H E F R O G S P I N A L C O R D
RICHARD C. CARLSEN* and LORNE M. MENDELL Department of Physiology and Pharmacology, Duke University Medical Center, Durham, N.C. 27710 (U.S.A.)
(Accepted July 28th, 1976)
SUMMARY Both lumbar dorsal root (DR) primary afferents and descending fibers in the lateral column (LC) make monosynaptic connections with ipsilateral lumbar motoneurons in the frog spinal cord. We have determined that LC fibers also make monosynaptic connections with thoracic motoneurons, while thoracic primary afferents do not. The central reflex time (CRT) for the lumbar D R - V R pathway was 2.5 ± 0.3 msec, but the C R T for the thoracic D R - V R pathway was 8.5 ± 2.6 msec. By using a conditioning-test paradigm we have been able to determine that the earliest sign of segmental synaptic transmission in thoracic motoneurons occurs only after a delay of 6.1 ± 0.9 msec. These results correlate very well with the different morphological characteristics of lumbar and thoracic segments. We have also investigated the central organization of lumbar and thoracic segments and found that the segmental polysynaptic input to motoneurons is more diffuse in thoracic than in lumbar segments. The intersegmental reflexes between thoracic and lumbar segments provide additional evidence for a more diffuse organization in thoracic segments.
INTRODUCTION The anuran spinal cord is composed of 10 spinal segments divided into 3 zones: the 3 brachial segments which are responsible for forelimb movement, the 4 lumbar segments which are responsible for hindlimb movement, and the 3 thoracic segments which provide the nerve supply for the trunk. Most neurophysiological investigations of the frog spinal cord have been concerned with the limb-moving segments 4-10, while
* Present address: Department of Human Physiology, University of California School of Medicine, Davis, Calif. 95616, U.S.A.
416
the thoracic segments have been largely neglected. However, in the courw ,)i" our investigation into the role peripheral structures play in influencing the deveh,pmeni hi' intraspinal synaptic connections ~*,~:', we have tk)und it necessary to determine the synaptic organization of normal thoracic spinal segments. The data reported here were obtained from our preliminary investigations using the isolated Rana pipicH.~ spinal cord. Additional evidence flom both the isolated and in situ Xenopu,v laevi, ~pinat cord suggests that similar synaptic relationships exist in this species as well. METHODS Experiments were performed using the isolated frog spinal cord preparation developed by Brookhart et al.:L Adult Rana pipic;ts of' either sex were obtained in the spring and summer from J. M. Hazen and Co. (Alburg, Vt.). The animals were housed in a semiaquatic environment ~t, maintained at room temperature (23 ! I C) and fed live crickets twice weekly. The isolated spinal cord was prepared essentially as specified by Brookhart et al. '~. The animals were placed on ice for 30 rain arid then pithed. The dorsal body wall. containing the vertebral colunm and urostyle, was removed and placed in a dissection chamber containing cold (10 'C) Ringer's solution 6,7, where a laminectom~, was performed. The spinal cord, with its roots intact, was removed from the ~ertebral canal and transferred to the recording chamber. The spinal cord was fixed to the surface ol' the chamber using minutien pins (Carolina Biological), and continuously perfused with a stream of cold Ringer's solution flowing at a rate o1'5-6 ml/min. ,'% thermistor placed adjacent to the spinal cord continuously monitored the temperature of the solution, which was maintained at 15 _ 1 C. The specified dorsal and ventral roots were placed on bipolar platinum hook electrodes, lifted above the surface of the bath, and covered with a layer of petroleum jelly thinned with mineral oil. A bipolar LC stimulating electrode, constructed of steel 000 insect pins insulated except at the tips, was placed on the lateral surface of the spinal cord between segments 3 and 4. Stimuli were rectangular pulses of 0.05-0.10 msec in duration. The final position of the stimulating electrode was adjusted to produce a short-latency, synchronous discharge from the ventral roots. This position was invariably over the lateral funiculus of the spinal cord" positioning the electrode over the dorsal funiculus, in close proximity to the dorsal column, produced a more asynchronous, longer-latency VR discharge Ventral root potentials were anaplified and recorded in the conventional m a n n e r Spinal segments have been numbered in accord with the nomenclature
41 5 Printed in The Netherlands
A C O M P A R I S O N OF T H E R E F L E X O R G A N I Z A T I O N OF T H O R A C I C A N D L U M B A R S E G M E N T S IN T H E F R O G S P I N A L C O R D
RICHARD C. CARLSEN* and LORNE M. MENDELL Department of Physiology and Pharmacology, Duke University Medical Center, Durham, N.C. 27710 (U.S.A.)
(Accepted July 28th, 1976)
SUMMARY Both lumbar dorsal root (DR) primary afferents and descending fibers in the lateral column (LC) make monosynaptic connections with ipsilateral lumbar motoneurons in the frog spinal cord. We have determined that LC fibers also make monosynaptic connections with thoracic motoneurons, while thoracic primary afferents do not. The central reflex time (CRT) for the lumbar D R - V R pathway was 2.5 ± 0.3 msec, but the C R T for the thoracic D R - V R pathway was 8.5 ± 2.6 msec. By using a conditioning-test paradigm we have been able to determine that the earliest sign of segmental synaptic transmission in thoracic motoneurons occurs only after a delay of 6.1 ± 0.9 msec. These results correlate very well with the different morphological characteristics of lumbar and thoracic segments. We have also investigated the central organization of lumbar and thoracic segments and found that the segmental polysynaptic input to motoneurons is more diffuse in thoracic than in lumbar segments. The intersegmental reflexes between thoracic and lumbar segments provide additional evidence for a more diffuse organization in thoracic segments.
INTRODUCTION The anuran spinal cord is composed of 10 spinal segments divided into 3 zones: the 3 brachial segments which are responsible for forelimb movement, the 4 lumbar segments which are responsible for hindlimb movement, and the 3 thoracic segments which provide the nerve supply for the trunk. Most neurophysiological investigations of the frog spinal cord have been concerned with the limb-moving segments 4-10, while
* Present address: Department of Human Physiology, University of California School of Medicine, Davis, Calif. 95616, U.S.A.
416
the thoracic segments have been largely neglected. However, in the courw ,)i" our investigation into the role peripheral structures play in influencing the deveh,pmeni hi' intraspinal synaptic connections ~*,~:', we have tk)und it necessary to determine the synaptic organization of normal thoracic spinal segments. The data reported here were obtained from our preliminary investigations using the isolated Rana pipicH.~ spinal cord. Additional evidence flom both the isolated and in situ Xenopu,v laevi, ~pinat cord suggests that similar synaptic relationships exist in this species as well. METHODS Experiments were performed using the isolated frog spinal cord preparation developed by Brookhart et al.:L Adult Rana pipic;ts of' either sex were obtained in the spring and summer from J. M. Hazen and Co. (Alburg, Vt.). The animals were housed in a semiaquatic environment ~t, maintained at room temperature (23 ! I C) and fed live crickets twice weekly. The isolated spinal cord was prepared essentially as specified by Brookhart et al. '~. The animals were placed on ice for 30 rain arid then pithed. The dorsal body wall. containing the vertebral colunm and urostyle, was removed and placed in a dissection chamber containing cold (10 'C) Ringer's solution 6,7, where a laminectom~, was performed. The spinal cord, with its roots intact, was removed from the ~ertebral canal and transferred to the recording chamber. The spinal cord was fixed to the surface ol' the chamber using minutien pins (Carolina Biological), and continuously perfused with a stream of cold Ringer's solution flowing at a rate o1'5-6 ml/min. ,'% thermistor placed adjacent to the spinal cord continuously monitored the temperature of the solution, which was maintained at 15 _ 1 C. The specified dorsal and ventral roots were placed on bipolar platinum hook electrodes, lifted above the surface of the bath, and covered with a layer of petroleum jelly thinned with mineral oil. A bipolar LC stimulating electrode, constructed of steel 000 insect pins insulated except at the tips, was placed on the lateral surface of the spinal cord between segments 3 and 4. Stimuli were rectangular pulses of 0.05-0.10 msec in duration. The final position of the stimulating electrode was adjusted to produce a short-latency, synchronous discharge from the ventral roots. This position was invariably over the lateral funiculus of the spinal cord" positioning the electrode over the dorsal funiculus, in close proximity to the dorsal column, produced a more asynchronous, longer-latency VR discharge Ventral root potentials were anaplified and recorded in the conventional m a n n e r Spinal segments have been numbered in accord with the nomenclature
