Journal of the Autonomic Nervous System, 8 (1983) 383-393
Splanchnic afferent input to the lateral reticular nucleus of the cat J. Perrin and J. Crousillat D~partement de Neurophysiologie v~g~tative, I.N.P.O1, C.N.R.S., 31 Chemin Joseph Aiguier, B.P. 71, 13277 Marseille Cedex 9, and D~partement de Bio~lectricit~, Universit~ de Provence, 13331 Marseille Cedex 3 (France)
(Received November 22nd, 1982) (Revised version receivedJanuary 20th, 1983) (Accepted June 16th, 1983)
K e y words: lateral reticular nucleus--splanchnic afferents--cat
Abstract The afferent input from splanchnic nerves to the lateral reticular nucleus (LRN) was studied in anesthetized cats. The activity of neurons of the parvi- and magnocellular regions of the nucleus was recorded by means of extracellular microelectrodes. The L R N neurons were stereotaxically located and identified by their response to antidromic stimulation of the cerebellar cortex. Activity of the L R N neurons studied was modified by electrical stimulation of the ipsi- and contralateral splanchnic nerves, and by mechanical stimulation of peritoneal receptors. Response latencies to stimulation of the splanchnic nerves were measured, and conduction velocities of peripheral fibers were determined; most of them were myelinated fibers with a small diameter (sub-groups of A- A78) and were connected with 'peritoneal movement mechanoreceptors'. Numerous convergences were observed. All of the tested neurons which responded to stimulation of the homolateral splanchnic nerve also responded to stimulation of the contralateral splanchnic and to diverse somatic stimulations. In 90% of the neurons tested, a splanchno-cortical convergence (sensory-somatic cortex) was observed. Two possible roles of splanchnic afferents that activate neurons of the L R N were considered: (1) they might, like other spinal inputs, influence L R N and cerebellar control of motor activity; and (2) they might also participate in cardiovascular regulations in which the L R N is involved.
0165-1838/83/$03.00 © 1983 Elsevier Science Publishers B.V.
The lateral reticular nucleus (LRN) is a relay to the cerebellum for peripheral and central information implicated in the control of motor activity. This nucleus receives cortical, fastigial and spinal afferents [6,7,10-14,20,21,37-39]. The LRN is considered to be a 'comparator' of command signals from the motor cortex and feedback signals from the spinal cord [7,21,37]. It is additionally a site of convergence for afferents from the hypothalamus and carotid sinus . These latter results suggest that this structure must also play a role in the control of cardiovascular regulations [421. The cerebellar cortex receives inputs from the visceral area which are conveyed by the splanchnic [2,3,24,29,30,37,40,41,43] and vagus nerves [18,25]. In a previous work we have shown that direct stimulation of splanchnic mechanoreceptors of the gastrointestinal tract and peritoneum modifies the cerebellar Purkinje cell discharge (vermis of lobule V and rostral part of lobule VI) . We have also shown that the inferior olive (I.O.) is a relay for these splanchnic afferents which project to the cerebellum via climbing fibers . It is now demonstrated here that the LRN is also a precerebellar relay for the splanchnic inputs via mossy fiber pathways. In this study the activity of LRN neurons was recorded by means of extracellular electrodes during splanchnic nerve stimulation and in some cases during direct stimulation of visceral mechanoreceptors. The type of peripheral fibers involved and the nature of the receptors were determined. As in the cerebellum, splanchno-somatic, splanchnocortical and viscero-visceral (splanchno-splanchnic and splanchno-vagal) convergences were observed. Our results suggest that the splanchnic afferents to the LRN might be involved in cerebellar control of somatic motor activity and in cardiovascular regulations involving the LRN. Preliminary communication of some of these results has been presented elsewhere .
Materials and Methods
The experiments were carried out on 17 adult cats of both sexes weighing 2-3.5 kg, anesthetized with a-chloralose (100 mg/kg, i.v.), after fluothane induction. Each animal was fixed in a stereotaxic apparatus, curarized (Flaxedil) and artificially ventilated. The rectal temperature was maintained between 37°C and 38°C by means of a regulated electric heating blanket. The left LRN was reached through a dorsal approach: the brainstem was exposed at the obex level. Recordings were performed by means of extracelhilar glass microelectrodes (KC1 3 M, 1 MI2) inserted stereotaxicaUy (Bergrnan's atlas). The angle of the electrode was 45 ° to the Horsley-Clarke plane (Fig. 1). These recordings were considered to be from cell bodies when the action potentials were diphasic. Stimulation of the left splanchnic nerve (0.5 ms, square-wave, 0.1-40 V) was achieved using two pairs of electrodes placed on the nerve at intrathoracic a n d / o r intra-abdominal levels. Each of these electrodes was constituted of 2 silver wires at
385 the bottom of a groove in a plexiglass holder. The nerve, covered with mineral oil in order to prevent drying and electrical diffusion, was left intact so as to allow testing by natural stimulation. The distance between the two pairs of electrodes was measured. The difference between the means of the response latencies obtained by threshold stimulation of one and then the other pair of electrodes was determined. It was thus possible to calculate the conduction velocity of the splanchnic peripheral fibers which excited the reticular neuron . In some experiments, the same technique was used to stimulate the contralateral splanchnic nerve and to determine its conduction velocity. In some instances the ipsilateral vagal nerve was electrically stimulated at the cervical level by means of a pair of electrodes similar to those used for the splanchnic nerves (0.5 ms, square-wave, 0.5-30 V). Electrical stimulation of skin and superficial tissues of the limb and thorax was performed (0.5 ms, square-wave, 0.5-5 V) using pairs of hypodermic needles as electrodes. The cerebellar cortex (lobes V and VI) was electrically stimulated (0.1 ms, square-wave, 0.5-4 V) by means of a silver mobile electrode in order to obtain antidromic responses of the LRN neurons studied; the indifferent electrode was in the temporal muscle. An identical technique was used to stimulate the somato-sensory areas SI and SII. Peritoneal splanchnic receptors were mechanically stimulated [15,35] in situ: displacement of whole organs (kidney, stomach), mobilization, stretching, stroking and punctate stimulation of the peritoneum were used.
Localization and identification of L R N neurons The recordings were obtained by means of microelectrodes inserted caudally to lhe obex (from 0 to 4 or 5 mm). The microelectrode tracks were made in vertical planes from 3.5 to 5 mm lateral to midline (Fig. 1). The responding neurons were located in the parvi- and magnocellular parts of the LRN, most of them being found in the ventral part of the nucleus. The location of these neurons was determined stereotaxically. Histological control, by Pontamine marking technique , was performed in some cases. The main criterion for the identification of the L R N neurons was an antidromic response evoked by stimulation of cerebellar cortex. One hundred and sixty neurons, which responded to electrical stimulation of the splanchnic nerve, were studied. Among them, 115 (72%) gave an antidromic response to cerebellar stimulation with stimulation threshold of less than 4 V and latencies less than 2.5 ms (Figs. 1 and 2B) (mean value 1.2 ms + 0.4). The areas of the cerebellar cortex, from which antidromic response were evoked, were located in the vermis and in the medial portion of the ipsilateral pars intermedia of lobules V and VI (in the rostral part of the latter). Some LRN neurons responded to stimulation of two or more homolateral areas, and also to stimulation of contralateral cerebellar cortical regions (Fig. 1). Those neurons from which antidromic responses were not obtained were studied
Fig. 1. Localization and identification of LRN neurons. On the left, schematic drawing of the recording and antidromic stimulation arrangements. This figure concerns sagittal plane 4.1 (Bergman's Atlas). Microelectrode (M.E.) is inserted at an angle of 45 ° to the Horsley-Clarke horizontal plane. Antidromic responses are evoked by stimulation of cerebellar cortex using a mobile unipolar electrode (A.E.). On the right, schematic drawing illustrating histological transverse section of the medulla performed 1.2 mm caudally to the obex. The mark indicates the localization of one neuron. Upper traces: response of this neuron to ipsilateral (CI) and contralateral (C2) antidromic stimulation of cerebellar cortex. Lower traces: response of this neuron to electrical stimulation of ipsilateral splanchnic nerve at threshold intensity (C3) and at an intensity equal to 3 times the threshold (C4). Each recording corresponds to a single sweep. Abbreviations: 5 S.P., alaminar spinal trigeminal nucleus; LRN, lateral reticular nucleus. o n l y if t h e y p r e s e n t e d the s a m e c h a r a c t e r i s t i c s as t h o s e w h i c h r e s p o n d e d to c e r e b e l lar s t i m u l a t i o n ( l o c a l i z a t i o n , visceral r e s p o n s e latency, t y p e of activity, c o n v e r g e n c e s , etc.).
Responses to electrical stimulation of the splanchnic nerve T h e r e s p o n s e s of L R N n e u r o n s e v o k e d b y electrical s t i m u l a t i o n of the s p l a n c h n i c n e r v e c o n s i s t e d o f a single spike o r a b u r s t o f 2 - 5 spikes. I n c r e a s i n g the i n t e n s i t y of the s t i m u l u s elicited a d e c r e a s e in r e s p o n s e l a t e n c y a n d a n i n c r e a s e in the n u m b e r of spikes (Fig. 1). T h e r e s p o n s e l a t e n c y at t h r e s h o l d s t i m u l a t i o n was d e t e r m i n e d for 147 n e u r o n s . T h e s e l a t e n c i e s r a n g e d b e t w e e n 5 a n d 60 m s ( m e a n v a l u e 25.1 ms ___ 1.1 (Fig. 2A)). A c c o r d i n g to t h e t y p e s of n e u r o n s , the results w e r e as follows: a n t i d r o m i c a l l y i d e n t i f i e d n e u r o n s , n = 106; m e a n l a t e n c y value, 25.3 ms + 1.1; n e u r o n s n o t a n t i d r o m i c a l l y i d e n t i f i e d n = 41; m e a n l a t e n c y value, 24.6 m s + 1.4.
Type of splanchnic fibers T h e c h a r a c t e r i s t i c s o f 92 s p l a n c h n i c fibers w h i c h e l i c i t e d L R N n e u r o n r e s p o n s e s were determined either by conduction velocity or by stimulation threshold measure-
i = sl
10 O 6
|1 = 75
I0 15 20 25
0 2 4 6 8 10 12 14
Fig. 2. Latencies and conduction velocities. Histograms showing distribution of latencies of responses evoked in L R N neurons by electrical stimulation. A: ipsilateral splanchnic nerve. Antidromically identified neurons and non-identified neurons (n] = 147). Hatched part: antidromically identified neurons ( n 2 = 106). B: cerebellar cortex (antidromic stimulation). C: contralateral cerebral cortex, areas SI (Cl) and SII (C2). Histogram showing distribution of velocities of peripheral fibers of ipsilateral splanchnic nerve projecting to the L R N D . Antidromically identified neurons and non-identified neurons (nt = 75). Hatched part: antidromicaUy identified neurons (n ~ = 57).
ment. Conduction velocity was determined for 75 of these fibers (Fig. 2D); for 11 of them it was slower than 2.4 m / s (unmyelinated fibers, type C), for the 64 others it ranged from 2.4 to 14 m / s (thin myelinated fibers, of the A group) (see ref. 36 for classification of afferent splanchnic fibers). The conduction velocity in the remaining 17 fibers was not measurable l; these were thought to be large AyS-fibers, or Aft fibers. The weak stimulation necessary for their activation ( < 0.3 V) permitted the identification of 3 of these fibers as Aft fibers (originating from Pacinian corpuscles in the peritoneum) .
Stimulation of splanchnic receptors The activity of 13 antidromically identified neurons, which responded to electrical stimulation of the splanchnic nerve, was elicited or modified by the activation of
i In rapid fibers, the difference between the latencies of the responses to stimulation by two pairs of electrodes placed on the nerve is too small to be measured with sufficient precision. Therefore, determination of the velocity is impossible unless interelectrode distances are great.
Fig. 3. Response of a L R N neuron to stimulation of a splanchnic peritoneal mechanoreceptor. This neuron did not exhibit any spontaneous activity. A: response to electrical stimulation of the splanchnic nerve (record is a single sweep). B: punctate stimulation. C: displacement of mesentery. Dot in A indicates artifact of electrical stimulation. Arrows in B and C indicate beginnings of mechanical stimulation.
mechanoreceptors (Fig. 3). These latter were slow-adapting movement receptors, localized in the mesentery, the gastric and perirenal peritoneum. These mechanoreceptors were activated by the displacement of whole organs (kidney, stomach) and intestinal segments. They were also sensitive to direct stimulation of the peritoneum (stretching, stroking, punctate stimulation). These mechanical stimulations increased the activity of most of these neurons; in 3 cases the activity decreased or even disappeared. Stimulation of large areas provoked activations of longer duration than did punctate stimulations. Viscero-somatic, viscero-visceral and viscero-cortical convergences on L R N neurons Viscero-somatic convergences
All 160 neurons studied (activated by splanchnic nerve stimulation) responded to electrical stimulation of the skin and superficial tissues of one or more limbs, and often to stimulation of the skin of the thorax (Fig. 4). Viscero-viscerai convergences
Electrical stimulation of the contralateral splanchnic nerve was performed in some animals. In these experiments, all the neurons (78) which responded to stimulation of the ipsilateral splanchnic nerve also responded to stimulation of the contralateral splanchnic nerve. The mean value of these contralateral responses was 25.5 ms + 1.6. The range of conduction velocities was identical to that of velocities measured for the ipsilateral nerve. The peripheral fibers of the contralateral nerve which elicited a L R N neuron response were mainly A78 type fibers, as in the ipsilateral nerve. Ipsilateral and contralateral peripheral fibers whose inputs converge on a common L R N neuron are mostly of the same type. However, convergences of inputs from one myelinated fiber of the Ay6 type in one nerve and from one unmyelinated fiber (C type) of another nerve were at times observed. Splanchno-vagal convergences are less frequent: 12 neurons among 63 tested (19%) presented this type of interaction (Fig. 4). Vagus nerve response latencies were highly variable from one neuron to another; they ranged between 4 and 45 ms.
1 A •
0.5 rnV I 20ms
Fig. 4. Examples of convergences induced by electrical stimulation. Neuron 1: A, ipsilateral splanchnic nerve; B, ipsilateral cerebellum (orthodromic response); C and D, somatosensory areas SI and SII respectively. Neuron 2: A, ipsilateral splanchnic nerve; B, ipsilateral vagus nerve; C, skin of ipsilateral hind limb; D, skin of the thorax. Neuron 3: This neuron is spontaneously active. A, ipsilateral splanchnic nerve; B, contralateral splanchnic nerve; in both cases stimulation elicits a long inhibition that is preceded by an activation. Dots indicate time of stimulation. Each recording corresponds to a single sweep.
Viscero-cortical convergences Convergences between inputs from the splanchnic nerves and from somato-sensory areas SI and SII were observed in 73 neurons, i.e. 90% of the neurons tested (Fig. 4). Most cortical evoked responses in L R N neurons, 69 in all had latencies of less than 30 ms (Fig. 2), but 4 had longer latencies, implying complex projection pathways. The mean values and standard errors obtained were 12.8 ms + 0.9 for SI and 12.1 ms ___0.8 for SII respectively.
Discussion The present study shows that L R N neuron activity can be influenced by electrical stimulation of the splanchnic nerve and by activation of peritoneal mechanoreceptors. We have previously demonstrated the existence of inputs from splanchnic mechanoreceptors to the cerebellum via mossy and climbing fibers . Additionally, we have shown that the splanchnic nerve projects to the inferior olive . The present results demonstrate d e a r l y that the L R N constitutes a relay for at least a portion of splanchnic afferents via mossy fibers.
390 The response latencies recorded in the LRN by electrical stimulation of the splanchnic nerve have a mean value of 25.3 ms _+ 1.1. This value is consistent with that which we obtained for the cerebellum (mean response latency for mossy fibers: 31 ms), taking into account the response latencies (ranged between 3 and 6 ms) recorded in the cerebellum by Murphy and Sabah  during LRN stimulation. The LRN neurons projecting to the cerebellum were identified by antidromic stimulation of lobules V and VI of the cerebellum. The most effective sites are located in the whole of lobule V and in the rostro-medial part of lobule VI. This observation is in agreement with the results obtained by several authors. In 1943, Brodal  suggested that the LRN projections on to the vermis were numerous. More recently, Brodal , Kunzle , Matshushita and Ikeda  have confirmed, by means of different histological techniques, that lobules V and VI receive axons from numerous LRN neurons. This cerebellar region includes the zone of splanchnic projection on the cerebellum which we studied previously , and corresponds in part to the termination areas of the spinoreticulocerebellar paths relayed through L R N which were described by Clendenin et al. [10,11]. The neurons situated in the ventral region of the LRN and which respond to splanchnic nerve stimulation are activated by antidromic stimulation (bilateral in most cases) of the vermis and the pars intermedia. Neurons of the dorso-lateral region are less numerous; some have bilateral, and others only ipsilateral projections. These results are in agreement with those of Crichlow , demonstrating the existence of bilateral LRN projections on the cerebellum, and those of Clendenin et al. , differentiating regions A and B of the LRN according to their ipsi- and bilateral cerebeUar projections. We demonstrate that the contingent of splanchnic fibers which project to the LRN is composed mainly of AyS-fibers; type C fibers which provoke a LRN neuron response after electric stimulation are scarce. Our observations were similar with regard to splanchnic afferents to the inferior olive, as well as mossy and climbing projections of the splanchnic nerve on the cerebellar cortex. It is known, however, that in the cat there is a high proportion of C fibers (approximately 2 / 3 of the number of A,/&type fibers ) in the afferent contingent of the splanchnic nerve. It can therefore be presumed that there is a preferential projection of A,/~-type fibers of the splanchnic nerve, and therefore of 'peritoneal mechanoreceptors of movement' to the relays of the spinocerebellar splanchnic (inferior olive and LRN) pathways, and on the cerebellum. The convergences observed between the splanchnic nerve and one or more limbs is not surprising, since the recordings were made in the bVFRT projection zone explored by Clendenin et at. , and Rosen and Scheid [37-39], who observed numerous convergences in cats' limbs after nerve and mechanoreceptor stimulation. The viscero-visceral (splanchno-splanchnic and splanchno-vagal) convergences observed at LRN level are similar to those found at the cerebellum level. In splanchnic nerves, interactions are primarily elicited by ipsi- and contralateral stimulation of the same types of fibers (AyS-type) connected with 'peritoneal mechanoreceptors of movement'. There are, however, splanchnic receptors of different types which project to the same LRN neuron. We found very few splanchno-vagal convergences in the
391 LRN. This result conforms with our observations: the vagal projections on lobules V and VI of the cerebellum via mossy fibers are scarce (unpublished data). The LRN neurons response latencies to cortical stimulations (areas SI and SII) are compatible with those which we recorded in the cerebellum after stimulation of these same areas, and conform with those reported by Allen and Tsukahara . It is possible to speculate about the physiological significance of splanchnic afferents to the LRN and cerebellum. It is known that the LRN receives signals from the motor cortex and the red nucleus concerning motor command. The LRN also receives information relative to the execution of this motor command (feedback signal) through spinoreticular pathways. The LRN has been thus considered as a comparator of command signals from the motor cortex and feedback signals from the spinal cord during ongoing movement [7,21]. This information is conveyed to the cerebellar cortex by way of mossy fibers, where it is employed in the cerebellar control of motor activity. A large number of LRN neurons considered to intervene in such information processes also receive, as shown here, splanchnic inputs from 'peritoneal movement receptors'. These receptors are activated by gastrointestinal tract contractions and by any movement or displacement of the animal. It can therefore be assumed that these visceral inputs are taken into account by the LRN and the cerebellum during somatic motor regulation. The LRN is also considered to be an integration site for hypothalamic cardiovascular and somatic afferents . Neurons of this nucleus project to cardio-acceleratory sites of the spinal cord [ 19]. Electrical stimulation of these neurons elicits increases in heart rate and arterial pressure . These results suggest that the LRN may play a role in somato-sympathetic reflexes, and that the afferents towards the LRN may contribute to the cardiovascular adjustings which occur during motor activity. Newman  however, observed modifications of arterial pressure as a result of mechanical stimulation of the gall-bladder. Ranieri et al.  observed increases in arterial pressure and heart rate during electrical stimulation of the central end of the splanchnic nerve. One may therefore expect that splanchnic inputs, coming from the gastrointestinal tract, the gall-bladder and the peritoneum may be integrated at the L R N level, and in this way, intervene in physiological cardiovascular regulation.
Acknowledgements The authors thank Andr~ Boyer for technical assistance and Fran~oise Farnarier for correcting the English.
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