Supraspinal regulation of spinal reflex discharge into cardiac sympathetic nerves

(1) In chloralose anaesthetized cats, reflex responses were recorded in inferior cardiac nerves following stimulation of intercostal nerves and hind limb afferent nerves. (2) In 80% of cats, a long latency reflex response alone was recorded, whereas, in the others, a short and long latency response was present to intercostal nerve stimulation. (3) In cats displaying only a long latency somatocardiac reflex response, damage to the ventral quadrant of the ipsilateral cervical spinal cord, through which runs a bulbospinal inhibitory pathway, resulted in the appearance of shorter latency reflexes to intercostal nerve stimulation. Lesions elsewhere in the cervical cord did not do this. (4) The characteristics of the early responses indicated that they were somatosympathetic reflexes and not dorsal root reflexes. (5) The early reflexes remained and the late reflex disappeared on subsequent complete transection of the spinal cord. The early reflexes were therefore spinal reflexes, and suppressed in the animal with cord intact. (6) Lesions at C4, which included a contralateral hemisection and a section of dorsal columns extending into the dorsal part of the lateral funiculus, abolished the inhibition of a sympathetic reflex that followed stimulation of some somatic afferent nerve fibres. These sections did not release the spinal reflex. Therefore, this reflex inhibition was not responsible for the suppression of the spinal somatosympathetic reflex. (7) The descending inhibitory influence on the segmental reflex pathway was not antagonized by strychnine, bicuculline or picrotoxin. (8) The possibility is discussed that the spinal reflex pathway into cardiac sympathetic nerves is tonically inhibited by a bulbospinal pathway originating from the classical depressor region of the ventromedial reticular formation.     

Tonic descending inhibition of the spinal cardio-sympathetic reflex in the cat

Electrical stimulation of the left inferior cardiac nerve elicited a two-component reflex potential (spinal and supraspinal reflexes) in the ipsilateral white ramus T3 from which recordings were made in chloralose-anaesthetised cats. Reversible interruption of all spinal pathways achieved by cooling the spinal cord at C2/C3 produced an enhancement of the spinal reflex and abolished the supraspinal reflex, the latter usually being the more prominent reflex potential prior to spinal cord block. The spinal cord block-induced increase in the amplitude of the spinal reflex was, however, less than the increase observed during stimulation of the somatic intercostal nerve T4. Recordings of the afferent volley following cardiac nerve stimulation and analysis of the stimulus-reflex response relationship in neuraxis-blocked cats indicated that the spinal reflex as determined here was activated by A delta afferent fibres. However, if stimulus strength was raised above C-fibre threshold, spinal cord block revealed in addition a late spinal reflex response. In some cases, the appearance of this late potential was accompanied by a secondary decline of the earlier spinal reflex potential, possibly indicating C-fibre-mediated afferent inhibition. Neither baroreceptor activation nor denervation had any effect on spinal reflex amplitudes. Pharmacologically, clonidine given i.v. to cats with a blocked neuraxis reduced the spinal reflex amplitudes to pre-block values, an action which could be antagonised by the subsequent administration of the alpha 2-adrenoceptor antagonist rauwolscine. When given to non-pretreated cats with intact neuraxis, however, neither rauwolscine nor its analog yohimbine were capable of inducing a persistent release from tonic inhibition. The results suggest that both purely visceral and somato-visceral reflexes are subject to tonic descending inhibition, but they do not support the hypothesis that a catecholamine is the responsible transmitter mediating this inhibition.     

Efferent activity in the phrenic nerve during the startle reflex in chloralose-anesthetized cats

Reflex activity in the phrenic nerve was studied in chloralose anesthetized cats during development of somatic startle reflexes in limb and lower intercostal nerves. It was shown that the main component of this activity during low-threshold reflexes evoked by acoustic, tactile and low-threshold somatic afferent stimulation was depression of phrenic inspiratory activity. The following reflex discharges were prevalent components of phrenic responses to high-threshold afferent stimulation: early, propriospinal (intercostal-to-phrenic reflex) and late, suprasegmental ones. The latter were of two types: inspiratory (observed mainly during inspiration in about 75% of experiments) and expiratory (observed during expiration in 25% of experiments) which could be classified as "phrenic startle reflexes". Modulation of all responses during the respiratory cycle was described. Structural characteristics of reflex responses evoked in the phrenic nerve by stimulation of various respiratory and nonrespiratory bulbar sites as well as their respiratory modulation have been analyzed. Organization of possible neurophysiological mechanisms of phrenic responses during startle reflexes is discussed.     

An analysis of reflex changes in excitability of phrenic,laryngeal, and intercostal motoneurons

An investigation was carried out in anesthetized cats to ascertain whether self-excitation of phrenic motoneurons is a specific or generalized reflex mechanism for motoneurons allied to respiration. Whereas stimulation of only caudal intercostal nerves evoked discharge of phrenic motoneurons (intercostal-to-phrenic reflex), stimulation of all intercostal nerves elicited discharges in the recurrent laryngeal nerve (intercostal-to-recurrent laryngeal reflex). Weak superior laryngeal nerve stimulation provoked short-latency discharges in the recurrent laryngeal nerve but inhibited on-going inspiratory activity in phrenic and external intercostal motoneurons. In the presence of self-excitation of phrenic motoneurons (phrenophrenic system), there was concomitant excitation of laryngeal motoneurons. In contrast, when self-excitation of laryngeal motoneurons occurred (laryngolaryngeal system) there was concomitant inhibition of inspiratory activity (phrenic and external intercostal motoneurons). Paired shocks delivered to superior laryngeal and intercostal nerves while recording from phrenic, recurrent laryngeal, and intercostal nerves failed to reveal convergent interaction. It is concluded that self-excitation is a generalized reflex mechanism for certain motoneurons allied to respiration.     

Late prolonged discharges in the motor nerves of the hindlimbs and their relationship to locomotor rhythmicity in thalamically immobilized cats]

Effects of flexor reflex afferents stimulation were investigated on high decerebrated curarized cats. Stimulation of ipsilateral flexor reflex afferents evoked late long-lasting discharges in flexor nerves. Contralateral flexor reflex afferents stimulation evoked late discharges both in extensor and flexor nerves. Transition from late discharges to rhythmic discharges was observed. Early segmental reflexes were tonically depressed in thalamic in comparison with acute spinal cats. A similar tonic depression of segmental reflexes took place in acute spinal cats after DOPA injection. Segmental reflexes were distinctly modulated during late and rhythmic discharges. On the basis of the data available possible central mechanisms of the observed changes of segmental reflexes are discussed.     

A gentle mechanical skin stimulation technique for inhibition of micturition contractions of the urinary bladder

Effects of gentle skin stimulation of various segmental areas on the micturition contractions of the urinary bladder were examined in anesthetized male rats. The bladder was expanded by infusing saline via urethral cannula until the bladder produced rhythmic micturition contractions as a consequence of rhythmic burst discharges of vesical pelvic efferent nerves. Gentle stimulation was applied for 1 min by slowly rolling on top of skin with an elastomer "roller". Rolling on the perineal area inhibited both micturition contractions and pelvic efferent discharges during and after stimulation. Stimulation of the hindlimb, abdomen and forelimb inhibited micturition contractions after stimulation ended, in this order of effectiveness. During stimulation of the perineal skin, the reflex increase in pelvic efferent discharges in response to bladder distension to a constant pressure was also inhibited up to 45% of its control response. The inhibition of the micturition contractions induced by perineal stimulation was abolished, to a large extent by the opioid receptor antagonist naloxone and completely by severing cutaneous nerves innervating the perineal skin. We recorded unitary afferent activity from cutaneous branches of the pudendal nerve and found that the fibers excited by stimulation were low-threshold mechanoreceptive Aβ, Aδ and C fibers. Discharge rates of afferent C fibers (7.9 Hz) were significantly higher than those of Aβ (2.2 Hz) and Aδ (2.9 Hz) afferents. The results suggest that low frequency excitation of low threshold cutaneous mechanoreceptive myelinated and unmyelinated fibers inhibits a vesico-pelvic parasympathetic reflex, mainly via release of opioids, leading to inhibition of micturition contraction.     

Inhibition and excitation of the nociceptive flexion reflex by conditioning stimulation of a peripheral nerve in the cat

A previous study in our laboratory showed a long-lasting, naloxone-reversible inhibition of the flexion reflex after prolonged repetitive stimulation of a peripheral nerve in the spinal cat. The present study employed a special pattern of conditioning stimulation for a shorter period (200 s) to determine the time course of the inhibition and the afferent fibers responsible for the inhibition. We stimulated the common peroneal nerve in 10 decerebrated and spinalized cats to elicit the flexion reflex, which we recorded as single-unit activity from filaments of the L7 ventral root. The C fiber-evoked late component of the flexion reflex was compared before, during, and after conditioning electrical stimulation applied to the tibial nerve. Stimulating the tibial nerve at an intensity that excited only A alpha beta fibers produced weak inhibition of the flexion reflex; increasing intensity above the threshold for A delta fibers produced much greater inhibition. Inhibition began during the first 10 s of conditioning stimulation and was maximum at about 100 s. Stimulation at a suprathreshold intensity for C fibers, however, produced an initial transient excitation, lasting 10 to 20 s, followed by inhibition. Intravenous injection of naloxone (0.05 mg/kg) produced no observable changes in this inhibition and excitation. These results suggest that conditioning stimulation of a peripheral nerve inhibits the flexion reflex. This inhibition has a short latency; the afferent fibers seem to be A delta fibers. In addition, input from afferent C fibers may trigger a mechanism that produces facilitation of the reflex. The differences in recovery time course and in sensitivity to naloxone suggest that two different mechanisms may be responsible for the fast-onset inhibition and the previously observed long-lasting inhibition produced after prolonged conditioning stimulation.     

The somato-adrenal medullary reflexes in rats

In chloralose-urethane-anesthetized rats, the effects of somatic stimulation on the adrenal sympathetic efferent nerve activity as well as the adrenal catecholamine secretion were examined. Single shock of the thoracic thirteenth spinal afferent nerve evoked reflex discharges in the adrenal sympathetic efferent nerve. The spinal and supraspinal reflex components evoked by the myelinated and unmyelinated afferent stimulation were identified. The adrenal nerve activity was usually increased reflexly by pinching of the lower chest or upper abdominal skin area in the central nervous system (CNS)-intact animals. Secretion of adrenal epinephrine was noted to be increased reflexly by pinching the lower chest or upper abdominal skin in the central nervous system intact animals.     

Somato-vesical reflexes in chronic spinal cats

The effects of afferent volleys in hindlimb cutaneous and muscle nerves on vesical tone and contractility and on the discharges in pelvic nerves to the bladder were measured in anesthetized CNS-intact and 2-19 months chronic spinal cats. In chronic spinal cats volleys in group III and IV fibers increased the tone of the quiet, empty bladder (excitatory somato-vesical reflex). The same volleys inhibited the slow, large, rhythmic micturition contractions of the expanded bladder (inhibitory somato-vesical reflex). In CNS intact cats single or short tetanic volleys induced a reflex discharge in pelvic vesical nerve branches with 3 distinct components. These reflexes could be observed during micturition contractions, not markedly between the contractions or when the bladder was empty and quiet. The latencies of the 3 components were 90, 320 and 770 ms, respectively. The two early components (AI- and A2-reflex) were evoked by volleys in group II and III hindlimb afferents. The late component (C-reflex) was induced by group IV volleys. In chronic spinal cats a group II and III-induced A-reflex (latency 90 ms) and a group IV-induced C-reflex (latency 340 ms) were observed. The central pathways and the physiological significance of the various somato-vesical reflexes are discussed.     

Neural organization of the viscero-cardiac reflexes

The reflex responses evoked in the postganglionic nerves to the heart were tested in chloralose-anaesthetized cats. Electrical stimulation of the A delta afferent fibres from the left inferior cardiac nerve evoked spinal and supraspinal reflex responses with the onset latencies of 36 ms and 77 ms respectively. The most effective stimulus was a train of 3-4 electrical pulses with the intratrain frequency of 200-300 Hz. Electrical stimulation of the high threshold afferent fibres (C-fibres) from the left inferior cardiac nerve evoked the reflex response with the onset latency of 200 ms. The C-reflex was present in intact animals and disappeared after spinalization. The most effective stimulus to evoke this reflex was a train of electrical pulses delivered at a frequency of 1-2 Hz with an intratrain frequency of 20-30 Hz. The most prominent property of the C-reflex was its marked increase after prolonged repeated electrical stimulation. We conclude that: (1) viscero-cardiac sympathetic reflexes may be organized at the spinal and supraspinal level; (2) viscero-cardiac sympathetic reflexes evoked by stimulation of the A delta and C afferent fibres from the left inferior cardiac nerve have different central organization.     

Responses in sympathetic nerves of the dog evoked by stimulation of somatic nerves

Responses in thoracic and renal sympathetic nerves evoked by electrical stimulation of cutaneous and muscle nerves in anaesthetized mongrel dogs were observed. Supramaximal stimulation of cutaneous nerves evoked two responses in both thoracic and renal nerves with latencies in the ranges 58--184 msec and 349--733 msec which are referred to as the early and late responses. It was shown that the early and late responses were evoked by group III and group IV afferent fibres respectively. Stimulation of muscle nerves of the forelimb and the hypoglossal nerve evoked smaller early responses which were considered to be due to activation of group III fibres and which had latencies in the range 92--157 msec. Supramaximal stimulation of muscle nerves in the hind limb failed to evoke any responses in approximately two-thirds of preparations and in the remainder only low level inconsistent early responses were observed. No matter how intense the stimuli applied to muscle nerves there were never any responses which could be related to the activation of group IV fibres.     

Reflex effects evoked by stimulation of hypoglossal afferent fibers

In pentobarbitone-anesthetized cats, electrical stimulation of the central ends of the main trunks of transected hypoglossal nerves evoked vascular (pressor or depressor) reactions, mydriasis, slow and deep breathing, and reflex activation of laryngeal and facial muscles. Stimulation of the central end of the transected ramus descendens hypoglossi also provoked reflex contraction of cricothyroideus. These reflexes may be elicited also after intracranial section of hypoglossal nerve roots, but not after intracranial section of ipsilateral vagal roots. The above reflexes were abolished by acute section of the ipsilateral hypoglossonodosal branch, but they may be reproduced by electrical stimulation of the central end of this anastomotic branch between hypoglossal nerve and nodose ganglion. Stimulation of the central end of one transected hypoglossus evoked reflex efferent discharges in contralateral hypoglossus and contraction of contralateral tongue muscles. Stimulation of the central end of one transected hypoglossal end-branch inhibited efferent discharges in another end-branch. The crossed hypoglossohypoglossal reflex and the ipsilateral reflex inhibition were abolished by section of the hypoglossonodosal branch or vagal roots at the stimulated side. We conclude that reflexes evoked by stimulation of peripheral hypoglossal nerve in cats are mediated by afferent fibers directed to the nodose ganglion and entering the brain stem via vagal roots.     

Role of somatic afferents in autonomic system control of the intestinal motility

(1) In anesthetized (chloralose-urethane) rats, strong mechanical stimuli which were applied to the abdominal skin always inhibited motility of the small intestine. This reflex is referred to as an 'inhibitory cutaneo-intestinal reflex'. Similar stimuli applied to the skin of the upper chest, neck, forepaws, or hindpaws, however, evoked the opposite effect, which is referred to as a 'facilitatory cutaneo-intestinal reflex'. (2) By recording the activity of efferent sympathetic nerves to the small intestine and by transecting intestinal sympathetic or parasympathetic nerves we found that the inhibitory cutaneo-intestinal reflex was largely due to an increase in intestinal sympathetic efferent activity, and that the facilitatory cutaneo-intestinal reflex was due to decrease in the intestinal sympathetic efferent nerve activity; both changes reflexly evoked. (3) The inhibitory cutaneo-intestinal reflex was shown to be a propriospinal reflex which was caused by excitation of group IV (unmyelinated) cutaneous afferent nerve fibers. On the other hand, the facilitatory cutaneo-intestinal reflex seemed to be mediated through supraspinal pathways, and was evoked by excitation of mainly group III (A-delta group) cutaneous afferent nerve fibers. (4) Interaction between the cutaneo-intestinal reflex and intestino-intestinal reflex was demonstrated. (5) The possibility of a dorsal root reflex contribution to cutaneo-intestinal reflex was eliminated. (6) Significance of the cutaneo-intestinal reflex in neural control of the gastro-intestinal tract was discussed.     

Electro-acupuncture stimulation to a hindpaw and a hind leg produces different reflex responses in sympathoadrenal medullary function in anesthetized rats

The effects of electro-acupuncture stimulation (EAS) of two different areas of a hindlimb with different stimulus intensities on sympathoadrenal medullary functions were examined in anesthetized artificially ventilated rats. Two needles of 160 microm diameter and about 5 mm apart were inserted about 5 mm deep into a hindpaw (Chungyang, S42) or a hind leg (Tsusanli, S36) and current of various intensities passed to excite various afferent nerve fiber groups at a repetition rate of 20 Hz and pulse duration of 0.5 ms for 30-60 s. Fiber groups of afferent nerves stimulated in a hindlimb were monitored by recording evoked action potentials from the afferents innervating the areas stimulated. The sympathoadrenal medullary functions were monitored by recording adrenal sympathetic efferent nerve activity and secretion rates of catecholamines from the adrenal medulla. EAS of a hindpaw at a stimulus strength sufficient to excite the group III and IV somatic afferent fibers produced reflex increases in both adrenal sympathetic efferent nerve activity and the secretion rate of catecholamines. EAS of a hind leg at a stimulus strength sufficient to excite the group III and IV afferent fibers produced reflex responses of either increases or decreases in sympathoadrenal medullary functions. All responses of adrenal sympathetic efferent nerve activity were lost after cutting the afferent nerves ipsilateral to the stimulated areas, indicating that the responses are the reflexes whose afferents nerve pathway is composed of hindlimb somatic nerves. It is concluded that electro-acupuncture stimulation of a hindpaw causes an excitatory reflex, while that of a hind leg causes either excitatory or inhibitory reflex of sympathoadrenal medullary functions, even if both group III and IV somatic afferent fibers are stimulated.     

Experimental study of a late response recorded from the thoracic wall after phrenic nerve stimulation

ObjectivesThe phrenic nerve cervical stimulation induces an early motor diaphragmatic M response that may be recorded from the 7th ipsilateral intercostal space (ICS). Some responses with prolonged latency and of unclear origin can be recorded from the same recording site. The aim of the study was to determine the electrophysiological characteristics and the neuroanatomical pathways underlying the long-latency responses (LLRs) recorded from the 7th ICS.MethodsWe studied seven healthy volunteers, five patients with spinal cord injury and five patients with diaphragmatic palsy. All underwent phrenic nerve conduction study. An LLR was sought for at different stimulation sites using various stimulus intensities.ResultsA polyphasic LLR was recorded from the 7th ICS in all healthy subjects. It was mainly elicited by nociceptive stimulations, not only of the phrenic, but also of the median nerves. Its latency was longer than 70 ms, with a wide inter- and intra-individual variability. Amplitude was highly variable and some habituation phenomenon occurred. The LLR was retained in most tetraplegic patients after phrenic nerve stimulation, but absent otherwise. It was present in all patients with diaphragmatic palsy after phrenic nerve stimulation.ConclusionThe LLR is likely to be produced by both intercostal and diaphragm muscles. It is a polysynaptic and multisegmental spinal response, probably conveyed by small-diameter nociceptive A-δ and/or C fibres and modulated by a supraspinal control.SignificanceThe LLR recorded from the chest wall may constitute, by analogy with the nociceptive component of the lower limb flexion reflex in humans, a protective and withdrawal spinal reflex response.     

Transmission of afferent information from urinary bladder, urethra and perineum to periaqueductal gray of cat

The micturition reflex pathway is a supraspinal pathway. Anatomical tracing evidence is compatible with an involvement of the periaqueductal gray (PAG) in the ascending limb of this reflex. We tested the involvement of the PAG in receiving urinary tract- or perineum-related information and attempted to characterize this ascending path in terms of what type of information is being conveyed. Electrical stimulation of the pelvic nerves, which carry afferent information from the urinary bladder, evoked maximum field potentials in the caudal third of the PAG, primarily in the dorsal part of the lateral PAG and in the ventrolateral PAG. Since the regions activated by pelvic nerve stimulation differed from those activated by stimulation of the sensory pudendal or superficial perineal nerves, it is possible that specific pathways for different nerve inputs to the PAG exist. Sacral spinal cord neurons ascending to the PAG were identified by antidromic activation and then tested for inputs from pelvic, sensory pudendal or superficial perineal nerves. Of 18 units identified, only five received inputs from any of the peripheral nerves tested and only two projecting neurons received a pelvic nerve input. Thus the PAG may receive inputs from bladder and perineum, but the small proportion of cells with direct projections to the PAG receiving inputs from our test nerves implies that the major part of this pathway is not directly related to lower urinary tract function.     

Activation of descending noradrenergic system by peripheral nerve stimulation

Noradrenaline (NA) release in the rat lumbar spinal cord (L3–4) in response to variable intensity, selective stimulation of large (A-beta), small myelinated (A-delta), and unmyelinated (C) afferent fibers was examined by in vivo microdialysis with high performance liquid chromatography and electrochemical detection. Application of 100 mM K⁺ solution via the dialysis probe increased NA in the dialysate. Thoracic segment transection rostral to the probe depressed the NA level. Transcutaneous stimulation of peripheral nerves had the following effects: 1) High intensity stimulation of afferent A-delta or C fibers increased spinal NA release, which was decreased by thoracic spinal cord transection. 2) Stimulation of afferent A-beta or A-delta fibers at low intensity did not affect the NA level. 3) High intensity stimulation of afferent A-beta fibers depressed NA release in half of the trials. Results indicate that many NA-containing nerve terminals that innervate the lumbar spinal cord originate from supraspinal structures. Somatic neural inputs from afferent C fibers and high-threshold A-delta, but not A-beta nor low-threshold A-delta fibers, activate the descending NA system and release the NA in the spinal cord. The descending NA system may participate in antinociception.     

Splenic, renal, and cardiac nerves have unequal dependence upon tonic supraspinal inputs

Stimulation of visceral receptors can lead to unequal reflex responses in splenic, renal and cardiac sympathetic nerves. Activity of splenic nerves is often more excited or less inhibited than that of cardiac or renal nerves. This study was undertaken to determine potential differences in resting discharge among these 3 nerves. Dependence upon supraspinal drive was evaluated by comparing the relative decrease in activity of these nerves in chloralose-anesthetized cats 30 min to 2 h following high cervical spinal cord transection. After this transection, discharge rates of cardiac and renal nerves were significantly depressed to less than 50% of initial values. In contrast, splenic nerve activity was not significantly affected. To determine if this sustained splenic nerve activity resulted from greater responsiveness to potential external sources of excitation, splenic, renal and cardiac neural responses to factors known to affect sympathetic discharge in spinal animals were compared. Neither increased arterial pressure, decreased arterial pressure, systemic hypercapnia and acidosis, nor thoracolumbar dorsal rhizotomy revealed specific inputs responsible for the preferential maintenance of splenic nerve activity in spinal cats. It was concluded that ongoing activity of splenic nerves is less dependent upon supraspinal sources of excitation than is activity of renal or cardiac nerves. The cause of this difference among these 3 components of sympathetic outflow remains to be determined.     

Three types of reticular neurons involved in the spinobulbo-spinal reflex of cats

Reticular neuron activity was recorded in 28 chloralosed cats in order to analyze the reflex arc of the spino-bulbo-spinal (SBS) reflex. Three types of reticular neurons, types I (input), II (output) and III (relay), were identified by unit discharges in response to stimulation of the sural nerve.

(1) Type I (input) neurons received spinal ascending volleys monosynaptically and responded to stimulation of the sural nerve with spikes of low amplitude and short latency. Unit spikes, however, were not produced by stimulation of the superficial radial nerve and the sensorimotor cortex. These input neurons were located in the dorsocaudal part of the medial bulbar reticular formation.

(2) Type II (output) neurons were part of the reticulospinal tract, which sends axons to the spinal cord, since these neurons exhibited antidromic spikes following stimulation of the ventrolateral funiculus of the spinal cord. Unit spikes were evoked by stimulation either to the sural or superficial radial nerves. These neurons were located in the ventrocaudal part of the medial bulbar reticular formation.

(3) Type III neurons included relay neurons. Unit spikes were evoked by stimulation of the sural nerve, superficial radial nerve and sensorimotor cortex. However, unit discharges were not obtained by antidromic stimulation to the reticulospinal tract. These neurons were distributed widely in the brain stem, both in the bulb and pons.

(4) Latency difference of unit discharges between input and output neurons was 3.5–5 msec, indicating the presence of interneurons (relays) between input and output neurons. Spikes of output neurons with 3.8–4.2 msec latency were observed following stimulation of the region where input neuron activity was found. We may conclude that three kinds of reticular neurons, input, relay and output, were involved in pathways of the SBS reflex.

Functional organization of the spinal reflex pathways from forelimb afferents to hindlimb motoneurons in the cat. II. Conditions of the interneuronal connections

The interneuronal conditionsof the descending pathways from forelimb afferents to hindlimb motoneurones were investigated by testing spatial interactions in these pathways and between these pathways and segmental lumbar reflex pathways. In high spinal unanaesthetized cats hindlimb motoneuroneswere intracellularly recorded and spatial interactions were tested between effects evoked by stimulation of pairs of ipsi- and contralateral forelimb nerves or pairs of a forelimb and an ipsilateral hindlimb nerve. The excitatory and late inhibitory pathways from forelimb afferents projecting to most of the hindlimb motoneurone pools, showed an interactive pattern which was distinctly different to the fast inhibitory pathway projecting specifically for ipsilateral forelimb afferents to flexor digitorum and hallucis longus (FDHL) motoneurones. Stimulation of homonymous or heteronymous pairs of two forelimb nerves of both sides evoked generally a distinct spatial facilitation of the excitatory and late inhibitory effects, while the specific early IPSPs to FDHL motoneurones were not facilitated. Paired stimulation of two forelimb nerves of one side only produced spatial facilitation of EPSPs or late IPSPs if low strength stimuli were used, using higher strength which induced larger effects, generally caused occlusion instead. In case of large IPSPs this may be due to the vicinity to the equilibrium potential. Except for an inhibition of cutaneous reflex pathways, the spatial interaction of the excitatory and late inhibitory pathways onto segmental lumbar reflex pathways was weak and variable. The fast inhibitory pathway to FDHL motoneurone showed a partial spatial facilitatory interaction with lumbar reflex pathways from cutaneous and group II muscle afferents. The second IPSP wave evoked by this pathway was inhibited by antidromic stimulation of the ventral root L7S1 and of the α-efferents of the antagonistic peroneal nerve. From the results conclusions are drawn on the interneuronal organization of the descending pathways from forelimb afferents to hindlimb motoneurones.