Reflex responses evoked in the adrenal sympathetic nerve to electrical stimulation of somatic afferent nerves in the rat

The present study was initiated to determine the role of somatic A (myelinated) and C (unmyelinated) afferent fibers in both responses of increases and decreases in adrenal sympathetic nerve activities during repetitive mechanical pinching and brushing stimulations of the skin in anesthetized rats with central nervous system (CNS) intact. Accordingly, changes in adrenal sympathetic nerve activity resulting from repetitive and single shock electrical stimulation of various spinal afferent nerves, especially the 13th thoracic (Th13) spinal nerve and the sural nerve, were examined in urethane/chloralose-anesthetized rats. Repetitive electrical stimulation of A afferent fibers in Th13 spinal or sural nerve decreased the adrenal nerve activity similarly as brushing stimulation of skin of the lower chest or hindlimb did, while repetitive stimulation of A plus C afferent fibers of those nerves increased the adrenal nerve activity as pinching stimulation of those skins did. Single shock stimulation of spinal afferent nerves evoked various reflex components in the adrenal nerve: an initial depression of spontaneous activity (the early depression); the following reflex discharge due to activation of A afferent fibers (the A-reflex); a subsequent reflex discharge due to activation of C afferent fibers (the C-reflex); and following post-excitatory depressions. These reflexes seem to be mediated mainly via supraspinal pathways since they were abolished by spinal transection at the C1-2 level. Although the supraspinal A- and C-reflexes could be elicited from stimulation of a wide variety of spinal segmental afferent levels, the early depression was more prominent when afferents at spinal segments closer to the level of adrenal nerve outflow were excited. It is suggested that the decreased responses of the adrenal nerve during repetitive electrical stimulation of A afferent nerve fibers are attributable to summation of both the early depression and post-excitatory depression evoked by single shock stimulation, while the increased responses during repetitive stimulation of A plus C afferent fibers are attributable to summation of the C-reflex after single shock stimulation. In spinalized rats, repetitive stimulation of Th13 always increased the adrenal nerve activities regardless of whether A fibers alone or A plus C fibers were stimulated, just as brushing and pinching of the lower chest skin always increased them. The increased responses in spinal animals seem to be related to the fact that single electrical stimuli of Th13 produced A- and C-reflexes of spinal origin without clear depressions.     

Post-tetanic facilitation of vasomotor reflexes elicited by electrical stimulation of spinal afferents

Summary Somato-vegetative reflexes elicited by electrical stimulation of various spinal afferents and the effect of conditioning were investigated in immobilized cats, conscious or anaesthetized with chloralose or urethane. Test stimuli (1–2–4 imp/s) were applied before and after higher frequency (30 imp/s) conditioning stimulation on the same (homotopic conditioning) or different (heterotopic conditioning) nerves. Changes in reflex blood pressure responses and in renal sympathetic efferent activity were evaluated.In case of homotopic conditioning facilitation of reflexes was observed in anaesthetized cats. It manifested itself in reversal of depressor reactions into biphasic or pressor ones and in the appearance of marked reflex discharges in the sympathetic efferent activity, if the testing stimulations involved A-fibre afferents only. Pressor reflexes elicited by testing stimulations of A+C-afferents were enhanced; in the renal neurograms second bursts of discharges (C-reflexes) appeared or, if they had been present, were enlarged.Facilitation failed 1. if conditioning stimulation was not strong enough to excite C-fibre afferents; 2. in case of heterotopic stimulations; 3. in conscious animals independently of the site of conditioning. Depression of reflexes could be observed if heterotopic conditioning was performed in urethane anaesthesia or in conscious animals.It is concluded that post-tetanic facilitation of sympathetic reflexes may be due to enhanced excitability of the first spinal interneurons; controlling role of supraspinal influences is admitted.     

Central pathways of some autonomic reflex discharges

1. Electrical stimulation of spinal sensory nerves evoked discharges in inferior cardiac and renal nerves. In the anaesthetized cat both an early and a late response could be recorded in each nerve.2. For any one afferent input the central delay of the late cardiac nerve response was significantly less than that of the late renal nerve response. The central delay of the early responses was similar for both nerves. In the spinal cat only the early response was present.3. Cooling the floor of the 4th ventricle abolished the late responses in renal nerves, but left reflex volleys in white rami and intercostal nerves unchanged.4. Stimulation in the brain stem evoked responses in both cardiac and renal nerves which had a shorter latency than the reflexes evoked in these nerves by stimulating dorsal roots.5. The late responses could be abolished by lesions in the cervical spinal cord.6. Such evidence led to the conclusion that there are two pathways for reflex discharge into inferior cardiac and renal nerves, one involving a supraspinal relay and the other confined to the spinal cord.     

Neuronal mechanisms of spino-bulbo-spinal activity

Mechanisms of spino-bulbo-spinal motor activity evoked by stimulation of somatic (intercostal nerves) and visceral (the greater splanchnic nerve) afferents were studied in cats anesthetized with chloralose. It was found that thoracic interneurons activated by spino-bulbo-spinal mechanisms are distributed mainly in Rexed's laminae VII and VIII. They are excited either by both segmental and spino-bulbo-spinal influences or only by the latter. This neuronal population seems to form the spinal (segmental) pathways of spino-bulbo-spinal reflexes. Supraspinal structures related to the transmission of spino-bulbo-spinal activity are widespread in both the bulbar and the pontine reticular formations. They comprise reticulo-spinal neurons as well as neurons without projections to the spinal cord. Reticulo-spinal axons of ventral (conduction velocities 16–120 m/s) and lateral (17–100 m/s) funiculi can possibly participate in descending transmission of spino-bulbo-spinal activity. Stimulation of certain brain stem structures, especially pontine structures, results in long lasting (300–800 ms) depression of spino-bulbo-spinal reflexes. Conditioning spino-bulbo-spinal volleys also produce long-lasting (300–850 ms) depression of spino-bulbo-spinal reflex activity, as well as of descending spino-bulbo-spinal activity and responses evoked by direct stimulation of the reticular formation. Evidence is presented that this depression can be localized at the supraspinal level. Possible mechanisms of initiation and inhibition of spino-bulbo-spinal reflex activity evoked by somatic and visceral afferents are discussed.     

Bilateral reflex effects on phrenic nerve activity in response to single-shock vagal stimulation

The bilateral reflex actions of vagus nerve afferent signals on phrenic efferent activity have been tested by unilateral graded single shock electrical stimulation. An early excitation (latency 3–5 msec) was more prominent in the phrenic nerve contralateral to the stimulated vagus. Spinal cord hemisection at C₃ eliminated both contralateral and ipsilateral responses: thus, both were mediated via descending tracts in the contralateral cord. A bilaterally symmetrical early inhibition (latency 8–12 msec) followed the early excitation. The electrical thresholds for evoking the early responses and the temperature for blocking these responses during graded vagal cooling were closely similar to the threshold and blocking temperature for pulmonary stretch receptor afferents. Higher stimulus strengths evoked a strong, bilaterally similar, late excitation (latency 12–20 msec) followed by a late inhibition. At very high stimulus strengths a third excitation (latency 25–30 msec) could appear. Sometimes these responses were followed by lowered phrenic activity for the remainder of inspiration. Single shock stimulation of the intact vagus nerve or of the peripheral end of the cut recurrent laryngeal nerve provoked. by the contraction of laryngeal muscles, a strong, short latency (12 msec) inhibition of phrenic activity mediated by superior laryngeal nerve afferents. The implications of these results with respect to the reflex pathways of the different responses and their possible integration in the central respiratory control mechanisms are discussed.     

Pelvic afferent reflex control of rectal motility and lumbar colonic efferent discharge mediated by the pontine sympatho-inhibitory region in guinea pigs

Rectal motility and the efferent discharge of lumbar colonic nerves (LCED) have previously been shown to be affected by reflex activity activated by rectal stimulation. The sensory limb of this reflex is represented by afferent fibers in pelvic nerves. The present study revealed that this reflex is modulated by supraspinal sympatho-inhibitory regions. Pelvic afferent stimulation led to rectal contraction through the withdrawal of a tonic inhibitory influence of lumbar colonic nerves. The supraspinal region responsible for this antagonism ofthe rectal-inhibitory colonic nerve activity was localized to the pons. Neither the intravenous administration of atropine nor that of guanethidine (and Eisai compound 865–123, another adrenergic neuron blocking agent) effected the ability of pelvic afferent stimulation to inhibit tonic discharge of lumbar colonic efferent nerves; nervertheless, both agents eliminated the mechanical response of the rectum to stimulation of pelvic afferents. These observations suggest that lumbar sympathetic nerves may tonically inhibit the release of acetylcholine from excitatory neurons in the rectal myenteric plexus. We conclude that descending fibers from the pons are activated as a result of pelvic afferent nerve stimulation. These descending pontine fibers in turn inhibit the firing of sympathetic lumbar colonic nerves. Removal of this tonic restraint leads to rectal contraction.     

Cutaneous Mechanical Stimulation Regulates Ovarian Blood Flow via Activation of Spinal and Supraspinal Reflex Pathways in Anesthetized Rats

The reflex effects of noxious mechanical stimulation of a hindpaw or abdominal skin on ovarian blood flow, and the reflex pathways involved in those responses were examined in anesthetized rats. Blood flow in the left ovary was measured using a laser Doppler flowmeter, and the activity of the left ovarian sympathetic nerve and mean arterial pressure (MAP) of the common carotid artery were recorded. Stimulation of the left or right hindpaw for 30 s produced marked increases in ovarian sympathetic nerve activity and MAP. Ovarian blood flow slightly decreased during the stimulation and then slightly increased after the stimulation. After the left ovarian sympathetic nerves were severed, the same stimulus produced a remarkable monophasic increase in ovarian blood flow that was explained by passive vasodilation due to a marked increase in MAP. After spinal transection at the third thoracic (T3) level, the responses of MAP, ovarian sympathetic nerve activity, and ovarian blood flow to hindpaw stimulation were nearly abolished. Stimulation of the abdomen at the right or left side for 30 s produced slight increases in ovarian sympathetic nerve activity and MAP. Ovarian blood flow slightly decreased during the stimulation and then slightly increased after the stimulation. After the ovarian sympathetic nerves were severed, the response of the ovarian blood flow changed to a monophasic increase due to an increase in MAP. After spinal transection, stimulation of the left abdomen produced a moderate increase in MAP, a remarkable increase in ovarian sympathetic nerve activity and a slight decrease in ovarian blood flow during the stimulation. In contrast, stimulation of the right abdomen produced a smaller response in ovarian sympathetic nerve activity during the stimulation while it increased the MAP to a similar degree. Ovarian blood flow slightly increased after the end of stimulation, which was explained as passive vasodilation due to the increase in MAP. In conclusion, stimulation of somatic afferents affects ovarian blood flow by inducing changes in ovarian sympathetic nerve activities and blood pressure. When stimulation was applied to a hindpaw whose segment of afferent input is far from the segment of the ovarian sympathetic nerves, it took a supraspinal reflex pathway. However, when stimulation was applied to the abdomen whose spinal segment of the afferent is close to the segment of the ovarian sympathetic nerve output, there are spinal segmental reflex pathways. The present results demonstrate that spinal reflexes depend on the laterality of the stimulus, while supraspinal reflexes do not depend on the laterality of the stimulus.     

Morphology and electrophysiology of superior laryngeal nerve afferents and postsynaptic neurons in the medulla oblongata of the cat.

Intra-axonal recordings were made from 24 afferent fibres of the superior laryngeal nerve in and around the nucleus tractus solitarius, in 26 pentobarbitone-anaesthetized cats. Conduction velocity ranged from 15 to 38 m/s. Four afferents were injected with horseradish peroxidase. They showed dense terminal arborization in the region of the ventral and ventrolateral subnuclei of the nucleus tractus solitarius, both rostral and caudal to the obex. Six other intra-axonal recordings were thought to originate from axons of neurons postsynaptic to superior laryngeal afferents; one of these was injected with horseradish peroxidase and showed a similar arborization pattern to that of the afferent axons. In the same region, intracellular recordings were made from 124 neurons which responded to superior laryngeal nerve stimulation with excitatory postsynaptic potentials (mean latency 2.7 +/- 1.0 ms). Ninety-nine of these neurons were thought to receive a monosynaptic input. The stimulation threshold evoking these responses was similar to that which inhibited phrenic nerve discharge. Eleven of the monosynaptically excited neurons were injected with horseradish peroxidase. They had fusiform or stellate somata and simple dendritic trees, radiating mainly in the transverse plane. In one experiment, in which both a superior laryngeal nerve afferent fibre and a neuron were labelled, afferent terminal varicosities were found in close apposition with the postsynaptic membrane of the injected neuron. Four of 14 (29%) tested neurons could be antidromically activated from the C3 spinal segment. The stimulus thresholds and onset latencies of the responses of superior laryngeal nerve afferents and medullary neurons to stimulation of the superior laryngeal nerve are consistent with their involvement in the reflex inhibition of respiratory neurons evoked by superior laryngeal nerve stimulation.     

Functional role of lumbar sympathetic nerves and supraspinal mechanism in the defecation reflex of the cat

The role of the lumbar sympathetic nerves and supraspinal mechanism in the defecation reflex was investigated in 30 adult cats and 6 kittens. One or two propulsive contractions, whose mean pressure evoked was more than about 90 cmH2O (adult cats) and 50 cmH2O (kittens), were induced in the rectum of all animals by rectal distension. These propulsive contractions could be generated at the descending and the transverse colons. The removal of the supraspinal influence by spinal transection at T13 or removal of pelvic afferents to the supraspinal center by spinal transection at L abolished the propulsive contractions. Successive lumbar sympathectomy restored the contractions. Lumbar sympathectomy and the successive removal of the supraspinal influence did not affect the propulsive contractions. In both cases, the final exclusion of the sacral segments by pithing of the spinal cord abolished the propulsive contractions. These results suggest that the sacral excitatory reflex mediated via pelvic nerves and the lumbar inhibitory reflex mediated via lumbar sympathetic nerves can function during rectal distension in spinal cats and that the lumbar inhibitory reflex is suppressed by the supraspinal sympathetic inhibitory reflex activated by pelvic afferents in intact cats, as in guinea pigs, resulting in propulsive contractions.     

Reflex and cerebellar influences on α and on `rhythmic' and `tonic' γ activity in the intercostal muscle

1. Efferent intercostal alpha and gamma activity and afferent intercostal muscle spindle activity were studied in decerebrate cats in response to stimulation of the anterior lobe of the cerebellum and to postural and other reflexes.2. Low threshold intercostal responses were elicited from lobuli IV and V of the anterior lobe of the cerebellum.3. The existence of two functionally different types of intercostal gamma neurones has been confirmed. These are the ;rhythmic' or ;specifically respiratory' gamma neurones, and the ;tonic' gamma neurones.4. In response to cerebellar stimulation, facilitatory, inhibitory and diphasic tetanic and post-tetanic effects were obtained from alpha and the two types of gamma fibres in both external and internal intercostal nerve branches.5. Generally both inspiratory and expiratory alpha and gamma activity was facilitated in response to tetanic stimulation at contralateral stimulus sites, and inhibited in response to stimulation of ipsilateral sites.6. ;Rhythmic' gamma activity appeared to be rather closely linked to the respiratory alpha activity but the balance between ;rhythmic' gamma and alpha was often changed in response to cerebellar stimulation, as indicated by the responses of primary muscle spindle afferents.7. The ;tonic' gamma neurones were as a rule more responsive to cerebellar stimulation than were the alpha and ;rhythmic' gamma neurones. Long-lasting post-tetanic effects were much more prominent in the ;tonic' gamma fibres than in the alpha or ;rhythmic' gamma fibres.8. ;Rhythmic' gamma activity was abolished after cervical transections of the cord. ;Tonic' gamma activity remained in the spinal preparations although usually at a different discharge rate.9. ;Tonic' gamma neurones were more responsive than the ;rhythmic' gamma neurones to the proprioceptive gamma reflex elicited by passive movements of the chest wall as well as to other spinal and supraspinal reflexes.10. Both ;dynamic' and ;static' gamma fibres seem to be represented in the group of ;tonic' intercostal gamma neurones.11. The significance of the results are discussed with regard to the following two main points: (a) integration of respiratory and postural functions of the intercostal muscles; (b) cerebellar control of the alpha-gamma balance in active contractions and passive relaxations.     

Spinal and supraspinal components of the reflex discharges into lumbar and thoracic white rami

1. In chloralose anaesthetized cats reflex discharges in thoracic and lumbar white rami were elicited by single shock stimulation of intercostal, spinal and hind limb nerves.2. In animals with an intact neuraxis single stimuli of sufficient strength usually elicited a white rami mass discharge having two distinct components. Following spinal transection only the late reflex component disappeared.3. The early (spinal) reflex component had its largest amplitude if the afferent volley entered the spinal cord at the same or an adjacent segment of the white ramus under observation, whereas the size of the late (supraspinal) component was rather independent of the segmental level of the afferent input.4. It was concluded that somatic afferent volleys have a twofold action on the sympathetic nervous system: a more generalized action via the supraspinal sympathetic reflex centres and a more circumscribed action on the preganglionic neurones at the segmental level.     

Electro-acupuncture stimulation effects on duodenal motility in anesthetized rats

The effect of electro-acupuncture stimulation (EAS) on duodenal motility was examined in anesthetized, artificially ventilated rats. EAS was applied to the abdominal area or to a hindpaw for 30 s at stimulus intensities of 0.1-10.0 mA with a stimulus frequency of 20 Hz. The duodenal motility was measured using the balloon method at a position about 1.5 cm caudal from the pylorus. Duodenal motility was inhibited by EAS at intensities of more than 5.0 mA (suprathreshold of group IV afferent excitation) when applied to the abdominal area. The duodenal inhibitory response existed after bilateral vagotomy or spinal transection, but was abolished by sectioning bilateral splanchnic nerves. Duodenal motility was facilitated by EAS at intensities of more than 2.0 mA (subthreshold of group IV, and suprathreshold for groups II+III afferent excitation) when applied to a hindpaw. The duodenal facilitatory response by EAS to a hindpaw existed after sectioning the splanchnic nerves, but disappeared after bilateral vagotomy or spinal transection. Furthermore, repetitive electrical stimulation of vagal efferent nerves enhanced duodenal motility, while repetitive electrical stimulation of the splanchnic efferent nerves inhibited the motility. It was concluded that the inhibitory response of duodenal motility elicited by EAS to the abdominal area is a spinal reflex response involving splanchnic inhibitory efferent nerves, and the enhanced response of duodenal motility by EAS to a hindpaw is a supraspinal reflex response involving vagal excitatory nerves.     

Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves

Summary The role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 m thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4–100 A, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region. These results demonstrate that extensive destruction of the region of the vlNTS: a) does not markedly affect the inspiratory terminating reflex associated with electrical stimulation of the vagus nerve in a current range selective for activation of pulmonary stretch receptor afferents, and b) abolishes the short-latency increase, but not the bilateral decrease or longer latency increase in phrenic motoneuronal discharge which follows stimulation of the superior laryngeal nerve. We conclude that respiratory neurons in the region of the vlNTS do not play an obligatory role in the respiratory phase transitions in this experimental preparation. Neurons in the vlNTS region may participate in other reflexes, such as the generation of augmented phrenic motoneuronal discharge in response to activation of certain superior laryngeal or vagus nerve afferents.     

Reflex changes in heart rate after mechanical and thermal stimulation of the skin at various segmental levels in cats

Heart rate can be changed by cutaneous stimulation. In anesthetized cats with the central nervous system intact, a reflex increase in heart rate was elicited after natural stimuli such as pinching (noxious mechanical stimulation), rubbing (non-noxious mechanical stimulation), warming and cooling (thermal stimuli) were applied to the skin of the neck, chest, abdomen or perineum. It was shown that this cutaneo-cardiac acceleration reflex was produced mainly by a reflex increase in the discharges of the cardiac sympathetic efferent nerves and partially by a reflex decrease in the discharges of the cardiac vagal efferent nerves. On the other hand, in spinal cats, only stimulation of the chest and abdominal skin produced a reflex increase in heart rate. A possible explanation of this difference between central nervous system intact and spinal cats is that a spinal, segmentally organized component of the cutaneo-cardiac acceleration reflex is dominated by a supraspinal, diffusely distributed component in central nervous system intact cats.     

Respiratory modulation of the cutaneous somatosympathetic reflex in normotensive (WKY) and in spontaneously hypertensive rats (SHR): species and strain-dependent patterns

In 6 normotensive Wistar-Kyoto (WKY) and 6 spontaneously hypertensive rats (SHRs) anesthetized with urethane and chloralose, paralyzed, artificially ventilated, vagotomized with carotid sinus nerves bilaterally cut, somatosympathetic reflex discharges were recorded in cervical and renal nerves by stimulating group II and III cutaneous afferents in the sural nerve. Only a long-circuited, late supraspinal component reflex discharge could be elicited. After averaging the responses evoked by random stimulation, the latency of the reflex discharge was significantly longer in the renal than in the cervical sympathetic nerve, equally in the WKY rat and in SHR. In WKY rats the peak of sympathetic discharge corresponded to early expiration, whereas in SHRs--to late inspiratory phase. The duration of the reflex discharge elicited in inspiration was greater in SHR than in WKY rats. In WKY rats stimuli applied during phrenic discharge produced a reflex response of longer latency and of reduced amplitude than those applied in expiration. In SHRs the latency of the reflex response in the sympathetic cervical nerve was shorter during inspiration than in expiratory phase. The timing of the sympathetic reflex responsiveness within respiratory cycle in SHR and in WKY rats corresponded to strain-dependent opposite respiratory synchronization pattern of the spontaneous sympathetic activity characterizing each strain. No respiratory modulation of the somatosympathetic reflex was observed in the renal nerve of SHR. It is concluded that both spontaneous and evoked sympathetic activity is synchronized differently in SHR and in WKY rats and this difference is both species- and strain-dependent.     

The influence of bulbospinal monoaminergic pathways on sympathetic nerve activity

1. Spontaneous and reflex activity was recorded from renal and splanchnic nerves and thoracic white rami during discrete electrical stimulation within the medulla oblongata of anaesthetized cats.2. Inhibition or excitation of spontaneous sympathetic nerve activity was obtained from several medullary regions.3. The long-circuited reflex elicited in renal nerves and the spinally mediated reflex discharge produced in white rami by single shock stimulation of intercostal nerves were inhibited by stimulation within the sympatho-inhibitory areas of the medulla.4. Activation of spontaneous sympathetic nerve activity or inhibition of spontaneous and reflex sympathetic nerve activity was obtained during electrical stimulation within the lateral funiculi of the cervical spinal cord in unanaesthetized decerebrate cats, spinalized at C1.5. There was a correlation between the position of some sympatho-inhibitory regions of the medulla and spinal cord and the position of the cell bodies and axons of descending monoamine-containing neurones.6. Intravenous administration of the precursor of noradrenaline, L-DOPA, to unanaesthetized decerebrate cats, spinalized at C1, was followed by a depression of spontaneous activity in renal nerves and reflex responses elicited in renal nerves and white rami.7. Similarly the precursor of 5-hydroxytryptamine, 5-HTP, caused a depression of reflex activity elicited in renal nerves and white rami, but had no effect on spontaneous renal nerve activity.8. It is suggested that there exist both noradrenergic and tryptaminergic pathways which descend to the spinal cord from the medulla and which are inhibitory to sympathetic outflow.     

On the gastrocecal inhibitory reflex in the rat

In rats anesthetized with urethane, the effects of distention of the stomach upon cecal motility and neural mechanisms which generate this effect were studied. Cecal motility was inhibited which generate this effect were studied. Cecal motility was inhibited when the pars glandularis of the stomach was distended by pressure ranging from 25 to 30 cm H2O. This inhibitory reflex was not affected by bilateral cervical vagotomy, but completely abolished following bilateral severance of the greater splanchnic nerves or after intravenous administration of guanethidine. After transection of the spinal cord at the level of the 5th thoracic segment the inhibitory reflex remained intact, but was abolished following pithing of the 6th thoracic segment and below. It may be concluded that the afferent and efferent path of the gastrocecal inhibitory reflex mainly pass through the greater splanchnic nerves and the reflex center is located in thoracic segments caudal to the 6th thoracic segment.     

The response of some sympathetic neurones to volleys in various afferent nerves

1. Responses of sympathetic neurones to various afferent inputs have been analysed both in anaesthetized cats with intact neuraxis and in spinal cats.2. In anaesthetized cats electrical stimulation of low threshold group III afferent fibres in skin and muscle nerves inhibited sympathetic neurones and gave depressor responses. The silent period following a sympathetic reflex discharge is most likely due to inhibition brought on by a particular subgroup within this afferent group.3. High threshold group III and group IV afferent fibres excited sympathetic neurones and elicited pressor responses.4. Sympathetic reflex arcs could be temporally facilitated during the somatic afferent induced inhibition, by group IV, and to a lesser extent by group III volleys.5. Section of the spinal cord shortened the time course and lessened the degree of group III inhibition suggesting that suprasegmental pathways are involved in the long-lasting depression following a reflex.6. Baroreceptor afferent stimulation inhibited group III- and group IV- evoked reflexes even with some temporal facilitation in the pathway.7. The special properties of the sympathetic reflex arcs and the relation of the results to other work on B.P. reflexes are discussed. It is suggested that the group III and IV muscle afferents have a chemoreceptor function and are responsible for mediation of the pressor reflex during muscle exercise.     

Correlated and uncorrelated high-frequency oscillations in phrenic and recurrent laryngeal neurograms

1. Power spectral analysis of phrenic and recurrent laryngeal (or efferent vagal) inspiratory discharge activity from anesthetized cats revealed a peak within the 60- to 110-Hz range in all spectra, plus a peak within the 40- to 60-Hz range in the laryngeal (and efferent vagal) spectra, and a peak less than 40 Hz in the phrenic spectra. 2. A 60- to 110-Hz peak was present in coherence spectra between the left and right phrenic neurograms, the left and right recurrent laryngeal (and efferent vagal) neurograms, and all combinations of phrenic-laryngeal (and phrenic-efferent vagal) pairs. It is concluded that the nearly-periodic oscillations represented by these peaks arise from a single source that projects functionally in parallel to many respiratory motor outputs. This source may be part of, or interact with, respiratory central pattern generation. 3. The 40- to 60-Hz oscillations in left and right recurrent laryngeal (and efferent vagal) neurograms were uncorrelated or occasionally were very weakly correlated. Thus it is unlikely that these oscillations arise from a common source such as a second respiratory central pattern generator. 4. The oscillations less than 40 Hz were weakly correlated between left and right phrenic neurograms. This correlation may be due substantially to spinal crossed-phrenic pathways. 5. It is proposed that both the 40- to 60-Hz oscillations in recurrent laryngeal neurograms and the oscillations below 40 Hz in phrenic neurograms originate in neural circuits associated with individual left or right recurrent laryngeal or phrenic motor outputs. 6. Our results do not support the interpretation that multiple peaks in phrenic and recurrent laryngeal power spectra are due to two respiratory central pattern generators whose outputs have parallel pathways to respiratory motoneurons.     

Topographic organization of monosynaptic reflexes in the cat spinal cord

The topographic organization of monosynaptic reflexes in the cat spinal cord has been studied by comparing the amplitude of reflex discharges recorded from ventral roots consequent to stimulation of dorsal roots entering the cord at different spinal segments. The results indicate that up to 80% of the potentiated monosynaptic reflex discharge recorded from a ventral root can be attributed to afferent input entering the spinal cord at the same segmental level. Moreover, within the same segment, afferents with a more rostral cord entry level exert a stronger synaptic effect on the more rostral portion of the corresponding ventral root.