Synaptic transmission in thoracic autonomic ganglia of the dog

Afferent stimulation of one canine thoracic cardiopulmonary nerve can generate compound action potentials in another ipsilateral cardiopulmonary nerve. These compound action potentials persist after acute decentralization of the middle cervical ganglion, indicating that they result from neural activity in the middle cervical ganglion and thoracic nerves. Changing the frequency of stimulation can alter the compound action potentials, suggesting that temporal facilitation or inhibition occurs in this middle cervical ganglion preparation. The compound action potentials can be modified by stimulation of sympathetic preganglionic fibers and by hexamethonium, atropine, phentolamine, propranolol, and (or) manganese. It thus appears that afferent cardiopulmonary nerves can activate efferent cardiopulmonary nerves via synaptic mechanisms in the stellate and middle cervical ganglia. It also appears that these mechanisms involve adrenergic and cholinergic receptors and are influenced by preganglionic sympathetic fibers arising from the cord.     

Functional anatomy of the canine mediastinal cardiac nerves located at the base of the heart

The major canine cardiopulmonary nerves which arise from the middle cervical and stellate ganglia and the vagi course toward the heart in the dorsal mediastinum where they form, at the base of the heart dorsal to the pulmonary artery and aorta, the dorsal mediastinal cardiac nerves. In addition, the left caudal pole and interganglionic nerves project onto the left lateral side of the heart as the left lateral cardiac nerve. These nerves contain afferent and (or) efferent axons which, upon stimulation, modify specific cardiac regions and (or) systemic pressure. In addition, with the exception of the left lateral cardiac nerve, stimulation of each of these nerves produces compound action potentials in the cranial ends of the majority of the major cardiopulmonary nerves demonstrating that axons in each dorsal mediastinal cardiac nerve interconnect with axons in the majority of the cardiopulmonary nerves. Axons in the left lateral cardiac nerve connect primarily with axons in the left caudal pole and left interganglionic nerves. The dorsal mediastinal nerves project distally onto the heart as coronary nerves accompanying the right or left coronary arteries. These innervated the ventricular myocardium which is supplied by their respective vessels. The left lateral cardiac nerve projects directly onto the lateral epicardium of the left ventricle. The dorsal mediastinal and left lateral cardiac nerves are the major sympathetic cardiac nerves. Thus, the cardiac nerves located in the mediastinum at the base of the heart are not simple extensions of cardiopulmonary nerves, but rather have a unique anatomy and function of their own.     

Direct recording of somatosensory evoked potentials in the vicinity of the dorsal column nuclei in man: their generator mechanisms and contribution to the scalp far-field potentials

Somatosensory evoked potentials (SEPs) in the vicinity of the dorsal column nuclei in response to electrical stimulation of the median nerve (MN) and posterior tibial nerve (PTN) were studied by analyzing the wave forms, topographical distribution, effects of higher rates of stimulation and correlation with components of the scalp-recorded SEPs. Recordings were done on 4 patients with spasmodic torticollis during neurosurgical operations for microvascular decompression of the eleventh nerve. The dorsal column SEPs to MN stimulation (MN-SEPs) were characterized by a major negative wave (N1; 13 msec in mean latency), preceded by a small positivity (P1) and followed by a large positive wave (P2). Similar wave forms (P1′-N1′-P2′) were obtained with stimulation of PTN (PTN-SEPs), with a mean latency of N1′ being 28 msec. Maximal potentials of MN-SEPs and PTN-SEPs were located in the vicinity of the ipsilateral cuneate and gracile nuclei, respectively, at a level slightly caudal to the nuclei. The latencies of P1 and N1 increased progressively at more rostral cervical cord segments and medulla, but that of P2 did not. A higher rate of stimulation (16 Hz) caused no effects on P1 and N1, while it markedly attenuated the P2 component. These findings suggest that P1 and N1 of MN-SEPs, as well as P1′ and N1′ of PTN-SEPs, are generated by the dorsal column fibers, and P2 and P2′ are possibly of postsynaptic origin in the respective dorsal column nuclei.The peak latency of N1 recorded on the cuneate nucleus was identical with the scalp-recorded far-field potential of P13–14 in all patients, while no scalp components were found which corresponded to P2. These findings support the previous assumption that the scalp-recorded P13–14 is generated by the presynaptic activities of the dorsal column fibers at their terminals in the cuneate nucleus.     

Characteristics of electrical activity in different sections of nerve endings in the frog

In experiments on frog sartorius neuromuscular preparations, the evoked electrical responses of nerve endings were recorded by extracellular microelectrodes. It was shown that in proximal parts of the nerve ending, the three-phase response (+ - +) with a high amplitude negative phase occurred due to motor nerve stimulation. With movement of the extracellular electrode in distal direction a certain increase of the initial positive phase and a significant decrease of the negative one were observed. At the end of the terminal that response transformed to the monophasic one (+). On local iontophoretic application of tetrodotoxin (TTX) to the recording site two components of the nerve ending response were revealed: TTX-insensitive and TTX-sensitive. A significant decrease of the TTX-sensitive component occurred along the course of the nerve ending. That component was absent from the distal synaptic areas. It is concluded that in frog nerve ending, the action potential propogates with decrement while depolarization of the end parts of the terminal is passive in nature.     

Spinal reflexes in the long-tailed stingray, Himantura fai

We have exploited the segregation of motor and sensory axons into peripheral nerve sub-compartments to examine spinal reflex interactions in anaesthetized stingrays. Single, supra-maximal electrical stimuli delivered to segmental sensory nerves elicited compound action potentials in the motor nerves of the stimulated segment and in rostral and caudal segmental motor nerves. Compound action potentials elicited in segmental motor nerves by single stimuli delivered to sensory nerves were increased severalfold by prior stimulation of adjacent sensory nerves. This facilitation of the segmental reflex produced by intense conditioning stimuli decreased as it was applied to more remote segments, to approximately the same degree in up to seven segments in the rostral and caudal direction. In contrast, an asymmetric response was revealed when test and conditioning stimuli were delivered to different nerves, neither of which was of the same segment as the recorded motor nerve: in this configuration, conditioning volleys generally inhibited the responses of motoneurons to stimuli delivered to more caudally located sensory nerves. This suggests that circuitry subserving trans-segmental interactions between spinal afferents is present in stingrays and that interneuronal connections attenuate the influence that subsequent activity in caudal primary afferents can have on the motor elements.     

Role of caudal ventrolateral medulla in reflex and central control of airway caliber.

We investigated the role played by the caudal ventrolateral (CVL) medulla in the reflex and central neural control of airway caliber in chloralose-anesthetized dogs. Changes in total lung resistance were evoked by four different stimuli. These changes were compared before and after bilateral injection of either ibotenic acid (75 nl; 100 mM) or cobalt chloride (75 nl; 50 mM) into the CVL medulla. The four stimuli used to change lung resistance were static muscular contraction, electrical stimulation of thin fiber afferents in the sciatic nerve, electrical stimulation of the posterior diencephalon, and hypoxia. The first three stimuli have been shown to decrease total lung resistance, whereas the latter stimulus has been shown to increase resistance. We found that injection of both ibotenic acid, which destroys cell bodies but not fibers of passage, and cobalt, which prevents synaptic transmission, either abolished or greatly attenuated the decrease in total lung resistance evoked by static contraction, by sciatic nerve stimulation, and by posterior diencephalic stimulation. We also found that injection of ibotenic acid and cobalt attenuated the reflex increase in lung resistance evoked by hypoxia. In control experiments, we found that bilateral injection of ibotenic acid into the dorsal medulla had no effect on the changes in total lung resistance evoked by these four stimuli. We conclude that the CVL medulla plays an important role in the reflex and central control of airway caliber.     

Synaptic potentials of motoneurons of the masseter muscle

Postsynaptic potentials of 93 motoneurons of the masseter muscle evoked by stimulation of different branches of the trigeminal nerve were studied. Stimulation of the most excitable afferent fibers of the motor nerve of the masseter muscle evoked monosynaptic EPSPs with a latent period of 1.2–2.0 msec, changing into action potentials when the strength of stimulation was increased. A further increase in the strength of stimulation produced an antidromic action potential in the motoneurons with a latent period of 0.9 msec. In some motoneurons polysynaptic EPSPs and action potentials developed following stimulation of the motor nerve to the masseter muscle. The ascending phase of synaptic and antidromic action potentials was subdivided into IS and SD components, while the descending phase ended with definite depolarization and hyperpolarization after-potentials. Stimulation of cutaneous branches of the trigeminal nerve, and also of the motor nerve of the antagonist muscle (digastric) evoked IPSPs with a latent period of 2.7–3.5 msec in motoneurons of the masseter muscle. These results indicate the existence of functional connections between motoneurons of the masseter muscle and its proprioceptive afferent fibers, and also with proprioceptive afferent fibers of the antagonist muscle and cutaneous afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 262–268, November–December, 1969.     

Inhibition of jaw opening reflexes by stimulation of the central gray matter and raphe nuclei in cats

The effect of stimulation of the mesencephalic central gray matter and raphe nuclei on jaw opening reflexes evoked by excitation of high-threshold (dental pulp) and low-threshold (A-alpha) fibers of the infraorbital nerve afferents was studied in cats anesthetized with chloralose and pentobarbital. The jaw opening reflex evoked by stimulation of the dental pulp was shown to be effectively suppressed by conditioning stimulation of the central gray matter and raphe nuclei. The reflex evoked by stimulation of low-threshold infraorbital nerve afferents also was depressed (but less deeply and for a shorter period than the reflex evoked by stimulation of the dental pulp) during stimulation of the raphe nuclei and caudal zone of the central gray matter, but was unchanged after stimulation of the points located in the rostral zone of the central gray matter. Application of single stimuli or bursts of five stimuli with a frequency of 100 Hz had no effect on the reflexes studied. Short-term stimulation with a burst of 10–20 stimuli with a following frequency of 200–400 Hz led to inhibition of the reflexes, which lasted 450–1000 msec. Long-term stimulation of the central gray matter and raphe nuclei for 30 sec with a frequency of 50 Hz caused inhibition of jaw opening reflexes evoked by stimulation of both high- and low-threshold afferents for 60 min. Impulses from the central gray matter and raphe nuclei thus have a mainly inhibitory action on the jaw opening reflex evoked by stimulation of high-threshold afferents, but they act less effectively on the reflex evoked by stimulation of low-thres-hold afferents. The duration of inhibition depends essentially on the parameters of stimulation.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 374–387, May–June, 1984.     

A simple in vitro method to study the trigeminal ganglion

This report describes a simple in vitro method to harvest and study the electrophysiological properties of the trigeminal ganglion of the rat. In suction electrode recordings from the proximal nerve end, two distinct peaks are identified in the compound action potential evoked by electrical stimulation of the distal nerve. gamma-Aminobutyric acid (GABA), added to the perfusion fluid, caused a positive shift in the DC potential (depolarization) and a partially selective decrease in the amplitude of the slow-conduction peak. Reversible antagonism of the GABA effect by picrotoxin and bicuculline suggests that both responses are receptor mediated. There was no response to carbamazepine or L-baclofen, suggesting that ganglionic polarization does not play a major role in the action of these drugs in trigeminal neuralgia.     

Morphological Characterization of Bushy Cells and Their Inputs in the Laboratory Mouse (Mus musculus) Anteroventral Cochlear Nucleus

Spherical and globular bushy cells of the AVCN receive huge auditory nerve endings specialized for high fidelity neural transmission in response to acoustic events. Recent studies in mice and other rodent species suggest that the distinction between bushy cell subtypes is not always straightforward. We conducted a systematic investigation of mouse bushy cells along the rostral-caudal axis in an effort to understand the morphological variation that gives rise to reported response properties in mice. We combined quantitative light and electron microscopy to investigate variations in cell morphology, immunostaining, and the distribution of primary and non-primary synaptic inputs along the rostral-caudal axis. Overall, large regional differences in bushy cell characteristics were not found; however, rostral bushy cells received a different complement of axosomatic input compared to caudal bushy cells. The percentage of primary auditory nerve terminals was larger in caudal AVCN, whereas non-primary excitatory and inhibitory inputs were more common in rostral AVCN. Other ultrastructural characteristics of primary auditory nerve inputs were similar across the rostral and caudal AVCN. Cross sectional area, postsynaptic density length and curvature, and mitochondrial volume fraction were similar for axosomatic auditory nerve terminals, although rostral auditory nerve terminals contained a greater concentration of synaptic vesicles near the postsynaptic densities. These data demonstrate regional differences in synaptic organization of inputs to mouse bushy cells rather than the morphological characteristic of the cells themselves.     

Effect of local depression of inhibition on electrical activity of the spinal cord

The dorsal cord, dorsal root, and focal potentials in response to peripheral nerve stimulation were investigated in rats with local depression of inhibition in the left or right half of the lumbar segments produced by the action of tetanus toxin. The investigation was carried out at the stage of poisoning when excitation of the neuron population with disturbed inhibition caused generalized excitation of spinal and bulbar motoneurons. Experiments on spinal animals showed that if a cutaneous nerve is stimulated on the side affected by the toxin these responses have a greater amplitude and a much longer duration than those evoked by stimulation of the opposite nerve or responses in healthy rats. The maximal increase in amplitude and duration of the negative component of the focal potential corresponding to the time of the increased P wave of the dorsal cord potential was found in the ventral quadrant on the side affected by the toxin. Besides evoked focal potentials, spontaneous rhythmic negative waves also were recorded in this area. The mechanisms of spread of seizure activity from the focus of depressed inhibition are discussed and the structures generating spreading seizure activity are identified.     

Waveform generation in Rhamphichthys rostratus (L.) (Teleostei,Gymnotiformes)

Rhamphichthys rostratus (L.) emits brief pulses (2 ms) repeated very regularly at 50 Hz. The electric organ shows a heterogeneous distribution of the electrocyte tubes and the occurrence of three electrocyte types (caudally innervated, rostrally innervated and marginallycaudally innervated). In the sub-opercular region the electric organ consists of a pair of tubes containing only caudally innervated electrocytes. At the abdominal region the EO consists of three pairs of tubes. Each pair contains one of the described electrocyte types. The number of electrocyte tubes increases toward the tail to reach nine or ten pairs in the most caudal segments. In the intermediate region most tubes contain doubly innervated electrocytes except the ventral pair that contains caudally innervated electrocytes. The caudal 25% contains exclusively caudally innervated electrocytes. The electric organ discharge consists of five wave components (V1 to V5). Electrophysiological data are consistent with the hypothesis that V1 results from the activity of the rostral faces of rostrally innervated electrocytes. V2 results from the activities of rostral faces of marginally-caudally innervated electrocytes while V3 results from the activities of caudal faces of most electrocytes. Curarization experiments demonstrated that V4 and V5 result from action potential invasion and are not directly elicited by neural activity.Abbreviations AEN1 anterior electromotor nerve 1 - AEN2 anterior electromotor nerve 2 - BMB boraxic methylene blue - CIE caudally innervated electrocytes - EMF electromotive force - EO electric organ - EOD electric organ discharge - I current amplitude - MCIE marginally-caudally innervated electrocytes - MT medial tubes - PEN posterior electromotor nerve - R _n internal impedance - RIE rostrally innervated electrocytes - Rl load resistor - SAT short abdominal tubes - V voltage amplitude     

Mechanisms of direct and neural excitability in electroplaques of electric eel

1. Current flow outward through the caudal, reactive membrane of the cell causes direct stimulation of the electroplaque. The electrical response in denervated as well as in normal preparations recorded with internal microelectrodes is first local and graded with the intensity of the stimulus. When membrane depolarization reaches about 40 mv. a propagated, all-or-nothing spike develops. 2. Measured with internal microelectrodes the resting potential is 73 mv. and the spike 126 mv. The latter lasts about 2 msec. and is propagated at approximately 1 M.P.S. 3. The latency of the response decreases nearly to zero with strong direct stimulation and the entire cell may be activated nearly synchronously. 4. Current flow inward through the caudal membrane of the cell does not excite the latter directly, but activation of the innervated cell takes place through stimulation of the nerve terminals. This causes a response which has a latency of not less than 1.0 msec. and up to 2.4 msec. 5. The activity evoked by indirect stimulation or by a neural volley includes a prefatory potential which has properties different from the local response. This is a postsynaptic potential since it also develops in the excitable membrane which produces the local response and spike. 6. On stimulation of a nerve trunk the postsynaptic potential is produced everywhere in the caudal membrane, but is largest at the outer (skin) end of the cell. The spike is initiated in this region and is propagated at a slightly higher rate than is the directly elicited response. Strong neural stimulation can excite the entire cell to simultaneous discharge. 7. The postsynaptic potential caused by neural or indirect stimulation may be elicited while the cell is absolutely refractory to direct excitation. 8. The postsynaptic potential is not depressed by anodal, or enhanced by cathodal polarization. 9. It is therefore concluded that the postsynaptic potential represents a membrane response which is not electrically excitable. Neural activation of this therefore probably involves a chemical transmitter. 10. The nature of the transmitter is discussed and it is concluded that this is not closely related to acetylcholine. 11. Paired homosynaptic excitation discloses facilitation which is not present when the conditioning stimulus is direct or through a different nerve trunk. These results may be interpreted in the light of the existence of a neurally caused chemical transmitter or alternatively as due to presynaptic potentiation. 12. The electrically excitable system of the electroplaque has two components. In the normal cell a graded reaction of the membrane develops with increasing strength of stimulation until a critical level of depolarization, which is about 40 mv. 13. At this stage a regenerative explosive reaction of the membrane takes place which produces the all-or-nothing spike and propagation. 14. During early relative refractoriness or after poisoning with some drugs (eserine, etc.) the regenerative process is lost. The membrane response then may continue as a graded process, increasing proportionally to the stimulus strength. Although this pathway is capable of producing the full membrane potential the response is not propagated. 15. Propagation returns when the cell recovers its regenerative reaction and the all-or-nothing response is elicited. 16. Excitable tissues may be classified into three categories. The axon is everywhere electrically excitable. The skeletal muscle fiber is electrically excitable everywhere except at a restricted region (the end plate) which is only neurally or chemically excitable. The electroplaque of the eel, and probably also cells of the nervous system have neurally and electrically excitable membrane components intermingled. The electroplaques of Raia and probably also of Torpedo as well as frog muscle fibers of the "slow" system have membranes which are primarily neurally and chemically excitable. Existence of a category of invertebrate muscle fibers with graded electrical excitability is also considered. 17. In the eel electroplaque and also probably in the cells of neurons, tests of the mode of neural activation carried out by direct or antidromic stimulation cannot reveal the neurally and chemically activated component. The data of such tests though they appear to prove electrical transmission are therefore inadequate for the detection and study of the chemically initiated process.     

Mapping of ventricular tachycardia induced by thoracic neural stimulation in dogs

To investigate ventricular tachycardias produced in healthy canine myocardium by stimulation of sympathetic ganglia or cardiac nerves, we simultaneously recorded a surface ECG and 63 ventricular electrograms in anesthetized open-chest dogs. Isochronal and isopotential maps were generated off-line by computer. Ventricular tachycardia with uniform beat-to-beat morphology was induced in 13 or 22 dogs by electrical stimulation of the left stellate ganglion (five experiments), the left middle cervical ganglion (four experiments), the left caudal pole cardiopulmonary nerve (two experiments), or the ventrolateral cardiac nerve (eight experiments). It was not inducible by stimulation of the right-sided major cardiopulmonary nerves or ganglia. In most instances the earliest measured electrical excitation occurred on the posterior aspect of the ventricles. Isochronal maps demonstrated a radial spread of the impulse away from the area of earliest excitation. Changes in the region of earliest excitation and (or) activation pattern were accompanied by changes in QRS morphology. The potential gradients measured between areas displaying positive and negative T waves on the anterior and left lateral aspects of the ventricles were significantly increased by ventrolateral cardiac nerve stimulation. However, the ventricular regions where these potential gradients existed differed from the regions of earliest excitation during ventricular tachycardia. These results demonstrate that the thoracic autonomic nervous system can induce repetitive ventricular excitation originating from consistent loci.     

Intersegmental variation of afferent pathways to giant interneurons of the earthworm,Lumbricus terrestris L.

Summary We have investigated the connectivity of four classes of mechanosensory afferents to giant interneurons in the earthwormLumbricus. Three of these classes of afferents change their specification for connection to medial giant (MGF) and lateral giant (LGF) fibers along the length of the animal. Near the caudal end, stimulation of touch, pressure and small tactile fibers generates excitatory post-synaptic potentials, epsp's, in the two LGF's but not in the MGF. Near the rostral end these afferents produce much smaller epsp's in the LGFs but produce large epsp's in the MGF. In the middle region of the animal an overlap region exists where both giant fibers receive approximately equal inputs from these afferents. The amplitude of these inputs are reduced compared to the maxima seen at either end. The fourth class of sensory afferents investigated, the stretch neurons, have no synaptic effect on the giant fibers anywhere in the nerve cord.These results explain at least part of the basis, in neuronal connectivity, for the differences in response to tactile stimulation of the head and tail segments previously characterized in terms of behavior and giant fiber impulse activity. In this system developmental mechanisms generating synaptic connectivity patterns have coded certain classes of homologous afferent neurons and interneurons to make different connections in different segments.Abbreviations MGF medial giant fiber - LGF lateral giant fiber - SN1 first segmental root - SN2 second segmental root - SN3 third segmental root - RIN giant interneuron     

Visual evoked potentials in the vicinity of the optic tract during stereotactic pallidotomy

We recorded visual evoked responses in eight patients with Parkinson's disease, using a depth electrode either at or below the stereotactic target in the ventral part of the globus pallidus internus (GPi), which is located immediately dorsal to the optic tract. Simultaneously, scalp visual evoked potentials (VEPs) were also recorded from a mid-occipital electrode with a mid-frontal reference electrode. A black-and-white checkerboard pattern was phase reversed at 1 Hz; check size was 50 min of arc. Pallidal VEPs to full field stimulation showed an initial positive deflection, with a latency of about 50 ms (P50), followed by a negativity with a mean latency of 80 ms (N80). The mean onset latency of P50 was about 30 ms. P50 and N80 were limited to the ventralmost of the GPi and the ansa lenticularis. Left half field stimulation evoked responses in the right ansa lenticularis region while right half field stimulation did not, and vice versa. These potentials thus seemed to originate posterior to the optic chiasm. The scalp VEPs showed typical triphasic wave forms consisting of N75, P100 and N145. The location of the recording electrode in the ansa lenticularis region did not modify the scalp VEP. These results suggest that P50 and N80 are near-field potentials reflecting the compound action potentials from the optic tract. Therefore, N75 of the scalp VEPs may represent an initial response of the striate cortex but not of the lateral geniculate nucleus.     

Control of phrenic nerve activity and blood pressure by the medullary raphe nuclei in cats

Electrical and chemical stimulation methods were used to determine the topographic organization of the medullary raphe nuclei (MRN) in controlling the systemic arterial blood pressure (BP) and phrenic nerve activities (PNA). Decerebrated, unanesthetized and bilateral vagotomized cats were used. Effective points in the MRN were systematically explored with constant current stimulation. We found stimulation of the rostral MRN produced a decrease in PNA amplitude and increase in BP and PNA frequency. Stimulation of the caudal MRN produced increases in BP and the amplitude and frequency of PNA. Microinjection of glutamate solution into the caudal or the rostral MRN points produced qualitatively similar results. Thus, we concluded that the caudal MRN neurons had excitatory connections whereas the rostral MRN neurons had excitatory and inhibitory connections to the cardiovascular preganglionic neurons and the phrenic nerve motoneurons.     

Analysis of the formation of evoked potentials in neurons of the rat olfactory cortex

Evoked focal potentials which were induced in vitro in a slice of olfactory tract by stimulation of the lateral olfactory tract (LOT) have been studied. The potential consisted of an initial biphasic wave, the compound action potential of LOT, population synaptic responses, and population spike. Functional significance and possible mechanisms of changes of different focal potential waves have been discussed.     

Innervation pattern and electric organ discharge waveform in Gymnotus carapo (Teleostei; Gymnotiformes)

The electrogenic organ (EO) of Gymnotus carapo has two main portions: a posterior region consisting of four bilaterally arranged electrocyte rows; and an anterior portion composed of only two. The lateral row (LR) of the anterior portion contains doubly innervated electrocytes with axon terminals from different nerves on their rostral and caudal faces. The LR is continuous with the most dorsal row of the caudal region. This row also contains doubly innervated electrocytes. The medial row (MR) electrocytes of the anterior region and ventral rows of the caudal region are exclusively caudally innervated. All caudal faces of the anterior or abdominal region are supplied by two nerves which originate from spinal roots VIII to XXI. Roots I to VII give origin to pure rostral nerves whose electromotor axons terminate on the rostral surfaces of the first seven LR electrocytes. A given doubly innervated electrocyte is supplied on its caudal face by a nerve originating several segments (usually seven) posterior to the spinal root supplying its rostral face. Transections of the spinal cord at the level of root VIII isolate the activity of the rostral surfaces of the first electrocytes. The EO discharge (EOD) then appears as a head negative deflection which arises from abdominally located electrocytes. Its monophasic character reveals that the activity remains restricted to the rostral electrocyte surfaces. Damage of the abdominal portion of the EO abolishes the first negative deflection of the normal pulse. Transections of the spinal cord at the level of root XXI isolate the activity of the whole abdominal portion of the EO. Since both doubly and singly innervated electrocytes remain active, the EOD appears biphasic. Comparative studies have shown that the EOD of Hypopomus sp. lacks any early negative wave and correspondingly all its electrocytes are exclusively caudally innervated.     

Effects of corticospinal tract stimulation on renal sympathetic nerve activity in rats with intact and chronically lesioned spinal cords

Sympathetic preganglionic neurons and interneurons are closely apposed (presumably synapsed upon) by corticospinal tract (CST) axons. Sprouting of the thoracic CST rostral to lumbar spinal cord injuries (SCI) substantially increases the incidence of these appositions. To test our hypothesis that these additional synapses would increase CST control of sympathetic activity after SCI, we measured the effects of electrical stimulation of the CST on renal sympathetic nerve activity (RSNA) and arterial pressure (AP) in alpha-chloralose-anesthetized rats with either chronically intact or chronically lesioned spinal cords. Stimuli were delivered to the CST at intensities between 25-150 muA and frequencies between 25 and 75 Hz. Stimulation of the CST at the midcervical level decreased RSNA and AP. These decreases were not mediated by direct projections of the CST to the thoracic spinal cord because we could still elicit them by midcervical stimulation after acute lesions of the CST at caudal cervical levels. In contrast, caudal thoracic CST stimulation increased RSNA and AP. Neither the responses to cervical nor thoracic stimulation were affected by chronic lumbar SCI. These data show that the CST mediates decreases in RSNA via a cervical spinal system but excites spinal sympathetic neurons at caudal thoracic levels. Because chronic lumber spinal cord injury affected responses evoked from neither the cervical nor thoracic CST, we conclude that lesion-induced or regeneration-induced formation of new synapses between the CST and sympathetic neurons may not affect cardiovascular regulation.