An intracellular study of the synaptic input to sympathetic preganglionic neurones of the third thoracic segment of the cat

In chloralose anaesthetized, paralyzed and artificially ventilated cats intracellular recordings were obtained from sympathetic preganglionic neurones (SPN) of the third thoracic segment of the spinal cord identified by antidromic stimulation of the white ramus T3. The synaptic input to SPNs was assessed, in cats with intact neuraxis or spinalized at C3, by electrical stimulation of segmental afferent fibres in intercostal nerves and white rami of adjacent thoracic segments and by stimulation of the ipsi- and contralateral dorsolateral funiculus and of the dorsal root entry zone of the cervical spinal cord. In both preparations SPNs showed on-going synaptic activity which predominantly consisted of excitatory post-synaptic potentials (EPSPs). Inhibitory post-synaptic potentials (IPSPs) were rarely observed. EPSPs were single step (5 mV) or, less frequently, large (up to 20 mV) summation EPSPs. The proportion of SPNs showing very low levels of on-going activity was markedly higher in spinal than in intact cats. Stimulation of somatic and sympathetic afferent fibres evoked early EPSPs (amplitude 3 mV, latency 5-22.3 ms), and late, summation EPSPs (amplitude up to 20 mV, latency 27-55 ms). Early and late EPSPs were evoked in nearly all SPNs in which this synaptic input was tested in the intact preparation (from 79-93% of the SPNs). In spinal cats, early EPSPs were evoked in 88% of the SPNs, whereas late EPSPs were recorded only in half of the neurones. No evidence for a monosynaptic pathway from these segmental afferent fibres to SPNs was obtained. In both intact and spinal cats, stimulation of the dorsolateral funiculus evoked early and late EPSPs in SPNs. Late EPSPs were recorded in 70% and 37% of the SPNs in intact and spinal cats, respectively. Early EPSPs, however, were evoked in all neurones. The early EPSPs evoked by stimulation of the dorsolateral funiculus had several components which are suggested to arise from stimulation of descending excitatory pathways with different conduction velocities. The following conduction velocities were calculated in intact (spinal) cats: 9.5-25 m/s (7.8-13.2 m/s), 5.7-9.5 m/s (5.5-7.8 m/s), 3.8-5.7 m/s (3.2-5.5 m/s), and 2.6-3.8 m/s (2.1-3.2 m/s). EPSPs of these various groups were elicited in a varying percentage in SPNs. EPSPs of the most rapidly conducting pathway were subthreshold for the generation of action potentials; some EPSPs of this group had a constant latency suggesting a monosynaptic pathway to SPNs. Stimulation of the dorsal root entry zone at the cervical level yielded essentially the same results as stimulation of the dorsolateral funiculus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Conduction velocity in spinal descending pathways of baro- and chemoreceptor reflex

Activity in the white rami T3 and L2 or 3 has been recorded and averaged with respect to excitation of the carotid sinus baroreceptor afferents produced by the pulsatile blood pressure (baroreceptor reflex) and with respect to brief trains of electrical stimuli exciting low threshold chemoreceptor afferents in the left carotid sinus nerve (chemoreceptor reflex). Experiments were performed on chloralose anaesthetized cats with both vago-depressor nerves cut. From the latency difference between the onset of the responses at the thoracic and their arrival at the lumbar level the spinal conduction velocity for the pathway of each reflex has been calculated. The baroreceptor reflex pathway has slower spinal conduction velocity 3.3 +/- 0.7 m/sec than the chemoreceptor pathway 5.5 +/- 0.9 m/sec. These results indicate that there are separate descending spinal pathways for the two types of reflexes.     

The involvement of serotonin neurones in the inhibition of renal nerve activity during desynchronized sleep

Numerous 5-hydroxytryptamine (5-HT)-containing cell bodies were visualized by fluorescence microscopy in the caudal brainstem rostal to the decussation of the pyramids in a region from which a desynchronized sleep-like pattern of sympathetic activity was obtained in a previous study. In unanaesthetized mid-collicular decerebrated cats recordings were made of sympathetic activity in a renal nerve. The inhibition of renal nerve activity occurring during desynchronized sleep-like state induced by physostigmine was attenuated significantly by procedures which interfered with the pathways from the 5-HT-containing neurones. Small cuts in the dorsolateral funiculus of the cervical spinal cord reduced the inhibition from43 ± 6%to14.0 ± 3%. Microinjection of 5,7-dihydroxytryptamine into cervical spinal cord reduced the serotonin content of the thoracic cord by 22.4% and attenuated the desynchronized sleep-like state inhibition of renal nerve activity by a similar amount. Depletion of serotonin withp-chlorophenylalanine significantly reduced the inhibition of renal nerve activity during the desynchronized sleep-like state, from42.5 ± 5%to10.0 ± 2.0%. It was suggested that serotonin-containing neurones are likely to be involved in the inhibition of renal nerve activity occurring during desynchronized sleep.     

Spinal sympathetic conduction velocity in humans

Simultaneous micro-electrode recordings of muscle sympathetic activity were made in the radial nerve at the mid-humerus level and the peroneal nerve at the fibular head in 8 healthy subjects. Sympathetic impulses occurred spontaneously in multi-unit bursts time-locked to the cardiac rhythm. There was a high degree of similarity between radial and peroneal neurograms with the radial bursts preceding corresponding peroneal ones by approximately 0.35 s. Utilizing this latency difference and previously determined values for peripheral sympathetic postganglionic conduction velocities, we calculated that the spinal conduction velocity for muscle sympathetic activity is 2.8 +/- 0.7 m/s (mean +/- SD). The result agrees with similar data from experimental animals.     

Motor conduction velocity in the human spinal cord: slowed conduction in multiple sclerosis and radiation myelopathy.

Transcutaneous electrical stimulation of the central nervous system was used to measure motor conduction velocity in the human spinal cord in 21 subjects aged 22 to 75 years (mean 55 years), none of whom had neurological disease. The motor conduction velocity between the sixth cervical (C6) and first lumbar (L1) vertebral levels was 67.4 +/- 9.1 m/s. This probably represents conduction velocity in the corticospinal tracts. In these subjects the motor conduction velocity in the cauda equina, between the first lumbar (L1) and fourth lumbar (L4) vertebral levels, was 57.9 +/- 10.3 m/s. In four of five patients with multiple sclerosis, all with corticospinal signs in the legs, motor conduction velocity between C6 and L1 was slowed (41.8 +/- 16.8 m/s), but cauda equina conduction was normal (55.8 +/- 7.8 m/s). Similar slowing of spinal cord motor conduction was found in a patient with radiation myelopathy. This method should provide a relevant, simple clinical test in patients with spinal cord disease.     

Reticulospinal tracts involved in the spino-bulbo-spinal reflex in cats

Twenty-three chloralosed cats were used to examine the spinal descending pathways of the spino-bulbo-spinal (SBS) reflex.Transection of the ventrolateral funiculus in the spinal cord at the thoracic level abolished the SBS reflex of caudal spinal segments ipsilateral to the transection, but did not abolish the ascending propriospinal reflex and SBS reflex of rostral segments.Unit discharges elicited in axons of the ventrolateral funiculus at L₃ by sural nerve stimulation had the appropriate latency for mediating the SBS reflex. These axons originated in the medial bulbar reticular formation, since the responses had a consistent short latency and followed repetitive stimulation of the bulbar reticular formation (up to a rate of 300 Hz). Conduction velocities of reticulospinal axons ranged widely from 20 to 120 m/sec. There were two peaks, fast (95 m/sec) and slow (35 m/sec). The fast conducting fibers showed oligo-spikes, high amplitude and wide distribution of latency. Almost 90% of the axons with slow conduction exhibited multi-spikes, low amplitude and narrow distribution.Unitary responses of the ventral rootlet at L₇ were elicited by sural nerve stimulation and correlated with the latency of the SBS reflex. The unitary response was also evoked by train pulse stimulation of the ventrolateral funiculus in the spinal cord. Conduction velocities of descending spinal tracts ranged from 20 to 60 (mean±S.D.,35 ± 8) m/sec.We may conclude that the descending spinal pathway of the SBS reflex is the slowly conducting reticulospinal tract which originates in the medial bulbar reticular formation and passes through the ventrolateral funiculus of the spinal cord.     

Electrophysiological determination of the axonal projections of single dorsal respiratory group neurons to the cervical spinal cord of cat

Antidromic microstimulation and orthodromic extracellular spike-triggered averaging were used to determine the axonal positions, divergence and terminations of 16 axons arising from bulbospinal, inspiratory (I) neurons. Activity from these neurons was recorded in the dorsal respiratory groups (DRG) of 12 cats. Systematic tracking was done both transversely and longitudinally in the contralateral fifth and sixth cervical segments of the spinal cord. Axonal position was determined by antidromically activating axons and by recording axonal field potentials. Thirteen axons were located in the lateral funiculus, two in the ventrolateral funiculus and one in the ventral funiculus. Axonal conduction velocity (CV) was calculated from conduction distance and conduction time, the latter defined as the interval of time from the recorded action potential in the medulla to the recorded averaged axonal potential in the spinal cord. Average (+/- S.D.) axonal CV was 52 +/- 15 m/s. Terminal potentials were recorded for 13 of these axons and were coincident with the location of evoked field potentials resulting from antidromic stimulation of phrenic motoneurons. In addition, terminal potentials from single I cells were recorded at multiple sites along the longitudinal axis of the phrenic motor column. These data indicate that axons of spontaneously active, DRG bulbospinal I cells descend predominantly in the lateral columns and diverge and terminate extensively within the phrenic motor column.     

Sympathetic neurons in the cat spinal cord projecting to the stellate ganglion.

Horseradish peroxidase (HRP) was injected into the stellate ganglia of anesthetized cats. After a 2-day survival time the spinal cord was processed for the HRP reaction to study the distribution of the preganglionic neurons which project to the stellate ganglia. HRP-labeled neurons in the cord were located exclusively on the side ipsilateral to the injected stellate ganglia and were concentrated in four distinct areas: (1) intermediolateral cell column (78.2%), (2) lateral funiculus (18.5%), (3) intercalated area and (4) central autonomic area (3.3%) for the latter 2 areas). Their distribution pattern in the very rostral pole of the thoracic sympathetic preganglionic cell column was different from the typical thoracic pattern in that a greater proportion of the cells were located in the lateral funiculus, and a few labeled cells were observed in the ventral horn. Longitudinally, a wide range (C8-T8) of spinal cord levels projected to the stellate ganglia, with a peak at the T2 level. These observations infer that a large population of axonal processes of sympathetic preganglionic neurons traveled several segments of the spinal cord through an intraspinal pathway before exiting a particular ventral root.     

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.     

Electrophysiological identification of spinally projecting neurons in the lateral reticular nucleus of the rat

Eighty-four neurons in the caudal ventrolateral medullary reticular formation were antidromically activated by the stimulation of the dorsolateral funiculus in 49 urethane-anesthetized rats. Of 76 neurons, 37 had no spontaneous discharge. Of the neurons that had spontaneous discharges, 80% had firing rates between 0.1 and 15 Hz. The average conduction velocity, determined among 70 neurons, was 15.20 +/- 1.23 m/s, and 87% had conduction velocities within the range of 2-30 m/s. This study further confirms the existence of spinally-projecting neurons in the lateral reticular nucleus (LRN) of the caudal medulla, and some of them are probably responsible for the descending controls of nociception from the LRN.     

Near-nerve needle sensory conduction study of the medial calcaneal nerve: New method and report of four cases of medial calcaneal neuropathy

There has been one previously published antidromic method for studying medial calcaneal nerve (MCN) conduction. However, the origin of the compound nerve action potentials (CNAPs) with this technique is uncertain because of the antidromic nature of stimulation. We report a new orthodromic method for MCN conduction study using the near-nerve needle technique. In 35 normal controls, maximum nerve conduction velocity (NCV) and negative-peak NCV of MCN were 42.4 +/- 3.9 m/s and 33.6 +/- 3.0 m/s, respectively. The amplitude of the CNAP was 4.1 +/- 2.2 micro V. We also report four cases of medial calcaneal neuropathy, three of which were confirmed by this technique. We conclude that the present technique is capable of recording the sensory nerve action potentials of MCN in isolation and confirming the diagnosis of medial calcaneal neuropathy electrophysiologically.     

Experimental uremic neuropathy. Part 1. Decreased nerve conduction velocity in rats

Acute and chronic renal failure were induced in adult male albino rats by conventional surgical methods. The glomerular filtration rate of rats with chronic uremia was reduced to approximately 15% of normal. Despite this drastic degree of renal failure, the motor nerve conduction velocity (MCV) increased from 47.5 +/- 6.4 m/s, preoperatively, to 55.6 +/- 4.0 m/s after 4 weeks. In rats with acute uremia, MCV decreased from 51.4 +/- 3.0 m/s to 45.2 +/- 2.7 m/s after 48 h. The conduction velocity, measured in vitro of excised sciatic nerves was 55.2 +/- 4.4 m/s in acute uremia and 64.7 +/- 4.7 m/s in a control group. There was no difference in the refractory period between these groups. It is concluded that this decrease in the nerve conduction velocity was not caused by changes in the composition of the extracellular fluid but rather by changes in the nodal membrane function.     

The spinal somatosensory evoked potentials in amyotrophic lateral sclerosis in relation to the spinal cord conduction velocities]

The lumbar-to-cervical conduction velocity (spinal cord conduction velocity, SCCV) was electrophysiologically studied in 14 patients with amyotrophic lateral sclerosis (ALS). The age of these patients ranged from 37 to 63, averaging 51.0 years old. We recorded the spinal somatosensory evoked potentials (SSEPs) from the surface electrodes at the level of the C2 spine and the T12 spine by the simultaneous stimulation of bilateral posterior tibial nerves. SCCV from the lumbar to cervical was measured from the latency difference between both SSEPs elicited at the each position. As the results, SCCVs were in the range of 50.6-66.6 (58.6 +/- 4.7: mean +/- SD) m/sec in normal age matched controls (18 adult volunteers, 46-63 years old, averaging, 52.7). On the other hand, in ALS patients, SCCVs were in the range of 42.1-67.1 (53.5 +/- 7.8: mean +/- SD) m/sec, values of which were lowered compared to those in normal subjects. These examination documented 4 out of 14 patients with ALS (28.6%) showing abnormalities beyond standard deviation. The vibration sense was checked by using 128 Hz tuning fork at the ankles, and for the quantitative measurement, a newly designed vibriometer being attached the piezoelectric accelerometer to the end of 128 Hz tuning fork was applied in 14 ALS patients. The vibration sense at the ankles was diminished in 6 patients, and 3 patients showed the abnormalities beyond 2 standard deviations. The degree of lowering in SCCVs among ALS patients were correlated with the degree of diminution of impaired vibration sense and the duration of illness, but were not correlated with the H/M ratio and the latency difference between T wave and H wave. Since SSEP impulses are transmitted in dorsal columns and dorsolateral fasciculus predominantly by large diameter and fast-conduction fibers, our results may suggest that, in ALS patients, spinal cord conduction velocities of ascending fibers mediating the dorsal columns and dorsolateral fasciculus are disturbed compared to those in normal subjects, and that the functional disturbance of ascending fibers mediating the dorsal columns and dorsolateral fasciculus plays the important role in the high rates of impaired vibration sense among ALS patients.     

Inhibition of nociceptive neurons in the shell region of nucleus ventralis posterolateralis following conditioning stimulation of the periaqueductal grey of the cat. Evidence for an ascending inhibitory pathway.

Recordings were made from neurons in the ventroposterior lateral nucleus (VPL) of urethane chloralose-anaesthetised cats, following both noxious mechanical stimulation of the integument, and electrical stimulation of the inferior cardiac nerve. The effects of stimulating the periaqueductal grey (PAG), or the nucleus raphe dorsalis (NRD) on the responses obtained from these units were also investigated. Units responding to noxious mechanical stimulation of the integument, with inferior cardiac nerve input, were found only around the periphery of the posterior half of VPL. Responses elicited from these units by electrical stimulation of the inferior cardiac nerve were inhibited following electrical stimulation of either the ventral PAG or the NRD. Inhibition following PAG or NRD stimulation could still be demonstrated after bilateral section of the dorsolateral funiculi at the junction between C3 and C4, although the degree of inhibition decreased. Responses elicited from these units by electrical stimulation of the anterolateral funiculus were also inhibited following PAG or NRD stimulation. These data suggest that PAG or NRD stimulation-produced inhibition of these units may be partially mediated by an ascending pathway, in addition to the well-known descending spinal pathways in the dorsolateral funiculi of the spinal cord.     

Some characteristics of sympathetic preganglionic Neurones in the rat

In anaesthetized rats sympathetic preganglionic neurones (SPN) were identified by their antidromic response to stimulation of the ipsilateral cervical sympathetic trunk (CST). Units were recorded at a depth of 0.75-1.1 mm from the dorsal surface of the spinal cord. The majority of SPN had axonal conduction velocities less than 1 m/s. Units could be routinely held for periods of up to 1 h. Spontaneously active SPN had discharge rates within the range 0.3-8.0 Hz. Application of horseradish peroxidase (HRP) to the central cut end of the CST resulted in the labelling of neurones only on the ipsilateral side, mainly in the intermediolateral cell column (IML) and the lateral funiculus and between the first and third thoracic (T) segments.     

Differential sensitivity of thick and thin fibers to HIV and therapy-induced neuropathy

The study assessed HIV-related and anti-retroviral therapy-induced neuropathy in myelinated and unmyelinated nerve fibers. One hundred consecutive HIV patients were examined clinically and standard nerve conduction velocities were measured. In addition, electrically induced sympathetic skin response (SSR) was assessed in the palms and soles. The difference in delay of SSR in palms and soles (DeltaSSR) was calculated as an indirect measure of C-fiber conduction velocity. Thick fiber conduction velocities significantly decreased with age and increasing stage of the disease, whereas no effect of stage was found for DeltaSSR (p=0.6). In contrast, medication of at least one of the most known neurotoxic drugs zalcitabine, stavudine, or didanosine did not result in significantly lower conduction velocities in thick fibers (51.29+/-3.4 m/s vs. 50.86+/-3.5 m/s), but was related to an increased DeltaSSR. DeltaSSR allows an indirect measurement of C-fiber conduction velocity. In HIV this measure of unmyelinated sympathetic fibers was most sensitive to anti-viral treatment whereas conduction velocity of myelinated somatic fibers was more sensitive to disease-related neuropathy. The results suggest that HIV neuropathy preferably affects myelinated and anti-retroviral therapy unmyelinated fibers.     

Identification and properties of sub-retrofacial bulbospinal neurones: a descending cardiovascular pathway in the cat

Experiments were performed on 21 chloralose-anaesthetized, paralyzed cats to identify electrophysiologically the bulbospinal vasomotor neurones of the rostral ventrolateral medulla. Neurones were recorded from a small region close to the ventral medullary surface (sub-retrofacial nucleus), where previous studies had localized points where microinjected excitant amino acids produced large pressor responses. A total of 157 neurones in this region were antidromically activated by electrical stimulation in the spinal cord, 132 from the ipsilateral dorsolateral funiculus at lower cervical level and 25 from the intermediolateral horn at T1. Of the cells tested, 33/34 were excited by iontophoresis of homocysteic acid, indicating that recordings originated from cell bodies rather than axons. The positions of 16 were marked by dye deposition, and located histologically to the sub-retrofacial nucleus. Stimulation of baroreceptors by inflating a carotid sinus blind sac to 200 mm Hg powerfully inhibited 41/48 bulbospinal neurones tested. The threshold for this effect was 80-100 mm Hg. Baroreceptor-sensitive neurones all had axonal conduction velocities between 2 and 8.8 m/s, and within this range, all but two cells were barosensitive. Barosensitive cells were unaffected by low threshold somatosensory stimulation. Baroreceptor stimulation was still able to silence these neurones' activity even when this was raised several-fold by iontophoresis of homocysteic acid. It is argued that sub-retrofacial bulbospinal neurones probably constitute a major descending vasomotor pathway, and that baroreceptors act by inhibiting them postsynaptically.     

Soleus H-reflex to S1 nerve root stimulation

H-reflexes in normals were elicited by percutaneous electrical and magnetic stimulation of proximal nerve roots at the cauda equina. H-M interval to S1 nerve root stimulation at the level of the S1 foramen was 6.8 +/- 0.33 ms, with side to side difference of 0.16 +/- 0.13 ms. Compression/ischemia of the sciatic nerve in the mid-thigh abolished the H-reflex to stimulation of the tibial nerve at the popliteal fossa when the H-reflex to S1 nerve root stimulation was preserved. The length of the S1 nerve root in human cadavers was measured to be 17.5 +/- 03 cm, providing an estimated dorsal root conduction velocity of 67.3 m/s and a ventral root conduction velocity of 54 m/s. We conclude that the H-M interval to S1 root stimulation can provide reliable measures of conduction within the spinal canal including proximal afferents, anterior horn cells and ventral roots.     

Conduction velocity and excitability of A and C fibers of the mesenteric nerve in the cat]

Conduction velocity and electrical excitability of mesenteric nerve fibres were studied in experiments on cats. These nerves were shown to consist of: 1) fibres with an excitation threshold of 0.06-0.10 V (stimuli duration being 0.1 ms) and with maximum conduction velocity of 48-85 m/s; 2) fibres with a threshold of 0.3-0.7 V and with conduction velocity from 8-10 to 33-39 7/s; 3) fibres with a threshold above 1 V and with conduction velocity from 1.8 to 7 m/s; 4) fibres with a threshold 6-8 V and conduction velocity below 1.8 m/s. The fibres of the first three groups are the afferent A-fibres. According to conduction velocity and wave amplitude in the compound action potential the fibres of the last group are divided into three subgroups. The first and third subgroups are shown to consist of afferent C-fibres; the second subgroup - of both afferent and postganglionic sympathetic C-fibres.