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

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

Field potentials evoked in rat primary somatosensory cortex (SI) by impulses in cutaneous A beta- and C-fibres

A projection of cutaneous C-afferent fibres to the contralateral primary somatosensory cortex (SI) was examined in the halothane-anaesthetized rat. Field potentials evoked by electrical stimulation of the right sural nerve were recorded at the surface of an intracortically within the left SI cortex. A late surface positive potential (latency 110-190 ms, mean 136 ms) was evoked by sural stimulation at a strength that activated A- and C-fibres. A selective anodal block of impulse conduction in A-fibres proximal to the stimulating electrodes showed that impulses in C-fibres generate the late potential also in the absence of a preceding A-fibre input. Stimulation of the sural nerve at two sites caused a shift in latency of the late potential corresponding to a conduction velocity of the primary afferent fibres of less than 1.0 m/s. The distribution of the C-fibre-evoked potential in SI was similar to that of the A beta-fibre evoked 'primary' potential suggesting that the investigated projection of cutaneous C-fibres to SI has a somatotopic organization.     

Longitudinal distribution of negative cord dorsum potentials following stimulation of afferent fibres in the left inferior cardiac nerve

Cord dorsum potentials were recorded along the spinal cord following electrical stimulation of afferent fibres of the left inferior cardiac nerve in chloralose anaesthetized cats. The potentials were more pronounced in spinal than in intact cats. Afferent fibres which generated cord dorsum potentials in the cervical spinal cord were localized mainly in T2 and T3 and to a smaller extent in C8 and T1 dorsal roots. The responses consisted of two waves: with short (7.0 ms; N3 wave) and long (56 ms; N4 wave) latency to the onset of potentials. N3 and N4 waves were generated by group III and group IV afferent fibres, respectively. The N3 wave was maximal at C8 and T1 spinal cord level and could be detected at least 5-6 segments rostrally from the level of afferent input responsible for its generation. The N4 wave could be detected at least 4 segments rostrally from its afferent fibre input. We conclude that afferent fibres from the left inferior cardiac nerve activate neurones in the cervical spinal cord. The implications of such finding are discussed.     

Three transverse dipolar generators in the human cervical and lumbo-sacral dorsal horn: evidence from direct intraoperative recordings on the spinal cord surface

In the context of the intraoperative study of spinal cord surface evoked potentials in patients operated upon for chronic pain and spasticity, we have undertaken an analysis of the dipolar dorso-ventral organization of surface spinal cord evoked potentials in man. Averaged evoked potentials to peripheral nerve electrical stimulations were obtained from the dorsal and ventral pial surface of the cervical and lumbo-sacral spinal cord (7 pairs from 5 patients), using a small silver ball macroelectrode, positioned during open neurosurgical approaches. We found that the dorsally recorded N13 and N24 waves reversed into ventral P13 and P24 waves respectively. A second negative potential, N2, and a late prolonged positivity, P, similarly reversed into a P2 and an N wave respectively. Our data add up to a collection of skin, oesophageal, epidural, pial and intramedullary recordings in man and animals to provide the evidence for a transverse dipolar organization of the human postsynaptic N13, N24 and N2 potentials, originating from deep layers of the cord dorsal horn, and for a similar organization of the P wave, which has been shown to correlate with presynaptic inhibition on primary afferent fibres.     

Interfering cutaneous stimulation and the muscle afferent contribution to cortical potentials

Cerebral potentials were recorded in response to selective stimulation using microelectrodes of muscle afferents in motor fascicles innervating the intrinsic muscles of the foot or at the motor point of abductor hallucis. The early components of these potentials (P40, N50 and P60) were consistently attenuated by continuous tactile stimulation of related skin areas and by electrical stimulation of digital nerves, timed so that the digital volley reached cortex approximately 5 msec before the muscle afferent volley. The same conditioning cutaneous inputs also attenuated the cerebral potentials evoked by selective stimulation of cutaneous afferents. These findings confirm that there are intermodality and intramodality interactions between low-threshold cutaneous and muscle afferents and between cutaneous afferents, respectively. The findings indicate that 'interference phenomena' (Kakigi and Jones 1986) can occur between different afferent modalities, and within any one modality, and cannot be used to determine the afferent species responsible for the test evoked potential.     

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)     

STUDIES ON the HUMAN EVOKED ELECTROSPINOGRAM

Evoked potentials from the human spinal cord were studied in 39 normal volunteers. Intrathecal recordings from lower cervical and lower thoracic intervertebral levels were made after the supramaximal stimulation of the median, ulnar and posterior tibial nerves, respectively. It was shown that the segmental cord potentials varied in shape and size according to the spatial relationship between the position of the electrode tip and the spinal cord and roots within the vertebral canal. Three main types of segmental evoked responses were obtained. One of them was recorded behind the cord dorsum and around the midline, and was composed of fast, sharp early, and slow, late components. This was called a CD potential and its first component was related to the activity of the ascending dorsal funiculus fibres. The second evoked response was the DR potential, and this triphasic compound action potential of very high amplitude and longer duration had no remarkable late component. It was recorded when the tip of the intrathecal electrode was lateral to the midline within the vertebral canal, and was then related mostly to activity of the spinal roots. Another kind of potential was called PH potential. It had a very small triphasic spike and two later components with prominent negativities being higher than the first spike. This potential might be related to the electrode tip position facing, and close to, the posterior horn of the spinal gray matter. Late components of the segmental evoked potentials were related to the pre- and post synaptic activity of the horizontally oriented fibre within the segmental gray matter of the posterior horn.     

Method of studying conduction velocity in sensory fibers of the peripheral nerves and spinal cord. The averaging technic

The author describes a method of measuring conduction velocity in sensory fibres based on averaged evoked potentials. In the method potentials are recorded with surface electrodes over the nerves of the upper extremity at three sites: wrist, elbow and branchial plexus. The further route of the central neuron was represented by two responses from the cervical spinal cord at the C2 level, and first component N1 of the cortical response. The method makes possible distinguishing of damage to the peripheral part from that to the central part of the sensory system. Checking of the method in 15 healthy subjects made possible determination of conduction velocity which was stable for all measuring points in the peripheral nerve and spinal cord and was 75 m/sec, while at the distance from the spinal cord to the cortex it was only 58 m/sec. The author discusses in detail the technique of recording and reading of evoked potentials, whose amplitudes may be below 0.5 microV and latency is read with an accuracy of 0.1 msec.     

Cerebral evoked potentials after stimulation of the posterior urethra in man.

Cerebral evoked potentials (EPs) were recorded in 25 neurologically normal subjects aged 22-73 years (mean 44.0) after stimulation of the posterior urethra (PU) and the pudendal nerve. After maximal PU stimulation 2 different configurations of the potential were found. In 12 cases a simple bi-triphasic wave form was recorded while in 12 cases there was a bifid form of the first negative wave. In 1 case identical EPs were recorded after PU and pudendal nerve stimulation. It was concluded that (1) PU stimulation excites fibres in the pudendal nerve at higher stimulation strength, resulting in a bifid wave form of the cerebral evoked potential in some individuals, (2) the most prominent negative peak, N1, with a latency of 102.1 +/- 13.2 msec, is the most reproducible part of the PU-evoked potential, (3) the N1 is probably transmitted through A delta fibres localized in the pelvic nerves, (4) there are differences between individuals concerning pudendal and pelvic nerve involvement in afferent innervation of the urethra.     

Presynaptic modulation of synaptic effectiveness of afferent and ventrolateral tract fibers in the frog spinal cord

In the isolated frog spinal cord, antidromic stimulation of motor nerves produces intraspinal field potentials with a characteristic spatial distribution. When recording from the ventral horn, there is a short latency (1–2 msec) response corresponding to activity generated by antidromic activation of motoneuron cell bodies and proximal dendrites. In addition, in the dorsal horn, a delayed wave (12–13 msec latency) corresponding in time with the negative dorsal root potential is also recorded. This wave (VR-SFP) is positive at the dorsal surface of the cord and inverts to negativity at more ventral regions. The negative VR-SFP is maximum between 300–500 μm depth from the dorsal surface and decays with increasing depth towards the motor nucleus. Six days after chronic section of the dorsal roots L7 to L9 in one side of the spinal cord, stimulation of the motor nerves on the deafferented side produces only the early response attributable to antidromic activation of motoneurons. No distinctive VR-SFPs are recorded at any depth within the cord. These findings are consistent with the interpretation that afferent fiber terminals are the current generators of the VR-SFP. The presynaptic and postsynaptic focal potentials recorded in the motor nucleus after stimulation of the ventrolateral tract, as well as the corresponding synaptic potentials electrotonically recorded from the ventral roots, are not depressed during conditioning stimulations which produce primary afferent depolarization. This contrasts with the depression of the presynaptic and post-synaptic focal potentials and synaptic potentials produced by stimulation of sensory fibers. It is concluded that, unlike the afferent fiber terminals, the terminals of the ventrolateral tract are not subjected to a presynaptic modulation of the type involving primary afferent depolarization.     

Evoked potentials recorded from scalp and spinous processes during spinal column surgery

Peroneal nerve evoked potentials were simultaneously recorded from scalp and from wire electrodes inserted into lumbar and thoracic spinous processes at multiple levels during surgery for correction of spinal column curvature in 43 patients. Spinal potentials progressively increased in latency rostrally. Over cauda equina and rostral spinal cord initially positive triphasic potentials were recorded. Over caudal spinal cord the response consisted of initial positive-negative diphasic potentials that merged with broad large negative and positive potentials. At rapid rates of stimulation, the initial diphasic component was stable but the subsequent potentials significantly diminished in amplitude. This suggests that the diphasic component reflects presynaptic activity arising in the intramedullary continuations of dorsal root fibers and that the subsequent components reflect largely postsynaptic activity. Scalp recordings at restricted bandpass (30-3000 c/sec) revealed well defined positive and negative potentials with mean peak latencies of 25.9 and 29.9 msec (PV-N1). The amplitudes and latencies of PV-N1 remained relatively stable throughout general anesthesia with halogenated agents which suggests that this component may be a reliable monitor of conduction within spinal cord afferent pathways during spinal surgery. Data are presented which suggest that selective filtering may help to distinguish faster frequency, synchronous axonal events from slower frequency, asynchronous axonal or synaptic events.     

Characteristics of sympathetic reflexes evoked by electrical stimulation of phrenic nerve afferents.

In chloralose-anaesthetized cats, sympathetic reflex responses were recorded in left cardiac and renal nerve during stimulation of afferent fibres in the ipsilateral phrenic nerve. In cardiac nerve, a late reflex potential with a mean onset latency of 75.6 +/- 13.8 ms was regularly recorded which, in 20% of the experiments, was preceded by an early, very small reflex component (latency between 35 and 52 ms). In contrast, in renal nerve only a single reflex component after a mean latency of 122.1 +/- 13.1 ms was observed. Bilateral microinjections of the GABA-agonist muscimol into the rostral ventrolateral medulla oblongata resulted in a nearly complete abolition of sympathetic background activity and in an 88% reduction of the late reflex amplitude with only small effects on the latency of the evoked potentials. Under this condition, an early reflex component was never observed to appear. After subsequent high cervical spinalization, the residual small potentials which persisted after bilateral muscimol injections were completely abolished and in cardiac nerve an early reflex potential with a mean latency of 45 +/- 10 ms was observed in all but one experiment. The early reflex was therefore referred to as a spinal reflex component which, however, is suppressed in most animals with an intact neuraxis. In the renal nerve a spinal response was only observed in one experiment after spinalization. The results suggest that sympathetic reflexes evoked by stimulation of phrenic nerve afferent fibres possess similar spinal and supraspinal pathways as previously described for somato-sympathetic and viscero-sympathetic reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Clinical application of laser evoked potentials using the Nd:YAG laser.

The clinical interest of a new type of laser evoked potentials (LEPs) using Nd:YAG laser was assessed in the diagnosis of peripheral neuropathies affecting the small-diameter nerve fibres, and of spinal cord lesions, affecting the spinothalamic tract. Twelve patients aged from 26 to 79 years with sensory neuropathies (n = 6) or spinal cord lesions (n = 6) underwent neurophysiological examination of the lower limbs comprising quantitative sensory testing, i.e., the determination of vibratory and thermal thresholds (VT and TT), somatosensory evoked potentials (SEPs) to electrical stimulation and Nd:YAG LEPs. VT and SEPs were used to assess large-diameter afferent nerve fibres and the lemniscal pathways while TT and LEPs were used to assess small-diameter afferent nerve fibres and the spinothalamic tract. In addition, patients with peripheral neuropathy underwent also standard nerve conduction studies to explore large fibres and the recording of sympathetic skin responses (SSRs) to explore small fibres, whereas motor evoked potentials were performed in patients with spinal cord lesion. LEPs were absent bilaterally in all patients with polyneuropathy, even when TT remained within the normal limits and SSRs were present. LEPs were absent after stimulation of the affected limb in all patients with a spinal cord lesion, and allowed to detect subclinical contralateral lesion in two cases. LEPs following Nd:YAG laser stimulation are sensitive in the diagnosis of peripheral and/or central nervous system disorders and they give complementary information as compared to routine electrophysiological tests.     

Electrophysiological actions of the rubrospinal tract in the anaesthetised rat

The rubrospinal tract (RST) of the rat is widely used in studies of regeneration and plasticity, but the electrophysiology of its spinal actions has not been described. In anaesthetised rats with neuromuscular blockade, a tungsten microelectrode was located in the region of the red nucleus (RN) by combining stereotaxis with recording of antidromic potentials evoked from the contralateral spinal cord. Single stimuli through this electrode typically elicited two descending volleys in the contralateral dorsolateral funiculus (DLF) separated by about 1 ms, and one volley recorded from the ipsilateral DLF. Latencies of the ipsilateral and the early contralateral volley were similar. The activation of these volleys depended on the location of the stimulation site in or near the RN. Evidence is adduced to show that: (a) the late contralateral volley is carried by fibres of RST neurones, synaptically activated; (b) the early contralateral volley is mostly carried by RST fibres stimulated directly; (c) the ipsilateral volley is sometimes carried by RST fibres from the RN on the side contralateral to the stimulus; (d) otherwise, either early volley may derive from fibres in other tracts. Synaptic potentials related to the volleys were recorded within the cervical enlargement and their distribution plotted on cross-sections of the spinal cord. These measurements suggest that the great majority of RST terminations are on interneurones in the intermediate region contralateral to the RN. Direct synaptic actions on motoneurones are likely to be weak. Stimulation parameters appropriate for specific activation of the RST in future studies are suggested.     

Properties of a spinal somatosensory evoked potential recorded in man.

Somatosensory evoked potentials were recorded from the skin overlying the cervical and lumbar spinal cord of man after stimulation of median and tibial nerves respectively. The early negative component (N11) of the cervical potential and the negative lumbar potential (N14) were studied. The spatial properties of N11 and N14 indicate a spinal cord origin. Evidence partly from threshold studies, shows that the low threshold cutaneous afferent fibres were responsible for activating the generators of the potentials. A conditioning test stimulation procedure supports a postsynaptic generator. It is concluded that N11 and N14 have properties similar to the cord dorsum potentials recorded in animals and probably have the same generator, the neurones of the dorsal horn.     

Effects of attention on auditory evoked middle latency potentials

The effects of attention on the auditory evoked middle latency potentials (MLPs) were examined in 23 normal subjects. Early positive-negative-positive-negative waves (about 8, 11, 15 and 18 msec peak latencies) and a late positive one (about 30 msec peak latency) were recorded from the unilateral mastoid-nose tip. The amplitudes of the early portions of MLPs to binaural 50 dB SL clicks during attention decreased as compared to those obtained when the subjects were not attending to the test stimuli. No significant difference was seen in the latency of any early wave.     

Effects of Attention on Auditory Evoked Middle Latency Potentials

Abstract: The effects of attention on the auditory evoked middle latency potentials (MLPs) were examined in 23 normal subjects. Early positive-negative-positive-negative waves (about 8, 11, 15 and 18 msec peak latencies) and a late positive one (about 30 msec peak latency) were recorded from the unilateral mastoid-nose tip. The amplitudes of the early portions of MLPs to binaural 50 dB SL clicks during attention decreased as compared to those obtained when the subjects were not attending to the test stimuli. No significant difference was seen in the latency of any early wave.     

Cortical evoked potentials following stimulation of the urinary bladder in man

Techniques are described for recording evoked potentials in humans in response to stimulation of the urinary bladder. Twenty patients with various disorders of their urinary function were investigated. Evoked cortical potentials following stimulation of the urinary bladder could be recorded from all patients except subjects with complete transverse spinal cord lesions. The response was most evident at the central site (Cz) and had its most prominent peaks at 45, 65, 82 and 102 msec. This technique may become a useful diagnostic tool in the study of neurogenic bladder dysfunctions.     

Properties of femoral venous afferent inputs and lumbosacral distribution of spinal evoked activity

The present studies were done to determine details of the anatomical and physiological characteristics of femoral-saphenous venous afferent input to the lumbar spinal cord. Gross anatomical examination revealed that afferent bundles could be seen coursing from the saphenous nerve to the femoral-saphenous vein. Compound action potentials elicited by femoral-saphenous venous afferent stimulation were recorded from the femoral nerve and in dorsal rootlets of the 6th lumbar cord segment. The compound action potentials included activity correlated with that of fibers conducting impulses at the rate of 31 to 61 m/s. Lumbar cord dorsum potentials elicited by femoral-saphenous venous afferent stimulation were abolished by rhizotomy of the most caudal rootlets of the 6th lumbar cord segment. L6 was also the cord segment from which the largest amplitude cord dorsum negative potentials were recorded, while action potentials with large late positive waves were recorded from more caudal cord segments. These observations suggested that the L6 segment contained the largest number of spinal neurons responding to primary femoral-saphenous venous afferent input, and that input reached the more caudal segments via intersegmental connections. A proposed physiological role of these afferents is briefly described.     

Interactions of somatosensory evoked potentials: simultaneous stimulation of two nerves.

To analyse the mechanism by which sensory inputs are integrated, interactions of somatosensory evoked potentials (SEPs) in response to simultaneous stimulation of two nerves were examined in 12 healthy subjects. Right, left and bilateral median nerves were stimulated in random order so that a precise comparison could be made among the SEPs. The arithmetical sum of the independent right and left median nerve SEPs was almost equal within 40 msec of stimulus onset to that evoked by the simultaneous stimulation of bilateral median nerves. However, a difference emerged after 40 msec. The greatest difference was recorded after 100 msec. Sensory information from right and left median nerves may interact in the late phase of sensory processing. Left median, left ulnar, and both nerves together were stimulated. The sum of the SEPs of left median and ulnar nerves was not equal to that evoked by the simultaneous stimulation of the two nerves even at early latencies. Differences between them were first recorded at 14-18 msec and became greater after 30-40 msec. It is suggested that the neural interactions between impulses in the median and ulnar nerves begin below the thalamic level.