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)     

Intracranial stimulation of the trigeminal nerve in man. II. Reflex responses.

The reflex responses evoked by direct electrical stimulation of the intracranial portion of the trigeminal nerve have been studied in 16 subjects undergoing percutaneous retrogasserian thermocoagulation for the treatment of trigeminal neuralgia affecting the second or third division. In the obicularis oculi muscle, early and late responses similar to the R1 and R2 components of the blink reflex were recorded. The former could be evoked only by stimulation of the second division and its latency was consistent with intermediately fast afferents. A late reflex (50-70 ms) was occasionally recorded from the anterior belly of the digastric muscle. The response was sometimes followed by a later activity and showed the features of a polysynaptic reflex. No response was obtained in the jaw elevators when fully relaxed. With the subject voluntarily clenching his teeth, both an early "H-like" response and two silent periods in the background EMG were obtained. The second silent period was similar in the muscles ipsi- and contralateral to intracranial stimulation, while the first silent period was longer in the ipsilateral muscles. Possible mechanisms contributing to the inhibition following stimulation of the mixed portion of the nerve are discussed.     

Neural organization of the viscero-cardiac reflexes

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

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

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

Spontaneous activity of cat spinal ganglion neurons in vivo

Recordings were made from L4-S1 spinal ganglion neurons of anesthetized cats while their associated dorsal root and the sciatic nerve were left intact, locally anesthetized, or locally anesthetized and sectioned. In all three experimental conditions spontaneous discharges were recorded. These discharges occurred in the absence of any electrical stimulation of the dorsal root or sciatic nerve, and were not due to peripheral exploration of receptive fields or sustained firing in joint or muscle afferents. The spontaneous discharges were relatively rhythmic, and their firing frequency ranged from 5 to 100 impulses per s. Interactions between spontaneous and electrically evoked discharges were observed that depended on the impulse's frequency of firing. High frequency discharges always abolished low frequency impulses regardless of whether these latter were spontaneous or evoked. Extra spikes and postspike events that followed impulses evoked by stimulation of the dorsal root or sciatic nerve were also recorded from some spinal ganglion neurons. These results suggest that spontaneous discharges may originate within the spinal ganglion itself, and that they can occur under normal circumstances.     

Split medulla preparation in the cat: arterial chemoreceptor reflex and respiratory modulation of the renal sympathetic nerve activity

The study was undertaken in order to assess the changes in sympathetic output in a split medulla preparation of the cat which, as shown earlier, has impaired respiratory rhythm generation. The effects of medullary midsagittal sections on renal sympathetic nerve firing were investigated in chloralose anesthetized, paralyzed and artificially ventilated cats. Recordings of phrenic and recurrent laryngeal nerve activity served as indices of central respiratory rhythm generation. Sections, 5 mm deep from the dorsal medullary surface and extending 6 mm rostrally and 3 mm caudally to the obex, did not produce any significant changes in heart rate, blood pressure or tonic renal sympathetic nerve firing levels. They decreased or abolished, however, the respiratory rhythmicity in renal sympathetic nerve which paralleled the reduction of inspiratory discharges in phrenic and recurrent laryngeal nerves, and abolished the carotid body chemoreceptor-sympathetic reflex. The inspiratory activity remaining after the sections could still be enhanced by chemoreceptor stimulation. The inhibitory baroreceptor and pulmonary stretch receptor sympathetic reflexes, and the central excitatory effect of CO2 on renal sympathetic nerve firing were not altered. The effects of electrical stimulation within the midsagittal plane of the medulla have shown that descending pathways from the medullary inspiratory neurons (or their medullary collaterals) do not participate in the facilitation of spinal preganglionic neurons during inspiration and in relaying the pulmonary stretch receptor inhibitory sympathetic reflex. A region located close to the obex was identified from which excitatory responses in renal sympathetic nerves, compatible with the response obtained by carotid sinus nerve stimulation, could be evoked. It is concluded that a lesion in the midline of the lower medulla at the level of the obex selectively destroys cells or pathways which relay the carotid body chemoreceptor-sympathetic reflex.     

Somato-vesical reflexes in chronic spinal cats

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

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

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

Projection of phrenic nerve afferents to the cat sensorimotor cortex

The projection of phrenic nerve afferents to the sensorimotor cortex was studied in cats. The results of these experiments demonstrate that stimulation of phrenic nerve afferents elicits cortical evoked potentials (CEPs) in the sensorimotor cortex of cats. Cortical foci for CEPs classified as primary were found in areas 3b, 3a and 4 gamma. These foci were located medial to forelimb and lateral to hindlimb afferent representations in the sensorimotor cortex.     

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.     

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)     

Discharge characteristics of sympathetic efferents to the knee joint of the cat

Sympathetic postganglionic neurons to the knee joint of the cat were studied to characterize the nerve supply and response to somatic stimulation. In halothane anesthetized cats, the sympathetic postganglionic units from a branch of medial articular nerve (MAN) were dissected. The other branch of MAN was left intact. Most of the central filaments of MAN showed spontaneous discharge. The frequency of the spontaneous discharge of single units ranged from 0.2 to 2.9 impulses per second. Cardiovascular rhythmic modulation was observed in most of the filaments tested. Phenylephrine-induced baroreceptor stimulation caused inhibition of the discharges. Repetitive stimulation of the lumbar sympathetic trunk or the peripheral cut end of MAN led to a decrease in the local temperature inside the joint. The frequency threshold for decreasing the temperature of the joint was approximately 1 Hz. Maximum effects were obtained with 5 Hz stimulation. Histograms of MAN sympathetic efferent fibre activity following electrical stimulation of afferent nerve fibres in the MAN exhibited two response periods. The first, of about 280 ms latency, was elicited by myelinated fibre excitation (the A-reflex) and the second, with a latency of approximately 700 ms, was evoked by unmyelinated fibre excitation (the C-reflex). Electrical stimulation of radial afferent nerve produced similar A- and C-reflex discharges in sympathetic fibres of MAN. Passive movement of the knee joint within its normal working range (flexion, extension or outward rotation) had very little effect on sympathetic efferent nerve activity in MAN, whereas noxious outward rotation of the joint produced a reflex increase in activity to about 140% of the prestimulus control level.(ABSTRACT TRUNCATED AT 250 WORDS)     

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

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

Effects of morphine on somatocardiac sympathetic reflexes in spinalized cats

The effects of morphine on sympathetic reflexes, recorded in the inferior cardiac nerve, to myelinated A and unmyelinated C afferent stimulation were tested in 17 acutely spinalized cats. Stable sympathetic A and C reflexes of short latency (approximately 30 ms and 140 ms in the case of the ulnar nerve, respectively) could be recorded in the inferior cardiac sympathetic nerve to stimulation of somatic A and C afferents in the ulnar and upper thoracic intercostal nerves, ipsilaterally. Spinal sympathetic A reflexes, which were primarily evoked from stimulation of A delta afferent fibers, could be elicited from more segmental levels than could sympathetic C reflexes. Additionally, smaller reflexes, only from A afferent fiber activation, were identified from stimulations on the contralateral side of the body. Small doses of morphine (0.02 mg kg-1, i.v.) proved to be ineffective at altering sympathetic A and C reflexes, while somewhat larger doses (0.2 mg kg-1, i.v.) produced a clear 62% decrease in C reflexes and a 33% decrease in A reflexes, Dosages of 1 and 2 mg kg-1 severely depressed both A and C reflexes. All of the above effects of morphine administration were completely and immediately reversible by naloxone (i.v.). The results are discussed with regard to the effects of morphine on sympathetic A and C reflexes in CNS intact, anesthetized cats.     

Phrenic nerve conduction studies: technical aspects and normative data

In our clinical work we have occasionally encountered difficulties (e.g., no response, concomitant brachial plexus stimulation) in performing phrenic nerve conduction studies. The aim of this study was to overcome these difficulties and obtain our own set of normative data. In 29 healthy volunteers (15 men), aged 21-65 years, phrenic nerve conduction studies were performed using bipolar surface stimulation electrodes and a standard recording montage. Stimulation just above the clavicle, between the sternal and clavicular heads of the sternocleidomastoid muscles, elicited responses at the lowest stimulation strength, without concomitant brachial plexus stimulation. M-wave amplitude and duration changed with respiration, whereas latency and area did not. The normative limit for M-wave latency was 8.0 ms (upper), for amplitude it was 0.46/0.33 mV (lower: inspiration/expiration), and for area it was 4.4 mVms (lower). We suggest a slight modification of the generally used position for phrenic nerve stimulation, and the use of M-wave latency and area (unaffected by the respiratory cycle) in future phrenic nerve conduction studies.     

Comparative study of sympathetic and parasympathetic discharges in the autonomic nerves during stimulation of different structures of the cat hypothalamus

Simultaneous recordings of hypothalamo-parasympathetic and hypothalamo-sympathetic evoked discharges in anesthetized cats demonstrated that both parasympathetic discharges in the pelvic nerve and sympathetic discharges in the splanchnic nerve of the lowest threshold and shortest latency could be obtained from stimulation of the posterior hypothalamus. The focus of the maximal neuronal activated elicited by stimulation of afferent fibres of visceral nerves and the focus evoking maximal efferent reactions of parasympathetic and sympathetic nerves were located in the same region of the postero-lateral hypothalamus. It is supposed (as a working hypothesis) that convergent polysensory neurons of the hypothalamus are also polyeffector divergent elements of the hypothalamo-visceral reflex system.     

Reciprocal and non-reciprocal action of the vagal and sympathetic nerves innervating the heart

Simultaneous recordings were made from vagal and sympathetic fibers innervating the heart in dogs anesthetized with chloralose. Reciprocal relationship between the two autonomic nerves was clearly seen in the baroreceptor reflex. Stimulation of chemoreceptors, however, evoked non-reciprocal responses of the two nerves; at the onset of the chemoreceptor reflex cardiac vagal and sympathetic discharges both increased, then, as baroreceptors became excited due to a pressor response, sympathetic nerve activity suddenly decreased while vagal discharges remained high, indicating the appearance of the reciprocal action typifying the baroreceptor reflex. Decrease in ventilatory volume and a slight increase in end-expired CO2 level augmented greatly both vagal and sympathetic discharges. As the phrenic-locked activity of the two nerves (i.e. the activity in vagus nerve occurs only in the absence of phrenic bursts while sympathetic discharges increase with phrenic bursts) increased, the alternate discharges between the two nerves became more conspicuous and the heart rate fluctuated with the respiratory (phrenic) rhythm. Thus, strong reciprocity between vagus and sympathetic can result in an oscillatory heart rate. When ventilatory volume was increased, both nerve activities decreased below control level. Mild hypoxia had similar effects to hypercapnia though changes in nerve activity were greater. When coactivation of vagal and sympathetic nerve was produced in reflex action, changes in vagal discharges occurred earlier and faster than in the sympathetic fibers. The magnitude of change in vagus activity was also far greater. The elimination of afferents in the vagi, the aortic and sinus nerves reduced cardiac vagal activity greatly. However, discharges were still present and occurred between phrenic bursts, indicating that the vagal "tone" is maintained centrally as well as peripherally by input from receptors in the cardiovascular system. The physiological significance of reciprocal and non-reciprocal control of vagal and sympathetic nerves innervating the heart was discussed.     

Descending lumbosacral cord potentials (DLCP) evoked by stimulation of the median nerve

In 22 normal human subjects, descending lumbosacral cord potentials (DLCP) were recorded intrathecally after stimulation of the median nerve at the elbow. The onset of DLCP is very short in latency (mean 12.1 ms) with a prominent sharp early positive peak (mean latency 13.7 ms) followed by a sharp negative peak (mean 17.6 ms). The amplitude of the first part of DLCP varied between 0.6 and 6.7 microV (mean 2.3 microV). The response was recorded most easily when the tip of intrathecal electrode was posterolaterally positioned. The threshold of the response was above or around the excitation threshold of the motor nerve fibers and it could not be produced by pure skin nerve stimulation. It resisted to subtetanic peripheral shocks. Mean peripheral conduction velocity responsible for the response was about 60.8 m/s. Some late and slower deflections appeared in many cases. It was concluded that the DLCP must have originated from the descending and very fast conducting propriospinal pathways located within the anterolateral funiculus which has an oligosynaptic anatomical organization. This response seemed to be the first direct evidence of interlimb reflex action between the arm and leg in man which is important in the coordination of movements and posture.     

The somato-adrenal medullary reflexes in rats

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

Intracranial stimulation of the trigeminal nerve in man. III. Sensory potentials.

Percutaneous electrical stimulation of the trigeminal root was performed in 18 subjects undergoing surgery for idiopathic trigeminal neuralgia or implantation of electrodes into Meckel's cave for recording of limbic epileptic activity. All subjects had normal trigeminal reflexes and evoked potentials. Sensory action potentials were recorded antidromically from the supraorbital (V1), infraorbital (V2) and mental (V3) nerves. In the awake subject, sensory potentials were usually followed by myogenic artifacts due to direct activation of masticatory muscles or reflex activation of facial muscles. In the anaesthetised and curarised subject, sensory potentials from the three nerves showed 1.4-2.2 ms onset latency, 1.9-2.7 ms peak latency and 17-29 microV amplitude. Sensory conduction velocity was computed at the onset latency (maximum CV) and at the peak latency (peak CV). On average, maximum and peak CV were 52 and 39 m/s for V1, 54 and 42 m/s for V2 and 54 and 44 m/s for V3. There was no apparent difference in CV between subjects with trigeminal neuralgia and those with epilepsy. A significant inverse correlation was found between CV and age, the overall maximum CV declining from 59 m/s (16 years) to 49 m/s (73 years). This range of CV is compatible both with histometric data and previous electrophysiological findings on trigeminal nerve conduction. Intraoperative intracranial stimulation is also proposed as a method of monitoring trigeminal function under general anaesthesia.