Investigations on the nervous mechanisms underlying the somatosensory cervical response in man.

The main features (amplitude, latency and shape) of the cervical activity evoked by stimulation of the median nerve, recorded throughout the cervical spine, have been concurrently investigated by monopolar, bipolar longitudinal and bipolar transverse recordings. In some subjects the derivation C7-Sn (suprasternal notch) has been employed as well. A comparative evaluation of the refractory period of each component of the cervical responses under investigation has been performed to differentiate presynaptic from postsynaptic events. Additional information has been obtained by cervical activity recorded by longitudinal and transverse bipolar derivations upon stimulation of the lower limb. It was thus demonstrated that both presynaptic and postsynaptic events were responsible for the cervical sensory evoked potential, as appearing when recorded against a cranial reference (that is the upper midfrontal region). The structures involved were the brachial plexus (N9), the cervical roots (P10 and a minor part of N11a), the dorsal columns both at caudal (N11a) and rostral (N11b) cervical levels, and the dorsal column nuclei (N13). However a contribution of the spinal segmental activity to the postsynaptic portion of the cervical response, more specifically to N13, should be considered as well, though direct evidence is still inadequate.     

The cervical somatosensory evoked potential in man: far-field, conducted and segmental components

Somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded from neck and scalp electrodes in 23 normal adults using cephalic and non-cephalic (knee) references simultaneously. With a cephalic reference, the neck SEP consisted of several 'negative' potentials that had the same latency at all recording locations. Simultaneous recordings from neck-scalp, neck-knee and scalp-knee derivations demonstrated that scalp far-field potentials significantly contributed to neck-to-scalp recordings and obscured the cervical SEPs. With a non-cephalic reference, the neck SEP consisted of a prominent positive wave (P9) followed by a large negative component (N13). A small positive potential, P10, seen in the lower neck, gradually increased in latency and amplitude from lower to upper neck and appeared as a P11 potential at upper cervical levels. In lower neck recordings, a negative wave, N11, was also present and in some subjects exhibited a latency shift from lower to upper neck. P9, P11 and N11 had a short refractory period suggesting a presynaptic origin whereas N13 had a longer refractory period indicating a postsynaptic generator. The consensus that P9 originates in the peripheral nervous system is consistent with its rapid recovery cycle. The bipolar characteristics of N11 and P11 as well as their latency shift and their short recovery cycle suggest that they reflect activity in the cervical dorsal columns. N13, that displayed no latency shift and had a longer recovery cycle, may originate in spinal cord dorsal horn interneurons.     

Skin and epidural recording of spinal somatosensory evoked potentials following median nerve stimulation: correlation between the absence of spinal N13 and impaired pain sense

Summary A clinical lesion study and intraoperative epidural recordings were made to test the origin and clinical significance of the spinal N13 and P13 of somatosensory evoked potentials (SEP) that follow median nerve stimulation. Intraoperatively, the respective peak latencies of spinal P13 and N13 coincided with those of the N1 component of the dorsal cord potential and its phase reversed positivity. On both the ventral and dorsal sides of the cervical epidural space, maximal amplitude was at the C5 vertebral level to which nerve input from the C6 dermatome is the main contributor. The modality of sensory impairment in the hand dermatome was examined in selected patients with cervical lesions, who showed such normal conventional SEP components as Erb N9, far-field P9, P11, P14, N18 and cortical N20, with or without loss of spinal N13. Statistically, the loss of spinal N13 was associated with decrease of pain sensation in the C6 dermatome. This was interpreted as being due to damage to the central grey matter of the cord, including the dorsal horn. Our results suggest the spinal N13 and P13 originate from the same source in the C6 spinal cord segment and that they are good indicators for the detection of centromedullary cervical cord damage.     

Epidurally recorded cervical somatosensory evoked potential in humans

Three slow wave components, P10, N13 and P18, can be seen in the cervical somatosensory evoked potential (CSEP) in response to median nerve stimulation recorded by an electrode in the epidural space at the dorsal aspect of the cervical spinal cord referenced to an electrode at the suprasternal notch. In the region of high CSEP amplitude, which extends over several cervical segments, the peak-to-peak amplitude is more than 10 microV, permitting observation of the CSEP slow waves in single, unaveraged records. The CSEP to finger nerve stimulation had a similar wave form and the same latencies (referred to the Erb's potential) as the CSEP to median nerve stimulation. The P10 activity is of presynaptic origin; it is generated in the brachial plexus, spinal roots and terminal branches of the primary sensory fibers. The N13 slow wave is of postsynaptic origin; however, the small wave on the ascending phase of this main postsynaptic component represents superimposed presynaptic activity. In bipolar epidural recordings, 3-5 fast waves are superimposed on the slow CSEP waves, which are of lower amplitude than the slow waves in unipolar recordings. The fast waves show a slight but progressive delay at the more rostral recording sites and are present even with high frequency stimulation, presumably reflecting activity in long ascending tracts. The surface recorded CSEP to median nerve stimulation is 4-7 times lower in amplitude than the CSEP in unipolar epidural recordings. The small wave on the ascending phase of N13 and the N13 peak of the unipolar epidural recordings had the same latencies as the surface N11 and N13 peaks.     

Subcortical somatosensory evoked potentials after median nerve and posterior tibial nerve stimulation in high cervical cord compression of achondroplasia

Children with achondroplasia may have high cervical myelopathy from stenosis of the cranio-cervical junction resulting in neurological disability and an increased rate of sudden death. To detect myelopathy we recorded somatosensory evoked potentials after median nerve (MN) and posterior tibial nerve (PTN) stimulation in 77 patients with achondroplasia aged 0.3–17.8 years (mean 2.7 years). In addition to the conventional technique of recording the cortical components and the central conduction time (CCT) we employed non-cephalic and mastoid reference electrodes to record the subcortical waveforms N13b and P13 (MN-SEP) as well as P30 (PTN-SEP), respectively, which are generated near the cranio-cervical junction. The findings were related to the MRI results. Thirty-four patients had abnormal MRI findings including spinal cord compression (n = 28) and/or myelomalacia (n = 24) at or below the cranio-cervical junction. The sensitivity of the MN-SEPs was 0.74 including all abnormal upper cervical cord MRI findings (specificity 0.98), and the sensitivity was 0.79 (specificity 0.92) for cervical cord compression, respectively. The sensitivity of the PTN-SEPs was 0.52 (specificity 0.93) for all abnormal MRI findings and 0.59 (specificity 0.92) for cervical cord compression. The subcortical SEPs N13b and P13 as well as P30 were more sensitive than the conventional recordings. The MN-SEPs, notably the subcortical tracings, are useful for the detection of cervical myelopathy in children with achondroplasia. The PTN-SEPs are less sensitive. However, the tibial nerve SEPs might contribute additional information from the lumbar or thoracic spinal cord, which was, however, not tested in this study.     

Anatomic origin of the cervical N13 potential evoked by upper extremity stimulation.

There is a large consensus, based on converging evidence, that N13 recorded at lower cervical levels has a segmental postsynaptic origin in the gray matter of the cervical cord and that because of the orientation of its dipole field, the Cv6-anterior cervical derivation should be used whenever the diagnostic problem requires that this potential be assessed selectively in terms of latency and amplitude. The diagnostic utility of the lower cervical N13 recording in dorsal horn deafferentation and in lesions at the Cv6-Cv8 metameric levels has been validated in all types of cervical cord lesions. Unfortunately, such clear-cut conclusions do not apply to the N13 potential recorded at upper cervical levels. Currently, this component is not considered to provide enough reliable information, in addition to P13-P14 scalp recordings, to be used routinely in the diagnosis of cervicomedullary lesions.     

Vector short-latency somatosensory-evoked potentials after median nerve stimulation

A new method has been developed for recording short-latency somatosensory-evoked potentials after median nerve stimulation. Negative electrical forces recorded with three orthodiagonal bipolar electrodes in the neck had a direction opposite to that of impulse conduction in the proximal peripheral and cervical somatosensory pathway. Sequential tracings of vectors opposite the electrical forces were made in three-dimensional display, thus reproducing the actual time sequence of electrical events in those structures. Fixed generators such as the subcortical nuclei were also analyzed with this technique, and multiple generators of N13 potential (N13a and N13b) were visualized. This technique may be useful in the functional evaluation of the somatosensory pathway in the cervical cord.     

Short-latency somatosensory evoked potentials in brain death

Summary Simultaneous recording of somatosensory evoked potentials to median nerve stimulation above the upper and lower neck in brain-dead patients revealed that all cervical responses were preserved in 10%, whereas a marked reduction in amplitude or even loss of N 13b at the level of the C2 spinous process was observed in 90%. Of the patients, 55% revealed an additional loss of N 13a, recorded at the level of the C7 spinous process; in 15% all cortical and spinal evoked potentials were missing, but Erb's point waves were still normal. These results suggest two different origins of the main negative waves (N 13a and N 13b), recorded above the upper and lower cervical spinal cord. N 13a (C7) is supposed to arise in the dorsal horn at the C6/7 level, N 13b (C2) in the cervicomedullary junction.     

Subpially recorded cervical spinal cord evoked potentials in syringomyelia.

Intraoperative spinal cord evoked potentials (SCEPs) to median nerve stimulation were detected subpially from the dorsal surface of the cervical spinal cord in 5 patients with cervical syringomyelia and were compared to normal SCEPs obtained from the unaffected side in 6 patients during intraoperative monitoring of dorsal root entry zone lesion. Normal SCEP began with a positive deflection P9 and a complex N11/N13 with several low amplitude short potentials superimposed on the N11/N13. The complex was followed by a second negative potential N2 and a late prolonged positivity, P. In the 4 patients in whom median nerve somatosensory evoked potentials (SEPs) were present preoperatively, SCEP consisted of the N11 potential and the following low amplitude short (LAS) potentials, while the N13 wave was missing. In the fifth patient, in whom the preoperative median nerve SEP was missing, SCEPs were of much lower amplitude and shorter duration than normal. The potentials N2 and P were not recorded in any of our 5 patients. Changes in N13 wave, N2 and P potentials noted in syringomyelia were presumed to be the result of destruction of the spinal cord dorsal horn neurons caused by spinal cord central cavitation.     

Intra-operative spinal cord neuromonitoring in patients operated on for intramedullary tumors and syringomyelia

Intra-operative neuromonitoring is of great help to the neurosurgeon and enables him to operate more precisely and with less risk of post-operative neurological deficit. The purpose of the study was to define the changes of the somatosensory evoked potentials (SEP) elements in intramedullary tumors and syringomyelia, and to show the correlation between the changes of the elements and the location of the lesions in the spinal cord. Thirty patients with pain syndromes, intramedullary tumors and syringomyelia were operated on the spinal cord. The methodology of the intra-operative neuromonitoring was based on electrical stimulation of peripheral mixed nerve (tibial and median), and on intra-operative subpial recording of conducted and interneuronic SEP from the dorsal surface of the spinal cord. The recordings in pain syndrome cases were normal, and were compared with recordings obtained in syringomyelia and tumors. The most stable element of the conducted SEP are the initial negative waves that originate in the spinocerebellar tract. Deterioration of the negative high amplitude potentials is the most sensitive indicator of damage to the somatosensory system of the spinal cord, and indicates damage to the dorsal columns. The N11 and N14 waves are the most stable elements of the cervical and lumbosacral interneuronic SEP, and most probably originate in the dorsal root entry zone. Changes of the N13 and N17 elements of interneuronic SEP suggest damage to the spinal cord gray matter, and are most frequently changed in intramedullary lesions. High frequency waves seen on N13 or N17 probably reflect the somatosensory long tracts, and are even better seen in syringomyelia and pure intramedullary tumors. Changes in the elements of interneuronic and conductive SEP enable us to localize the anatomical site of the lesion, which is of great help to the operating neurosurgeon.     

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.     

A comparative analysis of short-latency somatosensory evoked potentials in man, monkey, cat, and rat

The spatial and temporal properties of the early portion of the somatosensory evoked potential (SEP) were assessed in normal human subjects and in monkeys, cats, and rats. When recorded from comparable loci, the SEP evoked by median nerve stimulation was similar in waveform and topography in all species, suggesting direct correspondences between human and animal components. To assess the neural origins of this activity, simultaneous surface and depth recordings were obtained from cats and monkeys, and the effects of lethal doses of sodium pentobarbital were studied in all animals. Components identifiable in most humans and in the animals tested, and the structures thought to be their primary generators, are: N10, peripheral nerve at the level of the brachial plexus; N12a and N12b, primary afferent fibers at caudal and rostral levels of the cervical cord; N13a, dorsal horn; N13b, cuneate nucleus; N14, medial lemniscus; P15, n. ventralis posterolateralis; P16 and P18, uncertain; N20 or P20, primary somatosensory cortex.     

Noninvasive measurement of sensory action currents in the cervical cord by magnetospinography

ObjectiveTo obtain magnetic recordings of electrical activities in the cervical cord and visualize sensory action currents of the dorsal column, intervertebral foramen, and dorsal horn.MethodsNeuromagnetic fields were measured at the neck surface upon median nerve stimulation at the wrist using a magnetospinography system with high-sensitivity superconducting quantum interference device sensors. Somatosensory evoked potentials (SEPs) were also recorded. Evoked electrical currents were reconstructed by recursive null-steering beamformer and superimposed on cervical X-ray images.ResultsEstimated electrical currents perpendicular to the cervical cord ascended sequentially. Their peak latency at C5 and N11 peak latency of SEP were well-correlated in all 16 participants (r = 0.94, p < 0.0001). Trailing axonal currents in the intervertebral foramens were estimated in 10 participants. Estimated dorsal–ventral electrical currents were obtained within the spinal canal at C5. Current density peak latency significantly correlated with cervical N13–P13 peak latency of SEPs in 13 participants (r = 0.97, p < 0.0001).ConclusionsMagnetospinography shows excellent spatial and temporal resolution after median nerve stimulation and can identify the spinal root entry level, calculate the dorsal column conduction velocity, and analyze segmental dorsal horn activity.SignificanceThis approach is useful for functional electrophysiological diagnosis of somatosensory pathways.     

The human cervical and lumbo-sacral evoked electrospinogram. Data from intra-operative spinal cord surface recordings.

We have undertaken the analysis of the human 'evoked electrospinogram' during intra-dural surgical explorations in 20 patients. Averaged spinal cord surface evoked potentials to peripheral nerve electrical stimulation were obtained from various restricted loci on the pial surface of the cervical and lumbo-sacral spinal cord. The brachial plexus P9 potential and its lumbo-sacral counterpart P17 were recorded as ubiquitous initial far-field positivities. The pre-synaptic compound action potentials N11 and N21 dwelt on the ascending slope of N13 and N24 respectively. They were composed of 1-5 sharp peaks and collected from the dorsal and dorso-lateral positions mainly, on the cervical and lumbo-sacral cord respectively. They are thought to be generated in the proximal portion of the dorsal root, the dorsal funiculus and the afferent collaterals to the dorsal horn. Compound action potentials could also be gathered from the surface of the dorsal roots, the cervical N10 and lumbo-sacral N19 potentials. The large cervical N13 and lumbo-sacral N24 waves originate from a dorso-ventral post-synaptic dipole, generated in deep laminae of the dorsal horn during the activation of large diameter afferent fibers. These waves were maximal on the main entry cord segments of the stimulated nerves and fell off on the 1-4 more rostral and caudal segments. The N2 wave is the dorsal component of another post-synaptic dorso-ventral dipole generated in deep laminae of the dorsal horn but activated by medium diameter afferent fibers. The latest event was the N3 wave, also possibly part of a dorso-ventral post-synaptic dipole, and generated by cells in the dorsalmost and deep dorsal horn laminae during the activation of small diameter afferent fibers. The P wave was a prolonged positive deflection which carried the N2 and N3 waves. It is the manifestation of pre-synaptic inhibition on primary afferent fibers. A supra-segmental ascending spinal cord volley was also described, composed of a long succession of sharp and low voltage peaks.     

The effects of profound hypothermia on the cervical SEP in humans: evidence of dual generators

Somatosensory evoked potentials recorded over Erb's point and the cervical spine (at C2 and C7) were studied in a series of children undergoing cardiopulmonary bypass surgery with hypothermia alone (n = 15) or profound hypothermia and complete circulatory arrest (n = 15). A bifid response was recorded at normothermia or mild degrees of hypothermia at both C7 (N12a, N13a) and C2 (N12b, N13b). The differential responses of these components to profound hypothermia and ischaemia suggest that N12a and N12b represent components of the same dorsal root/dorsal column travelling wave, while N13a and N13b reflect postsynaptic activities which are thought to be generated at the dorsal horn and cuneate nucleus, respectively.     

Generators of human spinal somatosensory evoked potentials

Somatosensory evoked potentials recorded over the spine with a noncephalic reference following posterior tibial nerve stimulation have several components. (1) A stationary, synapse-dependent, negative potential (N22) occurs synchronously with a positive potential, P22, recorded ventral to the spinal cord and is localized to the lumbar region overlying the lumbar root entry zone. The N22/P22 complex is attributed to activation of interneurons in the dorsal gray of the lumbar cord. (2) A traveling negative potential with a gradually increasing latency may be recorded from the sacral to the cervical region. Its short refractory period indicates that it is not dependent on transmission across a synapse. This activity is attributed to transmission of the afferent volley through the lumbosacral plexus, roots, and the dorsal columns of the spinal cord. (3) N29, a stationary, synapse-dependent negative potential, localizes to the rostral cervical spine and is attributed to activation of the gracile nucleus relay cells. Following stimulation of the median nerve or fingers, the waveforms recorded over the cervical spine with a noncephalic reference include (1) the proximal plexus volley, a traveling negative potential reflecting transmission through the proximal brachial plexus and roots; (2) the dorsal column volley (DCV), the latency of which gradually increases from the caudal to rostral cervical region (the DCV is attributed to transmission of the afferent volley through the dorsal columns of the cervical cord); and (3) N13, a stationary negative waveform, with a long refractory period consistent with its dependence on transmission across a synapse. Experimental animal and human studies indicate that the N13 waveform is dependent on activity of at least two generator sites, namely the dorsal gray of the cervical cord and the cuneate nucleus.     

Chronically implanted electrodes for repeated stimulation and recording of spinal cord potentials

We have recorded and characterized the spinal cord evoked potentials (SCEPs) from the epidural space in the halothane-anesthetized rats. A group of 11 adult Wistar male rats was chronically implanted with two pairs of epidural electrodes. SCEPs were repeatedly elicited by applying electrical stimuli via bipolar U-shaped electrodes to the dorsal aspect of the spinal cord at C3-4 or Th11-12 levels, respectively. Responses were registered with the other pair of implanted electrodes, thus allowing us to monitor the descending (stimulation cervical/recording thoracic) and ascending SCEPs (stimulation thoracic/recording cervical). We studied the time-dependent changes of several SCEP parameters, among them the latency and amplitude of two major negative waves N1 and N2. During 4-weeks' survival, all major components of recordings remained stable and only minor changes in some parameters of the SCEPs were detected. We concluded that this technique enables repeated quantitative analysis of the conductivity of the spinal cord white matter in the rat. Our results indicate that SCEPs could be used in long-term experiments for monitoring progressive changes (degeneration/regeneration) in long projection tracts, primarily those occupying the dorsolateral quadrants of the spinal cord. These include projections that are of interest in spinal cord injury studies, i.e. ascending primary afferents, and important descending pathways including corticospinal, rubrospinal, reticulospinal, raphespinal and vestibulospinal tracts.     

Somatosensory evoked potentials after multisegmental upper limb stimulation in diagnosis of cervical spondylotic myelopathy.

Radial, median, and ulnar nerve somatosensory evoked potentials (SEPs) were recorded, with non-cephalic reference montage, in 38 patients with clinical signs of cervical myelopathy and MRI evidence of spondylotic compression of the cervical cord. Upper limb SEPs are useful in spondylotic myelopathy because SEPs were abnormal in all patients for at least one of the stimulated nerves and SEP abnormalities were bilateral in all patients but one. Reduction of the amplitude of the N13 potential indicating a segmental dysfunction of the cervical cord was the most frequent abnormality; it occurred in 93.4%, 84.2%, and 64.5% of radial, median, and ulnar nerve SEPs respectively. A second finding was that the P14 far-field potential was more sensitive than the cortical N20 potential to slowing of conduction in the dorsal column fibres. The high percentage of N13 abnormalities in the radial and median rather than in the ulnar nerve SEPs correlated well with the radiological compression level, mainly involving the C5-C6 vertebral segments. Therefore the recording of the N13 response is a reliable diagnostic tool in patients with cervical spondylotic myelopathy and P14 abnormalities, though less frequent, can be useful in assessing subclinical dorsal column dysfunction.     

Assessment of intraspinal and intracranial conduction by P30 and P39 tibial nerve somatosensory evoked potentials in cervical cord, brainstem, and hemispheric lesions.

In routine recordings of tibial nerve somatosensory evoked potentials (SEPs), a global central conduction time is evaluated by measuring the interval between the segmental spinal N22 potential, recorded in the lumbar region, and the cortical P39 potential. In this study, we tested the reliability of the scalp far-field P30 potential, which originates in the vicinity of the cervico-medullary junction, in order to evaluate separately intraspinal and intracranial conduction in normal subjects and patients with cervical cord and intracranial lesions. P30 and cortical P39 potentials were studied in 23 healthy subjects and in 70 patients with cervical cord (n = 47), brainstem (n = 11) or hemispheric lesions (n = 12) selected on the basis of neuroimaging--computed tomography (CT) or magnetic resonance (MR)--findings. Median nerve SEPs were also recorded in all patients. Of the several montages tested to obtain the P30 potential, the Fpz-Cv6 derivation gave the highest signal-to-noise ratio; it permitted to obtain a P30 potential that peaked at 29.2 +/- 1.6 ms in all normal subjects. P30 abnormalities were observed only in patients with cervical or cervico-medullary lesions; these were associated with a normal P39 in only two of 33 abnormal recordings. Conversely, P30 was consistently normal in lesions situated above the cervico-medullary junction whether associated with normal, delayed, or reduced P39. P30 abnormalities were subclinical in 42% of abnormal recordings. All patients with normal tibial and median nerve SEPs on both sides had normal touch, joint, and vibration sensation in the four limbs. There was a strong correlation between tibial nerve P30 and median nerve P14 data in the whole series of patients; both potentials behaved similarly in all cases of intracranial supramedullary lesions. Combined abnormalities of P30 and P39 potentials thus indicate that conduction is impaired at the spinal level and proved to be particularly informative for detecting spinal cord dysfunction in patients with neuroimaging evidence of a narrowed cervical canal. Recording of abnormal N13, P14, or P30 potentials provided evidence of a cervical cord dysfunction in 66% of patients who had a suspected spondylotic myelopathy. Recording of tibial nerve P30 potential has proven to give reliable and useful information when a separate assessment of intraspinal and intracranial somatosensory conduction is needed; it merits inclusion, as does the upper limb N13 potential, in the evaluation of patients whose MR image indicates cervical canal narrowing.     

Short latency potentials recorded from the neck and scalp following median nerve stimulation in man.

Short latency evoked potentials were recorded from sites overlying the cervical and thoracic vertebrae, the clavicles, mastoid processes and cerebral cortex, following percutaneous stimulation of median nerve fibres at the elbow, wrist and fingers in 23 normal subjects. At least four major early components each with simultaneous positive and negative constituents, plus the first component (N20) of the cortical response, were all found to be mediated by sensory afferent fibres with conduction velocity 65--75 m/sec in the forearm of one subject. Study of the distribution of these potentials, using reference electrodes located at Fz or over the lower part of the spine, has led to the proposal of generator sites in the brachial plexus (N9), spinal roots or dorsal columns (N11), spinal grey matter or brain stem (N13), and brain stem or thalamus (N14). Comparison with intrathecal recordings in man lends support to the view that N11 and N13 are generated in or adjacent to the spinal cord. It is hoped the findings may extend the clinical applications of a non-invasive technique for investigating the afferent sensory pathways in man.