S Ratto

Proprioceptive modulation of somatosensory evoked potentials during active or passive finger movements in man.

The effects of active and passive finger movements on somatosensory potentials evoked by stimulation of the median nerve at the wrist or of finger I were investigated in 15 healthy volunteers. As compared to the resting condition, both active and passive movements of the stimulated hand fingers induced a marked reduction in the amplitude of the primary cerebral response (N20-P25 complex) as well as of the N17 SEP component, which is supposed to reflect the activity of the thalamo-cortical radiation. The following cerebral SEP components, within 100 ms after the stimulus, were also depressed during motor activity. Neither N11 nor N13 components of the cervical response, reflecting the activation of dorsal columns and dorsal column nuclei respectively, were modified. The SEP changes induced by active or passive movements were absent after ischaemic block of large group I afferent fibers from the hand, thus suggesting the relevance of the feedback generated by such peripheral afferents during movement. The results indicate that the activation of peripheral receptors (probably muscle spindle endings) during both active and passive finger movement would induce a gating effect at both cortical and subcortical (thalamic) level, which might modulate selectively the different sensory inputs to the cortex.

New subcortical components of the cerebral somatosensory evoked potential in man

Two new components of the human SEP upon stimulation of the contralateral median nerve at the wrist have been identified. Such components have been called N16 and N17, according to their polarity and latency. N16 and N17, as well as the N14‐P15 complex, are generated by separate subcortical dipoles. Particularly, they are supposed to be far‐field reflections of the activity of the dorsal columns nuclei or the medial lemniscus (N14‐P15), the thalamus (N16) and the thalamo‐cortical radiation (N17). Moreover, it has been established that N14 is the very first intracranial component of the human SEP, the main peak of S wave and the preceding ones being extracranial in origin.

Assessment of motor neuron excitability in parkinsonian rigidity by the F wave

SummaryF-wave responses from abductor pollicis brevis muscle occurred more frequently, with a larger amplitude and longer duration in rigid parkinsonian patients than in age-matched normal controls. F-wave potentiation during voluntary contraction was impaired in parkinsonian patients. These findings suggest that spinal motor neuron excitability is enhanced in rigidity. F-wave amplitude was significantly correlated to the clinical evaluation of motor disability, so that the F wave may be regarded as a useful approach to quantitative evaluation of rigidity.

The effect of hand muscle vibration on the somatosensory evoked potential in man: an interaction between lemniscal and spinocerebellar inputs?

The effect of hand muscle mechanical vibration on the somatosensory potential (SEP) evoked by median nerve stimulation, was investigated in 10 healthy subjects. A marked decrease in the amplitude of the N17, N20 and P25 components of the cerebral SEP was observed, while the S11 and S13 components of the cervical response did not change. The amplitude reduction of the SEP components was larger when low frequency vibration was used. Recordings performed after cooling the hand further suggest that the reduction of the amplitude of the SEP components induced by vibration is likely to depend on activation of muscle receptors. These findings could reflect an interaction between limniscal and spino-cerebellar inputs, possibly occurring at the thalamo-cortical levels, a concept compatible with the hypothesis that muscle spindle afferents do contribute to kinaesthesia or position-sense.

Short-latency somatosensory evoked potentials in degenerative and vascular dementia.

Short-latency somatosensory evoked potentials (SEPs) were recorded from 54 patients with dementia as compared to 32 age-matched controls. SEPs were generally normal in patients with senile dementia of Alzheimer type, while patients with multi-infarct dementia showed a prolonged central conduction time, an increased latency of both N13 and N20 and a reduction of the primary cortical response amplitude. These findings suggest that recording SEPs may be useful in the differential diagnosis between degenerative dementia and multi-infarct dementia.

Electroneurographic correlates of the monosynaptic reflex: experimental studies and normative data.

The neurographic concomitants of the monosynaptic reflex, evoked either by electrical stimulation of the tibial nerve at the popliteal fossa or by percussion of the Achilles tendon, have been recorded from the sciatic nerve in the lower and middle thigh. Neurographic recordings were characterised by two travelling waves (P1 and P2), respectively increasing and decreasing in latency in the proximal direction, that showed the same chronological trend of the propagated action potentials concurrently recorded in the dorsal and ventral spinal roots at the lumbar level. At variance with P2, the speed of propagation of the P1 volley was stimulus-related, being faster on mechanical than on electrical stimulation, probably because in the latter case the latency of the fastest afferents is overestimated. The P2 volley is subserved by alpha-efferent fibres in either case as suggested, inter alia, by the strict parallelism between the P2 volley and the monosynaptic reflex under appropriate experimental conditions. Simultaneous recordings of spinal root and sciatic nerve action potentials allowed the direct assessment of afferent and efferent conduction velocities, both in the proximal (that is from the middle thigh to the spinal recording site and vice-versa) and in the distal (that is from the lower to the middle thigh recording site and vice versa) segments of the reflex arc. As expected, the speed of propagation of impulses was significantly higher in the proximal than in the distal segments, as well as in the afferent than in efferent limb of the monosynaptic pathway. The P1-P2 time interval was longer on mechanical than on electrical stimulation, probably due to the increased spinal delay of the T versus the H reflex. The present study provides a reliable method for the direct assessment of alpha-efferent as well as of Ia afferent group fibres conduction velocity, provided that in the latter case mechanical stimuli be used.

Spinal components of the cerebral somatosensory evoked response in normal man: the "S wave".

Responses evoked over the scalp and the neck by median nerve (or finger I) stimulation were concurrently recorded in 10 subjects. It was found that the first component of the cortical SEP consists of a small amplitude polyphasic wave (S wave) which could be recorded bilaterally upon unilateral stimulation. The polarity of the S wave varied according to the reference electrode position, at variance with the P15 component which remained constantly positive. It is therefore correct to assume that different generators are responsible for these two potentials.

A reassessment of sensory evoked potential parameters in multiple sclerosis: a discriminant analysis approach.

The sensitivity of the different parameters (absolute latency, interwave latency, latency asymmetry, amplitude) of both cervical and cerebral responses evoked by stimulation of the median nerve at the wrist was assessed in patients with multiple sclerosis by discriminant analysis. The peak latency of N13 or N20 SEP components or both was found to be more sensitive than their amplitude, provided that a preliminary covariation with the height of the subjects was performed. The measurement of latency asymmetry between the two sides increased the tests sensitivity, while amplitude asymmetry turned out to be of little diagnostic value. A linear discriminant function with four variates (that is mean amplitude, mean latency, latency asymmetry and height of the subject) was computed to summarise the information provided by the different parameters to give a rapid and exact method for the assessment of SEP abnormalities in multiple sclerosis patients.

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.

Conducted and segmental components of the somatosensory cervical response.

Cervical responses evoked by stimulation of the median nerve have been concurrently recorded from C7--Fz and C7--Sn (suprasternal notch). The existence of two different waveforms (RI and RII) has been confirmed. RI (from C7--Fz) consists of four negative peaks (N9, N11, N13, N14) followed by a large positive deflection (P16). RII (from C7--Sn) is characterised by an early positive--negative spike (P1--N1a) followed by a slow negative--positive wave (N1b--P2). The study of the most relevant parameters (polarity, latency and refractory period) of each component of RI and RII did not indicate whether the generators underlying RI differ from those responsible for RII. However, stimulation of the lower limb, which does not involve segmental events at cervical level, showed a clearcut difference: no response was recorded from C7--Sn, while evoked activity equivalent to RI was obtained from C7--Fz. Therefore it is suggested that RII is entirely generated by segmentally evoked potentials while RI is mainly due to conducted potentials.