Ionic permeability of the opossum sciatic nerve perineurium, examined using electrophysiological and electron microscopic techniques.

A parallel electrophysiological and electron microscopic study was used to assess the ionic permeability of the sciatic nerve perineurium of the opossum Monodelphis domestica. The electrophysiological method was used to monitor permeability to K(+), followed by combined electron microscopy and X-ray probe analysis to monitor permeability to the electron-dense tracer lanthanum. Isolated but intact nerves were mounted in a 'grease gap' chamber for extracellular measurement of DC potential and compound action potential (CAP). Challenge with 100 mM [K(+)] Ringer was used to assess the K(+) permeability of the perineurium, since a change in DC potential (DeltaDC) under these conditions reflected changes in the axonal resting membrane potential. There was no detectable change in DC potential or CAP to the first K(+) challenge (n=71 nerves) indicating negligible K(+) permeability under control conditions. The inflammatory mediators histamine 0.1-40 mg/ml (1. 3-130 mM), bradykinin (0.1-4.7 mM) and 5HT (serotonin) 0.1-5.0 mg/ml (0.5-23.5 mM) caused no measurable DeltaDC on subsequent challenge with 100 mM [K(+)] Ringer, indicating no effect on perineurial K(+) permeability. In nerves exposed to the bile salt sodium deoxycholate (DOC, 6 min, 4 mM), challenge with elevated K(+) Ringer caused a dose-dependent DeltaDC in the range 10-100 mM [K(+)] (1.67+/-0.17 mV in 100 mM [K(+)], n=20), indicating increased perineurial permeability caused by DOC, but the response was smaller than that previously reported for the frog perineurium. Lanthanum was observed in the outer layers of the perineurium, but was not seen to penetrate the endoneurium in any of the nerves examined (n=51), even after DOC application. This study shows that the combined electrophysiological and electron microscopic technique for monitoring ionic permeability can be applied to mammalian nerve, and suggests that the opossum perineurium is more resistant to tight junction opening by chemical modulators than is the frog perineurium.

Ionic permeability of the frog sciatic nerve perineurium: parallel studies of potassium and lanthanum penetration using electrophysiological and electron microscopic techniques.

The isolated sciatic nerve of the frog Rana temporaria was used for a parallel electrophysiological and electron microscopic examination of the ionic permeability of the perineurium, one component of the blood-nerve barrier. Nerves mounted in a grease-gap chamber for electrophysiological recording showed negligible changes in DC potential (Delta DC) or compound action potential on challenge with 100 mM K(+) Ringer, evidence that the perineurium was tight to K(+). In preparations then fixed and exposed to 5 mM lanthanum in the fixative, and examined in the electron microscope, electron-dense lanthanum deposits were seen between perineurial lamellae, but lanthanum was not detectable within the endoneurium, confirming that the perineurium was also tight to lanthanum. Absence of lanthanum penetration was confirmed by X-ray analysis of electron microscopic sections. In nerves exposed to 2 mM sodium deoxycholate (DOC) in the recording chamber, then challenged with high [K(+)], a moderate increase in perineurial K(+) permeability (P(K)) was observed, but lanthanum was still excluded. Exposure of nerves to 4 mM DOC caused a greater increase in perineurial potassium permeability, and the two nerves with the greatest permeability (P(K) > 1 x 10(-5) cm x sec(-1)) also showed detectable lanthanum within the endoneurium. The results indicate that DOC causes a dose-dependent increase in tight junctional permeability in the perineurium, and that the electrophysiological monitoring of K(+) penetration is a more sensitive measure of small ion permeability than electron microscopical analysis using lanthanum as tracer. Vesicular profiles observed in perineurial lamellae did not form open channels for ion flux across the perineurium in control nerves, or in those exposed to DOC. In preparations where lanthanum reached the endoneurium, lanthanum was observed in dense deposits in the extracellular spaces around nodes of Ranvier, and in the outer mesaxon cleft, but did not penetrate the internodal periaxonal space, the myelin intraperiod line, or the Schmidt-Lanterman incisures, in contrast to observations in mammalian nerves. The apparent differences in accessibility of the internodal periaxonal space in frog and mammalian axons are discussed in relation to axonal physiology. The study illustrates the value of parallel electrophysiological and electron microscopic examination in elucidating the properties of extracellular ionic pathways and their role in neural function.

Cortical focusing is an alternative explanation for improved sensory acuity on an amputation stump.

The ability to localize a sensory stimulus on the body surface (locognosia) has been investigated in normal subjects in a quest to find an explanation for the reported findings of improved sensory acuity on an amputation stump. We have shown that when attending to a smaller area of skin, during the testing procedure, locognosia improves (P < 0.001) by a similar degree to that seen in amputees. Such selective attention is likely to occur in upper limb amputees as they have a reduced area of skin on which to focus during sensory testing. This represents a further explanation for improved sensory acuity on an amputation stump without implicating plasticity of connections within the somatosensory cortex.

Visual event related potentials modulated by contextually relevant and irrelevant olfactory primes.

Visual evoked potentials were recorded from 16 scalp locations on 10 young subjects during presentation of a series of high-quality photographs on a computer screen. The photographs consisted of equal numbers of pictures of fruit (citrus and non-citrus fruits), flowers (roses and other flowers) and objects (e.g. buildings, vehicles, animals etc.). Every picture was different in order to avoid repetition effects. The pictures were presented under four odour conditions: no odour, rose odour, jasmine odour and citrus odour. In order to keep the subjects alert they were asked to make categorizing decisions for the visual stimuli (e.g. flower or fruit). No decision was required concerning the relationship between the visual stimulus and the odour. As expected, the N400 peak was more negative when the picture stimulus did not match the odour. It is hypothesized that the N400 peak can be used as a measure of relatedness of a sensory stimulus to a previous or on-going prime, irrespective of the mode of the stimuli.

Effects of the bile salt sodium deoxycholate, protamine, and inflammatory mediators on the potassium permeability of the frog nerve perineurium.

An electrophysiological method was used to measure the potassium permeability (PK) of the perineurium of the sciatic nerve of frogs Rana temporaria and R. pipiens. Isolated but intact nerves were mounted in a grease-gap chamber, and compound action potential and DC potential monitored. Change in the DC potential (delta DC) in response to challenge with 100 mM [K+] Ringer was used to assess the K+ permeability of the perineurium, since change in DC potential under these conditions reflected changes in the axonal resting potential. The permeability of the perineurium was calculated from the published calibration curve relating delta DC to bathing [K+] in desheathed nerves of Abbott et al. (1997). In the control condition, PK was < 1.1 x 10(-6) cm.s-1. The bile salt sodium deoxycholate (DOC, 1-4 mM) caused a dose-dependent increase in PK, which reached a maximum of 1.7 x 10(-5) cm.s-1 after 2-min exposure to 4 mM DOC, but access of K+ to the endoneurial compartment was more restricted after DOC than after desheathing. Protamine phosphate (1 mM) and protamine sulphate (0.1-5 mg/ml equals 0.125-6.25 mM) had no effect on PK. Neither histamine (0.4-40 mg/ml), bradykinin (0.1-5 mg/ml) nor serotonin (5-hydroxytryptamine, 0.1-5 mg/ml) affected PK. The frog nerve perineurium appears to be relatively insensitive to chemical agents and inflammatory mediators, in contrast to the endothelial cells forming the endoneurial blood-nerve barrier and the blood-brain barrier.

Single fibre electromyographic changes in man after organophosphate exposure.

1. Neuromuscular (NM) changes resulting from organophosphate exposure are known to be complex. After severe acute poisoning recovery from initial depolarisation paralysis may be followed in a limited number of cases by onset of a non-depolarisation paralysis (the Intermediate Syndrome). It is not clear whether this block arises subclinically in all cases of poisoning as a sequel to the initial depolarisation. 2. Single fibre electromyography (SFEMG) is a sensitive clinical neurophysiological technique allowing detection of subclinical changes at the neuromuscular junction. In the study reported it has been used to examine changes in NM transmission in the forearm of fit volunteers exposed to a low level of sarin (isopropyl methyl phosphonofluoridate). 3. Small changes in SFEMG were seen at three hours and three days after an exposure sufficient to cause a reduction in red cell acetyl cholinesterase to 60% of normal. The SFEMG changes were not accompanied by any clinical neuromuscular symptoms or signs and returned to normal 2 years after exposure. 4. The results indicate that there are reversible subclinical changes compatible with the development of non-depolarising NM block without frank clinical expression. In the small population examined there were individual variations in response which may reflect differences in safety margin at the neuromuscular junction.

Electroencephalographic study of divers with histories of neurological decompression illness.

OBJECTIVE: To determine whether divers with histories of neurological decompression illness are electroencephalographically distinguishable from non-divers. METHODS: The electroencephalograms (EEGs) from 68 divers with histories of neurological decompression illness and 45 non-diver controls were examined independently by two clinical neurophysiologists. RESULTS: The diver and non-diver groups were electroencephalographically indistinguishable. CONCLUSION: There is no electroencephalographic evidence for the existence of cerebral dysfunction in divers with histories of decompression illness.

Neurophysiological assessment of divers with medical histories of neurological decompression illness.

OBJECTIVE: To examine the possibility that subclinical damage may persist after clinical recovery from neurological decompression illness. METHODS: The neuraxes of 71 divers with medical histories of neurological decompression illness and 37 non-diver controls were examined by recording the somatosensory evoked potentials produced on stimulation of the posterior tibial and median nerves. RESULTS: Although the tests gave some objective support for the presence of "soft" residual neurological symptoms and signs, no evidence was given for the presence of subclinical damage. CONCLUSIONS: The contention that neurological damage persists after full clinical recovery from the neurological decompression illness was not supported.

Reversible motor and sensory neurophysiological abnormalities in cauda equina claudication.

A case of cauda equina claudication with canal stenosis is presented. Neurophysiological studies show reversible changes during symptomatic and asymptomatic phases. The somatosensory evoked potential from the tibial nerve was reduced in amplitude. Central motor conduction time (CMCT) after transcranial magnetic stimulation of the brain was reversibly prolonged in the symptomatic phase. Reversible CMCT changes have not been previously shown. The findings are discussed in the light of the pathophysiology of ischaemic nerve.

Fluoxetine, amitriptyline and the electroencephalogram.

Electroencephalograms recorded before and after 4 weeks treatment of depressed patients with fluoxetine or amitriptyline were assessed visually and by power spectrum analysis blind to patient, treatment and whether the recordings were carried out before or after treatment. No significant between-group differences in alpha, beta or theta activity were found on visual assessment. Power spectrum analysis revealed a significant decrease in the amount of beta activity at week 4. There was no EEG evidence of drowsiness or epileptiform activity in either of the treatment groups.

Magnetic stimulation in the determination of lumbosacral motor radiculopathy.

Magnetic stimulation was utilised to diagnose lumbosacral motor radiculopathy non-invasively. Magnetic coil stimulation estimated peripheral motor nerve conduction time (MNCT) which, used in combination with F response, allowed calculation of "motor root conduction time (MRCT)," response being recorded from abductor hallucis. Twenty-five normal controls and 26 patients with lumbar spondylosis were studied. The mean interside difference (left minus right) of MRCT in the control was +0.06 msec (range: -0.88 to +0.74 msec). On clinical and radiological grounds, patients with spondylosis were grouped into those with: (I) no lumbosacral root compression, (II) root compression without motor sign, and (III) root compression with motor deficit. All patients in group III and 36% of cases of group II had MRCT significantly prolonged on the affected side.

Neurophysiological monitoring of acute neurological decompression illness.

Somatosensory evoked potentials recorded over the scalp in response to posterior tibial nerve stimulation may provide a useful means of assessing the integrity of the neuraxis during the treatment of neurological decompression illness (DCI). The neurophysiological assessment of a case of DCI is described.

The perceptions of force and of movement in a man without large myelinated sensory afferents below the neck.

1. Motor memory and the sense of effort have been investigated in a man with a complete large fibre sensory neuropathy for over 16 years. The perceptions of pain, heat, cold and muscular fatigue remained but he was without perceptions of light touch and proprioception below the neck. 2. The subject was able to discriminate weights held in the hand with an accuracy only slightly worse than control subjects (20 g in 200 g) when forearm movement and visual inspection were allowed. With eyes shut however he could only distinguish a weight of 200 g from 400 g. It is concluded that a crude sense of effort remains which may have a peripheral origin. 3. A limited motor memory was also present, which allowed him to maintain a posture or continue a simple repetitive movement. No novel movement was possible without visual feedback. 4. Differences in movement ability between this subject and others with similar if less pure sensory neuropathies are ascribed to rehabilitation.

On the late failure of spinal cord stimulation for deafferentation pain.

Two cases are described in which spinal cord stimulation was effective in abolishing previously intractable deafferentation pain for a number of years, but in which late failure occurred for non-technical reasons. A possible explanation for this is advanced; namely that the dorsal column fibres have altered electrical properties due to a form of transganglionic degeneration.

A cerebral hemisphere influence on cutaneous vasomotor reflexes in humans.

Cutaneous vasomotor reflexes (CVR), the transient waves of cutaneous vasoconstriction after stimuli, such as a cough, were recorded from the digits of patients with unilateral frontoparietal lesions using a laser doppler flowmeter. Vasoconstriction was 6-15 times greater on the fingers contralateral to the lesions, an effect which was independent of the temperature difference between limbs. CVRs may be tonically inhibited under normal circumstances by control from the cerebral hemispheres.