Cortical projection of impulses in myelinated cutaneous afferent nerve fibres of the cat

The sensory neurons of touch

The somatosensory system decodes a wide range of tactile stimuli and thus endows us with a remarkable capacity for object recognition, texture discrimination, sensory-motor feedback and social exchange. The first step leading to perception of innocuous touch is activation of cutaneous sensory neurons called low-threshold mechanoreceptors (LTMRs). Here, we review the properties and functions of LTMRs, emphasizing the unique tuning properties of LTMR subtypes and the organizational logic of their peripheral and central axonal projections. We discuss the spinal cord neurophysiological representation of complex mechanical forces acting upon the skin and current views of how tactile information is processed and conveyed from the spinal cord to the brain. An integrative model in which ensembles of impulses arising from physiologically distinct LTMRs are integrated and processed in somatotopically aligned mechanosensory columns of the spinal cord dorsal horn underlies the nervous system's enormous capacity for perceiving the richness of the tactile world.

Opioid-mediated muscle afferents inhibit central motor drive and limit peripheral muscle fatigue development in humans

We investigated the role of somatosensory feedback from locomotor muscles on central motor drive (CMD) and the development of peripheral fatigue during high-intensity endurance exercise. In a double-blind, placebo-controlled design, eight cyclists randomly performed three 5 km time trials: control, interspinous ligament injection of saline (5K(Plac), L3-L4) or intrathecal fentanyl (5K(Fent), L3-L4) to impair cortical projection of opioid-mediated muscle afferents. Peripheral quadriceps fatigue was assessed via changes in force output pre- versus postexercise in response to supramaximal magnetic femoral nerve stimulation (DeltaQ(tw)). The CMD during the time trials was estimated via quadriceps electromyogram (iEMG). Fentanyl had no effect on quadriceps strength. Impairment of neural feedback from the locomotor muscles increased iEMG during the first 2.5 km of 5K(Fent) versus 5K(Plac) by 12 +/- 3% (P < 0.05); during the second 2.5 km, iEMG was similar between trials. Power output was also 6 +/- 2% higher during the first and 11 +/- 2% lower during the second 2.5 km of 5K(Fent) versus 5K(Plac) (both P < 0.05). Capillary blood lactate was higher (16.3 +/- 0.5 versus 12.6 +/- 1.0%) and arterial haemoglobin O(2) saturation was lower (89 +/- 1 versus 94 +/- 1%) during 5K(Fent) versus 5K(Plac). Exercise-induced DeltaQ(tw) was greater following 5K(Fent) versus 5K(Plac) (-46 +/- 2 versus -33 +/- 2%, P < 0.001). Our results emphasize the critical role of somatosensory feedback from working muscles on the centrally mediated determination of CMD. Attenuated afferent feedback from exercising locomotor muscles results in an overshoot in CMD and power output normally chosen by the athlete, thereby causing a greater rate of accumulation of muscle metabolites and excessive development of peripheral muscle fatigue.

Vibrotactile coding capacities of spinocervical tract neurons in the cat

The response characteristics and tactile coding capacities of individual dorsal horn neurons, in particular, those of the spinocervical tract (SCT), have been examined in the anesthetized cat. Twenty one of 38 neurons studied were confirmed SCT neurons based on antidromic activation procedures. All had tactile receptive fields on the hairy skin of the hindlimb. Most (29/38) could also be activated transynaptically by electrical stimulation of the cervical dorsal columns, suggesting that a common set of tactile primary afferent fibers may provide the input for both the dorsal column-lemniscal pathway and for parallel ascending pathways, such as the SCT. All but 3 of the 38 neurons studied displayed a pure dynamic sensitivity to controlled tactile stimuli but were unable to sustain their responsiveness throughout 1s trains of vibration at vibration frequencies exceeding 5-10 Hz. Stimulus-response relations revealed a very limited capacity of individual SCT neurons to signal, in a graded way, the intensity parameter of the vibrotactile stimulus. Furthermore, because of their inability to respond on a cycle-by-cycle pattern at vibration frequencies >5-10 Hz, these neurons were unable to provide any useful signal of vibration frequency beyond the very narrow bandwidth of approximately 5-10 Hz. Similar limitations were observed in the responsiveness of these neurons to repetitive forms of antidromic and transynaptic inputs generated by electrical stimulation of the spinal cord. In summary, the observed limitations on the vibrotactile bandwidth of SCT neurons and on the precision and fidelity of their temporal signaling, suggest that SCT neurons could serve as little more than coarse event detectors in tactile sensibility, in contrast to DCN neurons the bandwidth of vibrotactile responsiveness of which may extend beyond 400 Hz and is therefore broader by approximately 40-50 times than that of SCT neurons.

Effects of a dorsal column lesion on temporal processing within the somatosensory system of primates

A dorsal column (DC) lesion has lasting effects on behavioral tasks that require temporal processing of tactile information (e.g., frequency and duration discrimination). The present experiments describe physiological correlates of these deficits in temporal discrimination. Compound action potentials evoked by electrocutaneous stimulation were recorded from the major white matter subdivisions of the spinal cord in anesthetized monkeys, and relationships between stimulation frequency and evoked potential (EP) amplitude were determined for the ascending pathways. At 10 pulses per second (Hz) EPs recorded in the lateral spinal columns were attenuated slightly (by 15% or less, relative to 1.5 Hz), whereas potentials recorded from the DCs were not attenuated. The attenuation increased with stimulation frequencies up to 50 Hz, reaching 80% for the anterolateral column and 38% for the dorsolateral column, but only 15% for the DC. Epidural EPs were recorded, before and after interruption of the contralateral DC, from awake animals with electrodes chronically implanted over primary somatosensory cortex (SI). Following the lesion. EP responses to 1.5-Hz stimulation were 46% of preoperative responses to the same stimulus. At 10 Hz, EP amplitudes were attenuated even more, to 27% of the preoperative amplitude at 1.5 Hz. Principal components analysis was employed to quantify alterations in EP conformation and stimulus frequency was varied from 1.5 to 10 Hz, before and after a DC lesion. Interruption of the DC resulted in a significant decrease in the information provided by the EP about changes in stimulus frequency. EPs were also recorded from different locations along the anterior-posterior dimension of the hindlimb region of SI in lightly anesthetized animals. Principal components analysis revealed that there was less information present in the EP about changes in stimulus frequency (1.5-10 Hz) at all recording locations in animals with a DC lesion, compared with the cortex of normal animals. The DC lesion significantly decreased the amplitude of cortical EPs evoked by repetitive stimulation. At 10 Hz the EP was nearly buried in noise, consistent with behavioral deficits in discrimination of the duration of 10 Hz stimulation following interruption of the DC. Also, significantly less information was present in the cortical EPs about changes in stimulus frequency in the absence of intact DCs, which is consistent with deficits in frequency discrimination. This reduction could be explained in part by a lesser capacity of spinal pathways in the lateral column to follow repetitive stimulation above 10 Hz. However, more rostral manifestations of a DC lesion, at either the thalamus or the cortex, are likely to contribute to the reduced capacity of animals with DC lesions to make temporal discriminations.

Somatosensory evoked potentials induced by stimulating a variable number of nerve fibers in rat

Somatosensory evoked potentials (SEPs) were recorded from rat spinal cord (sSEPs) and cerebral cortex (cSEPs). Stimulus sites included either one or both sural nerve branches having different fiber populations (group A), or distal to a lesion of controlled size of the sural nerve made 1 week earlier (group B). In the two groups of animals, amplitudes of SEPs correlated with the quantity of large myelinated nerve fibers. Peak latencies of sSEPs in group A related to the ratio of sizes of transmitting fibers. sSEPs and cSEPs in both groups A and B could be recorded in a reproducible fashion by stimulating sural nerve branches or lesioned nerve trunks containing only 100 or less nerve fibers greater than 4 microns in size. Thus, presence of sSEPs or cSEPs after stimulation distal to a lesion site does not insure that many nerve fibers have continuity with the central nervous system (CNS).

Nociceptive C fibre input to the primary somatosensory cortex (SI). A field potential study in the rat

In the present study, noxious thermal stimulation of the skin with short pulses of CO2-laser radiation was used to identify a cutaneous nociceptive C fibre input to SI and investigate the organization of this input in halothane-nitrous oxide anaesthetized rats. Noxious CO2-laser stimulation of the glabrous skin of the hindpaw consistently evoked late surface positive field potentials in SI (average onset latency 226 ms, peak latency 296 ms). It was demonstrated that these late potentials were evoked by an input from nociceptive C fibres, using a combination of latency measurements, anodal block of A fibre conduction and graded intensities of stimulation. Compared to the tactile evoked potentials in SI, the nociceptive C fibre evoked potentials were more widespread and exhibited a crude somatotopical organization. Intracortically, both tactile and nociceptive C fibre evoked potentials reversed polarity and exhibited a peak negativity in laminae III-IV. The nociceptive C fibre evoked potentials exhibited an additional peak negativity in laminae V-VI. The latter potential had a different time course as compared to the nociceptive C fibre potential evoked in laminae III-IV. In conclusion, an input from cutaneous nociceptive C fibres to SI was demonstrated for the first time in animal experiments. The input to SI from tactile receptors and cutaneous C nociceptors were differently organized.

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.

Effects on median nerve SEPs of tactile stimulation applied to adjacent and remote areas of the body surface

Study of the influence of continuous tactile stimulation on somatosensory evoked potentials (SEPs) following electrical stimulation of the median nerve revealed an effect due to interfering input from both adjacent and remote regions of the body surface. The distribution of the effect was demonstrated by subtracting the 'interference' from the 'control' response to derive a 'difference' wave form. Tactile stimulation of the thumb ipsilateral to the stimulated median nerve produced a difference wave form in which a marked phase reversal was apparent between pre- and post-central areas for 2 complexes, at latencies of approximately 20 and 30 msec. It is proposed that this may have been due to partial 'saturation' of a generator in the hand region of area 3b in the primary somatosensory cortex (SI), which was then unable to respond fully to the median nerve impulse. A similar effect was observed when the interfering stimulus was applied to the ipsilateral little finger, possibly reflecting a process of 'surround inhibition.' Tactile stimulation of more remote regions (principally the face and contralateral hand) resulted in consistent difference wave forms in which the early components (less than 30 msec latency) had scalp distributions differing from one another but consistent with influence on generators in the face or hand region of the second somatosensory cortex (SII). Later potentials consistently identifiable in the difference wave forms were similar for all locations of the interfering stimulus apart from the ipsilateral thumb and were distributed in accordance with a proposed generator in the parietal 'association' cortex.

Concussive head injury producing suppression of sensory transmission within the lumbar spinal cord in cats

This study examines the effects of concussive levels of a fluid-percussion head injury on sensory transmission within the lumbar spinal cord of the cat. Primary afferent depolarization (PAD) was suppressed for 2 to 5 minutes following injury, as assessed by dorsal root potentials and augmentation of antidromic dorsal root potentials, both evoked by stimulation of adjacent dorsal roots. Polysynaptic reflex discharges in ventral root potentials evoked by dorsal root stimulation were also profoundly suppressed during this same period, even when spontaneous and monosynaptic reflex discharges were facilitated. Changes in PAD produced by injury were abolished by spinal cord transection, but were not affected by midpontine transection. These findings suggest that concussive head injury can produce suppression of segmental sensory transmission by neurally mediated processes involving the bulbar brain stem. Recordings of dorsal root resting potentials, antidromic dorsal root potentials, and reductions of antidromic dorsal root potentials induced by tetanic root stimulation indicated that depressed segmental sensory function produced by injury was due to suppression of postsynaptic interneuronal transmission rather than to excitability changes in primary afferent fibers. Somatosensory cortical potentials evoked by dorsal root stimulation were profoundly depressed at the same time as segmental sensory transmission was suppressed, suggesting that suppressed segmental sensory transmission may also contribute to suppression of ascending sensory transmission. It is hypothesized that transmission failure of interneuronal systems in the initial period following insult may be a general response occurring in wide areas of the central nervous system, and not restricted to areas to which mechanical stress is directly applied. This response pattern may result from indiscriminate activation of interconnected excitatory and inhibitory elements of interneuronal systems.

Convergence in the somatosensory pathway between cutaneous afferents from the index and middle fingers in man

Average short-latency cerebral potentials were recorded from the parietal scalp to mechanical stimulation of the index and middle fingers and to electrical stimulation of the digital nerves in normal subjects. The early components of the cerebral potential, representing the arrival of the afferent volley at the sensorimotor cortex, were studied during stimulation of the fingers separately and together. When strong or moderate stimuli were used there was a suppressive interaction between the afferent input from the two fingers with either electrical or mechanical stimulation. During simultaneous stimulation of both fingers the size of the early component of the cerebral potential was less than predicted by simple addition of the potentials produced by stimulation of the fingers individually. When very weak stimuli, close to the level necessary for detection by the subject, the input from the two fingers produced additive or facilitatory interactions in the early components of the cerebral potential. These results suggest that there is convergence between the afferent inputs from the index and middle fingers along the somatosensory pathway. At levels of stimulation comparable to those which produced facilitation in the electrophysiological studies, simultaneous stimulation to both fingers was detected significantly more frequently than would be expected from the detection of stimulation to individual fingers.

The termination in the mesencephalon of fibres from the lateral cervical nucleus. An anatomical study in the cat

The termination in the dorsal mesencephalon of fibres from the lateral cervical nucleus (LCN), recently discovered with the retrograde tracing technique, was investigated by both the degeneration method and anterograde transport of lectin-conjugated horseradish peroxidase. Both strategies gave similar results. The termination area was found to be situated at the level of transition between the superior and inferior colliculi and comprises mainly the intercollicular nucleus and the deep and intermediate layers of the posterolateral pole of the superior colliculus. The functional implication of the demonstrated projection from LCN is discussed in relation to somatosensory activity in the mesencephalic tectum.

Dissimilar influences of some injectable anaesthetics on the responses of reticulo-spinal neurones to inhibitory transmitters in the lamprey

1 Intracellular recordings were made from identified bulbar reticulo-spinal neurones in the medulla of lamprey ammocoetes. Responses to iontophoretically applied inhibitory transmitters were measured as changes in membrane potential and input resistance. 2 Dose-dependent alterations in the responses to gamma-aminobutyric acid (GABA) and glycine during bath application of injectable anaesthetic drugs were measured; the compounds used were pentobarbitone, ketamine, metomidate and the steroid mixture alphaxalone/alphadolone (Saffan). 3 GABA responses were potentiated by pentobarbitone (1-3 X 10(-4) M) and prolonged by ketamine (3.7 X 10(-5) M); but depressed by high concentrations (10(-3) M) of all drugs, as well as by anaesthetic concentrations of alphaxalone (1-3 X 10(-5) M). 4 Glycine responses were depressed by alphaxalone (1-3 X 10(-5) M) and by supra-anaesthetic concentrations of ketamine (3.7 X 10(-4) M) and metomidate (1.8 X 10(-3) M). No drug potentiated the glycine responses. 5 In the absence of an effect common to the 4 anaesthetics, it is concluded that neither potentiation nor inhibition of all GABA or glycine responses is an essential feature of anaesthesia. However, effects comparable to those described here may contribute to the overall clinical picture during anaesthesia of higher vertebrates. The findings do not support the notion that all anaesthetic agents act on biological membranes by a single mechanism.

Spinal cord pathways mediating somatosensory evoked potentials

Using a CO2 laser, discrete thoracic spinal cord lesions were made in cats anesthetized with ketamine and xylazine (Rompun). Differences in cortical somatosensory evoked potentials (SEP's) produced with high-intensity stimulation (20 times the motor threshold) of each posterior tibial nerve determined for nine different combinations of unilateral spinal cord lesions. The results of these studies show that nerve fibers in the ipsilateral dorsal column, the ipsilateral dorsal spinocerebellar tract, and the contralateral ventrolateral tracts with respect to the side of leg stimulation, contribute to cortical SEP's. A lesion of the dorsal spinocerebellar tract affected only the early waves (less than 30 msec) of the SEP from leg stimulation ipsilateral to the side of the lesion, whereas a solitary lesion of the ventrolateral tract caused changes primarily in the amplitude of later waves (greater than 30 msec) of the SEP produced by contralateral leg stimulation. Lesions involving one-half of the dorsal column caused changes in the amplitude of both the early and late waves produced by stimulation ipsilateral to the side of the lesion. The effects of various combinations of lesions on the cortical SEP's were not additive, which indicates significant interaction between afferent pathways. These findings suggest that high-intensity peripheral nerve stimulation, which activates both C and A fibers, could be used intraoperatively to assess spinal cord function with more accuracy than the current practice of using a stimulus strength of twice the motor threshold. The importance of using anesthetic agents that do not depress cortical activity (which may affect the later components of the SEP) is also emphasized.

The relationship between the size of a muscle afferent volley and the cerebral potential it produces

This study examined the relationship between the size of an afferent neural input produced by electrical stimulation of the posterior tibial nerve at the ankle and the size of the early components of the evoked cerebral potential. For five of six subjects the first peak of the afferent neural volley recorded in the popliteal fossa was uncontaminated by either motor efferents or cutaneous afferents. This was established by measuring the conduction times of motor fibres in the posterior tibial nerve and cutaneous fibres in the sural and posterior tibial nerves over the ankle to popliteal fossa segment. It is likely therefore that the first peak of the afferent volley contained predominantly, if not exclusively, activity in rapidly conducting afferents from the small muscles of the foot. The size of the two earliest components of the cerebral potential did not increase in direct proportion to the size of the afferent volley which produced it. The early components of the cerebral potential reached a maximum when the responsible muscle afferent volley was less than 50% of its maximum.

Effect of superficial radial nerve stimulation on the activity of nigro-striatal dopaminergic neurons in the cat: role of cutaneous sensory input

The release of 3H-dopamine (DA) continuously synthesized from 3H-tyrosine was measured in the caudate nucleus (CN) and in the substantia nigra (SN) in both sides of the brain during electrical stimulation of the superficial radial nerve in cats lightly anaesthetized with halothane. Use of appropriate electrophysiologically controlled stimulation led to selective activation of low threshold afferent fibers whereas high stimulation activated all cutaneous afferents. Results showed that low threshold fiber activation induced a decreased dopaminergic activity in CN contralateral to nerve stimulation and a concomitant increase in dopaminergic activity on the ipsilateral side. Stimulation of group I and threshold stimulation of group II afferent fibers induced changes in the release of 3H-DA mainly on the contralateral CN and SN and in the ipsilateral CH. High stimulation was followed by a general increase of the neurotransmitter release in the four structures. This shows that the nigro-striatal dopaminergic neurons are mainly--if not exclusively--controlled by cutaneous sensory inputs. This control, primarily inhibitory in the side contralateral to the stimulation, seems rather non-specific when high threshold cutaneous fibers are also activated. Such activations could contribute to reestablish sufficient release of DA when the dopaminergic function is impaired as in Parkinson's disease.

The effects of noxious heat on responses of spinocervical units to low intensity cutaneous stimuli

The responses of spinocervical neurons to sinusoidal hair displacements were studied during and in the absence of radiant heating of parts of the hindpaw to noxious levels (45-65 degrees C). Noxious heat usually increased background discharge and lowered the signal-to-noise ratio at low frequencies of hair displacement. At higher frequencies over 20 Hz, this ratio was slightly depressed for half of the cells, and dramatically increased for the others. Similar effects were found when the heating was off the receptive field for hair displacement, which suggests a central cause for these effects.

The effect of the selective activation of different peripheral nerve fiber groups on the somatosensory evoked potentials in the cat

Contralateral and ipsilateral cortical somatosensory evoked potentials to selective activation of different diameter sural nerve fiber groups were recorded in cats. Activation of alpha fibers evoked a well defined positive-negative primary potential at the contralateral somatosensory area II. Activation of delta fibers in addition to alpha fibers resulted in another positive-negative wave of comparable amplitude with a latency approximately twice that of the early component. Isolation of the peripheral input to the delta fibers alone, by blocking the alpha fiber activity with the use of triangular pulses, caused the complete disappearance of the initial wave, leaving the second with no latency or amplitude changes. This demonstrated that the latter potential was evoked by delta fibers. Ipsilateral responses followed the same pattern, however with higher peripheral thresholds, longer latencies and smaller amplitudes. No cortical potentials could be recorded to activation of C fibers. The results of this study are at variance with previous observations about the cortical responses to small myelinated nerve fiber activity. The rationale of the use of triangular pulses for selectively activating nerve fiber groups is also discussed.

Effects of spinal cord lesions on somatic evoked potentials altered by interactions between afferent inputs

In cats anesthetized with alpha-chloralose, somatic evoked potentials (SEP) were recorded in response to electrical stimulation of surgically isolated peripheral nerves. Selected surgical lesions were made at T9-L1 spinal cord and were histologically verified. Two stimulus magnitudes were used to activate peripheral nerves, one only exciting the large fibers and another exciting the small fibers as well. Control SEPs were recorded in response to stimulation of both large and small fibers of the radial nerve. The later components (latencies greater than 40 msec) of this SEP were suppressed when evoked 100 msec after application of a conditioning stimulus (CS) to the large fibers of either peroneal nerve. Bilateral transection of the dorsal columns and spinocervical tracts eliminates these effects. Increasing the CS intensity to include small diameter fibers again resulted in reduction of the later components of the SEP. This interaction was largely eliminated if the transection was extended to include mid-lateral cord tracts. These results suggest that the SEP can be influenced by small fiber afferent activity conducted in mid-ventrolateral spinal cord in the absence of the dorsal columns and spinocervical tracts. Alterations in the forelimb-evoked SEP by a conditioning hindlimb stimulus is a sensitive indicator of spinal cord integrity. This method may be used to assess whether low spinal injury spares ventrolateral columns.

Simultaneous measurements of microflow and evoked potentials in the somatomotor cortex of the cat brain during specific sensory activation

The behaviour of both microflow and evoked potentials was investigated in the right somatomotor cortex of the cat (anaesthetized with chloralose) during electrical stimulation of the contralateral left forepaw. Frequency, amplitude, and time of stimulation were varied. Using the local hydrogen clearance method the changes of microflow were continuously monitored in the same cortical area from which the evoked potentials were recorded. The experiments have shown that activation of the somatomotor cortex by somatic stimulation of the contralateral forepaw results in changes of microflow which clearly correlate to the side and amplitude of the primary evoked potentials. An increase in flow as well as in amplitude of the potentials depends on the stimulation parameters. The changes of microflow are limited to a small area of 1--2 mm in diameter. We conclude that a tight coupling of flow to functional activity exists in the microcirculatory range.

Facilitation of synaptic transmission by general anaesthetics

1. The actions of five structurally different intravenous and inhalation anaesthetics (alphaxalone/alphadolone, halothane, ketamine, methohexitone, and pentobarbitone) have been studied on synaptic transmission through the cuneate nucleus of the dorsal column-lemniscal afferent pathway in the decerebrate cat. 2. Synaptic input and output were estimated from antidromic and orthodromic potentials, which were evoked by either afferent volleys from the periphery or micro-electrode excitation of the presynaptic fibre terminals in the cuneate and recorded at forelimb nerves and the medial lemniscus. 3. Each of the anaesthetic agents potentiated the efficiency of synaptic transmission, as shown by the elevation of input-output curves constructed from the integrals of the potentials evoked by varying intensities of either peripheral or cuneate stimulation. 4. The excitability of the afferent terminals, as measured at the peripheral nerves by the antidromic responses to micro-electrode stimulation, was depressed by the anaesthetics. Post-synaptic excitability, which was assessed from the direct lemniscal response to intra-nuclear stimulation, did not appear to change. 5. Hypotensive states (mean arterial levels less than 60 torr) produced depolarization of presynaptic terminals and depression of synaptic efficiency and transmission; these changes opposed the primary effects of the general anaesthetics. 6. It is concluded that anaesthetics do not depress activity at all synapses of the central nervous system. Their facilitatory action on cuneate transmission is attributed to an enhanced release of excitatory transmitter; the underlying mechanism may be hyperpolarization of the primary afferent terminals, secondary to an increase in K+ conductance.

The effects of interaction between large and small diameter fiber systems on the somatosensory evoked potential

The effect of interaction between large and small diameter fiber systems on the somatosensory evoked potential (SEP) was studied in anesthetized cats. Activation of large diameter fibers of the peroneal or radial nerves eliminates the late components of the SEP produced by stimulation of all fibers in the contralateral median or radial nerves. The inhibitory effects of a selective conditioning stimulus to the large diameter fibers of the peroneal nerve on the radial nerve evoked SEP was eliminated by bilateral transection of the dorsal column and spino-cervical tracts. However, interaction could still be obtained following transection when both large and small diameter fibers in the peroneal nerve were stimulated. The results of this study support the hypothesis that a correlation exists between activity in different fiber groups in afferent nerves, their conduction pathways through the cord, and the components of the cortical evoked potential.

Input-output relation of transmission through cuneate nucleus

1. In decerebrate cats, micro-electrodes were inserted into the cuneate nucleus to stimulate afferent terminals with single shocks of varying intensities. Estimates of the input and output of the nucleus were obtained by integrating antidromic responses in forelimb cutaneous nerves and orthodromic responses in the medial lemniscus. 2. Input-output curves were normally very non-linear, reflecting the high synaptic potency of small inputs. They were fitted readily by power functions, with exponents averaging 0-50. 3. The normal input-output relation rapidly disappeared after interruption of the blood supply. A loss of synaptic efficiency of small inputs was indicated by curves with exponents of greater than or equal to 1; this was associated with a sharp increase in terminal excitability. 4. Within the range of surface temperature 30-40 degrees C, warming made the input-output curves steeper but reduced terminal excitability, whereas cooling had the opposite effect. The efficiency of transmission was thus inversely correlated with terminal excitability. 5. The non-linear shape of cuneate input-output curves is probably not determined by inhibitory control, since picrotoxin depressed rather than enhanced outputs. 6. On the other hand, pentobarbitone made the input-output curves markedly steeper and tended to lower terminal excitability.

The effects of graded forelimb afferent volleys on acetylcholine release from cat sensorimotor cortex

1. The acetylcholine (ACh)-releasing system in the cerebral cortex of pentobarbital anaesthetized cats was investigated by examining the effect of graded afferent volleys in forelimb nerves on ACh release from the sensorimotor cortices contralateral and ipsilateral to the site of stimulation. 2. Cortical ACh release was determined by bio-assay of neostigmine-containing perfusates which had been in contact with the cortical surfaces for 5-10 min periods. 3. Afferent volleys, generated by stimuli that were effective in activating as many fibres of a fibre group as possible without stimulating fibres in the group with the next highest threshold for activation, were monitored from dorsal roots C7 or C8 before entering the spinal cord. 4. Stimulation of the deep (DR) and superficial (SR) radial nerves and the radial (R) nerve proximal to the junction of the DR and SR were effective in enhancing ACh release only when either group III or groups III and IV fibres were included in the afferent volley. 5. The rates of ACh release from the primary receiving area of the sensorimotor cortex contralateral to the site of stimulation did not differ from those from the same area of the ipsilateral sensorimotor cortex. 6. The pertinence of this data to the various hypotheses concerning the nature of the ACh-releasing pathways to the cerebral cortex is discussed.

Reversible spinal cord trauma: a model for electrical monitoring of spinal cord function

✓ Cortical evoked potentials in anesthetized cats were recorded by a noninvasive averaging technique as a means of estimating spinal cord damage. Graded pressure on the spinal cord produced reversible blocking of these potentials. With this type of trauma, block of motor transmission through the cord paralleled the block of sensory transmission, and each seemed to be a sensitive indicator of spinal cord function. The possible use of such monitoring in anesthetized patients undergoing spinal operations is discussed.