Recordings of polymodal single C-fiber nociceptive afferents following mechanical and argon-laser heat stimulation of human skin

Recordings were made in the peroneal nerve of healthy volunteer subjects from C-mechano-heat (CMH) nociceptors (n=25) with their receptive fields in the skin on the dorsum of the foot. The investigation focused on afferent single C-fiber activity induced by short (200 ms) high-intensity argon-laser light pulses projected to localized spots of the skin. Cutaneous heat stimulation with the argon laser, 2–3 times the activation threshold, induced inter-burst spike frequencies in the nerve, reaching 50 Hz, while mechanical stimulation 10–20 times threshold only evoked frequencies reaching 10 Hz. The decrease in conduction velocity of action potentials in the C-fiber afferents following mechanical and heat stimulation was closely related to the degree of activation. Following a laser pulse of 200 ms, a spike pattern with highly reproducible inter-spike intervals was evoked with a fast saturation. On the contrary, a high variability in the number of action potentials evoked by both heat and mechanical stimuli was found, depending on the location of stimuli within the receptive field. A relation between the conduction velocity and the peak firing within the spike train following laser stimulation was detected. Heat and mechanical stimulation activated single C-fibers in matching spots within the same skin areas, in line with the assumption that the two modalities in the CMH-fibers share matching morphological cutaneous substrates. No correlation was found in thresholds or excitability to mechanical and heat stimulation, respectively. This suggests that subsets of receptors exist within nerve endings of the cutaneous receptive fields, with the ability to generate action potentials independent of heat and mechanical stimuli. Unexpectedly, no signs of sensitization or other inflammatory responses were observed after repeated laser pulses; on the contrary, a rapidly developing fatigue was observed when single spots were repeatedly stimulated. However, no fatigue was observed if neighboring spots were stimulated, indicating a localized generator of the fatigue. In each subject, a good correlation was observed between the reported pain sensation and the activity evoked in the afferent C-fibers by the laser. However, the magnitude of the reported pain sensation to comparable degrees of C-fiber activation showed a high variability between different subjects. A fairly good subjective estimate of the afferent-fiber activation was observed when skin spots from 3- down to 1-mm diameter were stimulated. In a few individuals, no painful sensation was reported when the stimulated spots were reduced to 1-mm diameter, despite the occurrence of multiple spikes in single C-fiber afferents, amplifying the importance of spatial summation in the perception of pain. Received: 14 December 1997 / Accepted: 24 March 1998     

Speed and temperature dependences of mechanotransduction in afferent fibers recorded from the mouse saphenous nerve

Here we have systematically characterized the stimulus response properties of mechanosensitive sensory fibers in the mouse saphenous nerve. We tested mechanoreceptors and nociceptors with defined displacement stimuli of varying amplitude and velocity. For each sensory afferent investigated we measured the mechanical latency, which is the delay between the onset of a ramp displacement and the first evoked spike, corrected for conduction delay. Mechanical latency plotted as a function of stimulus strength was very characteristic for each receptor type and was very short for rapidly adapting mechanoreceptors (<11 ms) but very long in myelinated and unmyelinated nociceptors (49-114 ms). Increasing the stimulus speed decreased mechanical latency in all receptor types with the notable exception of C-fiber nociceptors, in which mean mechanical latency was not reduced less, similar100 ms, even with very fast ramp stimuli (2,945 microm/s). We examined stimulus response functions and mechanical latency at two different temperatures (24 and 32 degrees C) and found that stimulus response properties of almost all mechanoreceptors were not altered in this range. A notable exception to this rule was found for C-fibers in which mechanical latency was substantially increased and stimulus response functions decreased at lower temperatures. We calculated Q(10) values for mechanical latency in C-fibers to be 5.1; in contrast, the Q(10) value for conduction velocity for the same fibers was 1.4. Finally, we examined the effects of short-term inflammation (2-6 h) induced by carrageenan on nociceptor and mechanoreceptor sensitivity. We did not detect robust changes in mechanical latency or stimulus response functions after inflammation that might have reflected mechanical sensitization under the conditions tested.     

Excitation of primate spinothalamic neurons by cutaneous C-fiber volleys.

1. The responses of spinothalamic tract cells in the lumbosacral spinal cords of anesthetized monkeys were examined following electrical stimulation of the sural nerve or the application of noxious thermal and mechanical stimuli to the skin on the lateral aspect of the foot. 2. The spinothalamic tract neurons were classified as wide dynamic range (WDR), high-threshold (HT), or low-threshold (LT) cells on the basis of their responses to mechanical stimuli. 3. All of the WDR and HT spinothalamic tract cells tested responded to volleys in A- and C-fibers. However, strong C-fiber responses were more common in HT than in WDR cells. 4. The responses atributed to C-fibers were graded with the size of the C-fiber volley. The latencies of the responses attributed to C-fibers indicated that the fastest afferents involved had a mean conduction velocity of 0.9 m/s. The responses remained after anodal blockade of conduction in A-fibers. 5. Temporal summation of the responses of spinothalamic tract cells was demonstrated both to brief trains of stimuli at 33 Hz and to single stimuli repeated at 1- to 2-s intervals. The latter phenomenon is often called "windup." 6. The responses of several spinothalamic tract cells to noxious heat pulses could still be elicited during anodal blockade of conduction in A-fibers. Similarly, it was possible to demonstrate an excitatory action of noxious mechanical stimuli despite interference with conduction in A-fibers by anodal current. 7. The cells investigated were located either in the marginal zone or in the layers of the dorsal horn equivalent to Rexed's laminae IV-VI in the cat. The cells were generally activated antidromically from the caudal part of the ventral posterior lateral nucleus of the thalamus.     

Modulation of heat evoked nociceptive withdrawal reflexes by painful intramuscular conditioning stimulation

Convergence between cutaneous heat nociceptors and muscles afferents was investigated by applying a phasic, conditioning electrical stimulus to the tibialis anterior muscle (a train of five 1 ms pulses over 21 ms) at varying time intervals relative to a thermal test stimulus used for evoking the withdrawal reflex in humans. The 200 ms thermal stimulus was applied on the dorsum of the foot at an intensity of two times the pain threshold. The conditioning electrical stimulus was applied at an intensity of two times the pain threshold via a set of intramuscular needle electrodes. The conditioning-test interval was varied between –400 ms and 8,000 ms at 17 different intervals. The mean reflex onset latency of reflexes evoked by thermal stimuli alone was 354 ± 9 ms. A facilitation of the reflex was seen when the conditioning stimulus was applied 275 ms (174 ± 30% compared to control) and 300 ms (162 ± 32% compared to control) after the test stimulus onset indicating sensory convergence between muscle afferents (group I–III) and cutaneous Aδ heat nociceptors arriving simultaneously at the spinal cord.     

Hyper-responsivity in a subset of C-fiber nociceptors in a model of painful diabetic neuropathy in the rat

While clinical characteristics of diabetic painful neuropathy are well described, the underlying electrophysiological basis of the exaggerated painful response to stimuli, as well as the presence of spontaneous pain, are poorly understood. In order to elucidate peripheral contributions to painful diabetic neuropathy, we quantitatively evaluated the function of C-fibers in a rat model of painful diabetic neuropathy, diabetes induced by the pancreatic beta-cell toxin streptozotocin. While there was no significant effect of diabetes on conduction velocity, mechanical threshold or spontaneous activity, the number of action potentials in response to sustained threshold and suprathreshold mechanical stimuli was significantly increased in the diabetic rats. Moreover, there was a clustering of responses of C-fibers in diabetic rats; while two-thirds of C-fibers fired at the same mean frequency as C-fibers in control rats, one-third of C-fibers in diabetic rats were markedly hyper-responsive, demonstrating a threefold increase in firing frequency. The high-firing-frequency C-fibers in rats with diabetes also had faster conduction velocity than the low-firing-frequency C-fibers in rats with diabetes or in C-fibers in control rats. The hyper-responsiveness was characterized by a selective increase of the shortest interspike intervals (<100ms) in the burst component (first 10s) of the response to a sustained suprathreshold stimulus; in the plateau phase (last 50s) of the response to a 60-s suprathreshold stimulus, we found a selective increase of interspike intervals between 100 and 300ms in hyper-responsive C-fibers in rats with diabetes. The hyper-responsiveness did not correlate with mechanical threshold, presence of spontaneous activity or location of the fiber's receptive field. In summary, in an established model of painful diabetic neuropathy in the rat, a subset of C-fibers demonstrated a marked hyper-responsiveness to mechanical stimuli. The subset was also found to have a greater mean conduction velocity than the fibers not demonstrating this hyper-responsivity. The present findings suggest that study of individual neurons in vitro may allow elucidation of the ionic basis of enhanced nociception in diabetic neuropathy.     

Firing properties of chopper and delay neurons in the lateral superior olive of the rat

 Neurons in the lateral superior olivary nucleus (LSO) respond to acoustic stimuli with the ”chopper response”, a regular repetitive firing pattern with a short and precise latency. In the past, this pattern has been attributed to dendritic integration of synaptic inputs. We investigated a possible contribution of intrinsic membrane properties using intracellular recording techniques in a tissue slice preparation. We found two electrophysiological classes of neurons in the LSO. Chopper neurons responded to depolarizing current pulses with a single onset spike at short, precise latency close to threshold and with repetitive, regular, but accommodating discharges at greater current intensities. An emphasis of response onset and subsequent rate accommodation resulted from the activation of a voltage- and time-dependent sustained outward rectification in a range depolarized from rest. Responses to hyperpolarizing pulses were characterized by an inward rectification, which caused a depolarizing voltage sag in a range negative to –65 mV. Peristimulus time histograms were multimodal, and discharge regularity was evident in narrow unimodal interspike interval time histograms and low coefficients of variation. The accommodation time course was usually fit best by two exponentials with time constants of τ₁=3–8 ms and τ₂=32–97 ms. Delay neurons responded with a regular repetitive firing to depolarization by current pulses. However, repetitive spike discharge occurred with a prolonged, variable delay of 25–180 ms. High current intensities evoked an additional onset spike with short, precise latency. Activation of a transient outward conductance in the depolarized voltage range caused an early repolarization, which terminated as a depolarizing ramp, reaching spike threshold after the delay. Flat peristimulus time histograms characterized the repetitive discharge in spite of narrow unimodal interspike interval time histograms and low coefficients of variation. Intracellular neurobiotin injections revealed morphological differences between these classes. Chopper neurons were large and fusiform, with a bipolar dendritic distribution oriented perpendicular to the curvature of the LSO. Delay neurons were small and spherical, with highly branched tortuous dendritic arbours of bipolar origin and variable orientation. Chopper and delay neurons are probably LSO principal cells and lateral olivocochlear efferent neurons, respectively. Our findings suggest that the pattern of firing activity of LSO neurons to sound, in vivo, is determined to a large extent by intrinsic membrane properties. Somato-dendritic integration of synaptic inputs are fundamental to the encoding of interaural sound differences, but membrane non-linearities play an important role in determining postsynaptic response patterns. Received: 3 December 1997 / Accepted: 28 July 1998     

Effects of neonatal C-fiber depletion on the integration of paired-whisker inputs in rat barrel cortex

In the present study we used computer-controlled mechanical displacement of paired whiskers in normal and C-fiber-depleted rats to quantitatively examine the role of C-fibers in the receptive field properties of barrel cortical cells. In rodents when adjacent whiskers are stimulated prior to the main whisker responses to the main whisker are inhibited, the degree of inhibition being a function of the inter-deflection intervals. The adjacent-whisker-evoked inhibition of barrel cells in normal and C-fiber-depleted rats using neonatal capsaicin treatment were examined by stimulation of the adjacent whisker zero, 10, 20, 30, 50 and 100 ms prior to the main whisker deflection. C-fiber depletion reduced the suppressive effect of paired whisker stimulation at all of the tested inter-stimulus intervals without changing response latencies. The main effect was observed during the later phase of response (about 13–17 ms from stimulus onset) and not during the initial responses (7–12 ms). These results suggest that the inhibitory receptive field properties of low-threshold mechanical somatosensory cells are influenced by C-fibers.     

Anodally focused polarization of peripheral nerve allows discrimination of myelinated and unmyelinated fiber input to brainstem nuclei

We investigated the ability of a novel direct current (DC) polarization technique to block selectively the conduction in peripheral myelinated nerve fibers and allowing propagation in only unmyelinated fibers. In anesthetized adult rats, distal branches of the sciatic nerve (caudal cutaneous sural and tibial nerves) were exposed for electrical stimulation of A- and C-fibers. Two specially fabricated trough electrodes of different size and surface area were placed onto the sciatic nerve. Through these proximal electrodes a controlled ramped DC was timed to coincide with the arrival of A- and C-fiber action potentials, evoked electrically at the distal nerves or naturally from the foot or ankle, with the intent of blocking propagation in A-fibers while allowing C-fiber throughput. Neuronal recordings were made both peripherally (proximal sciatic nerve fascicles or L5 dorsal roots) and centrally (single cells in the nucleus gracilis or nucleus reticularis gigantocellularis). The DC polarization was shown to block conduction in myelinated A-fibers effectively, while allowing conduction in the unmyelinated C-fibers, without activation of fibers via the DC polarization itself. This was dependent upon the following factors: electrode polarity, onset rate of polarization, peak amplitude of polarization, distance between polarizing electrodes, size difference between polarizing electrodes, and gross nerve size. These experiments demonstrate that anodally focused DC polarization, applied utilizing two trough electrodes of different sizes, is capable of effectively, reversibly, and reproducibly blocking conduction in myelinated A-fibers evoked either electrically or naturally, while still allowing conduction to occur in the unmyelinated C-fiber population. In the context of experimental usage, we have demonstrated blocking of low-threshold A-fiber, but not C-fiber, mediated inputs to the caudal brainstem. This technique should find wide application in studies involving the processing of information conveyed centrally by the unmyelinated C-fiber afferent population, including discriminating afferent responses to peripheral stimuli, the role of C-fiber input in reflex activity, and the plasticity following injury or other manipulations. Received: 14 November 1997 / Accepted: 3 March 1998     

Different mechanical transduction mechanisms for the immediate and delayed responses of rat C-fiber nociceptors.

1. In this electrophysiological study, action potentials from single C-fibers were recorded in fine filaments teased from the rat saphenous nerve. We evaluated the effect of pharmacological agents on the responses of C-fiber mechanoheat nociceptors (C-MH; n = 53) after sustained suprathreshold and subthreshold stimuli. 2. Sustained suprathreshold mechanical stimuli elicit an immediate burst of activity that quickly adapts to a low-level firing that is maintained during the stimulus. Sustained subthreshold stimuli activate C-MHs after a delay and elicit a constant, low-level firing. 3. Gentamicin, a known suppressor of mechanosensory cell activity, blocked the initial rapid burst response to suprathreshold stimuli (n = 11) but had no effect on the adaptive low-level firing. The latency of the delayed activation of C-MHs induced by sustained subthreshold stimuli was not affected by gentamicin. 4. Sphingosine, a protein kinase inhibitor, increased the latency of the delayed activation of C-MHs (n = 7) to sustained subthreshold stimuli; phorbol 12-myristate 13-acetate (TPA), a protein kinase C activator, decreased the latency of the delayed activation of C-MHs (n = 9); and 4 alpha-phorbol, an inactive isomer of TPA, had no effect on the latency of the delayed activation (n = 7). Sphingosine, TPA, and 4 alpha-phorbol had no affect on the initial burst response induced by suprathreshold stimuli. 5. K+ channel blockers, 4-aminopyridine (n = 9) and noxiustoxin (n = 5), decreased the latency of the delayed activation of C-MHs to sustained subthreshold stimuli but had no effect on the initial burst response of C-MHs to suprathreshold stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activity evoked by A- and C-afferent fibers in rat dorsal horn neurons and its relation to a flexion reflex

The responses of 56 neurons recorded in the lumbosacral spinal cord of halothane-anesthetized rats were studied following the application of mechanical stimuli to the skin on the lateral aspect of the paw or electrical stimulation of the sural nerve. Only neurons driven by A- and C-fiber stimulation were considered. The evoked activity in a nerve supplying flexor muscles, the common peroneal nerve, was also recorded to evaluate possible relations between neuronal events and reflex discharges. To quantify the reflex output we also recorded the activity of 12 motoneurons. Four different populations of dorsal horn neurons activated by C-fibers could be distinguished. The neurons were classified on the basis of their responses to mechanical stimuli and of their location in the dorsal horn. Class 1 neurons were driven by nonnoxious stimulation only. Neurons driven by nonnoxious stimuli and noxious stimuli were denoted class 2S (superficial to the location of the maximal A-beta-fiber-evoked field potentials) or class 2D (deep to the same potential). Class 3 neurons were driven by noxious stimuli only. The functional characteristics of these four classes of neurons differed in many respects. The latency for the A-beta-fiber-evoked discharge was, on average, 2 ms longer in class 2S than in class 2D neurons, indicating a polysynaptic A-beta input to the former class of neurons. The C-fiber-evoked neuronal discharge often showed time-locked peaks of activity during the interval 120-170 ms. Such peaks of activity occurred, in general, later in class 2D neurons (mean, 157 ms) than in class 2S (mean, 137 ms) or in class 3 (mean, 140 ms), suggesting that the different classes received C-fiber input via partially different routes. The responses to repeated C-fiber stimulation also differed markedly among the four classes. After 16 single electrical stimulations (100 T (T = threshold strength for activating A-beta-afferents), 1 Hz), the C-fiber-evoked discharge in class 2D neurons was increased by 196%, whereas the corresponding value for those in classes 2S, 3, and 1 was 41, 24, and 38%, respectively. Ten of 14 class 2D neurons showed a simultaneous increase of the A-fiber-evoked discharge, indicating an increased excitability of these neurons after repeated impulses in C-fiber afferents. An early reflex discharge (latency, 6-10 ms) was evoked in the common peroneal nerve by electrical stimulation of the sural nerve.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of odor stimulation on antidromic spikes in olfactory sensory neurons

Spikes were evoked in rat olfactory sensory neuron (OSN) populations by electrical stimulation of the olfactory bulb nerve layer in pentobarbital anesthetized rats. The latencies and recording positions for these compound spikes showed that they originated in olfactory epithelium. Dual simultaneous recordings indicated conduction velocities in the C-fiber range, around 0.5 m/s. These spikes are concluded to arise from antidromically activated olfactory sensory neurons. Electrical stimulation at 5 Hz was used to track changes in the size and latency of the antidromic compound population spike during the odor response. Strong odorant stimuli suppressed the spike size and prolonged its latency. The latency was prolonged throughout long odor stimuli, indicating continued activation of olfactory receptor neuron axons. The amounts of spike suppression and latency change were strongly correlated with the electroolfactogram (EOG) peak size evoked at the same site across odorants and across stimulus intensities. We conclude that the curve of antidromic spike suppression gives a reasonable representation of spiking activity in olfactory sensory neurons driven by odorants and that the correlation of peak spike suppression with the peak EOG shows the accuracy of the EOG as an estimate of intracellular potential in the population of olfactory sensory neurons. In addition, these results have important implications about traffic in olfactory nerve bundles. We did not observe multiple peaks corresponding to stimulated and unstimulated receptor neurons. This suggests synchronization of spikes in olfactory nerve, perhaps by ephaptic interactions. The long-lasting effect on spike latency shows that action potentials continue in the nerve throughout the duration of an odor stimulus in spite of many reports of depolarization block in olfactory receptor neuron cell bodies. Finally, strong odor stimulation caused almost complete block of antidromic spikes. This indicates that a very large proportion of olfactory axons was activated by single strong odor stimuli.     

Altered temporal pattern of mechanically evoked C-fiber activity in a model of diabetic neuropathy in the rat

While enhanced nociceptor activity has been demonstrated in models of painful peripheral neuropathy, analyses of activity pattern, which could play a role in the symptoms experienced as well as help elucidate underlying mechanism, are still limited. We evaluated the pattern of C-fiber activity, in response to mechanical and chemical stimuli, in a rat model of diabetes induced by a pancreatic beta-cell toxin, streptozotocin (STZ). In diabetic rats the number of action potentials produced by threshold and suprathreshold (10 g) sustained (60 s) mechanical stimuli was elevated in approximately half of C-fibers. These high-firing C-fibers demonstrated a disproportionate increase in interspike intervals (ISIs) between 100 and 199 ms, compared with low-firing diabetic and control C-fibers. The co-efficient of variability (CV2), a frequency independent measure of ISI variability, was also greater in high-firing fibers, compared with control fibers. Unexpectedly, instantaneous frequency of the initial burst of activity during the first second was lower in high-firing fibers, even though the average frequency over the last 59 s was significantly higher. The number of action potentials evoked by a noxious chemical stimulus, 300 and 600 mM KCl, injected adjacent to the mechanical receptive field was also significantly increased in C-fibers from diabetic rats and mechanically high-firing fibers had more action potentials in response to KCl than control fibers and a disproportionate increase in ISIs between 100 and 199 ms for responses to chemical stimuli appeared only in mechanically high-firing C-fibers, compared with the mechanically low-firing diabetic or control C-fibers. There was, however, no corresponding change in CV2 or instantaneous frequency plots for the response to chemical stimulation in mechanically high-firing fibers, as there was in the response to mechanical stimulation. Our data demonstrate specific changes in firing pattern of high-firing C-fibers in the rat model of painful neuropathy produced by STZ-diabetes that might contribute to the symptoms experienced by patients.     

Microneurography in rats: A minimally invasive method to record single C-fiber action potentials from peripheral nerves in vivo

Microneurography is a method suitable for recording intraneural single or multiunit action potentials in conscious subjects. Microneurography has rarely been applied to animal experiments, where more invasive methods, like the teased fiber recording technique, are widely used. We have tested the feasibility of microneurographic recordings from the peripheral nerves of rats. Tungsten microelectrodes were inserted into the sciatic nerve at mid-thigh level. Single or multiunit action potentials evoked by regular electrical stimulation were recorded, digitized and displayed as a raster plot of latencies. The method allows unambiguous recording and recognition of single C-fiber action potentials from an in vivo preparation, with minimal disruption of the nerve being recorded. Multiple C-fibers can be recorded simultaneously for several hours, and if the animal is allowed to recover, repeated recording sessions can be obtained from the same nerve at the same level over a period of weeks or months. Also, single C units can be functionally identified by their changes in latency to natural stimuli, and insensitive units can be recognized as ‘silent’ nociceptors or sympathetic efferents by their distinctive profiles of activity-dependent slowing during repetitive electrical stimulation, or by the effect on spontaneous efferent activity of a proximal anesthetic block. Moreover, information about the biophysical properties of C axons can be obtained from their latency recovery cycles. Finally, we show that this preparation is potentially suitable for the study of C-fiber behavior in models of neuropathies and nerve lesions, both under resting conditions and in response to drug administration.     

Short latency inputs to phrenic motoneurones from the sensorimotor cortex in the cat

Summary Short latency responses were recorded from C₅ phrenic roots and intracellularly from phrenic motoneurones following stimulation of the pericruciate cortex or medullary pyramids in cats anaesthetized with Nembutal or chloralose-urethane. Focal stimulation of the cortical surface (single pulses, 0.5–2 ms, 0.3–8 mA) during inspiration evoked EPSPs (latency 4.7 ± 1.7 ms, rise time 1.9 ± 1.1 ms, amplitude 0.22 to 3.94 mV) in 42% of motoneurones studied (n = 107). The EPSPs were absent, or on average 60% smaller, following stimulation during expiration. In all but two motoneurones, during both inspiration and expiration, hyperpolarizing potentials were observed either following the initial depolarization or alone. They could be reversed by hyperpolarizing current or chloride injection. Stimulation of the pyramidal tract at mid medullary level (1 to 3 pulses, 0.2 ms) evoked short latency excitation in phrenic motoneurones only with currents of more than 200 A. Smaller stimuli applied to the medial reticular formation above the pyramidal tract evoked excitation (onset latency 1.5–3.2 ms) in which the earliest part was probably monosynaptic. These results show that the corticospinal responses in phrenic motoneurones are both excitatory and inhibitory. They are not transmitted through the pyramidal tract and are at least disynaptic. Excitation evoked from the medullary pyramidal tract can be explained by current spread beyond the pyramidal tract fibres.     

Peripheral and transcortical loops activated by electrical stimulation of the tibial nerve in the monkey

Peripheral and supraspinal loops activated by electrical stimulation of the tibial nerve were studied in alert monkeys. Weak conditioning stimuli below the threshold for muscle contraction and strong conditioning stimuli which elicited a direct motor response and an H-reflex were applied to the tibial nerve, while soleus EMG was recorded. The motoneuronal excitability was measured with a test H-reflex for different intervals between conditioning and test stimuli. A facilitation of the motoneuronal pool occurred with a latency of about 50 ms for weak, as well as strong, conditioning stimuli. The facilitation was superimposed on a long-lasting inhibition which was more pronounced when strong stimuli were used. The shape of the excitability curve of the motoneurons after strong conditioning stimuli was studied before and after various lesions. The shape of the curve did not change after an ipsilateral cerebellectomy except that the facilitation was more pronounced. A few days after pyramidotomy, the facilitation diminished in size but it recovered to its initial size after 3 months. Spinal hemisection did not abolish the facilitation. We concluded from these results that a peripheral loop was activated when conditioning stimuli above the threshold for a direct motor and reflex response were applied. The facilitation might be mediated by the muscle contraction and following activation of muscle afferents. Superimposed on thus loop is a previously demonstrated transcortical loop of similar latency.     

The effects of task type and task requirements on the dissociation of skin conductance responses and secondary task probe reaction time

Although task-irrelevant events elicit smaller skin conductance responses (SCRs) than do task-relevant events, secondary task probe reaction time (RT) is often slower during the former. Three experiments (N = 48 in each) examined the effects of task demands, instructions, and stimulus discriminability on this dissociation effect. SCRs were larger to task-relevant stimuli in all experiments regardless of experimental manipulation. Subjects in Experiment I counted either all tones of one pitch (high/low group) or longer-than-usual tones of one pitch (longer group). There was more RT slowing during task-irrelevant tones at a 250-ms probe position in the high/low group and at a 150-ms probe position in the longer group. Experiment 2 employed differential Pavlovian conditioning in which the offset of task-relevant stimuli (CS+) coincided with the onset of a shock stimulus. Half the subjects were told which stimulus would be followed by shock (information group), whereas the others received no information (no-information group). Increased RT slowing during CS? was restricted to the no-information group. Experiment 3 employed visual conditioned stimuli that were easy or difficult to discriminate. RT slowing at 4,000 ms was greater during CS+, whereas there was a tendency for more RT slowing during CS? at 150 ms. There was no effect for CS discriminability. The results suggest that during both simple discrimination and during Pavlovian conditioning, task-irrelevant stimuli are more actively processed than task-relevant stimuli within the first 250 ms of stimulus presentation.     

Fear potentiated startle at short intervals following conditioned stimulus onset during delay but not trace conditioning

The latency of conditioned fear after delay and trace conditioning was investigated. Some argue that delay conditioning is not dependent on awareness. In contrast, trace conditioning, where there is a gap between the conditioned stimulus (CS) and the unconditioned stimulus (US), is assumed to be dependent on awareness. In the present study, a tone CS signaled a noise US presented 1000 ms after CS onset in the delay conditioning group. In the trace conditioning group, a 200-ms tone CS was followed by an 800-ms gap prior to US presentation. Fear-potentiated startle should be seen at shorter intervals after delay conditioning compared to trace conditioning. Analyses showed increased startle at 30, 50, 100, and 150 ms after CS onset following delay conditioning compared to trace conditioning. This implies that fear-relevant stimuli elicit physiological reactions before extended processing of the stimuli occur, following delay, but not trace conditioning.     

Segmental and descending influences on intraspinal thresholds of single C-fibers.

1. The effect was studied of various conditioning stimuli on the threshold of single C-fibers near their spinal terminals. Spikes were recorded in L6 and L7 dorsal root ganglia of cats. A stimulating electrode in the superficial dorsal horn delivered periodic pulses whose widths were adjusted automatically to near threshold for antidromic spike production. Most units were classified according to their adequate cutaneous stimuli, as C-mechanoreceptors, high-threshold mechanoreceptors, or polymodal nociceptors. 2. Orthodromic activity in all units increased their threshold for up to several minutes; the maximum and rate of decay depended on the amount of activity. This phenomenon parallels the hyperpolarizing afterpotential of C-fibers in peripheral nerve and, we suggest, is probably due to the aftereffect of impulses. 3. Cutaneous conditioning stimuli were applied for 10-20 s near the receptive fields of tested units, but without activating them. During the brushing of skin hair, all threshold changes were decreases; during pinching most changes were increases; during noxious heating the numbers of increases and decreases were similar. It will be necessary to analyze the responses of postsynaptic cells in order to know the physiological significance of these threshold changes. 4. Stimulation in the nucleus raphe magnus caused in half the units higher intraspinal thresholds. If this result is causally related to the previously reported inhibition of neuronal responses in the dorsal horn by the nucleus raphe magnus (NRM), then increased thresholds could reflect either direct presynaptic inhibition or facilitation of inhibitory connections. 5. No correlation between receptive-field classification and the response of terminals to natural cutaneous stimulation or stimulation of the NRM could be discovered. However, the terminals of all kinds of C-fibers differ from A-fibers in their reaction to noxious cutaneous and NRM stimulation, suggesting they are subject to a different system of control.     

Simulation of the Interaction Between Muscle Fiber Conduction Velocity and Instantaneous Firing Rate

In this study, the relationships between the early and late afterpotentials and velocity and amplitude recovery functions (VRF and ARF) in skeletal muscle were examined using model simulation. A mathematical model of the muscle fiber action potential, that incorporated a tubular slow potassium conductance, was developed and used to simulate muscle fiber action potentials at a range of interpulse intervals. The slow potassium conductance produced an afterhyperpolarization which resulted in supernormal action potential conduction velocity and amplitude for interpulse intervals >7 ms. Increasing the number of conditioning stimuli caused a further increase in conduction velocity and amplitude, and an additional phase of supernormality, with a peak at approximately 100 ms. Positive correlations between instantaneous firing rate and both conduction velocity and amplitude were also observed during simulation of repetitive stimulation of the muscle fiber. The relationships were eliminated when the slow potassium conductance channel was removed from the model. The results suggest that an afterhyperpolarization, possibly due to a slow tubular potassium conductance, could cause the VRF and ARF observed in muscle. They additionally suggest that the positive correlations between instantaneous firing rate, conduction velocity, and amplitude are directly related to the VRF and ARF.     

Differences in responses to 70 dB clicks of cerebellar units with simple versus complex spike activity: (i) In medial and lateral ansiform lobes and flocculus; and (ii) Before and after conditioning blink conditioned responses with clicks as conditioned stimuli

Activity was recorded from 554 cerebellar units in eleven conscious cats to determine if responses to 70 dB clicks differed in units with simple and complex spike discharges. Effects of region of recording and behavioral state (with click used as a conditioned stimulus for conditioning) were also assessed. Cells with only simple spikes were distinguished from cells that had the following types of complex spike events: Type I—simple or initial spike followed >1 ms by multiple spikes with baseline displacement (classical complex spikes), Type II—followed ≤1 ms by spikes with or without baseline displacement (spikes in the absolute refractory period should arise from a separate site of initiation), and Type III—followed by spikes and displacement too close to the baseline noise to distinguish as Type I or II. Among the groups mean baseline activity was greatest in cells with Type I complex spikes, least in cells with Type III complex spikes, and greater in Type II cells than simple cells. Significant increases in activity within 32 ms of presenting clicks were found in the groups of Type II cells and simple cells. These appear to be the main auditory responsive cells of the cerebellar regions studied. Activity of Type II cells best reflected the temporal properties of the click; responses of simple cells had slower onsets (except in flocculus) and longer durations. Responses to click in Type II and simple cells differed in recordings from: (i) lateral ansiform lobe (lateral crus I and portions of crus II), (ii) medial ansiform lobe (medial crus I), and (iii) flocculus. The largest mean responses above baseline in the first 32 ms after click were found in Type II cells of the lateral ansiform lobe with onsets of 8–16 ms. Magnitudes of response differed before and after conditioning and backward conditioning. In the lateral ansiform lobe, the <32 ms response to click was greater in Type II than simple cells in each state, but showed a greater increase above baseline after backward conditioning when conditioned responses were not produced than after conditioning. The onset of increased activity to click conditioned stimuli in Type II cells of the lateral ansiform region preceded the onset of the blink conditioned response after conditioning, consisted almost entirely of simple spikes, and reflected an increase in magnitude of response as opposed to an increased number of responsive units. After conditioning, an increased number of units in the flocculus responded to click conditioned stimuli in the 16–24 ms post stimulus period. Of the 16 cells with an onset of increased activity at this time, eight showed only simple spike activity. Seven of the remaining eight cells (all Type II) showed a significant increase in conditioned stimulus-evoked complex spiking above the low (usually <1/s) baseline level of complex spike discharges.

The findings support the conclusions that cerebellar units can respond rapidly enough to acoustic stimuli to play a role in auditory as well as motor processing and that the responses to 70 dB clicks differ among cells with simple and complex spike discharges. The differences are influenced substantially by the region of cerebellar recording and the behavioral state. The findings in cells of the flocculus offer the first evidence that complex as well as simple spike activity can contribute to an increased probability of discharge to click as a conditioned stimulus after conditioning.