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L M Mendell

Publications and source records attributed to L M Mendell.

At least 55 records · Page 3Linked to original sources
PubMed1992

Neonatal Anti-NGF Treatment Reduces the Adelta- and C-Fibre Evoked Vasodilator Responses in Rat Skin: Evidence That Nociceptor Afferents Mediate Antidromic Vasodilatation.

Electrical stimulation of cutaneous nerve Adelta-fibres can lead to increases in skin blood flow (Jänig and Lisney, J. Physiol. (Lond.), 415, 477 - 486, 1989). Here we have examined this phenomenon in adult rats treated neonatally with antisera to nerve growth factor between postnatal days 2 and 14. This treatment forces many Adelta nociceptor afferents to take on the phenotype of low-threshold D-hair afferents (Lewin et al., J. Neurosci., 12, 1896 - 1905, 1992). In animals treated this way we found a parallel decrease in the ability of Adelta-fibres to increase skin blood flow. The increase in blood flow evoked by C-fibre stimulation was also reduced, but no change was seen in the ability of C-fibres to elicit neurogenic extravasation in skin. These data may be taken as evidence that in rats, amongst the cutaneous Adelta-fibres, nociceptor but not D-hair afferents are capable of producing blood flow changes in the skin.

Journal Article↗
PubMed1992

Somal membrane properties of physiologically identified sensory neurons in the rat: effects of nerve growth factor.

1. Intracellular recordings were made in situ from physiologically identified dorsal root ganglion (DRG) cells in untreated rats aged 5-8 wk and in rats treated from birth to 5 wk of age with nerve growth factor (NGF) or antisera against NGF (anti-NGF). 2. As demonstrated in cats, the shape of the somal action potential (AP) of DRG cells of normal rats is correlated with peripheral receptor type. Cells that innervate high-threshold mechanoreceptors (HTMRs) and thus respond to noxious stimulation of skin or deep tissue in the periphery have long-duration APs characterized by an inflection on the falling limb of the spike. Cells that innervate low-threshold mechanoreceptors (LTMRs) have briefer APs that lack the inflection. Somal APs of neurons supplying HTMRs tend to be larger in amplitude, have slower peak rates of rise, and on average have longer afterhyperpolarizations than those innervating LTMRs. 3. It was also found that the somal APs of HTMRs were not blocked by 200 microM tetrodotoxin (TTX) applied directly to the surface of the ganglion. In contrast, those of LTMRs were rapidly and irreversibly blocked. Despite the difference in the sensitivity of the soma, axonal conduction in both types of cells was abolished by TTX. 4. Chronic treatment with NGF resulted in an increase in duration of the falling limb of the spike compared with untreated control animals or animals treated with preimmune rabbit serum. This was true only in cells that had long duration APs to begin with, i.e., HTMRs. LTMRs were unaffected by the treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1992

On the role of nerve growth factor in the development of myelinated nociceptors.

We have previously demonstrated that administration of antisera against NGF (anti-NGF) can have profound effects on developing primary afferents (Ritter et al., 1991). Chronic administration of anti-NGF to rats beginning on the day of birth results in a severe depletion of cutaneous A delta high-threshold mechanoreceptors (HTMRs) from the sural nerve. Here we have carried out further experiments in order to define the period of time over which this change in the cutaneous afferent population can be produced, and to investigate a possible mechanism for the change. Treatment with anti-NGF from postnatal day (PND) 0-14 resulted in a depletion of cutaneous A delta HTMRs from the sural nerve and also a 20% loss of sensory neurons. However, treatment from PND 2-14 produced an identical deficit of HTMRs without any accompanying cell death. Thus, the depletion of cutaneous A delta HTMRs can be achieved in the absence of cell death induced by anti-NGF treatment. It was also found that a 7 d treatment from PND 4-11 was sufficient to reproduce this effect, but that 7 d treatments earlier (PND 2-9) or later (PND 7-14) within the first 2 weeks were much less effective. This critical period, PND 4-11, corresponds to a period of anatomical change in the innervation of the skin, from epidermal innervation to primarily dermal innervation (Fitzgerald, 1967; Reynolds et al., 1991). In every case where anti-NGF treatment reduced the proportion of HTMRs, there was a reciprocal increase in the proportion of sensitive A delta hair follicle (D-hair) afferents. We hypothesize that in the absence of NGF, developing cutaneous A delta HTMRs do not die but innervate novel targets in the dermis and become D-hair afferents instead.

Animals↗
PubMed1991

Requirement for nerve growth factor in the development of myelinated nociceptors in vivo.

In adult animals, sensory neurons innervating the skin are phenotypically diverse. We have now investigated whether nerve growth factor (NGF) has a physiological role in the development of this diversity. We gave antisera against NGF to rats from postnatal day 1 (PND 1) to adulthood (5 weeks). We found a virtually complete depletion of high threshold mechanoreceptors conducting in the A delta range (2-13 ms-1) in the sural nerve. This afferent type, normally present in large numbers, appeared to have been replaced by D-hair afferents, sensitive mechanoreceptors which normally are relatively rare. NGF deprivation had this effect only in early postnatal life; treatment from postnatal day 14 to adulthood had no effect. We conclude that the presence of NGF postnatally in skin is necessary for the proper phenotypic development of A delta cutaneous nociceptors.

Afferent Pathways↗
PubMed1991

Modulation of synaptic transmission at Ia-afferent fiber connections on motoneurons during high-frequency stimulation: role of postsynaptic target.

1. High-frequency stimulation of single group Ia-fibers results in modulation of excitatory postsynaptic potential (EPSP) amplitude recorded in target motoneurons. This can be either positive (EPSP amplitude increases in response to successive stimuli in the high-frequency burst) or negative (decrease in EPSP amplitude). We have investigated whether the magnitude of modulation is associated with the stimulated afferent, the responding motoneuron, or the amplitude of the EPSP. 2. In agreement with previous findings, we found that positive modulation tends to occur at connections generating small EPSPs and negative modulation, at those producing large EPSPs. Because large EPSPs generally are evoked in motoneurons with low values of rheobase, we found, as anticipated, that connections on low rheobase motoneurons are prone to negative modulation during high-frequency stimulation, whereas those on high rheobase motoneurons (which tend to generate small EPSPs) are prone to positive modulation. 3. In experiments where the projection of multiple afferents to a single motoneuron was studied, we found that amplitude modulation was similar despite differences in EPSP amplitude. Thus in a given motoneuron there is no relationship between modulation and amplitude, in contrast to the existence of such a relationship in the population of connections as a whole. 4. In the converse experiments where the projection of single afferents to multiple motoneurons was studied, we found marked variability in the modulation patterns with clear indications that amplitude and modulation are correlated as in the entire population of Ia/motoneuron connections. 5. We tested the constancy of modulation patterns evoked in a given motoneuron by comparing the modulation patterns evoked in motoneurons by single fibers, and by stimulation of the heteronymous nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Interaction of baseline synaptic noise and Ia EPSPs: evidence for appreciable negative correlation under physiological conditions.

1. In the anesthetized cat, simultaneous intracellular recordings from pairs of spinal motoneurons were undertaken to see whether the amplitude of single-fiber excitatory postsynaptic potentials (EPSPs) in both cells fluctuated in a coordinated manner that would indicate correlative mechanisms at either pre- or post-synaptic level. Although these recordings revealed correlated fluctuations in the baseline, the single-fiber Ia/EPSPs recorded with the spike-triggered averaging technique exhibited no correlated fluctuations and, unexpectedly, virtually no increase in baseline variance associated with the EPSP. However, the fact that these experiments were carried out under conditions of high baseline synaptic noise (i.e., with muscle stretch) may have influenced the outcome because of interaction between EPSP and synaptic noise, and this possibility was evaluated explicitly. 2. A given connection was studied under low noise by electrically stimulating a single Ia fiber in the absence of muscle stretch. The same connection was analyzed under conditions of high noise by activating the fiber and all other stretch receptor afferents with muscle stretch and by using spike-triggered averaging to extract the EPSP. The differences in mean EPSP amplitude at a given connection under conditions of low noise and high noise were minimal. 3. Fluctuations in EPSP amplitude were then determined to see whether these were influenced by presence of baseline synaptic noise and whether the interaction was nonlinear. Two methods were used to measure EPSP fluctuations: measurement of the variance associated with the EPSP, and determination by the use of deconvolution methods of the discrete amplitude components associated with the EPSP. 4. An increase in baseline variance was observed during the EPSP evoked under low noise conditions at all six connections studied in this way. This increase disappeared at two of these connections when examined under high noise. This may help to explain the results obtained in pairs of motoneurons. 5. The deconvolution results were used to calculate the variance of the noise-free EPSP. This was found to differ from the variance of the EPSP amplitude distribution measured directly from the change in baseline variance associated with the EPSP. Analytic techniques suggested that this difference could be explained in most cases by negative correlation between the EPSP and baseline synaptic noise. These considerations led to an analytic method to assess the reliability of the deconvolution result. 6. Simulation studies revealed that the baseline variance increase associated with the EPSP is also highly dependent on the correlation between signal and noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Modulation of synaptic transmission at Ia-afferent connections on motoneurons during high-frequency afferent stimulation: dependence on motor task.

1. Monosynaptic excitatory postsynaptic potentials (EPSPs) were evoked in medial gastrocnemius motoneurons by maximal group Ia stimulation of the heteronymous lateral gastrocnemius-soleus nerve in anesthetized cats. Three different patterns of high-frequency stimulation were delivered to the nerve, and the EPSPs were averaged in register (1, 2, . . ., n) for each. 2. One pattern ("Burst") consisted of 32 shocks delivered every 2 s at an interstimulus interval of 6 ms (167 Hz). The second pattern ("Stepping") was a frequency-modulated burst of 52 shocks derived from a recording of a spindle during stepping and was delivered every 2 s. The third pattern ("Paw Shake") was from an extensor spindle afferent recorded during rapid paw shake and was delivered in groups of six bursts with an interburst interval of 75 ms and a 3-s pause between groups of six bursts. The EPSPs in each burst were averaged in register (1, 2, . . ., n) so that the relative amplitude of each EPSP in the burst could be ascertained. The EPSP produced by low-frequency stimulation of the nerve (18 Hz) was also recorded for each motoneuron. 3. The initial EPSP in most bursts was larger than the EPSP measured as a result of low-frequency stimulation. This potentiation, defined as the ratio of the amplitude of the initial EPSP of the response to that of the low-frequency control, was found to vary systematically as a function of amplitude of the control EPSP as well as the stimulus paradigm used.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthesia↗
PubMed1991

Tuning of spinal networks to frequency components of spike trains in individual afferents.

Cord dorsum potentials (CDPs) evoked by primary afferent fiber stimulation reflect the response of postsynaptic dorsal horn neurons. The properties of these CDPs have been shown to vary in accordance with the type of primary afferent fiber stimulated. The purpose of the present study was to determine the relationships between frequency modulation of the afferent input trains, the amplitude modulation of the evoked CDPs, and the type of primary afferent stimulated. The somata of individual primary afferent fibers were impaled in the L7 dorsal root ganglion of alpha-chloralose-anesthetized cats. Action potentials (APs) were evoked in single identified afferents via the intracellular microelectrode while simultaneously recording the response of dorsal horn neurons as CDPs, or activity of individual target interneurons recorded extracellularly or intracellularly. APs were evoked in afferents using temporal patterns identical to the responses of selected afferents to natural stimulation of their receptive fields. Two such physiologically realistic trains, one recorded from a hair follicle and the other from a slowly adapting type 1 receptor, were chosen as standard test trains. Modulation of CDP amplitude in response to this frequency-modulated afferent activity varied according to the type of peripheral mechanoreceptor innervated. Dorsal horn networks driven by A beta afferents innervating hair follicles, rapidly adapting pad (Krause end bulb), and field receptors seemed "tuned" to amplify the onset of activity in single afferents. Networks driven by afferents innervating down hair follicles and pacinian corpuscles required more high-frequency activity to elicit their peak response. Dorsal horn networks driven by afferents innervating slowly adapting receptors including high-threshold mechanoreceptors exhibited some sensitivity to the instantaneous frequency, but in general they reproduced the activity in the afferent fiber much more faithfully. Responses of synaptically coupled dorsal horn neurons belonging to either hair follicle or SA1 fiber-driven networks to frequency-modulated input were in agreement with the CDP results, confirming that CDP amplitude modulation is a true reflection of EPSP amplitude modulation in at least a subset of dorsal horn neurons comprising the network.

Action Potentials↗
PubMed1990

Correlation of monosynaptic field potentials evoked by single action potentials in single primary afferent axons and their bouton distributions in the dorsal horn.

The relationship between structure and function of the projections of single identified primary cutaneous axons was investigated by recording cord dorsum potentials at 4 sites in response to electrical stimulation of the single axon and visualizing the boutons of the axon stained by intracellular injection of horseradish peroxidase. The rostrocaudal extent of boutons differed from fiber to fiber ranging from 4.14-11.50 mm; their location in the dorsal horn also varied in agreement with the known somatotopy of the presynaptic neuropil and dorsal horn neurons. Rostrocaudal distributions of cord dorsum potentials and boutons of individual fibers revealed good agreement. Cord dorsum potential amplitude and length of the spinal projection were positively correlated with number of boutons, but no correlation with bouton density was found. The spinal projection of afferents innervating slowly adapting type 1 mechanoreceptors exhibited a greater rostrocaudal extent (mean: 8.48 mm) than those innervating rapidly adapting mechanoreceptors (i.e., hair follicle and field receptors: mean: 5.87 mm). Although the mean total number of boutons was greater for axons with slowly adapting receptors (7,250/fiber) than for axons of rapidly adapting receptors (4,677/fiber), no differences in the longitudinal density of boutons (boutons/mm) were observed. Likewise, summed amplitudes of cord dorsum potentials at the 4 recording electrodes were larger for SA1 afferents than for those of field and hair follicle afferents. A major role for the number of boutons in determining these differences is supported by the finding that the calculated average contribution per bouton to cord dorsum potentials (expressed as an amplitude coefficient a) was similar for slowly and rapidly adapting afferents. No evidence was found for regions in which boutons did not contribute to the cord dorsum potential.

Action Potentials↗
PubMed1989

Mismatches between peripheral receptor type and central projections after peripheral nerve regeneration.

The central projections of axotomized fibers were assessed to investigate how well these matched the modality of the reinnervated receptor. Our results using intrafiber injection of horseradish peroxidase (HRP) and cord dorsum potentials evoked by intracellular stimulation of single afferents indicate that regenerating fibers can innervate inappropriate receptors. Mismatches included: (i) reinnervation of muscle receptors by cutaneous afferents, and vice versa; (ii) reinnervation of low threshold cutaneous receptors by nociceptive afferents; and (iii) reinnervation of slowly adapting receptors by fibers normally supplying rapidly adapting receptors.

Action Potentials↗
PubMed1988

Modulation of EPSP amplitude during high frequency stimulation depends on the correlation between potentiation, depression and facilitation.

High frequency bursts were delivered every 2 s to single group Ia fibers in anesthetized cats. The modulation of EPSP amplitude in target motoneurons was determined. Changes in EPSP amplitude during the high frequency stimulation (facilitation or depression) were correlated with those occurring 2 s after the burst (potentiation). Connections displaying high levels of potentiation exhibited more depression resulting in a net negative modulation. Negative amplitude modulation was greatest at connections on low rheobase ('small') motoneurons that develop large EPSPs. Positive modulation occurred at a subset of connections on high rheobase ('large') motoneurons that exhibit small EPSPs. We suggest that these synaptic properties permit the appropriate amount of depolarization to be delivered to the heterogeneous elements of the motoneuron pool during high frequency Ia fiber activity characteristic of the step cycle.

Action Potentials↗
PubMed1988

The spinal projection of individual identified A-delta- and C-fibers.

1. Recordings were made from individual sensory neurons with an A-delta peripheral conduction velocity, either intrasomally in the L7 dorsal root ganglion, or extracellularly in Lissauer's Tract or in the dorsal root close to the root entry zone. The spinal projection of these afferents was assessed by their antidromic response to stimulation of the dorsal columns (DC) or Lissauer's Tract (LT) at the L5/L6 border. The adequate stimulus was also ascertained. 2. A-delta-fibers could be divided into two groups: high-threshold mechanoreceptors from either skin or muscle (HTMRs) and low-threshold mechanoreceptors (LTMs), primarily Down Hairs. A third group of cells recorded intrasomally had broad spikes with shoulders on the downstroke characteristic of A-delta-nociceptors and were so classified provisionally, although no adequate stimulus could be identified. HTMRs and broad spike cells projected either in DC or LT, but LTMs projected only in DC, never in LT. About one-quarter of both groups failed to project rostrally as far as L5/L6. 3. Cells with unmyelinated axons recorded intrasomally were found to supply either low-threshold or high-threshold mechanoreceptors. Unlike A-delta-cells, all these cells had broad spikes with shoulders on the downstroke. Proportionally fewer C-fibers than A-delta-fibers projected as far as one segment rostral from their root entry zone. Of those that did, axons supplying low-threshold mechanoreceptors projected only in DC, whereas those innervating high-threshold mechanoreceptors could project either through LT or DC. 4. A-delta-fibers supplying LTMs and HTMRs exhibited a similar reduced conduction velocity was reduced even further in the spinal cord but much more for HTMRs than for LTMs. For C-fibers the conduction velocity decrease was more substantial in the dorsal root for HTMRs than for LTMs. 5. These findings suggest that axons innervating different peripheral receptors exhibit characteristic cellular properties. They confirm that the primary afferent component of Lissauer's Tract is specialized as a "pain pathway" but also indicate that the dorsal columns may play some role in the transmission of nociceptive information.

Afferent Pathways↗
PubMed1988

Properties of somata of spinal dorsal root ganglion cells differ according to peripheral receptor innervated.

1. Intracellular recordings were made in the somata of dorsal root ganglion cells in the L7 or S1 DRG in cats anesthetized with alpha-chloralose. The properties of the action potentials (amplitude, duration, peak rate of rise), duration of afterhyperpolarization (AHP), magnitude of inward rectification, and axonal conduction velocity were measured. The adequate stimulus was determined, and the extent to which these properties are correlated was investigated. 2. All cells with receptive fields could be classified as mechanoreceptors. Most cells with A-beta-axons (greater than 36 m/s) could be activated by gentle mechanical stimulation but a small minority with conduction velocity in the low end of the A-beta-range were nociceptors. Cells with A-delta-axons (2-36 m/s) innervated either the very sensitive Down hair follicles (D-hairs) or high-threshold mechanoreceptors (HTMRs). In addition a group of A-delta-fibers was found for which no receptive field could be described. Their spikes, AHPs, and membrane properties were indistinguishable from those of cells supplying HTMRs (see below) and they were lumped together with A-delta-HTMRs. 3. A-beta-neurons exhibited smaller, briefer spikes than A-delta-neurons, even those supplying D-hairs. Peak rate of rise (dV/dt)max and inward rectification were significantly larger in A-beta-cells than in A-delta s, whereas AHP duration and input resistance were smaller. However, the values of these parameters in cells of a given conduction velocity range were generally associated with receptor type. 4. A-delta-HTMRs exhibited spikes of greater amplitude and duration, longer AHP duration, and smaller inward rectification than D-hairs. The long duration of these spikes was due largely to a prominent hump on their descending limb. Input resistance was similar in both groups of cells. 5. A-beta-HTMRs differed from A-beta-cells innervating low threshold receptors in the same general way that A-delta-HTMRs differed from D-hairs. However, A-beta-LTMRs supplying different receptor types (e.g., slowly adapting type I, Pacinian corpuscles, etc.) exhibited no correlation between receptor type and electrophysiology of the soma. 6. These differences in spike parameters occur at the level of the membrane rather than in the degree of somal invasion because the largest amplitude spikes also exhibited the slowest time course (i.e., in HTMRs). Systematic variation in AHP duration and inward rectification also suggest differences in the proportions of ionic channel types among these cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Functional specialization of central projections from identified primary afferent fibers.

1. Somata of primary afferent fibers were impaled in the L7 or S1 dorsal root ganglion in cats anesthetized with alpha-chloralose. Individual cells (n = 182) were characterized according to receptive field (RF) and by the peripheral mechanoreceptor they innervated. They were then stimulated intracellularly while recording the evoked cord dorsum potentials (CDPs) simultaneously at four sites. CDPs were recorded in response to single fiber stimulation while varying both the frequency and the numbers of action potentials (APs) evoked per trial. Stimulus parameters included: 1) single APs evoked at both high (18 Hz) and low (0.67 Hz) frequencies, 2) pairs of APs (50-ms ISI) delivered at 0.67 Hz, and 3) trains of four APs (20-ms ISI) also delivered at 0.67 Hz. The properties of the CDPs and their relationship to receptor type innervated by the fiber were determined. 2. CDPs evoked by 18-Hz stimulation consisted of an axon fiber spike followed at a short latency [600 +/- 9.9 (SEM) microseconds] by the onset of a monophasic negative wave. A-beta-afferent fibers innervating slowly adapting type 1 and 2 receptors (SA1 and SA2) evoked the largest amplitude CDPs followed by field and hair follicle afferents while A-delta-fibers rarely produced measurable CDPs at this frequency of stimulation. 3. The magnitude of the CDP varied at the four recording sites with clear evidence in individual experiments that the projections of individual fibers are somatotopically organized. 4. CDPs evoked by 0.67-Hz stimulation had the same onset latency but were larger in amplitude and had longer time-to-peak and durations than those elicited by high frequency stimulation. Under these conditions A-beta-fibers innervating hair follicles produced the largest CDPs followed by field receptors and SA1s and SA2s. A-beta- and A-delta-fibers innervating high threshold mechanoreceptors (HTMRs) and A-delta-fibers innervating down hair follicles (D-hairs) produced CDPs of amplitude and duration similar to those evoked by slower A-beta-fibers. 5. The response to a test stimulus delivered 50 ms after a conditioning stimulus to the same single fiber was always depressed in the case of A-beta-fibers supplying rapidly adapting receptors. The conditioning stimulus exerted relatively little effect on the response to the test stimulus for A-beta- fibers innervating slowly adapting receptors. The test response to stimulation of A-beta- and A-delta- fibers innervating HTMRs was consistently facilitated while stimulation of A-delta- D-hairs evoked either marked facilitation or slight depression.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1988

Physiological aspects of synaptic plasticity: the Ia/motoneuron connection as a model.

Damage to peripheral nerves results in substantial changes in the function of spinal synapses that mediate the monosynaptic reflex. These alterations consist independently of those caused by axotomy of the postsynaptic neuron and those produced by axotomy of the presynaptic neuron. Synaptic transmission is depressed following interruption of either limb of the segmental reflex loop. These reductions in EPSP amplitude are largely reversed following reinnervation of the peripheral tissue by the cut fibers, whether or not the regenerating fiber finds the correct muscle. Alterations in synaptic transmission following damage to the spinal cord itself are more variable than those noted after peripheral nerve damage. Although segmental EPSPs are generally enhanced following spinal injury (if in fact changes do occur), these increases are superimposed on motoneuron hyperpolarization, making the net changes in reflex transmission unpredictable. Furthermore, the extent of motoneuron depolarization is also influenced by the amount of temporal summation of EPSPs during the high-frequency activation of group Ia fibers known to occur, for example, during normal walking. Because EPSP amplitude is not constant during such activation and is modulated differently at connections on different motoneurons, changes in effective levels of synaptic transmission after such lesions are very difficult to predict. Thus it is not surprising that it is not possible to make close correlations between changes in amplitude of EPSPs and reflex behavior following injury (but see chapter by M.E. Goldberger and M. Murray, this volume). These alterations in synaptic transmission can occur at a considerable physical distance from the site of injury, peripheral or central. They illustrate the interdependence in elements of the nervous system, which can also be seen as changes in properties of motoneurons themselves (e.g., input resistance, after-hyperpolarization, rheobase) following many of these injuries. Altered cell properties complicate the functional interpretation of synaptic changes obtained after injury but must be considered in evaluating the sequelae of lesions in the peripheral and central nervous system.

Animals↗
PubMed1987

Distribution of potentiation following short high frequency bursts to motoneurons of different rheobase.

Potentiation of composite EPSPs has been studied at group Ia fiber/alpha motoneuron connections using a short burst of conditioning stimuli [32 shocks with 6 ms interstimulus interval (ISI)]. Potentiation reached its peak (range 1.2-2.0 X control value) 100-150 ms following the burst. Potentiation decayed slowly and was still present 2 s after the burst. High frequency burst stimulation of a nerve to a synergist muscle did not potentiate the response to stimulation of the homonymous nerve. Three independent sets of measurements suggest that the time course of decline of potentiation depends on the amount of transmission during the potentiated state. First, connections on high rheobase motoneurons exhibited more peak potentiation than those on low rheobase motoneurons but potentiation decayed more rapidly in the former than in the latter. Second, increasing the frequency of the conditioning burst enhanced peak potentiation but the rate of decay of this potentiation also increased. Third, potentiated EPSPs exhibited more low frequency depression than unpotentiated ones at the same connection suggesting that low frequency stimulation during the potentiated state could elevate the rate of decay of potentiation. The short burst paradigm could cause peak potentiation similar in magnitude to that evoked by long, high frequency trains (studied here at the same connection) but with much less of an increase in latency and rise time of the potentiated EPSP. The magnitude of potentiation was unrelated to changes in EPSP latency and rise time. These findings indicate that potentiation can act to modulate EPSP amplitude under conditions of normal motor behaviour when spindle afferents fire in patterns similar in duration and frequency to those used in the present experiments.

Animals↗
PubMed1986

The rostral projection of small diameter primary afferents in Lissauer's tract.

The rostral projection of A delta and C primary afferents in Lissauer's tract (LT) was studied by assessing how far, rostrally, axons entering at a given level could be activated by electrical stimulation. Sural A delta primary afferents in LT exhibited a rostral projection at least as far as the L3/L4 border, 4 segments rostral to their segment of entry (S1 dorsal root). C-fibers from the sural nerve projected in LT less than 2 segments rostral to their segment of entry. It is concluded that small afferent fibers can project appreciable distances along the spinal cord to bring them into regions not normally responsive to natural stimulation of the receptive field of such fibers.

Afferent Pathways↗