Flexor Withdrawal Research Articles

Repeated exposure to sham testing procedures reduces reflex withdrawal and hot-plate latencies: Attenuation of tonic descending inhibition?

Five days repeated exposure of experimentally naive rats to the experimental environment and to sham nociceptive testing procedures (‘habituation’) reduced the latency for reflex withdrawal of the hindpaw from hot water (49°C) by 43%, to that of spinalised habituated or novice animals. Hot-plate (50°C) paw lick latencies were reduced equally (40%) by habituation or parachlorophenylalanine, and were increased 32% by d,l-5-hydroxytryptophan. Neither drug affected hot-plate latencies of habituated animals. Naloxone had no effect on flexor withdrawal or hot-plate latencies in either novice or habituated animals. These results suggest that habituation substantially attenuates tonic serotonergic inhibition of spinal nociceptive transmission.

Inhibitory and facilitatory effects from the peroneal nerve onto the soleus H-reflex in normal and spinal man

The effects of conditioning stimulation of a mixed nerve in the leg, the common peroneal nerve (CPN), on the ipsilateral soleus H-reflex were compared with the effects of stimulating its cutaneous branch, the superficial peroneal nerve (SPN), in two groups of subjects — normals and patients with spinal spasticity subsequent to a clinically complete transection of the spinal cord. Condition-test delays of 20 msec to 2 sec, measured from the end of the 20 msec train (3 pulses at 100 Hz), were investigated. In normal subjects, CPN stimulation at 1.4 × MT profoundly depressed the soleus H-reflex. There was an initial depression (peak 40–90 msec) followed by a slow recovery which was incomplete at condition-test delays of 2 sec. One-half of the subjects showed a late facilitation, or disinhibition, peaking at 170–190 msec. The inhibitory effects were attributed to activation of low threshold, groups I and II, muscle afferents because stimulation of the SPN, at 1.5 × threshold for a compound action potential recorded from the CPN, had only facilitatory effects on the soleus H-reflex. Facilitation occurred at condition-test delays of 30–190 msec. The cutaneous stimulation was presumed to activate the largest, Aβ, cutaneous afferents as it elicited a weak paraesthesia on the dorsum of the foot. The results suggested that cutaneous afferents may have contributed to the late facilitation seen with CPN conditioning stimulation. In spinal cord-lesioned subjects, CPN stimulation depressed the soleus H-reflex but the decrease was less and the recovery was faster and more complete than in normals. The magnitude of the initial depression at 20–100 mse varied with the severity of the spasticity, subjects with mild spasticity showing less of a depression. Weak cutaneous conditioning stimulation either had no effect or produced a slight depression of the soleus H-reflex, providing clear evidence that transmission in the pathways mediating the facilitatory effects of cutaneous afferents onto extensor motoneuronal pools is depressed in spinal spasticity. This may shift the balance of activity toward the flexor motoneurones, thus favouring the development of, for example, flexor spasms and flexor hypertonia. Since inhibitory effects from cutaneous stimulation are associated with activation of higher threshold afferents in normal man, the present results may reflect a decrease in the threshold for flexor withdrawal reflexes commonly associated with spasticity of spinal origin.

Synaptic actions of cutaneous Aδ and C fibers on primate hindlimb α-motoneurons

Postsynaptic potentials evoked in hindlimb α-motoneurons by stimulation of a cutaneous nerve (sural) with finely graded stimulus strengths were analyzed in the primate, monitoring the spinal cord potentials and afferent nerve volleys in the sural nerve. It was observed that activities in Aαβ, Aδ and C fibers of the cutaneous nerve elicited characteristic excitatory and/or inhibitory postsynaptic potentials (EPSPs and/or IPSPs) with different latencies and durations in extensor and flexor motoneurons. Volleys in Aδ fibers of the cutaneous nerve produced EPSPs in 57% of flexor and 31% of extensor motoneurons tested, whereas IPSPs were produced by Aδ volleys in 41% of flexor and 62% of extensor motoneurons. EPSPs with longer latencies and longer durations were evoked by cutaneous C fiber volleys in 55% of flexor and 34% of extensor motoneurons, whereas IPSPs due to C volleys were recorded in 9% of flexor and 14% of extensor motoneurons. Aαβ and Aδ volleys caused motoneurons to fire in several instances, and some motoneurons discharged repetitively during the depolarizations evoked by activities in C fibers of the nerve. Central latency for transmission in interneuronal chains in the spinal cord was estimated from the onset of the cord potential (N3 wave) to the onset of the postsynaptic potential evoked by Aδ volleys. Ranges of central latencies of the EPSPs and IPSPs evoked by Aδ volleys were 2.0–7.0 ms and 3.5–8.5 ms, respectively. It is postulated that there may be at least two interneurons interposed in the excitatory reflex pathway from Aδ afferent fibers to motoneurons and the Aδ inhibitory pathway may involve longer interneuronal chains. In a few motoneurons, however, sural volleys with strengths sufficient to activate Aδ fibers produced EPSPs with a central latency of about 1 ms, suggesting activation of a disynaptic segmental pathway with one interposed interneuron. Stimulation of the sural nerve with strengths sufficient to activate cutaneous C fibers produced slow negative cord dorsum potentials with long latencies. It is proposed that primate motoneurons, which show characteristic postsynaptic potentials evoked by cutaneous Aδ and C fiber volleys, may provide a suitable model for analyzing the role of high threshold cutaneous afferent fibers not only in the flexor withdrawal reflex but also in motor control functions.

Substance P and calcitonin gene-related peptide synergistically modulate the gain of the nociceptive flexor withdrawal reflex in the rat

A comparison has been made between the effects of intrathecally administered l-glutamate, l-aspartate, substance P and calcitonin gene-related peptide (CGRP) on the excitability of rat flexor α-motoneurons activated monosynaptically by Ia afferents and polysynaptically by high mechanothreshold cutaneous afferents. At doses that do not modify the monosynaptic reflex, substance P, CGRP and some factor released from sural C-fibres increase the excitability of the nociceptive flexion reflex for prolonged periods in a multiplicative fashion. The excitatory amino acids have no such action. We suggest that one role for C-afferent neuropeptides is a long-acting gain modulation of nociceptive inputs into the spinal cord.

Relative effectiveness of C primary afferent fibers of different origins in evoking a prolonged facilitation of the flexor reflex in the rat

Changes in the excitability of the hamstring flexor withdrawal reflex produced by conditioning stimuli applied to C-afferent fibers of different origins have been examined in the decerebrate spinal rat. In the absence of conditioning stimuli, the flexor reflex elicited by a standard suprathreshold mechanical stimulus to the toes is stable when tested repeatedly for hours. Three categories of conditioning stimuli have been used in an attempt to modify the excitability of the flexor reflex; electrical stimulation of a cutaneous (sural) nerve or a muscle (gastrocnemius-soleus) nerve at C-fiber strength; the application of mustard oil, a chemical irritant that activates chemosensitive C-afferents, to the skin or injected intramuscularly and intraarticularly; and the indirect activation of high-threshold muscle afferents by fused tetanic contractions of the tibial muscles. Conditioning stimuli of an intensity sufficient to activate C-afferent fibers result in a heterosynaptic facilitation of the flexor motoneuronal response to the standard test input, which lasts from 3 min to more than 3 hr, depending on the stimulus and the C-afferents activated. Pretreatment of the sciatic nerve with the C-fiber neurotoxin capsaicin abolishes all the postconditioning facilitations, which is an indication that it is likely that it is C-afferents that are primarily responsible for the facilitatory effects of the conditioning stimuli, although some A delta afferents may contribute. Capsaicin pretreatment does not modify the reflex response to the test stimulus. The most prolonged increase in the excitability of the flexor reflex resulted from intraarticular injections of 5 microliter mustard oil. Using the subsequent injection of lignocaine intraarticulary, it was found that the prolonged facilitation of the reflex is triggered by the afferent input generated by the conditioning stimulus and does not require an ongoing input for its maintenance. These results indicate that there is a spectrum of central changes in the stimulus response relations of the spinal cord resulting from the activation of C-fibers of different origins. The prolonged duration of some of these changes means that the peripheral activation of C-afferents will modify the functional response of the spinal cord to other inputs applied long after the conditioning input, and this may be responsible for some of the sensory and motor alterations found after peripheral tissue injury.

Morphine-sensitive and morphine-insensitive actions of C-fibre input on the rat spinal cord

A dose-dependent differential action of morphine on the hamstring flexor withdrawal reflex recorded in flexor α-motoneurones has been found in the decerebrate-spinal rat. At low doses (0.5 mg/kg) morphine pretreatment reduces the prolonged heterosynaptic facilitation of the reflex that follows brief (20 s) C-primary afferent fibre conditioning stimuli, without modifying the activation of the motoneurones by C-fibre input or noxious peripheral stimuli. Moderate doses (5 mg/kg) of morphine are required to suppress the C-fibre-induced excitability increase once it is established by the conditioning stimulus but only very high doses (20 mg/kg) reduce the direct reflex activation of the flexor motoneurones by either C-fibre input or noxious stimuli. The relevance of these findings to the antinociceptive and analgesic actions of morphine are discussed.

Cutaneous receptive field and morphological properties of hamstring flexor alpha‐motoneurones in the rat.

Intracellular recordings have been made from twenty antidromically identified posterior biceps femoris/semitendinosus (p.b.s.t.) hamstring flexor alpha-motoneurones in the decerebrate-spinal rat. The hamstring motoneurones had either low or no spontaneous background activity. In nineteen of the twenty cells high-frequency phasic responses could be elicited by stimulation of the ipsilateral hind paw with firm pressure or pinch. There was no response to light touch or brush. Contralateral cutaneous mechanoreceptive fields with higher thresholds and weaker responses were present in 70% of the motoneurones. Noxious heating of the ipsilateral hind paw produced excitatory responses in six of eight cells tested and two of these cells also responded to heating of the contralateral hind paw. Stimulation of the ipsilateral sural nerve at graded strengths that successively activated A beta, A delta and C afferents produced excitatory post-synaptic potentials (e.p.s.p.s) at progressively longer latencies in the motoneurones. The C-fibre induced e.p.s.p. lasted up to 200 ms. Horseradish peroxidase was injected into ten motoneurones and in seven cases full reconstructions of dendritic field, cell body and axon could be made. In agreement with previous reports from studies in the cat, the dendritic fields of rat motoneurones are very extensive in the rostrocaudal, mediolateral and dorsoventral planes. The general pattern of dendritic branching for each motoneurone in this functionally homogeneous population was uniformly organized. Three major spatial orientations were always present: a rostrocaudally restricted series of dendrites emerging from the cell body and directed dorsolaterally towards the dorsolateral funiculus with branches in the lateral dorsal horn, a laterally, and a ventromedially directed series of branches arranged obliquely in the ventral horn, both of which were distributed rostrocaudally for equal distances from the cell body. Many of these dendritic branches terminated within the lateral and ventral white columns. Although the sizes of the rat flexor motoneurones' somas (51 +/- 4.9 micron, S.E., n = 10) were similar to those of cat lumbosacral alpha-motoneurones, the tip-to-tip rostrocaudal extent of their dendritic fields (1130 +/- 34 micron, S.E., n = 7) was half that reported in the cat. These results are discussed in terms of the organization of the cutaneous flexor withdrawal reflex in the rat.

The cutaneous contribution to the hamstring flexor reflex in the rat: an electrophysiological and anatomical study

The location and properties of the cutaneous receptive fields responsible for detecting the flexor withdrawal reflex in the posterior head of biceps femoris (pBF) and semitendinosus (ST) components of the hamstring muscle have been examined in unanaesthetized decerebrate rats, spinalized at T10-T11. Single α-motoneurone efferents were recorded from the nerve to pBF and the principal head of ST and their responses to ipsi- and contralateral hindlimb skin stimulation investigated. The efferents to both muscles characteristically ha had a low or absent background discharge and they all had mechanoreceptive fields on the ipsilateral foot. The mechanical threshold of these fields was high with no response to light touch or brush. Fifty-four percent of these units also had a smaller and weaker contralateal mechanoreceptive field. The only apparent difference between ST and pBF efferents was that more ST efferents had contralateral fields than pBF units. Noxious, hot and cold thermal stimuli applied to the ipsilateral foot activated 56% of the efferents. Mustard oil, a chemical irritant, produced a long-lasting flexor response when applied to the ipsilateral foot. The responses of these efferents to stimulation of Aβ, Aδ and C cutaneous afferents in the sural nerve were also studied. Short latency reflexes were elicited in all efferents by Aβ inputs, longer latency reflexes were elicited in 64% by Aδ inputs and very long latency refponses with long afterdischarges were found in 73% of the units to C inputs. Retrograde labelling of the hamstring motoneurones with WGA-HRP indicated that they lay in ventrolateral lamina IX extending from the caudal portion of the third lumbar segment to the junction of the 5th and 6th lumbar segments. Transganglionic labelling of small diameter primary afferent terminals in the dorsal horn of cutaneous nerve innervating the foot revealed that the longitudinal distribution corresponded closely with that of the hamstring motor nucleus. The flexor reflex in the spinal rat provides a useful model therefore, for studying hiow the input in nociceptive afferents is processed and transformed within the spinal cord, to produce appropriate outputs.

Postsynaptic potentials evoked in cat hindlimb motoneurons by C-fiber volleys

Postsynaptic potentials were recorded from flexor and extensor motoneurons in the lumbosacral enlargement of unanesthetized, decerebrate cats spinalized at an upper lumbar level in response to volleys in C-fibers in cutaneous or mixed peripheral nerves. It is proposed that these potentials contribute to the reflex activity of motoneurons engaged in the flexor withdrawal reflex.

Functional plasticity of the flexor withdrawal reflex in the rat following peripheral tissue injury

Functional plasticity of the flexor withdrawal reflex in the rat following peripheral tissue injury

Spinal-cord concussion in frogs a study of reflex changes

A reliable experimental model of the spinal frog, employing the flexor withdrawal reflex to a noxious stimulus, has been used to study the response to single and repeated concussing blows over the spinal column. It is evident that caudally placed blows depress the reflex response, whereas more cephalad blows facilitate the response. The results of studies using repeated concussing blows support the concept that neuronal units may completely lose their function without suffering permanent structural damage.

Classical conditioning of the flexion reflex in spinal cat: Features of the reflex circuitry

Classically conditioned facilitation of the flexor withdrawal reflex of spinal cat occurs in knee and ankle flexor muscles but not in a flexor muscle of the toes. Furthermore, the spinal circuity activated by a component of the conditioned stimulus (Aα cutaneous fibers) is not by itself involved in the reflex conditioning. The results suggest that increase in both cutaneous afferent output and motoneuron excitability may be eliminated as mechanisms contributing to conditioning and point to certain interneuronal pools as the locus of learning in this preparation.

Antinociceptive effect of peripheral segmental electrical stimulation in the rat

We have investigated the effect of transcutaneous electrical stimulation, applied to the base of the tail, on the flexor withdrawal response of the rat, following immersion of the tail in water at 49°C. In intact rats, electrical stimulation delayed the response to the controlled noxious stimulus, the delay depending on the duration of stimulation and on the frequency and pulse width of the stimulus. The prolongation in reaction time produced by stimulation at 100 Hz, 0.2 msec for 30 min at a voltage sufficient to cause slight fibrillation of the tail muscles, but no escape behaviour, was equal to that produced by 7.5 mg/kg of morphine. The specific narcotic antagonist naloxone (1 mg/kg) virtually abolished the effect of the electrical stimulation. After recovery from the operation, rats with complete spinal transection at the 10th or 11th thoracic vertebra exhibited a flexor withdrawal response to the noxious stimulus behaviourally simpler and of shorter latency than that exhibited by the intact animals. Peripheral electrical stimulation again delayed the response, and naloxone prevented the delay, but the antinociceptive action of the stimulation was quantitatively less than that observed in intact animals. Morphine also had a correspondingly reduced effect in the spinal animals. Depletion of 5-hydroxytryptamine levels by systemic pretreatment with parachlorphenylalanine significantly diminished the effect of both electrical stimulation and morphine administration in intact rats, but not in spinal rats. Our results support the concept that peripheral electrical stimulation excites naloxone-dependent antinociceptive mechanisms at both spinal and supraspinal levels. The supraspinal mechanism, but not the spinal mechanism, is associated with 5-hydroxytryptamine.

Effects of stimulus intensity on the habituation of flexor withdrawal activity mediated by the functionally transected human spinal cord

Ten patients with a chronic, transverse lesion of the cervical spinal cord were submitted to two daily sessions involving a relatively high- or low-intensity habituating stimulus applied unilaterally to a midplantar site at the rate of 1/sec. Test stimuli of relatively high, intermediate, and low intensity were applied before and after habituation training. Response measures were based upon the integrated electromyographic activity of the tibialis anterior muscle. Analyses of both absolute and relative responses measures obtained during and at the termination of habituation training indicated that the degree of habituation was inversely related to the intensity of stimulation. The incremental growth in responsiveness observed during the first few repetitions of the habituating stimuli was greater for strong stimulation than for weak. Analyses of responses to test stimuli applied before and after habituating training suggested that the effects of the strong habituating stimulus generalized more to a posthabituation test stimulus of intermediate intensity than did the effects of the weak habituating stimulus.

Blockade of intracranial self-stimulation by antipsychotic drugs: failure to correlate with central alpha-noradrenergic blockade.

The involvement of central alpha-noradrenergic receptors in intracranial self-stimulation (ICSS) was studied. Dose-response curves were established for the blockade of ICSS by the antipsychotic drugs chlorpromazine, thioridazine, clozapine, and pimozide and the alpha-antagonist phenoxybenzamine. Antagonism of the facilitation, produced by the central alpha-agonist clonidine, of flexor withdrawal reflexes in the reserpinized spinal rat was used to assess the central alpha-blocking potency of the same drugs, and dose-response curves were established. No correlation was found between central alpha-blockade, as reflected by the ED50 for blockade of clonidine-facilated spinal reflexes, and the ED50 for blockade of ICSS. Pimozide blocked ICSS at doses virtually devoid of central alpha-blocking activity, while phenoxybenzamine was a potent alpha-antagonist and a weak blocker of ICSS. The lack of correlation between central alpha-blockade and decreased ICSS suggests that alpha-receptors are not critically involved in self-stimulation behavior.

The effect of p-CPAand lesions of the dorsal raphe nucleus on habituation of the flexor withdrawal reflex

The effect of p-CPAand lesions of the dorsal raphe nucleus on habituation of the flexor withdrawal reflex

Dishabituation of flexor withdrawal activity mediated by the functionally transected human spinal cord

Dishabituation of lower limb flexor activity was studied in 8 patients with a chronic, functionally complete transection of the cervical spinal cord. The EMG activity of the right tibialis anterior muscle and the associated dorsiflexion of the right ankle was habituated to criterion by applying a 40-msec pulse train at 500-msec intervals to the medial plantar surface of the right foot. The dishabituating stimulus, a pulse train to the dorsal aspect of the right foot or the posterior plantar surface, was then applied. After a 500-msec interval, the habituating stimulus was applied once again to determined the extent of dishabituation. A 4-sec stimulus-free interval was then imposed, followed by an additional presentation of the habituating stimulus. The results indicated that dishabituation was achieved in all subjects, but that following the stimulus-free interval, responsiveness to the habituating stimulus fell to the level observed prior to applying the dishabituating stimulus. This outcome was interpreted as being consistent with the view that dishabituation does not represent a disruption of the underlying habituation process, and instead is due to a transient excitatory state that is superimposed upon the habituation process. Additional findings pertained to the effects of using other stimulus sites and to varying the interval between the dishabituating stimulus and the subsequent presentation of the habituating stimulus.

Habituation of a monosynaptic response in vertebrate central nervous system: lateral column-motoneuron pathway in isolated frog spinal cord.

ArticlesHabituation of a monosynaptic response in vertebrate central nervous system: lateral column-motoneuron pathway in isolated frog spinal cordP. B. Farel, D. L. Glanzman, and R. F. ThompsonP. B. Farel, D. L. Glanzman, and R. F. ThompsonPublished Online:01 Nov 1973https://doi.org/10.1152/jn.1973.36.6.1117MoreSectionsPDF (2 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformationCited ByThe development of serotonergic raphespinal projections in Xenopus laevisInternational Journal of Developmental Neuroscience, Vol. 4, No. 5Activation of brainstem serotoninergic pathways decreases homosynaptic depression of monosynaptic responses of frog spinal motoneuronsBrain Research, Vol. 280, No. 2The nictitating membrane response: An electrophysiological study of the abducens nerve and nucleus and the accessory abducens nucleus in rabbitBrain Research, Vol. 258, No. 2Habituation of the Hoffmann reflexBrain Research, Vol. 220, No. 2Disruption of vertebrate monosynaptic habituation by ethyl alcoholBrain Research, Vol. 212, No. 1The effects of hyposmolality on spinal cord activityLife Sciences, Vol. 28, No. 8Effects of changes in osmolality on spinal cord activityExperimental Neurology, Vol. 68, No. 3Alterations in spontaneous miniature potential activity during habituation of a vertebrate monosynaptic pathwayBrain Research, Vol. 189, No. 2Evidence against conduction failure as the mechanism underlying monosynaptic habituation in frog spinal cordBrain Research, Vol. 174, No. 2The bulbo-spinal indoleaminergic pathway in the frogBrain Research, Vol. 172, No. 2Inhibition of the monosynaptic responses in frog spinal motoneuronsNeuroscience, Vol. 4, No. 7Habituation of the nictitating membrane reflex response in the intact frogPhysiology & Behavior, Vol. 22, No. 6Effect of temperature on habituation of the LC-VR reflex of the frog spinal cordPhysiology & Behavior, Vol. 22, No. 4Reflex activity of regenerating frog spinal motoneuronsBrain Research, Vol. 158, No. 2Action of Mg2+ at low concentrations on the response to habituating stimuli and spontaneous activity of frog spinal cord motoneuronsBrain Research, Vol. 142, No. 1The efects of Ca2+ and Mg2+ on the habituating LCVR reflex of the frog spinal cordBrain Research, Vol. 139, No. 1The pharmacology of the amphibian spinal cordProgress in Neurobiology, Vol. 11, No. 1Brainstem reticular formation mechanisms subserving generalized seizures: Effects of convulsants and anticonvulsants on sensory-evoked responsesProgress in Neuro-Psychopharmacology, Vol. 2, No. 4Monosynaptic habituation in the vertebrate forebrain: The dentate gyrus examined in vitroBrain Research, Vol. 115, No. 3Long-term and short-term plasticity in the CA1, CA3, and dentate regions of the rat hippocampal sliceBrain Research, Vol. 110, No. 3Ultrastructural observations in the frog spinal cord in relation to the generation of primary afferent depolarizationNeuroscience Letters, Vol. 2, No. 3Criteria for distinguishing between monosynaptic and polysynaptic transmissionBrain Research, Vol. 105, No. 1The relation between monosynaptic spinal reflex amplitudes and some EEG alpha activity parametersElectroencephalography and Clinical Neurophysiology, Vol. 40, No. 3Sensitization and habituation of the plantar cushion reflex in catsBrain Research, Vol. 103, No. 2The effects of 5,6-dihydroxytryptamine in the amphibian spinal cord using silver staining techniquesBrain Research, Vol. 78, No. 2The effect of p-CPAand lesions of the dorsal raphe nucleus on habituation of the flexor withdrawal reflexBrain Research, Vol. 77, No. 3Dual processes control response habituation across a single synapseBrain Research, Vol. 72, No. 2Persistent increase in synaptic efficacy following a brief tetanus in isolated frog spinal cordBrain Research, Vol. 66, No. 1 More from this issue > Volume 36Issue 6November 1973Pages 1117-1130 Copyright & PermissionsCopyright © 1973 the American Physiological Societyhttps://doi.org/10.1152/jn.1973.36.6.1117PubMed4543415History Published online 1 November 1973 Published in print 1 November 1973 Metrics

Effect of scopolamine and atropine on habituation of the flexor withdrawal reflex

The effects of intraperitoneal injection of scopolamine and atropine on habituation of the flexor withdrawal reflex response in conscious rats were investigated. Scopolamine (200 and 500 μg/kg) caused a marked depression of reflex responses which habituated to a lesser degree than responses in control animals. Reasons are given for rejecting this finding as representing support for the hypothesis that habituation is dependent on cholinergic mechanisms. It was found that the establishement, but not the retention of habituation was impaired by administration of atropine (30 mg/kg).

Triggering of habituation: Reconsidered

Habituation of the flexor withdrawal reflex was studied in conscious rats. In control experiments in which 300 stimuli were applied (interstimulus interval 10 sec) there was found to be a linear begative correlation between log response and the number of stimuli. In other experiments stimuli were given in two trains separated by a 15 min pause. The first train contained 20, 40, 60, or 80 stimuli. The initial responses to the second train were significantly greater than the control responses recorded an identical period of time after the first stimulus in the experiment. In experiments in which the first train contained 40, 60, or 80 stimuli, a significant degree of recovery occurred during the 15 min pause. The initial rates of habituation to the second train in these experiments was greater than rates observed to the corresponding stimuli in the control series. The rates of decrement to subsequent stimuli however, were not different from control values. The data were discussed with reference to the hypothesis that habituation could be triggered by initial experience.