Contribution of unmyelinated afferents from the sural nerve to spinal reflex activity

Inhibitory effects from various types of dorsal column and raphe magnus stimulations on nociceptive withdrawal flexion reflexes

Most of the clinical and research reports agree about the analgesic effects of dorsal column (DC) stimulation, but there is no unanimity about the neural mechanisms involved in this stimulation. The aim of the present study was to compare the effects of segmental and rostral activation of the DCs and to investigate whether these effects are mediated through a brainstem spinal loop. Decerebrate-decerebellate cats were subjected to selective DC lesions at C(1) and C(3) spinal cervical levels and their reflex reactions to natural or electrical nociceptive stimuli were monitored either as withdrawal flexion reflexes or as motorneuronal discharges. Conditioning stimulation was performed as train of shocks (100 Hz, for 1 to 10 min or 300 Hz for 30 ms) applied on the DCs either rostral (DCr) or caudal (DCc) to the spinal lesions or on the raphe magnus (RM). Conditioning trains for 5-10 min applied on DCr inhibited the withdrawal flexion reflexes recorded as toe flexion (90% of the control). Comparisons of the effects of DCr, DCc or RM of conditioning stimuli were made on the discharges of 110 motorneurons recorded in isolated ventral root fibers. Conditioning stimulation applied to DCc produced short lived inhibition (in about 60%) or facilitation (in about 30% of the neurons) while DCr or RM conditioning produced inhibition in 90% of neurons which outlasted the duration of the conditioning trains. It was also shown that repetitive application of conditioning train on either DCr or RM resulted in longer duration of inhibition than that observed following DCc conditioning. We conclude that the stronger inhibition of motorneuronal discharges, evoked by nociceptive stimuli, is obtained by rostral activation of the DCs and that long term effects of DCst are mediated through a DC-brainstem-spinal loop.

Relationship between cardiovascular neurones and descending antinociceptive pathways in the rostral ventrolateral medulla of the cat

It has been previously reported that injection of neuroexcitatory compounds into the rostral ventrolateral medulla (RVLM) can produce an inhibition of nociceptive reflexes, often associated with a rise in arterial blood pressure. The aim of this study was to determine whether the subretrofacial (SRF) nucleus, which is a highly circumscribed group of cells within the RVLM known to play a major role in cardiovascular regulation also has an antinociceptive function. In barbiturate-anaesthetised and paralysed cats, unilateral microinjections of the neuroexcitatory compound sodium glutamate (8-20 nl of 0.5 M solution) into the SRF nucleus produced large increases in mean arterial pressure but had only small and inconsistent effects on the simultaneously measured ventral root responses to stimulation of primary afferent C-fibres. On the other hand, glutamate microinjections into RVLM sites closely adjacent to the SRF nucleus, or into the nucleus raphe magnus, produced powerful inhibition of the C-fibre evoked response in the ventral root which was accompanied by no or only small changes in arterial pressure. It is concluded that the SRF pressor cells do not exert any control over nociceptive spinal reflexes, but that such a function may be served by cells in closely adjacent parts of the RVLM. Moreover, the method of recording C-fibre evoked responses in ventral roots as a measure of the magnitude of nociceptive spinal reflexes, combined with the glutamate microinjection procedure, was shown to have a sufficient resolution to allow an accurate mapping of the location of antinociceptive cell groups within the ventrolateral medulla.

Morphine-sensitive late components of the flexion reflex in the neonatal rat

The 8-15-day-old rat spinal cord was isolated together with peripheral nerves innervating a hindlimb. Multiunit neural discharges in response to electrical stimulation of a cutaneous nerve (sural, plantar of superficial peroneal nerve) were recorded from a flexor nerve (deep peroneal nerve or nerve innervating the hamstring muscles). Attempts were made to find relations between the magnitude of the flexion reflex discharges and the sizes of the volleys in the myelinated or unmyelinated afferent fibers. The neonatal flexion reflex discharges due to myelinated fiber volleys were exaggerated when compared with those in the adult rats. Higher stimulus strengths recruited later components of the flexion reflex discharges. The observed increment of the flexion reflex discharges was precisely associated with the recruitment of unmyelinated afferent fibers in the nerve. These late flexion reflex discharges were shown to be depressed by the opiate analgesic morphine in a naloxone-reversible manner.

Selective inhibition of nociceptive flexion reflex discharge by the kappa agonist bremazocine

In unanaesthetized decerebrate spinal cats, bremazocine (0.012-0.2 mg/kg, i.v.) selectively inhibited the late C-fibre reflex discharge, recorded in sectioned lumbo-sacral ventral root filaments, after supramaximal electrical stimulation of the ipsilateral sural or common peroneal nerve. This action was naloxone-reversible. The results suggest that activation of kappa opioid receptors in the spinal cord inhibits the integration of nociceptive, but not locomotor, flexion reflexes.

Synaptic actions of cutaneous A delta and C fibers on primate hindlimb alpha-motoneurons

Postsynaptic potentials evoked in hindlimb alpha-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 A alpha beta, A delta 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 A delta fibers of the cutaneous nerve produced EPSPs in 57% of flexor and 31% of extensor motoneurons tested, whereas IPSPs were produced by A delta 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. A alpha beta and A delta 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 A delta volleys. Ranges of central latencies of the EPSPs and IPSPs evoked by A delta 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 A delta afferent fibers to motoneurons and the A delta inhibitory pathway may involve longer interneuronal chains. In a few motoneurons, however, sural volleys with strengths sufficient to activate A delta 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 A delta 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.

Inhibition of nociceptive withdrawal flexion reflexes through a dorsal column-brainstem-spinal loop

In decerebrate-decerebellate cats, dorsal column stimulation (DCst) rostral to dorsal funicular cuts, to prevent antidromic activation of afferent dorsal column fibers, inhibited spinal flexion reflexes evoked by nociceptive stimuli. The effects of DCst above the cuts were compared to those below the cuts. Our findings indicate that the analgesic effects of DCst can be attributed to activation of a DC-brainstem-spinal loop in addition to antidromic activation of spinal 'gating' mechanisms.

Effects of a distant noxious stimulation on A and C fibre-evoked flexion reflexes and neuronal activity in the dorsal horn of the rat

In the halothane-anaesthetized rat, the responses of 49 neurons in the lumbo-sacral cord and the reflex discharge in the common peroneal nerve following electrical stimulation of the sural nerve were recorded in order to study possible relations between neuronal events and reflex nerve discharges. A distant noxious stimulus (to activate Diffuse Noxious Inhibitory Controls (DNIC) of Le Bars et al.) was used as a conditioning stimulus. Only the responses of neurons receiving an input from both A and C fibres were studied. The neurons were classified as class 1 (low threshold mechanoreceptive input only, n = 2), class 2 (nonnoxious and noxious inputs, n = 34) or class 3 (responding to noxious stimuli only, n = 13). During conditioning stimulation the C fibre evoked discharge was inhibited in 32 out of 34 class 2 neurons. The A fibre-evoked discharge was simultaneously inhibited in 29 of these neurons. The main effect of the distant noxious stimulation on the C fibre evoked neuronal discharge was to decrease the discharge by a constant number of spikes, independent of the level of evoked activity. Only one class 3 neuron was inhibited during conditioning stimulation and none of the class 1 cells were influenced by DNIC. During conditioning stimulation the late and prolonged C fibre evoked reflex nerve discharge (latency 160-200 ms, duration up to several hundred ms) was strongly depressed. Concomitantly, a short-lasting reflex nerve discharge appeared over the interval 115-160 ms. This released reflex nerve discharge (RR) had a constant latency. There was no simultaneous change of the A beta evoked reflex nerve discharge. After the end of the distant noxious stimulation the late C fibre evoked reflex nerve discharge (latency 160-200 ms) recovered. Concomitantly, the RR disappeared. The possibility that the class 2 neurons and the class 3 neurons are intercalated in different reflex pathways is discussed.

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 alpha-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 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 contralateral 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 A beta, A delta and C cutaneous afferents in the sural nerve were also studied. Short latency reflexes were elicited in all efferents by A beta inputs, longer latency reflexes were elicited in 64% by A delta inputs and very long latency responses 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 nerves innervating the foot revealed that the longitudinal distribution corresponded closely with that of the hamstring motor nucleus. The flex-or reflex in the spinal rat provides a useful model therefore, for studying how 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.