Activation of descending noradrenergic system by peripheral nerve stimulation

Noradrenaline (NA) release in the rat lumbar spinal cord (L3–4) in response to variable intensity, selective stimulation of large (A-beta), small myelinated (A-delta), and unmyelinated (C) afferent fibers was examined by in vivo microdialysis with high performance liquid chromatography and electrochemical detection. Application of 100 mM K⁺ solution via the dialysis probe increased NA in the dialysate. Thoracic segment transection rostral to the probe depressed the NA level. Transcutaneous stimulation of peripheral nerves had the following effects: 1) High intensity stimulation of afferent A-delta or C fibers increased spinal NA release, which was decreased by thoracic spinal cord transection. 2) Stimulation of afferent A-beta or A-delta fibers at low intensity did not affect the NA level. 3) High intensity stimulation of afferent A-beta fibers depressed NA release in half of the trials. Results indicate that many NA-containing nerve terminals that innervate the lumbar spinal cord originate from supraspinal structures. Somatic neural inputs from afferent C fibers and high-threshold A-delta, but not A-beta nor low-threshold A-delta fibers, activate the descending NA system and release the NA in the spinal cord. The descending NA system may participate in antinociception.     

Responses of inferior salivatory neurons to stimulation of trigeminal sensory branches

A total of 84 single inferior salivatory neurons was identified by antidromic stimulation of the tympanic nerve. Their responsiveness was tested to stimulation of the ipsilateral infraorbital, lingual, and inferior alveolar nerves in urethane-chloralose-anesthetized cats. The conduction velocities of preganglionic fibers of inferior salivatory neurons ranged from 2.2 to 9.1 m/s, and 54% of those neurons responded with spikes to stimulation of at least one of the infraorbital, lingual, or inferior alveolar nerves (responsive type neurons). The latencies of spike responses to stimulation of the trigeminal sensory branches ranged from 4.0 to 21.0 ms, which were shorter than those of superior salivatory neurons. Impulses of both A-beta and A-delta afferent fibers of the trigeminal nerve were found to be effective for activation of inferior salivatory neurons. The convergence of excitatory inputs from more than one sensory nerve was found in most of the responsive type neurons (73%).     

Decremental conduction in a mammalian peripheral nerve

Conduction velocities of the A-beta, A-delta and C-fibers in isolated vagal nerves were measured before, during, and after the nerve was exposed to sodium-deficient hypotonic solutions. The solution was kept at 23 degrees C. Recorded at a conduction distance of 18 mm and with the nerve in a 0.25% NaCl solution, the conduction velocity of the A-beta component in the response was found to decrease at an average rate of 1.07 m/s per min; of the A-delta, 0.35 m/s per min and of the C-fibers, 0.020 m/s per min. Thus, the A-delta component in the response would disappear in about 29 min; the C-component, in 49 min; and the A-beta component, in 54 min. Recorded at a distance of 36 mm, the decrease in conduction velocity of the A-beta was 1.40 m/s per min, of the A-delta, 0.43 m/s per min, and of the C-fibers, 0.023 m/s per min. Thus, recorded at this conduction distance, the A-delta would disappear in 20 min; the C-component, in 36 min; and the A-beta component, in 42 min. These findings indicate that when decremental conduction occurs in a mammalian peripheral nerve exposed to these solutions, the decrement is most apparent in the A-delta component, then the C-fiber component, and finally the A-beta component in the nerve. Similar observations were obtained in nerves exposed to 0.5% NaCl solution except that the rate of decrement was slower.     

Code models of sensory information from the skin analyzer periphery]

Experimental studies on the relations between the afferent flows in A-beta and A-delta and C-fibres of a cutaneous nerve for different stimulations of cutaneous receptors provided the basis for modelling of various digital and graphical codes of the known and unknown sensations.     

Excitability characteristics of the A- and C-fibers in a peripheral nerve.

The saphenous nerve of the cat was stimulated with square pulses, triangle waves, sine waves, and haversine waves of varying intensities and durations. We found that haversine waves, with the anode proximal to the cathode in relation to the recording electrode, were most effective in producing “break” responses. At a given intensity and duration of the haversine waves for stimulation, only the response of C-fibers in the nerve was recorded. The strength/duration curves show that the rheobase of the C-fibers was 0.033 ma; and of the A-fibers, 0.0166 ma. The C-fibers had a chronaxy of 1.5 msec; the A-delta fibers, 0.45 msec; and the A-beta fibers, 0.020 msec. The average conduction velocity of the C-fibers was 0.92 m/sec; of the A-delta fibers, 11.2 meters per second; and of the A-beta fibers, 85.8 m/sec. The temperature of the nerves in the experiment was 29–30 C.     

Analysis of the selective nature of sensory nerve stimulation using different sinusoidal frequencies

This study examines the ability to selectively activate different nerve fibers in a finger by using different sinusoidal stimulation frequencies. Specifically, the stimulation of A-beta, A-delta, and C-fibers is looked into, and responses from each of three different stimuli (5 Hz, 250 Hz, and 2000 Hz) are compared. Action potential (AP) responses from the different nerve fibers are simulated. Activation thresholds are determined for each fiber type. The resulting firing frequencies are compared with thresholds found in the literature to determine the stimulating signal amplitude at which sensations begin to be perceived for each stimulation frequency. Results indicate that while selective stimulation of C-fibers and A-beta fibers appears to be possible with 5 Hz and 2000 Hz, respectively, selective stimulation of A-delta fibers may not be possible due to them requiring a higher stimulating signal amplitude to cause the nerve to reach the physiological threshold than A-beta fibers for 250 Hz. Thus, selective stimulation of the three types of nerve fibers may not be possible for all three examined frequencies.     

Suppression of the jaw-opening reflex by conditioning A-delta fiber stimulation and electroacupuncture in the rat

Selective conditioning stimulation of A-delta fibers in the peripheral nerve with triangular pulses as well as electroacupuncture to the hind leg (5 Hz, 15 min) strongly suppressed the jaw-opening reflex in a lightly anesthetized rat, whereas maximal A-beta fiber stimulation with rectangular pulses was almost ineffective. Hence, the participation of A-delta fibers in electroacupuncture analgesia is suggested.     

A fMRI study of brain activations during non-noxious and noxious electrical stimulation of the sciatic nerve of rats

An acute pain animal model for fMRI study would provide useful spatial and temporal information for studying the supraspinal nociceptive neuronal responses. The aim of the present study was to investigate whether the nociceptive responses in different brain areas can be differentiated by using functional magnetic resonance imaging (fMRI) in anesthetized rats. Functional changes in brain regions activated by noxious or non-noxious stimuli of the sciatic nerve were investigated using fMRI in a 4.7 T MR system in alpha-chloralose anaesthetized rats. To determine the electrical intensity for noxious and non-noxious stimuli, compound action potential recording was employed to reveal the type of fibers activated by graded electrical stimulation of sciatic nerve. It showed that innocuous A-beta fibers were excited by two times the muscle twitch threshold and nociceptive A-delta and C fibers were recruited and excited by 10 and 20 times threshold, respectively. A series of four-slice gradient echo images were acquired during innocuous (two times threshold) and noxious (10 and 20 times threshold) stimuli in a 4.7 T MR system. Contralateral somatosensory cortex was the most prominent brain area activated by innocuous stimuli. Both signal intensity and activated areas were significantly increased in the somatosensory cortex, cingulate cortex, medial thalamus and hypothalamus during noxious stimuli. These four brain areas activated by noxious stimuli were significantly suppressed by prior intravenous injection of morphine (5 mg/kg). The present findings demonstrated that the difference of the innocuous and nociceptive responses in the brain could be detected and localized by an in vivo spatial map using fMRI. Results suggest that fMRI may be an invaluable tool for studying pain in anesthetized animals.     

Evidence for two populations of rat spinal dorsal horn neurons excited by urinary bladder distension

Spinal L6-S2 dorsal horn neurons of cervical spinal cord-transected, decerebrate female rats were characterized using urinary bladder distension (UBD) as a visceral stimulus. Constant pressure, phasic, graded (20-80 mm Hg, 20 s) air UBD was delivered via a transurethral catether and extracellular single-unit recordings obtained from all neurons excited by UBD. Responses to graded UBD and noxious/non-noxious cutaneous stimuli were determined in 258 neurons which could be stratified into two groups based on their effect of a counterirritation stimulus: Type I neurons (n=112) were inhibited by noxious pinch presented in a non-segmental field; Type II neurons (n=146) were not similarly inhibited. Both Types of neurons were identified in both superficial and deep recording sites and demonstrated graded responses to graded UBD. All UBD-excited neurons had convergent cutaneous receptive fields (RFs) excited by non-noxious and/or noxious stimuli. As a group, Type I neurons had a period of decreased activity following termination of the distending stimulus whereas Type II neurons typically had a sustained afterdischarge. UBD-evoked activity in Type II neurons was inhibited more than similar activity in Type I neurons by both intravenous morphine and lidocaine. These results support the assertion that at least two different populations of spinal dorsal horn neurons exist which encode for a stimulus of urinary bladder distension. These populations are an analogue to previously characterized, similar neuronal populations excited by colorectal distension and suggest that they are representative of the overall phenomenon of visceral sensory processing, a component of which is nociception.     

Origins of spontaneous and noxious stimuli-evoked miniature EPSCs in substantia gelatinosa

The origins of spontaneous and noxious stimuli-evoked glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in substantia gelatinosa (SG) neurons were investigated by using whole-cell voltage-clamp technique on adult rat spinal cord slice. The properties of mEPSCs of SG neurons from rats either neonatally capsaicin-treated or sciatic nerve ligated showed no difference from those of intact SG neurons, indicating independence of spontaneous mEPSCs on primary afferent fibers. In the presence of tetrodotoxin (TTX), capsaicin, which noxiously stimulated fine primary afferent fibers, caused increase of the mEPSCs frequency, but did not affect the amplitude profiles or mean amplitudes. TTX affected neither the spontaneous mEPSCs nor capsaicin-induced mEPSCs frequency increase. The results suggest that spontaneous mEPSCs in SG are mediated by presynaptic spontaneous glutamate release predominantly originating from interneuron terminals rather than from primary afferent terminals; under noxious stimulation, however, mEPSCs frequency increase is mediated by primary afferent excitation.     

Role of cervical neurons in propriospinal inhibition of thoracic dorsal horn neurons.

We previously reported that electrical or glutamate stimulation of the cervical spinal cord elicits a 40-60% decrease in renal sympathetic nerve activity (RSA) in the anesthetized rats. This sympatho-inhibition was possible, however, only after transection of the spinal cord at C1 or GABAergic inhibition of neurons in the rostral ventrolateral medulla. We postulated that cervical neurons inhibit RSA by inhibiting the activity of spinal interneurons that are antecedent to sympathetic preganglionic neurons (SPNs), and that these interneurons may be, in turn, excited by afferent signals. In this study, we tested the hypothesis that cervical neurons can inhibit visceroceptive thoracic spinal neurons. We recorded the spontaneous and evoked activity of 45 dorsal horn neurons responsive to splanchnic stimulation before, during, and after chemical or electrical stimulation of the cervical spinal cord in chloralose-anesthetized spinal rats. Cervical spinal stimulation that inhibited RSA also inhibited the spontaneous and/or evoked activity of 44 dorsal horn neurons. In addition to inhibiting splanchnic-evoked neuronal responses, cervical stimulation also inhibited responses, in the same neurons, evoked by noxious heat or light brushing of receptive dermatomes. We concluded that cervical neurons participate in propriospinal inhibition of afferent transmission and that this inhibitory system may be involved in controlling the access of afferent information to SPNs.     

The 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.¹⁹) 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 Aβ 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.     

Paraventricular hypothalamic oxytocinergic cells responding to noxious stimulation and projecting to the spinal dorsal horn represent a homeostatic analgesic mechanism

The participation of the hypothalamic paraventricular nucleus (PVN) in an endogenous central mechanism of analgesia has been observed using rats in various experimental procedures including electrophysiological and behavioral tests. However, little is known about the PVN neuronal responses to noxious stimulation. The only data available indicate a c-fos increase after noxious visceral stimulations. Our electrophysiological recordings of single PVN cells showed that, out of 223 cells, 79 responded to noxious mechanical and/or thermal stimuli, and another 10 responsive cells were found in the Reuniers thalamic nucleus. These cells responded only to noxious stimuli mainly in the ipsilateral hind limb but we also observed cells responding to stimulation of both hind limbs and also the tail. Mechanical stimulation was most effective but some cells could respond to both mechanical and thermal noxious stimuli. Some of the responding PVN cells were identified by antidromic stimulation in the ipsilateral lumbar dorsal horn spinal cord. Finally, in order to document the nature of the neurotransmitter and the projection to the spinal cord of the PVN cells that responded to noxious stimulation, we used a juxtacellular approach to record and stain some neurons and found them to be oxytocinergic by immunofluorescence procedures. The PVN cells activated by noxious stimuli may suppress the peripheral incoming afferent A-delta and C fibers, completing a circuit involved in diffuse endogenous analgesia. This mechanism strongly suggests that the PVN participates in a homeostatic mechanism involved in pain and analgesia.     

Properties of functionally identified nociceptive and nonnociceptive facial primary afferents and presynaptic excitability changes induced in their brain stem endings by raphe and orofacial stimuli in cats

The activity of 221 single primary afferent units was recorded extracellularly in the trigeminal (V) ganglion of chloralose-anaesthetized cats to examine the receptive field properties of nonnociceptive and nociceptive cutaneous afferents and the effect of conditioning stimulation of the raphe system and orofacial afferents on the antidromic excitability of their brain stem endings in V subnucleus caudalis. In addition to slowly adapting and rapidly adapting low-threshold mechanosensitive afferents, we functionally identified three classes of cutaneous nociceptive afferents: these included A-delta high-threshold mechanoreceptive afferents (A-delta HTMs), C-fiber high-threshold mechanoreceptive afferents (C-HTMs), and C-polymodal nociceptive afferents (CPNs). Most of the CPNs could be activated by light tactile stimuli as well as by heavy pressure and pinch and noxious radiant heat applied to their mechanoreceptive field which usually involved a localized spot (approximately 1 mm in diameter) of skin. In contrast, the C-HTMs and A-delta HTMs could not be activated by radiant heat stimuli although some did show sensitization which was also a feature of the CPNs; they did respond to noxious mechanical stimulation of a localized area of skin. We noted that orofacial conditioning stimulation could produce an increase in antidromic excitability which was considered a reflection of primary afferent depolarization (PAD) in both nociceptive and nonnociceptive afferents innervating the cat's facial skin; nonnoxious mechanical stimuli and electrical stimuli were particularly effective in the low-threshold mechanosensitive afferents and noxious mechanical and high-intensity electrical stimuli were especially effective in the cutaneous nociceptive afferents. Raphe conditioning stimulation also was very effective in inducing PAD in these nociceptive afferents; however, the raphe conditioning effects were not limited to these nociceptive afferents since PAD was also frequently demonstrated in the low-threshold mechanosensitive afferents.     

Calcitonin: brainstem microinjection but not systemic administration inhibits spinal nociceptive transmission in the cat

In anaesthetized cats, nociceptive responses of lumbar dorsal horn neurons were studied during administration of salmon calcitonin (sCT). Systemic sCT administration (4–95IU/kg i.v.) produced no change in neuronal responses produced by noxious skin heating or by impulses evoked electrically in afferent C-fibres. Responses to skin heating were reduced during electrical stimulation in the brainstem, but the efficacy of this descending inhibition was not altered by systemic sCT administration. In contrast, noxious heat responses were clearly reduced by microinjection of sCT into the mesencephalic periaqueductal grey or the medullary raphe regions. These results suggest that calcitonin or a related peptide could act at specific brainstem sites to inhibit the spinal transmission of nociceptive information.     

Influence of limb temperature on cutaneous silent periods

ObjectiveThe cutaneous silent period (CSP) is a spinal inhibitory reflex mediated by small-diameter afferents (A-delta fibers) and large-diameter efferents (alpha motoneurons). The effect of limb temperature on CSPs has so far not been assessed.MethodsIn 27 healthy volunteers (11 males; age 22–58 years) we recorded median nerve motor and sensory action potentials, median nerve F-wave and CSPs induced by noxious digit II stimulation in thenar muscles in a baseline condition at room temperature, and after randomly submersing the forearm in 42 °C warm or 15 °C cold water for 20 min each.ResultsIn cold limbs, distal and proximal motor and sensory latencies as well as F-wave latencies were prolonged. Motor and sensory nerve conduction velocities were reduced. Compound motor and sensory nerve action potential amplitudes did not differ significantly from baseline. CSP onset and end latencies were more delayed than distal and proximal median nerve motor and sensory latencies, whereas CSP duration was not affected. In warm limbs, opposite but smaller changes were seen in nerve conduction studies and CSPs.ConclusionThe observed CSP shift “en bloc” towards longer latencies without affecting CSP duration during limb cooling concurs with slower conduction velocity in both afferent and efferent fibers. Disparate conduction slowing in afferents and efferents, however, suggests that nociceptive EMG suppression is mediated by fibers of different size in the afferent than in the efferent arm, indirectly supporting the contribution of A-delta fibers as the main afferent input.SignificanceLimb temperature should be taken into account when testing CSPs in the clinical setting, as different limb temperatures affect CSP latencies more than large-diameter fiber conduction function.     

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.     

Tooth pulp-evoked activity in the spinal trigeminal nucleus caudalis of cat: comparison to primary afferent fiber, reflex, and sensory responses

Tooth pulp-evoked single-neuron responses were recorded in the spinal trigeminal nucleus caudalis of the cat. The thresholds to monopolar electric pulses of various durations (0.2 to 20 ms) were determined using a constant current stimulator. With stimulus pulse durations of 10 to 20 ms, the thresholds were comparable with those of primary afferent A-fibers, although the most sensitive primary afferent fibers had lower thresholds. Primary afferent C-fibers had higher thresholds than the postsynaptic neurons studied. The threshold for the tooth pulp-elicited jaw-opening response was obtained at a lower stimulus intensity than the liminal response in most postsynaptic neurons of this study. The threshold rise of the postsynaptic trigeminal neurons with decreasing stimulus pulse duration (from 5 to 0.2 ms) was much steeper than that of primary afferent A-fibers or jaw-opening response. The strength-duration curves for tooth pulp-elicited pain sensations in man resemble those of spinal trigeminal neurons. Sixty-two percent of the units had a threshold elevation during a noxious pinch of the tail. The results indicate that the activation of postsynaptic trigeminal neurons requires a considerable temporal summation of primary afferent impulses. The jaw reflex thresholds cannot be explained by the properties of the neurons in the subnucleus caudalis of the trigeminal tract. The results support the concept that dental pain is based on the activation of spinal trigeminal nucleus caudalis neurons receiving their input from intradental A-fibers.     

Oxytocin actions on afferent evoked spinal cord neuronal activities in neuropathic but not in normal rats

A hypothalamic oxytocinergic-descending pathway that reaches the dorsal horn of the spinal cord has been well documented and recently related to states of pain and analgesia. In order to study the action of the neuropeptide oxytocin (OT) on pain-related responses, we compared dorsal horn neuronal responses to electrical and mechanical stimulation of receptive fields in normal and neuropathic rats. Spinal nerve (L5 and L6) ligation (Chung rats) was used to produce experimental neuropathy. Single unit activity was recorded at the L4-L5 level from neurons identified as wide dynamic range presenting latency responses corresponding to A-beta, A-delta, C fibers and also exhibiting post-discharge, and wind-up. We tested intrathecally applied doses of 0.05, 0.1, 1, 2, 5, 10 I.U. of OT. Minor effects on responses to electrical stimulation were present in normal rats. Mechanical responses evoked by von Frey filaments were slightly reduced in normal animals. In neuropathic rats a dose of 1 I.U. produced a significant reduction in C-fibers and post-discharge activities, and doses of 2 I.U. caused a further, pronounced reduction in post-discharge, wind-up, and input values. However, the most marked change was the post-discharge reduction at 10 and 20 min after OT administration. Mechanical responses were significantly reduced in terms of their discharge rate response in neuropathic rats. The contrasting results obtained in normal and neuropathic rats revealed an important distinction between these animals and indicate that plastic changes occur as a consequence of nerve damage. In neuropathic rats, mechanisms involving ascending noxious information to the paraventricular nuclei and descending OT activities could be altered so sensitizing the OT receptors of the spinal dorsal horn cells and could explain our observations. Our results point out an anti-algesic OT effect in neuropathic rats.     

Responses of neurons in ventroposterolateral nucleus of primate thalamus to urinary bladder distension.

The purpose of this study was to examine effects of a noxious visceral stimulus, urinary bladder distension (UBD), on cells in the ventroposterolateral (VPL) nucleus of anesthetized monkeys. We hypothesized that processing of visceral information in the VPL nucleus of the thalamus is similar to spinothalamic tract (STT) organization of visceral afferent input. Urinary bladder distension excites sacral and upper-lumbar STT cells that have somatic input from proximal somatic fields; whereas, thoracic STT cells are inhibited by UBD. Extracellular action potentials of 67 neurons were recorded in VPL nucleus. Urinary bladder distension excited 22 cells, inhibited 9 cells, and did not affect activity of 36 cells. Seventeen of 22 cells excited by UBD also received convergent somatic input from noxious squeeze of the hip, groin, or perineal regions. No cells activated only by innocuous somatic stimuli were excited by UBD. Five of 9 cells inhibited by UBD had upper-body somatic fields. There was a significant tendency for VPL neurons excited by UBD to have proximal lower-body somatic fields that were excited by noxious stimulation of skin and underlying muscle (P less than 0.001). Antidromic activation of 4 thalamic neurons affected by UBD showed that visceral input stimulated by UBD reached the primary somatosensory (SI) cortex.