- Email: [email protected]
Modular organization of the head retraction responses elicited by electrical painful stimulation of the facial skin in humans
Clinical Neurophysiology 126 (2015) 2306–2313
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Modular organization of the head retraction responses elicited by electrical painful stimulation of the facial skin in humans Mariano Serrao a,b, Francesca Cortese a, Ole Kæseler Andersen c, Carmela Conte b, Erika G. Spaich c, Gaia Fragiotta a, Alberto Ranavolo d,⇑, Gianluca Coppola e, Armando Perrotta f, Francesco Pierelli a a
Department of Medical-Surgical Sciences and Biotechnologies, Sapienza University of Rome, Corso della Repubblica 79, 04100 Latina, Italy Laboratory of Movement Analysis, Policlinico Italia, Piazza del Campidano 6, 00162 Rome, Italy Center for Sensory–Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7-D3, DK-9220 Aalborg, Denmark d INAIL, Department of Occupational and Environmental Medicine, Epidemiology and Hygiene, Via Fontana Candida 1, 00040 Monteporzio Catone, Rome, Italy e G.B. Bietti Foundation IRCCS, Department of Neurophysiology of Vision and Neurophthalmology, Via Livenza 3, 00198 Rome, Italy f IRCCS Neuromed, Via Atinense 18, 86077 Pozzilli, Isernia, Italy b c
a r t i c l e
i n f o
Article history: Accepted 28 January 2015 Available online 19 February 2015 Keywords: Trigeminocervical reflexes Head retraction reflexes Modular organization Reflex receptive field Withdrawal reflex Electromyography Kinematics
h i g h l i g h t s We found a well-defined modular organization of the head retraction reflexes (HRRs). HRR are related to withdrawal strategies aimed at protecting the face. Central nervous system may exploit trigeminocervical reflexes synergies to simplify head and neck
motor control.
a b s t r a c t Objective: To explore whether the trigeminocervical reflexes (TCRs) show a reflex receptive field organization in the brainstem. Methods: The facial skin of 16 healthy subjects was electrically stimulated at nine sites reflecting the distribution of the three branches of the trigeminal nerve. The reflex-evoked EMG responses were measured bilaterally from the neck muscles and the head and neck kinematic reactions were detected. Results: TCRs are site dependent. There was a vertical gradient in the magnitude of the reflex responses. EMG and kinematic reflexes were larger when evoked from ophthalmic and maxillary sites than from mandibular ones. The reflex responses exhibited a crossed right–left behavior. Stimulation of the lateral sites evoked larger reflex responses in the contralateral trapezium muscle as well as head rotation and neck bending away from the stimulated side. Conclusion: This modular arrangement of the TCRs seems to be related to withdrawal strategies aimed at protecting the face from injuries, in accordance with the functional role that each group of muscles plays in head and neck motion. Significance: It is likely that the CNS may exploit the neck muscle synergies revealed by the painful stimulation of the skin face in order to control the head and neck movements. Ó 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Nociceptive withdrawal reflexes (NWRs) are polysynaptic, multisegmental spinal responses that generate coordinated ⇑ Corresponding author. Tel.: +39 06 94181267; fax: +39 06 94181410. E-mail addresses: [email protected] (M. Serrao), francesca.cortese05@ libero.it (F. Cortese), [email protected] (O.K. Andersen), [email protected] (C. Conte), [email protected] (E.G. Spaich), [email protected] (G. Fragiotta), [email protected] (A. Ranavolo), [email protected] (G. Coppola), [email protected] (A. Perrotta), [email protected] (F. Pierelli).
muscle synergies aimed at withdrawing a limb from a potential source of injury (Sandrini et al., 2005). The pattern of withdrawal reflex-mediated muscle recruitment depends closely on the site of the stimulation. In animal models, Schouenborg and Kalliomäki (1990) discovered a well-defined modular organization of the neurons mediating NWRs: each muscle or group of muscles (module) was found to have a separate cutaneous receptive field corresponding to the skin region that is withdrawn when the muscle contracts. In this way, stimulation within a given receptive field induces reflex responses
http://dx.doi.org/10.1016/j.clinph.2015.01.029 1388-2457/Ó 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
producing an optimal limb retraction from the source of the stimulation, whereas stimulation outside the receptive field may result in inhibition of a reflex in the same muscles. Andersen and colleagues (Andersen et al., 1999) showed that a similar modular organization is also present in the human lower limb. Proximal muscles were found to have large receptive fields while more distally located muscles had smaller receptive fields covering, for example, only a part of the foot. Stimulation of the dorsolateral side of the foot evoked inversion as the dominant ankle movement along with plantar flexion (functional extension) through activation of the gastrocnemius, soleus and tibialis anterior muscles (Sonnenborg et al., 2001), whereas stimulation of the plantar side of the foot evoked dorsal flexion as the dominant ankle movement through activation of the tibialis anterior muscle. Furthermore, stimulation applied to the distal, medial sole resulted in inversion (correlated with tibialis anterior activity), whereas stimulation of the distal, lateral sole of the foot evoked eversion (Andersen et al., 1999). Nociceptive withdrawal reflexes may also involve the head and face. Indeed, the so-called trigeminocervical reflexes (TCRs) may be considered the electrophysiological counterpart of the head retraction reflexes (HRRs) that protect the face and the head against potential injury (Sartucci et al., 1986; Serrao et al., 2003). Anatomical studies in animal models have demonstrated the presence, and role, of projections from the sensory trigeminal complex to the pedunculopontine nucleus and reticular formation nuclei. These projections have been shown to be important in head orientation in space (Meredith et al., 1992; Sasaki et al., 2004), in the coordination of the neck and proximal limbs and orienting head movements (Cowie et al., 1994; Sugiuchi et al., 2004), in postural cervical tone adaptation after external perturbation (Prentice and Drew, 2001; Drew et al., 2004), in startle reactions to unexpected auditory stimuli (Davis et al., 1982; Yeomans and Frankland, 1995), and in reactions to painful stimuli (Inglis and Winn, 1995). The existence of TCRs in healthy subjects suggests that, in humans too, there is a close functional relationship between the trigeminal sensory system and cervical motor neurons. Since, to date, the central organization of the reflex pathway has not been studied, it remains to be established whether HRRs, like spinal reflexes, show a modular organization. Unveiling a modular organization of HRRs would provide important information on the functional organization of the trigeminal sensory pathways into the brainstem. Were such an organization to be confirmed, it could be hypothesized that the central nervous system (CNS) exploits the modules present in the brainstem to select the muscle synergies needed for specific tasks involving the head and neck (e.g. orientation and postural changes) in the same way as it exploits the modules involved in movement of the limbs (Bizzi et al., 2000). The main hypothesis underlying the present study was that cutaneous nociceptive stimulation of a localized facial area preferentially activates a specific group of neck muscles, i.e. those able to produce the optimal withdrawal response. Given the importance of the eye area, we expected reflexes evoked by stimulation of the skin innervated by the trigeminal ophthalmic branch and by the maxillary branch serving the eye area to be more pronounced than those evoked by the trigeminal mandibular branch. Furthermore, the pattern of muscle responses may differ upon stimulation of facial skin in midline as opposed to lateral areas. It was envisaged that stimulation of midline areas would induce neck–head retraction responses, whereas stimulation of lateral areas would evoke neck bending and/or head rotation responses, according to the side of stimulation. To test these hypotheses, the modulation pattern of major neck muscle responses following bilateral nociceptive trigeminal nerve stimulation was assessed. Nerves were stimulated at different
2307
facial skin sites corresponding to the ophthalmic, maxillary and mandibular innervation areas. 2. Methods 2.1. Participants Sixteen right-handed healthy subjects, aged 23–41 years, 10 males and 6 females, gave their written informed consent and participated in this study. The study conformed to the standards set by the latest revision of the Declaration of Helsinki. In particular, none of them had any uncorrected visual, as assessed with Snellen visual acuity test, or auditory, as assessed with pure tone audiometry test, deficits. The experimental procedures had local ethics committee approval. 2.2. Technique 2.2.1. Electrical stimulation We used an electrical stimulator (Digitimer DS7A, UK) synchronized to a data acquisition and analysis interface (CED Power 1401, Cambridge Electronic Design, UK). Male subjects were required to shave several hours prior to the experiment to reduce the bias of differences in skin thickness and resistance due to facial hair. The skin of the face was cleaned with alcohol and was stimulated through standard Ag/AgCl surface bipolar electrodes (Medelec, Oxford, UK; diameter 1 cm, 1 cm inter-electrode distance) applied to nine different sites. The sites were chosen according to the distribution of the three branches of the trigeminal nerve (ophthalmic, maxillary and mandibular) and thus showed a mediolateral arrangement (see Fig. 1). For each electrode position, the electrode was moved slightly in case the evoked sensation indicated direct nerve stimulation (with the sensation radiating to the innervation territory of the nerve branch). Trains of electrical stimuli composed of three pulses, each of 1 ms duration (inter-pulse interval 5 ms), were used to evoke the TCRs (Serrao et al., 2010). Individual pain thresholds (PTs) were assessed at each stimulation site using a staircase method that consisted of three series of ascending and
Fig. 1. Experimental setup. Facial skin stimulation sites. Trigeminocervical responses were evoked by distributed electrical stimulation of the face using surface electrodes at distinct locations. Three midline (glabella, supra- and infra-lips; 1, 4, 7) and six lateral (two supra-orbital and two infra-orbital and two at lip angles) skin sites were stimulated. The location of the sites reflected the anatomical distribution of the three branches of the trigeminal nerve (ophthalmic, maxillary and mandibular). Examples of trigeminocervical reflexes in the rectified EMG signals are depicted.
2308
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
descending stimulus intensities. The threshold was determined as the average of the peaks and troughs. The stimulation intensity was then adjusted to 2xPT for each of the nine stimulation sites. Five reflex responses were obtained per site, meaning that each subject was submitted to a total of 45 stimuli. The sequence of the stimulation sites was randomized for each participant. The subjective intensity of the painful sensation elicited by the electrical stimulation was rated by the subjects on an 11-point numerical scale (Numerical Rating Scale, NRS), ranging from 0 = no pain to 10 = unbearable pain. 2.2.2. EMG recordings EMG activity was recorded through pairs of Ag/AgCl surface electrodes (Medelec, Oxford, UK; diameter 1 cm, distance between the electrodes 2 cm), placed over the semispinalis capitis (SSC), upper trapezium (TRP) and sternocleidomastoid (SCM) muscles bilaterally. The skin area over the muscle was cleaned with alcohol. The electrodes placed over the SSC were positioned at the level of the C3 and C7 vertebrae on the midline in a belly-tendon configuration (Serrao et al., 2003). For recording from the SCM, the electrodes were placed on the anterior border midway between the mastoid process and sternoclavicular junction, while recordings from the TRP were performed with the electrodes placed in accordance with European recommendations for surface electromyography (SENIAM) (Hermens et al., 1999). A disposable reference electrode, embedded in an armband, was placed over the distal extremity of the right forearm. 2.2.3. Kinematic recordings An optoelectronic motion analysis system (SMART-E System, BTS, Milan, Italy) (Ferrigno and Pedotti, 1985), consisting of eight infra-red ray cameras (operating at 300 Hz) designed to detect movements in a three-dimensional space, was used. Nine markers (diameter: 15 mm) coated in reflective aluminum powder were placed on the subject according to the following specifications: head: left and right temples and a third marker located on the posterior side; neck: C2 and two markers on the lateral side; shoulder: left and right acromion processes and C7. 2.2.4. Experimental protocol The experimental protocol began by settling the participant comfortably on a chair. Before starting the formal measurements, each subject underwent an initial training session to familiarize him/her with the electrical stimulation and pain rating procedures and thus to reduce any effects due to arousal and/or anxiety. Thereafter, the six EMG electrodes and the nine reflective markers were applied, and the stimulation electrodes were positioned on the facial skin, checking the sensation evoked at each site. Electrical stimuli were then delivered to the different skin sites in a random sequence with random inter-stimulus intervals of at least 40 s to avoid reflex habituation. The entire experimental session lasted about 2 h per subject. 2.3. Data analysis 2.3.1. EMG analysis The EMG analysis was based on the activity in a window from 300 ms before until 300 ms after the delivery of the electrical stimulus. The signals were band-pass filtered at 20–2500 Hz and the EMG signal was rectified. Before proceeding with the formal recordings several different head and neck positions were tried in order to minimize neck muscle activity. The onset latency of the reflex was identified in the poststimulus EMG window as a response with an amplitude greater than at least the mean plus 3 times the SD of the pre-stimulus background EMG activity for at least 30 ms. The offset latency
was identified when the EMG activity returned to and remained below that level for at least 30 ms. All measurements were checked by visual inspection. The root mean square (RMS) amplitude in the pre- and post-stimulus time windows was calculated. The EMG reflex activity was measured between onset and offset latencies and the reflex magnitude was expressed as the difference between post- and pre-stimulus RMS for each investigated muscle. Five individual reflex responses were averaged for each stimulation site.
2.3.2. Kinematic analysis To evaluate the head, neck and shoulder kinematics cardan angles were used, calculated as flexion–extension, rotation and lateral bending angles of the head, and flexion–extension and lateral bending angles of the neck. The movements considered for further analyses were: (i) head and neck flexion–extension on stimulation of the midline facial skin sites (1, 4, 7); (ii) head and neck flexion–extension, head and neck lateral bending and head rotation on stimulation of the lateral facial skin sites (right: 2, 6 and 8; left: 3, 5 and 9).
2.4. Statistical analysis The Kolmogorov–Smirnov test was used to test whether data followed a normal distribution. Subsequently, one-way ANOVA for repeated measures or the Friedman test was used to evaluate the effect of the stimulation site on the neurophysiological variables (latencies and magnitudes) and NRS scores. In the ANOVA analyses, a Greenhouse–Geisser correction was used when necessary to deal with violations of sphericity. Sites were compared both according to a horizontal (right–left) criterion, i.e. within each of the three trigeminal branches (2 vs 3, 5 vs 6, and 8 vs 9) and following a vertical criterion, i.e. between the three trigeminal branches (comparing the midline sites 1, 4 and 7; the lateral right sites 2, 6 and 8; and the lateral left sites 3, 5 and 9). Post-hoc analysis with Bonferroni test was used to multiple comparisons. In order to look for a possible influence of the level of precontraction of the neck muscles, a one-way repeated measures ANOVA was used to evaluate the differences in the pre-stimulus EMG activity between stimulation sites. The TCR magnitudes detected at each facial skin site were interpolated within a bi-dimensional grid. Then, for each muscle, a two-dimensional interpolation was superimposed onto a map of the face. The results were plotted on contour plots to provide a picture of how the reflex magnitude changes according to skin stimulation sites, to allow a visual understanding of the modular organization of the reflex. We analyzed the kinematic responses of the head and neck both as categorical (flexion–extension, left–right lateral bending, left– right rotation) and continuous range of motion (ROM) variables. In order to detect the presence/absence pattern of reflex-induced mechanical responses we measured the kinematic ‘‘reflex probability’’ (expressed as a percentage), defined as the number of trials in which reflex responses were detected in each type of movement (head and neck flexion–extension, head and neck left–right lateral bending, left–right head rotation). The reflex was considered present if the ROM value, in a time window of 300 ms after the stimulus, was greater than the mean plus 3 times the SD of the background level in a time window of 300 ms preceding the stimulus. Chi-square and repeated measures ANOVA were used to compare the categorical and continuous variables, respectively. Mean and standard error were calculated. A p-value <0.05 was considered significant.
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
3. Results 3.1. Stimulus intensity The mean stimulation intensities for each site were: site 1: 53.2 ± 23.1 mA, site 2: 50.4 ± 14.3 mA, site 3: 54.3 ± 18.7 mA, site 4: 56.6 ± 21.3, site 5: 56.4 ± 15.3 mA, site 6: 55.6 ± 18.9, site 7: 62.9 ± 27.1, site 8: 65.3 ± 27.1; site 9: 67.2 ± 30.0. 3.2. NRS scores The mean 5.8 ± 0.3; site 6.0 ± 0.4; site 5.7 ± 0.5. No effect (p > 0.05).
NRS scores for each stimulation site were: site 1: 2: 5.8 ± 0.3; site 3: 5.9 ± 0.4; site 4: 6.0 ± 0.5; site 5: 6: 5.7 ± 0.4; site 7: 5.8 ± 0.5; site 8: 5.8 ± 0.4; site 9: of stimulation site on NRS scores was observed
3.3. EMG reflex responses 3.3.1. Reflex latencies A significant effect of stimulation site on NWR latencies was found for all muscles (TRP right: main effect, F(3.841, 57.609) = 11.020, p < 0.001; TRP left: main effect, F(8, 120) = 7.906, p < 0.001; SCM right: main effect, F(4.062, 60.920) = 7.076, p < 0.001; SCM left: main effect, F(3.731, 55.970) = 6.616, p < 0.01; SSC right: main effect, F(4.069, 60.879) = 2.775, p < 0.001; SSC left: main effect, F(3.398, 50.964) = 3.787, p < 0.01). Pairwise comparisons revealed significant lower latency values in all muscles on both sides after stimulation of the ophthalmic and maxillary midline sites compared with the mandibular ones (sites 1 and 4 > 7), as well as after stimulation of the ophthalmic and maxillary lateral sites compared with the mandibular ones (sites 2 and 6 > 8, and 3 and 5 > 9) (Fig. 2). In the right TRP muscle, significantly lower latency values were found after stimulation of the lateral left ophthalmic and maxillary
2309
sites compared with the right ones (3 > 2 and 5 > 6, respectively) and vice versa in the left TRP muscle (2 > 3 and 6 > 5, respectively) (Fig. 2). No significant differences were found for TRP and SSC muscles (all, p > 0.05).
3.3.2. Reflex magnitudes A significant effect of stimulation site on NWR magnitude was found for all the muscles investigated [TRP right: main effect, F(2.654, 30.802) = 9.107, p < 0.001, TRP left: main effect, F(2.659, p = 0.005; SCM right: main effect, F(3.145, 39.888) = 7.257, p = 0.024, SCM left: main effect, F(2.461, 47.178) = 6.123, 36.916) = 8.144, p = 0.012; SSC right: main effect, F(8, 120) = 11.350, p < 0.001, SSC left, main effect, F(4.022, 60.337) = 6.095, p < 0.001]. Pairwise comparisons revealed significantly higher magnitudes in the TRP, SCM and SSC muscles on both sides after stimulation of the ophthalmic and maxillary midline sites compared with the mandibular ones (sites 1 and 4 > 7), as well as after stimulation of the ophthalmic and maxillary lateral sites compared with the mandibular ones (sites 2 and 6 > 8, and 3 and 5 > 9) (Fig. 3). In the right TRP muscle, significantly higher magnitudes were found after stimulation of the lateral left ophthalmic, maxillary and mandibular sites compared with the right ones (3 > 2, 5 > 6, and 9 > 8, respectively) and vice versa in the left TRP muscle (2 > 3, 6 > 5 and 8 > 9, respectively) (Fig. 3). In the left SCM, significant higher magnitude values were found after stimulation of the left ophthalmic site (3 > 2), whereas no significant differences between left and right stimulation sites were found in the right SCM, although a trend for a similar behavior was observed (Fig. 3). No significant differences between left and right stimulation sites were found in SSC muscles within the three branches that were stimulated. No significant effect of pre-stimulus EMG activity was found in any of the muscles between all recorded sweeps (all, p > 0.05).
Fig. 2. Pairwise post hoc analyses. Comparisons (in the vertical and horizontal directions) of TCRs latencies recorded at midline and lateral facial skin sites in different muscles (TRP, SCM and SSC), both on the right and on the left. ⁄p < 0.05, ⁄⁄p < 0.01.
2310
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
Fig. 3. Pairwise post hoc analyses. Comparisons (in the vertical and horizontal directions) of TCRs amplitudes recorded at midline and lateral facial skin sites in different muscles (TRP, SCM and SSC), both on the right and on the left. ⁄p < 0.05, ⁄⁄p < 0.01.
Fig. 4 shows the contour plot of the reflex magnitudes. A reflex magnitude gradient in the vertical direction can be observed for both the midline and lateral skin sites of each muscle. Thus, higher reflex amplitudes were observed at both the ophthalmic and the maxillary sites, whereas lower values were found at the mandibular sites. In the horizontal direction, a well-defined left–right gradient can be noted in the TRP muscles: higher values were observed at left skin sites and lower values at right skin sites in the right TRP and vice versa in the left TRP (Fig. 3). 3.4. Kinematic reflex responses Fig. 4 provides a qualitative illustration of the head and neck kinematic response patterns considered in this study. Neck extension was the main response observed following stimulation of all nine sites. Indeed, significantly higher reflex probability and larger ROM values were found for neck extension compared with neck flexion responses after stimulation of every skin site (Table 1). Larger responses were found after stimulation of both the midline and the lateral ophthalmic and maxillary skin sites than after stimulation of the corresponding mandibular sites (midline sites: 1 and 4 > 7; right sites: 2 and 6 > 8; left sites: 3 and 5 > 9). Head rotation and neck lateral bending were the most common kinematic responses observed after stimulation of the lateral sites (Table 1, Fig. 5). In particular, higher reflex probability values and larger ROM values were observed for left head rotation compared with right head rotation and for neck bending to the left compared with neck bending to the right after stimulation of the right lateral
sites (sites 2, 6, 8). Opposite results were found after stimulation of the left lateral sites (sites 3, 5, 9) (Table 1, Fig. 5). Larger responses were found for the ophthalmic and maxillary sites than for the mandibular ones (2 and 6 > 8 on the right and 3 and 5 > 9 on the left, respectively, see Table 1).
4. Discussion In contrast to the old Sherringtonian concept of the flexion withdrawal reflex, recent studies have clearly demonstrated that the NWR is organized as a series of reflexes based on a precise relationship between cutaneous receptive field, dorsal horn neuronal receptive field and muscle biomechanics (Schouenborg and Weng, 1994; Weng and Schouenborg, 1998; Levinsson et al., 1999; Andersen et al., 1999, 2001, 2003; Sonnenborg et al., 2000; Schouenborg, 2002, 2003). In the light of this evidence, each withdrawal reflex is closely related to the area effectively withdrawn by the reflex contraction and should be regarded as a functional module (Petersson et al., 2003; Grillner, 2004). The present study demonstrates the existence of a similar modular organization of human HRRs. Our findings clearly showed that TCRs were site dependent: (i) there was a vertical gradient in the reflex responses. EMG and kinematic reflexes (neck extension) were larger when evoked from ophthalmic and maxillary sites than from mandibular ones; (ii) the reflex responses exhibited a crossed right–left behavior, which was, however, only clearly evident in the TRP muscles in terms of EMG responses. Stimulation of the lateral sites evoked larger reflex responses in the contralateral TRP
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
2311
Fig. 4. Contour plot of reflex magnitudes. Plot of reflex magnitudes showing mean reflex receptive fields in 16 healthy subjects in different muscles (TRP, SCM and SSC) on the right (right column) and left (left column) sides. Grand mean TCR maps were averaged across five trials in all subjects. The white circles indicate the stimulation sites. TCR maps were derived by two-dimensional interpolation of the TCR amplitudes (RMS). Reds indicate higher reflex magnitude whereas the different shades of blue indicate lower magnitudes.
muscles as well as head rotation and neck bending away from the stimulated side. Neck extension was the most common kinematic response observed after stimulation of the midline facial skin sites (88– 95% of the recorded trials) (Table 1 and Fig. 5). Such a neck extension may be explained by a bilateral activation of the TRP muscles (neck extensors) after stimulation of midline sites. Conversely, the reflex-mediated activation of both the SSC (head extensors) and SCM (head flexors) muscles might explain the inconsistent kinematic behavior of the head. Indeed, in most of the trials, the head showed either no relevant movement or a flexion or extension response (in 20–38% of the trials). Hence, this apparent co-contraction can be interpreted as an attempt, by co-activating antagonist
muscles, to stabilize the head over the neck during an abrupt neck extension reaction. Larger EMG reflexes with shorter latencies and larger kinematic responses were found when stimulating ophthalmic and maxillary skin sites than when stimulating the mandibular region. This difference might be explained by the importance of protecting the eye and nose regions. Alternatively, the explanation may lie in the role of the jaw in chewing. Reflex-mediated EMG silent periods have been documented in the temporal and masseter muscles (Ongerboer de Visser et al., 1990; Svensson et al., 1998). These inhibitory responses serve to arrest ongoing chewing to prevent potential injuries to the mouth. Thus, competitive excitatory and inhibitory inputs may partially explain the attenuated EMG and
2312
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
Table 1 Kinematic reflex responses of the head and neck after painful stimulation of the nine facial skin sites. Percentage values from 756 trials. Sites
Head
Neck
Sagittal plane
1 2 3 4 5 6 7 8 9
Transverse plane
Sagittal plane
Flex.
Ext.
Right bend.
Frontal plane Left bend.
Right rot.
Left rot.
Flex.
Ext.
Frontal plane Right bend.
Left bend.
31.8% 3.17 ± 2.2° 15.0% 2.78 ± 1.7° 38.9% 3.54 ± 2.8° 30% 3.68 ± 1.9° 36.8% 2.95 ± 1.2° 14.4% 4.03 ± 2.2° 38.9% 4.88 ± 3.0° 20.0% 5.22 ± 2.4° 25.0% 3.44 ± 0.4°
31.8% 4.02 ± 1.3° 10.0% 3.30 ± 1.9° 32.2% 3.57 ± 1.9° 40% 2.92 ± 1.5° 31.5% 4.71 ± 4.2° 16.7% 4.30 ± 3.1° 32.2% 2.83 ± 0.4° 27.0% 3.74 ± 2.5° 30.0% 2.68 ± 0.3°
–
–
–
–
–
0% – 11.1% 1.91 ± 0.1° –
0% – 88.9% 3.96 ± 1.8° –
73.7% 3.73 ± 2.8° 0% – –
10.0% 2.99 ± 0.1° 86.4% 2.90 ± 1.3° –
80.0% 3.03 ± 2.3° 0%
2.1% 2.12 ± 0.7° 27.8% 1.87 ± 0.3° –
0% – 22.2% 1.68 ± 0.8° –
90.5% 3.26 ± 1.4° 0%
0%
95.2% 2.66 ± 1.3° 0%
0%
–
83.3% 6.21 ± 3.2° –
0% – 5.8% 2.33 ± 0.5°
6.7% 1.54 ± 0.7° 5.3% 1.57 ± 0.2°
0% – 71.4% 3.22 ± 1°
71.4% 3.10 ± 1.8° 4.8% 1.61 ± 0.1°
90.5% 5.53 ± 1.5° 65.0% 4.16 ± 1.2° 68.2% 5.17 ± 2.4° 95.0% 6.11 ± 4.1° 71.4% 5.70 ± 3.2° 74.4% 5.84 ± 2.9° 88.2% 6.81 ± 3.3° 77.0% 5.07 ± 2.5° 65.0% 4.24 ± 1.9°
–
5.0% 1.55 ± 0.5° 33.3% 2.67 ± 0.6° –
14.3% 2.06 ± 1.5° 30.0% 2.45 ± 0.6° 13.6% 3.25 ± 0.8° 0% – 4.8% 2.24 ± 0.5° 5.6% 2.81 ± 0.1° 0% – 0% – 15.0% 1.64 ± 0.3°
– 0% 75.0% 2.24 ± 1.4°
–
88.9% 4.48 ± 1.8° – 84.6% 2.55 ± 1.2° 0%
Bold type indicates statistical significance.
Fig. 5. Head and neck kinematic pattern responses. Illustrative representation of the kinematic pattern of head and neck responses according to the facial skin stimulation sites.
kinematic responses when stimulation was delivered to the mandibular region compared with the other facial sites (Table 1 and Fig. 4). Stimulation of the lateral facial sites was found to evoke larger EMG reflex responses with shorter onset latencies in the contralateral TRP muscles than in the ipsilateral ones (Table 1, Figs. 3 and 4). We found also larger EMG responses in the left SCM muscle after ipsilateral ophthalmic stimulation (site 3) compared to the contralateral stimulation (site 2). Furthermore, contour plot analysis of reflex magnitude changes showed that the patterns recorded in the SSC muscles were similar to those recorded in the TRP muscles and those recorded in the right SCM muscle was similar to those recorded in the left SCM muscle, even though the difference were statistically less consistent (Fig. 4). This lateral reflexmediated activation of the neck muscles fits well with the observed kinematic responses, which were characterized by neck extension associated with a neck bending and head rotation away from the stimulated site. The lack of a statistical difference in SCM reflex magnitude, i.e. between right and left maxillary and mandibular sites (Fig. 3), suggests that other muscles, such as the rectus anterior capitis, longus capitis, and splenius capitis, are likely involved in nocifensive head rotation responses. It further suggests that the reflex-activated SCM may also have a stabilizing role in the sagittal plane to avoid abrupt and excessively large flexion–extension movements during withdrawal responses. In the present study we have investigated withdrawal reflexes in the neck muscles at rest. However, it is possible that a more
relevant modulation of the pattern of the reflex responses may be revealed in relation with different rest head–neck positions which could further support the evidence of a functional avoiding modular organization of the nociceptive TCR in humans. Further studies are needed to investigate this aspect. The particular organization of TCR receptive fields may serve a dual purpose, given that the interneuronal network mediating withdrawal reflexes is, itself, modulated by the transmission of descending motor commands to the target motor neurons (Sandrini et al., 2005). First of all, it contributes to the organization of the nociceptive pathway. In this regard, the TCRs may be a useful tool to investigate on the central sensitization mechanisms. In the nociceptive system, the central sensitization process involves spinal neurons in which expansion/rearrangement of the cutaneous reflex receptive fields has been observed. Indeed, it has been observed that the modular organization of withdrawal reflexes can be drastically modified in animals with spinal sensitization (Clarke and Harris, 2001; Harris and Clarke, 2003) as well as in patients with chronic pain disorders (Neziri et al., 2010; Biurrun Manresa et al., 2013a,b). Furthermore, patients with spinal cord injury also show enlarged nociceptive reflex receptive fields (Andersen et al., 2004; Spaich et al., 2005; Biurrun Manresa et al., 2013a,b), which suggests that descending pathway lesions are associated with a lack of module specificity in the spinal cord. It can be speculated that in chronic pain syndromes, the TCRs lack their normal receptive field organization and the associated rearrangement in the brainstem could result in widespread pain at the level of the face and head. Further studies on this topic in clinical settings are needed. In this regard, TCRs could be used as an instrument for identifying pharmacological targets able to increase synaptic efficacy and possibly restore the normal modular organization within the brainstem. Nociceptive withdrawal reflexes have also been used as a neural window onto the spinal mechanisms activated during movement (see Sandrini et al., 2005 for review). Thus and here we come to the second role, TCRs may serve other functions in addition to purely protective ones. Recent studies have shown that in order to balance the degree of freedom of the neuromuscular-skeletal system the CNS simplifies the control of movements by reducing the number of computing variables. Evidently, the CNS achieves this by selecting, activating, and combining a small set of muscles into synergies/modules (Tresch et al., 1999; D’Avella et al., 2003; Ivanenko et al., 2004).
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
It has been suggested that withdrawal reflexes may form the basic building blocks for limb movements (Duysens et al., 2013). Approximately 20 muscles participate in the execution of head movements in mammals (Napier and Napier, 1985; Richmond et al., 2001). Such high number of muscles would clearly make it extremely complex for the CNS to control head and neck movement. A limited group of motor neurons, possibly the same ones involved in head and neck withdrawal responses, may be activated by descending motor commands in order also to produce more subtle reactions such as postural adjustments in a meaningful motor context (i.e. orienting movements). 5. Conclusions This study discloses the existence of anatomical–functional connections between specific facial skin areas and motor neurons innervating the head and neck muscles, in addition to the wellknown ‘‘onion skin’’ somatotopic representation of the face in humans and non-humans primates (Bushnell et al., 1984; Dubner et al., 1989; Gybels and Sweet, 1989; Maixner et al., 1989; Craig et al., 1999). Acknowledgement The contribution of the Fondazione Bietti in this paper was supported by Ministry of Health and Fondazione Roma. Authors declare no funding for the present research. Conflict of interest: none. References Andersen OK, Sonnenborg FA, Arendt-Nielsen L. Modular organization of human leg withdrawal reflexes elicited by electrical stimulation of the foot sole. Muscle Nerve 1999;22:1520–30. Andersen OK, Sonnenborg FA, Arendt-Nielsen L. Reflex receptive fields for human withdrawal reflexes elicited by non-painful and painful electrical stimulation of the foot sole. Clin Neurophysiol 2001;112:641–9. Andersen OK, Sonnenborg F, Matjacic´ Z, Arendt-Nielsen L. Foot-sole reflex receptive fields for human withdrawal reflexes in symmetrical standing position. Exp Brain Res 2003;152:434–43. Andersen OK, Finnerup NB, Spaich EG, Jensen TS, Arendt-Nielsen L. Expansion of nociceptive withdrawal reflex receptive fields in spinal cord injured humans. Clin Neurophysiol 2004;115:2798–810. Biurrun Manresa JA, Neziri AY, Curatolo M, Arendt-Nielsen L, Andersen OK. Reflex receptive fields are enlarged in patients with musculoskeletal low back and neck pain. Pain 2013a;154:1318–24. Biurrun Manresa JA, Nguyen GP, Curatolo M, Moeslund TB, Andersen OK. Probabilistic model for individual assessment of central hyperexcitability using the nociceptive withdrawal reflex: a biomarker for chronic low back and neck pain. BMC Neurosci 2013b;14:110. Bizzi E, Tresch MC, Saltiel P, d’Avella A. New perspectives on spinal motor systems. Nat Rev Neurosci 2000;1:101–8. Bushnell MC, Duncan GH, Dubner R, He LF. Activity of trigeminothalamic neurons in medullary dorsal horn of awake monkeys trained in a thermal discrimination task. J Neurophysiol 1984;52:170–87. Clarke RW, Harris J. The spatial organization of central sensitization of hind limb flexor reflexes in the decerebrated, spinalized rabbit. Eur J Pain 2001;5: 175–85. Cowie RJ, Smith MK, Robinson DL. Subcortical contributions to head movements in macaques. I. Connections of a medial pontomedullary head-movement region. J Neurophysiol 1994;72:2665–82. Craig AD, Zhang ET, Blomqvist A. A distinct thermo receptive subregion of lamina I in nucleus caudalis of the owl monkey. J Comp Neurol 1999;404:221–34. D’Avella A, Saltiel P, Bizzi E. Combinations of muscle synergies in the construction of a natural motor behavior. Nat Neurosci 2003;6:300–8. Davis M, Gendelman DS, Tischler MD, Gendelman PM. A primary acoustic startle circuit: lesion and stimulation studies. J Neurosci 1982;2:791–805. Drew T, Prentice S, Schepens B. Cortical and brainstem control of locomotion. Prog Brain Res 2004;143:251–61. Dubner R, Kenshalo Jr DR, Maixner W, Bushnell MC, Oliveras JL. The correlation of monkey medullary dorsal horn neuronal activity and the perceived intensity of noxious heat stimuli. J Neurophysiol 1989;62:450–7.
2313
Duysens J, De Groote F, Jonkers I. The flexion synergy, mother of all synergies and father of new models of gait. Front Comput Neurosci 2013;13:7–14. Ferrigno G, Pedotti A. ELITE: a digital dedicated hardware system for movement analysis via real-time TV signal processing. IEEE Trans Biomed Eng 1985;32:943–50. Grillner S. Muscle twitches during sleep shape the precise muscles of the withdrawal reflex. Trends Neurosci 2004;27:169–71. Gybels JM, Sweet WH. Neurosurgical treatment of persistent pain. Physiological and pathological mechanisms of human pain. Pain Headache 1989;11:1–402. Harris J, Clarke RW. Organisation of sensitisation of hind limb withdrawal reflexes from acute noxious stimuli in the rabbit. J Physiol 2003;546:251–65. Hermens HJ, Freriks B, Merletti R, Hagg G, Stegeman D, Block J, et al. SENIAM: european recommendations for surface electromyography. Enschede, The Netherlands: Roessingh Research and Development; 1999. Inglis WL, Winn P. The pedunculopontine tegmental nucleus: where the striatum meets the reticular formation. Prog Neurobiol 1995;47:1–29. Ivanenko YP, Poppele RE, Lacquaniti F. Five basic muscle activation patterns account for muscle activity during human locomotion. J Physiol 2004;556:267–82. Levinsson A, Garwicz M, Schouenborg J. Sensorimotor transformation in cat nociceptive withdrawal reflex system. Eur J Neurosci 1999;11:4327–32. Maixner W, Dubner R, Kenshalo Jr DR, Bushnell MC, Oliveras JL. Responses of monkey medullary dorsal horn neurons during the detection of noxious heat stimuli. J Neurophysiol 1989;62:437–49. Meredith MA, Wallace MT, Stein BE. Visual, auditory and somatosensory convergence in output neurons of the cat superior colliculus: multisensory properties of the tecto-reticulo-spinal projection. Exp Brain Res 1992;88:181–6. Napier JR, Napier PH. The natural history of primates. Cambridge, MA: The MIT Press; 1985. Neziri AY, Haesler S, Petersen-Felix S, Müller M, Arendt-Nielsen L, Manresa JB, Andersen OK, Curatolo M. Generalized expansion of nociceptive reflex receptive fields in chronic pain patients. Pain 2010;151:798–805. Ongerboer de Visser BW, Cruccu G, Manfredi M, Koelman JH. Effects of brainstem lesions on the masseter inhibitory reflex. Functional mechanisms of reflex pathways. Brain 1990;113:781–92. Petersson P, Waldenstrom A, Fahraeus C, Schouenborg J. Spontaneous muscle twitches during sleep guide spinal self-organization. Nature 2003;424:72–5. Prentice SD, Drew T. Contributions of the reticulospinal system to the postural adjustments occurring during voluntary gait modifications. J Neurophysiol 2001;85:679–98. Richmond FJ, Singh K, Corneil BD. Neck muscles in the rhesus monkey. I. Muscle morphometry and histochemistry. J Neurophysiol 2001;86:1717–28. Sandrini G, Serrao M, Rossi P, Romaniello A, Cruccu G, Willer JC. The lower limb flexion reflex in humans. Prog Neurobiol 2005;77:353–95. Sartucci F, Rossi A, Rossi B. Trigemino cervical reflex in man. Electromyogr Clin Neurophysiol 1986;26:123–9. Sasaki S, Yoshimura K, Naito K. The neural control of orienting: role of multiplebranching reticulospinal neurons. Prog Brain Res 2004;143:383–9. Schouenborg J, Kalliomäki J. Functional organization of the nociceptive withdrawal reflexes. I. Activation of hindlimb muscles in the rat. Exp Brain Res 1990;83:67–78. Schouenborg J, Weng HR. Sensorimotor transformation in a spinal motor system. Exp Brain Res 1994;100:170–4. Schouenborg J. Modular organisation and spinal somatosensory imprinting. Brain Res Brain Res Rev 2002;40:80–91. Schouenborg J. Somatosensory imprinting in spinal reflex modules. J Rehabil Med 2003:S73–80. Serrao M, Rossi P, Parisi L, Perrotta A, Bartolo M, Cardinali P, Amabile G, Pierelli F. Trigemino-cervical-spinal reflexes in humans. Clin Neurophysiol 2003;114: 1697–703. Serrao M, Coppola G, Di Lorenzo C, Di Fabio R, Padua L, Sandrini G, Pierelli F. Nociceptive trigeminocervical reflexes in healthy subjects. Clin Neurophysiol 2010;121:1563–8. Sonnenborg FA, Andersen OK, Arendt-Nielsen L. Modular organization of excitatory and inhibitory reflex receptive fields elicited by electrical stimulation of the foot sole in man. Clin Neurophysiol 2000;111:2160–9. Sonnenborg FA, Andersen OK, Arendt-Nielsen L, Treede RD. Withdrawal reflex organisation to electrical stimulation of the dorsal foot in humans. Exp Brain Res 2001;136:303–12. Spaich EG, Arendt-Nielsen L, Andersen OK. Repetitive painful stimulation produces an expansion of withdrawal reflex receptive fields in humans. Artif Organs 2005;29:224–8. Sugiuchi Y, Kakei S, Izawa Y, Shinoda Y. Functional synergies among neck muscles revealed by branching patterns of single long descending motor-tract axons. Prog Brain Res 2004;143:411–21. Svensson P, Bakke M, Michler L, Arendt-Nielsen L, Møller E. Trigeminal responses evoked by painful electrical stimulation. J Oral Rehabil 1998;25:128–34. Tresch MC, Saltiel P, Bizzi E. The construction of movement by the spinal cord. Nat Neurosci 1999;2:162–7. Weng HR, Schouenborg J. On the cutaneous receptors contributing to withdrawal reflex pathways in the decerebrate spinal rat. Exp Brain Res 1998;118:71–7. Yeomans JS, Frankland PW. The acoustic startle reflex: neurons and connections. Brain Res Brain Res Rev 1995;21:301–14.
Contents lists available at ScienceDirect
Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph
Modular organization of the head retraction responses elicited by electrical painful stimulation of the facial skin in humans Mariano Serrao a,b, Francesca Cortese a, Ole Kæseler Andersen c, Carmela Conte b, Erika G. Spaich c, Gaia Fragiotta a, Alberto Ranavolo d,⇑, Gianluca Coppola e, Armando Perrotta f, Francesco Pierelli a a
Department of Medical-Surgical Sciences and Biotechnologies, Sapienza University of Rome, Corso della Repubblica 79, 04100 Latina, Italy Laboratory of Movement Analysis, Policlinico Italia, Piazza del Campidano 6, 00162 Rome, Italy Center for Sensory–Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7-D3, DK-9220 Aalborg, Denmark d INAIL, Department of Occupational and Environmental Medicine, Epidemiology and Hygiene, Via Fontana Candida 1, 00040 Monteporzio Catone, Rome, Italy e G.B. Bietti Foundation IRCCS, Department of Neurophysiology of Vision and Neurophthalmology, Via Livenza 3, 00198 Rome, Italy f IRCCS Neuromed, Via Atinense 18, 86077 Pozzilli, Isernia, Italy b c
a r t i c l e
i n f o
Article history: Accepted 28 January 2015 Available online 19 February 2015 Keywords: Trigeminocervical reflexes Head retraction reflexes Modular organization Reflex receptive field Withdrawal reflex Electromyography Kinematics
h i g h l i g h t s We found a well-defined modular organization of the head retraction reflexes (HRRs). HRR are related to withdrawal strategies aimed at protecting the face. Central nervous system may exploit trigeminocervical reflexes synergies to simplify head and neck
motor control.
a b s t r a c t Objective: To explore whether the trigeminocervical reflexes (TCRs) show a reflex receptive field organization in the brainstem. Methods: The facial skin of 16 healthy subjects was electrically stimulated at nine sites reflecting the distribution of the three branches of the trigeminal nerve. The reflex-evoked EMG responses were measured bilaterally from the neck muscles and the head and neck kinematic reactions were detected. Results: TCRs are site dependent. There was a vertical gradient in the magnitude of the reflex responses. EMG and kinematic reflexes were larger when evoked from ophthalmic and maxillary sites than from mandibular ones. The reflex responses exhibited a crossed right–left behavior. Stimulation of the lateral sites evoked larger reflex responses in the contralateral trapezium muscle as well as head rotation and neck bending away from the stimulated side. Conclusion: This modular arrangement of the TCRs seems to be related to withdrawal strategies aimed at protecting the face from injuries, in accordance with the functional role that each group of muscles plays in head and neck motion. Significance: It is likely that the CNS may exploit the neck muscle synergies revealed by the painful stimulation of the skin face in order to control the head and neck movements. Ó 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Nociceptive withdrawal reflexes (NWRs) are polysynaptic, multisegmental spinal responses that generate coordinated ⇑ Corresponding author. Tel.: +39 06 94181267; fax: +39 06 94181410. E-mail addresses: [email protected] (M. Serrao), francesca.cortese05@ libero.it (F. Cortese), [email protected] (O.K. Andersen), [email protected] (C. Conte), [email protected] (E.G. Spaich), [email protected] (G. Fragiotta), [email protected] (A. Ranavolo), [email protected] (G. Coppola), [email protected] (A. Perrotta), [email protected] (F. Pierelli).
muscle synergies aimed at withdrawing a limb from a potential source of injury (Sandrini et al., 2005). The pattern of withdrawal reflex-mediated muscle recruitment depends closely on the site of the stimulation. In animal models, Schouenborg and Kalliomäki (1990) discovered a well-defined modular organization of the neurons mediating NWRs: each muscle or group of muscles (module) was found to have a separate cutaneous receptive field corresponding to the skin region that is withdrawn when the muscle contracts. In this way, stimulation within a given receptive field induces reflex responses
http://dx.doi.org/10.1016/j.clinph.2015.01.029 1388-2457/Ó 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
producing an optimal limb retraction from the source of the stimulation, whereas stimulation outside the receptive field may result in inhibition of a reflex in the same muscles. Andersen and colleagues (Andersen et al., 1999) showed that a similar modular organization is also present in the human lower limb. Proximal muscles were found to have large receptive fields while more distally located muscles had smaller receptive fields covering, for example, only a part of the foot. Stimulation of the dorsolateral side of the foot evoked inversion as the dominant ankle movement along with plantar flexion (functional extension) through activation of the gastrocnemius, soleus and tibialis anterior muscles (Sonnenborg et al., 2001), whereas stimulation of the plantar side of the foot evoked dorsal flexion as the dominant ankle movement through activation of the tibialis anterior muscle. Furthermore, stimulation applied to the distal, medial sole resulted in inversion (correlated with tibialis anterior activity), whereas stimulation of the distal, lateral sole of the foot evoked eversion (Andersen et al., 1999). Nociceptive withdrawal reflexes may also involve the head and face. Indeed, the so-called trigeminocervical reflexes (TCRs) may be considered the electrophysiological counterpart of the head retraction reflexes (HRRs) that protect the face and the head against potential injury (Sartucci et al., 1986; Serrao et al., 2003). Anatomical studies in animal models have demonstrated the presence, and role, of projections from the sensory trigeminal complex to the pedunculopontine nucleus and reticular formation nuclei. These projections have been shown to be important in head orientation in space (Meredith et al., 1992; Sasaki et al., 2004), in the coordination of the neck and proximal limbs and orienting head movements (Cowie et al., 1994; Sugiuchi et al., 2004), in postural cervical tone adaptation after external perturbation (Prentice and Drew, 2001; Drew et al., 2004), in startle reactions to unexpected auditory stimuli (Davis et al., 1982; Yeomans and Frankland, 1995), and in reactions to painful stimuli (Inglis and Winn, 1995). The existence of TCRs in healthy subjects suggests that, in humans too, there is a close functional relationship between the trigeminal sensory system and cervical motor neurons. Since, to date, the central organization of the reflex pathway has not been studied, it remains to be established whether HRRs, like spinal reflexes, show a modular organization. Unveiling a modular organization of HRRs would provide important information on the functional organization of the trigeminal sensory pathways into the brainstem. Were such an organization to be confirmed, it could be hypothesized that the central nervous system (CNS) exploits the modules present in the brainstem to select the muscle synergies needed for specific tasks involving the head and neck (e.g. orientation and postural changes) in the same way as it exploits the modules involved in movement of the limbs (Bizzi et al., 2000). The main hypothesis underlying the present study was that cutaneous nociceptive stimulation of a localized facial area preferentially activates a specific group of neck muscles, i.e. those able to produce the optimal withdrawal response. Given the importance of the eye area, we expected reflexes evoked by stimulation of the skin innervated by the trigeminal ophthalmic branch and by the maxillary branch serving the eye area to be more pronounced than those evoked by the trigeminal mandibular branch. Furthermore, the pattern of muscle responses may differ upon stimulation of facial skin in midline as opposed to lateral areas. It was envisaged that stimulation of midline areas would induce neck–head retraction responses, whereas stimulation of lateral areas would evoke neck bending and/or head rotation responses, according to the side of stimulation. To test these hypotheses, the modulation pattern of major neck muscle responses following bilateral nociceptive trigeminal nerve stimulation was assessed. Nerves were stimulated at different
2307
facial skin sites corresponding to the ophthalmic, maxillary and mandibular innervation areas. 2. Methods 2.1. Participants Sixteen right-handed healthy subjects, aged 23–41 years, 10 males and 6 females, gave their written informed consent and participated in this study. The study conformed to the standards set by the latest revision of the Declaration of Helsinki. In particular, none of them had any uncorrected visual, as assessed with Snellen visual acuity test, or auditory, as assessed with pure tone audiometry test, deficits. The experimental procedures had local ethics committee approval. 2.2. Technique 2.2.1. Electrical stimulation We used an electrical stimulator (Digitimer DS7A, UK) synchronized to a data acquisition and analysis interface (CED Power 1401, Cambridge Electronic Design, UK). Male subjects were required to shave several hours prior to the experiment to reduce the bias of differences in skin thickness and resistance due to facial hair. The skin of the face was cleaned with alcohol and was stimulated through standard Ag/AgCl surface bipolar electrodes (Medelec, Oxford, UK; diameter 1 cm, 1 cm inter-electrode distance) applied to nine different sites. The sites were chosen according to the distribution of the three branches of the trigeminal nerve (ophthalmic, maxillary and mandibular) and thus showed a mediolateral arrangement (see Fig. 1). For each electrode position, the electrode was moved slightly in case the evoked sensation indicated direct nerve stimulation (with the sensation radiating to the innervation territory of the nerve branch). Trains of electrical stimuli composed of three pulses, each of 1 ms duration (inter-pulse interval 5 ms), were used to evoke the TCRs (Serrao et al., 2010). Individual pain thresholds (PTs) were assessed at each stimulation site using a staircase method that consisted of three series of ascending and
Fig. 1. Experimental setup. Facial skin stimulation sites. Trigeminocervical responses were evoked by distributed electrical stimulation of the face using surface electrodes at distinct locations. Three midline (glabella, supra- and infra-lips; 1, 4, 7) and six lateral (two supra-orbital and two infra-orbital and two at lip angles) skin sites were stimulated. The location of the sites reflected the anatomical distribution of the three branches of the trigeminal nerve (ophthalmic, maxillary and mandibular). Examples of trigeminocervical reflexes in the rectified EMG signals are depicted.
2308
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
descending stimulus intensities. The threshold was determined as the average of the peaks and troughs. The stimulation intensity was then adjusted to 2xPT for each of the nine stimulation sites. Five reflex responses were obtained per site, meaning that each subject was submitted to a total of 45 stimuli. The sequence of the stimulation sites was randomized for each participant. The subjective intensity of the painful sensation elicited by the electrical stimulation was rated by the subjects on an 11-point numerical scale (Numerical Rating Scale, NRS), ranging from 0 = no pain to 10 = unbearable pain. 2.2.2. EMG recordings EMG activity was recorded through pairs of Ag/AgCl surface electrodes (Medelec, Oxford, UK; diameter 1 cm, distance between the electrodes 2 cm), placed over the semispinalis capitis (SSC), upper trapezium (TRP) and sternocleidomastoid (SCM) muscles bilaterally. The skin area over the muscle was cleaned with alcohol. The electrodes placed over the SSC were positioned at the level of the C3 and C7 vertebrae on the midline in a belly-tendon configuration (Serrao et al., 2003). For recording from the SCM, the electrodes were placed on the anterior border midway between the mastoid process and sternoclavicular junction, while recordings from the TRP were performed with the electrodes placed in accordance with European recommendations for surface electromyography (SENIAM) (Hermens et al., 1999). A disposable reference electrode, embedded in an armband, was placed over the distal extremity of the right forearm. 2.2.3. Kinematic recordings An optoelectronic motion analysis system (SMART-E System, BTS, Milan, Italy) (Ferrigno and Pedotti, 1985), consisting of eight infra-red ray cameras (operating at 300 Hz) designed to detect movements in a three-dimensional space, was used. Nine markers (diameter: 15 mm) coated in reflective aluminum powder were placed on the subject according to the following specifications: head: left and right temples and a third marker located on the posterior side; neck: C2 and two markers on the lateral side; shoulder: left and right acromion processes and C7. 2.2.4. Experimental protocol The experimental protocol began by settling the participant comfortably on a chair. Before starting the formal measurements, each subject underwent an initial training session to familiarize him/her with the electrical stimulation and pain rating procedures and thus to reduce any effects due to arousal and/or anxiety. Thereafter, the six EMG electrodes and the nine reflective markers were applied, and the stimulation electrodes were positioned on the facial skin, checking the sensation evoked at each site. Electrical stimuli were then delivered to the different skin sites in a random sequence with random inter-stimulus intervals of at least 40 s to avoid reflex habituation. The entire experimental session lasted about 2 h per subject. 2.3. Data analysis 2.3.1. EMG analysis The EMG analysis was based on the activity in a window from 300 ms before until 300 ms after the delivery of the electrical stimulus. The signals were band-pass filtered at 20–2500 Hz and the EMG signal was rectified. Before proceeding with the formal recordings several different head and neck positions were tried in order to minimize neck muscle activity. The onset latency of the reflex was identified in the poststimulus EMG window as a response with an amplitude greater than at least the mean plus 3 times the SD of the pre-stimulus background EMG activity for at least 30 ms. The offset latency
was identified when the EMG activity returned to and remained below that level for at least 30 ms. All measurements were checked by visual inspection. The root mean square (RMS) amplitude in the pre- and post-stimulus time windows was calculated. The EMG reflex activity was measured between onset and offset latencies and the reflex magnitude was expressed as the difference between post- and pre-stimulus RMS for each investigated muscle. Five individual reflex responses were averaged for each stimulation site.
2.3.2. Kinematic analysis To evaluate the head, neck and shoulder kinematics cardan angles were used, calculated as flexion–extension, rotation and lateral bending angles of the head, and flexion–extension and lateral bending angles of the neck. The movements considered for further analyses were: (i) head and neck flexion–extension on stimulation of the midline facial skin sites (1, 4, 7); (ii) head and neck flexion–extension, head and neck lateral bending and head rotation on stimulation of the lateral facial skin sites (right: 2, 6 and 8; left: 3, 5 and 9).
2.4. Statistical analysis The Kolmogorov–Smirnov test was used to test whether data followed a normal distribution. Subsequently, one-way ANOVA for repeated measures or the Friedman test was used to evaluate the effect of the stimulation site on the neurophysiological variables (latencies and magnitudes) and NRS scores. In the ANOVA analyses, a Greenhouse–Geisser correction was used when necessary to deal with violations of sphericity. Sites were compared both according to a horizontal (right–left) criterion, i.e. within each of the three trigeminal branches (2 vs 3, 5 vs 6, and 8 vs 9) and following a vertical criterion, i.e. between the three trigeminal branches (comparing the midline sites 1, 4 and 7; the lateral right sites 2, 6 and 8; and the lateral left sites 3, 5 and 9). Post-hoc analysis with Bonferroni test was used to multiple comparisons. In order to look for a possible influence of the level of precontraction of the neck muscles, a one-way repeated measures ANOVA was used to evaluate the differences in the pre-stimulus EMG activity between stimulation sites. The TCR magnitudes detected at each facial skin site were interpolated within a bi-dimensional grid. Then, for each muscle, a two-dimensional interpolation was superimposed onto a map of the face. The results were plotted on contour plots to provide a picture of how the reflex magnitude changes according to skin stimulation sites, to allow a visual understanding of the modular organization of the reflex. We analyzed the kinematic responses of the head and neck both as categorical (flexion–extension, left–right lateral bending, left– right rotation) and continuous range of motion (ROM) variables. In order to detect the presence/absence pattern of reflex-induced mechanical responses we measured the kinematic ‘‘reflex probability’’ (expressed as a percentage), defined as the number of trials in which reflex responses were detected in each type of movement (head and neck flexion–extension, head and neck left–right lateral bending, left–right head rotation). The reflex was considered present if the ROM value, in a time window of 300 ms after the stimulus, was greater than the mean plus 3 times the SD of the background level in a time window of 300 ms preceding the stimulus. Chi-square and repeated measures ANOVA were used to compare the categorical and continuous variables, respectively. Mean and standard error were calculated. A p-value <0.05 was considered significant.
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
3. Results 3.1. Stimulus intensity The mean stimulation intensities for each site were: site 1: 53.2 ± 23.1 mA, site 2: 50.4 ± 14.3 mA, site 3: 54.3 ± 18.7 mA, site 4: 56.6 ± 21.3, site 5: 56.4 ± 15.3 mA, site 6: 55.6 ± 18.9, site 7: 62.9 ± 27.1, site 8: 65.3 ± 27.1; site 9: 67.2 ± 30.0. 3.2. NRS scores The mean 5.8 ± 0.3; site 6.0 ± 0.4; site 5.7 ± 0.5. No effect (p > 0.05).
NRS scores for each stimulation site were: site 1: 2: 5.8 ± 0.3; site 3: 5.9 ± 0.4; site 4: 6.0 ± 0.5; site 5: 6: 5.7 ± 0.4; site 7: 5.8 ± 0.5; site 8: 5.8 ± 0.4; site 9: of stimulation site on NRS scores was observed
3.3. EMG reflex responses 3.3.1. Reflex latencies A significant effect of stimulation site on NWR latencies was found for all muscles (TRP right: main effect, F(3.841, 57.609) = 11.020, p < 0.001; TRP left: main effect, F(8, 120) = 7.906, p < 0.001; SCM right: main effect, F(4.062, 60.920) = 7.076, p < 0.001; SCM left: main effect, F(3.731, 55.970) = 6.616, p < 0.01; SSC right: main effect, F(4.069, 60.879) = 2.775, p < 0.001; SSC left: main effect, F(3.398, 50.964) = 3.787, p < 0.01). Pairwise comparisons revealed significant lower latency values in all muscles on both sides after stimulation of the ophthalmic and maxillary midline sites compared with the mandibular ones (sites 1 and 4 > 7), as well as after stimulation of the ophthalmic and maxillary lateral sites compared with the mandibular ones (sites 2 and 6 > 8, and 3 and 5 > 9) (Fig. 2). In the right TRP muscle, significantly lower latency values were found after stimulation of the lateral left ophthalmic and maxillary
2309
sites compared with the right ones (3 > 2 and 5 > 6, respectively) and vice versa in the left TRP muscle (2 > 3 and 6 > 5, respectively) (Fig. 2). No significant differences were found for TRP and SSC muscles (all, p > 0.05).
3.3.2. Reflex magnitudes A significant effect of stimulation site on NWR magnitude was found for all the muscles investigated [TRP right: main effect, F(2.654, 30.802) = 9.107, p < 0.001, TRP left: main effect, F(2.659, p = 0.005; SCM right: main effect, F(3.145, 39.888) = 7.257, p = 0.024, SCM left: main effect, F(2.461, 47.178) = 6.123, 36.916) = 8.144, p = 0.012; SSC right: main effect, F(8, 120) = 11.350, p < 0.001, SSC left, main effect, F(4.022, 60.337) = 6.095, p < 0.001]. Pairwise comparisons revealed significantly higher magnitudes in the TRP, SCM and SSC muscles on both sides after stimulation of the ophthalmic and maxillary midline sites compared with the mandibular ones (sites 1 and 4 > 7), as well as after stimulation of the ophthalmic and maxillary lateral sites compared with the mandibular ones (sites 2 and 6 > 8, and 3 and 5 > 9) (Fig. 3). In the right TRP muscle, significantly higher magnitudes were found after stimulation of the lateral left ophthalmic, maxillary and mandibular sites compared with the right ones (3 > 2, 5 > 6, and 9 > 8, respectively) and vice versa in the left TRP muscle (2 > 3, 6 > 5 and 8 > 9, respectively) (Fig. 3). In the left SCM, significant higher magnitude values were found after stimulation of the left ophthalmic site (3 > 2), whereas no significant differences between left and right stimulation sites were found in the right SCM, although a trend for a similar behavior was observed (Fig. 3). No significant differences between left and right stimulation sites were found in SSC muscles within the three branches that were stimulated. No significant effect of pre-stimulus EMG activity was found in any of the muscles between all recorded sweeps (all, p > 0.05).
Fig. 2. Pairwise post hoc analyses. Comparisons (in the vertical and horizontal directions) of TCRs latencies recorded at midline and lateral facial skin sites in different muscles (TRP, SCM and SSC), both on the right and on the left. ⁄p < 0.05, ⁄⁄p < 0.01.
2310
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
Fig. 3. Pairwise post hoc analyses. Comparisons (in the vertical and horizontal directions) of TCRs amplitudes recorded at midline and lateral facial skin sites in different muscles (TRP, SCM and SSC), both on the right and on the left. ⁄p < 0.05, ⁄⁄p < 0.01.
Fig. 4 shows the contour plot of the reflex magnitudes. A reflex magnitude gradient in the vertical direction can be observed for both the midline and lateral skin sites of each muscle. Thus, higher reflex amplitudes were observed at both the ophthalmic and the maxillary sites, whereas lower values were found at the mandibular sites. In the horizontal direction, a well-defined left–right gradient can be noted in the TRP muscles: higher values were observed at left skin sites and lower values at right skin sites in the right TRP and vice versa in the left TRP (Fig. 3). 3.4. Kinematic reflex responses Fig. 4 provides a qualitative illustration of the head and neck kinematic response patterns considered in this study. Neck extension was the main response observed following stimulation of all nine sites. Indeed, significantly higher reflex probability and larger ROM values were found for neck extension compared with neck flexion responses after stimulation of every skin site (Table 1). Larger responses were found after stimulation of both the midline and the lateral ophthalmic and maxillary skin sites than after stimulation of the corresponding mandibular sites (midline sites: 1 and 4 > 7; right sites: 2 and 6 > 8; left sites: 3 and 5 > 9). Head rotation and neck lateral bending were the most common kinematic responses observed after stimulation of the lateral sites (Table 1, Fig. 5). In particular, higher reflex probability values and larger ROM values were observed for left head rotation compared with right head rotation and for neck bending to the left compared with neck bending to the right after stimulation of the right lateral
sites (sites 2, 6, 8). Opposite results were found after stimulation of the left lateral sites (sites 3, 5, 9) (Table 1, Fig. 5). Larger responses were found for the ophthalmic and maxillary sites than for the mandibular ones (2 and 6 > 8 on the right and 3 and 5 > 9 on the left, respectively, see Table 1).
4. Discussion In contrast to the old Sherringtonian concept of the flexion withdrawal reflex, recent studies have clearly demonstrated that the NWR is organized as a series of reflexes based on a precise relationship between cutaneous receptive field, dorsal horn neuronal receptive field and muscle biomechanics (Schouenborg and Weng, 1994; Weng and Schouenborg, 1998; Levinsson et al., 1999; Andersen et al., 1999, 2001, 2003; Sonnenborg et al., 2000; Schouenborg, 2002, 2003). In the light of this evidence, each withdrawal reflex is closely related to the area effectively withdrawn by the reflex contraction and should be regarded as a functional module (Petersson et al., 2003; Grillner, 2004). The present study demonstrates the existence of a similar modular organization of human HRRs. Our findings clearly showed that TCRs were site dependent: (i) there was a vertical gradient in the reflex responses. EMG and kinematic reflexes (neck extension) were larger when evoked from ophthalmic and maxillary sites than from mandibular ones; (ii) the reflex responses exhibited a crossed right–left behavior, which was, however, only clearly evident in the TRP muscles in terms of EMG responses. Stimulation of the lateral sites evoked larger reflex responses in the contralateral TRP
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
2311
Fig. 4. Contour plot of reflex magnitudes. Plot of reflex magnitudes showing mean reflex receptive fields in 16 healthy subjects in different muscles (TRP, SCM and SSC) on the right (right column) and left (left column) sides. Grand mean TCR maps were averaged across five trials in all subjects. The white circles indicate the stimulation sites. TCR maps were derived by two-dimensional interpolation of the TCR amplitudes (RMS). Reds indicate higher reflex magnitude whereas the different shades of blue indicate lower magnitudes.
muscles as well as head rotation and neck bending away from the stimulated side. Neck extension was the most common kinematic response observed after stimulation of the midline facial skin sites (88– 95% of the recorded trials) (Table 1 and Fig. 5). Such a neck extension may be explained by a bilateral activation of the TRP muscles (neck extensors) after stimulation of midline sites. Conversely, the reflex-mediated activation of both the SSC (head extensors) and SCM (head flexors) muscles might explain the inconsistent kinematic behavior of the head. Indeed, in most of the trials, the head showed either no relevant movement or a flexion or extension response (in 20–38% of the trials). Hence, this apparent co-contraction can be interpreted as an attempt, by co-activating antagonist
muscles, to stabilize the head over the neck during an abrupt neck extension reaction. Larger EMG reflexes with shorter latencies and larger kinematic responses were found when stimulating ophthalmic and maxillary skin sites than when stimulating the mandibular region. This difference might be explained by the importance of protecting the eye and nose regions. Alternatively, the explanation may lie in the role of the jaw in chewing. Reflex-mediated EMG silent periods have been documented in the temporal and masseter muscles (Ongerboer de Visser et al., 1990; Svensson et al., 1998). These inhibitory responses serve to arrest ongoing chewing to prevent potential injuries to the mouth. Thus, competitive excitatory and inhibitory inputs may partially explain the attenuated EMG and
2312
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
Table 1 Kinematic reflex responses of the head and neck after painful stimulation of the nine facial skin sites. Percentage values from 756 trials. Sites
Head
Neck
Sagittal plane
1 2 3 4 5 6 7 8 9
Transverse plane
Sagittal plane
Flex.
Ext.
Right bend.
Frontal plane Left bend.
Right rot.
Left rot.
Flex.
Ext.
Frontal plane Right bend.
Left bend.
31.8% 3.17 ± 2.2° 15.0% 2.78 ± 1.7° 38.9% 3.54 ± 2.8° 30% 3.68 ± 1.9° 36.8% 2.95 ± 1.2° 14.4% 4.03 ± 2.2° 38.9% 4.88 ± 3.0° 20.0% 5.22 ± 2.4° 25.0% 3.44 ± 0.4°
31.8% 4.02 ± 1.3° 10.0% 3.30 ± 1.9° 32.2% 3.57 ± 1.9° 40% 2.92 ± 1.5° 31.5% 4.71 ± 4.2° 16.7% 4.30 ± 3.1° 32.2% 2.83 ± 0.4° 27.0% 3.74 ± 2.5° 30.0% 2.68 ± 0.3°
–
–
–
–
–
0% – 11.1% 1.91 ± 0.1° –
0% – 88.9% 3.96 ± 1.8° –
73.7% 3.73 ± 2.8° 0% – –
10.0% 2.99 ± 0.1° 86.4% 2.90 ± 1.3° –
80.0% 3.03 ± 2.3° 0%
2.1% 2.12 ± 0.7° 27.8% 1.87 ± 0.3° –
0% – 22.2% 1.68 ± 0.8° –
90.5% 3.26 ± 1.4° 0%
0%
95.2% 2.66 ± 1.3° 0%
0%
–
83.3% 6.21 ± 3.2° –
0% – 5.8% 2.33 ± 0.5°
6.7% 1.54 ± 0.7° 5.3% 1.57 ± 0.2°
0% – 71.4% 3.22 ± 1°
71.4% 3.10 ± 1.8° 4.8% 1.61 ± 0.1°
90.5% 5.53 ± 1.5° 65.0% 4.16 ± 1.2° 68.2% 5.17 ± 2.4° 95.0% 6.11 ± 4.1° 71.4% 5.70 ± 3.2° 74.4% 5.84 ± 2.9° 88.2% 6.81 ± 3.3° 77.0% 5.07 ± 2.5° 65.0% 4.24 ± 1.9°
–
5.0% 1.55 ± 0.5° 33.3% 2.67 ± 0.6° –
14.3% 2.06 ± 1.5° 30.0% 2.45 ± 0.6° 13.6% 3.25 ± 0.8° 0% – 4.8% 2.24 ± 0.5° 5.6% 2.81 ± 0.1° 0% – 0% – 15.0% 1.64 ± 0.3°
– 0% 75.0% 2.24 ± 1.4°
–
88.9% 4.48 ± 1.8° – 84.6% 2.55 ± 1.2° 0%
Bold type indicates statistical significance.
Fig. 5. Head and neck kinematic pattern responses. Illustrative representation of the kinematic pattern of head and neck responses according to the facial skin stimulation sites.
kinematic responses when stimulation was delivered to the mandibular region compared with the other facial sites (Table 1 and Fig. 4). Stimulation of the lateral facial sites was found to evoke larger EMG reflex responses with shorter onset latencies in the contralateral TRP muscles than in the ipsilateral ones (Table 1, Figs. 3 and 4). We found also larger EMG responses in the left SCM muscle after ipsilateral ophthalmic stimulation (site 3) compared to the contralateral stimulation (site 2). Furthermore, contour plot analysis of reflex magnitude changes showed that the patterns recorded in the SSC muscles were similar to those recorded in the TRP muscles and those recorded in the right SCM muscle was similar to those recorded in the left SCM muscle, even though the difference were statistically less consistent (Fig. 4). This lateral reflexmediated activation of the neck muscles fits well with the observed kinematic responses, which were characterized by neck extension associated with a neck bending and head rotation away from the stimulated site. The lack of a statistical difference in SCM reflex magnitude, i.e. between right and left maxillary and mandibular sites (Fig. 3), suggests that other muscles, such as the rectus anterior capitis, longus capitis, and splenius capitis, are likely involved in nocifensive head rotation responses. It further suggests that the reflex-activated SCM may also have a stabilizing role in the sagittal plane to avoid abrupt and excessively large flexion–extension movements during withdrawal responses. In the present study we have investigated withdrawal reflexes in the neck muscles at rest. However, it is possible that a more
relevant modulation of the pattern of the reflex responses may be revealed in relation with different rest head–neck positions which could further support the evidence of a functional avoiding modular organization of the nociceptive TCR in humans. Further studies are needed to investigate this aspect. The particular organization of TCR receptive fields may serve a dual purpose, given that the interneuronal network mediating withdrawal reflexes is, itself, modulated by the transmission of descending motor commands to the target motor neurons (Sandrini et al., 2005). First of all, it contributes to the organization of the nociceptive pathway. In this regard, the TCRs may be a useful tool to investigate on the central sensitization mechanisms. In the nociceptive system, the central sensitization process involves spinal neurons in which expansion/rearrangement of the cutaneous reflex receptive fields has been observed. Indeed, it has been observed that the modular organization of withdrawal reflexes can be drastically modified in animals with spinal sensitization (Clarke and Harris, 2001; Harris and Clarke, 2003) as well as in patients with chronic pain disorders (Neziri et al., 2010; Biurrun Manresa et al., 2013a,b). Furthermore, patients with spinal cord injury also show enlarged nociceptive reflex receptive fields (Andersen et al., 2004; Spaich et al., 2005; Biurrun Manresa et al., 2013a,b), which suggests that descending pathway lesions are associated with a lack of module specificity in the spinal cord. It can be speculated that in chronic pain syndromes, the TCRs lack their normal receptive field organization and the associated rearrangement in the brainstem could result in widespread pain at the level of the face and head. Further studies on this topic in clinical settings are needed. In this regard, TCRs could be used as an instrument for identifying pharmacological targets able to increase synaptic efficacy and possibly restore the normal modular organization within the brainstem. Nociceptive withdrawal reflexes have also been used as a neural window onto the spinal mechanisms activated during movement (see Sandrini et al., 2005 for review). Thus and here we come to the second role, TCRs may serve other functions in addition to purely protective ones. Recent studies have shown that in order to balance the degree of freedom of the neuromuscular-skeletal system the CNS simplifies the control of movements by reducing the number of computing variables. Evidently, the CNS achieves this by selecting, activating, and combining a small set of muscles into synergies/modules (Tresch et al., 1999; D’Avella et al., 2003; Ivanenko et al., 2004).
M. Serrao et al. / Clinical Neurophysiology 126 (2015) 2306–2313
It has been suggested that withdrawal reflexes may form the basic building blocks for limb movements (Duysens et al., 2013). Approximately 20 muscles participate in the execution of head movements in mammals (Napier and Napier, 1985; Richmond et al., 2001). Such high number of muscles would clearly make it extremely complex for the CNS to control head and neck movement. A limited group of motor neurons, possibly the same ones involved in head and neck withdrawal responses, may be activated by descending motor commands in order also to produce more subtle reactions such as postural adjustments in a meaningful motor context (i.e. orienting movements). 5. Conclusions This study discloses the existence of anatomical–functional connections between specific facial skin areas and motor neurons innervating the head and neck muscles, in addition to the wellknown ‘‘onion skin’’ somatotopic representation of the face in humans and non-humans primates (Bushnell et al., 1984; Dubner et al., 1989; Gybels and Sweet, 1989; Maixner et al., 1989; Craig et al., 1999). Acknowledgement The contribution of the Fondazione Bietti in this paper was supported by Ministry of Health and Fondazione Roma. Authors declare no funding for the present research. Conflict of interest: none. References Andersen OK, Sonnenborg FA, Arendt-Nielsen L. Modular organization of human leg withdrawal reflexes elicited by electrical stimulation of the foot sole. Muscle Nerve 1999;22:1520–30. Andersen OK, Sonnenborg FA, Arendt-Nielsen L. Reflex receptive fields for human withdrawal reflexes elicited by non-painful and painful electrical stimulation of the foot sole. Clin Neurophysiol 2001;112:641–9. Andersen OK, Sonnenborg F, Matjacic´ Z, Arendt-Nielsen L. Foot-sole reflex receptive fields for human withdrawal reflexes in symmetrical standing position. Exp Brain Res 2003;152:434–43. Andersen OK, Finnerup NB, Spaich EG, Jensen TS, Arendt-Nielsen L. Expansion of nociceptive withdrawal reflex receptive fields in spinal cord injured humans. Clin Neurophysiol 2004;115:2798–810. Biurrun Manresa JA, Neziri AY, Curatolo M, Arendt-Nielsen L, Andersen OK. Reflex receptive fields are enlarged in patients with musculoskeletal low back and neck pain. Pain 2013a;154:1318–24. Biurrun Manresa JA, Nguyen GP, Curatolo M, Moeslund TB, Andersen OK. Probabilistic model for individual assessment of central hyperexcitability using the nociceptive withdrawal reflex: a biomarker for chronic low back and neck pain. BMC Neurosci 2013b;14:110. Bizzi E, Tresch MC, Saltiel P, d’Avella A. New perspectives on spinal motor systems. Nat Rev Neurosci 2000;1:101–8. Bushnell MC, Duncan GH, Dubner R, He LF. Activity of trigeminothalamic neurons in medullary dorsal horn of awake monkeys trained in a thermal discrimination task. J Neurophysiol 1984;52:170–87. Clarke RW, Harris J. The spatial organization of central sensitization of hind limb flexor reflexes in the decerebrated, spinalized rabbit. Eur J Pain 2001;5: 175–85. Cowie RJ, Smith MK, Robinson DL. Subcortical contributions to head movements in macaques. I. Connections of a medial pontomedullary head-movement region. J Neurophysiol 1994;72:2665–82. Craig AD, Zhang ET, Blomqvist A. A distinct thermo receptive subregion of lamina I in nucleus caudalis of the owl monkey. J Comp Neurol 1999;404:221–34. D’Avella A, Saltiel P, Bizzi E. Combinations of muscle synergies in the construction of a natural motor behavior. Nat Neurosci 2003;6:300–8. Davis M, Gendelman DS, Tischler MD, Gendelman PM. A primary acoustic startle circuit: lesion and stimulation studies. J Neurosci 1982;2:791–805. Drew T, Prentice S, Schepens B. Cortical and brainstem control of locomotion. Prog Brain Res 2004;143:251–61. Dubner R, Kenshalo Jr DR, Maixner W, Bushnell MC, Oliveras JL. The correlation of monkey medullary dorsal horn neuronal activity and the perceived intensity of noxious heat stimuli. J Neurophysiol 1989;62:450–7.
2313
Duysens J, De Groote F, Jonkers I. The flexion synergy, mother of all synergies and father of new models of gait. Front Comput Neurosci 2013;13:7–14. Ferrigno G, Pedotti A. ELITE: a digital dedicated hardware system for movement analysis via real-time TV signal processing. IEEE Trans Biomed Eng 1985;32:943–50. Grillner S. Muscle twitches during sleep shape the precise muscles of the withdrawal reflex. Trends Neurosci 2004;27:169–71. Gybels JM, Sweet WH. Neurosurgical treatment of persistent pain. Physiological and pathological mechanisms of human pain. Pain Headache 1989;11:1–402. Harris J, Clarke RW. Organisation of sensitisation of hind limb withdrawal reflexes from acute noxious stimuli in the rabbit. J Physiol 2003;546:251–65. Hermens HJ, Freriks B, Merletti R, Hagg G, Stegeman D, Block J, et al. SENIAM: european recommendations for surface electromyography. Enschede, The Netherlands: Roessingh Research and Development; 1999. Inglis WL, Winn P. The pedunculopontine tegmental nucleus: where the striatum meets the reticular formation. Prog Neurobiol 1995;47:1–29. Ivanenko YP, Poppele RE, Lacquaniti F. Five basic muscle activation patterns account for muscle activity during human locomotion. J Physiol 2004;556:267–82. Levinsson A, Garwicz M, Schouenborg J. Sensorimotor transformation in cat nociceptive withdrawal reflex system. Eur J Neurosci 1999;11:4327–32. Maixner W, Dubner R, Kenshalo Jr DR, Bushnell MC, Oliveras JL. Responses of monkey medullary dorsal horn neurons during the detection of noxious heat stimuli. J Neurophysiol 1989;62:437–49. Meredith MA, Wallace MT, Stein BE. Visual, auditory and somatosensory convergence in output neurons of the cat superior colliculus: multisensory properties of the tecto-reticulo-spinal projection. Exp Brain Res 1992;88:181–6. Napier JR, Napier PH. The natural history of primates. Cambridge, MA: The MIT Press; 1985. Neziri AY, Haesler S, Petersen-Felix S, Müller M, Arendt-Nielsen L, Manresa JB, Andersen OK, Curatolo M. Generalized expansion of nociceptive reflex receptive fields in chronic pain patients. Pain 2010;151:798–805. Ongerboer de Visser BW, Cruccu G, Manfredi M, Koelman JH. Effects of brainstem lesions on the masseter inhibitory reflex. Functional mechanisms of reflex pathways. Brain 1990;113:781–92. Petersson P, Waldenstrom A, Fahraeus C, Schouenborg J. Spontaneous muscle twitches during sleep guide spinal self-organization. Nature 2003;424:72–5. Prentice SD, Drew T. Contributions of the reticulospinal system to the postural adjustments occurring during voluntary gait modifications. J Neurophysiol 2001;85:679–98. Richmond FJ, Singh K, Corneil BD. Neck muscles in the rhesus monkey. I. Muscle morphometry and histochemistry. J Neurophysiol 2001;86:1717–28. Sandrini G, Serrao M, Rossi P, Romaniello A, Cruccu G, Willer JC. The lower limb flexion reflex in humans. Prog Neurobiol 2005;77:353–95. Sartucci F, Rossi A, Rossi B. Trigemino cervical reflex in man. Electromyogr Clin Neurophysiol 1986;26:123–9. Sasaki S, Yoshimura K, Naito K. The neural control of orienting: role of multiplebranching reticulospinal neurons. Prog Brain Res 2004;143:383–9. Schouenborg J, Kalliomäki J. Functional organization of the nociceptive withdrawal reflexes. I. Activation of hindlimb muscles in the rat. Exp Brain Res 1990;83:67–78. Schouenborg J, Weng HR. Sensorimotor transformation in a spinal motor system. Exp Brain Res 1994;100:170–4. Schouenborg J. Modular organisation and spinal somatosensory imprinting. Brain Res Brain Res Rev 2002;40:80–91. Schouenborg J. Somatosensory imprinting in spinal reflex modules. J Rehabil Med 2003:S73–80. Serrao M, Rossi P, Parisi L, Perrotta A, Bartolo M, Cardinali P, Amabile G, Pierelli F. Trigemino-cervical-spinal reflexes in humans. Clin Neurophysiol 2003;114: 1697–703. Serrao M, Coppola G, Di Lorenzo C, Di Fabio R, Padua L, Sandrini G, Pierelli F. Nociceptive trigeminocervical reflexes in healthy subjects. Clin Neurophysiol 2010;121:1563–8. Sonnenborg FA, Andersen OK, Arendt-Nielsen L. Modular organization of excitatory and inhibitory reflex receptive fields elicited by electrical stimulation of the foot sole in man. Clin Neurophysiol 2000;111:2160–9. Sonnenborg FA, Andersen OK, Arendt-Nielsen L, Treede RD. Withdrawal reflex organisation to electrical stimulation of the dorsal foot in humans. Exp Brain Res 2001;136:303–12. Spaich EG, Arendt-Nielsen L, Andersen OK. Repetitive painful stimulation produces an expansion of withdrawal reflex receptive fields in humans. Artif Organs 2005;29:224–8. Sugiuchi Y, Kakei S, Izawa Y, Shinoda Y. Functional synergies among neck muscles revealed by branching patterns of single long descending motor-tract axons. Prog Brain Res 2004;143:411–21. Svensson P, Bakke M, Michler L, Arendt-Nielsen L, Møller E. Trigeminal responses evoked by painful electrical stimulation. J Oral Rehabil 1998;25:128–34. Tresch MC, Saltiel P, Bizzi E. The construction of movement by the spinal cord. Nat Neurosci 1999;2:162–7. Weng HR, Schouenborg J. On the cutaneous receptors contributing to withdrawal reflex pathways in the decerebrate spinal rat. Exp Brain Res 1998;118:71–7. Yeomans JS, Frankland PW. The acoustic startle reflex: neurons and connections. Brain Res Brain Res Rev 1995;21:301–14.