Topographic variations of the relationship of the sciatic nerve and the piriformis muscle and its relevance to palsy after total hip arthroplasty

The aim of this paper was to study the anatomical relationship between the piriformis muscle and the sciatic nerve with regard to the possibility of neurological deficit after THA. The incidence of anatomical variation of both structures is 15-30% in the literature. The authors studied 91 cadavers and found an atypical relationship in 19 cases (20.9%). In this study individual variations were found with the following frequency: The sciatic nerve exits below the piriformis muscle in 79.1% of the cases. The sciatic nerve separates into two divisions above the piriformis, one branch passing through the muscle, the other below it (14.3%). An unsplit nerve passes through the piriformis muscle in 2.2%. The nerve separates into two divisions above the piriformis, one branch exiting above the muscle and passing along its dorsal aspect, the second exiting distally below the muscle in 4.4%. The most common reasons for sciatic nerve injury in surgery of the hip joint are direct injuries, ischemia of the nerve tissue, compression or excessive distraction of the nerve, compression by bone cement, thermal damage during cement polymerization, injury during THA dislocation, compression by hematoma, bone prominence or an implanted acetabular component. According to the presented anatomical study, overstretching of the nerve itself or its branches in the area of the pelvitrochanteric muscles after their release from their origin can be another mechanism. Such overstretching can appear in the presence of some of the aforementioned anatomical variants.     

Piriformis muscle: clinical anatomy and consideration of the piriformis Syndrome

Patients with lumbosacral and buttock pain provide tacit support for recognizing the piriformis muscle as a contributing factor to the pain (piriformis syndrome). One hundred and twelve cadaveric specimens were observed to elucidate the anatomical variations of the piriformis muscle referred to the diagnostic and treatment of the piriformis syndrome. The distance between the musculotendinous junction and the insertion was measured and the piriformis categorized into three types: Type A (71, 63.39%): long upper and short lower muscle belly; Type B (40, 35.71%): short upper and long lower muscle belly; Type C (1, 0.9%): fusion of both muscle bellies at the same level. The diameter of the piriformis tendon at the level of the musculotendinous junction ranged from 3 to 9 mm (mean: 6.3 mm). The piriformis showed the following possible fusions with adjacent tendons. In type one (60, 53.57%) a rounded tendon of the piriformis reached the upper border of the greater trochanter. In type two (33, 29.46%) it first joined into the gemellus superior tendon and at last both fused with the obturator internus tendon and inserted into the medial surface of the greater trochanter. A fusion of the piriformis, obturator internus and gluteus medius tendon with the same insertion area as above was observed in type three (15, 13.39%) and finally in type four (4, 3.57%) the tendon fused with the gluteus medius to reach the upper surface of the greater trochanter. Based on this survey anatomical causes for the piriformis syndrome are rare and a more precise workup is necessary to rule out more common diagnosis.     

Double superior gemellus together with double piriformis and high division of the sciatic nerve

Summary We report a case with double superior gemellus and double piriformis mm. associated with the sciatic n. dividing high and passing between the two piriformis mm. in the same lower extremity. This abnormality has not previously been described in the literature. As many musculoskeletal structures may be involved in sciatica, the supernumerary superior gemelli and piriformis mm. may exert pressure on the sciatic n. and this should be taken into account by clinicians.

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Association de muscles jumeau supérieur et piriforme doubles à une division haute du nerf sciatique

Résumé Nous rapportons le cas de l'association de muscles jumeau supérieur et piriforme doubles à une division haute du n. sciatique. Ce nerf avait la particularité de passer entre les deux mm. piriformes. Cette anomalie n'a pas été signalée dans la littérature. De la même manière qu'un certain nombre de structures musculo-squelettiques sont susceptibles de provoquer une névralgie sciatique, la présence de mm. jumeau supérieur et piriforme surnuméraires est susceptible de comprimer le n. sciatique. Aussi cette anomalie doit-elle être connue des cliniciens.

     

Double superior gemellus together with double piriformis and high division of the sciatic nerve

We report a case with double superior gemellus and double piriformis mm. associated with the sciatic n. dividing high and passing between the two piriformis mm. in the same lower extremity. This abnormality has not previously been described in the literature. As many musculoskeletal structures may be involved in sciatica, the supernumerary superior gemelli and piriformis mm. may exert pressure on the sciatic n. and this should be taken into account by clinicians.     

The neurovascular and muscular anomalies of the gluteal region an atypical pudendal nerve

In two cases, one male and one female, muscular anomalies together with neurovascular variations were encountered in the gluteal regions, in each cadaver on the same side. In the male cadaver, there was a double piriformis muscle and high division of the sciatic nerve. In the female cadaver, in addition to these anomalies, the superior and inferior gemelli and obturator internus muscles, and the internal pudendal vessels and pudendal nerve passed behind the sacrotuberous ligament. Although duplication of the piriformis and high division of the sciatic nerve have been reported previously, to the best of our knowledge the other anomalies have not yet been reported. The abnormal relationship of the internal pudendal vessels and the pudendal nerve with the sacrotuberal ligament, as in our case, may cause venous congestion, arterial obstruction, dysfunction of penile erection and perineal neuralgia. These anomalies of the gluteal region are not only of academic interest, but may be of practical importance for surgical intervention in the area.     

The neurovascular and muscular anomalies of the gluteal region: an atypical pudendal nerve

Summary In two cases, one male and one female, muscular anomalies together with neurovascular variations were encountered in the gluteal regions, in each cadaver on the same side. In the male cadaver, there was a double piriformis muscle and high division of the sciatic nerve. In the female cadaver, in addition to these anomalies, the superior and inferior gemelli and obturator internus muscles, and the internal pudendal vessels and pudendal nerve passed behind the sacrotuberous ligament. Although duplication of the piriformis and high division of the sciatic nerve have been reported previously, to the best of our knowledge the other anomalies have not yet been reported. The abnormal relationship of the internal pudendal vessels and the pudendal nerve with the sacrotuberal ligament, as in our case, may cause venous congestion, arterial obstruction, dysfunction of penile erection and perineal neuralgia. These anomalies of the gluteal region are not only of academic interest, but may be of practical importance for surgical intervention in the area.

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Anomalies musculaires et neurovasculaires de la région glutéale : un nerf pudendal atypique

Résumé Sur deux sujets — un homme et une femme — l'association d'anomalies musculaires et de variations neurovasculaires ont été observées dans la région glutéale. Sur chacun des cadavres, toutes ces particularités étaient situées du même côté. Chez le cadavre mâle, il y avait duplication du muscle piriforme et division haute associée du nerf sciatique. Chez le cadavre femelle, outre ces anomalies, les muscles jumeaux supérieur et inférieur, le muscle obturateur interne, ainsi que les vaisseaux et le nerf pudendal, passaient en arrière du ligament sacrotubéral. Si la duplication du muscle piriforme et la division haute du nerf sciatique ont déjà été rapportées, à notre connaissance les autres anomalies n'ont encore jamais été signalées. Les rapports atypiques des vaisseaux honteux internes et du nerf honteux avec le ligament sacro-tubéral comme dans notre cas, pourraient induire une congestion veineuse voire une obstruction artérielle et entraîner un dysfonctionnement du pénis et des algies périnéales. Ces anomalies de la région glutéale n'ont pas un simple intérêt académique mais peuvent être d'importance pratique pour la chirurgie de la région.

     

Double gluteus maximus muscle with associated variations in the gluteal region

Summary We report bilateral muscular and neurovascular anomalies of the gluteal region in a cadaver. On the right side, the gluteus maximus muscle had two parts, one of which was fibrous and the other muscular. In addition, there were duplicated piriformis muscles and high division of the sciatic nerve. The common peroneal nerve passed between the two parts of the piriformis muscle, and the tibial nerve emerged from under the lower piriformis muscle (infrapiriform foramen). At the same time the internal pudendal vessels and pudendal nerve passed over the sacrotuberous ligament on the left side. The double piriformis muscles and high division of the nerve are known as an anomaly which is believed to cause a nerve compression syndrome called the syndrome of the piriformis muscle. To the best of our knowledge anomalies of the gluteus maximus muscle and pudendal structures have not yet been reported. This complex anomaly should be kept in mind in connection with intramuscular injections of the gluteal region, the piriformis syndrome, and the surgery of this region.

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Muscle grand fessier dédoublé et variations associées de la région fessière

Résumé Les auteurs rapportent les anomalies musculaires et vasculo-nerveuses observées sur les deux régions fessières d'un même cadavre. À droite, le muscle grand fessier présentait deux portions, l'une fibreuse, l'autre musculaire. S'y associaient un m. piriforme double et une division haute du nerf sciatique. Le nerf fibulaire commun traversait le muscle piriforme entre ses deux chefs, le nerf tibial émergeait audessous de la partie inférieure du muscle piriforme (foramen infra-piriforme) (canal sous-pyramidal). Parallèlement, les vaisseaux honteux internes et le nerf honteux passaient au-dessus du ligament sacro-tubéral du côté gauche. Le m. piriforme double et la division haute du nerf sciatique sont des anomalies connues. La première est accusée d'être à l'origine d'une compression nerveuse appelée «syndrome du muscle piriforme». À notre connaissance l'association de l'anomalie du muscle grand fessier et des éléments honteux internes que nous présentons n'a encore jamais été décrite. Il faut avoir présente à l'esprit cette variation complexe lors d'anomalies dans la réalisation d'injections intramusculaires fessières, devant un syndrome du piriforme et lors de la chirurgie dans la région fessière.

     

Double gluteus maximus muscle with associated variations in the gluteal region

Summary We report bilateral muscular and neurovascular anomalies of the gluteal region in a cadaver. On the right side, the gluteus maximus muscle had two parts, one of which was fibrous and the other muscular. In addition, there were duplicated piriformis muscles and high division of the sciatic nerve. The common peroneal nerve passed between the two parts of the piriformis muscle, and the tibial nerve emerged from under the lower piriformis muscle (infrapiriform foramen). At the same time the internal pudendal vessels and pudendal nerve passed over the sacrotuberous ligament on the left side. The double piriformis muscles and high division of the nerve are known as an anomaly which is believed to cause a nerve compression syndrome called the syndrome of the piriformis muscle. To the best of our knowledge anomalies of the gluteus maximus muscle and pudendal structures have not yet been reported. This complex anomaly should be kept in mind in connection with intramuscular injections of the gluteal region, the piriformis syndrome, and the surgery of this region.     

Comparison of two stretching methods and optimization of stretching protocol for the piriformis muscle

Piriformis syndrome is an uncommon diagnosis for a non-discogenic form of sciatica whose treatment has traditionally focused on stretching the piriformis muscle (PiM). Conventional stretches include hip flexion, adduction, and external rotation. Using three-dimensional modeling, we quantified the amount of (PiM) elongation resulting from two conventional stretches and we investigated by use of a computational model alternate stretching protocols that would optimize PiM stretching.Seven subjects underwent three CT scans: one supine, one with hip flexion, adduction, then external rotation (ADD stretch), and one with hip flexion, external rotation, then adduction (ExR stretch). Three-dimensional bone models were constructed from the CT scans. PiM elongation during these stretches, femoral neck inclination, femoral head anteversion, and trochanteric anteversion were measured. A computer program was developed to map PiM length over a range of hip joint positions and was validated against the measured scans.ExR and ADD stretches elongated the PiM similarly by approximately 12%. Femoral head and greater trochanter anteversion influenced PiM elongation. Placing the hip joints in 115° of hip flexion, 40° of external rotation and 25° of adduction or 120° of hip flexion, 50° of external rotation and 30° of adduction increased PiM elongation by 30–40% compared to conventional stretches (15.1 and 15.3% increases in PiM muscle length, respectively).ExR and ADD stretches elongate the PiM similarly and therefore may have similar clinical effectiveness. The optimized stretches led to larger increases in PiM length and may be more easily performed by some patients due to increased hip flexion.     

Abnormal extrapelvic course of the inferior gluteal artery

At its extrapelvic course the inferior gluteal a. is found to be strictly related to the sciatic n. This relationship has been described in a general way, emphasizing its medial localization in respect to the nerve. Clinicosurgical reports describe cases of aneurysms of the inferior gluteal a. on its extrapelvic course and subsequent compression at the nerve. In order to get further details on the relationship between these two structures, 80 gluteal regions from 40 cadavers of adult Brazilian individuals, 29 males and 11 females, were dissected. The inferior gluteal a. was found medial to the sciatic n. in 62 cases (77.5%); in the 18 remaining (22.5%) the trunk of the artery or one of its branches perforated the nerve. Of these, 14 (77.8%) were males and 4 (22.2%) females. This disposition was found 8 times (44.4%) on the right and 10 (55.6%) on the left side; was unilateral in 4 individuals (1 on the right and 3 on the left side) and bilateral in 7. The course of the inferior gluteal a. through the sciatic n. and/or the presence of aneurysms of this artery should be considered as a possible cause of nerve compression.     

神经电生理对臀部肌注致坐骨神经损伤的诊断价值

目的：探讨神经电生理检测对臀部肌注致坐骨神经损伤的诊断意义。方法：对34例患者进行腓总神经、胫神经的神经传导速度（NCV）检测，并对胫骨前肌、腓肠肌、股二头肌长短头、椎旁肌L4-S1进行针极肌电图检测。结果：34例病例中，腓总神经传导速度NCV异常29例（85．3％），胫神经13例（38．2％），腓总神经、胫神经同时异常8例（23．5％）。经统计学处理，与正常对照组比较差异有显著意义（P〈0．01）。肌电图示，有失神经电位改变者：胫前肌为29例（85．3％），股二头肌短头为29例（85．3％），腓肠肌为13例（38．2％），股二头肌长头为13例（38．2％）。结论：神经电生理检测对臀部肌注致坐骨神经损伤的诊断可提供客观依据。     

Variations of the piriformis and sciatic nerve with clinical consequence: A review

The deep gluteal region is often encountered when performing injections, when performing surgery such as total hip replacements, or diagnosing problems of this region or lower limbs using clinical or imaging techniques. Previously, the prevalence figures of piriformis and sciatic nerve anomalies have ranged from 1.5 to 35.8% in dissected specimens. This study systematically reviews and meta‐analyses the prevalence of piriformis and sciatic nerve anomalies in humans using previously published literature. A further review is conducted regarding the anatomical abnormalities present in surgical case series of procedures for patients suffering from piriformis syndrome. After pooling the results of 18 studies and 6,062 cadavers, the prevalence of the anomaly in cadavers was 16.9%; 95% confidence interval (CI) 16.0–17.9%. The prevalence of the piriformis and sciatic nerve anomaly in the surgical case series was 16.2%, 95% CI: 10.7–23.5%. The difference between the two groups was not found to be significant 0.74%; 95% CI: ?5.66 to 7.13; P = 0.824. Because of the high likelihood of an anomaly being present in a patient, clinicians and surgeons should be aware of the potential complications this anomaly may have on medical or surgical interventions. Furthermore, because the prevalence of the anomaly in piriformis syndrome patients is not significantly different from what is thought to be a normal population, it indicates that this anomaly may not be as important in the pathogenesis of piriformis syndrome as previously thought. Clin. Anat. 23:8–17, 2010. © 2009 Wiley‐Liss, Inc.     

周围神经65KD蛋白在损伤性大鼠坐骨神经中的表达

目的：研究周围神经６５ＫＤ蛋白在损伤性大鼠坐骨神经中的分布与表达。方法：取损伤１５ｄ后的大鼠坐骨神经近侧端及远侧端,冰冻切片,与抗６５ＫＤ蛋白单克隆抗体与切片孵育后,加羊抗小鼠ＩｇＧＨＲＰ,ＤＡＢ反应成色。结果：大鼠坐骨神经远侧端及近侧端中均有６５ＫＤ蛋白阳性免疫反应产物,阳性免疫反应产物位于雪旺氏细胞。阳性免疫反应产物的密度及强度在神经的远侧端大于近侧端。结论：神经诱向因子６５ＫＤ蛋白由雪旺氏细胞产生。在损伤后神经近、远侧端都存在６５ＫＤ蛋白,远侧端的强度大于近侧端,这一浓度梯度可能是诱导再生的神经纤维定向生长过程中的一个重要因素。     

Anatomical variations between the sciatic nerve and the piriformis muscle: a contribution to surgical anatomy in piriformis syndrome

Purpose

To detect the variable relationship between sciatic nerve and piriformis muscle and make surgeons aware of certain anatomical features of each variation that may be useful for the surgical treatment of the piriformis syndrome.

Methods

The gluteal region of 147 Caucasian cadavers (294 limbs) was dissected. The anatomical relationship between the sciatic nerve and the piriformis muscle was recorded and classified according to the Beaton and Anson classification. The literature was reviewed to summarize the incidence of each variation.

Results

The sciatic nerve and piriformis muscle relationship followed the typical anatomical pattern in 275 limbs (93.6 %). In 12 limbs (4.1 %) the common peroneal nerve passed through and the tibial nerve below a double piriformis. In one limb (0.3 %) the common peroneal nerve coursed superior and the tibial nerve below the piriformis. In one limb (0.3 %) both nerves penetrated the piriformis. In one limb (0.3 %) both nerves passed above the piriformis. Four limbs (1.4 %) presented non-classified anatomical variations. When a double piriformis muscle was present, two different arrangements of the two heads were observed.

Conclusions

Anatomical variations of the sciatic nerve around the piriformis muscle were present in 6.4 % of the limbs examined. When dissection of the entire piriformis is necessary for adequate sciatic nerve decompression, the surgeon should explore for the possible existence of a second tendon, which may be found either inferior or deep to the first one. Some rare, unclassified variations of the sciatic nerve should be expected during surgical intervention of the region.     

Injection of the sciatic nerve with TMEV: a new model for peripheral nerve demyelination

Demyelination of the human peripheral nervous system (PNS) can be caused by diverse mechanisms including viral infection. Despite association of several viruses with the development of peripheral demyelination, animal models of the condition have been limited to disease that is either autoimmune or genetic in origin. We describe here a model of PNS demyelination based on direct injection of sciatic nerves of mice with the cardiovirus, Theiler's murine encephalomyelitis virus (TMEV). Sciatic nerves of FVB mice develop inflammatory cell infiltration following TMEV injection. Schwann cells and macrophages are infected with TMEV. Viral replication is observed initially in the sciatic nerves and subsequently the spinal cord. Sciatic nerves are demyelinated by day 5 post-inoculation (p.i.). Injecting sciatic nerves of scid mice resulted in increased levels of virus recovered from the sciatic nerve and spinal cord relative to FVB mice. Demyelination also occurred in scid mice and by 12 days p.i., hindlimbs were paralyzed. This new model of virus-induced peripheral demyelination may be used to dissect processes involved in protection of the PNS from viral insult and to study the early phases of lesion development.     

Interleukin-1 beta promotes sensory nerve regeneration after sciatic nerve injury

Nerve injury brings about axonal disconnection, and thus axonal extension is one of the important steps for nerve regeneration. Expression of the pro-inflammatory cytokine interleukin-1 beta (IL-1beta) is increased at the early stage of nervous system injury, and previously IL-1beta has been reported to promote neurite outgrowth by inhibiting RhoA activity in vitro. However, the effect of IL-1beta on axonal extension in vivo has not been obvious. Now we examine whether IL-1beta takes advantages on sciatic nerve regeneration. Sciatic nerves of rats are transected and sutured, and IL-1beta or PBS is locally administered for 2 weeks. Although IL-1beta does not influence on motor functional recovery, it promotes sensory functional recovery, estimated by toe pinch test, and increases the number and the area of neurofilament-positive axons at 12 weeks compared with PBS. Moreover IL-1beta, which promotes Schwann cell proliferation and thus may inhibit myelination, does not impair remyelination, estimated by myelin basic protein. These findings suggest that IL-1beta may contribute to sensory nerve regeneration following sciatic nerve injury by promoting axonal extension.     

Microanatomical structure of the human sciatic nerve

Background  Sciatic nerve is the largest peripheral nerve of the human body. It gives motor and sensitive innervation for the most of lower limb. The aim of the present investigation was revealing his fascicular pattern in relation to microanatomic morphometric characteristics of its connective tissue sheaths. Methods  The material consisted of sciatic nerve slices, excised from 17 cadavers of humans aging 8–93 years. After routine histologic processing and light microscopic examination of the preparations, morphometric analysis was performed at magnifications of 40 and 630×. Results  Sciatic nerve showed to be polyfascicular nerve type, with the group pattern of nerve fascicless distribution. The number of fascicless ranged from 27 to 70, whereas the number of fascicless per square millimeter was 1–4. Morphometric and correlation analysis confirmed the significant increase of whole sciatic nerve cross section area, which was associated with the significant increase of its epi- and perineural connective tissue sheaths. Interfascicular sciatic nerve domains of elderly persons contained more adipose tissue. Moreover, already detected loss and degeneration of the large myelinated nerve fibers within fascicles was accompanied by the significant increase of endoneural connective tissue. Conclusions  In conclusion, our study revealed comparative connective tissue enlargement of human sciatic nerve in the course of aging. These phenomena might influence on result of injured nerve’s surgical reparations. We interpret this finding as non-specific compensatory phenomenon elicited by loss of thickest myelinated nerve fibers, higher vulnerability of remaining ones, and age-dependent decrease of connective tissue elasticity.     

Curcumin protects the dorsal root ganglion and sciatic nerve after crush in rat

The goal of this study was to quantify the histological changes in the dorsal root ganglion (DRG) and the sciatic nerve in rats subjected to sciatic nerve crush (SNC) following curcumin treatment. The rats were divided into four groups, each including five animals, and underwent the following intervention: group I: control animals which received olive oil; group II: sham-operated animals whose skin of the posterior thigh was opened, sutured, and received the vehicle; group III: SNC animals which received the vehicle; and group IV: SNC plus curcumin (100 mg/kg/day) solved in the vehicle. On the 28th day, the fifth lumbar DRG and sciatic nerve were removed. Volume of the ganglion, mean cell volume, total volume of DRG cells (A- and B-cells), and total surface of DRG cells, total number, diameter, and area of the myelinated nerve fibers were estimated using stereological methods. Except for the volume of the ganglion, all other parameters were decreased after nerve crush. In curcumin-treated rats, these parameters decreased, but to a lesser extent, and the values were significantly higher than in the non-treated SNC group (p < 0.04).It can be concluded that in rats after crush, curcumin has a protective effect on the DRG and sciatic nerve.     

Fetal developmental change in topographical relationship between the human lateral pterygoid muscle and buccal nerve

In adults, the lateral pterygoid muscle (LPM) is usually divided into the upper and lower heads, between which the buccal nerve passes. Using sagittal or horizontal sections of 14 fetuses and seven embryos (five specimens at approximately 20-25 weeks; five at 14-16 weeks; four at 8 weeks; seven at 6-7 weeks), we examined the topographical relationship between the LPM and the buccal nerve. In large fetuses later than 15 weeks, the upper head of the LPM was clearly discriminated from the lower head. However, the upper head was much smaller than the lower head in the smaller fetuses. Thus, in the latter, the upper head was better described as an 'anterior slip' extending from the lower head or the major muscle mass to the anterior side of the buccal nerve. The postero-anterior nerve course seemed to be determined by a branch to the temporalis muscle (i.e. the anterior deep temporal nerve). At 8 weeks, the buccal nerve passed through the roof of the small, fan-like LPM. At 6-7 weeks, the LPM anlage was embedded between the temporobuccal nerve trunk and the inferior alveolar nerve. Therefore, parts of the LPM were likely to 'leak' out of slits between the origins of the mandibular nerve branches at 7-8 weeks, and seemed to grow in size during weeks 14-20 and extend anterosuperiorly along the infratemporal surface of the prominently developing greater wing of the sphenoid bone. Consequently, the topographical relationship between the LPM and the buccal nerve appeared to 'change' during fetal development due to delayed development of the upper head.     

Jaw muscle response to stimulation of type II somatosensory afferents of limbs in the rat

Convergence of various afferent inputs onto brainstem neurones may play an important role in the regulation of trigeminal motor activity. In particular, previous studies suggest that, besides sensory inputs arising from the orofacial region, extratrigeminal information may modulate jaw muscle function. In the present study the actions exerted on masseter and digastric muscles by the activation of somatosensory afferents coming from fore- and hind limbs were examined. The electromyographic activity (EMG) of masseter and digastric muscles was recorded in 20 anaesthetised rats, and EMG responses to single and paired electrical stimulation of common radial and sciatic nerves, at a threshold intensity for the activation of group II afferent fibres, were studied. The stimulation induced an excitatory response in both masseter and digastric muscles bilaterally. Ipsi- and contralateral radial nerve stimulation evoked masseter responses at latencies of 13.8±2.4 ms and of 18.0±2.6 ms, respectively, and digastric responses 1.6±0.4 ms later. Ipsi- and contralateral sciatic nerve stimulation elicited masseter responses at latencies of 21.4±2.6 ms and of 23.3±2.0 ms, respectively, and digastric responses 2.0±0.2 ms later. The same masseter and digastric motor units were excited by both radial and sciatic nerve stimulation; this suggests a convergence of somatosensory inputs arising from fore- and hind limbs on the same pool of masseter and digastric motoneurones. Paired stimulation of the two nerves did not induce any summation of the responses; this finding suggests that the two inputs, reaching a common relay station, could give rise either to occlusion or to inhibitory interactions. Spinotrigeminal relationship evidenced in this study may be involved in the coordination of jaw and limb movements. Electronic Publication