LUMBOSACRAL PLEXUS OF THE MONGOLIAN GERBIL ( Meriones unguiculatus Milne-Edwards , 1867 ) PLEXO LOMBOSSACRAL DO GERBIL ( Meriones unguiculatus Milne-Edwards , 1867 )

The Mongolian gerbil, belonging to the Muridae family, is characterized as an economic asset in the pet market due to its easy management. Current research analyzes and describes the origin of the nerves that make up the lumbosacral plexus in the species, foregrounding comparative anatomy, especially among wild rodents. The experiment, approved by the Committee of Institutional Bioethics, involved 15 animals which had been used in other studies and were donated to the Laboratory of Applied Animal Morphophysiology of the Federal Rural University of the Semi-Arid, Mossoró, Rio Grande do Norte, Brazil. The animals were fixed in a water solution of formaldehyde 10%. After 72 hours, they were dissected, the abdominal cavity was opened and eviscerated, and the psoas minor, psoas major and the quadratus lumborum muscles were removed to inspect the nerves that form the lumbosacral plexus. Results were given in percentages and grouped in tables. The lumbosacral plexus of the Mongolian gerbil frequently derived from the ventral roots of the last thorax nerves, from the seven lumbar nerves and from the four sacral nerves (Type II – T12-S4). The nerves of the gerbil ́s lumbosacral plexus are the iliohypogastricus, ilioinguinalis, cutaneus femoris lateralis, genitofemoralis, femoralis, obturatorius, ischiadicus, gluteus cranialis, pudendus, gluteus caudalis and rectales caudales nerves. Information on the origin of the nerves of the gerbil ́s lumbosacral plexus and their description is relevant for clinical and surgical studies, and for the application of techniques or anesthetic drugs.


INTRODUCTION
The Mongolian gerbil or squirrel is a rodent native to the desert regions of Northeastern China and Mongolia.The docile, fecund and easily managed animal has singular characteristics which are highly important for the pet market and for experimental models in studies on the application of drugs, such as local, peridural and spinal anesthesia (LICHTENBERGER;JEFF, 2007;GAERTNER et al., 2008).The gerbil may present morphofunctional characteristics which are highly relevant for comparative studies with other mammals, including human beings (SANTOS et al., 2003).
Specialized literature provides scanty reports on the anatomy of the nervous system of the species (FREWEIN, 1978;KÜNZEL, 1985;CASTRO-PACHECO et al., 2001;KUCHINKA et al., 2008), particularly on the lumbosacral innervation.The literature (LOPES et al., 2012;OLIVEIRA et al., 2014;TONINI et al., 2014) describes the lumbosacral plexus as a structure formed by the interaction between the lumbar and the sacral plexuses, generally made up of nerve roots derived from the last lumbar vertebra and from the first sacral vertebra, innerving the pelvis and the region´s viscera.
Lesions in the lumbosacral plexus or in associated nerves may cause loss of spinal reflexes and conscious proprioception in the pelvis, atrophy, paralysis and loss of flection or voluntary extension of the muscles and even the inability to bear body weight (LORENZ; KORNEGAY, 2006).Information on the formation of the plexus is not merely a contribution for the study of comparative anatomy but also provides adequate descriptions on the clinical, surgical and anesthetic behavior to veterinarians.Due to scarcity of data on the anatomy of the species, particularly on the nervous system, current paper will contribute towards the use of the animals as experimental models.Therefore, this study was undertaken to analyze and to describe the origin of the nerves that make up the lumbosacral plexus in the Mongolian gerbil, foregrounding comparative anatomy, especially among wild rodents.

MATERIAL AND METHODS
Fifteen adult animals (10 males and 5 females) were used.They originated from previous studies performed in the Laboratory of Animal Parasitology of the Federal Rural University of the Semi-Arid (Protocol CEUA n. 04/2014, Process n. 23091.005376/2013-67)and donated to the Laboratory of Applied Animal Morphophysiology.
The animals were fixed in formaldehyde solution 10% and dissected after 72 h.A median longitudinal incision from the xiphoid cartilage to the anus was performed so that the abdominal cavity could be accessed and the animals eviscerated.The m. psoas major, m. psoas minor and the m.lumbar quadratus were exposed and removed.The pubis bone was removed by a scissor and the muscles close to the intervertebrate foramens were dissected.
Cotton pads moistened in a solution of hydrogen peroxide 2% were placed under the nerves for 12 h.
Photographs of the different nerve arrangements that compose the lumbosacral plexus were taken to illustrate the results.Data were grouped in tables as percentages and analyzed immediately.The nomenclature was based on the International Committee on Veterinary Gross Anatomical Nomenclature (2012).
Fisher´s exact test was employed to calculate probability when left and right antimeres of males and females were paired.

RESULTS
Dissection revealed that the gerbil´s lumbosacral plexus has two formations: Type 1 with T 12 -S 3 nerve roots (Figure 1A) and Type II with T 12 -S 4 roots (Figure 1B).The ilioinguinalis nerve is frequently formed by L 1 branches only, or by L 2 branches and in certain cases may receive a small contribution of L 1 .On the right antimere, the nerve passes obliquely in a caudal direction close to the medial section of the right kidney, close to the renal hilum.The nerve passes in a caudal direction on the left antimere and runs ventrally to the left supra-renal gland.
The cutaneus femoris lateralis nerve is formed by the bifurcation of the ventral branches L 1 and L 2 or only from L 2 branch.The iliohypogastricus, ilioinguinalis and cutaneus femoris lateralis nerves run ventrally and on the surface to the psoas minor muscle.
The genitofemoralis nerve derives from L 2 or from L 2 -L 3 .After its formation, the nerve emerges between the psoas minor and psoas major muscles.
The femoralis nerve has a variable origin and is derived frequently from L 3 -L 4 branches or L 2 -L 4 .The obturatorius nerve is frequently made up of L 3 -L 4 roots.The ischiadicus nerve originated from the joint branches of L 4 -L 6 .
The gluteus cranialis nerve is a relatively slim nerve, scarcely visible, and lies dorsally and intertwined to the ischiadicus nerve frequently formed by L 5 -L 6 .The gluteus caudalis nerve emerges frequently from the sacral ventral branches of S 1 -S 2 or only from S 2 .
The pudendus nerve is formed by a nexus between the nerve roots of L 7 -S 1 , or it may originate only from L 7 .
The rectales caudales nerve runs caudally and ventrally along the sacral vertebrae.It is covered by sacrocaudal ventral muscles with the innervation of the sphincter ani externus and the levator ani muscles.The nerve originated frequently from S 1 -S 4 in the specimens analyzed (Table 2).

DISCUSSION
The gerbil´s lumbar and sacral skeletontopy has seven lumbar vertebrae and four sacral vertebrae.Several variations in the number of lumbar and sacral vertebrae usually occur in rodents.In their studies on rock cavies, Lacerda et al. (2006) reported that the number varies between 6 and 7 lumbar vertebrae, while AYDIN et al. (2009) registered only four lumbar vertebrae for the porcupine.On the other hand, Oliveira et al. (2014) studying Spix's toothed-cavy observed variations in the number of sacral vertebrae, namely, between three and four, corroborating information that the amount of vertebrae of the spinal column may vary in rodents.
The factors that make up the lumbosacral plexus vary according to the number of vertebrae that constitute the lumbar and sacral regions.The gerbil´s plexus is mainly represented by branches from T 12 -S 4 .On the other hand, Greene (1963) shows that the lumbosacral plexus in rats derives from the last thorax nerve (T 13 ), from all lumbar nerves (L 1 -L 6 ) and also from the sacral nerves (S 1 , S 2 ).
Contrastingly to what has been reported on the paca (TONINI et. al., 2014), there were no significant variations between the origin of the plexus and gender.
Results for the gerbil under analysis differed from those described for the guinea-pig (COOPER; SCHILLER, 1975) since nerves derived from L 3 -L 6 and S 1 -S 3 , featuring separately the lumbar and sacral trunk.They also differed from those on the paca (TONINI et al., 2014), formed from L 4 -S 3 ; from those of the rock cavy (LACERDA et al., 2006) and Spix's yellow toothed-cavy (OLIVEIRA et al., 2014) derived from L 5 -S 3 .Difference may be related to the acquisition of morphological characteristics throughout the evolution process which may have differentiated the cavid from the murid rodent groups or they may be related to such behavior as great agility, greater capacity for cramming, climbing trees or jumping.
The gerbil revealed iliohypogastricus, ilioinguinalis and genitofemoralis nerves as branches of the lumbar plexus, described as normal by Warwick & Williams (1995) for human beings.Bolk (1898), quoted by El-Assy (1966), showed that the first root in the human lumbosacral plexus is either T 12 or L 1 , similar to results on the Mongolian squirrel.
The gerbil´s iliohypogastricus nerve originates from the ventral branch of the last thorax nerve (T 12 ), differing from that in the rat (GREENE, 1963) and in the porcupine (AYDIN et al., 2009) formed by T 13 and T 15 , respectively.
Gerbil´s ilioinguinalis nerve is frequently derived from L 1 .The nerve in the red squirrel (AYDIN, 2010) and in the chinchilla (MARTINEZ-PEREIRA; RICKES, 2011) is formed by roots derived from L 3 and demonstrated a nerve arrangement different from that of the gerbil.
The cutaneus femoris lateralis nerve has a similar formation as that of the anteater (CRUZ et al., 2014), differing from reports on the chinchilla (MARTINEZ-PEREIRA; RICKES, 2011) formed exclusively from L 4 , and on the paca (TONINI et al., 2014) derived from L 5 .
The gerbil´s obturatorius nerve is formed by L 3 and L 4 , as in the seal (CASTRO et al., 2009).The obturatorius nerve in the ocelot (LOPES et al., 2012) and in the paca (TONINI et al., 2014) derives from L 5 and L 6 , or rather, different from that of the gerbil.
The gluteus cranialis nerve originates from L 5 -L 6 , different from that of the rock cavy (LACERDA et al., 2006) and Spix's yellow toothed-cavy (OLIVEIRA et al., 2014), with its origin in L 7 .It also differs from the porcupine´s nerve (AYDIN et al., 2009) which derives from L 3 -L 4 in males and from L 3 in females.
The gerbil´s gluteus caudalis nerve frequently originates from the roots of S 1 and S 2 .This is different from reports on the rock cavy (LACERDA et al., 2006) derived from L 7 and S 1 , and on the paca (TONINI et al., 2014).
The rectales caudales nerve has a plurisegment origin, frequently from S 1 -S 4 .In the porcupine (AYDIN et al., 2009) it originates from the branches of S 2 , the nerve derives from S 2 and S 3 in the dog (GETTY, 1986) and in the chinchilla (MARTINEZ-PEREIRA; RICKES, 2011).

CONCLUSIONS
The gerbil´s lumbosacral plexus is derived from nexuses between the ventral branches of the spinal nerves that emerge from the last thorax vertebra up to the fourth sacral vertebra (T 12 -S 4 ).

Table 1 .
Origin of the gerbil´s lumbosacral plexus.Data are given for the right (RA) and left (LA) antimeres.

Table 2 .
Origin of nerves of the lumbosacral plexus of the gerbil distributed on the right (RA) and left (LA) antimeres.

Table 3 .
Origin of the brachial plexus nerves in different mammalian species.