Peripheral nerve lesions and plexopathies are common in routine clinical practice and surgery. Injuries to the lumbar spinal cord and cauda equina can compromise quality of life and incapacitate the individual. Therefore, knowledge of the origin, route, and destination of the plexus components is needed for intravenous or intramuscular injections and locoregional anesthesia and surgical interventions in order to diminish the risk of iatrogenic injury and to help in the examination and diagnosis of injuries. The mapping provided can correlate each nerve to the target organ, allowing the neurological injury to be identified and treatment and rehabilitation managed.

In this review, the topography, origin, and distribution of the nerves of the lumbar plexus are compared among vertebrates. This comparative approach to the neuroanatomy of the peripheral nervous system helps us to understand the functional aspects of neural structure design. For this reason, the anatomical descriptions in this review include mammals used as experimental models for studies of plexopathies and peripheral nerve injuries, such as the rat (Rattus norvegicus), the guinea pig (Cavia porcellus), the chinchilla (Chinchilla lanigera), the rabbit (Oryctolagus cuniculus), dogs, cats, pigs, and nonhuman primates. The comparative perspective on mammalian plexus design includes an analysis of lower tetrapods, although fewer available reports are available on amphibians and reptiles or even birds than on mammals. These descriptive analyses elucidate the remarkable variation of limb anatomy and modes of locomotion among tetrapods.

27.2 Lumbar Spinal Cord and Nerves

The topography of the caudal end of the spinal cord (intumescentia lumbalis and conus medullaris) varies widely among mammals and other species and is of considerable interest in clinical and surgical medicine. These portions are important in procedures for epidural anesthesia or analgesia, ^{1,​ 2} collection of cerebrospinal fluid, ³ and injection of radiopaque substances during radiographic procedures. ⁴ This explains why there are morphological and morphometric descriptions of this structure in many animals. Precise descriptions of the spinal cord end in terms of the characteristics of each species are necessary for refining the aforementioned procedures. However, the topographies of the intumescentia lumbalis and conus medullaris between the last lumbar vertebra (basis) and the first sacral one (apex) vary with species and age. The topographies of these structures in different species are compared in ▶ Table 27.1.

Owing to the differential growth rates of vertebral column and spinal cord, the more caudal lumbar nerves constitute the cauda equina. This consists of nerve roots located inside the spinal canal of the lumbar and sacral spine, together with the medullar cone and terminal filament, the location varying among species. The cauda equina is located laterocaudally to the conus medullaris. An injury in this region can involve several nerves, since the cauda equina contains numerous nerve roots in the small area between L3 and S3 and includes the main nerves of the lumbar and sacral plexuses.

Unlike the spinal cord in mammals, the spinal cord in birds extends along the length of the vertebral canal including the coccygeal region; it is as long as the whole vertebral column and decreases in diameter caudally. ^{5,​ 6} Although its segmentation is the same as in mammals, the cervical portion is longer and variable among avian species, while the thoracic portion is very short and the sacral cord is also longer, as in the coccygeal region. There are two intumescences in birds, cervical and lumbar, and only birds display a glycogen body lying in the spinal cord in the area of the lumbosacral sinus. ^{5,​ 6} Glycogen body cells are of glial origin, possibly from astrocytes, and have undergone extreme differentiation, but neither the origin of these cells nor the function of the glycogen body is wholly clear.

The reptilian spinal cord extends throughout the vertebral canal, filling only 50% of the lumen in alligators and 29 to 34% in several lizard species. ⁷ Although the reptilian spinal cord has a segmented organization as in other vertebrates, it lacks some of the functional regionalization seen in mammals. It tends to be larger near the brainstem, and the cervical and sacral regions are larger in cross-section, corresponding to the brachial and lumbosacral plexuses, though there is no lumbar region in turtles, ^{8,​ 9} lizards, or crocodiles. ^{7,​ 10} The cords in snakes and limbless lizards lack the brachial and sacral intumescences. ⁷ The spinal cord varies among amphibians, being more differentiated in anurans than in urodeles. Anurans have a relatively short spinal cord with 11 segments and conspicuous intumescentiae cervicalis and lumbalis. ¹¹ The spinal cord terminates in a relatively long and slender cone, which represents the remnant of the premetamorphosis caudal portion. ^{11,​ 12} In reptiles, no lateral horn can be distinguished, but the large area of gray matter between the horns is composed of interneurons. ⁷ In chelonians, the ventral horn is reduced in the midtrunk because there are fewer motor neurons since trunk musculature is lacking. ⁹

The number of spinal nerves varies among species according to the number of vertebrae. However, the numbers of segments and nerves in the cervical spinal cord are the same in almost all species. For example, humans have five lumbar segments and nerves ¹³; nonhuman primates vary between four and seven ^{14,​ 15,​ 16}; dogs have seven. ¹⁷ From T1, all spinal nerves emerge below their corresponding vertebrae, even those that constitute the cauda equina. The numbers of lumbar spinal nerves that correspond to spinal segments for some species are shown in ▶ Table 27.2. Although the neuroanatomy and cytoarchitecture of the mammalian cerebral cortex (pallial domains) differs considerably from that of birds, the peripheral nervous system is very similar in cytoarchitecture features and anatomical arrangement. In birds, the spinal segments and nerves are identified in relation to the cervical, thoracic, lumbar, sacral, and coccygeal regions. However, as stated in the Nomina Anatomica Avium, ^{18,​ 19} the best way to determine the spinal nerves is to count the number of vertebrae, starting at the base of the skull and proceeding caudally. Because the number of vertebral segments varies widely across taxa and distinct regional boundaries are lacking, this same method is used for other tetrapods such as amphibians and reptiles (▶ Table 27.2).

The lumbar nerves comprise five pairs of spinal nerves (the last thoracic and the first four lumbar) emerging between the last thoracic vertebra and, eventually, the first lumbar vertebra, but there are species differences. Each lumbar nerve is organized as a typical spinal nerve divided into anterior and posterior primary divisions and a visceral communicating branch to the sympathetic trunk. The primary posterior or dorsal division emits a medial branch to the multifidus muscles and skin, a lateral superior branch to the sacrospinalis muscles and skin of the gluteal region, and a lateral inferior branch to the sacrospinalis muscles. The primary anterior (or ventral) division constitutes the iliohypogastricus, ilioinguinalis, genitofemoralis, cutaneus femoris lateralis, femoralis, obturatorius, and truncus lumbosacralis nerves, constituted by the last lumbar and the first two sacral nerves. Small recurrent branches innervate the spinal dura mater.

The lumbar autonomic innervation to the abdominal and pelvic viscera is complex. The sympathetic lumbar nerves (nervi splanchnici lumbales) originate from L1–L2 but receive contributions of neurons from the last lumbar ganglia. Because of this, intestinal and renal pain can be a source of lower back pain and, if the intestines or uterus are involved, the pain signal can manifest in the pelvic limb via the ischiadicus nerve, although a visceral dysfunction is responsible. ^{13,​ 20} On the other hand, severe caudal lumbar lesions block the central inhibition of micturition and result in urinary incontinence, since the parasympathetic reflex of bladder emptying usually persists. ^{13,​ 20} The sympathetic innervation should survive parasympathetic loss; detrusor sphincter dyssynergy can result in the loss of detrusor muscle activity and abnormally strong contractions of the urethra sphincter muscle.

Because the lumbar plexus formation varies in the location of the medullary conus and cauda equina (▶ Table 27.1), occasional lesions cranial to L4–L5 can produce signs of lumbosacral disease. For example, a lesion affecting the L4–L6 roots is likely to produce sensory-motor deficit in the extensors and adductors of the thigh (femoral and obturator nerves, respectively) and partially affect the cranial and caudal gluteus nerves. However, authors disagree about the formation and division of the lumbar and lumbosacral nerves, especially in nonhuman animals. Anatomy textbooks mention three possible configurations: (1) a lumbar plexus (nervi iliohypogastricus, ilioinguinalis, genitofemoralis, cutaneus femoris lateralis, femoralis, and obturatorius); (2) a sacral plexus (nervi gluteus cranial, gluteus caudal, ischiadicus, pudendus, and rectales caudales or inferiores); and (3) the lumbar nerves and the ischiadicus constituting the lumbosacral trunk. ^{17,​ 21,​ 22,​ 23} Another difference is the presence of unisegmentary nerves (iliohypogastricus and ilioinguinalis: 22) in nonhuman species, while in humans all lumbar nerves comprise two or more spinal segments. ²⁴ However, some axon fibers that innervate the pelvic limb via the lumbar and/or sacral nerves could originate from various cord segments, allowing each segment to participate more in the formation of nerve trunks and terminal nerves. Thus, axonal tracing has frequently been used to determine the exact axons and cell bodies that constitute each nerve.

On the other hand, several studies describe variations in the formation and distribution of the lumbar nerves. Since the terms prefixed (proximal or superior or cranial), ordinary (normal or median-fixed), and postfixed (distal or inferior or caudal) were introduced by Sherrington, ²⁵ the variations in lumbar plexus and nerve origins in humans have been extensively revised. The ordinary form comprises the L1 to L4 nerves. When a communicating branch from T12, also known as the subcostal nerve, joins the first lumbar trunk (L1), it constitutes a prefixed plexus, ^{26,​ 27,​ 28}

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