Lab Instructions:

Lumbosacral plexus

The nerves of the pelvic limb originate from the lumbosacral plexus. The lumbosacral plexus is diffuse and consists of the ventral branches of the lumbar and sacral spinal nerves. Unlike the brachial plexus, the spinal nerves that contribute roots to the lumbosacral plexus have variability within and between species. Broadly speaking, spinal nerves of L3-S3 (i.e., third lumbar spinal nerve to the third sacral spinal nerve) contribute to the lumbosacral plexus. We will not identify these roots in the cadaver.

Caudal gluteal

The caudal gluteal nerve passes over the ischiatic notch medial to the middle gluteal muscle and enters the medial surface of the superficial gluteal muscle. It is the sole innervation of the superficial gluteal muscle. It has a variable origin from the seventh lumbar and first two sacral spinal nerves.

Cranial gluteal

The cranial gluteal nerve passes over the greater ischiatic notch, crosses the lateral surface of the ilium at the origin of the deep gluteal muscle, and innervates the middle and deep gluteal mm., as well as the tensor fasciae latae m. It arises from the ventral branches of the sixth and seventh lumbar nerves, and the first sacral spinal nerve.

Caudal cutaneous femoral

The caudal cutaneous femoral nerve arises from the sacral plexus and is united to the pudendal for most of its intrapelvic course. The caudal cutaneous femoral nerve follows the caudal gluteal artery to the level of the ischiatic tuberosity, where it becomes superficial near the attachment of the sacrotuberous ligament and terminates in the skin on the proximal caudal half of the thigh.

Sciatic

The sciatic n. arises from the last two lumbar and first two sacral spinal nerves. Small branches leave within the pelvis to supply the internal obturator, gemelli, and quadratus femoris mm. Do not dissect these branches. The sciatic nerve passes caudally over the hip medial to the greater trochanter, craniomedial to the ischiatic tuberosity, and then distally caudal to the femur on the lateral side of the adductor muscle. Unnamed branches leave the sciatic nerve at the level of the hip and innervate the biceps femoris, semitendinosus, and semimembranosus mm.

The sciatic nerve terminates in the thigh as the common fibular and tibial nerves. There are lateral and caudal cutaneous sural nerves that arise from the fibular and tibial components of the sciatic nerve, respectively, and supply the skin on the lateral and caudal surfaces of the crus.

Common Fibular

The common fibular nerve arises primarily from the ventral branches of the sixth and seventh lumbar spinal nerves. It passes laterally, deep to the thin terminal part of the biceps femoris m. and crosses over the superficial surface of the lateral head of the gastrocnemius m. and proximal fibula. It then courses between the lateral digital flexor muscle caudally and the fibularis longus m. cranially to enter the muscles on the cranial side of the crus. Here the common fibular nerve divides into superficial and deep branches (named the superficial and deep fibular nerves, respectively). These nerves innervate the flexor muscles of the tarsus and the extensor muscles of the digits. These include the cranial tibial, fibularis longus, and long digital extensor mm.

The superficial fibular nerve leaves the lateral portion of the parent nerve below the stifle, where it lies between the lateral digital flexor m. (lateral head of the DDF m.) caudally and the fibularis longus m. cranially.  At the beginning of the distal third of the crus, it becomes subcutaneous and accompanies the cranial branch of the saphenous artery. Distal to the tarsus the superficial fibular nerve forms dorsal common digital nerves that innervate the paw.

The deep fibular nerve arises from the cranial surface of the parent nerve. It enters the muscles on the cranial part of the crus and courses distally in association with the cranial tibial artery. In the proximal half of the tarsus, they both lie in a groove formed by the tendons of the long digital extensor muscle laterally and by the cranial tibial muscle medially.

At the tarsus the nerve divides into dorsal metatarsal nerves that continue distally to innervate the dorsal aspect of the paw along with the dorsal common digital nerves from the superficial fibular nerve.

Tibial

The tibial nerve arises primarily from the ventral branches of the seventh lumbar and first sacral spinal nerves and is the caudal portion of the sciatic nerve. It separates from the common fibular nerve in the thigh. At the stifle, it passes between the two heads of the gastrocnemius m. The tibial nerve supplies the caudal muscles of the crus, which include the extensors of the tarsus and flexors of the digits. It innervates both heads of the gastrocnemius m., superficial digital flexor m., popliteus m., and both deep digital flexor mm. The tibial n. emerges from the deep surface of the medial head of the gastrocnemius m. and continues distally along the medial side of the caudal surface of the tibia.

We’ll now shift our attention proximally back up to the medial aspect of the thigh.

Obturator

The obturator nerve arises from the fourth, fifth, and sixth lumbar spinal nerves. It is formed within the caudomedial portion of the iliopsoas muscle. It leave the muscle dorsomedially, runs caudoventrally along the body of the ilium, penetrates the medial side of the levator ani muscle and leave the pelvis by passing through the cranial part of the obturator foramen. It supplies the adductors muscles of the limb; i.e., the external obturator, pectineus, gracilis, and adductor mm.

Femoral

The femoral nerve arises primarily from the fourth, fifth, and occasionally sixth lumbar spinal nerves. It supplies branches to the iliopsoas muscle, and enters the quadriceps muscles between the rectus femoris and vastus medialis mm. and supplies all four heads of the quadriceps (rectus femoris m., vastus medialis/lateralis/intermedius mm.).

Saphenous

Within the iliopsoas m., the saphenous nerve arises from the cranial side of the femoral n. and passes through the femoral triangle along with the femoral artery and vein. The saphenous n. and/or femoral n. innervates the sartorius m. The cutaneous portion of the saphenous n. supplies the skin on the medial side of the thigh, the stifle, crus, tarsus, and remainder of the paw.

Autonomous Zones of the Pelvic Limb and Pes

You are not required to dissect the nerves of the pes.  However, you are responsible for identifying them on models and for naming the autonomous zones of the foot and their corresponding nerves.

Proximal to the tarsocrural joint, the tibial nerve divides into the medial and lateral plantar nerves. These cross the tarsus medial to the tuber calcanei through the tarsal tunnel and terminate as the plantar common digital and plantar metatarsal nerves, respectively, which are sensory to the plantar surface of the paw. The tarsal tunnel is formed by the sustentaculum tali dorsally, the calcanean tuber laterally, and the flexor retinaculum on the plantar side. It contains the tendon of the lateral head of the deep digital flexor with its synovial sheath, the medial and lateral plantar nerves, and the caudal branch of the saphenous artery. We will come back to these medial and lateral plantar nerves in the equine distal limb. For our purposes in the carnivore, we will focus on the tibial nerve providing cutaneous sensory innervation to the plantar surface of the pes.

The cutaneous innervation of the dorsal surface of the pes (and indeed the tarsus and the cranial crus) is provided by superficial branch of fibular n.