Visual Anatomy 視覚解剖学

下殿神経　（仙骨神経叢の枝）　（かでんしんけい、英： inferior gluteal nerve ）

　　　（ Wikipedia、イラスト掲載サイト）

　下殿神経とは

　「 Wikipedia 」には以下のような解説文が見られる。

　「 The nerve may also send a branch to the posterior femoral cutaneous nerve. 」

　「日本人体解剖学 (上巻) 」では以下のように解説している。

「運動性。第5腰神経から第2仙骨神経から起こり、下殿動・静脈、坐骨神経および後大腿皮神経とともに大坐骨孔で梨状筋の下、すなわち梨状筋下孔を通り小骨盤に出て、主として大殿筋の内面に達しこの筋に分布する。」

腰仙骨神経叢

模型図

仙骨神経叢の枝

腰神経

仙骨神経

　仙骨神経叢の枝

　以下は仙骨神経叢の枝を簡単に表した図となる。

　以下は「 Wikipedia 」の解説文となる。

「 The largest muscle of the posterior hip, gluteus maximus, is innervated by the inferior gluteal nerve.[1] It branches out and then enters the deep surface of the gluteus maximus, the principal extensor of the thigh, and supplies it.
【 origin 】
The muscle is supplied by the inferior gluteal nerve which arises from the dorsal branches of the ventral rami of the fifth (L5), the first (S1) and second (S2) sacral nerves.[2]
The lumbosacral trunk, which is made up of L5 and a small branch of L4, effectively connects the lumbar and sacral plexuses.[3] The lower branches of the L4 and the L5 nerves enter the sacral plexus.
The sacral plexus is formed by the lumbosacral trunk, the first to third sacral ventral rami, and part of the fourth, the remainder of the last joining the coccygeal plexus. The sacral plexus is formed in the pelvis in front of the piriformis muscle.[3]
The sacral plexus is formed anterior to the piriformis muscle and gives rise to the sciatic nerve, the superior and inferior gluteal nerves, and the pudendal and posterior femoral cutaneous nerves.[3]
However, most of the sacral plexus nerves are scarcely recognizable, because they leave the pelvis through the greater sciatic foramen. From the pelvis, the anterior primary branches of the nerves entering the plexus (the first sacral nerve being a particularly large one) and a mass of nerves on the piriformis can be recognized.[4]
【 course 】
The inferior gluteal nerve leaves the pelvis through the greater sciatic foramen and runs underneath the piriformis muscle. It then divides into muscular branches to supply the gluteus maximus that pass posteriorly into the deep surface of the gluteus maximus muscle.[5]
The inferior gluteal nerve is superficial to the sciatic nerve. It has been described as having multiple branches with subsequent innervation of the overlying gluteus maximus.[1]
The inferior gluteal nerve entered the deep surface of gluteus maximus very inferiorly. At the lower border of the piriformis muscle, the nerve turns backward and divides into upward and downward diverging branches, which enter the gluteus maximus. The nerve may also send a branch to the posterior femoral cutaneous nerve.[3]
The inferior gluteal nerve reliably emerged inferior to the piriformis muscle. The branching characteristics of the nerve falls into two broad categories.One category is short stalks which came under the piriformis and then gave rise to all of the terminal branches of the nerve which spanned the muscle of the gluteus maximus. The number of branches from the stalk ranged from four to six.[1] The second category occurs as a partial split of the stalk that occurred proximal to the coverage of the piriformis. There were two to three divisions of the inferior gluteal nerve under the piriformis that would further divide close to the insertion of the nerve into the actual muscle belly.[1]
The nerve was always seen close to and medial to the sciatic nerve when it left the sacral plexus inferior to the piriformis. In all specimens, the nerve entered the deep surface of gluteus maximus approximately 5 cm from the tip of the greater trochanter of the femur and entered the deep surface of gluteus maximus over the inferior one-third of the muscle belly.[2]
The inferior gluteal nerve is accompanied by the inferior gluteal artery, a branch of the anterior trunk of the internal iliac artery.[2]
However the relationship between the inferior gluteal nerve and artery was found to be unpredictable.[1] No consistent relationship between the inferior gluteal artery and the inferior gluteal nerve was observed in current studies.
There is a relationship between the common stalk of the inferior gluteal nerve and external anatomic landmarks. The targeted region should be aimed inferior to the most prominent aspect of the greater trochanter, and medial to the landmark of the ischial tuberosity, at the depth of the posterior border of the proximal femur. Triangulating using these three coordinates, one can reliably reach the source of the inferior gluteal nerve. This will result in maximal stimulation of the gluteus maximus musculature when using electrical stimulation for the purpose of prevention of pressure ulcers.[1]
The sciatic nerve (L4 to S3), the largest nerve of the body, immediately leaves the pelvis through the greater sciatic foramen, below the piriformis. The superior gluteal nerve passes backward through the greater sciatic foramen, above the piriformis: the inferior gluteal nerve also passes backward through the greater sciatic foramen but below the piriformis."[4]

【 function 】

　The major function of the gluteus maximus is to extend the flexed thigh and bring it inline with the trunk. It may prevent the forward momentum of the trunk from producing flexion at the supporting hip during bipedal gait. It is intermittently active in the walking cycle and in climbing stairs and continuously active in strong lateral rotation and abduction of the thigh and also stabilizes the femur on the tibia when the knee extensors are relaxed. In addition to this, the gluteus maximus has an important role during some activities like running or standing up.[5]
The gluteus maximus, a large muscle with numerous attachments, is a powerful extensor of the thigh or of the trunk lower limbs are in a fixed position. The gluteus maximus is also a strong hip stabilzer. It extends thigh at the hip, assists in laterally rotating the thigh.[6] It contracts at heel-strike, slowing forward motion of trunk by arresting flexion of the hip and initiating extension. This movement prevents trunk from falling forward.[6][7]
Surprisingly, however, the gluteus maximus is not important posturally, is relaxed when one is standing, and is little used in walking. It is employed in running, climbing, and rising from a sitting or stooped position. It also controls flexion at the hip upon sitting down.[6]」