Lumbosacral Plexus

Question 1

contrast upper and lower limb rotation

Answer 1

• Upper limbs rotate laterally 90*
  
  • Flexor faces anteriorly
• Lower limbs rotate medially 180*
  
  • Flexor at joints below hip face posteriorly
  • Twisted appearance of the lumbar dermatomes due to limb rotation during embryologic development

Question 2

What is the lumbar plexus and lumbosacral plexus?

What are the spinal segments?

Answer 2

Two major plexi that contribute to innervation of the lower limb:

• The lumbar plexus
  
  • L2, L3, and L4 ventral rami will split into anterior and posterior divisions (like the three trunks of the brachial plexus)
  • 2 major nerves arise from the lumbar plexus to innervate the lower limb: the femoral nerve and the obturator nerve
• Lumbosacral plexus
  
  • ​S2, S3
• The combined lumbar and lumbosacral plexi contain ventral rami from L2 through S3; L2 and L3 tend to innervate muscles that act more at the hip, whereas S2 and S3 tend to innervate muscles in the foot.
• Also nerves from torso block - iliohypogastric and lateral femoral cutaneous

Question 3

What 2 major terminal nerves arise from the lumbar plexus,

and what do they innervate?

Answer 3

• The femoral nerve is c​omposed of posterior division fibers, and innervates the anterior compartment of the thigh
  
  • Posterior division fibers from the plexus end up in anterior position in the thigh due to the rotation.
• The obturator nerve is composed of anterior division fibers, and innervates the medial compartment of the thigh (again, the apparent discrepancy is due to rotation).
• The sciatic nerve is actually the common fibular nerve (aka common peroneal) + tibular nerve
  
  • ​Common fibular = posterior divisions of L4-S2
  • Tibular = anterior divisions of L4-S3

Question 4

proximal to distal gradient

Answer 4

• Similar to the brachial plexus, there is a proximal-to-distal gradient of innervation.
• The combined lumbar and lumbosacral plexi contain ventral rami from L2 through S3
  
  • L2 and L3 tend to innervate muscles that act more at the hip
  • Whereas S2 and S3 (lumbosacral) tend to innervate muscles in the foot

Question 5

where do the femoral nerve and obturator nerve exit from?

Answer 5

• The femoral nerve exits the abdominopelvic cavity by passing under the inguinal ligament, along with the femoral artery and vein.
• The obturator nerve exits the pelvic cavity through the obturator foramen

Question 6

Femoral nerve:

• Fiber type?
• Spinal cord segments?
• Myotome?
• Dermatome?

Answer 6

• Posterior division fibers
• L2, L3, L4 fibers –> act more proximally, at hip and knee
• Innervates the anterior compartment of the thigh due to lower limb rotation.
  
  • There are 3 major muscles in the anterior thigh – the pectineus, sartorius and quadriceps femoris
    
    • Sartorius muscle, a flexor of the thigh at the hip and a flexor of the leg at the knee. It also causes external rotation of the thigh.
    • Quadriceps muscles (rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis). These muscles cause extension of the leg at the knee; the rectus femoris also crosses the hip joint and is a flexor of the thigh at the hip.
    • Pectineus - a flexor and adductor of the thigh at the hip.
• Dermatome = anterior thigh, medial leg
  
  • The dermatome of the femoral nerve = the skin of the anterior to anteromedial thigh.
  • As the femoral nerve reaches the inferior aspect of the thigh, it continues as a cutaneous-only nerve, the saphenous nerve, which provides cutaneous innervate to the anteromedial leg.
    
    • (Note the parallelism to the musculocutaneous nerve of the upper limb, which becomes the lateral cutaneous nerve!)

Question 7

sartorius muscle:

• What is it innervated by?
• What actions does it do, where?

Answer 7

• Innervated by femoral nerve (posterior fibers of L2-L4)
• 3 actions:
  
  • Flexor of the thigh at the hip
  • Flexor of the leg at the knee
  • External rotation of the thigh

Question 8

anterior thigh - what are the 3 major muscles?

What are they innervated by?

Answer 8

• There are 3 major muscles in the anterior thigh – the pectineus, sartorius and quadriceps femoris
• All innervated by the femoral nerve
  
  • Sartorius muscle = a flexor of the thigh at the hip and a flexor of the leg at the knee. It also causes external rotation of the thigh.
  • Quadriceps muscles = rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis. These muscles cause extension of the leg at the knee; the rectus femoris also crosses the hip joint and is a flexor of the thigh at the hip.
  • Pectineus = a flexor and adductor of the thigh at the hip.

Question 9

quadriceps:

What are the 4 muscles?

Innervation?

Action?

Answer 9

Quads =

• Rectus femoris
• Vastus lateralis
• Vastus intermedius
• Vastus medialis
• All innervated by femoral nerve (L2-L4)
• All cause extension of the leg at the knee
• The rectus femoris also crosses the hip joint and is a flexor of the thigh at the hip

Question 10

obturator nerve

• Fiber type?
• Spinal cord segments?
• Myotome?
• Dermatome?

Answer 10

• Anterior division fibers, L2-L4 (act proximally at hip and knee)
• Innervates the medial compartment of the thigh due to lower limb rotation.
  
  • The medial compartment consists of three adductors: the adductor brevis, the adductor longus, and the adductor magnus.
    
    • All adductors of the thigh at the hip. The adductor magnus also acts similar to a hamstring muscle in that it can extend the thigh at the hip and flex the leg at the knee.
• Also innervates the obturator externus, an adductor and lateral rotator of the thigh at the hip, as well as the gracilis, which acts similarly to the adductor magnus (an adductor with a secondary hamstring-like function).
• Dermatome = a small ovoid area on the inferomedial thigh

Question 11

medial thigh muscles

What are the 3 muscles, what do they do?

Innervation?

Answer 11

• Medial thigh is innervated by obturator nerve
• The medial compartment of thigh contains the hip adductor muscles, the gracilis and the obturator externus. All adductors.
• The medial compartment consists of three adductors:
  
  • adductor brevis
  • adductor longus
  • adductor magnus.
  • These 3 muscles are all adductors of the thigh at the hip.
    
    • The adductor magnus also can extend the thigh at the hip and flex the leg at the knee

Question 12

What are the 2 muscles that the obturator nerve innervates

that don’t have “adductor” in name?

Answer 12

• The obturator nerve also innervates:
  
  • obturator externus – adductor and lateral rotator of the thigh at the hip
  • gracilis – acts similarly to the adductor magnus, as an adductor with a secondary hamstring-like function: can extend the thigh at the hip and flex the leg at the knee

Question 13

Sciatic nerve

• Part of what plexus?
• Why is it a misnomer?
• Spinal segments?
• Anterior and posterior divisions?
• Innervation?

Answer 13

• lumbosacral plexus (L4-S3)
• misnomer, bc actually two different nerves that happen to ride within the same connective tissue sheath.
• The posterior divisions of L4 through S2 come together to form a common fibular nerve.
  
  • Innervates anterior and lateral leg
• The anterior divisions of L4 through S3 come together to form the tibial nerve.
  
  • Innervates posterior thigh and leg
• Thus, the sciatic nerve truly represents the common fibular and tibial nerves running together from their origin in the pelvis until they divide from each other at the superior edge of the popliteal fossa (the “knee pit”).
• Sciatic nerve exits just inferior to the piriformis muscle, gluteal region

Question 14

lumbosacral plexus

• spinal segments?
• Nerves?
• Exit point?

Answer 14

• L4, L5, S1, S2, S3
• Innervates lower limb via sciatic nerve, which is actually:
  
  • Common fibular nerve - posterior fibers, for anterior and lateral leg
    
    • Branches into deep and superficial
  • Tibular nerve - anterior fibers, for posterior thigh and leg
• Lumbosacral plexus exits the pelvic cavity through the greater sciatic foramen
  
  • An opening in the posterior pelvis bounded by the ilium (superior and anterior), sacrotuberous ligament (posteromedial), and sacrospinous ligament (inferior)

Question 15

Where does the sciatic nerve split into the tibial nerve and common fibular nerve?

Answer 15

• At the popliteal fossa (pit of the knee)
• The tibial nerve continues straight down (anterior divisions for posterior leg and thigh)
• Whereas the common fibular nerve swings laterally to pass over the neck of the fibula (posterior divisions for anterior and lateral leg)

Question 16

tibial nerve

• Fibers?
• Spinal cord segments?
• Myotome?
• Dermatome?

Answer 16

• The tibial nerve contains anterior division fibers, but innervates the posterior compartment of the thigh due to lower limb rotation.
• L4 through S3 fibers, which act throughout the length of the lower limb.
• The posterior compartment of the thigh consists of the hamstring muscles:
  
  • All of the true hamstrings (those that cross both the hip and knee joints) are innervated by the tibial nerve, and cause extension of the thigh at the hip and flexion of the leg at the knee. These include the semitendinosus, semimembranosus, and the long head of the biceps femoris.
  • The short head of the biceps femoris (not a true hamstring) is innervated by the common fibular nerve.
• Also responsible for plantar flexion (foot on the gas pedal) because innervates
  
  • Need plantar flexion in order to stand on toes
• At foot, tibial nerve divides into lateral plantar nerve and medial plantar nerve
• Myotome: posterolateral leg, and plantar foot

Question 17

What are the true hamstrings?

Answer 17

• True hamstrings cross both the hip and knee joints
  
  • innervated by the tibial nerve
  • cause extension of the thigh at the hip and flexion of the leg at the knee.
  • These include the semitendinosus, semimembranosus, and the long head of the biceps femoris.
• The short head of the biceps femoris (not a true hamstring bc only crosses knee) is innervated by the common fibular nerve. It flexes the knee.

Question 18

muscles of posterior leg

Answer 18

• Mosly plantar flexors
• Innervated by tibial nerve
• Plantaris, Gastrocnemius, Soleus = major plantar flexors
• Popliteus = small muscle, unlocks the knee (small rotation of femur, to initiate motion of flexion at the knee)
• Tibialis posterior, flexor digitorum longus, flexor hallucis longus = plantar flexion, some inversion of foot

Question 19

tibial nerve at the foot

Answer 19

• Splits into medial and lateral plantar nerves
  
  • Swings under the medial malleolus (inner ankle) to innervate the intrinsic muscles of the plantar foot

Question 20

dermatome of tibial nerve

Answer 20

(see pic)

• The dermatome of the tibial nerve includes the posterolateral leg (the red and green sections of the leg pic), as well as the plantar foot (all but the blue segment in the foot pic).

Question 21

common fibular nerve

(aka common peroneal)

• Fiber type?
• Spinal cord segments?
• Myotome?
• Dermatome?

Answer 21

• Posterior division fibers
• L4, L5, S1, S2 –> act on knee and below
• Only innervates one posterior muscle, and more predominantly the lateral and anterior compartments of the leg, due to lower limb rotation.
• The common fibular nerve rides over the neck of the fibula, and then divides into superficial and deep fibular nerves.
  
  • The common fibular nerve itself innervates only the short head of the biceps femoris.
  • The superficial fibular nerve innervates the lateral compartment of the leg (fibularis longis and brevis), which cause eversion of the foot.
  • The deep fibular nerve innervates the anterior compartment of the leg, which causes dorsiflexion and some inversion of the foot.

Question 22

Dorsal foot muscles can be thought of as a continuation of what compartment?

Function?

Innervation?

Answer 22

The muscles of the dorsal foot should be thought of as

continuations of the anterior compartment of the leg,

bc they have the same functions and innervation.

• Anterior compartment of the leg & dorsal foot causes dorsiflexion and some inversion of the foot
• Innervated by deep fibular nerve

Question 23

What are the branches of common fibular (peroneal) nerve?

What do they innervate?

Answer 23

• The common fibular nerve rides over the neck of the fibula, and then divides into superficial and deep fibular nerves.
  
  • common fibular nerve: innervates only the short head of the biceps femoris.
  • superficial fibular nerve: innervates the lateral compartment of the leg (fibularis longis and brevis), which cause eversion of the foot.
  • deep fibular nerve: innervates the anterior compartment of the leg (tibialis anterior, extensor digitorum longus, extensor hallucis longus and fibularis tertius), which causes dorsiflexion and some inversion of the foot.
• Dermatome = anterolateral leg, dorsum of foot
• In dermatome pic: Deep fibular nerve does fip flop web (feet deep in the sand!) and superficial fibular nerve innervates grey area. Brown area is branch off common fibular. Red area, edge of foot, is actually branch off tibial.

Question 24

What are the nerves of the lower limb? (5)

Anterior/posterior?

Answer 24

• Femoral nerve - posterior (anterior compartment of thigh)
• Obturator nerve - anterior (medial compartment of thigh)
• Sciatic nerve - a/p
  
  • The posterior divisions form a common fibular nerve
    • Common fibular branches into deep and superficial (for thigh. Deep is anterior compartment, superficial is lateral compartment.)
  • Anterior divisions form the tibial nerve - innervates posterior leg and thigh

Question 25

Which nerve of the lower limb innervates both the thigh and leg?

Answer 25

* The **tibial nerve innervates both the thigh and leg** * Anterior fibers of sciatic nerve, for posterior compartments of thigh and leg

Question 26

What are the 3 compartments of the thigh? 3 compartments of the leg? (Muscles in the same compartment tend to share the same innervation and blood supply.)

Answer 26

Thigh: * **Anterior compartment - Femoral nerve** * **Medial compartment - Obturator nerve** * **Posterior compartment - Tibial nerve** * Except the short head of the biceps, which is innervated by common fibular nerve * Blood supply for all = **deep femoral artery**. Posterior compartment gets the perforating branches. Leg: * **Anterior compartment - Deep fibular nerve** * ​Anterior tibial artery * **Lateral compartment - Superficial fibular nerve** * **​**Fibular artery * **Posterior compartment - Tibial nerve** * **​**Posterior tibial artery * **​**(All the blood supply are divisions of the popliteal artery, which is the name for the femoral artery once it passes through adductur hiatus.)

Question 27

posterior compartment of the leg * Muscles? * Innervation? * Action?

Answer 27

* The posterior leg is the largest of the three compartments. Collectively, the muscles in this area **plantarflex and invert the foot**. * All innervated by the **tibial nerve**, a terminal branch of the sciatic nerve. * There are 4 muscles in the deep compartment of the posterior leg: * The **popliteus**, acts only on the knee joint. * The remaining three muscles (**tibialis posterior, flexor hallucis longus and flexor digitorum longus**) act on the ankle and foot

Question 28

What innervates the posterior compartment of the thigh and leg? Which muscle in the posterior compartment of the thigh is the exception?

Answer 28

* The posterior compartment of the thigh and leg is mostly innervated by the **tibial nerve** * The short head of the biceps femoris (in the thigh) is an exception * **Short head is innervated by the common fibular nerve**

Question 29

Which artery provides blood supply to lower limb?

Answer 29

**Femoral artery** (The femoral artery is a continuation of the external iliac artery, which simply switches its name when it passes the inguinal ligament.)

Question 30

branches of the femoral artery

Answer 30

Femoral artery branches... **P**ut **M**y **L**eg **D**own **P**lease * *P**rofundus femoris (deep femoral artery) * *M**edial circumflex femoral artery * *L**ateral circumflex femoral artery * *D**escending genicular arteries * *P**erforating arteries * The femoral artery gives off a **profunda (deep) femoral artery**, which is the major blood supply for the thigh. * It gives off a **medial** and **lateral circumflex femoral artery**, which create an anastomotic connection around the superior shaft of the humerus. * _The_ _medial is of particular note, because it passes up the neck of the femur to serve as a blood supply of the femoral head. Thus, a fracture through the femoral neck may cause a loss of blood supply to the femoral head and associated avascular necrosis_. * The profunda (deep) femoral artery also gives of **perforating branches** to provide a blood supply for the posterior thigh. * The remainder of the femoral artery continues through the thigh and changes its name when it passes through the adductor hiatus, an opening at the inferior edge of the adductor magnus muscle. It becomes known as the **popliteal artery** at that point. * Popliteal divides into: * **anterior tibial artery =** blood supply to the anterior compartment of the leg * **posterior tibial artery =** blood supply to the posterior compartment of the leg * **fibular artery =** blood supply to the lateral compartment of the leg

Question 31

Where does the femoral artery change names?

Answer 31

* At the inguinal ligament - above that, used to be the **external iliac artery** * At the adductur hiatus - becomes known as **popliteal artery** * Adductur hiatus is an opening at the inferior edge of the adductur magnus muscle. * The popliteal divides into an **anterior tibial artery** (supplies anterior compartment of the leg), **posterior tibial artery** (supplies posterior compartment of the leg), and **fibular artery** (supplies lateral compartment of the leg)

Question 32

What could result from a fracture through the femoral neck?

Answer 32

* Damage to the medial circumflex femoral artery --\> **necrosis of femoral head** * The medial passes up the neck of the femur to serve as a blood supply of the femoral head. * **A fracture through the femoral neck may cause a loss of blood supply to the femoral head and associated avascular necrosis.** * (Medial anastomoses with the lateral. They're branches off the profunda, which is a branch off the femoral.)

Question 33

Answer 33

## Footnote The answer is (C). Matching anterior/posterior divisions with anatomic location of innervation in the lower limb is a bit more difficult than in the upper limb, due to the more extensive rotation of the lower limb. In this case, the obturator nerve is composed of anterior division fibers of L2 through L4 (A)The common fibular nerve contains posterior division fibers of L4 through S2. (B)The femoral nerve contains posterior division fibers of L2 through L4. (C)(correct answer) (D)There are a few issues with this answer. The sciatic nerve is not truly a terminal nerve of the lumbosacral plexus, per se, as much as it is an anatomic structure composed of two terminal nerves in the same connective tissue sheath (common fibular and tibial). It will contain posterior divisions of L4 through S2 in the common fibular nerve, and anterior divisions of L4 through S3 in the tibial nerve. The range given here is too extensive. (E)The tibial nerve contains anterior division fibers of L4 through S3.

Question 34

Answer 34

## Footnote A: The hamstring muscles reside in the posterior thigh. The semitendinosus, semimembranosus, and long head of the biceps femoris are considered “true” hamstrings because they cross both the hip and knee joints. They are all innervated by the tibial nerve. The short head of the biceps femoris (not a “true” hamstring) is innervated by the common fibular nerve. (A)(correct answer) (B)It is the femoral artery that changes its name when it passes through the adductor hiatus (to become the popliteal artery). (C)The superficial fibular nerve provides most of the cutaneous innervation to the dorsal foot; the tibial nerve provides cutaneous innervation to the posterolateral leg and most of the plantar surface of the foot. (D)The tibial nerve is composed of anterior division fibers from L4 through S3. (E)The everters of the foot are found in the lateral compartment of the leg, which are innervated by the superficial fibular nerve.

Question 35

Answer 35

B: This question stem describes a **femoral hernia** (in which a loop of bowel herniates under the inguinal ligament and into the superior thigh). The **femoral nerve** also runs through this space, and so would be most at risk in this clinical scenario. (A)The deep fibular nerve exits the pelvis posteriorly via the greater sciatic foramen (as part of the sciatic nerve). (B)(correct answer) (C)The obturator nerve exits the pelvis anteromedially via the obturator foramen. (D)The superficial fibular nerve exits the pelvis posteriorly via the greater sciatic foramen (as part of the sciatic nerve). (E)The tibial nerve exits the pelvis posteriorly via the greater sciatic foramen (as part of the sciatic nerve).

Question 36

Answer 36

E: **Dorsiflexion** is accomplished by the anterior compartment of the leg, which is innervated by the deep fibular nerve. It is the **common fibular nerve that passes over the neck of the fibula, and then splits to form the superficial and deep fibular nerves** (both of which may be affected in this case, so there could also be problem with foot eversion.) (A)While the common fibular nerve innervates the short head of the biceps femoris, this is before it reaches the neck of the fibula. Additionally, the other (true) hamstrings are innervated by the tibial nerve. (B)Extension of the leg at the knee is accomplished by the femoral nerve, which innervates the quadriceps muscles. (C)Unlocking of the knee is mostly the role of the popliteus muscle, which is innervated by the tibial nerve (posterior compartment of the leg). (D)Plantar flexion is accomplished by the tibial nerve, which innervates the posterior compartment of the leg. (E)(correct answer)

Question 37

Answer 37

## Footnote B: The **dermatome of the deep fibular nerve is restricted to the webspace between the first and second toes (the “flip-flop**” distribution). While part of the lower limb, this area is not part of the anatomic leg. (A)In addition to the fact that the common fibular nerve contains the superficial fibular nerve, it also gives off a lateral sural nerve that provides innervation to the skin of the lateral leg. (B)(correct answer) (C)The femoral nerve continues into the leg as the saphenous nerve, a cutaneous nerve that innervates the skin of the medial leg. (D)The superficial fibular nerve innervates the skin of the anterolateral leg (and, for choice (A), is a part of the common fibular nerve). (E)The tibial nerve innervates the skin of the posterolateral leg.

Question 38

Answer 38

## Footnote The answer is (C). The medial circumflex femoral artery runs up the femoral neck (which is fractured in this x-ray) and may be lesioned in a femoral neck fracture. This puts the femoral head at risk for avascular necrosis, and must be repaired surgically. (A)The femoral artery passes more medially, under the inguinal ligament, and would be an unusual injury in this setting. (B)The lateral circumflex femoral artery passes around the superior shaft of the femur. (C)(correct answer) (D)The popliteal artery is found much more inferiorly, in the popliteal fossa. (E)The profunda femoral artery runs through the thigh; while it gives off the medial circumflex femoral artery, it itself is generally not in close approximation with the femoral neck.

Question 39

Answer 39

The answer is (C). **The lateral compartment of the leg receives it blood supply from the fibular nerve, and its innervation from the superficial fibular nerve.** (A)The posterior compartment of the thigh receives its blood supply from the perforating branches of the profunda (deep) femoral artery, and its innervation from the tibial nerve (mostly; the short head of the biceps femoris is innervated by the common fibular nerve). (B)The posterior compartment of the leg receives its blood supply from the posterior tibial artery, and its innervation from the tibial nerve. (C)(correct answer) (D)The anterior compartment of the leg receives its blood supply from the anterior tibial artery, and its innervation from the deep fibular nerve. (E)The dorsal foot (which is essentially an extension of the anterior compartment of the leg) receives its blood supply from the anterior tibial artery (called the dorsalis pedis once it crosses the ankle joint) and its innervation from the deep fibular nerve.

Question 40

lateral compartment of leg: Muscles? Blood supply? Innervation?

Answer 40

The lateral compartment of the leg: * Peroneus longus * Peroneus brevis * Blood supply from **fibular artery** * Innervation from the **superficial fibular nerve**.

Question 41

posterior compartment of thigh: Muscles? Innervation? Blood supply?

Answer 41

* Muscles = **hamstrings** (biceps femoris, semitendinosus and semimembranosus) * Innervation = **tibial nerve** * **​**(mostly; the short head of the biceps femoris is innervated by the common fibular nerve) * Blood supply = **perforating branches of the profunda (deep) femoral artery**

Question 42

posterior compartment of leg * Muscles? * Innervation? * Blood supply?

Answer 42

* Muscles = superficial (calf) and deep: * Superficial muscles of posterior leg = calf muscles = **gastrocnemius, soleus and plantaris** * Deep muscles of posterior leg = **popliteus, tibialis posterior, flexor digitorum longus and flexor hallucis longus** * Innervation = **tibial nerve** * Blood supply = **posterior tibial artery**

Question 43

anterior compartment of leg * Muscles? * Innervation? * Blood supply?

Answer 43

* Muscles = **tibialis anterior, extensor digitorum longus, extensor hallucis longus and fibularis tertius** * Innervation = **deep fibular nerve** * Blood supply = **anterior tibial artery**

Question 44

dorsal foot * Blood supply? * Innervation?

Answer 44

* _The dorsal foot is essentially an extension of the anterior compartment of the leg_ * Blood supply = **anterior tibial artery** (called the **dorsalis pedis** once it crosses the ankle joint) * Innervation = **deep fibular nerve**