PNS Lecture 4: Lumbar and Sacral Plexuses, Dermatomes

lumbar plexus- overview

-arises from ventral rami of L1-L4

-branches into anterior/posterior divisions

-main branches: femoral nerve, obturator nerve

femoral nerve

-posterior divisions of L2-L4

-innervates iliacus (L2-L3): hip flexion

-innervates anterior thigh muscles: quadriceps (L2-L4), sartorius (L2-L3)- hip flex, ER, abd, knee flex

-innervates 1/2 pectineus (L2-L3): hip add, flex, IR

-branches: anterior femoral cutaneous nerve, saphenous nerve (sensory innervation all the way to foot)

obturator nerve

-anterior divisions of L2-L4

-innervates adductor muscles (medial thigh): adductor longus/brevis/magnus, gracilis (L2-L4), pectineus (L2-L3)- hip add, flex, IR

-innervates obturator externus (L3-L4)

smaller branches of lumbar plexus

iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous nerve

iliohypogastric nerve

L1

-internal/external obliques, transverse abdominis

ilioinguinal nerve

L1

-internal/external obliques, transverse abdominis, inguinal canal

genitofemoral nerve

L1-L2

-external genitalia, proximal anterior thigh

lateral femoral cutaneous nerve

posterior divisions L2-L3

-lateral proximal thigh (sensory)

sacral plexus- overview

-from spinal nerves L4-S4

-lumbosacral trunk- posterior divisions of L4-L5

-branches innervate: buttocks + lower extremities, pelvis, perineum

sciatic nerve

L4-S3- includes tibial and common fibular nerve

tibial nerve

anterior division of L4-S3

-innervates most muscles of posterior thigh (except short head of biceps femoris), leg, and foot (plantar surface)

-branches include medial + lateral plantar nerves, sural nerve

plantar nerves (medial and lateral)

-branch of tibial nerve

-innervates intrinsic foot muscles, skin of sole of foot

sural nerve

-branch of tibial nerve

-innervates skin of posterior leg

tibial nerve- injury?

-biceps femoris- long head (L5-S2): knee flexion/LR/hip ext

-semimembranosus (L5-S2): knee flexion/MR/hip ext

-semitendinosus (L5-S2): knee flexion/MR/hip ext

-gastrocnemius (S1-S2): PF in knee ext, knee flex

-soleus (S1-S2): PF

-popliteus (L4-S1): knee flex w/ med tibial rot (open), knee flex w/ lat rot of femur (closed)

-plantaris (S1-S2): PF, knee flex

-tibialis posterior (S2-S3): PF, inv

-flexor digitorum longus (S2-S3): flex toes 2-5 at MTP, IPs, inv, PF

-flexor hallucis longus (S2-S3): flexion of big toe at MTP, IP, inv, PF

-flexor digitorum brevis (medial plantar n S2-S3): flex toes 2-5 at MTP, PIP

skin of sole of foot, posterior leg

common fibular nerve

posterior division of L4-S2

-innervates short head of biceps femoris (L4)

-branches into superficial and deep fibular nerve

-superficial fibular nerve: lateral leg muscles, skin on dorsum of foot

-deep fibular nerve: anterior compartment of leg

superficial fibular nerve

-innervates skin on dorsum of foot

-fibularis longus: L5-S2, PF, eversion

-fibularis brevis: L5-S2, PF, eversion

deep fibular nerve

-tibialis anterior (L4-L5): DF, inv

-extensor hallucis longus (L4-L5): extend big toe at MTP, IP

-extensor digitorum longus (L4-L5): extend toes 2-5 at MTP, IP

-fibularis tertius (L4-L5): DF, eversion

-extensor digitorum brevis (L5-S1): extend toes 2-4 at MTP, IPs

-extensor hallucis brevis (L5-S1): extend great toe at MTP

other large branches of sacral plexus

-superior gluteal nerve

-inferior gluteal nerve

-pudendal nerve

-posterior femoral cutaneous nerve

superior gluteal nerve

posterior divisions L4-S1

-gluteus medius: hip ext/ER/IR/abd

-gluteal minimus: hip ext/ER/IR/abd

-tensor fascia latae: hip abd/IR/flex

inferior gluteal nerve

posterior divisions L5-S2

-gluteus maximus: hip ext/ER/abd

pudendal nerve

anterior divisions of S2-S4

-pelvic floor muscles (levator ani) and perineum structures

posterior femoral cutaneous nerve

posterior divisions of S1, anterior division S2-S3

-sensory innervation of posterior thigh, medial posterior leg

cutaneous innervation of lower extremity

-common fibular nerve = lateral leg, skin on dorsum of foot

-posterior femoral cutaneous nerve = posterior thigh, medial posterior leg

-tibial nerve = posterior leg, sole of foot

-femoral nerve (anterior femoral cutaneous nerve) = anterior medial thigh

-lateral femoral cutaneous nerve = anterior lateral thigh

-genitofemoral nerve = proximal, anterior thigh

-saphenous nerve = medial down to ankle

-obturator nerve = small portion of medial thigh

dermatome

area of skin innervated by cutaneous branches of a single spinal nerve

-trunk dermatomes are nearly horizontal

-limb dermatomes run more longitudinally along limb (variable overlap)

upper limb dermatomes

nerves from C5-T2

lower limb dermatomes

nerves from L1-S2 (S3)

-lumbar nerves = primarily skin on anterior surface

-sacral nerves = primarily skin on posterior surface