MSK - The Sciatic Nerve

Question 1

What are the nerve roots of the sciatic nerve?

Answer 1

L4-S3

Question 2

What are the motor functions of the sciatic nerve?

Answer 2

Innervates the muscles of the posterior thigh and the hamstring portion of the adductor magnus

Indirectly innervates (via its terminal branches) the muscles of the leg and foot

Question 3

What are the sensory functions of the sciatic nerve?

Answer 3

No direct sensory function

Indirectly innervates (via its terminal branches) the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot

Question 4

What is the sciatic nerve derived from?

Answer 4

Derived from the lumbosacral plexus

Question 5

What does the sciatic nerve enter the gluteal region via?

Answer 5

Greater sciatic foramen

Question 6

Where does the sciatic nerve exit the greater sciatic foramen in relation to the piriformis muscle?

What is its course after this?

Answer 6

Inferior to the piriformis muscle

Descends in the plane between the superficial and deep groups of gluteal muscles:

Posteriorly - To the deep gluteal muscles (obturator internus, gemmelus inferior and superior and quadriceps femoris)

Anteriorly - to gluteus maximus and the superficial group of gluteal muscles

Question 7

After descending in the plane between the superficial and deep groups of gluteal muscles, where does the sciatic nerve continue?

What does it give rise to?

Answer 7

Continues down the posterior thigh

Gives rise to motor branches for the hamstring muscles

Question 8

Where does the sciatic nerve terminate?

How does it terminate?

Answer 8

Apex of the popliteal fossa

Terminates by bifurcating into the tibial and common fibular nerves

Question 9

Where else could the sciatic nerve terminate in around 12% of people?

Why?

Answer 9

Separate as they leave the pelvis

In reality, the sciatic nerve is two individual nerves bundled together in the same connective tissue sheath; the tibial nerve and the common fibular nerve.

Question 10

What muscles does the sciatic nerve innervate in the gluteal region?

Answer 10

None!

Although it does pass through there

Question 11

What region of muscles does the sciatic nerve innervate?

It also innervates a portion of another muscle, which muscle and what portion?

Answer 11

Posterior compartment of the thigh

Hamstring portion of the adductor magnus

Question 12

The sciatic nerve also innervates other muscles via its terminal branches

What muscles does the tibial nerve innervate?

Answer 12

Tibial Nerve – the muscles of the posterior leg (calf muscles), and some of the intrinsic muscles of the foot

Question 13

The sciatic nerve also innervates other muscles via its terminal branches

What muscles does the common fibular nerve innervate?

Answer 13

Common Fibular Nerve – the muscles of the anterior leg, lateral leg, and the remaining intrinsic foot muscles

Question 14

In total, what does the sciatic nerve and all of its terminal branches innervate in terms of motor function?

Answer 14

Posterior thigh, all of the leg and all of the foot

Question 15

What are the direct cutaneous sensory innervations of the sciatic nerve?

Answer 15

It does not have a direct sensory cutaneous innervation!

However it does an indirect sensory innervation, via its terminal branches

Question 16

What are the sensory innervations of the tibial nerve?

Answer 16

Innervates the postero-lateral and antero-lateral sides of the leg and the plantar surface of the foot (the sole)

Question 17

What are the sensory innervations of the common fibular nerve?

Answer 17

Innervates the lateral leg and the dorsal surface of the foot

Question 18

Label this diagram to show the sensory innervations of the sciatic nerve

Answer 18

Question 19

Why must a clinician take into account the sciatic nerve’s anatomical course?

Answer 19

When administering intramuscular injections into the gluteal region

Question 20

The gluteal region can be divided into quadrants using 2 lines, marked by bony landmarks

What are the two lines and what bony landmarks do they use?

Answer 20

Vertically - Highest point on the iliac crest

Horizontally - Passes through the vertical line half way between the highest point on the iliac crest and ischial tuberosity

Question 21

What quadrant does the sciatic nerve pass through and therefore which quadrant should intramuscular injections be given?

Answer 21

Passes through the lower medial quadrant

Intramuscular injections are given only in the upper lateral quadrant of the gluteal region