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Sciatica Part 1 : Anatomy

Posted May 13 2012 7:38am
To learn about sciatica, you need to know a little about the anatomy of the sciatic nerve. Below is the anatomy of the sciatic nerve explain. Now, I appreciate that not everyone is a geek about this stuff like I am, so I summarized the anatomy at the end of the post. If you want to skip all the scientific mumbo-jumbo then click here: skip to summary

The sciatic nerve arise from the nerve roots of L4 through to S3 It is the longest nerve in the body, and is about 2cm thick. It is essentially a combination of two nerves the common peroneal nerve (L4-S2) and the tibial nerve (L4-S3) wrapped in a common sheath. Occasionally (~10%), the two nerves around enclosed in a common sheath and one part may go through the piriformis muscles.

The nerve exits the pelvis below through the infrapiriform foramen, beneath the gluteus maximus and biceps femoris, along the posterior aspects of the adductor muscles, down the the knee joint. Once the sciatic nerve reaches the knee it splits and the two nerves within go their separate ways.


Before bifurcation at the knee, the sciatic nerve innervates the semitendinosus, semimembranosus, the short head of the biceps femoris, the hamstring part of the adductor magnus

The peroneal nerve runs through the lateral edge of the popliteal fossa to the head of the fibula and wraps around the neck of the fibula and enters the peroneus longus. Inside the muscle, the nerve divides into the superficial and deep peroneal nerves.
The superficial nerve is mostly a sensory nerve and runs the length of the peroneus longus to the back of the foot. It innervates the peroneus longus and the peroneous brevis, and also provides sensation to the dorsum of the foot (except the first web space).

The deep peroneal nerve is mostly a motor nerve and it turns to the front of the lower leg, innervating te anterior compartment of the lower leg on it's way down the lateral surface of the tibialis anterior and branches off into lateral and medial terminal branches at the front of the ankle. It also provides sensation to the first web space.
anterior view knee
posterior view of knee
The tibial nerve travels straight down the back of the leg underneath the gastrocnemius, behind the achilles tendon and wraps behind the medial malleous. Underneath the ankle it branches off into the medial plantar nerve and the lateral plantar nerve, the nerves. It innervates the popliteus and the posterior compartment of the lower leg.

The medial plantar nerve innervates the first lumbrical, adductor hallucis, flexor digitorum brevis, and flexor hallucis brevis muscles and provide sensation to the plantar surface of the foot. The lateral plantar nerver innervates the quadratus plantae, flexor digiti minimi, adductor hallucis, interossei, lumbricals, and the abductor digiti minimi. Together these nerves provide sensation to the plantar surface of the foot.

Summary

Here is a brief about the anatomy of the sciatic nerve
  • The Sciatic nerve is a combination of two nerves
    • The tibial nerve
    • the common peroneal nerve
    • it innervates the hamstrings and other muscle in the back of the thigh and provides sensation to the back of the thigh.
  • Once the sciatic nerve reaches the back of the knee it splits and these two nerves go their separate ways
    • The tibial nerve runs straight down the back of the leg to the inside ankle
      • the tibial nerve then branches off into two smaller nerves that go into the foot
      • it innervates the calf muscles and small muscles in the foot, and provides sensation to the bottom of the foot.
    • The common peroneal nerve enters into the long peroneal muscle where it splits again into the superficial peroneal nerve and the deep peroneal nerve
  • The superficial peroneal nerve is mostly sensory and it runs down the length of the long peroneal muscle, close to the skin, all the way to the back of the foot. It innervates the peroneus longus and peroneus brevis, and provides sensation to the top of the foot. 
  • The deep peroneal nerve turns and goes more to the front of the lower leg. It runs down the outside of the anterior tibial muscle and ends on the outside of the foot. It innervates the extensor muscles on the lower leg, and provides sensation between the big toe web space.
The sciatic nerve provides sensation to the back of the thigh, and the entire lower leg except for the medial aspect, as well as most of the foot. It also provides motor function to this movements. The symptoms you get will depend on where the nerve is compromised. The reason that back pain commonly has sciatica as a symptom is because it compromises the nerve directly at the origin of the nerve, but we'll get more into that in the coming post.

Part 2 - Signs and Symptoms




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