Anatomy
Lower Limb
You are asked to review a 54 year old man who presents complaining of lower back pain with radiation down the right leg. You are concerned about sciatic nerve root compression. The sciatic nerve receives nerve fibres from which of the following nerve roots:
Answer:
The sciatic nerve is derived from the lumbosacral plexus and receives fibres from L4 - S3.Sciatic Nerve
Anatomy / Lower Limb / Hip Joint and Gluteal Region / Innervation of Lower Limb / Posterior Thigh
Last Updated: 21st April 2023
The sciatic nerve is derived from the lumbosacral plexus and receives fibres from L4 - S3.
Table: Anatomical Overview of the Sciatic Nerve
| Nerve | Sciatic |
|---|---|
| Nerve roots | L4 – S3 |
| Motor supply | Posterior thigh muscles, hamstring portion of adductor magnus, all of the muscles in the leg and foot |
| Sensory supply | Skin on the lateral leg and foot, the heel and the dorsum and sole of the foot |
| Motor loss in injury | Weak extension of hip and flexion of knee, loss of dorsiflexion and plantarflexion of ankle, loss of inversion and eversion of foot, foot drop with high-stepping gait |
Anatomical Course
The sciatic nerve leaves the pelvis and enters the gluteal region via the greater sciatic foramen inferior to the piriformis muscle before descending through the gluteal region and entering the posterior thigh.
In the posterior compartment of the thigh, the sciatic nerve lies on the adductor magnus muscle and is crossed by the long head of the biceps femoris muscle.
Within the posterior thigh, the sciatic nerve gives rise to branches to the hamstring muscles and adductor magus.
The sciatic nerve terminates at the apex of the popliteal fossa by dividing into the tibial (anterior divisions of L4 – S3) and the common fibular (posterior divisions of L4 – S2) nerves.
![By Henry Vandyke Carter [Public domain], via Wikimedia Commons](https://mrcemsuccess.com/wp-content/uploads/2017/01/Sciatic-Nerve.png)
Sciatic Nerve. (Image by Henry Vandyke Carter [Public domain], via Wikimedia Commons)
Function
Motor Supply:
The sciatic nerve innervates all of the muscles in the posterior compartment of the thigh (biceps femoris, semimembranosus and semitendinosus) and the hamstring portion of the adductor magnus. Through its terminal branches, it indirectly innervates all of the muscles in the leg and the foot.
Sensory Supply:
The sciatic nerve has no direct sensory function but through its terminal branches, it indirectly supplies the skin of the posterolateral leg and foot, the heel and both the dorsum and sole of the foot.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
