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Neurology of the Lower Limb:
Aims:

Lower Limb Dermatomes:
-

The area of skin supplied by a single spinal nerve is called a dermatome
...


-

The axial line is the line at which the dermatomes DO NOT cross each other
[no overlap]
...


-

Dermatomes L1-L5 extend as a serried of bands from the posterior midline of
the trunk into the limbs passing laterally + inferiorly around the limb to its
anterior + medial parts
...

➢ L3 is over the KNEE
...


-

Dermatomes S1+S2 pass inferiorly down the posterior aspect of the limb +
separate at the ankle to pass to the lateral + medial margins of the foot
...

➢ S1 supplies the sole of the foot
...

➢ SO WALK on S1 and SIT on S3
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-

These nerves can be tested to see if there is injury to part of that nerve
...


-

Obturator Nerve = L2 – 4
➢ This gives a small contribution to the medial inner aspect of the thigh
...


-

Direct sacral plexus contribution (S2+S3) = posterior cutaneous nerve of the
thigh which supplies posterior thigh
...

➢ COMMON PERONEAL NERVE supplies the anterior-lateral part of the
leg + dorsum of foot
...

❖ The sural nerve is a branch of the tibial nerve but also has a
communication from the common peroneal nerve
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❖ The sural nerve runs close to the small saphenous vein
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-

There is a TABLE which helps to learn the innervations of the different
muscles, starting from the hip and working down to the foot
...

➢ Flexion = L2 + 3 [Psoas major is L1-3]
➢ Extension = L4 + L5

-

Knee 
➢ Extension (this is towards the body) = L3 + 4
➢ Flexion = L5 + S1

-

Ankle 
➢ Extension (dorsi-flexion) = L4 + L5

➢ Flexion (plantar flexion) = S1 + S2
-

The foot does not follow the pattern =
➢ Inversion = L4 + L5
➢ Eversion = L5 + S1
➢ Intrinsic muscles = S2

-

So essentially the first movement is of the hip supplied by L2 + L3, the next
backward movement is the next two spinal segments along
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The Lumbar Plexus:
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The lumbar plexus has an anterior division = obturator nerve and a posterior
division = femoral nerve
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-

The main branches are:
➢ Iliohypogastric
➢ Ilioinguinal
➢ Genito-femoral

➢ Lateral Femoral Cutaneous (L2 + L3)
➢ Femoral (L2 – L4)
➢ Obturator (L2 – L4)
➢ Branch to lumbosacral trunk (L4 + L5) and sacral plexus
➢ There are also branches which are muscular to the adjacent psoas,
quadratus lumborum + iliacus muscles
...


-

The genito-femoral is on the ANTERIOR surface of PSOAS MAJOR
...


Femoral Nerve:
-

Largest branch of the lumbar plexus
...


-

It pierces psoas major and runs inferiorly and laterally within it to eventually
emerge between psoas major + iliacus muscles just above the inguinal
ligament
...


-

The femoral nerve then passes lateral to the femoral vessels + outside of the
femoral sheath [NOT in it]
...

➢ To iliacus, pectineus, Sartorius + quadriceps
...

➢ Intermediate and medial cutaneous nerves of the thigh
➢ Saphenous nerve to inside of leg + foot
...
Can be injured during catheterisation of femoral
artery
...


➢ A common risk factor is lying in the LITHOTOMY position in surgery [on
the back with the thighs and legs flexed]
...
No quads means
extension of the knee is lost  can check this by using the patella jerk
reflex where the quadriceps tendon is tapped
...

➢ This descends through the femoral triangle and lateral to the
femoral sheath
...

➢ It then passes inferiorly at the front + supplies the skin + fascia
of the anterior + medial aspects of the KNEE, LEG + FOOT
...


➢ The saphenous nerve is PURELY SENSORY and is contributed to
by L3 + L4
...

➢ Dysfunction of this nerve will cause;
❖ Loss of sensation extending from below the knee down to
the medial shin + to arch of foot
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Obturator Nerve:
-

Derived from the anterior divisions of the lumbar plexus [femoral is from the
posterior]
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-

It also has a cutaneous sensory patch on the lower medial side of thigh (L2)
...

This can cause referred pain
...

2) In runs down to the obturator canal which is an opening in the obturator
membrane which covers the obturator foramen
...


-

The obturator nerve can also have neuropathies:
➢ Injury is rare but may occur due to pelvic fractures or pelvic masses
...


-

An accessory obturator may occur in 15% of people and arises from L3 –
normally supplies the pectineus muscle + sometimes hip joint
...


-

Course of the nerve:
1) The nerve is derived from the posterior divisions of the anterior rami of
the lumbar plexus
...

3) It then runs down and laterally along the anterior surface of iliacus
...
It
reaches a subcutaneous position by passing through this muscle
...


-

COMPRESSION of this nerve = meralgia paraesthetic = can be caused by tight
jeans or entrapment in inguinal ligament if it passes through it
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-

This plexus is formed in front of the piriformis muscle
...
There are 12
branches in total
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-

The ANTERIOR ones included the TIBIAL nerve which is associated with the
original flexor surface of the leg so actually supplies most of the posterior
muscles of thigh + has motor + sensory branches to the leg calf + planter
surface of foot
...

➢ It passes almost dorsally between the upper border of greater sciatic
notch and the upper border of piriformis muscle
...

➢ It also supplies gluteus medius + tensor fascia lata + hip joint
...

➢ The nerve arises as the plexus passes through the greater sciatic
foramen
...
ONLY MUSCLE
SUPPLIED BY THIS NERVE!!

Sciatic Nerve:

-

The sciatic nerve is the LARGEST nerve in the body
...


-

COMMON FIBULAR NERVE:
➢ Derived from the posterior divisions of the nerves entering the sacral
plexus
...

➢ In the middle of the thigh it gives off its first branch to biceps femoris
...

➢ Sometimes pierces the piriformis muscle + can be compressed here =
piriformis syndrome
...

➢ Arises from L4-S3
...


➢ It continues to the leg and foot to supply the muscles and skin of the
posterior aspect of the leg + plantar surface sole of foot
...


-

BUT the sciatic nerve DOES supply the knee joint
...

➢ This commonly occurs when a branch of the sciatic nerve is compressed
at the base of the spine
...

➢ In people aged 20-40 years a herniated or prolapsed disc in the spinal
column which then presses on the nerve root
...

➢ In older people, sciatica can be caused by bony changes in the spine such
as osteoarthritis
...
Pregnancy can also cause it
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❖ This is done via the straight-leg raising test which involves the
patient laying flat on their back
...

❖ This stretches the sciatic nerve around the buttock
...

❖ Extra pain should occur when the nerve is stretched further by
dorsi-flexion of the foot
...


-

It is the MOST SUPERFICIAL of the 3 main
components of the popliteal fossa (nerve, vein +
artery)
...

➢ It splits from the sciatic nerve behind the
biceps femoris muscle
...

➢ At first the semi-membranosus muscle covers
the tibial nerve
...
Here it is covered by the converging heads of the Gastrocnemius
muscle
...

➢ It then goes to the posterior compartment of the leg deep to the deep
transverse intermuscular septum but posterior to the deep flexor
muscles
...

➢ It passes down on the tibialis posterior muscle with the posterior tibial
vessels
...

➢ It gives a branch to ankle joint
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Common Fibular Nerve:
-

This is the LATERAL and SMALLER of the terminal branches of the sciatic
nerve
...


-

It leaves the fossa by passing superficially the the lateral

head

of

Gastrocnemius
...

-

The common fibular nerve passes over the posterior aspect of the HEAD OF
FIBULA before winding around the lateral surface of the neck of this bone
...


-

It then goes deep to the top part of the fibularis longus muscle
...


-

This nerve also gives off a lateral sural cutaneous nerve to the skin on the
calf and the peroneal communicating branch of sural nerve

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