4.4 Brachial and Lumbosacral Plexus Organization

4.4 Brachial and Lumbosacral Plexus Organization

Quick Answer: The brachial plexus is typically C5–T1: roots → upper, middle, lower trunks → divisions → lateral, posterior, medial cords → terminal branches (musculocutaneous, median, ulnar, radial, axillary). The lumbar plexus (L1–L4) gives femoral, obturator, and lateral femoral cutaneous nerves; the sacral plexus (L4–S3) gives sciatic nerve, which divides into tibial and common fibular (peroneal) nerves. Plexopathy can drop SNAPs; preganglionic root lesions may spare them.

Outline topic I.D.3 is the entire plexus map. OpenExamPrep teaches both plexuses in this section because a Registered Nerve Conduction Study Technologist (R.NCS.T.) laboratory that can find an ulnar sensory potential must also know why a sural sensory potential is requested in a suspected lumbosacral plexopathy. Independent teaching for candidates using the American Association of Electrodiagnostic Technologists (AAET) outline is the aim. This is not a statement of official AAET review or partnership.

Brachial plexus: roots and the prefix/postfix traps

Typical contributing roots (ventral rami) are C5, C6, C7, C8, and T1. They exit between the anterior and middle scalene muscles (the interscalene triangle) as the roots of the plexus—already ventral rami, not the dorsal/ventral spinal roots of section 4.3. Keep the language clean on an exam item: spinal root versus plexus root (ventral ramus).

A prefixed plexus takes a large C4 contribution and often a smaller T1 contribution. A postfixed plexus takes a large T2 contribution and often a smaller C5 contribution. These variants are favorite traps. A prefixed lower trunk may behave as if T1 is less important; a postfixed upper trunk may look as if C5 is missing from a lesion that would otherwise be called upper trunk. When a textbook myotome and a patient's findings disagree at the edges of C5 or T1, prefix/postfix is on the differential before you invent a new nerve.

Long thoracic nerve (C5–C7, serratus anterior) and dorsal scapular nerve (C5, rhomboids) arise from the root level, proximal to the trunks. They can be involved in true root or very proximal plexus disease and spared in infraclavicular cord lesions. That proximal-versus-distal contrast is localization, not needle technique: you still do not perform needle EMG as an R.NCS.T., but you understand why a physician might needle serratus or rhomboid and why your SNAPs still matter.

Trunks, divisions, and cords

Trunks form in the posterior triangle of the neck:

• Upper trunk: C5 and C6
• Middle trunk: C7 alone
• Lower trunk: C8 and T1

Each trunk splits into an anterior division and a posterior division, usually behind the clavicle. Anterior divisions supply flexor compartments conceptually; posterior divisions supply extensor compartments. You do not record from a division with a surface montage, but the wiring explains the cords.

Cords are named for their position around the axillary artery in the axilla:

• Lateral cord: anterior divisions of upper and middle trunks
• Posterior cord: posterior divisions of all three trunks
• Medial cord: anterior division of the lower trunk

Terminal branches and the handbook ulnar example

Musculocutaneous nerve continues from the lateral cord (C5–C7). It innervates coracobrachialis, biceps, and brachialis, then continues as the lateral antebrachial cutaneous (LABC, lateral cutaneous nerve of the forearm) sensory nerve. Biceps CMAP and LABC SNAP are upper-trunk/lateral-cord tools.

Median nerve has two cord contributions: lateral root of median (lateral cord, mostly C6–C7 sensory to the thumb and index, motor to forearm pronators and finger flexors) and medial root of median (medial cord, C8–T1, including abductor pollicis brevis). Exam trap: median sensory NCS is not a lower-trunk test; median motor NCS to thenar muscle is a lower-trunk/medial-cord test. Mixing those two median studies is a classic localization error.

Ulnar nerve is the sample handbook pathway you should be able to recite without drawing: C8–T1 roots → lower trunk → medial cord → ulnar nerve. It does not receive a lateral-cord contribution. Ulnar sensory to digit 5 and ulnar motor to abductor digiti minimi or first dorsal interosseous are lower-trunk/medial-cord studies. A lesion of upper trunk leaves ulnar studies intact. A lower-trunk lesion hits ulnar and also the medial antebrachial cutaneous (MABC) nerve, which arises from the medial cord and is not inside the ulnar nerve at the elbow. That is why MABC SNAP is the partner study when you are deciding lower trunk versus ulnar nerve at the elbow: both drop in lower trunk/medial cord; MABC is spared in a lesion at the cubital tunnel.

Radial nerve is the large continuation of the posterior cord (C5–T1 contributions, clinically often C6–C8 in the forearm extensors). Superficial radial SNAP and extensor CMAP assess posterior cord and the C7-heavy middle trunk, with C6 and C8 edges.

Axillary nerve leaves the posterior cord (C5–C6) to deltoid and teres minor. It is an upper-trunk/posterior-cord nerve, not a lower-trunk nerve.

Sensory NCS: upper trunk versus lower trunk

When the question is upper trunk (burner/stinger, Erb-type pattern, C5–C6 plexopathy), the sensory studies that live in that neighborhood are:

• Lateral antebrachial cutaneous SNAP (musculocutaneous continuation, C5–C6, lateral cord, upper trunk)
• Median sensory SNAP to thumb or index (C6–C7, lateral cord, upper and middle trunks)
• Radial sensory SNAP (C6–C7, posterior cord, upper and middle trunks)

When the question is lower trunk (true neurogenic thoracic outlet pattern, Klumpke-type pattern, C8–T1 plexopathy, medial cord), the sensory studies that live there are:

• Ulnar sensory SNAP (digit 5, C8, lower trunk, medial cord)
• Medial antebrachial cutaneous SNAP (T1-heavy, lower trunk, medial cord)

SNAPs that do not decide upper versus lower trunk include a normal sural study (wrong plexus) and a median sensory that you misread as a thenar motor equivalent. Combine motor maps with these SNAPs: biceps and deltoid CMAPs for upper trunk; ulnar and APB CMAPs for lower trunk. Horner syndrome plus lower-trunk NCS abnormalities points toward T1 proximal involvement (root, spinal nerve, or very proximal lower trunk with sympathetics), which is clinical context rather than a waveform cutoff.

Lumbosacral plexus: lumbar plexus (L1–L4)

The lumbar plexus forms within psoas major from ventral rami L1–L4 (often with a T12 contribution to the upper branches). Major nerves for NCS and localization:

• Iliohypogastric and ilioinguinal (L1) and genitofemoral (L1–L2): abdominal wall and groin sensory/motor. Rarely a standard SNAP montage; know they exist so you do not call every groin sensory complaint a femoral neuropathy.
• Lateral femoral cutaneous nerve (LFCN, L2–L3): purely sensory to the anterolateral thigh. Entrapment at the inguinal ligament is meralgia paresthetica, a postganglionic mononeuropathy. An L2 or L3 root lesion that numbs the lateral thigh may spare an LFCN SNAP for the same DRG reason as in section 4.3, though LFCN studies are technically fussy.
• Femoral nerve (L2–L4): iliacus, then quadriceps; sensory continuation is the saphenous nerve to the medial leg. Femoral motor to vastus medialis and saphenous SNAP are the lumbar-plexus/femoral tools. A femoral neuropathy at the inguinal ligament drops those studies and spares the obturator-innervated adductors; a lumbar plexopathy may hit both femoral and obturator.
• Obturator nerve (L2–L4): adductor compartment and a small medial-thigh sensory patch. Harder to record as a SNAP; the localization value is clinical and in physician needle EMG of adductors versus quadriceps.

Anterior-versus-posterior division language repeats here: obturator is an anterior-division lumbar nerve; femoral is a posterior-division lumbar nerve. You do not need to record a division. You need to know that femoral plus obturator plus LFCN is lumbar plexus, not sciatic.

Sacral plexus, lumbosacral trunk, and sciatic division

The sacral plexus forms on the piriformis from ventral rami L4–S3 (sometimes S4). The lumbosacral trunk is the L4–L5 contribution that crosses the pelvic brim to join S1–S3. It is a vulnerable cable in obstetric injury, pelvic hematoma, and some pelvic masses: an L4–L5 deficit that looks like a sciatic or fibular problem but is intraspinal-sparing and SNAP-reducing may be trunk or plexus, not an L5 root.

Sciatic nerve (L4–S3) leaves through the greater sciatic foramen. Inside it, tibial (anterior division, L4–S3) and common fibular / common peroneal (posterior division, L4–S2) fascicles are already separate. That is why a partial sciatic lesion can look like an isolated fibular neuropathy until you study short head of biceps femoris (physician needle) or recognize that tibial and superficial fibular / sural SNAPs do not fit a lesion at the fibular head alone.

Terminal NCS landmarks:

• Tibial nerve: plantar intrinsic CMAPs (abductor hallucis), sural SNAP (with a contribution from the communicating fibular branch; still the workhorse S1-region sensory study).
• Common fibular nerve: tibialis anterior and extensor digitorum brevis CMAPs, superficial fibular SNAP.
• Superior and inferior gluteal nerves (gluteus medius/minimus and gluteus maximus) and posterior femoral cutaneous nerve arise from the sacral plexus proximal to the sciatic exit pattern you stimulate at the ankle. They help a physician separate plexus from a sciatic lesion in the thigh; they are not routine surface SNAPs.

Plexopathy versus root: the DRG rule applied to both plexuses

Section 4.3 built the rule. Apply it here without new magic numbers and without invented laboratory cutoff scores.

A C7 radiculopathy may numb the middle finger yet leave a median digit-3 SNAP present, because the DRG and peripheral median sensory axon are intact. A middle-trunk plexopathy can flatten that SNAP. A C8–T1 radiculopathy may spare ulnar and MABC SNAPs; a lower-trunk plexopathy often reduces them. An L5 radiculopathy typically spares the superficial fibular SNAP; an lumbosacral plexopathy or a postganglionic fibular lesion can reduce it. An S1 radiculopathy typically spares the sural SNAP; a sacral plexopathy or tibial/sciatic lesion can reduce it. An L4 radiculopathy may spare the saphenous SNAP; a lumbar plexopathy or femoral neuropathy can reduce it.

Paraspinal involvement, when the physician needles it, argues for root or spinal-nerve level (dorsal ramus still in play). Plexus lesions typically spare paraspinals. You do not place those needles. You provide SNAP and CMAP maps that make the DRG and ramus story testable.

If you remember only one brachial sentence, remember: ulnar nerve = C8–T1, lower trunk, medial cord. If you remember only one lumbosacral sentence, remember: sciatic = L4–S3 from the sacral plexus, then tibial and fibular, with L4–L5 reaching it through the lumbosacral trunk. If you remember only one conduction sentence, remember: postganglionic plexus lesions can drop SNAPs; preganglionic root lesions may not.