Muscles of the Back
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Direct answer
A patient who cannot shrug after a neck node biopsy has lost the accessory nerve in the trapezius; a patient whose scapula wings after axillary surgery has lost the long thoracic nerve to serratus anterior — and both single-best answers come from one rule: the extrinsic muscles of the back are supplied by ventral rami through named peripheral nerves (trapezius by the accessory nerve with C3–C4, latissimus dorsi by the thoracodorsal nerve, the rhomboids by the dorsal scapular nerve), whereas every true intrinsic muscle of the back is supplied by the dorsal rami of spinal nerves. The intrinsic group runs from superficial to deep as erector spinae (iliocostalis, longissimus, spinalis), transversospinalis (semispinalis, multifidus, rotatores), and the deepest segmental muscles. Posteriorly, the suboccipital triangle hides the vertebral artery and the first dorsal ramus.
What you must remember
- Extrinsic nerve pairs: trapezius — accessory nerve for motor, C3–C4 for proprioception; latissimus dorsi — thoracodorsal nerve (C6–C8), at risk in mastectomy; levator scapulae and rhomboids — dorsal scapular nerve (C5); serratus anterior — long thoracic nerve (C5–C7), the winged scapula of axillary surgery.
- Erector spinae: the chief extensor, arising from a common tendon on the sacrum, iliac crest and lumbar spinous processes, splitting into iliocostalis (lateral), longissimus (intermediate) and spinalis (medial) columns from skull to sacrum.
- Transversospinalis: obliquely running fibres filling the groove between spinous and transverse processes — semispinalis spanning five or six vertebrae, multifidus spanning about three and best developed in the lumbar region, rotatores spanning one or two.
- Suboccipital triangle: bounded by rectus capitis posterior major, obliquus capitis superior and obliquus capitis inferior, roofed by semispinalis capitis; its floor on the posterior arch of the atlas carries the vertebral artery in its groove and the suboccipital nerve (C1 dorsal ramus).
- Greater occipital nerve: the dorsal ramus of C2, emerging below obliquus capitis inferior — the nerve of occipital neuralgia and a target in migraine and headache anatomy.
- Triangle of auscultation: bounded superomedially by the lateral border of trapezius, laterally by the medial border of the scapula and inferiorly by the upper border of latissimus dorsi; the floor is the sixth and seventh intercostal spaces — best heard with the arms crossed forward.
- Lumbar triangle of Petit: between the iliac crest base, the posterior border of external oblique anteriorly and the anterior border of latissimus dorsi posteriorly — the site of the inferior lumbar hernia and of the classical approach to the renal tract.
Choosing a route to the kidney
Percutaneous nephrostomy and the old lumbar incision both exploit the same window. With the patient prone or in the kidney position, the surgeon works through the triangle of Petit — an avascular gap where the latissimus dorsi and external oblique part company over the iliac crest, with internal oblique forming a partial floor. Above and deep to this region, the three flank layers — external oblique, internal oblique and transversus abdominis — are split in line with their fibres, and the latissimus dorsi is retracted or incised at its lower border; the serratus posterior inferior is reflected with the deep fascia. No major intercostal or subcostal nerve trunk should be sacrificed at this level, since the subcostal and iliohypogastric nerves run between internal oblique and transversus and their injury leaves flank numbness or a weak abdominal wall. The erector spinae is pushed posteriorly rather than divided, its common tendon on the sacrum marking the medial limit of the approach.
Nerve-muscle pairs the screeners test
NBE extracts this region as matched pairs and as named triangles. The pair list rewards anyone who can recite the winged scapula trio — long thoracic (serratus, winging at rest and on wall push), dorsal scapular (rhomboids, winging with slight lateral shift) and accessory nerve (trapezius, drooping shoulder with weak elevation). The triangle list names auscultation and Petit. The classic trap is broader: candidates forget that dorsal rami supply only intrinsic back muscles and the overlying skin, never limb muscles — so a stem describing gluteal or shoulder girdle weakness cannot be answered by a dorsal ramus.
Frequently asked questions
Which nerve supplies the trapezius?
The spinal accessory nerve provides motor supply, with C3 and C4 conveying proprioception — injury after lymph node biopsy abolishes shrugging.
What are the boundaries of the suboccipital triangle?
Rectus capitis posterior major superomedially, obliquus capitis superior superolaterally and obliquus capitis inferior inferiorly, with the vertebral artery and suboccipital nerve on the posterior arch of the atlas at its floor.
What are the three columns of erector spinae?
From lateral to medial, iliocostalis, longissimus and spinalis, arising from a common tendinous origin on the sacrum, iliac crest and lumbar spinous processes.
Which nerve is derived from the dorsal ramus of C2?
The greater occipital nerve, which supplies the posterior scalp and emerges below obliquus capitis inferior — the nerve implicated in occipital neuralgia.
What is the clinical significance of the lumbar triangle of Petit?
It is a weak area of the posterior abdominal wall through which inferior lumbar hernias occur and through which the kidney is approached in lumbar surgery.