5.2 Myology of the Axial Skeleton

Why axial myology is its own 17%

The official Spinal Anatomy bullets for myology of the axial skeleton are face and head, back and neck, thorax and abdomen, pelvis and perineum, and development. Appendicular myology (rotator cuff, forearm compartments, hand, thigh) already sat in General Anatomy. Here the exam wants the muscles that move the skull, vertebral column, ribs, abdominal wall, and pelvic floor — plus the cranial-nerve muscles of the face that live on the skull.

/practice/nbce-part1Practice questions with detailed explanations

Face and head

Muscles of mastication (first pharyngeal arch, V3)

Four muscles. Three close the jaw; one opens it.

Lateral pterygoid is the only mastication muscle that depresses (opens) the mandible. Gravity and suprahyoids help, but on a four-option item, opening = lateral pterygoid. Unilateral lateral and medial pterygoid action swings the chin to the contralateral side. Buccinator is not a muscle of mastication; it is facial expression (CN VII) and keeps the bolus on the teeth. The parotid duct pierces buccinator opposite the upper second molar.

Muscles of facial expression (second arch, CN VII)

They insert into skin, which is why the face moves in sheets. Motor branches of CN VII inside the parotid plexus: temporal, zygomatic, buccal, marginal mandibular, cervical (Two Zebras Bit My Cookie). Orbicularis oculi closes the eye (palpebral part blinks, orbital part squeezes); loss is an open eye and corneal risk in facial palsy. Orbicularis oris closes the mouth. Zygomaticus major smiles. Frontalis (belly of occipitofrontalis) raises the brow; galea aponeurotica joins frontalis to occipitalis. Platysma is the thin cervical sheet from clavicle to mandible and mouth; it is CN VII, not ansa cervicalis. Corrugator supercilii knits the brow. Levator palpebrae superioris is not facial expression — it is CN III, with a sympathetic superior tarsal (Müller) assist.

Extraocular muscles

Six extraocular plus levator palpebrae. Innervation mnemonic LR6 SO4 AO3: lateral rectus CN VI, superior oblique CN IV, all others CN III.

Clinical testing uses the H pattern: from adduction, up tests inferior oblique and down tests superior oblique; from abduction, up tests superior rectus and down tests inferior rectus. Part I wants innervation and the trochlea of superior oblique more than orthoptic nuance.

Tongue and palate

Extrinsic tongue muscles: genioglossus (protrudes; the muscle that, if weak, lets the tongue fall back and, if unilateral, lets the tongue deviate toward the lesion on protrusion), hyoglossus (depresses and retracts), styloglossus (retracts and elevates). Those three are CN XII. Palatoglossus is the exception: CN X (pharyngeal plexus). Intrinsic tongue muscles (longitudinal, vertical, transverse) are also CN XII. Palatal levator veli palatini is X (except tensor veli palatini, which is V3 and hooks around the pterygoid hamulus). Stylopharyngeus is the only muscle innervated solely by CN IX.

Back and neck

Layers of the back

Erector spinae (sacrospinalis) from lateral to medial: iliocostalis, longissimus, spinalis — "I Love Spine." Each has regional names (lumborum/thoracis/cervicis for iliocostalis; thoracis/cervicis/capitis for longissimus and spinalis). They arise from a broad lumbar aponeurosis on sacrum, iliac crest, and lumbar spines. Iliocostalis inserts on rib angles and cervical TPs; longissimus on TPs and mastoid (longissimus capitis); spinalis on spinous processes.

Transversospinalis runs from transverse processes up and medially to spinous processes. Superficial to deep, and long to short:

Unilateral transversospinalis rotates the trunk to the opposite side (TP toward SP of a vertebra above on the same side pulls that SP toward the TP, rotating the body contralaterally). Erector spinae unilaterally same-side bends; splenius unilaterally same-side rotates the head.

Splenius capitis from nuchal ligament and C7–T3 spines to mastoid and lateral superior nuchal line; splenius cervicis from T3–T6 spines to C1–C3 TPs. Together they are the spinotransversales group — the fiber direction opposite transversospinalis.

Suboccipital muscles (all C1 dorsal ramus — the suboccipital nerve)

The suboccipital triangle is RCP major (superomedial), OCI (inferolateral), OCS (superolateral). RCP minor is not a border. Floor is the posterior atlanto-occipital membrane and posterior arch of C1. Roof is semispinalis capitis (deep) and trapezius/splenius (superficial). Contents: vertebral artery in the C1 groove (third part, horizontal) and suboccipital nerve (C1 dorsal ramus) which innervates the four muscles. The greater occipital nerve (C2 dorsal ramus) crosses below OCI, then pierces semispinalis capitis and trapezius to the scalp — occipital neuralgia anatomy, still a Part I identification fact. C1 dorsal ramus is usually motor-only to the triangle; C2 is the big sensory nerve of the posterior scalp.

Sternocleidomastoid and scalenes

Sternocleidomastoid (SCM) has a sternal head from the manubrium and a clavicular head from the medial clavicle; both insert on the mastoid process and lateral superior nuchal line. Unilateral action: ipsilateral lateral flexion and contralateral rotation of the head (the face turns away). Bilateral: flex the neck (anterior fibers) or extend the head at AO if the cervical spine is stable. Innervation: spinal accessory nerve (CN XI) for motor, C2–C3 ventral rami for proprioception. CN XI crosses the posterior triangle on levator scapulae after leaving SCM.

Scalenes:

The interscalene (scalene) triangle between anterior and middle scalene, sitting on rib 1, transmits the brachial plexus trunks and the subclavian artery. The subclavian vein does not go through that triangle — it is anterior to anterior scalene with the phrenic nerve. That three-dimensional fact is how Part I distinguishes thoracic-outlet anatomy without turning the item into a diagnosis question.

Prevertebral muscles (ventral rami): longus colli, longus capitis, rectus capitis anterior, rectus capitis lateralis. They flex the head and neck and form the floor of the retropharyngeal space. Sympathetic cervical ganglia lie on their longus colli surface.

Hyoid muscles

Suprahyoid:

Infrahyoid (strap) muscles: sternohyoid, omohyoid, sternothyroid, thyrohyoid. Ansa cervicalis (C1–C3) innervates the first three; thyrohyoid is C1 hitchhiking on CN XII, like geniohyoid. Omohyoid has superior and inferior bellies with a sling on the clavicle; inferior omohyoid splits the posterior triangle.

Thorax and abdomen

Intercostals. External intercostals run inferomedially ("hands in pockets") from the tubercles of the ribs to the costochondral junctions, then continue as the external intercostal membrane. They elevate ribs in inspiration. Internal intercostals run inferolaterally, from sternum to angles, and are active in forced expiration (interosseous part); the interchondral internal fibers can help inspiration. Innermost intercostals are a deep incomplete layer. The neurovascular plane is between internal and innermost — VAN in the costal groove. Transversus thoracis on the inner sternum depresses costal cartilages. Levatores costarum (dorsal rami C8–T11) from C7–T11 TPs to the rib below are small inspiratory muscles and proprioceptors. Serratus posterior superior (C7–T3 spines to ribs 2–5) and inferior (T11–L2 spines to ribs 9–12) are intermediate back muscles innervated by intercostal nerves.

Diaphragm is the primary muscle of inspiration: dome of skeletal muscle from xiphoid, costal margin, and lumbar crura (right crus L1–L3, left crus L1–L2) plus arcuate ligaments. Central tendon receives the muscle. Innervation: phrenic nerves C3–C5 (motor and most sensation to the central diaphragmatic peritoneum/pleura); peripheral diaphragm sensation is lower intercostals. Apertures:

"I 8 10 Eggs At 12" remains legal on Part I. Left phrenic pierces the muscle to reach the undersurface; it does not share the caval opening.

Anterolateral abdominal wall, superficial to deep: external oblique (inferomedial, becomes inguinal ligament from ASIS to pubic tubercle, and the external spermatic fascia), internal oblique (inferolateral in the upper wall, becomes cremaster in males), transversus abdominis (horizontal, deepest, transversalis fascia inside). The three aponeuroses form the rectus sheath around rectus abdominis. Above the arcuate line, internal oblique splits around rectus; below it, all three aponeuroses pass anterior to rectus and only transversalis fascia is posterior — a weak site for hernia, still an anatomy fact. Rectus has three to four tendinous intersections fused to the anterior sheath. Pyramidalis is inconstant, tenses linea alba, T12. Innervation of the wall is T7–T12 (subcostal) and L1 (iliohypogastric, ilioinguinal).

Posterior abdominal wall: psoas major from T12–L5 bodies, discs, and TPs to the lesser trochanter (with iliacus as iliopsoas); lumbar plexus is embedded in psoas. Psoas minor (inconstant) to pectineal line. Quadratus lumborum from iliac crest and iliolumbar ligament to the 12th rib and L1–L4 TPs; it is a hip-hiker and stabilizer of the 12th rib for diaphragm pull. Innervation T12–L4 ventral rami.

Pelvis and perineum

Pelvic diaphragm = levator ani + coccygeus (ischiococcygeus).

Innervation: pudendal nerve (S2–S4) plus direct sacral branches (S3–S4) to levator ani. Piriformis (sacrum to greater trochanter) and obturator internus (obturator membrane to greater trochanter) form the walls, not the floor; the pudendal nerve exits the greater sciatic foramen below piriformis, then re-enters the perineum through the lesser sciatic foramen into Alcock's (pudendal) canal on obturator internus.

Perineum is a diamond: pubic symphysis, ischial tuberosities, coccyx. A line between the tuberosities divides an anterior urogenital triangle and a posterior anal triangle. Superficial pouch (inferior to the perineal membrane): ischiocavernosus (covers the crura), bulbospongiosus (covers the bulb; compresses the urethra), superficial transverse perineal. Deep pouch: external urethral sphincter, deep transverse perineal (and in many females compressor urethrae / sphincter urethrovaginalis). External anal sphincter is in the anal triangle (subcutaneous, superficial, deep parts) and is pudendal (inferior rectal branches). Perineal body is the fibromuscular node where these muscles meet — the obstetric landmark between vagina and anus.

Development of axial muscle

Somites split into epimere (dorsal, epaxial, dorsal-ramus true back muscles) and hypomere (ventral, hypaxial, ventral-ramus body-wall and limb muscles). That is why multifidus is dorsal ramus and rectus abdominis is ventral ramus even though both flex or extend the trunk.

Pharyngeal (branchial) arches supply the head muscles that are not extraocular somite muscles:

Tongue muscles are occipital somites that drag CN XII with them as they migrate; palatoglossus stays with the palatal X innervation. Extraocular muscles are preotic somites / somitomeres with CN III, IV, and VI. Diaphragm muscle is cervical myotomes C3–C5 that migrate down, which is why the phrenic nerve arises in the neck and why referred diaphragmatic pain can hit the shoulder (C3–C5 dermatomes). The septum transversum, pleuroperitoneal folds, and esophageal mesentery close the diaphragm; a posterolateral (Bochdalek) defect is a failed pleuroperitoneal fold — a development item that still belongs under myology of the thorax.