6 - Trunk Suboccipital Triangle

• Torso muscles can be divided into muscles of the trunk walls, muscular diaphragms, and muscles of the head and neck
• Muscles of the trunk walls: somatic muscles following a segmental pattern
• Muscular diaphragms include thoracic, pelvic, and urogenital diaphragms, larynx and pharyngeal constrictors and suprapleural membranes
• Muscles of the head and neck are derived from embryonic somites but mostly from embryonic branchial arches and have a complex organization

Muscles of the Trunk Wall

• The muscles of the anterior and posterior walls of the trunk consist of bilaterally paired longitudinal muscle groups
• Intrinsic back muscles are the posterior paired longitudinal groups which attach to the lever arms of the vertebrae
• Intrinsic back muscles either extend the thorax or extend the pelvis
• Intrinsic muscles of the back attach to the lever arms (only) of the vertebrae
• The deep muscles of the back are responsible for maintenance of posture and movements of the head and vertebral column, referred to as the erector spinae muscle mass
• The deep muscles are named according to their relationship with the surface of the body
• Classified into three groups: superficial, intermediate, and deep
• Superficial and intermediate muscle groups are extrinsic muscles of the back that deal primarily with limb movement and respiration

Superficial Extrinsics

• They connect the upper limb to the trunk
• There are two layers
• Layer one includes the trapezius and latissimus dorsi
• Layer two includes the levator scapulae, rhomboid minor and rhomboid major

Intermediate Extrinsics

• This layer of muscle lies under cover of the rhomboids superiorly and the latissimus dorsi inferiorly
• Consists of two flat inspiratory muscles segmentally innervated by the dorsal rami of the intercostal nerves
• The serratus posterior superior are thin sheets of fibers directed inferolaterally to the outer surfaces of ribs 2-5 beyond the angles, and it is a feeble elevator of the ribs
• The serratus posterior inferior arising inferiorly from the spines of T11-L2 with the fibers directed superolaterally to the surfaces of ribs 8-12 beyond their angles, and it is a depressor or fixator of the lower ribs

Deep (Intrinsic) Muscles of the Back:

• This is a superficial layer of intrinsics

The Splenius Muscles

• It has a bandage-like appearance that extends over the sides and back of the neck
• Divided into cranial and cervical portions sharing a common origin into half of the ligamentum nuchae as well as the spinous processes of T1-6
• The splenius capitis inserts into the mastoid process and lateral 1/3 of the superior nuchal line on the occipital bone
• The splenius cervicis inserts into the posterior tubercles of the transverse processes of C1-4 deep to the levator scapulae
• Acting alone, the splenius muscles will laterally flex and rotate the head to the same side, while acting together they extend the head and neck

Intermediate Layer of Intrinsics

• The erector spinae is the chief extensor of the spine and its composite parts are all innervated segmentally
• Forms a massive bulge on either side of the vertebral column and lies in a fascial compartment deep to the thoracolumbar fascia
• Has a common inferior origin to the sacrum, iliac crest and sacroiliac ligament
• From this common origin 3 vertical columns arise
  • The spinalis muscle (medial column) extends from spinous process to spinous process
  • The longissimus extends to the transverse processes and ribs
  • The iliocostalis (medial column) extends to the angles of the ribs
• Each column has three parts dependent on location but they aren't defined functionally
• These muscles extend the head and vertebral column and can unilaterally flex it

Deep Layer of Intrinsics

• Removal of the erector spinae reveals muscles referred to as the transversospinal group
• These muscles extend obliquely from the transverse process below to the spines or laminae above, missing several vertebrae between origin and insertion
• The semispinalis has three parts: capitis, cervicis, and thoracis
• The semispinalis is more superficial of the two transversospinals and misses 4 to 6 vertebrae between origin and insertion
• The multifidus is several bundles of muscles that cover the laminae of the vertebrae
• Is superficial to the rotatores and misses one or two vertebrae between origin and insertion
• The rotatores are the deepest muscles in the groove between spinous and transverse processes, spanning along the whole column but are best visualized in the thoracic region
• The next two members of the deep intrinsic muscles of the back are short muscles that extend between adjacent vertebrae
• The intertransverse are lateral flexors linking adjacent transverse processes
• The interspinales are extensors uniting the spinous processes of adjacent vertebrae
• These last two groups are best developed in the lumbar and cervical regions

Suboccipital Triangle

• The suboccipital triangle is a triangular area around the articulation between the occipital bone of the skull and the superior end of the vertebral column (C1 - the atlas and C2 - the axis), and it is deep to the semispinalis capitis
• It is important clinically because of its main constituent which is the vertebral artery
• The vertebral artery passes medially in a horizontal plane after ascending through the transverse foramina of the cervical vertebrae, and before it accesses the cranial cavity through the foramen magnum (also in the occipital bone)
• When the vertebral artery diameter narrows due to atherosclerosis, prolonged turning of the head or looking upward (hyperextension) may cause dizziness due to impaired blood flow in this region

Boundaries:

• Roof: semispinalis capitis muscle
• Floor: atlanto-occipital membrane
• Lateral: superior oblique muscle
• Medial: rectus capitis major
• Inferior: inferior oblique muscle
• There are four small muscles in this region
  • Rectus capitis (posterior) major: extends from C2 to the skull
  • Rectus capitis (posterior) minor: extends from C1 to the skull
• These are both postural muscles, but can also rotate the head to the same side unilaterally, extend it when acting bilaterally
  • Superior oblique: extends from C1 to the skull
  • Inferior oblique: extends from C2 to C1 (pulls on the atlas)

Contents

• The vertebral artery entering the foramen magnum and lying horizontally in a groove on the superior surface of the posterior arch of the atlas (C1)
• The suboccipital nerve (the dorsal ramus of C1)

Anatomy questions covering back muscles and related anatomical structures such as the transverse foramen, suboccipital triangle, vertebral artery and the atlas. Questions also address the primary function of the intrinsic and extrinsic muscles of the back.