Musculature: Functional Groups

2 Musculature: Functional Groups

2.1 The Muscles of the Trunk Wall: Their Origin and Function

Overview of the Trunk Wall Muscles

The muscles of the trunk wall in the strict sense consist of the intrinsic back muscles and the muscles of the chest and abdominal wall. In the broad sense, they include the muscles of the pelvic floor (which form the inferior boundary of the abdominopelvic cavity) and the diaphragm (which separates the thoracic and abdominal cavities). Besides the muscles of the trunk wall, the back and thorax also contain muscles of the shoulder girdle and upper limb that migrated to the trunk during the course of development (nonintrinsic back and thoracic muscles). Examples are the thoracohumeral muscles anteriorly, the spinohumeral muscles laterally and posteriorly, and the spinocostal muscles. Other muscles that have migrated to the trunk, such as the trapezius, are derived from the mesenchyme of the branchial arches (branchial musculature). They are innervated by cranial nerves (the trapezius by the accessory nerve) and were incorporated secondarily into the trunk wall (see p. 300 ff).

A Trunk wall muscles in the strict sense

Intrinsic back muscles

Lateral tract

• Sacrospinal system

– Iliocostalis

– Longissimus

• Spinotransverse system

– Splenius

• Intertransverse system

– Intertransversarii

– Levatores costarum

Medial tract

• Spinal system

– Interspinales

– Spinalis

• Transversospinal system

– Rotatores breves and longi

– Multifidus

– Semispinalis

Short nuchal and craniovertebral joint muscles (suboccipital muscles)

• Rectus capitis posterior major

• Rectus capitis posterior minor

• Obliquus capitis superior

• Obliquus capitis inferior

Prevertebral neck muscles (belong topographically to the group of deep neck muscles but act mainly on the cervical spine)

• Longus capitis

• Longus colli

• Rectus capitis lateralis

• Rectus capitis anterior

Muscles of the thoracic cage and wall

• Intercostal muscles

• Transversus thoracis

• Subcostal muscles

• Scalene muscles (belong topographically to the group of deep neck muscles but are functionally related to thoracic breathing)

Muscles of the abdominal wall

Anterolateral abdominal muscles

• External oblique

• Internal oblique

• Transversus abdominis

Anterior (straight or strap) abdominal muscles

• Rectus abdominis

• Pyramidalis

Posterior (deep) abdominal muscles

• Quadratus lumborum

• Psoas major (belongs functionally to the hip muscles, see p. 482)

B Trunk wall muscles in the broad sense

Muscles of the pelvic floor and perineum

Pelvic diaphragm

• Levator ani muscle

– Puborectalis

– Pubococcygeus

– Iliococcygeus

– Coccygeus

Deep perineal muscles

• Deep transverse prineal (in male)

• External urethral sphincter

• Compressor urethra (in female)

• Urethrovaginal sphincter (in female)

Superficial perineal muscles

• Superficial transverse perineal

• Bulbospongiosus

• Ischiocavernosus

• External anal sphincter

Diaphragm

• Costal part

• Lumbar part (crura)

• Sternal part

C Muscles that migrated secondarily to the trunk wall (described in the Upper Limb unit, p. 240)

Spinocostal muscles (see also p. 166)

• Serratus posterior superior

• Serratus posterior inferior

Spinohumeral muscles between the trunk and shoulder girdle

• Rhomboid major and minor

• Levator scapulae

• Serratus anterior

• Subclavius

• Pectoralis minor

• Trapezius

Spinohumeral muscles between the trunk and arm

• Latissimus dorsi

Thoracohumeral muscles

• Pectoralis major

* The suboccipital muscles in the strict sense are the short or deep nuchal muscles that are counted among the intrinsic back muscles (because they are innervated by a posterior ramus). The rectus capitis anterior and lateralis muscles are not classified as intrinsic back muscles because they are innervated by anterior rami, even though they are also suboccipital in their location.

Origin of the Trunk Wall Muscles

The striated muscles of the trunk wall (including the muscles of the diaphragm and pelvic floor), like the limb muscles, develop embryologically from the myotomes of the somites (see p. 6) and are therefore called the somatic muscles. In all, approximately 42 to 44 pairs of segmental somites are formed in the paraxial mesoderm between the 20th and 30th day of development. Five occipital, 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal somites are formed in a craniocaudal sequence (see D). Some of these somites regress with further development, particularly the first occipital somites and most of the coccygeal somites, so that the number of original somites is greater than the number of subsequent vertebral segments. The boundary between the head and neck runs through the fifth pair of occipital somites. At the end of the sixth week of development, the somite myotomes migrate in the dorsoventral direction and become separated into a dorsal part (epimere or epaxial muscles) and a ventral part (hypomere or hypaxial muscles) (E). While the epaxial muscles develop into intrinsic (local) back muscles and retain their original location, the hypomere develops into the anterolateral muscles of the chest and abdominal wall and the limb musculature (F). As the myotomes become segregated, the spinal nerves undergo a corresponding division into a dorsal (posterior) ramus for the epaxial muscles and a ventral (anterior) ramus for the hypaxial muscles (see Ea). The original segmental (metameric) arrangement of the trunk muscles mostly disappears with further development. It persists only in the deep layers of the intrinsic back muscles (e.g., the rotator, interspinal, and intertransverse muscles) and the thoracic muscles (e.g., the internal and external intercostal muscles), while the superficial portions of the myotomes fuse together to form long, continuous muscles (“polymerization”) in which only the neurovascular supply still exhibits the original segmental arrangement (see F).

D Somites in a 5-week-old human embryo

Right lateral view. The somites formed from the paraxial mesoderm are classified as preotic (shown in blue and green) or postotic (shown in red), meaning that they are located cranial or caudal to the ear rudiment. The somatic muscles develop from postotic somites. Segmentation into different somites is not observed in the preotic region, which contains the rudiments for the branchiogenic pharyngeal arch muscles and the extraocular muscles. The structures in this region are innervated by cranial nerves (after Boyd, Hamilton, and Mossmann, quoted in Starck).

E Transverse sections through a 6-week-old human embryo

a Transverse section at the level of a limb bud.

b Transverse section through the abdominal wall.

Muscle precursor cells with replicative capacity located at the level of the limb buds migrate from the myotomes into the limb buds, and the myotome that remains in these regions develops into intrinsic back muscles. With further growth of the limb bud, the muscle tissue differentiates into a dorsal rudiment (blastema) for the extensor muscles and a ventral rudiment for the flexor muscles of the upper and lower limbs (see p. 20). As in the hypomere, the limb muscles are innervated by ventral (anterior) spinal nerve rami (brachial plexus and lumbosacral plexus, see pp. 362 and 536).

Note the different innervation of the epaxial muscles (dorsal [posterior] ramus) and hypaxial muscles (ventral [anterior] ramus).

F Diagram of the principal muscle groups in an 8-week-old human embryo

Right lateral view. Red = somatic muscles, blue = branchiogenic (branchial arch) muscles, green = extraocular muscles (after Boyd, Hamilton, and Mossmann, quoted in Starck).

2.2 The Intrinsic Back Muscles: Lateral Tract

A Lateral tract: schematic of the sacrospinal system

a Iliocostalis muscles.

b Longissimus muscles.

B Lateral tract: schematic of the spinotransverse and intertransverse systems

a Splenius muscles.

b Intertransversarii and levatores costarum muscles.

Iliocostalis muscles (*see p. 147, opposite, lower right)
Origin:	① Iliocostalis lumborum: sacrum, iliac crest, superficial layer of the thoracolumbar fascia ② Iliocostalis thoracis: seventh to twelfth ribs ③ Iliocostalis cervicis: third to seventh ribs
Insertion:	• Iliocostalis lumborum: sixth to twelfth ribs, deep layer of thoracolumbar fascia, transverse processes of upper lumbar vertebrae • Iliocostalis thoracis: first to sixth ribs • Iliocostalis cervicis: transverse processes of C4–C6 vertebrae
Action:	Entire muscle: bilateral contraction extends the spine, unilateral contraction bends the spine laterally to the same side.
Innervation:	Lateral branches of posterior rami of spinal nerves C8–L1
Longissimus muscles
Origin:	④ Longissimus thoracis: sacrum, iliac crest (common tendon of origin with iliocostalis), spinous processes of lumbar vertebrae, transverse processes of lower thoracic vertebrae ⑤ Longissimus cervicis: transverse processes of T1–T6 vertebrae ⑥ Longissimus capitis: transverse processes of T1–T3 vertebrae and transverse and articular processes of C4–C7 vertebrae
Insertion:	• Longissimus thoracis: second to twelfth ribs, costal processes of lumbar vertebrae, transverse processes of thoracic vertebrae • Longissimus cervicis: transverse processes of C2–C5 vertebrae • Longissimus capitis: mastoid process of temporal bone
Action:	• Entire muscle: bilateral contraction extends the spine, unilateral contraction bends the spine laterally to the same side. • Longissimus capitis: bilateral contraction extends the head, unilateral contraction flexes and rotates the head to the same side.
Innervation:	Lateral branches of posterior rami of spinal nerves C1–L5

Splenius muscles
Origin:	⑦ Splenius cervicis: spinous processes of the T3–T6 vertebrae ⑧ Splenius capitis: nuchal ligament, spinous processes of the C7–T3 vertebrae
Insertion:	• Splenius cervicis: transverse processes of C1 and C2 • Splenius capitis: lateral superior nuchal line, mastoid process
Action:	Entire muscle: bilateral contraction extends the cervical spine and head, unilateral contraction flexes and rotates the head to the same side.
Innervation:	Lateral branches of dorsal rami of spinal nerves C1–C6
Intertransversarii
Origin and insertion:	⑨ Intertransversarii mediales lumborum: course between adjacent mammillary processes of all lumbar vertebrae ⑩ Intertransversarii laterales lumborum: course between adjacent costal processes of all lumbar vertebrae ⑪ Intertransversarii posteriores cervicis: course between adjacent posterior tubercles of the C2–C7 vertebrae • Intertransversarii anteriores cervicis: course between adjacent anterior tubercles of the C2–C7 vertebrae
Action:	• Bilateral contraction stabilizes and extends the cervical and lumbar spine. • Unilateral contraction bends the cervical and lumbar spine laterally to the same side.
Innervation:	Posterior rami of the spinal nerves for intertransversarii mediales lumborum, intertransversarii laterales lumborum, and intertransversarii posteriores cervicis; anterior rami of the spinal nerves for intertransversarii anteriores cervicis
Levatores costarum
Origin:	⑫ Levatores costarum breves: transverse processes of the C7 and T1–T11 vertebrae ⑬ Levatores costarum longi: transverse processes of the C7 and T1–T11 vertebrae
Insertion:	• Levatores costarum breves: costal angle of the next lower rib • Levatores costarum longi: costal angle of second lower rib
Action:	• Bilateral contraction extends the thoracic spine. • Unilateral contraction bends the thoracic spine to the same side, rotates it to the opposite side.
Innervation:	Posterior rami of the spinal nerves

C Lateral tract of the intrinsic back muscles: the sacrospinal system (iliocostalis and longissimus muscles)

D Lateral tract of the intrinsic back muscles: the spinotransverse system (splenius muscles) and intertransverse system (intertransversarii and levatores costarum muscles)

* The schematic muscle diagrams with accompanying tables on p. 146 are intended to give a systematic overview of the muscles and their actions. The drawings on the right-hand page are intended to display the muscles as they would appear in a dissection. Not all the structures listed in the tables on p. 146 are labeled in the drawings above, as they are not all visible in the views shown.

2.3 The Intrinsic Back Muscles: Medial Tract

A Medial tract of the intrinsic back muscles: schematic of the spinal system

The interspinales and spinalis muscles.

Interspinales muscles
Origin and insertion:	① Interspinales cervicis: course between the spinous processes of adjacent cervical vertebrae ② Interspinales lumborum: course between the spinous processes of adjacent lumbar vertebrae
Action:	Extends the cervical and lumbar spine
Innervation:	Posterior rami of the spinal nerves
Spinalis muscles
Origin:	③ Spinalis thoracis: lateral surface of the spinous processes of the T10–T12 and L1–L3 vertebrae ④ Spinalis cervicis: spinous processes of the C5–C7 and T1–T2 vertebrae
Insertion:	• Spinalis thoracis: lateral surface of the spinous processes of the T2–T8 vertebrae • Spinalis cervicis: spinous processes of the C2–C5 vertebrae
Action:	• Bilateral contraction extends the cervical and thoracic spine. • Unilateral contraction bends the cervical and thoracic spine to the same side.
Innervation:	Posterior rami of the spinal nerves

B Medial tract of the intrinsic back muscles: schematic of the transversospinal system

a Rotatores breves and longi.

b Multifidus.

c Semispinalis.

Rotatores breves and longi
Origin and insertion:	⑤ Rotatores breves: from transverse process of thoracic vertebrae to spinous process of the next higher vertebra ⑥ Rotatores longi: from transverse process of thoracic vertebrae to spinious process of the vertebra two levels higher
Action:	• Bilateral contraction extends the thoracic spine. • Unilateral contraction rotates it to the opposite side.
Innervation:	Posterior rami of the spinal nerves
Semispinalis muscles
Origin:	⑧ Semispinalis thoracis: transverse processes of the T6–T12 vertebrae ⑨ Semispinalis cervicis: transverse processes of the T1–T6 vertebrae ⑩ Semispinalis capitis: transverse processes of the C3–T6 vertebrae
Insertion:	• Semispinalis thoracis: spinous processes of the C6–T4 vertebrae • Semispinalis cervicis: spinous processes of the C2–C5 vertebrae • Semispinalis capitis: occipital bone between the superior and inferior nuchal lines
Action:	• Bilateral contraction extends the thoracic spine, cervical spine, and head (stabilizes the craniovertebral joints). • Unilateral contraction bends the head, cervical spine, and thoracic spine to the same side and rotates them to the opposite side.
Insertion:	Posterior rami of the spinal nerves