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Muscular System

Dr. NimirDr. Safaa

ObjectivesUnderstand the development of muscles (skeletal,

cardiac and smooth).Explain somite formation.Describe the development of limb musculature.Enlist the derivatives of Primaxial & Abaxial

domains.Define the relation of muscle with its nerve supply.Understand the development of skull.Understand the development of limbs.Explain the mechanism of limb innervation.Discuss the anomalies of the limbs.Understand the development of vertebrae.Explain the anomalies of vertebrae.Understand the development of ribs & sternum.

Development Of Muscular System

With the exception of some smooth muscle tissue the muscular system develops from the mesodermal germ layer and consists of :

SkeletalSmoothCardiac muscle

Skeletal muscle is derived from Paraxial mesoderm,which forms somites from the occipital to the sacral regions and somitomeres in the head.

Smooth muscle differentiates from splanchnic mesoderm surrounding the gut and its derivatives and from ectoderm (pupillary, mammary gland, and sweat gland muscles).

Cardiac muscle is derived from splanchnic mesoderm surrounding the heart tube.

Somite formation:Somites develop from

paraxial mesoderm as somitomeres.

Seven somitomeres will form the head musculature

The rest somitomeres will become somites:

4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal pairs.

The first occipital and the last five to seven coccygeal somites later disappear.

Somites form the axial skeleton.

The ventral region of each somite forms the sclerotome (bone-forming cells for the vertebrae and ribs).

Cells in the upper region of the somite form the dermatome and two muscle-forming areas at the ventrolateral (VLL) and dorsomedial (DML) lips (or edges).

Cells from these two areas form muscle cells ventral to the dermatome, thereby forming the dermomyotome.

Some cells from VLL migrate into parietal layer of lateral plate mesoderm. Here they form:

Infrahyoid, abdominal wall (rectus abdominus, internal and external oblique, and transversus abdominus), and limb muscles.

The remaining cells in the myotome form muscles of the back, shoulder girdle, and intercostal muscles.

The border between each somite and the parietal layer of lateral plate mesoderm called the lateral somitic frontier.

The lateral somitic frontier separates two mesodermal domains in the embryo:

1-The primaxial domain that comprises the region around the neural tube and contains only somite-derived (paraxial mesoderm) cells.

2- The abaxial domain that consists of the parietal layer of lateral plate mesoderm together with somite cells that have migrated across the lateral somitic frontier.

Muscle cells that cross the frontier (those from the VLL edge of the myotome) and enter the lateral plate mesoderm comprise the abaxial muscle cell precursors.

Muscle cells that remain in the paraxial mesoderm and do not cross the frontier (the remaining VLL cells and all of the DML cells) comprise the primaxial muscle cell precursors.

Origins of Muscles from Abaxial and Primaxial Precursors

Abaxial PrimaxialInfrahyoid Scalenes Cervical region

GeniohyoidPrevertebral

Pectoralis major and minor

Intercostals Thoracoabdominal region

External obliqueInternal obliqueTransversus abdominusSternalisRectus abdominusPelvic diaphragmDistal limb muscles Rhomboids Upper limb

Levator scapulaeLatissimus dorsi

All lower limb muscles Lower limb

Origins of the Craniofacial MusclesInnervation Muscles Mesodermal OriginOculomotor (III) Superior, medial,

ventral rectiSomitomeres 1 and 2

Trochlear (IV) Superior oblique Somitomere 3Trigeminal (V) Jaw closing Somitomere 4Abducens (VI) Lateral rectus Somitomere 5Facial (VII) Jaw opening, other

second archSomitomere 6

Glossopharyngeal (IX)

Stylopharyngeus Somitomere 7

Intrinsic laryngeals

Intrinsic laryngeals Somites 1 and 2

Hypoglossal (XII)

Tongue Somites 2 to 5a

a Somites 2 to 5 constitute the occipital group (somite 1 degenerates for the most part).

Limb musculature:Appears about the seventh

week of development as mesenchyme near the base of the limb buds.

The mesenchyme is derived from dorsolateral cells of the somites.

Connective tissue that dictates the pattern of muscle formation, is derived from the parietal layer of lateral plate mesoderm, which also gives rise to the bones of the limb.

Innervation of skeletal musclesThe new description of muscle

development (primaxial and abaxial domains) differs from the old concept (epimeres and hypomeres ), which was based on innervation.

The new description is based on the actual embryological origin of muscle cells.

Epaxial (above the axis) muscles (back muscles) are innervated by dorsal primary rami, whereas hypaxial (below the axis) muscles (body wall and limb muscles) are innervated by ventral primary rami.

Congenital anomalies:Partial or complete

absence of one or more muscles is common.Examples are:

Total or partial absence of the pectoralis major muscle (Poland anomaly) .

Similarly, the palmaris longus, the serratus anterior, and the quadratus femoris muscles may be partially or entirely absent.

Partial or complete absence of abdominal musculature is called prune belly syndrome .

Poland anomaly. The left pectoralis major muscle is absent.

Prune belly syndrome: a distended abdomen

from atrophy of abdominal wall musculature.