L.13 Axial Muscles

Axial Muscles Overview

  • Axial muscles are divided into two main groups: epaxials (back) and hypaxials (front).

1. Metamerism

A. In Fish

  • Repeated muscle blocks attached to repeated vertebrae allow for lateral undulation.

    • Epaxials (red): Dorsal muscles.

    • Hypaxials (white): Ventral muscles.

  • Separated by horizontal skeletal myoseptum.

B. In Tetrapods

  • Locomotion by limbs leads to changes in the metameric pattern.

  • While the metameric pattern is obscured or changed due to limbs, it is retained in some musculature.

    • Epaxial Muscles: Retain metameric serratus muscles, intertransversii (strap-like, no myosepta).

    • Hypaxial Derivatives: Metameric pattern lost.

2. Embryonic Origins of Metamerism

  • Arises from paraxial mesoderm (somite).

  • Segmented somites begin at the neck and progress to the tail.

    • Mesoderm Composition: Composed of three distinct populations of mesenchyme:

    • Dermomyotome: Can differentiate into dermatome (skin) and myotome (body muscle).

    • Sclerotome: Forms vertebrae.

3. Axial Muscle Origins

  • Somites give rise to mesenchymal cells that stream into the body wall, migrating ventrally and undergoing cell divisions.

4. Anatomy of Trunk and Tail Muscles in Fish

  • Myomeres are separated by myosepta, with one spinal nerve per myomere.

  • The horizontal skeletal myoseptum attaches to dorsal ribs, stretching between vertebrae and skin along the body length.

5. Trunk and Tail Muscles of Various Species

A. Intermediate Stages to Life on Land

  • Necturus (amphibian):

    • Retains epaxial and hypaxial as segments, with hypaxial in three layers.

  • Sphenodon (reptile):

    • Epaxial: Includes muscles like latissimus dorsi and trapezius, retaining metamerism as serratus.

    • Hypaxial: Unsegmented sheets (external oblique, internal oblique, transverse).

    • Loss of horizontal skeletal myoseptum.

B. Organization of Axial Muscles in Mammals

  • Epaxial Muscles:

    • Amphibians retain metamerism, referred to as dorsalistrunci.

    • Amniotes exhibit long bundles extending over multiple body segments, functioning to extend and straighten the vertebral column and facilitate lateral and dorsal-ventral body flexion.

    • Four Groups:

    1. Intervertebrals (not shown)

    2. Longissimus dorsi

    3. Spinalis dorsi

    4. Iliocostales

6. Specific Epaxial Muscles of Amniotes

A. Intervertebrals

  • Deepest epaxials maintaining primitive metamerism.

    • Four Types:

    1. Intertransversarii: Extending between successive transverse processes of vertebrae.

    2. Interspinales: Connecting two neural spines of vertebrae.

    3. Interarcuales: Linking two neural arches of vertebrae.

    4. Interarticulares: Connecting zygopophyses of vertebrae.

B. Longissimus dorsi

  • Longest epaxial muscle, courses from head to tail.

C. Spinalis dorsi

  • Connects head to trunk.

    • Capitis Bundles: Insert on skull.

    • Cervicus Bundles: Insert on neck.

    • Dorsi Bundles: Insert on trunk (in mammals, distinct longissimus bundles).

D. Iliocostales

  • Located lateral to longissimus; thin sheets arising from ilium to insert on ribs; dominant in reptiles, aiding in lateral undulation.

7. Epaxial Specializations in Various Animals

  • Turtles and Birds:

    • Trunk epaxials are not well developed due to fused thoracic vertebrae (thorax in turtles) and a fused synsacrum.

    • Both have well-developed neck epaxials enables substantial movement.

  • Snakes:

    • Utilizes dorsal-ventral flexion and lateral undulation, resulting in developed epaxials.

8. Hypaxial Muscles of the Trunk

  • Three Main Groups:

    1. Oblique Sheets: Lateral body wall (internal and external) - running from ribs to linea alba.

    2. Transverse Sheets: Lateral body wall (parietal muscles) - spanning ribs and ilium to linea alba and pubis.

    3. Rectus Abdominis: Located on either side of the linea alba, extending from sternum to pubis (commonly referred to as the '6-pack').

    • Supports the abdominal sling and plays a role in respiration, and helps compress viscera for:

    • Egg laying

    • Delivery of mammalian young

    • Emptying digestive tract

  • Hypaxials Overview:

    • Includes both obliques (external oblique as superficial, internal oblique as deep) and transverse (deepest being transverse abdominis).

    • Also contains external and internal intercostals, with the latter being vestigial in turtles.

9. Hypaxial Muscles in Specific Species

  • Focus on Necturus (mud puppy) which retains epaxial muscle masses.

10. Role of Muscles in Structural Integrity

  • Consider all these muscles as straps holding anatomical structures together.

11. Hypaxial: Rectus Abdominis

  • Assists in flexing the trunk.

A. Species Variability

  • Urodeles (like Necturus) exhibit segmental structures.

  • Anurans and Amniotes (such as cats): possess transverse tendonous inscriptions.

12. Mammalian Diaphragm

  • Originates from mesenchyme at the level of the 3rd to 5th cervical spinal nerves, migrating caudally into the somatopleure, which separates thoracic and abdominal cavities.

    • Central Tendon: Surrounded by muscles:

    • Sternal Portion: Attached to xiphoid process.

    • Costal Portion: On the caudalmost ribs.

    • Vertebral Portion: Anchored to lumbar vertebrae.

    • Crura: Triangular masses anchoring to lumbar vertebrae.

  • Essential for mammalian suction breathing.

    • Represents an evolutionary adaptation aiding air breathing and vocalization, effectively separating thoracic and abdominal cavities.