Axial Musculature: Thorax and Abdomen

Question 1

Muscles of the Thorax

Answer 1

Diaphragm - main muscle of respiration
--inspiration
-->diaphragm contracts (depresses)
-->increases size of thoracic cavity
--expiration
-->diaphragm relaxes
-->thoracic cavity decreases in size
Origin: inside body wall and ribs
Insertion: central tendon

Intercostals - respiratory muscles (accessory)
--external intercostals
-->elevate ribs, inspiration
--internal intercostals
-->depress ribs, expiration (forced)
Origin: ribs
Insertion: ribs

Question 2

Muscles of the Abdominal Wall

Answer 2

Rectus abdominus, External Oblique, Internal Oblique, Transversus Abdominus

Question 3

Rectus Abdominus (RA)

Answer 3

Midline; “6-pack”

action: flexes trunk
fibers run superior/inferior

Origin: pubis
Insertion: ribs and xiphoid process

Question 4

External Oblique (EO) and Internal Oblique (IO; deep to EO)

Answer 4

action: flex and rotate trunk
fibers run obliquely (same direction as intercostals)

Origin: lower ribs
Insertion: pelvis and linea alba

Question 5

Transversus Abdominus (T) - deep to IO

Answer 5

action: compress abdominal cavity
deepest abdominal muscle; fibers horizontally

Origin: posterior body wall
Insertion: anterior body wall

Question 6

Pelvic Diaphragm

Answer 6

3 layers of muscles that form the pelvic floor:

• -levator ani muscles
• -coccygeus muscle
• -pelvic fascia

Spans from ischium and pubis to sacrum and coccyx
Support pelvic viscera (organs)
Also contains 2 hiatuses (holes) to allow passage of urethra/vagina and rectum

Origin: pelvic outlet
Insertion: midline

Question 7

Intrinsic Back Muscles

Answer 7

Erector spine muscles (includes) [lateral to medial]:

• -iliocostalis
• -longissimus
• -spinalis

Actions:

• -bilateral: extend trunk, neck, head
• -unilateral: laterally flex trunk, neck, head

Origin: pelvis
Insertion: ribs, vertebrae, cranium

Question 8

“Spare Tire” mnemonic [deepest to superficial]

Answer 8

Transverse abdominus
Internal oblique
Rectus abdominus
External oblique