The musculoskeletal system: Axial

Question 1

What is the musculoskeletal system? Origin?

Answer 1

• bones and muscles that move body around

- mesodermal origin

Question 2

What is the axial skeleton composed of?

Answer 2

vertebrae; sternum/ribs; skull

Question 3

Functions of the axial skeleton?

Answer 3

• protection of CNS (spinal chord/brain) and viscera (thoracic and abdominal)
• breathing movements (ribs and sternum)
• support and movement

Question 4

Name + number vertebrae in humans

Answer 4

• 7 cervical (neck)
• 12 thoracic
• 5 lumbar
• 5 sacral (large, fused into single mass, support weight/pelvis)
• 3-5 coccygeal (tail - tiny, fused)

Question 5

What is the atlas joint?

Answer 5

• atlas = C1 (has transverse ligament)
• axis = C2 (has dens = odontoid process)
• flexion-extension + pivot joint

Question 6

Why is the lumbar spine vulnerable?

Answer 6

-lies between wide thorax and wide pelvis
-heavily loaded
(region that often collapses in elderly due to osteoporosis)

Question 7

What is osteoporosis?

Answer 7

• eating away of bone in spine
• bone lost faster than can be repaired
• sponge-like vertebrae = collapse of spine (particularly in elderly)

Question 8

What is the most variable part of the vertebral column? What is its function?

Answer 8

• the coccyx = what’s left of the tail

- pelvic floor muscles attach to coccyx (important for strengthening after childbirth)

Question 9

What are the axial muscles? What movements do they allow?

Answer 9

-epaxial muscles
-deep back muscles (dorsal to ribs/vertebrae)
-extend spine
-hypaxial muscles
-intercostal + abdominal wall muscles (ventral to
ribs/vertebrae)
-flex spine

Question 10

What is the difference between vertebrates and invertebrates?

Answer 10

-invertebrates: lack vertebral column
(insects, jellyfish, flatworms, starfish…)
-vertebrates: animals with vertebrae

Question 11

Describe earliest vertebrate ancestors + their evolution

Answer 11

-earliest vertebrate ancestors had no vertebrae
-body stiffened by flexible mesodermal rod - notochord
(lies immediately below nerve cord)

-nerve chord and notochord then became enclosed in vertebrae = stronger, allow more complex muscles + movements

Question 12

How does vertebral formation occur?

Answer 12

sclerotome from somites
-forms vertebral (neural arches)
-forms vertebral body (notochord in middle)
(=around neural tube)

Question 13

What are intervertebral discs?

Answer 13

• in between vertebral bodies
• contribution from sclerotome + notochord
• tough fibrous outer ring
• soft central nucleus pulposus = remnant of notochord

Question 14

What is a disc herniation?

Answer 14

• outer fiber can tear = part of nucleus pulposus comes out; pushes against spinal nerves-“slipped disc”
• often in lumbar region (vulnerable)

Question 15

How does resegmentation keep muscle-verterba relationships?

Answer 15

• each vertebra formed by portions of 2 adjacent somites

- means that vertebra joined up by muscle (myotome from 1 somite attached to 2 parts of dif. vertebrae it will form)

Question 16

What is metameric segmentation?

Answer 16

• repetition of segments
• found in invertebrates: worms, centipedes (each segment has same structure)
• also obvious in long-bodied vertebrates: snakes

Question 17

Give signs of segmentation in human development

Answer 17

• somites
• muscle blocks (myotome) innervated by segmentally arranged spinal nerves
• dermatomes - segmental sensory pattern to the skin (each dermatome receives sensory nerve supply from 1 segmental spinal nerve)

-most obvious in thorax: intercostal muscles of chest, segmental pattern of blood vessels, of nerves to muscle and skin, ribs/vertebrae and sternum

Question 18

What is vertebral regionalisation?

Answer 18

-Bony fish:
similar vertebrae, undifferentiated muscle flex body

-Early tetrapod to primitive amniote
-vertebral regionalisation: muscles differentiate for
simple limb movements (move onto land, need
support)

• Therapsid
  
  • distinct neck = head mobility
  • muscles differentiate for limbs/breathing mvts
  • loss of ribs = lumbar region

(throughout process: more differentiation in vertebral column = more dif types of vertebrae)

Question 19

What variation is there in the cervical + lumbar region in mammals?

Answer 19

-all mammals have 7 cervical vertebrae, but in dif sizes (ex: giraffes)
(=evolutionary constraint, while reptiles and bird show great variation)

-differences in length of lumbar region in mammals
=different locomotor strategies
(ex: giraffes: few lumbar vertebrae
but cats: longer lumbar region: flexibility, speed, agility)

Question 20

What bones do somites give rise to?

Answer 20

-occipital bone + vertebrae

Question 21

When does specification of segmental identity take place?

Answer 21

-identity (cervical or thoracic or lumbar…) determined before somite formation
-transplanting “thoracic” presomitic paraxial mesoderm into “cervical” mesoderm
=vertebrae with ribs in cervical region
=transplant does not change cell fate

Question 22

What is homeosis?

Answer 22

• anatomical change where one segment/body part replaced by another
• William Bateson FRS - observed in insects

Question 23

What are homeotic mutations?

Answer 23

-homeotic mutations
=mutations caused by homeosis
-Calvin Bridges
-2 recessive mutations:
bithorax (bx)=anterior haltere into wing
bithoraxoid (bxd)=posterior haltere into posterior
wing

Question 24

What are antennapedia mutations in drosophila?

Answer 24

• transformation of antennae (sex organs) into legs (normally expressed in thorax)
• gain of function mutation

Question 25

How many homeotic genes and complexes in drosophila melanogaster?

Answer 25

Drosophila
-8 homeotic genes
-found close to each other on Xsome B
-in 2 complexes:
   antennapedia complex (5) + bithorax complex (3)

Question 26

How many homeotic genes and complexes in tribolium (flower beetle)?

Answer 26

Tribolium (flower beetle):

• 9 homeotic genes
• only 1 complex (typical in insects)