Bio 205, Lecture 14

Muscles I

This is an outline of the lecture. All of this material is in Kardong. Use this outline to highlight the material in Kardong that you should know.

Developmental origin

• head mesoderm - extrinsic eye muscles, branchiomeric muscles
• myotome - skeletal muscle of body wall and appendages, hypobranchial mm.
• splanchnic mesoderm - smooth muscle of viscera, cardiac muscle of heart
• general mesenchyme - smooth muscle of vessels

Classification

1. control: voluntary vs. involuntary
2. location: Visceral vs. somatic
3. general microscopic appearance
   1. Striated
      1. Skeletal - syncitium formed by fusion of multiple cells, hence multinucleate, long, striated
      2. cardiac - single nuclei, short and joined by intercalated disks, striated
   2. smooth - not striated
4. color: red vs.white (more tomorrow)

anatomical organization

• muscle surrounded by epimysium, composed of fasicles
• fasicle surrounded by perimysium, composed of individual muscle cells or fibers
• fibers surrounded by endomysium
• within fibers are organelles like nucleus, mitochondria, and myofibrils, which are bundles of contractile proteins

The sarcomere of skeletal muscle
(thanks to the biology department at Davidson College for this movie)

• defined laterally by z-bands
  • A band
  • I band
  • H zone
• Thick-thin filament overlap
• major proteins
  • myosin
  • actin
  • tropomyosin
  • troponin
  • titin

Contraction

1. synapse
2. depolarization
3. SR release of Ca++
4. Ca++ binds to troponin
5. troponin moves tropomyosin off of actin so that myosin binding site on actin is exposed
6. myosin binds ATP, cocks head and binds to actin, hydrolyzes ATP, and pivots

Steps 4-6 are seen here:

(click here to see quicktime version)
(thanks to the SDSU human physiology site and Roger Sabbadini for this movie)

General trends

1. Branchiomeric muscles in head divide and specialize
   1. hyomandibular arch muscles
      • constrictor colli in non-mammalian tetrapods
      • The CC differentiates into the many facial muscles of mammals
2. differentiation of primitive myomeric organization into discrete muscles
   • Fish myomeres are divided by horizontal septum into epaxial and hypaxial
   • amphibians have not differentiated the epaxial muscles much but the hypaxials are differentiated into three superifical to deep layers
     1. transversus
     2. internal oblique
     3. external oblique
   • epaxial muscles in amniotes have specialized into medio-lateral groups
     1. transversospinalis
     2. longissimus
     3. iliocostalis
3. differentiation of appendicular muscles
   1. primitive pattern - dorsal and ventral muscle blocks (dorsal: scapula, ilium; ventral; coracoid, ischium-pubis)
   2. rotation in mammals
      1. forelimb - clockwise rotation (dorsal mass becomes caudal)
         • pronate forearm in order to redirect digits
      2. hindlim - anticlockwise rotation (dorsal becomes ventral)

Effect of posture on the kinematics of secondarily aquatic tetrapods - an example of historical or phylogenetic constraints. Essentially, because of the derived posture in mammals in which the limbs are underneath the body instead of sprawled to the side, the axial muscles now emphasize dorsoventral and not lateral motion. Thus, in mammals that have become secondarily aquatic, the typical motion of the dorsoventral, either along the body as in an otter, or typically confined to the caudal end as in manatees, and cetaceans (whales and dolphins). In contrast, non-mammalian tetrapods have a sprawled posture and emphasize lateral axial motions. Hence, secondarily aquatic non-mammalian tetrods undulate laterally (like fish), as seen in snakes, crocodiles, the siren and other aquatic amphibians, etc. These differences in axial undulation (dorsoventral or lateral) are not adaptive in any way but simply reflect the design of the animal prior to re-invading the water.