Lecture Set 19

Question 1

Describe the structure of muscle/levels of organization

Answer 1

muscle is made up of muscle fibers. Each muscle fiber is made up of myofibrils. Each myofibril is made of sarcomeres

Question 2

Describe the formation of myofibrils during embryogenesis

Answer 2

myoblasts are dividing, until expression of a muscle-specific gene required for terminal differentiation. The myoblasts fuse through cell-to-cell adhesion proteins

After that, the muscle cells don’t ever divide. Cell mass added through recruitment of more myoblasts, individual nuclei can be added or lost to fibers. Satellite cells = muscle stem cells and provide self-renewing source of myoblasts

Question 3

Describe the structure of the sarcomere

Answer 3

A band –> length of myosin fibers
H zone –> area of myosin not overlapped by actin filaments
I band –> area of actin filaments not overlapped by myosin fibers
M line –> connection point between myosin fibers, contains myomesin
Z line –> defines sarcomere

Question 4

Describe the structure of myosin

Answer 4

Has 2 heavy chains, each chain has a head. Each chain has 2 light chains wrapped around the neck to provide structural support

Question 5

Describe the different proteins

Answer 5

myomesin –> connects two strands of myosin
troponin –> binds to tropomyosin, have regulatory roles
cap Z –> caps actin at + end
tropomodulin –> caps actin at - end
titin –> prevent muscle from tearing, within myosin filament and attaches to Z disc
nebulin –> binds to actin, binds actin to Z disc

Question 6

Describe the polarity of actin filaments

Answer 6

Have + end at the Z line

Question 7

Describe the sliding filament model

Answer 7

As muscle contracts, myosin heads walk towards the + end of actin (towards z disc) and this causes increased overlap between filaments leading to contraction

Question 8

Describe the contraction cycle

Answer 8

ATP binds to myosin in place of ADP, causes relaxation of myosin to low-energy configuration. Hydrolysis = move to high-energy configuration. Loss of Pi = tightly bound to actin, loss of ADP = power stroke back to low energy conformation

Question 9

How does Ca2+ play a role in contraction?

Answer 9

tropomyosin covers up the myosin binding sites on actin. When Ca binds to troponin, it pulls the tropomyosin away so that myosin can bind. Ca released through action potential that runs down T-tubule and activates voltage-activated Ca channel, also release through sarcoplasmic reticulum

Question 10

Describe some differences between skeletal muscle, cardiac muscle, and smooth muscls

Answer 10

Cardiac muscle doesn’t have sarcomeres, they have intercalated discs. Contraction modulated by electrical signals send from SA node, depolarization

Smooth muscle doesn’t have sarcomeres, no multinucleation. They have dense bodies wherever actin filaments connect, no striations, different contraction