Mechanism Of Muscular Contraction

Question 1

organize the structure of muscle

Answer 1

Contractile protein filaments are tethered within myofibrils –> myofibers or cells–> fascicles –> muscles

Question 2

how does adult muscle growth occur

Answer 2

Adult muscle growth from some fusion of satellite cells into fibers, but mostly from synthesis of new myofibrillar proteins

Question 3

differentiate embryonic, neonatal and adult myogenesis

Answer 3

embryonic forms the primary myogeneis which fuse to form the synthesis of new myofibrillar proteins forming scaffolds

neonatal is responsible for establishing the stellate cell niche as well as adult fiber type specification

adult is where the BM is assembled and you get the formation of the NMJ

Question 4

what is the significance of the I band

Answer 4

the region where the thin filaments are not super imposed by the thick ones

Question 5

what protein is responsible for tethering the thick and thin filaments

Answer 5

titin

Question 6

which fibers (I or II) have the most myoglobin content?

which fibers are largest

which one are the most red in color

Answer 6

fiber type 1

fiber type II are twice as large

fiber 1 due to myoglobin (IIb stain the lightest)

Question 7

what kind of channel is the receptor on the motor end plate

Answer 7

NA/K pump which pumps Na in and K out

Question 8

which part of the NMJ does the esterase lie

Answer 8

motor end plate

Question 9

what antiporter is responsible for the activation of the ACH release

what symporter is responsible for the activation of the ACH release

Answer 9

Ca- ACH release from presynaptic terminal

NA on the presynaptic terminal

Question 10

what is the reason for latency phase in the electrochemical signaling of the NMJ

Answer 10

Latency due to time needed for Ca2+release from SR

Question 11

how does the requires stimulus to fire change with the fiber size

Answer 11

smaller fibers are from small motor units and they require less stimulus to fire…. also remeber that the fast twich fibers are larger

the integrated of multiple fiber types allows graded contractile response

Question 12

define fiber summation versus frequency summation

Answer 12

fiber summation Motor units may be recruited according to size, leading to stronger contraction

frequency summation Repeated stimulation at frequencies that do not allow time for sufficient calcium reuptake will produce stronger contraction with each stimulus (frequency summation) and result ultimately in a state of sustained contraction (tetanus/tetany)

Question 13

what allows muscles to maintain constant tension

Answer 13

alternate firing of the motor units

Question 14

describe the electrochemical basis for skeletal muscle contraction

Answer 14

depolarization activated the voltage-gated calcium channel, the dihydropyridine receptor (DHPR)–> brings in ca which then activates RYR1 (CSCR) on SR = release of ca into cytosole and into T tubule from SERCA = contraction

Question 15

what stores the SR calcium

Answer 15

SERCA

Question 16

What is the cascade of events that take place for a muscle to fire

Answer 16

Nerve AP

nerve release

muscle AP

muscle relase of Ca from SR

muscle contraction

Question 17

what does the interaction of sliding filaments produce

Answer 17

cross bridge cycling

Question 18

name something that helps the filaments in the myosin actin complex stay tethered in close proximity

Answer 18

myosin-binding protein C

Question 19

how does contraction occur on the molecular level

Answer 19

this cross bidge cycling is ATP dependent and Ca regulated

ATP binding decreases the affinity

1- ATP binds to myosin and reduces its affinity for actin, allowing release of the rigor state.

2- Once ATP binds, it is hydrolyzed to ADP and a phosphate, both of which remain bound to the myosin head.

3- Once calcium becomes available, it binds troponin C and causes the conformational shift in tropomyosin that will allow actin-myosin binding.

4- The weak initial binding of actin and myosin in the presence of calcium triggers release of the phosphate residue –> concomittent strong binding of actin/myosin and triggers a power stroke.

3- ADP is released during the power stroke as well, leaving the myosin bound once more to actin.

Removal of calcium leaves myosin and actin chains bounds together until the next ATP binds the myosin head. This completes the cycle, and means we have now described both electrochemical and physiochemical properties of skeletal muscle that allow the cross bridge cycling that we see as muscle contraction, and have completed the excitation contraction coupling that ties the electrical, chemical, and mechanical responses in skeletal muscle.

If no ATP is present, the bound myosin retains its high affinity for actin, and will remain locked to it in a state of rigor. This is not likely to happen in a living being, as ATP levels are tightly managed and ATP is thus available.