Lecture 9.3 main points

Question 1

briefly describe the events involved in initiating muscle conttraction

Answer 1

nerve impulse reaches axon terminal, voltage gated calcium channels open, Ach is released to the synaptic cleft, Ach binds to receptors in sarcolemma which opens ligand gated Na and K channels, end plate potential causes voltage gated Na channels to open causing depolarization, if threshold is reached an AP is activated causing propagation across the sarcolemma down into the T-Tubules. DHP receptors (proteins) in the t-tubules change shape and activate the ryanodine receptors causing a release of calcium from the SR into the sarcoplasm

Question 2

when intracellular (sarcoplasmic) calcium concentrations are low the muscle fiber is in a relaxed state, why is this

Answer 2

because tropomyosin blocks the active sites of myosin binding sites on the thin filaments. Myosin heads of the thick filament are required to bind to the thin filaments to cause contraction

Question 3

when sarcoplasmic calcium increases due to release of Ca from the SR, what does calcium bind to the in the sarcomere

Answer 3

calcium binds to troponin C of the troponin-tropomyosin complex

Question 4

what change occurs in the thin filament due to the binding of calcium and troponin C?
what can the thick filament do?

Answer 4

troponin changes shape and moves tropomyosin away from the myosin binding sites

myosin can then bind to actin and undergo power stroke causing sarcomere shortening

Question 5

what happens when nervous system (motor neuron) stimulation of the skeletal muscle ceases

Answer 5

no more calcium pumping into sarcoplasm therefore no Ach release and no AP stimulation

Question 6

what is the name of the SR pump that removes calcium from the sarcoplasm

Answer 6

SERCA pumps

Question 7

what are the names of the pumps in the sarcolemma that remove calcium from the sarcomplasm

Answer 7

Ca pumps

Question 8

how is the position of the troponin-tropomyosin complex affected by low sarcoplasmic calcium

Answer 8

low sarcoplasmic calcium causes the tropomyosin to block myosin binding sites on actin on thin filaments

Question 9

how is the position of the troponin-tropomyosin complex affected by high sarcoplasmic calcium

Answer 9

the troponin C binds to calcium and moves tropomyosin away from the myosin blocking sites

Question 10

define cross bridge formation

Answer 10

myosin heads bind to actin of thin filaments. Sliding of thin filaments with respect to the thick begins as power stroke of the myosin head takes place
cross bridges from and break several times ratcheting thin filaments toward the center of sarcomere
this causes shortening of the muscle fiber

Question 11

briefly describe the process of cross bridge cycling and the change in location of the z-discs relative to the M- line

Answer 11

z-disc is pulled toward the m-line. This causes the I bands to shorten, the Z discs move closer, the H zones disappear, A bands of adjacent sarcomeres move closer together but the A length stays the same

Question 12

what sarcoplasmic ion concentration allows for cross bridge cycling to continue in the contracting muscle cell

Answer 12

as long as sarcoplasmic Ca stays high and adequate ATP is present

Question 13

in order for cross bridging between a g-actin subunit of the thin filament and the myosin head of the thick filament to form. what two molecules must be bound to the myosin head

Answer 13

ADP and an inorganic phosphate

Question 14

define power stroke and what molecules are released in the process

Answer 14

when inorganic phosphate is released changing the shape of the myosin head this initiates the power stroke, then ADP is released
where the myosin head pivots and pulls thin filament toward the M line
release ADP and Pi in the process

Question 15

how does the myosin head release from the g-actin subunit of the thin filament post power stroke

Answer 15

ATP attaches to myosin head weakening the link between actin and myosin and cross bridge detaches

Question 16

how is the myosin re-energized post detachment

Answer 16

energy from the hydrolysis of ATP into ADP and inorganic phosphate cocks the myosin head into a high energy state

Question 17

what two molecules must be bound to myosin head for it to be in the high affinity state for the binding site on the g-actin subunit

Answer 17

inorganic phosphate and ADP

Question 18

what molecule initiates power-stroke when released from the myosin head

Answer 18

inorganic phsophate

Question 19

the release of which molecule completes the process of power-stroke of the myosin head

Answer 19

ADP

Question 20

which molecules must bind to the myosin head in order for it to release from its binding site on the g-actin subunit of the thin filament

Answer 20

ATP

Question 21

what must myosin do to ATP in order to reset back into high affinity state

Answer 21

hydrolyze ATP into ADP and Pi

Question 22

when sarcomeres shorten and elastic filaments are stretched what is produced

Answer 22

muscle tension

Question 23

what particular features relating to the position of the thin and thick filaments determine the tension a muscle fiber is capable of producing

Answer 23

dependent on the amount of overlap between the thin and thick filaments

Question 24

how is max tension affected if the muscle fiber is too short (under-stretched)? why is this?

Answer 24

there is too much overlap between thin and thick filaments, tension is decreased when the muscle is shortened, thin filaments begin to overlap

Question 25

how is max tension affected if the muscle fiber is too long (over-stretched)? why is this?

Answer 25

there is too little overlap between thin and thick filaments therefore not enough cross bridges can be formed causing tension to decrease the more a muscle is stretched

Question 26

define optimal resting length and why is produces max tension in the muscle fiber when contraction occurs

Answer 26

allows for maximum number of actin/myosin cross bridge formations, allows for max tension generated during contraction
-muscles rest at this length

Question 27

how long post expiration (after death) does rigor mortis begin to occur

Answer 27

3-4 hours post death, maximum stiffness around 12 hours post death, can take 48-60 hours to subside

Question 28

describe the process of rigor mortus as it relates to sarcoplasmic calcium control, decrease in available ATP, and cross- bridge cycling

Answer 28

when the heart stops, blood flow stops and oxygen/nutrient delivery is halted, therefore ATP production declines causing cell death
ATP is no longer available to bind to the myosin head for detachment of myosin head from active site on actin. ATP is no longer available to power the calcium pumps that remove calcium from the sarcoplasm into the terminal cisternae. This means that intracellular calcium stays high enough to allow the active sites on thin filaments to remain unblocked

Question 29

how can rigor mortis be used by law enforcement and medical professionals

Answer 29

can be used to determine time of death