lec 9- muscle and movement part 2

Question 1

what are the contractile structures in muscle fibre called?

Answer 1

myofibrils

Question 2

what are myofibrils made of?

Answer 2

sarcomeres

Question 3

what two proteins are sarcomeres made of?

Answer 3

Thin filaments- actin
thick filaments- myosin
the filaments overlap in the dark bands but not light bands

Question 4

what are thin filaments composed of and what are they bound too?

Answer 4

two coiled chains of globular actin, one end of each filament is bound to z disks which anchor the filament

Question 5

what are thick filaments composed of?

Answer 5

multiple strands of myosin

Question 6

what happens to the filaments during contraction?

Answer 6

the filaments slide past one another during contraction, the sarcomere shortens with no change in the thick and thin filaments themselves

Question 7

myosin has a pair of subunits that have?

Answer 7

tails that are coiled around one another and whose heads are bent to the side

Question 8

what does the head region of the myosin bind to?

Answer 8

the head region of myosin binds to the actin molecule, it catalyzes the hydrolysis of ATP

Question 9

what is the 4 step process of generating force by actin and myosin?

Answer 9

1. ATP binds to myosin, causing conformational change resulting in release from actin
2. ATP is hydrolyzed to ADP and inorganic phosphate causing myosin head to straighten and bind to new actin subunit down the filament, head is now cocked and ready for power stroke
3. inorganic phosphate is released, causing head to bend back to original conformation resulting in a power stroke that moves the entire thin filament relative to the thick filament
4. ADP is released, and the myosin filament is ready for binding to another ATP molecule

Question 10

what does more overlap of thick and thin filaments result in?

Answer 10

more contractile force

Question 11

what does overlap of thick and thin filaments depend on?

Answer 11

length of sarcomeres as measured by distance between z disks

Question 12

which length generates the most force?

Answer 12

intermediate length

Question 13

what is the structure of the sarcomere?

Answer 13

thick filament in center is surrounded by 6 thin filaments, each head is attached for a short time, thin and thick filaments overlap in two regions of sarcomere

Question 14

what initiates muscle contraction?

Answer 14

calcium ions

Question 15

what proteins mediate the initiation of contraction?

Answer 15

tropomyosin and troponin

Question 16

what occurs in relaxed muscles?

Answer 16

tropomyosin blocks the myosin binding sites on actin causing myosin-actin interaction to not occur, the muscle relaxes

Question 17

what occurs in initiation of contraction?

Answer 17

when intracellular Ca2+ increases and binds to troponin, it results in the troponin-tropomyosin complex to move and expose the myosin binding sites on actin, allowing myosin to bind

Question 18

what increases intracellular Ca2+ levels in muscle cells?

Answer 18

a signal from a motor protein

Question 19

what is the mechanism called that increases calcium levels in the muscle cell?

Answer 19

excitation-contraction coupling

Question 20

what part of the motor neuron interacts with the muscle cell?

Answer 20

the synapse, called the neuromuscular junction

Question 21

what occurs at the neuromuscular junction during initiation of contraction?

Answer 21

synaptic transmission results in depolarization of the muscle which spreads through T-tubules to the interior of the fibre

Question 22

what are the steps to initiation of contraction?

Answer 22

1. action potential triggers release of acetylcholine (ACh) from motor neuron to synaptic cleft
2. ACh diffuses across synaptic cleft and binds to ligand-gated ion channels, activation of ACh receptors allows sodium influx, depolarizing the muscle cell and causing an action potential to occur in muscle cell
3. action potential spreads and goes through T-tubules to enter interior of cell
4. Ca2+ channels in sarcoplasmic reticulum are opened due to action potentials in T-tubules, causing calcium to diffuse into cytoplasm where sarcomeres are located
5. calcium causes myosin binding sites to be exposed which enables contraction

Question 23

what are the steps in opening calcium channels?

Answer 23

1. AP depolarization activates DHPR in T-tubule membrane
2. and then DHPR activation results in opening of RyR which allows calcium flux into cytoplasm

Question 24

what are the two ways of calcium channels opening in muscle cells?

Answer 24

depolarization induces calcium release- skeletal muscle
DHPR triggers calcium release which triggers RYR to induce calcium release- cardiac muscle

Question 25

how does termination of contraction occur?

Answer 25

ACh neurotransmitter is diffused, digested or taken up. acetylcholinesterase digests ACh. calcium ATPase pumps calcium back into sarcoplasmic reticulum

Question 26

how do some chemical weapons attack the neuromuscular junction?

Answer 26

they inhibit acetylcholinesterase from digesting ACh which results in uncontrolled activation of ACh

Question 27

what is rigor mortis?

Answer 27

calcium leaks exposing myosin binding sites on actin, however due to death, ATP numbers are low causing myosin to not be released from actin, hence freezing of sarcomeres causing rigid muscles