Excitation Contraction Coupling

Question 1

what does the action potential depolarization cause at the terminal button?

Answer 1

voltage gated calcium channels open and the elevated levels cause vesicles of ACh to fuse with the membrane

Question 2

where does ACh from the motor neuron bind and what does this cause?

Answer 2

binds to channels in the muscle membrane and depolarizes the motor endplate with influx of Na and a minute amount of K efflux

Question 3

what is the consequence of depolarization at the motor endplate?

Answer 3

opens voltage gated sodium channels that form an action potential down the muscle cell

Question 4

how is ACh cleared from the synaptic cleft??

Answer 4

acetylcholinesterase inactivates the released acetylcholine.

Question 5

what is the pathology of Myasthenia Gravis? How can it be treated?

Answer 5

autoantibodies target nicotinic ACh receptors at the neuromuscular junction
edrophonium chlorie decreases inactivation of ACh and increases the availability at the synapse

Question 6

what are the symptoms of Myasthenia Gravis?

Answer 6

muscle fatigability that gets worse later in the day. hallmark symptoms are diploplia (double vision), ptosis and weakness of the smile

Question 7

where, in the excitation contraction pathway, can disease mechanisms inhibit?

Answer 7

at the motor neuron, (along the axon and in the cell body), at the sarcomere, at the neuromuscular junction and in the muscle.

Question 8

what is an example of a motor neuron disease?

Answer 8

ALS (motor neuron death in spine)

Question 9

what is an example of a demyelenating disease?

Answer 9

Guilain-Barre syndrome (autoimmune response against myelin)

Question 10

what is an example of a neuromuscular junction disease?

Answer 10

Myasthenia gravis (autoimmune response against ACh receptor)

Question 11

what is an example of a sarcomere disease?

Answer 11

malignant hyperthermia (mutation in Ryr1 causing excessive calcium release in muscle)

Question 12

what is an example of a muscle disease?

Answer 12

muscular dystrophy (reduced attachment to muscle membrane)

Question 13

how does depolarization cause calcium release in muscle?

Answer 13

the T tubule is depolarized and ryanodine receptors in the sarcoplasmic reticulum open calcium channels

Question 14

how much does Ca concentration increase after an action potential in a muscle cell?

Answer 14

from 10^-7 to 10^-5

Question 15

where is the internal store of calcium inside the muscle? what are the implications of this reserve?

Answer 15

the sarcoplasmic reticulum

the cell can contract without extracellular calcium

Question 16

what causes relaxation of the muscle cell?

Answer 16

calcium is transported back into the SR by ATPase ion pump

Question 17

what binds calcium in the SR?

Answer 17

calreticulin and calsequestrin

Question 18

what is the skeletal muscle triad composed of?

Answer 18

one T tubule (invagination of the cellular membrane) and 2 sarcoplasmic reticulum cisternae

Question 19

how are the cisternae attached to the T tubule in the muscle triad?

Answer 19

by direct coupling of L-type calcium channels in the T tubule membrane to ryanodine receptors embedded in the SR membrane

Question 20

what molecular changes does depolarization of the T tubule membrane cause?

Answer 20

change in the structure of the dihydropyridine receptor (L type Ca receptor) that opens a Ca channel gate in the ryanodine receptor in the SR membrane

Question 21

what is ryanodine? What is its effect at higher and lower concentrations?

Answer 21

plant alkaloid that binds to SR calcium release channels (ryanodine receptors)
at very small concentrations it opens the receptors
at higher concentrations it closes the receptors

Question 22

what is calcium induced calcium release? what effect does this cause?

Answer 22

ryanodine receptor is stiumated to open by the presence of cytoplasmic calcium
this causes calcium release along the entire SR, not just at the triad

Question 23

what is the major mechanism for calcium removal in the muscle cell?

Answer 23

SR Ca ATPase pumps Ca back into the SR which binds to calreticulin and calsequestrin

Question 24

what two mechanisms of calcium removal from the sarcoplasm extrudes calcium into the ECF?

Answer 24

plasma membrane Calcium ATPase that pumps one Ca from the cell in exchange for one ATP.
Na Ca exchanger lets 3 sodium into cell to remove one calcium

Question 25

what is the temporal relationship among the action potential, calcium transient and tension development in the fast twitch fiber?

Answer 25

less than 5 ms between the action potential and Ca release. shorter delay between calcium release and tension buildup but tension builds slowly (varies for different types of fibers)

Question 26

what is twitch duration directly related to in muscle cells?

Answer 26

calcium transient duration- becomes briefer in faster twitch fibers

Question 27

what type of summation is seen in muscle twitches? what effect does it have on tension?

Answer 27

muscle twitches show temporal summation that builds the tension over time

Question 28

what is tetanus? what is the difference between fused and unfused tetanus?

Answer 28

a maintained contraction in response to repetitive stimulation unfused tetanus is when the stimuli are far enough apart to cause the tension to oscillate and fused tetanus is caused when there is no time between oscillations for the tension to decrease

Question 29

what is the strength of contraction of skeletal muscle graded by?

Answer 29

rate coding (frequency of stimulation) and recruitment of additional motor units

Question 30

what is treppe? is it related to summation

Answer 30

the steady increase in tension in successive muscle twitches. unrelated to summation because each twitch relaxes back to zero before the next twitch

Question 31

what is a presumed cause for treppe?

Answer 31

Ca reuptake results in an increase in Ca concentration (increased crossbridge formation)

Question 32

what is a motor unit? how many motor neuron is each muscle cell innervated by?

Answer 32

one motor neuron and all myofibers innervated by it | only one

Question 33

how many muscle fibers are controlled by a single large motor unit? in the smallest motor unit?

Answer 33

2000 fibers vs 3

Question 34

what type of muscle fibers are in small and large motor units?

Answer 34

fast twitch fibers usually comprise large motor units and slow twitch muscle usually makes up small motor units.

Question 35

are large motor units typically recruited automatically? in what situation are only small motor units required?

Answer 35

no, thy are recruited after small motor units need more force. small motor units only when force requirement is low and control demands are high

Question 36

what types of fibers are slow twitch and what are their properties?

Answer 36

type 1 fibers. they are fatigue resistant, myoglobin rich, glycogen poor, use oxidative metabolism and have many mitochondria

Question 37

what are the two types of fast twitch fibers and how do they differ?

Answer 37

Type 2a and 2b fibers. 2a are more like slow twitch fibers with more glycogen. 2b fibers are very fast twitch, fatigable, and glycolytic in their metabolism

Question 38

what is the sequence of energy usage in muscle cells related to usage duration?

Answer 38

ATP, phosphocreatine, anaerobic glucose metabolism, oxidative metabolism of glucose and oxidative metabolism of other organic compounds

Question 39

how long does intramuscular ATP last during contraction? phosphocreatine?

Answer 39

1-2 seconds | 5-8 seconds

Question 40

how long does the muscle use anaerobic metabolism and what causes it to cease?

Answer 40

a minute | limited by lactic acid formation, not glucose amount

Question 41

how does phosphocreatine provide energy to the muscle cell?

Answer 41

by transferring a phosphate to ADP

Question 42

what causes muscle pain (burn)? what is this called in the heart?

Answer 42

anaerobic glycolysis producing lactic acid | in heart, called angina

Question 43

is lactic acid in muscle always bad?

Answer 43

no, it may have a role in regulating muscle excitability during extreme exertion

Question 44

what causes delayed onset muscle soreness?

Answer 44

muscle damage and possibly inflammatory response

Question 45

what is muscle fatigue and what causes it?

Answer 45

decline in tension as a result of previous activity | caused by buildup of lactic acid and phosphates (not due to low ATP)

Question 46

what is the adaptive advantage of muscle fatigue?

Answer 46

to prevent rigor that would result from a low ATP level

Question 47

what is high frequency and low frequency fatigue? what are the causes?

Answer 47

high- accompanies short duration high intensity exercise because of a failure of action potential. low- low intensity long duration exercise. due to buildup of lactic acid and phosphate (slow recovery)