Module 5

Question 1

Describe the breakdown (largest to smallest) of muscle structure

Answer 1

Whole muscles –> fasciculi, fascicle –> muscle cells/fibers –> myofibrils –> thin and thick myofilaments –> thin = actin, troponin and tropomyosin, thick = myosin

Question 2

Define sarcolemma

Answer 2

the muscle cell membrane surrounding the muscle cell, over which the action potential is transmitted

Question 3

Describe what transverse (T) tubules do

Answer 3

extend down the muscle cell, conducting the action potential deep into the cell to contractile proteins

Question 4

Describe the sarcoplasmic reticulum and its function

Answer 4

mesh-like network of tubes surrounding the myofibrils, stores Ca++ ions (essential for contraction)

Question 5

Describe terminal cisternae

Answer 5

membranous enlargement of the SR close to the T tubules, one on either side of T tubule; releases Ca++ during excitation-contraction coupling

Question 6

Describe the structure of thin myofilaments

Answer 6

mostly globular protein actin, with special binding site for myosin; also tropomyosin, covers actin binding site when muscle is at rest; also troponin with 3 subunits: A (binds actin), T (binds tropomyosin), and C (binds Ca++); structured like a double helix/bead necklace

Question 7

Describe the structure of thick myofilament

Answer 7

made up of myosin (long, bendable tail, two heads that can attach to myosin binding sites on actin) with two binding sites on each had, one for actin and the other to split ATP

Question 8

Define Z disk

Answer 8

where thin myofilaments are anchored, central disk where they fork in a Y shape

Question 9

Define M line

Answer 9

central line, thick myofilaments have an M line at their center and extend on either end, contraction pulls towards the M line

Question 10

Define sarcomere

Answer 10

the smallest functional contractile unit of a muscle cell; the region from one Z disk to another; during contraction, sarcomere shortens

Question 11

Define I band

Answer 11

region with only thin myofilaments, appears lighter, creates banded/striated look in muscle cell

Question 12

Define A band

Answer 12

region with thick myofilament, appears darker and creates banded/striated appearance; during contraction

Question 13

Describe the sliding filament theory

Answer 13

head of myosin binds to actin and forms a crossbridge; myosin undergoes shape change, swinging head and producing a power stroke, sliding actin past the myosin (neither filament shortens, but sarcomere does)

Question 14

Define excitation-contraction coupling

Answer 14

the process by which an action potential in the cell membrane (sarcolemma) excites the muscle cell to produce a muscle contraction

Question 15

Describe the events of excitation-contraction coupling

Answer 15

AP generated in NMJ spreads over sarcolemma and down T tubules to core muscle cells –> AP travels close to SR and opens Ca++ channels, releasing Ca++ from terminal cisternae –> Ca++ binds troponin C on thin myofilament, causing tropomyosin to reveal actin binding site for myosin, allowing binding and powerstroke

Question 16

Describe the relaxation of a muscle

Answer 16

AP stops, Ca++ no longer diffuses out of SR, special pumps bring Ca++ back into SR (up [ ] gradient, requires ATP), without Ca++ in cytoplasm of muscle cell, tropomyosin covers actin binding sites, so myosin cannot bind, no powerstroke, muscle relaxes

Question 17

Describe why rigor mortis occurs

Answer 17

no more ATP in muscle cell, myosin cannot release itself from actin, degradation of SR releases Ca++, allowing crossbridges to form, contracting muscle, but no ATP so cannot break bonds until cell degrades

Question 18

Name 4 functions of ATP in muscle contraction

Answer 18

Splitting of ATP positions myosin head to bind to actin, powers the power stroke by releasing ADP and Pi, causes crossbridge to dissociate by binding new ATP molecule, powers active transport of Ca++ back into SR

Question 19

Define motor unit

Answer 19

a motor neuron and all the muscle cells/fibers it causes to contract; one motor neuron will contact several muscle cells, but each muscle cell is innervated by only one motor neuron

Question 20

Define motor unit recruitment

Answer 20

the progressive activation of motor units resulting in a more forceful muscle contraction

Question 21

Define muscle twitch

Answer 21

simplest and smallest muscle contraction, the result of one action potential in the motor neuron; duration between 10 and 100ms

Question 22

Define latent period

Answer 22

the time delay from when the action potential occurred on the motor neuron and when the muscle contracted; caused by all the events at the NMJ, generation of AP on muscle cell, and interaction of myofilaments

Question 23

Define summation of twitch contraction

Answer 23

stimulating muscle to contract again before fully relaxed, triggering another twitch on top of the first, resulting tension in muscle will be doubled

Question 24

Describe how to increase the force of muscle contraction

Answer 24

increase number of APs per second (frequency) down motor neuron; twitches are mechanical events and can be summed (unlike APs)

Question 25

Define maximal tetanic contraction

Answer 25

at high frequencies, summation of individual contractions can produce a plateau in the muscle tension