Muscle & Skeletal System

Question 1

Muscles are made of ____, which are made of ____, which are made of ____.

Answer 1

muscle bundles
muscle fibers
myofibrils

Question 2

What are filaments?

Answer 2

long, narrow bundles of contractile proteins that interact with one another to cause muscle fibers to shorten

Question 3

What two filaments make up the sarcomere?

Answer 3

actin (thin filament)
myosin (thick filament)

Question 4

What is the sarcomere?

Answer 4

the functional unit of muscle

Question 5

What is the role of tropomyosin in the sarcomere?

Answer 5

blocks myosin binding sites between actin and myosin

Question 6

What is the role of troponin in the sarcomere?

Answer 6

moves tropomyosin away from mysoin binding sites, allowing cross bridges to form and muscle to contract

Question 7

When myofibrils stretch to longer lengths, the actin-myosin overlap ____.

Answer 7

decreases

Question 8

When myofibrils contract to shorter lengths, the actin-myosin overlap ____.

Answer 8

increases

Question 9

longer muscle length –> ____
shorter muscle length –> ____

Answer 9

relaxation
contraction

Question 10

What is the cross-bridge?

Answer 10

the binding of the myosin head to actin at a specific site between myosin and actin filaments

Question 11

What is the energy source of muscle contraction?

Answer 11

conversion of chemical energy released by ATP hydrolysis into force and kinetic energy

Question 12

What is the first stage of the cross-bridge cycle?

Answer 12

ATP is hydrolized into ADP and Pi, and myosin head is cocked back.

Question 13

What is the second stage of the cross-bridge cycle?

Answer 13

• troponin units on actin are bound by Ca2+ following a nerve impulse
• tropomyosin is moved away, which exposes the myosin-binding sites on actin
• myosin head releases Pi to bind to actin via myosin-binding sites
• cross-bridge is formed

Question 14

What is the third stage of the cross-bridge cycle?

Answer 14

the mysoin head and actin filament stimulate a power stroke via the sliding-filament model, and the muscle contracts

Question 15

What is the fourth stage of the cross-bridge cycle?

Answer 15

as the muscle contracts, ADP bound to the myosin head is released

Question 16

What is the fifth stage of the cross-bridge cycle?

Answer 16

ATP binds to the myosin head, which halts the contraction, and the myosin head detaches from actin

Question 17

What happens at the motor neuron in the neuromuscular junction?

Answer 17

1. action potential reaches the axon terminals of the motor neuron
2. motor neuron is depolarized, which opens its voltage-gated Ca2+ channels
3. Ca2+ facilitates the release of ACh neurotransmitters into the motor endplate

Question 18

What happens at the motor endplate and muscle cell in the neuromuscular junction?

Answer 18

1. ACh binds to ligand-gated Na+ channels on the muscle cell, which triggers the influx of Na+ ions into the muscle cell
2. This causes depolarization of the muscle cell
3. Depolarization causes the voltage-gated Ca2+ channels in the sarcoplasmic reticulum to open, releasing Ca2+ into the filaments
4. Ca2+ binds to troponin, which then moves away from tropomyosin to expose myosin binding sites
5. Cross bridge is formed

Question 19

How does muscle return to its relaxed state following a contraction?

Answer 19

1. AChE breaks down ACh
2. Ca2+ is transported back to the SR
3. tropomyosin blocks the myosin-binding sites on actin

Question 20

What is a twitch contraction?

Answer 20

a single action potential that only exerts one wave of force

Question 21

What is force summation?

Answer 21

when a second action potential arrives before the muscle has relaxed, causing a greater force to be produced

Question 22

Where does Ca2+ go during a twitch contraction?

Answer 22

muscle pumps Ca2+ back into the SR

Question 23

Where does Ca2+ go during force summation?

Answer 23

it remains within the myofibrils to activate more cross-bridges

Question 24

What is tetanus contraction?

Answer 24

muscle contraction of sustained force in which muscle force reaches a plateau at sufficiently high frequency

Question 25

What is slow-twitch?

Answer 25

found in muscles that contract slowly and consume ATP slowly

i.e. long distance runners

Question 26

What is fast-twitch?

Answer 26

generate force quickly, produce rapid movement, and consume ATP more quickly

i.e. sprinters

Question 27

Slow-twitch muscle fibers are the color ____ due to high ____.
Fast-twitch muscle fibers are the color ____ due to low ____.

Answer 27

red; myoglobin
white; myoglobin

Question 28

How does botox work?

Answer 28

prevents the release of ACh, which prevents muscle contraction and causes stiffness

Question 29

Excitotoxic shock causes the muscles to ____.
Flaccid paralysis causes the muscles to ____.

Answer 29

stiffen; relax