9-2. Muscle Contraction

Question 1

sliding filament theory

Answer 1

in a relaxed muscle fiber, the thin and thick filaments overlap only partially, but during contraction, the thin filaments slide past the thick ones towards the center of the sarcomere, which shortens

Question 2

What are the steps of the sliding filament theory?

Answer 2

1. cross bridge attachment - myosin heads (cross bridges) attach to active (myosin binding) sites on the actin > high energy position (“cocked”)
2. power stroke - myosin head swings forward, pulling the actin filament toward center of the sarcomere
3. cross bridge detachment - ATP binds to myosin head, enabling to to release the hold on actin and swing back to cocked position as ATP is hydrolized
4. cross bridge attaches to another active site further along the actin, and sequence repeats

Question 3

What is the result of the cross bridge attachment, power stroke, and detachment?

Answer 3

thin filaments pulled to center of sarcomere > Z lines pulled closer together > shortens > effect multiplied by thousands of sarcomeres in a single fiber

Question 4

A strongly contracted muscle may shorten up to ___ it’s resting length.

Answer 4

1/2

Question 5

energy for contraction

Answer 5

comes from ATP, which enables the cross bridges to detach and swing back to cocked position

Question 6

rigor

Answer 6

in the absence of ATP, cross bridges don’t detach and muscles become rigid

Question 7

rigor mortis

Answer 7

following death, ATP production stops along with all other metabolic activities. Cross bridges remain locked in place and all skeletal muscles become rigid

Question 8

Why don’t cross bridges form in a relaxed muscle?

Answer 8

myosin-binding sites on the actin are covered by the troponin-tropomyosin complex, preventing the cross bridges from binding.

covered because there is a low concentration of calcium ions in the sarcoplasm

Question 9

Where is calcium stored in a muscle?

Answer 9

sarcoplasmic reticulum

Question 10

What signals contraction?

Answer 10

stimulation by a nerve

Question 11

motor neuron

Answer 11

nerve cell that stimulates muscle tissue - as neuron enters muscle, it divides into many branches so that each muscle fiber gets its own branch

Question 12

neuromuscular junction

Answer 12

connection between a motor neuron and a muscle fiber

Question 13

synaptic cleft

Answer 13

tiny space between end of neuron and surface of muscle cell (region of sarcolemma called motor end plate)

Question 14

synapic vescile

Answer 14

when a nerve impulse reaches the end of the neuron, these small sacs in the end of the neuron release a chemical into the cleft by exocytosis

Question 15

What neurotransmitter is released by motor neurons?

Answer 15

acetylcholine (ACh)

Question 16

receptors

Answer 16

binding sites on the sarcolemma - when ACh binds, an electrical current travels along the surface of the muscle cell

Question 17

motor unit

Answer 17

a motor neuron and all the muscle fibers it stimulates. The average number of fibers per motor unit is 150

Question 18

Summarize the events in a muscle contraction

Answer 18

1. nerve impulse travels to the end of a motor neuron, causing the synaptic vesicles to release ACh, which diffuses across the synaptic cleft and binds to receptors on the sarcolemma. This generates a muscle impulse that travels along the sarcolemma and down the transverse tubules
2. impulse causes sarcoplasmic reticulum to release stored calcium ions
3. calcium ions bind to troponin, pulling the troponin-tropomyosin complex out of the blocking position on the actin and exposing myosin binding sites
4. cross bridges form as myosin heads attach to the actin on the binding sites
5. myosin heads swing forward, causing the thin filaments to slide past the thick filaments towards the center of the sarcomere
6. ATP causes cross bridges to detach and swing back into their cocked position as ATP is hydrolyzed. They then reattach to another binding site, and the process repeats.
7. Z lines are drawn closer together and sarcomere shortens. As the thousands of sarcomeres in a fiber shorten, the muscle contracts

Question 19

What enzyme breaks down ACh?

Answer 19

acetylcholinesterase - released by sarcolemma into cleft

Question 20

Summarize the steps of a contraction stopping

Answer 20

1. after the nerve impulse stops, the ACh in the cleft must be destroyed or it will continue to exite the muscle fiber
2. sarcolemma releases acetylcholinesterase into the cleft to break down the ACh
3. muscle impulse (electricity on sarcolemma) stops
4. calcium ions are actively transported back into the sarcoplasmic reticulum for storage
5. troponin resumes its original shape, and the T-T complex moves back to cover the myosin binding sites on the actin
6. with the cross bridges broken, the filaments slide back to their relaxed position and the sarcomeres return to their resting length (and the muscle relaxes)

Question 21

twitch

Answer 21

rapid, jerky response to a single threshold stimulus - contracts quickly, then relaxes

Question 22

What are the three phases of a twitch?

Answer 22

1. latent period (flat line - brief time between stiumulus and beginning of contraction)
2. contraction period (upward curve - as cross bridges are pulling thin filaments past thick and muscle is shortening)
3. relaxation period (downward curve - as muscle relaxes, Ca2+ is pumped back into the sarcoplasmic reticulum)

Question 23

tetanus

Answer 23

smooth, sustained contraction produced by a series of rapid stimuli - the second stimulus is applied before the muscle has relaxed from the first

Question 24

Most voluntary actions invovle short-tern ____ contractions

Answer 24

tetanic (resulting in smooth, coordinated movements)

Question 25

treppe

Answer 25

“staircase effect” - the same strength stimulus is repeatedly applied to a muscle, it produces a slightly stronger contraction each time (enzymes are speeding up with the heat)

Question 26

The principle behind warming up in athletics is based on ___ contractions

Answer 26

treppe

Question 27

isotonic contraction

Answer 27

movement without tension or resistance - thense ion in muscle remains constant as it shortens

Question 28

isometric contraction

Answer 28

tension increases during contraction, but the muscle neither shortens nor lengthens

Question 29

Too much resistance causes ____ contraction; no resistance causes ____ contraction.

Answer 29

isometric; isotonic

Question 30

muscle tone

Answer 30

slightly contracted state of skeletal muscles - doesn’t produce active movement, but keeps the muscles firm, healthy and ready to respond to stimulation