Lecture 16- How do muscles contract?

Question 1

What does the skeleton and muscles make up?

Answer 1

The musculoskeletal system

Question 2

What are embryonic muscle cells called?

Answer 2

Myoblasts

Question 3

What are muscle fibres?

Answer 3

Multinucleate muscle cells

Question 4

What do muscles consist of?

Answer 4

Muscle fibres bundled together by connective tissue

Question 5

What does a single muscle fiber consist of?

Answer 5

Many nucleus
Plasma membrane (sarcolemma)
Mitochondria
Myofibrils

Question 6

Muscle contraction is the result of interactions between which two contractile proteins?

Answer 6

Actin- thin filaments

Myosin- thick filaments

Question 7

What are myofibrils?

Answer 7

Bundles of actin and myosin filaments

Question 8

What are myosin filaments held in place by?

Answer 8

Titin

Question 9

What are the repeating units that make up a myofibril called?

Answer 9

Sacromeres

Question 10

What does titin connect to?

Answer 10

The full length of the sarcomere, from Z to Z line

Question 11

Where does resistance to stretch come from in relaxed muscle?

Answer 11

Titin

Question 12

What is anchored to the Z line, other than titin?

Answer 12

Actin

Question 13

What is the A band?

Answer 13

The band that contains all of the myosin filaments

Question 14

What is in the H zone?

Answer 14

Region where the actin and myosin does not overlap (myosin side)

Question 15

What is in the I zone?

Answer 15

Region of actin where myosin does not overlap

Question 16

What is the M band?

Answer 16

Contains a protein to help hold the myosin filaments in their regular arrangements

Question 17

What happens when muscles contract to the sarcomeres?

Answer 17

They shorten

Question 18

What happens to the H and I zone when muscles contract?

Answer 18

They shorten

Question 19

What happens to the z lines as a muscle contracts?

Answer 19

Move towards the A band (A band is where they overlap)

Question 20

What is the name of the theory of muscle contraction called?

Answer 20

Sliding filament theory

Question 21

What causes filaments to slide?

Answer 21

Actin-myosin interactions

Question 22

How is muscle contraction initiated?

Answer 22

Action potential from a motor neuron at the neuromuscular junction

Question 23

What is a motor unit?

Answer 23

All the muscle fibers activated by one motor neuron

Question 24

How does actin-myosin interactions cause muscles to slide past each other?

Answer 24

Myosin heads bind to specific sites on actin molecules to form cross bridges
Myosin changes conformation, actin filament slides 5-10 nm

Question 25

What may one muscle have?

Answer 25

Multiple motor units

Question 26

How is strength of muscle contraction increased?

Answer 26

- Increased rate of firing of motor neuron | - More motor neurons firing

Question 27

What does a myosin filament consist of?

Answer 27

Myosin molecules which consist of a linear polypeptide chain and a globular head

Question 28

What does an actin filament consist of?

Answer 28

Actin monomer, tropomyosin, troponin

Question 29

What does it mean that muscle cells are excitable?

Answer 29

The plasma membrane can conduct action potentials

Question 30

What molecule is released at the motor neuron to open ion channels in the motor end plate?

Answer 30

Acetylcholine- a neurotransmitter

Question 31

How do action potentials travel deep within muscle fibers?

Answer 31

T tubules

Question 32

What are T tubules?

Answer 32

Transverse tubules | Descend into the sarcoplasm (muscle fiber cytoplasm)

Question 33

What do T tubules run close to?

Answer 33

Sacroplasmic reticulum

Question 34

What is in the sarcoplasmic reticulum?

Answer 34

A high concentration of Ca2+ at rest | Ca2+ pumps

Question 35

What happens when an action potential reaches the receptor proteins?

Answer 35

The Ca2+ channels open, calcium ions flow out of sarcoplasmic reticulum, triggers actin-myosin interactions

Question 36

What is troponin?

Answer 36

A protein with three sub units: one binds actin, one binds myosin, one binds Ca2+

Question 37

What does tropomyosin do at rest?

Answer 37

Blocks binding sites on actin

Question 38

What happens to troponin when Ca2+ is released?

Answer 38

They bind, conformation of troponin change

Question 39

What happens once troponin has bound to Ca2+?

Answer 39

Tropomyosin and troponin bind, tropomyosin twists to expose actins active site

Question 40

How does contraction of a muscle stop?

Answer 40

Ca2+ pumps remove Ca2+ from the sarcoplasm, contraction stops

Question 41

How is cardiac muscle similar and different to skeletal muscle?

Answer 41

Also striated Cells are smaller and only have one nucleus Cells branch and interdigitate to withstand high pressures

Question 42

What provides mechanical adhesions between cardiac muscle cells?

Answer 42

Intercalated discs

Question 43

What initiates coordinated heart contractions?

Answer 43

Pacemaker, conducting cells

Question 44

What does it mean that the heart is myogenic?

Answer 44

It generates the heartbeat its self

Question 45

How else is heart rate determined?

Answer 45

The autonomic nervous system

Question 46

How do cardiac muscles contract?

Answer 46

DHP proteins in T tubules are Ca channels ryanodine receptors are ion gated calcium channels, sensitive to calcium ions Action potential causes Ca2+ to flow into sarcoplasm from T tubules, increase opens ca channels in sarcoplasmic reticulum, initiates contraction

Question 47

TL;DR, how do cardiac muscles contract?

Answer 47

Ca2+- induced Ca2+ release

Question 48

How are smooth muscle cells arranged?

Answer 48

In sheets, electrical contact is via gap junctions- action potential in one cell spreads to others in sheet

Question 49

What happens when smooth muscles are stretched?

Answer 49

Cells depolarise and fire action potentials to start contraction

Question 50

How does smooth muscle contract?

Answer 50

Ca2+ influx to sarcopasm stimulated by hormones, stretching or action potentials Ca2+ binds to calmodulin- activates myosin kinase which phosphorylates myosin heads- bind and release actin

Question 51

Explain how the hypothesis that stretch and parasympathetic stimulation induce contraction in gut smooth muscle can be tested experimentally.

Answer 51

Incubate a strip of smooth muscle in a saline bath | Measure action potentials and force of contraction.

Question 52

Explain the method of this experiment.

Answer 52

The muscle is anchored to a device which applies a stretch force An electrode detects membrane and action potentials which is recorded In experiment 2, acetylcholine or norepinephrine is dripped on

Question 53

What are the results of this experiment?

Answer 53

Stretching depolarises the membrane which causes action potentials which activates contractile membrane Neurotransmitters alter membrane resting potential to alter rate at which action potentials are fired

Question 54

What is the minimum unit of skeletal contraction?

Answer 54

A twitch

Question 55

What factors determine the force generated by an entire muscle?

Answer 55

- Number of activated motor units | - Frequency of firing of motor units