Module 5: Muscles

Question 1

What are muscles?

Answer 1

Biological machines that utilize chemical energy from breakdown and metabolism of food to perform useful work.

Question 2

What are the three kinds of muscle cells?

Answer 2

1) Skeletal muscle
2) Smooth muscle
3) Cardiac muscle

Question 3

Where is smooth and cardiac muscle found?

Answer 3

Smooth muscle is found within walls of blood vessels, airways, various ducts, urinary bladder, uterus, and the digestive tract. Cardiac muscle is found in the heart.

Question 4

How many different muscles does the body contain?

Answer 4

Over 600.

Question 5

What are the three principal functions these muscles perform?

Answer 5

1) Movement
2) Heat production
3) Body support and posture

Question 6

What is another name for skeletal muscle?

Answer 6

Striated muscle.

Question 7

Describe the breakdown of skeletal muscle.

Answer 7

Whole muscles are made up of bundles of fasciculi. Each fascicle is made up of groups of muscle cells or fibers. Each muscle cell contains many bundles of myofibrils. Each myofibril contains thin and thick myofilaments. Thin myofilaments contain mostly the protein actin along with troponin and tropomyosin. Thick myofilaments contain the protein myosin. Interaction of thin and thick myofilaments results in muscle contraction.

Question 8

What are groups of fasciculi surrounded by?

Answer 8

A white connective tissue called perimysium.

Question 9

How many nuclei do muscle cells (or fibers) have?

Answer 9

More than one.

Question 10

What are muscle cells surrounded by?

Answer 10

Sarcolemma, the muscle membrane. This is over which the action potential is transmitted.

Question 11

What are the small tube-like projections in the sarcolemma called? Where do they extend into? What is their function?

Answer 11

Transverse (T) tubules that extend down into the cell. They conduct action potential deep into the cell where the contractile proteins are located.

Question 12

What contains the contractile proteins?

Answer 12

Myofibrils.

Question 13

What are the contractile proteins?

Answer 13

Thin (actin) and thick (myosin) myofilaments.

Question 14

What surrounds the myofibrils? What does it contain?

Answer 14

A mesh-like network of tubes called sarcoplasmic reticulum (SR) that contains Ca2+ ions. These ions are essential for contraction.

Question 15

What is located at either end and continuous with the SR?

Answer 15

Terminal cisternae, a membranous enlargement of the SR. This is located close to the T tubule (where action potential travels).

Question 16

What are the thin myofilaments predominantly composed of?

Answer 16

Actin, a globular protein.

Question 17

What site does each actin molecule contain?

Answer 17

A special binding site for the other contractile protein, myosin.

Question 18

How is the backbone of thin myofilaments formed?

Answer 18

Many actin molecules are strung together and then twisted to form backbone.

Question 19

What are the long protein strands found on the thin myofilaments called?

Answer 19

Tropomyosin.

Question 20

What are the 3 subunits the regulatory protein, troponin, is made up of? What does each subunit bind to?

Answer 20

Troponin A (binds to actin), troponin T (binds to tropomyosin), and troponin C (binds to Ca2+).

Question 21

What does the troponin complex do at rest?

Answer 21

Holds the tropomyosin over the myosin binding sites.

Question 22

What happens when Ca2+ binds to the troponin C subunit?

Answer 22

Tropomyosin is pulled off the myosin binding sites by the troponin.

Question 23

What are the thick myofilaments made up of?

Answer 23

The protein myosin.

Question 24

What is the structure of myosin?

Answer 24

Myosin has a long, bendable tail and two heads.

Question 25

What sites do the heads of myosin have?

Answer 25

Each head has 1 actin binding site and 1 ATP binding site (ATP is split).

Question 26

What does the splitting of the ATP do?

Answer 26

Splitting of ATP releases energy to the myosin that powers the contraction of the muscle.

Question 27

How are the groups of myofilaments arranged along the length of the myofibril from one end of the muscle cell to the other?

Answer 27

In a repeating pattern (thin, thick, thin, thick, and so on).

Question 28

From what does each group of thin myofilaments extend outward from in opposite directions?

Answer 28

Z disk (or Z line), where they are anchored.

Question 29

From what does each group of thick myofilaments extend outward from?

Answer 29

M line, where they are attached.

Question 30

What is the region from one Z disk to another called?

Answer 30

Sarcomere. This is the smallest functional contractile unit of the muscle cell.

Question 31

What are the regions that contain thick filaments called? What about the regions that contain only thin filaments? How does each band appear.

Answer 31

A bands and I bands, respectively. A bands appear darker, while I bands appear lighter.

Question 32

What is the theory used to describe muscle contraction?

Answer 32

Sliding Filament Theory.

Question 33

How does muscle contraction occur?

Answer 33

When the head of a myosin molecule attaches to binding site on actin and forms a crossbridge, myosin undergoes a change in shape. The change in shape causes myosin head to swing, producing a power stroke. Power stroke slides actin past the myosin.

Question 34

Do the lengths of the thin or thick filaments shorten during muscle contraction?

Answer 34

No.

Question 35

What does excitation-contraction coupling refer to?

Answer 35

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

Question 36

Describe the process of excitation-contraction coupling.

Answer 36

Action potential generated at neuromuscular junction will spread out over sarcolemma and down T-tubules into the core of muscle cell. Action potential travels very close to sarcoplasmic reticulum (SR) and will open CA2+ channels, causing release of Ca2+ from terminal cisternae of SR. Ca2+ will bind to troponin C on thin myofilaments, causing tropomyosin to uncover myosin binding sites on actin. Myosin will now be able to attach to actin and power stroke will occur.

Question 37

Describe the process of the relaxation of muscle.

Answer 37

Once action potentials stop, Ca2+ will no longer diffuse out of SR. Special calcium pumps rapidly pump Ca2+ back into SR (up it’s concentration gradient; this requires ATP). Without Ca2+ present in cytoplasm of muscle cell, tropomyosin will cover myosin binding sites again. Myosin will be unable to bind to actin and power strokes will not occur.

Question 38

Describe the events that occur during a single crossbridge formation between actin and myosin.

Answer 38

1) Splitting of ATP to ADP and inorganic phosphate (Pi) releases energy to myosin and prepares myosin head for activity.
2) Formation of crossbridge when Ca2+ binds to troponin C. This rolls tropomyosin off myosin binding sites.
3) Power stroke occurs when myosin head bends and slides the thin myofilaments of actin over the thick myofilaments of myosin. ADP and Pi molecules are released from myosin head.
4) New ATP molecule binds to myosin heads.

Question 39

What causes rigor mortis?

Answer 39

Rigor mortis begins 3-4 hours after death and muscles become stiff for ~12 hours. Stiffness slowly disappears over next 24-48 hours.

Rigor mortis results from loss of ATP (dead cells do not produce ATP). Slow degradation of SR releases Ca2+. Myosin binds to actin, forming crossbridge between thin and thick myofilaments; this produces rigid conditions of the muscle. Without ATP, bonds cannot be broken until further cell degeneration (decomposition) dissolves linkage.

Question 40

What are two ways the muscle can alter the force of contraction?

Answer 40

1) Recruit motor units

2) Through summation of twitch contractions

Question 41

What is a motor unit?

Answer 41

A motor neuron and all of the muscle cells/fibers it causes to contract.

Question 42

How many motor neurons is each muscle cell innervated by?

Answer 42

Only one motor neuron, but each motor neuron innervates many muscle cells.

Question 43

Differentiate between a large motor unit and a small motor unit.

Answer 43

A large motor unit has a motor neuron in contact with a large number of muscle cells (up to ~200), while a small motor unit has a motor neuron only in contact with a few muscle cells.

Question 44

Example: You are holding a bucket while it fills up. The bucket gets heavier and heavier. What is taking place in your arm muscles?

Answer 44

When bucket was empty, only a few motor units (hence, only a few muscle cells) were contracting to hold bucket up. As bucket got heavier, it was necessary to activate more muscle cells. This required the activation of more motor units. This is motor unit recruitment.

Question 45

What is motor unit recruitment?

Answer 45

The progressive activation of motor units resulting is a more forceful contraction.

Question 46

What is a muscle twitch?

Answer 46

The simplest and smallest muscle contraction. This is the result of one action potential in the motor neuron. This single action potential will excite muscle cell and cause release of enough Ca2+ from SR to cause a very small contraction of muscle.

Question 47

What is the duration of a muscle twitch? Compare this to the duration of an action potential.

Answer 47

Duration of twitch can vary from 10 to 100 milliseconds. Duration of an action potential is ~2 milliseconds.

Question 48

What would happen if you stimulated the muscle with action potential, causing muscle twitch, and then stimulated it again before the muscle fully relaxed?

Answer 48

This would trigger another twitch and would add on top of the first one (tension of muscle would double). This can take place because the twitch is a type of mechanical event relying on protein interactions, so it can be added.

Therefore, you can increase the force of contraction by increasing number of action potentials per second (increasing frequency).

Question 49

What happens at high frequency of action potentials to the summation of twitch contractions?

Answer 49

This will produce a maximal tetanic contraction, a plateau in muscle tension.

Question 50

What is the equipment used to study muscle contractions?

Answer 50

1) Electronic stimulator that generates an electrical impulse that will cause the muscle contraction. Frequency of impulses can be adjusted using an events per second dial. Length of impulse can be adjusted using a duration dial. Strength of impulse can be adjusted using amplitude voltage dial.
2) Impulses will travel from stimulator through wires to electrodes on forearm of subject.

Question 51

What happens to muscle contractions as voltage (strength) is increased?

Answer 51

More motor units are recruited. This causes muscle contractions get stronger.

Question 52

What happens to muscle contractions as frequency of impulses is increased?

Answer 52

Summation of muscle twitches. Number of contractions per second increases. Increasing frequency also makes the muscle contractions stronger because they are being added on top of each other.

Question 53

What happens to muscle contractions if both voltage and frequency are increased?

Answer 53

Muscle contractions will be very strong because motor unit recruitment and summation of twitches are working together.

Question 54

Does the length of the muscle cell change during contraction?

Answer 54

Yes, the sliding of the thin and thick myofilaments brings Z lines closer together. Because this takes place throughout all the sarcomeres in every muscle cell, the whole muscle shortens during contraction.