Skeletal muscle & smooth muscle

Question 1

What determines the strength of a muscle contraction?

Answer 1

1. Muscle cross-sectional area
2. Number (and type) of fibers within the muscle that are stimulated to contract.
3. Frequency of muscle fiber stimulation
4. Thickness of each muscle fiber
5. Resting length of muscle fibers
6. Velocity of movement

Question 2

What determines the strength of a muscle contraction?

Answer 2

1. Number of muscle fibers stimulated to contract

2. Type of muscle fibers stimulated to contract.

Question 3

Define multiple motor unit summation.

Answer 3

Different numbers of motor units are brought into play to produce gradations of strength

Question 4

Define the size principle.

Answer 4

1. When a weak contraction is desired, smaller motor units are recruited.
2. When a stronger contraction is desired, progressively larger motor units are recruited.

**HOWEVER, this is reversed with electrical stimulation modality

Question 5

Msucle fiber type is determined by what?

Answer 5

By the motor neuron

Question 6

A given somatic motor neuron innervates muscle fibers of how many types?

Answer 6

ONLY ONE TYPE

Question 7

Which fibers are slow-twitch and what fibers are fast twitch?

Answer 7

1. Slow twitch = Type 1 fibers

2. Fast twitch = Type II fibers

Question 8

Differences in conduction velocity are due to.

Answer 8

1. Conduction velocity of motor neurons innervating muscle fibers.
2. Different myosin ATPase isoenzymes.

Question 9

Postural muscles have higher proportion of what type of muscle fibers?

Answer 9

Type I fibers because they are resistant to fatigue.

Question 10

Why are skeletal muscle multinucleate?

Answer 10

Because each muscle cell is formed from the union of several embryonic myoblast cells

Question 11

Describe the process of muscle contraction.

Answer 11

1. Globular head of myosin (ATPase) splits ATP to ADP and Pi BEFORE myosin can bind to actin
2. ADP and Pi remain bound to myosin until the myosin heads attach to actin
3. When cross bridges are formed (myosin binds to actin), Pi is released, causing a conformation change in myosin, resulting in a power stroke
4. Power stroke causes filaments to slide. ADP is released only when myosin binds to new ATP at the end of the power stroke
   * Release of ADP causes the cross bridge bond to break (myosin is released from actin).
5. ATP is hydrolyzed, causing myosin head to return to its original conformation.

Question 12

Contraction cycles must be repeated several times… why?

Answer 12

Because a single stroke of all the cross bridges would shorten a muscle only about ~1% of its resting length.

Question 13

Describe the motor end plate at NMJ.

Answer 13

A motor end plate is a specialized region of the sarcolemma at the NMJ with an increase of nACh receptors.

Binding of ACh to nACh receptors produces an end-plate potential. If this end-plate potential is sufficient, threshold occurs, and an action potential is produced.

Question 14

What is terminal cisternae?

Answer 14

It is where most calcium ion in relaxed muscle is stored within expanded portions of the SR

Question 15

Terminal cisternae are separated by what?

Answer 15

A narrow gap transverse tubules (T tubules)

Question 16

What happens when action potentials arrive in the T tubules?

Answer 16

SR releases calcium.

Question 17

Describe the structure of thins filaments.

Answer 17

1. F-actin is composed of globular subunits of actin called G-actin
2. F-actin is arranged in a double row & twisted to form a helix

Question 18

What is tropomyosin?

Answer 18

Protein that lies within the groove between the F-actin chains.

Question 19

What is troponin?

Answer 19

Complex of 3 proteins that is attached to tropomyosin.

Question 20

What is the role of calcium in muscle contraction?

Answer 20

1. In relaxed muscle, tropomyosin blocks the binding site on actin for the myosin head.
2. AP in muscle cell causes calcium release from the ST
3. Calcium binds to troponin
4. This triggers tropomyosin-troponin complex to move.
5. Active binding site on actin is now revealed.
6. Cross bridges can now bind to actin and begin power strokes.

Question 21

How does muscle relaxation occur?

Answer 21

1. ACh is broken down by ACh-ase within the NMJ.
   * *this enzyme breaks down ACh into acetate and choline.
2. ST stops releasing Calcium & IMMEDIATELY resequesters calcium that was just released.

Question 22

Drugs that INHIBIT ACh-ase do what?

Answer 22

It allows ACh to stay in the synaptic cleft LONGER

Question 23

What are the ADRs for drugs that inhibit ACh-ase?

Answer 23

DUMBBELSS

1. Diarrhea
2. Urination
3. Miosis
4. Bronchoconstriction
5. Bradycardia
6. Excitation (skeletal mm and CNS)
7. Lacrimation
8. Salivation
9. Sweating

Question 24

Define muscle fatigue.

Answer 24

The inability to maintain a particular muscle tension when a contraction is sustained.

Question 25

During a maximal contraction, fatigue is most likely due to what?

Answer 25

Most likely due to accumulation of extracellular potassium

Question 26

During moderate exercise, fatigue is most likely due to what?

Answer 26

It's MULTIFACTORIAL (metabolic, electrophysiological, and CNS changes)

Question 27

What does endurance training do to muscle adaptations to exercise?

Answer 27

It increases the lactate threshold, which means it prolongs the time until an increasing proportion of energy must be derived from anaerobic glycolysis