Muscular System

Question 1

What happens when an ACh is releases at a NMJ of skeletal muscle (4 steps)

Answer 1

1) Net sodium entry triggers AP, 2) AP in T-tubule alters shape of DHP receptor, 3) DHP opens RyR Ca release channel in SR and Ca enters cytoplasm, 4) Power stroke/contraction-relaxation cycle

Question 1

What are the steps in the power stroke cycle

Answer 1

1) Rigor state: Myosin bound to actin at 45 degree angle, 2) ATP binds to myosin, causing it to dissociate from actin, 3) Myosin hydrolyzes ATP, leaving ADP bound to it, 4) Myosin head swigs to bind to new actin at 90 degree angle, 5) Resting state: Phosphate released to initiate power stroke, myosin head rotates and pushes actin past, 6) End of power stroke, myosin head releases ADP and resumes rigor state

Question 2

What does Ca bind to in skeletal muscle

Answer 2

Troponin

Question 3

What does tropomyosin do in skeletal muscle

Answer 3

Blocks myosin but rotates out of the way to expose the actin binding site

Question 4

What stops the contraction-relaxation cycle

Answer 4

ACh is broken down in synaptic cleft by acetylcholinesterase and Ca is put back into SR by Ca++ ATPase uniporter

Question 5

What are type I skeletal muscle fibers (7 characteristics)

Answer 5

Slow-oxidative: smallest, slowest, longest, fatigue resistant, least force, uses O2, dark red

Question 6

What are type IIa skeletal muscle fibers (6 characteristics)

Answer 6

Fast-twitch oxidative-glycolytic: Intermediate force and size, fast, fatigue resistant, short, glycolytic but can become more oxidative

Question 7

What are type IIb/x skeletal muscle fibers (7 characteristics)

Answer 7

Fast-twitch glycolytic: Fastest, largest, most force, short, easily fatigued, more anaerobic than IIa (convert pyruvate into lactose), pale

Question 8

What factors influence muscle force production

Answer 8

Sarcomere and fiber length (~2.1-2.2um), and summation/frequency of stimulation (activation level)

Question 9

What are single twitches

Answer 9

Muscle relaxes completely between stimuli

Question 10

What is summation

Answer 10

Stimuli close together that don’t allow muscles to relax fully

Question 11

What is unfused tetanus

Answer 11

Muscle tension oscillates around peak force

Question 12

What is fused tetanus

Answer 12

Muscle tension flatlines at peak force based on Ca release rate that saturates the cross bridge binding sites (all occupied)

Question 13

What is a motor unit

Answer 13

A somatic motor neuron and all the muscle fibers (same type) it innervates

Question 14

What are the characteristics of motor units

Answer 14

Vary in number of muscle fibers, all fibers are of the same type, force mainly determined by number of fibers

Question 15

How do muscles vary muscle force

Answer 15

NS uses frequency coding and twitch summation, as well as motor unit recruitment (smaller and slower recruited first to prevent fatigue)

Question 16

What is hypertrophy

Answer 16

Muscle fibers get bigger because they add more actin and myosin and thereby can produce more crossbridges

Question 17

What is atrophy

Answer 17

Muscle loss due to decreased use

Question 18

What is isotonic contraction

Answer 18

Any contraction where the muscle changes length, two kinds (concentric and eccentric)

Question 19

What is concentric contraction

Answer 19

When muscle force is greater than the load, the muscle shortens

Question 20

What is eccentric contraction

Answer 20

When muscle force is less than the load, the muscle lengthens (muscle tries to slow lengthening)

Question 21

What is an isometric contraction

Answer 21

The muscle contracts but doesn’t change length because the force is equal to the load

Question 22

What are the 3 muscle load-velocity relationships

Answer 22

Increasing the load decreases velocity of shortening, maximal velocity of concentric contraction when load=0, isometric contraction when load=muscle force

Question 23

What are characteristics of smooth muscle

Answer 23

No sarcomeres or striations, has actin and myosin, can contract on its own, under endocrine and autonomic control

Question 24

What initiates contraction in smooth muscle

Answer 24

Signal ligands (membrane receptors) or depolarization or stretch (membrane channels)

Question 25

What happens when Ca from the ECF binds to membrane receptors in smooth muscle

Answer 25

Increases IP3, activates IP3-R on SR to release calcium

Question 26

What happens in smooth muscle once Ca is released from the SR (4 things)

Answer 26

1) Binds to calmodulin (CaM), 2) activates myosin light chain kinase (MLCK) 3) Phosphorylates light chains in myosin heads and increases myosin ATPase activity, 4) Active myosin cross bridges slide along actin to create muscle tension

Question 27

How does smooth muscle relax

Answer 27

Myosin light chain phosphatase or Ca removed by ATPase or NA+ antiporters

Question 28

What are the 6 characteristics of cardiac muscle

Answer 28

Only in the heart, striated, short, branched, and electrically-linked (in intercalated disks), contract spontaneously, influenced by endocrine and autonomic systems, EC coupling like skeletal muscle

Question 29

What is the the pathway of cardiac EC coupling (7 steps)

Answer 29

1) AP enters from adjacent cell, 2) Voltage-gated Ca channels open and Ca enters, 3) Ca induces Ca release through RyR channels, 4) Local release sparks Ca, 5) Summed Ca sparks create Ca signal, 6) Ca binds to troponin to start contraction, 7) Relaxation when Ca unbinds and is pumped back into SR for storage (also exchanged with Na by NCX antiporter)

Question 30

How are smooth muscle fibers organized

Answer 30

In oblique bundles

Question 31

Which muscle fibers are the fastest at contracting

Answer 31

Skeletal, then cardiac, then smooth