Chapter 8: Skeletal Muscle Structure and Function

Question 1

from bigger picture to smaller details, describe muscle structure

Answer 1

muscle —> fascicles —> muscle fibers (cells) —> myofibrils —> sarcomeres —> thick and thin filaments

Question 2

how many nuclei do skeletal muscle fibers have

Answer 2

many nuclei among the entire length of the muscle fiber

Question 3

where are subsarcolemmel (SS) mitochondria located? purpose?

Answer 3

directly beneath the cell membrane; produce cellular energy needed to maintain active transport of ions across sarcolemma (to establish a concentration gradient for action potentials)

Question 4

where are intermyofibrillar mitochondria (IMF) located? purpose?

Answer 4

located near the myofibrillar (contractile) proteins; provide energy needed to sustain muscle contraction

Question 5

purpose of satellite cells

Answer 5

increase number of nuclei in muscle fibers during muscle growth

Question 6

what does an increase in # of nuclei allow for muscle fibers?

Answer 6

greater protein synthesis, which is important for muscle growth in response to strength training

Question 7

non-endocrine functions of skeletal muscle

Answer 7

-force production for locomotion and breathing
-force production for postural support
-heat production during cold stress (shivering thermogenesis)

Question 8

endocrine functions of skeletal muscle

Answer 8

produce myokines and IL-6 during contraction, promote anti-inflammatory environment

Question 9

purpose of myokines

Answer 9

stimulate glucose uptake and fatty acid oxidation, promote blood vessel growth in muscle, promote liver glucose production and triglyceride breakdown

Question 10

purpose of IL-6 during exercise

Answer 10

anti-inflammatory effects which reduces chronic inflammation

Question 11

what is advantageous about an anti-inflammatory environment induced by exercise?

Answer 11

reduces chronic inflammation, reduces risk of heart disease, type 2 diabetes and certain cancers

Question 12

muscles that decrease joint angles

Answer 12

flexors

Question 13

muscles that increase joint angles

Answer 13

extensors

Question 14

describe static exercise

Answer 14

muscle exerts force without changing length (i.e. pulling against immovable object or postural muscles)

Question 15

describe concentric dynamic exercise

Answer 15

muscle shortens during force production

Question 16

describe eccentric dynamic exercise

Answer 16

muscle produces force but length increases, this is associated with muscle fiber injury and soreness

Question 17

define motor unit

Answer 17

motor neuron and all the muscle fibers it innervates

Question 18

role of somatic motor neurons of the PNS

Answer 18

carry neural messages from spinal cord to skeletal muscles

Question 19

define neuromuscular junction

Answer 19

junction between motor neuron and muscle fiber

Question 20

define motor end plate

Answer 20

pocket formed around motor neuron by the sarcolemma of the muscle fiber

Question 21

describe the sliding filament model

Answer 21

muscle shortening occurs due to the movement of the thin filament over the thick filament

Question 22

during muscle contraction, there is a reduction in distance between the __ lines of the sarcomere

Answer 22

Z lines

Question 23

5 stages of excitation-contraction coupling

Answer 23

1) action potential stimulates Ach release from alpha motor neuron at the neuromuscular junction
2) Ach induces an AP in muscle fiber; AP spreads down sarcolemma and T-tubules
3) AP stimulates Ca2+ release from sarcoplasmic reticulum
4) Ca2+ initiates tropomyosin displacement, revealing myosin-binding sites
5) myosin forms cross-bridges with actin, producing force

Question 24

7 stages of cross-bridge cycling

Answer 24

1) myosin binding sites on the actin molecule are covered in resting fibers
2) the AP releases calcium from SR
3) Ca2+ binds troponin; tropomyosin moves, revealing binding sites
4) cross bridge forms when myosin heads bind actin
5) myosin head pivots, moving actin, ADP and Pi dissociate from myosin head
6) myosin head releases actin when a fresh ATP binds
7) myosin head bends next actin; ATP is cleaved to ADP + Pi and the cycle repeats

Question 25

what is a muscle cramp

Answer 25

spasmodic, involuntary muscle contractions often associated with prolonged, high intensity exercise

Question 26

why does the electrolyte depletion and dehydration theory fall short in explaining muscle cramps?

Answer 26

dealing with electrolyte imbalance/dehydration is a full body phenomenon that doesn’t necessarily have to do with one single muscle, also stretching/massaging helps relieve cramps

Question 27

describe the altered neuromuscular control theory for muscle cramps

Answer 27

there is abnormal spinal reflex activity due to fatigue which results in increased excitatory activity of muscle spindles and reduced inhibitory effect of Golgi tendon organ

Question 28

role of muscle spindles, specifically in regards to muscle cramps

Answer 28

muscle spindles detect stretch of the muscle, then sensory neurons conduct APs to the spinal cord and synapse with alpha motor neurons, stimulation of alpha motor neurons causes the muscle to contract to resist being stretched to far (muscle spindle activity is increased during a muscle cramp)

Question 29

role of Golgi tendon organ, specifically with regard to muscle cramps

Answer 29

Golgi tendon organs detect tension applied to a tendon, sensory neurons conduct APs to spinal cord and synapse with inhibitory interneurons that synapse with alpha motor neurons, inhibition of the alpha motor neurons causes muscle relaxation, relieving tension (decreased Golgi tendon activity during a muscle cramp)

Question 30

what relieves muscle cramps and why does this work?

Answer 30

passive stretching, because it activates the Golgi tendon organ and inhibits motor neurons in the spinal cord, resulting in muscle relaxation

Question 31

type I fibers

Answer 31

slow-twitch fibers, slow-oxidative fibers

Question 32

type IIa fibers

Answer 32

intermediate fibers, fast-oxidative glycolytic fibers

Question 33

type IIx fibers

Answer 33

fast-twitch fibers, fast-glycolytic fibers

Question 34

3 biochemical properties differing between muscle fiber types

Answer 34

1) oxidative capacity 2) type of myosin ATPase isoform 3) abundance of contractile proteins

Question 35

what is meant by oxidative capacity? which muscle fiber type has the highest?

Answer 35

number of capillaries, mitochondria, and amount of myoglobin; type I has the highest oxidative capacity

Question 36

what does the type of myosin ATPase isoform determine? which muscle fiber type has the fastest isoform?

Answer 36

speed of ATP degradation; type IIx

Question 37

which muscle fiber type has the most abundance of contractile proteins?

Answer 37

type IIx

Question 38

4 contractile properties differing between muscle fiber types

Answer 38

1) speed of contraction (Vmax) 2) maximal force production 3) maximal power output 4) muscle fiber efficiency

Question 39

what is speed of contraction dependent on? which muscle fiber type has the fastest speed of contraction?

Answer 39

dependent on rate of SR releasing calcium and ATPase activity, type IIx has the fastest speed of contraction

Question 40

what is maximal force production dependent on? which fiber type has the highest max force production?

Answer 40

dependent on the force per unit of cross-sectional area; type IIx has the highest max force production

Question 41

how do men and women differ in terms of muscle mass?

Answer 41

men have significantly more skeletal muscle mass than women

Question 42

how do men and women differ in absolute force production?

Answer 42

men are stronger than women in terms of absolute force production

Question 43

how to men and women differ in terms of relative force production?

Answer 43

there is no apparent sex or age differences in fiber distribution, so men and women have the same relative force production

Question 44

what does maximal power output depend on? what type of muscle fibers have the highest maximal power output?

Answer 44

power = force x velocity of shortening type IIx

Question 45

what is muscle fiber efficiency dependent on? which muscle fiber type is the most efficient?

Answer 45

how much ATP is used to generate force (lower ATP, more efficient); type I are the most efficient

Question 46

power athletes such as sprinters generally have higher percentage of

Answer 46

fast fibers

Question 47

endurance athletes such as distance runners generally have a higher percentage of

Answer 47

slow fibers

Question 48

3 factors that determine the amount of force produced during a muscle contraction

Answer 48

1) types and number of motor units recruited 2) initial muscle length 3) nature of the neural stimulation of motor units

Question 49

describe Type S (slow) motor neurons

Answer 49

smallest, innervate type I muscle fibers

Question 50

describe Type FR (fast, fatigue resistant) motor neurons

Answer 50

innervate type IIa muscle fibers

Question 51

describe type FF (fast, fatigable) motor neurons

Answer 51

largest of the motor neurons, innervate type IIx muscle fibers

Question 52

describe the recruitment pattern of motor neurons during graded exercise

Answer 52

type S —> type FR —> type FF

Question 53

describe the muscle fiber type recruitment pattern

Answer 53

type I —> type IIa —> type IIx

Question 54

what is the size principle with regard to motor neuron recruitment?

Answer 54

progressive recruitment of motor units begins with the smallest motor unit and progressively gets to larger motor units

Question 55

how does increasing stimulus affect the motor units recruited?

Answer 55

recruits more motor units and thus produces more force

Question 56

describe the length-tension relationship in skeletal muscle

Answer 56

there is an “ideal” length for force generation which allows for maximal cross bridge formation; not too much overlap and not too little

Question 57

describe how frequency of stimulation affects the force produced in muscles

Answer 57

increasing frequency results in summation of simple twitches until tetanus is finally reached

Question 58

how can force production be additive?

Answer 58

increased Ca2+ availability, not enough time to fully resequester it within the sarcoplasmic reticulum

Question 59

what is delayed onset muscle soreness (DOMS) due to? what is it NOT due to?

Answer 59

microscopic tears in muscle fibers or connective tissue, not due to buildup of lactic acid

Question 60

timeline of DOMS

Answer 60

sarcomere damage (hours to days) —> immune cell infiltration (days) —> satellite cell activation (days to weeks)

Question 61

mechanism of DOMS

Answer 61

1) structural damage to muscle fibers 2) membrane damage 3) calcium leaks out of SR 4) protease activation- breakdown of cellular proteins 5) inflammatory response 6) edema and pain

Question 62

describe the repeated bout effect on DOMS

Answer 62

following recovery, another bout of the same exercise results in minimal injury (soreness)