Exam 2- Skeletal Muscle Mechanics

Question 1

___ is the muscle’s response to a single stimulus

Answer 1

twitch

Question 2

describe the 3 components of a twitch

Answer 2

1- latent period (latency)- slight delay b/w when you give the stimulus and the muscle contracts- involves AP traveling down neuron axon, transmission of AP from terminal to muscle membrane, and initiation of contraction by AP on muscle membrane itself (latency very short, only a few ms)

2- contraction phase- contraction/tension

3- relaxation phase- goes back to normal

Question 3

when a muscle is responding to a series of stimuli, you get something called ___, which is the ___

Answer 3

tetany

the maximum sustained contraction of the muscle

Question 4

with tetany, the muscle is being stimulated so rapidly that…

Answer 4

instead of contract & relax, it contracts and stays contracted at its maximum strength

Question 5

what is the difference b/w muscle tension and load

Answer 5

tension: the force generated by a muscle contraction

load: the force of resistance exerted against a muscle (ex: weights; resistance of weight against gravity when walking up stairs)

Question 6

what are the 2 major types of muscle contraction and what defines them

Answer 6

isotonic and isometric

relationship b/w tension and load defines them

Question 7

describe isotonic muscle contraction

Answer 7

tension is greater than the load (lifting a 1 pound weight: muscles generate 1lb tension, muscles generate enough tension to overcome the resistance –> load is moved)

• once muscle tension exceeds the load, remains constant throughout the remainder of the contraction
• typical sliding filament mechanism: sarcomeres shorten, muscle contract, load is moved, then muscle realaxes, sarcomeres go back to normal length)

Question 8

what is the most typical form of muscle contraction

Answer 8

isotonic- tension greater than load

Question 9

describe isometric muscle contraction

Answer 9

load is greater than the max tension your muscles can generate (and can’t be moved or the muscle is held in a fixed position- either way load can’t be moved)

• as muscle tries to move the load, more and more cross-bridges form, instead of sarcomere/muscle shortening, muscle tension keeps increasing until it maxes out, sarcomeres do not really shorten
• sarcomeres stay ~ the same length, but muscle tension increases

Question 10

in isotonic contraction, __ shorten and __ remains the same

Answer 10

sarcomeres
tension

Question 11

at the cellular level, muscle contraction is ___, meaning every time you get an AP, get contraction of…

Answer 11

all-or-none
of the same strength

Question 12

although muscle membrane produce a typical all-or-none action potential, what happens with intact muscles?

Answer 12

intact muscles can respond with a wide range of graded responses (diff amt of tension) depending on the degree of resistance of the load

ex: if lifting weights, generate a lot of force vs. in surgery, generate small force

Question 13

muscles will generate graded responses to 2 types of stimuli:

Answer 13

1- change stimulus intensity –> graded response

2- change stimulus frequency –> graded response

Question 14

muscles generate a graded response to changes in stimulus intensity, describe motor unit recruitment/summation

Answer 14

a motor unit is one motor neuron and all the muscle fibers that it innervates, motor neuron has 1 axon but multiple terminals- so can activate multiple motor cells

• motor nerve is made of many motor neurons (not all axons have the same diameter in a single motor nerve- diameter determines speed of AP AND threshold voltage as well)- threshold voltage is directly proportional to axon diameter (small diameter have the lowest threshold and will be stimulated first)

Question 15

when talking about a muscles graded response to changing stimulus intensity, what directly proportional relationship is the basis of this?

Answer 15

axon diameter directly proportional to threshold voltage- small diameter have lowest threshold & are stimulated first

Question 16

describe how different stimulus intensities change a muscles contraction

Answer 16

• if have low stimulus intensity –> below threshold & no firing neurons –> no muscle contraction
• intensity reaches threshold –> threshold for only the smallest diameter neurons –> get their motor fibers contracting –> very small contraction b/c only a few have been activated
• increase stimulus more –> more motor neurons firing –> more muscles contracting –> more tension
• once reach max stimulus –> all neurons firing –> max tension

Question 17

the stimulation of more motor units leads to….

this depends on what?

Answer 17

a more forceful muscle contraction

the relation of the stimulus to threshold which activates diff motor units depending on axon diameter

Question 18

motor unit size and number depend on…

Answer 18

the type of muscle and its function

ex: large muscles (gastrocnemius) vs. small muscles (eye and larynx)

Question 19

describe how motor units size –> function changes in large and small muscles

Answer 19

in large muscles (gastrocnemius) –> large motor unit: 1 neuron which controls 2,000 muscle fibers –> but relatively few motor units in number
- these are adapted for burst speed and power (0-100 quickly), very coarse control of contraction

in small muscles that need precise and small movements (muscles of eye and larynx) –> small motor units: 1 neuron controls 15-20 muscle fibers –> relatively high number of motor units
- adapted for very fine, precise movements, very fine control of graded contractions

Question 20

graded responses of muscle contraction in response to changes in stimulus frequency, what property is the basis of this?

Answer 20

absolute refractory period (property of AP that corresponds to duration of the AP itself) –> will not generate another AP during absolute refractory, but this only lasts 1 ms, the muscle contraction itself lasts much longer, about 40-50 ms)

Question 21

changes in stimulus frequency have what specific graded responses due to absolute refractory period?

Answer 21

muscle membrane can be stimulated again during the 1st muscle contraction b/c membrane is out of absolute refractory period after 1 ms and is now responsive again –> causes a second contraction to occur before it has time to relax from the first –> so tension generated from 2nd adds on top of the residual tension remaining from 1st contraction

• if you stimulate faster and faster, the 3rd and 4th contractions add on top –> more and more tension as it is stimulated more (tension adds in a stair-step manner called treppe, (however it still has some time to relax, incomplete fusion)
• eventually, if u stimulate with high enough frequency so that muscle does not have any time to relax at all, complete fusion, reaches maximum sustained period of contraction called tetany (eventually, muscles will fatigue)

Question 22

we refer to tetany as tetany, not ___

Answer 22

tetanus

Question 23

as a part of the graded response a muscle contraction has in response to increase in stimulus frequency, adding each individual wave of contraction on top of each other is called ___

Answer 23

temporal summation/wave summation

Question 24

name the 4 metabolic fiber types of skeletal muscle

Answer 24

1- tonic fibers
2- slow phase (slow twitch) fibers
3- fast phase glycolytic fibers
4- fast phase oxidative fibers

Question 25

describe tonic fibers

Answer 25

very slow to contract, but can maintain the muscle contraction (tone) for a long time - more common in non-mammalian vertebrates, helps maintain posture (plants can stay shut, predators can hold their posture) - membranes are poorly excitable, so they are slow to contract - 1 AP coming to these membranes will not result in a twitch - has to have a series (train) of AP's from a motor neuron to cause a response- graded contraction - the membranes do not conduct AP's well and do not generate their own AP's, so need polysynaptic innervation- single muscle cell has to be innervated at many points in order to be stimulated/contracted - contract slowly b/c has type I ATPase (low turnover myosin ATPase) (small changes to an ATPase creates diff ATPase isoforms, type 1 ATPase is one of them) --> it splits ATP slowly --> as a result, the tonic fibers go through their contraction and reset cycle very slowly - b/c they use ATP slowly, they almost never run out, so they are very resistant to fatigue, hold posture a very long time)

Question 26

tonic fibers characterized as ____ to contract, but can ____ the muscle contraction/___ for a long time

Answer 26

slow maintain posture

Question 27

what ATPase do tonic fibers use

Answer 27

type I (low turnover myosin) ATPase- splits ATP slowly, so contract and reset slowly, however b/c splits ATP slowly, almost never runs out/fatigues, holds posture for long time

Question 28

slow phase are also called ___ fibers

Answer 28

slow twitch

Question 29

describe slow phase fibers

Answer 29

contract slowly & fatigue slowly - typical mammalian posture muscles- back and spine (ex: soleus in lower leg- helps maintain body posture during standing, walking, throwing) - have a relatively long sarcomere length which makes them slow, sarcomeres have to be shortened/tightened before they start generating force - have a reduced SR (their calcium delivery and uptake is slow)- contract & relax more slowly; irregular T-tubule system - typical excitable membrane- 1 AP will produce a twitch, but produces a slow twitch ~100 ms in duration - has type I (slow turnover myosin) ATPase- uses ATP slowly, once it's contracted, can stay contracted, resists fatigue - has high concentrations of myoglobin (oxygen transport molecule)- pulls oxygen out of blood and stored in muscle so during long periods of contraction, the muscle does not go anaerobic - myoglobin is red so this is called red muscle - have high blood flow at rest

Question 30

which fibers of skeletal muscle are called red muscle

Answer 30

slow phase/slow twitch fibers

Question 31

what ATPase does slow phase fibers use

Answer 31

type 1 (low turnover myosin) ATPase- uses ATP slowly, contracts slowly, resists fatigue

Question 32

slow phase fibers have high concentrations of ___, which makes them this color muscle, __

Answer 32

myoglobin red

Question 33

describe fast phase glycolytic fibers

Answer 33

- contract rapidly & fatigue quickly - adapted for speed & power: sprinters, weightlifters - short sarcomere length- as soon as stimulated, will generate tension - extensive SR system- rapid calcium cycles (deliver and pull out quickly) - well-developed T-tubule system, prominent triads - typically the largest diameter fibers- generate the most power - fast twitch (15 ms), type IIb (highest turnover myosin) ATPase - few myoglobin (fatigue quickly) few mitochondria --> white muscle - energy from glycolysis very inefficient - high levels of parvalbumin- helps make calcium cycles faster - high levels of creatine phosphate for energy reserve- can recycle ADP back to ATP quickly - not used for typical motor activity- only intense short bursts of speed or strength

Question 34

which fibers of skeletal muscle typically have the largest diameter, and thus have what effect

Answer 34

fast phase glycolytic generate the most power

Question 35

which fibers of skeletal muscle are called white muscle? and why

Answer 35

fast phase glycolytic b/c no myoglobin and no/few mitochondia

Question 36

fast phase glycolytic fibers have high levels of ___ which makes rapid calcium transients & high levels of ___ for energy reserve

Answer 36

parvalbumin creatine phosphate

Question 37

which ATPase does fast phase glycolytic fibers use?

Answer 37

type IIb highest turnover myosin ATPase

Question 38

describe fast phase oxidative fibers

Answer 38

- contract quickly, fatigue slowly - adapted for rapid, repetitive movements- marathon runners - 1 ATP generates a fast twitch - type IIa (high turnover myosin) ATPase - high density mitochondria (red/brown muscle) - energy from TCA and ox. phos. very efficient

Question 39

these fibers are for sprinters/weightlifters these are for marathoners

Answer 39

fast phase glycolytic fast phase oxidative

Question 40

which ATPase does fast phase oxidative fibers use

Answer 40

type IIa (high turnover myosin) ATPase

Question 41

fast phase oxidative fibers have this color muscle b/c of why

Answer 41

red/brown high density of mitochondria