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Physiology > L9 - Skeletal Muscle > Flashcards

L9 - Skeletal Muscle Flashcards

1
Q

What is the sarcolemma of a muscle fiber?

A

the cell membrane of a muscle fiber

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2
Q

How big are muscle fibers?

A

muscle fibers are huge by cell standards: ~10-100µm in diameter, and can be up to 30cm in length!

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3
Q

What makes muscle cells “excitable”?

A

the sarcolemma has similar voltage-gated channels to a neuron and can transmit action potentials, making skeletal muscle cells “excitable cells.”

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4
Q

What are T-tubules? What do they enable?

A

continuations of the sarcolemma which extend deep inside the muscle fiber (think of poking your finger into dough).
these enable the action potential in the sarcolemma to reach the inner workings of the fibre

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5
Q

What is the sarcoplasmic reticulum? What does it store at high concentrations through active transport?

A

a modified type of endoplasmic reticulum found exclusively in muscle cells
stores Ca2+

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6
Q

What are myofibrils? What happens to the muscle when the myofibrils shorten and how are the individual sarcomeres shortened?

A

long chains of proteins within each muscle fiber composed of repeating sarcomeres
the muscle contracts due to a shortening of these myofibrils as the thick and thin filaments slide across one another to shorten the individual sarcomeres

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7
Q

What are thick filaments composed of within the myofibrils and how do they shorten the muscle fiber?

A

myosin ATPase

use ATP to pull themselves along the thin filament and shorten the muscle fibre

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8
Q

What are thin filaments composed of within the myofibrils and how do they shorten the muscle fiber?

A

thin filaments are primarily composed of actin

the myosin proteins from the thick filaments bind to actin to pull themselves along and shorten the muscle fibre

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9
Q

What is a sarcomere and what do they do?

A

the basic unit of muscle

contract together to shorten the entire fibre

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10
Q

What is each myofibril composed of?

A

repeating sarcomeres lined up in a row

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11
Q

What does the A band contain?

A

the “anistropic” band contains the entire length of one set of thick filaments

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12
Q

What does the M line represent and what does it contain?

A

the mittelscheibe (german for “middle disc”) represents the middle of the sarcomere, and contains many cytoskeletal connective elements holding the sarcomere in place

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13
Q

What does the I band represent and how does the size of this band change?

A

the “isotropic” band represents the region of the thin filaments that are not overlapping with thick filaments
the size of this band changes as the sarcomere shortens and the region of overlap increases

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14
Q

What does the H zone represent and how does the size of this band change?

A

the heller zone (german for “brighter”), represents the region of thick filaments which do not overlap thin filaments
size changes as the sarcomere shortens or lengthens and the overlap between thick and thin filaments changes with it

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15
Q

What does the Z disk represent and what does it serve as?

A

the zwischenscheibe disk (german for “disk in between”) represents the segment between two adjoining sarcomeres
serves as an “anchor point” between sarcomeres as they shorten

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16
Q

Where are myofibrils found? What are they composed of and how do they shorten the individual sarcomeres?

A

within each muscle fibre are long chains of proteins called myofibrils, composed of repeating sarcomeres
the muscle contracts due to a shortening of these myofibrils as the thick and thin filaments slide across one another to shorten the individual sarcomeres

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17
Q

What is a motor unit defined as?

A

a single alpha motor neuron, and all the muscle fibres it innervates

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18
Q

Is one action potential in the alpha neuron guaranteed to to generate one action potential in the membrane (sarcolemma) of each muscle fiber that neuron innervates? Why?

A

yes, this is due to the nature of the neuromuscular junction

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19
Q

What will happen once the alpha motor neuron for an individual motor unit is recruited to fire an action potential?

A

all the fibres of that same motor unit will contract

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20
Q

What is a twitch?

A

the small muscle contraction resulting from a single action potential

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21
Q

What are the broad categories of muscle fiber types?

A

slow-twitch (Type I) and fast-twitch (Type II)

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22
Q

Do hands have more or less muscle fibers per motor unit in comparison to quadriceps?

A

less because this allows for greater precision of movement

23
Q

Which category of muscle fibers have a longer twitch duration? What is the twitch duration?

A

slow-twitch muscle fibres have a longer twitch duration than fast-twitch fibres
the amount of time between the commencement of twitch contraction and relaxation

24
Q

Which category of muscle fibers contract with a higher velocity?

A

fast-twitch fibers

25
Q

What are the characteristics of slow-twitch fibers?

A

higher oxidative capacity, are fuelled by more abundant capillary beds, and hence are far more fatigue-resistant than fast-twitch fibres

26
Q

What are the characteristics of fast-twitch fibers?

A

have a higher glycolytic capacity, and are more prone to fatigue

27
Q

How does a muscle contract with greater force? By contracting active motor units harder or recruiting more motor units?

A

recruiting more motor units

28
Q

Are slow-twitch or fast-twitch muscle fibers always recruited first?

A

slow-twitch muscle fibers

29
Q

At what point is the maximum contractile force determined?

A

when all motor units in that muscle have been recruited and are contracting

30
Q

Why can we pick up pencils all day but not buses?

A

because slow-twitch muscle fibers are very fatigue-resistant while fast-twitch muscle fibers are not

31
Q

Why must the “decision making” process of muscle fiber recruitment be made at the level of the alpha motor neurons?

A

because if we initiate an action potential in the alpha motor neuron, we are guaranteed action potentials, and therefore contractions in all of the muscle fibers that neuron innervates within the motor unit

32
Q

Which characteristic of alpha motor neurons leads to the activation of only a select group? How does this work?

A

the alpha neurons are different sizes, which varies their responsiveness to incoming EPSPs
this works because neurons with smaller cell bodies will be more sensitive/responsive to incoming EPSPs because those EPSPs are automatically initiated closer to the axon hillock

33
Q

What adjustment must the cortical neuron make to recruit 2, then all three of the motor units it innervates if some are different sizes?

A

increase the frequency of action potentials

34
Q

Which alpha-motor neuron must be innervating the slow-twitch muscle fibers? (the smallest one, the intermediate one or the largest one)

A

the smallest one because neurons with smaller cell bodies will be more sensitive/responsive to incoming EPSPs as those EPSPs are automatically initiated closer to the axon hillock

35
Q

What is the specialised graded potential called in the neuromuscular junction?

A

an end plate potential (EPP)

36
Q

How is the NMJ able to generate such a high-amplitude graded potential?

A

ACh is released in an extraordinarily high volume and there is an extraordinarily high density of nicotinic receptors waiting for it
this high number of nicotinic receptors interacting with the high volume of ACh leads to a much larger voltage change in the motor end plate compared with a normal CNS synapse

37
Q

What allows there to be such a high density of nicotinic receptors on the motor end plate?

A

the motor end plate contains grooves, or corrugations, which increase surface area to maximise the number of nicotinic receptors it can fit

38
Q

How do we ensure that the large volume of ACh released does not initiate more than one EPP?

A

an enzyme, acetylcholinesterase (AChE), is present within the NMJ to break down ACh immediately into acetate and choline

39
Q

Why does the action potential need to get into the middle of the cell?

A

an action potential needs to initiate a change deep within the fibre itself to cause a contraction

40
Q

How does the sarcolemma enable the action potential to get into the middle of the cell?

A

the sarcolemma contains invaginations called t-tubules, which are just continuations of the membrane which extend into the middle of the fibre
the action potential travels along the membrane, into these t-tubules, which are then connected to cellular machinery which initiate the contraction

41
Q

What are t-tubules coupled to? What does this structure do?

A

a structure called the sarcoplasmic reticulum, which surrounds the sarcomere, and stores a high concentration of calcium ions

42
Q

How are t-tubules and the sarcoplasmic reticulum connected?

A

connected mechanically by ryanodine receptors

43
Q

What are ryanodine receptors activated by and what does their activation lead to?

A

activated by the action potential travelling along the t-tubule -> conformational change -> Ca2+ release from the sarcoplasmic reticulum

44
Q

What encases the sarcomere and what is the sarcomere made up of?

A

encased by sarcoplasmic reticulum

the sarcomere is made up of overlapping thick (myosin) and thin (actin) filaments

45
Q

What are thin (actin) filaments anchored by?

A

anchored to disks called Z-disks

46
Q

What are the thin filaments surrounded by?

A

the thin filaments are surrounded by a long cable-like protein called tropomyosin, which is dotted with smaller proteins called troponin

47
Q

What does tropomyosin do in the absence of Ca2+?

A

blocks the binding sites on the actin filament, prohibiting myosin from binding to it

48
Q

What happens when Ca2+ is released from the sarcoplasmic reticulum?

A

Ca2+ binds to troponin, which causes tropomyosin to shift its position on the actin filament, revealing the myosin binding sites

49
Q

What happens when myosin binding sites are revealed?

A

the myosin head, using ATP as an energy source, begins to “ratchet” along the actin filament, pulling one overlapping filament over the other in a process called the crossbridge cycle

50
Q

What is the end point of the ratcheting process?

A

the end point of the ratcheting process is that the Z-disks are pulled closer together, causing a shortening of the sarcomere, hence the muscle fibre
this shortening is the muscle contraction

51
Q

How is the muscle able to relax after contraction?

A

the sarcoplasmic reticulum constantly takes the calcium back up using a Ca2+-ATPase pump
this Ca2+ reuptake enables the tropomyosin to slide back over the myosin binding sites, and the muscle fibre relaxes, ceasing the contraction

52
Q

What is tetanus?

A

when peak calcium concentration and peak contractile activity is reached within the muscle fibers

53
Q

What is summation?

A

when calcium concentration and contractile activity builds upon previous signals resulting in a larger contraction

54
Q

What determines twitch duration?

A

calcium reuptake speed

Physiology (33 decks)

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