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Human Physiology > Week 4: Skeletal Muscle > Flashcards

Week 4: Skeletal Muscle Flashcards

1
Q

what is the make up of a muscle

A

-> muscle
-> fascicles
-> muscle fibres
-> myofibrils
-> thick/thin filaments

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2
Q

what is a motor unit

A
  • provides interaction between the nervous system and the muscles
  • comprised of a motoneuron, axon and muscle fibres
  • action potentials initiate the contractions in muscle fibres
  • there ate different sizes of motoneurons depending on size and function of said muscle
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3
Q

what is a neuromuscular junction

A
  • where the motor units connect with muscles
  • synaptic cleft: where the neurotransmitters diffuse in order to produce an action potential on the muscle fibre
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4
Q

what in the process for muscle contraction

A
  • motor neurons action potential arrives at axon terminal
    • depolarises plasma membrane
  • opening Ca2+ channels
    • Ca2+ ions diffuse into axon terminal
    • Ca2+ binds to proteins
  • synaptic vesicles release Ach
  • Ach diffuses from axon terminal to motor end plate, binding to nicotinic receptors
  • binding of ach opens an ion channel
    • Na+ and K+ can pass through these channels (electrochemical gradient across plasma membrane means more Na+ moves in than K+ out)
  • local depolarisation of the motor end plate
  • muscle fibre action potentials initiated
  • propagation (end plate potential)
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5
Q

what is excitation-contraction coupling

A
  • the sequence of events by which an action potential in the plasma membrane activate the force generating mechanisms
  • an action potential in a skeletal muscle fibre lasts 1 to 2 ms and is over before signs of mechanical activity begin
  • mechanical activity following an action potential may last 100ms or more (depending on availability of intracellular Ca2+)
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6
Q

what is a sarcomere

A

it is made up of actin (thin) and myosin (thick) filaments

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7
Q

how is a binding site exposed

A

calcium exposes cross bridge binding sites allowing contractions to occur via action potentials travelling into the muscle

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8
Q

what occurs at a relaxed muscle

A
  • low Ca2+
  • cross bridge cannot bind with actin because tropomyosin is covering the binding site (troponin holds tropomyosin over binding site)
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9
Q

what occurs at an active muscle

A
  • high Ca2+
  • Ca2+ binds to troponin -> tropomyosin moves away from cross-bridge binding sites -> actin binds to cross bridge
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10
Q

what proteins help with muscle contraction

A
  • two proteins are responsible for linking the membrane action potential with calcium release in the cell
    • dihydropyridine (DPP) receptor (membrane)
    • ryanodine receptor (sarcoplasmic reticulum)
  • removal of Ca2+ from the cytosol requires energy
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11
Q

what is the sliding filament mechanism

A
  • shortening of the muscle is the result of certain parts of the actin and myosin filaments interacting with each other
  • actin filaments slide over myosin filaments
  • typically, muscle shortening involves one end of the muscle remaining at a fixed position while the other end shortens toward it
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12
Q

what is a cross-bridge cycle

A
  • energised myosin cross bridges on the thick filaments bind to actin
  • cross bridges binding triggers release of ATP hydrolysis products from myosin, producing angular movement
  • ATP bound to myosin, breaking link between actin and myosin -> cross bridge dissociate
  • ATP bound to myosin is split energising the myosin cross bridge
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13
Q

what is ATPase

A

an enzyme which determines the speed of ATP hydrolysis and resulting sarcomere shortening velocity

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14
Q

what is muscle tension

A

the force that a muscle exerts on the joint when it is contracting is called the tension of the muscle

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15
Q

what is muscle load

A

the force that is exerted on a muscle by an object is called the load of the muscle

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16
Q

how do muscle tension and load link to contractions

A

muscle tension must exceed the load in order for the muscle fibres to shorten and therefore move the object that is responsible for the load. If muscle tension does not exceed the load then the muscle will either remain at the same length or it will lengthen

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17
Q

what are the types of muscular contraction

A
  • concentric
  • eccentric
  • isometric
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18
Q

what is a concentric contraction

A
  • constant load
  • muscle shortens
  • tension > load
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19
Q

what is an eccentric contraction

A
  • muscle length increases
  • load > tension
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20
Q

what is an isometric contraction

A
  • constant muscle length
  • free object: load = tension
  • fixed object: load => tension
21
Q

what are single fibre contractions

A
  • the mechanical response of a muscle fibre to a single action potential is known as a twitch
  • after the action potential, there is a latent period (few ms) before the tension in the muscle fibre begins to increase
  • the time interval from the beginning of tension development (at the end of the latent period) to the peak tension is the contraction time
22
Q

what does an increasing load cause in a concentric contraction

A
  • the latent period will increase
  • the velocity of shortening will slow down
  • the total duration of the twitch will become shorter
  • the distance shortened to become less
23
Q

what is the load-velocity relationship

A
  • in the absence of a load, a shortening contraction reaches its maximum shortening velocity
  • when the load increases to the point where the muscle is not able to move it, then the contraction becomes isometric
  • when the load increases beyond the peak tension that a muscle can produce the contraction becomes lengthening (eccentric)
24
Q

what is the frequency-tension relationship

A
  • stimuli are separated by more time than total contraction time. all the tensions are similar
  • a stimulus is introduced halfway during the contraction time causing the muscle tension to rise further
  • two stimuli are introduced very close together resulting in a higher tension with a smooth curve
  • the increase of muscle tension from successive action potentials is called summation
  • when successive stimulations result in a sustained contraction, the contraction is called tetanus
  • the muscle fibre has time to partially relax before the next stimulation. the development of tension oscillates
  • the muscle fibre has no time to relax between stimulations. the development of tension is smooth and continuous
25
Q

what is the length-tension relationship

A
  • shorter muscle provides very little tension
  • optimum flexion e.g. elbow 90 degrees, has high tension
  • fully extended has little tension
26
Q

what are the fibre types differing characteristics

A
  • primary mechanism used to produce ATP
  • type of motor neuron innervation
  • type of myosin heavy chain expressed
27
Q

what are the different fibre types

A
  • IIx
    • fast twitch
    • fast glycolitic fibres
  • IIa
    • intermediate fibres
    • fast oxidative glycolytic fibres
  • I
    • slow twitch
    • slow oxidative fibres
28
Q

what is the spread like for the different fibre types

A
  • most muscles have a mixed composition
  • different fibres with different properties
  • referred to as ‘fast’ (II) and ‘slow’ (I) twitch
  • fast twitch equally sub divided (IIx and IIa)
    • on average 45-55% type I fibres in arm and leg muscles
    • no gender differences in fibre distribution
    • large intra individual variation
    • trend in distribution consistent across muscle groups
29
Q

how are muscle fibres categorised

A
  • muscle fibres are categorised based on how fast they contract and the metabolic pathways that they utilise to produce ATP
    • slow oxidative fibres
      • low ATPase activity, highly oxidative
    • fast oxidative fibres
      • high ATPase activity, highly oxidative / moderately glycolytic
    • fast glycolytic fibres
      • high ATPase activity, highly glycolytic
30
Q

what mechanisms are involved in muscle fatigue

A
  • conduction failure
    • caused by potassium accumulation in the T-tubules fast recovery
  • lactic acid buildup
    • acidic movement in muscle affects the physiological functioning of proteins and the mechanisms involved in calcium release and reuptake
  • inhibition of cross bridge cycling
    • accumulation of ADP and Pi in muscle fibres slows down the cross bridge cycling by preventing the release of cross bridges from actin molecules
  • fuel substrates
    • muscle glycogen, blood glucose, dehydration
  • central command fatigue
    • failure to propagate signals from the brain to the motor neurons
31
Q

what is VO2 max

A

maximal oxygen consumption (rate at which muscles consume oxygen)

32
Q

what techniques are there to determine muscle fibre type

A
  • colour of fibre
  • ENUG identification of motor units
  • fibre speed and oxidative capacity
  • myosin heavy chain isoform
  • genomic nomenclature
33
Q

what is a muscle biopsy

A
  • uses a thick needle to take out a small section of a persons muscle
34
Q

in what order are motor units recruited

A

motor units are recruited in order from smallest to largest

35
Q

what is the equation for increased force

A

an increase in force = an increase in discharge rate + an increase in recruitment + and increased contractile force

36
Q

what is the equation for increased strength

A

increased strength = increase in motor unit recruitment and firing frequency + increase in muscle mass

37
Q

what is hypertrophy

A
  • increase in muscle fibre size
  • due to the addition of contractile proteins in the muscle cell
  • protein synthesis > protein breakdown
  • depends on:
    • initial strength
    • duration of the training program
    • training technique
38
Q

what are the components of resistance training

A
  • time under tension
  • intensity
  • sets
  • repetitions
  • velocity
  • exercise order
  • recovery between sets
  • frequency
  • exercise type
39
Q

hypertrophy vs hyperplasia

A
  • increase in the number of muscle fibres
  • there is evidence of hyperplasia in animals
  • however, there is not enough evidence to support hyperplasia in humans
40
Q

what is satellite cell proliferation

A
  • satellite cell activation and proliferation
  • chemotaxis to injured fibre
  • fusion to damaged myofibre (hypertrophy)
  • alignment and fusion to produce new myofibres (hyperplasia)
  • regenerated myofiber with central nuclei
41
Q

what does mTOR stand for

A

mammalian target of rapamycin

42
Q

what does anabolism mean

A
  • muscle gain
  • rate of MPS > MPB
43
Q

what does catabolism mean

A
  • muscle loss
  • rate of MPS < MPB
44
Q

what does MPS stand for

A

muscle protein synthesis

45
Q

what does MPB stand for

A

muscle protein breakdown

46
Q

what does an increase in MPS cause

A
  • food (combined essential amino acids + insulin response)
  • exercise
47
Q

what does an increase in MPB cause

A
  • starvation
  • stress
  • injury/illness
  • exercise
48
Q

what does ACSA mean

A

area perpendicular to a muscles longitudinal axis

49
Q

what is the calculation for PCSA

A

(mass mass x cosine of the pennation angle) / (fibre length x muscle density)

Human Physiology (11 decks)

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