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Physiology > Skeletal Muscle Physiology > Flashcards

Skeletal Muscle Physiology Flashcards

1
Q

Describe the cellular components of skeletal muscle.

A
  • Muscle fibres
    • Single cells
    • Multinucleated
    • Surrounded by the sarcolemma
  • Myofibrils
    • Contractile elements
    • Surrounded by the sarcoplasm
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2
Q

What is the role of titin molecules in skeletal muscle physiology?

A

They keep actin and myosin filaments in place

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3
Q

What lies in the extracellular spaces between myofibrils of skeletal muscle?

A

Sarcoplasmic reticulum

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4
Q

What is the trajectory of T tubules in skeletal muscle?

A

They lead from the exterior of the muscle fibre to the inside which is important for conducting the electrical signal to the centre of the muscle fibre.

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5
Q

What causes the striated pattern of skeletal muscle?

A

Regular organisation of the contractile proteins

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6
Q

What part of skeletal muscle is this?

A

A band

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7
Q

What part of skeletal muscle is this?

A

I band

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8
Q

What components make up the sarcomere?

A

The sarcomere is made up of a complete A band and the 2 halves of the I bands adjacent to it.

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9
Q

Describe the change in conformation of the sarcomere during skeletal muscle contraction.

A

The I band (light band) shortens in space during contraction and the 2 Z lines move closer to each other.

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10
Q

What results from the release of large quantities of calcium ions by the sarcoplasmic reticulum at the muscle membrane?

A

Attractive forces are initiated between the actin and myosin filaments, causing them to slide along each other. This it the contractile process.

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11
Q

What is the source of the energy used in muscle contraction?

A

The high energy bonds in ATP molecules which are degraded to ADP to liberate energy.

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12
Q

What are the molecular characteristics of myosin?

A
  • The head also functions as ATPase enzyme:
    • This allows the head to cleave ATP and use derived energy to energise contraction.
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13
Q

What are the molecular characteristics of actin?

A
  • In resting state, tropomyosin molecules lie on top of the active sites so attraction cannot occur between actin and myosin.
  • Troponin complex (3 loosely bound protein subunits) likely attaches the tropomyosin to actin.
  • Troponin has strong affinity for calcium ions and likely initiates the contraction process.
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14
Q

Describe the interaction of 1 myosin filament, 2 actin filaments and calcium ions during contraction.

A
  • Active sites on the normal actin filament of the relaxed muscle are inhibited or physically covered by the troponin-tropomyosin complex.
  • Therefore, the sites annot attach to the heads of the myosin filaments to cause contraction.
    • I.e. for contraction to take place, the inhibitory effect of the troponin-tropomyosin complex must itself be inhibited.
  • Large amounts of calcium ions inhibits the inhibitory effect.
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15
Q

Describe the ‘walk-along’ theory of contraction.

A

The heads of two cross-bridges attaching to and disengaging from active sites of an actin filament.

The heads of the cross-bridges bend back and forth and step-by-step walk along the actin filament, pulling the ends of two successive actin filaents toward the centre of the myosin filament.

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16
Q

What are the steps of the ‘walk-along’ muscle contraction?

A
  • Head attachment (of cross bridges) simultaneously causes changes in the intramolecular forces between the head and arm of its cross-bridge.
    • The new alignment of forces cause the head to tilt toward the arm and to drag the actin filament along with it - the power stroke.
  • Immediately after tilting, the head then automatically breaks away from the active site.
  • The head then returns to its extended direction. In this position, it combines with a new active site further down along the actin filament; the head then tilts again to cause a new power stroke, and the actin filament moves another step.
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17
Q

Describe the length tension relationship.

A

The amount of actin/myosin overlap determines tension developed by the contracting muscle.

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18
Q

What is active tension of skeletal muscle?

A

Tension increase resulting from contraction (decreases as muscle is stretched beyond its normal length).

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19
Q

ATP is used as the energy source for contraction of skeletal muscle. What are the sources of energy for rephosphorylation?

A
  • Phosphocreatine
  • Glycolysis of glycogen stored in muscle cells
  • Oxidative metabolism to liberate ATP
20
Q

Describe the role of phosphocreatine in muscle contraction.

A
  • Source of energy for rephosphorylation.
  • Has a high energy phosphate bond similar to ATP, with a slightly higher amount of free energy.
  • It is instantly cleaved. Released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP.
  • The total amount of phosphocreatine in muscle fibres is also small - ~5x as great as the ATP.
    • Combined energy of both the stored ATP and PCr in the muscle can cause maximal muscle contraction for only 5-8 seconds.
21
Q

Describe the role of glycolysis in muscle contraction.

A
  • Rapid enzymatic breakdown of glucose into glycogen.
  • Used to reconstitute both ATP and phosphocreatine.
  • Can occur in the absence of oxygen, therefore contraction can be for many seconds (up to a minute) even when oxygen delivery from blood is unavailable.
  • The rate of formation of ATP by glycolytic process is ~2.5x as rapid as ATP formation in response to cellular foodstuffs reacting with oxygen.
  • But, end products of glycolysis accumulate in muscle cells so glycolysis loses its capability to sustain maximal muscle contraction after ~1 minute.
22
Q

Describe the role of oxidative metabolism in muscle contraction.

A
  • Oxidative metabolism (combining oxygen with end products of glycolysis and other cellular foodstuffs) to liberate ATP.
  • >95% of all energy used by muscles for sustained, long-term contraction is derived this way.
  • Foodstuffs are carbohydrates, fats and proteins.
  • For extremely long-term maximal muscle activity (many hours) the greatest proportion of energy comes from fats, but for periods of ~2-4 hours, half the energy can come from stored CHOs.
23
Q

Describe isotonic muscle contration.

A

Shortening of muscle at a constant tension.

24
Q

Describe isometric muscle contraction.

A

Contraction of muscle at a constant length. The muscle does not shorten during contraction.

25
Q

What detemrines the speed of contraction of skeletal muscle?

A

Vmax of myosin ATPase.

Can be high Vmax or low Vmax (determines whether fast or slow muscle fibre). Most muscles contain both types of fibre, but proportions differ.

All fibres in a particular motor unit will be of the same type (fast or slow).

26
Q

Desribe the twitch contraction of fast white fibres.

A
  • High Vmax
  • Rapid cross bridge cycling
  • Rapid rate of shortening (fast fibre)
27
Q

Describe the twitch contraction of slow fibres.

A
  • Low Vmax
  • Slow cross bridge cycling
  • Slow rate of shortening (slow fibre)
28
Q

Describe slow fibres of skeletal muscle.

A
  • Oxidative
  • Small diameter
  • High myoglobin content
  • High capillary density
  • Many mitochondria
  • Low glycolytic enzyme content
  • Fatigue resistant
29
Q

Describe fast fibres of skeletal muscle.

A
  • Glycolytic
  • Large diameter
  • Low myoglobin content
  • Low capillary density
  • Few mitochondria
  • High glycolytic enzyme content
  • Fatigue easily
30
Q

What are the roles of fast, slow and intermediate fibres in muscle contraction?

A
  • In any muscle there is a mixture of slow and fast fibres.
  • Motor units containing slow fibres will be recruited first to power ‘normal’ contractions.
  • Fast fibres help out when partially forceful contraction is required.
  • Fast twitch and slow twitch fibres cannot remodel - they stay the same.
31
Q

Describe the innervation of a motor unit.

A

All muscle fibres in a motor unit are innervated by a single nerve fibre.

32
Q

What is summation with respect to muscle contraction?

A

Adding together of individual twitch contractions to increase intensity of overall muscle contraction.

33
Q

What are the characteristics of small motor units?

A
  • As few as 10 fibes per unit
  • Precise control
  • Rapid reacting
34
Q

Wht are the characteristics of large motor units?

A
  • As many as 1000 fibres per unit
  • Coarse control
  • Slow reacting
35
Q

What is force summation?

A

Increase in contraction intensity as a result of the additive effect of individual twitch contractions.

Can either be multiple fibre summation or frequency summation.

36
Q

Describe multiple fibre summation.

A

Increase in contraction intensity resulting from an increase in the number of motor units contracting simltaneously (fibre recruitment).

37
Q

Describe frequency summation.

A

Increase in contraction intensity resulting from an increase in the frequency of contraction of a single motor unit.

38
Q

Describe what happens during frequency summation of twitches and tetanus.

A
  • Myoplasmic Ca2+ falls (initiating relaxation) before development of maximal contractile force.
  • If the muscle is stiumlated before complete relaxation has occured, the new twitch will sum with the previous one.
  • If AP frequency is sufficiently high, the individual contractions are not resolved and a ‘fused tetanus’ contraction is recorded.
39
Q

Describe hypertrophy of skeletal muscle

A
  • Common
  • Caused by near maximal force development (weight lifting)
  • Increase in actin and myosin
  • Myofibrils split
  • Increased force generation
  • No change in shortening capacity or maximum velocity of contraction
40
Q

Describe hyperplasia of skeletal muscle

A
  • Rare
  • Formation of muscle fibres
  • Can be caused by endurance training
  • Increased force generation
  • No change in shortening capacity or maximum velocity of contraction
41
Q

Describe lengthening of skeletal muscle

A
  • Normal
  • Occurs with normal growth
  • No change in force development
  • Increased shortening capacity
  • Increased maximum contraction velocity
42
Q

What are the causes of atrophy of skeletal muscle?

A
  • Denervation / neuropathy
  • Tenotomy
  • Sedentary lifestyle
  • Plaster cast
  • Space flight (zero gravity)
43
Q

What happens to muscle performance following atrophy?

A
  • Degeneration of contractile proteins
  • Decreased maximal force of contraction
  • Descreased velocity of contracton
44
Q

Wht is excitation-contraction coupling?

A
  • A way to link muscle excitation (the depolarisation of the AP) to Ca2+ release from the sarcoplasmic reticulum.
45
Q

Describe the sequence of events in excitation-contraction coupling.

A
  • Action potential in T-tubule causing a conformational change in voltage-sensing dihydropyridine receptors.
  • Ca2+ release channels open in terminal cisternae of sarcoplasmic reticulum and allow Ca2+ to rapidly diffuse in sarcoplasmic reticulum and initiate muscle contraction.
  • With repolarisation, conformation change in DHP receptor closes Ca2+ release channels and Ca2+ is transported from sarcoplasm to sarcoplasmic reticulum by adenosine triphosphate - dependent calcium pump.

Physiology (10 decks)

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