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Musculoskeletal > Skeletal Muscle Physiology II > Flashcards

Skeletal Muscle Physiology II Flashcards

1
Q

What two factors does gradation of skeletal muscle tension depend on?

A

The number of muscle fibres contracting within the muscle, tension developed by each contracting muscle fibre

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2
Q

What do motor units of muscle allow?

A

Simultaneous contraction of a number of muscle fibres

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3
Q

What is motor unit recruitment?

A

Stronger contraction is achieved by stimulation of more motor units

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4
Q

What does asynchronous motor unit recruitment do?

A

Helps prevent muscle fatigue when during submaximal contractions

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5
Q

What does the tension developed by each contracting muscle fibre depend on?

A

Frequency of stimulation, summation of contraction, length of muscle fibre at onset of contraction, thickness of muscle fibre

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6
Q

How long is the action potential of skeletal muscle in comparison to the resulting twitch?

A

The action potential is much shorter

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7
Q

What does having an action potential shorter than the resulting twitch allow skeletal muscle to do?

A

Allows summation of twitches to bring about a stronger contraction through repetitive fast stimulation of skeletal muscle

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8
Q

What is tetanus?

A

Maximal sustained contraction caused by muscle fibre being stimulated so rapidly that it doesn’t have time t relax

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9
Q

Why can cardiac muscle not be tetanised?

A

It is prevented by the long refractory period

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10
Q

What happens if a skeletal muscle is stimulated once?

A

A single contraction called a twitch is produced

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11
Q

Why are single twitches not very useful?

A

They produce little tension and aren’t useful in bringing about meaningful skeletal activity

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12
Q

What happens if skeletal muscle receives a second stimulation before it has time to completely relax?

A

The second response adds to the first and a greater muscle tension is developed

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13
Q

How does increasing the frequency of stimulation affect skeletal muscle tension?

A

It increases tension (important mechanism for modulating the force of contraction)

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14
Q

When can maximal tetanic contraction be achieved?

A

When the muscle is at optimal length before the onset of contraction

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15
Q

What does developed tension depend on?

A

Initial length of the skeletal muscle fibre

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16
Q

Why can maximal tetanic contraction be achieved when the muscle is at optimal length?

A

There is optimal overlap of myosin and actin cross bridge binding sites

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17
Q

What mechanism can be used to explain skeletal muscle tension?

A

Sliding filaments mechanism

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18
Q

What is the resting length of the skeletal muscle equal to?

A

Approximately its optimal length

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19
Q

What does skeletal muscle tension occur as a result of?

A

Cross-bridge cycling (contractile component of mechanism)

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20
Q

How is skeletal muscle tension transmitted to the bone?

A

Via the stretching and tightening of muscle, connective tissue and tendons (elastic component of mechanism), depends on whether muscle changes length

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21
Q

What are the two types of muscle contraction?

A

Isotonic contraction and Isometric contraction

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22
Q

What is isotonic contraction?

A

Used for body movements and for moving objects, muscle tension remains constant as the muscle length changes

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23
Q

What is isometric contraction?

A

Used for supporting objects in fixed positions and for maintaining body posture, muscle tension develops at constant muscle length

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24
Q

How is muscle tension transmitted to bone in both isotonic and isometric contraction?

A

Via the elastic component of muscle

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25
Q

What happens to the velocity of muscle shortening as the load increases?

A

Velocity of muscle shortening decreases

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26
Q

What are the main differences between different types of skeletal muscle fibres?

A

Enzymatic pathways for ATP synthesis
The resistance to fatigue (muscle fibres with greater capacity to synthesise ATP)
Activity of myosin ATPase

27
Q

What does the activity of myosin ATPase determine?

A

Speed at which energy is made available for cross bridge cycling (e.g speed of contraction)

28
Q

How many types of muscle fibre are present at one motor unit?

A

Only one type of muscle fibre at each unit

29
Q

What are the different metabolic pathways that supply ATP?

A

Transfer of high energy phosphate from creatine phosphate to ADP (immediate source of ATP)
Oxidative phosphorylation (main source when O2 is present)
Glycolysis (main source when O2 not present)

30
Q

What are the three types of skeletal muscle fibres?

A

Slow oxidative (type I) fibres, fast oxidative (type IIa) fibres, fast glycolytic (type IIx) fibres

31
Q

What are some features of slow oxidative muscle fibres?

A

Also known as slow-twitch fibres, are used mainly for prolonged relatively low work aerobic activities

32
Q

What are some features of fast oxidative muscle fibres?

A

Also known as immediate twitch fibres, use both aerobic and anaerobic metabolism, useful in prolonged relatively moderate work activities

33
Q

What are some features of fast glycolytic muscle fibres?

A

Also known as fast twitch fibres, use anaerobic metabolism, mainly used for short term high intensity activities

34
Q

How is purposeful skeletal muscle activity produced?

A

By input from a variety of sources that influence the activities of motor units

35
Q

Where do motor nerves receive input from to produce purposeful skeletal muscle activity?

A

Receive input from the brain and inputs to a variety of receptors (input can be either excitatory or inhibitory)

36
Q

What is a reflex action?

A

Stereotyped response to a specific stimulus, simplest form of co-ordinated movement

37
Q

What are the neural pathways of reflexes important in?

A

Localising lesions in the motor system

38
Q

Where are reflex pathways located?

A

Various levels of the motor system

39
Q

What is the stretch reflex?

A

Simplest monosynaptic spinal reflex

40
Q

What are the functions of the stretch reflex?

A

Serves as a negative feedback that resists passive change in muscle length of muscle, helps maintain posture

41
Q

What is the sensory receptor of the stretch reflex?

A

Muscle spindle = activated by muscle stretch

42
Q

What does stretching cause in the stretch reflex?

A

Stretching the muscle spindle increases firing in the afferent neurons

43
Q

Where do afferent neurons synapse in the stretch reflex?

A

Synapse in the spinal cord with the alpha motor neurons (efferent limb of stretch reflex) that innervate the stretched muscle

44
Q

What does activation of the stretch reflex cause?

A

Contraction of the stretched muscle

45
Q

What co-ordinates the stretch reflex?

A

Simultaneous relaxation of antagonist muscle

46
Q

How can you elicit the stretch response?

A

Tapping the muscle tendon with a rubber hammer = rapidly stretches the muscle resulting in its contraction

47
Q

What are some examples of tendon reflexes elicited by rubber hammer?

A

Knee jerk = L3 and L4, femoral nerves
Ankle jerk = S1 and S2, tibial nerves
Biceps jerk = C5 and C6, musculocutaneous nerve
Brachioradialis = C5 and C6, radial nerve
Triceps jerk = C6 and C7, radial nerve

48
Q

What are muscle spindles?

A

Sensory receptors for stretch reflex, collection of specialised muscle fibres

49
Q

What are muscle spindles known as?

A

Intrafusal fibres

50
Q

What are extrafusal fibres?

A

Ordinary muscle fibres

51
Q

Where are muscle spindles found?

A

Within the belly of muscles, run parallel to ordinary muscle fibres

52
Q

What are annulospiral fibres?

A

Sensory nerve endings of muscle spindles

53
Q

What happens when the muscle and muscle spindles are stretched?

A

Discharge from spindle sensory endings increases

54
Q

What supplies muscle spindles?

A

Efferent (motor) nerves, called gamma (g) motor neurons

55
Q

What do g-motor neurons do within muscle spindles?

A

Adjust the level of tension to maintain the spindle’s sensitivity when their muscle shorten during muscle contraction

56
Q

Does the contraction of intrafusal muscle fibres contribute to the overall strength of muscle contraction?

A

No

57
Q

What can cause impairment of skeletal muscle function?

A

Intrinsic disease of muscle, disease of NMJ, disease of lower motor neurons which supply the muscle, disruption of input to motor nerves

58
Q

What are some causes of intrinsic muscle diseases?

A

Genetically determined myopathies, acquired myopathies

59
Q

What are some features of genetically determined myopathies?

A

Characteristic microscopic changes leading to reduced contractile ability of muscles, chronic degeneration of contractile elements (muscular dystrophy), abnormalities in muscle membrane ion channels (myotonia)

60
Q

What are the kinds of acquired myopathies?

A

Inflammatory (polymyositis), non-inflammatory (fibromyalgia), endocrine (Cushing’s syndrome, thyroid), toxic (alcohol, statins)

61
Q

What are some symptoms of muscle diseases?

A

Muscle weakness/tiredness, delayed relaxation after voluntary contraction (myotoria), muscle pain (myalgia), muscle stiffness

62
Q

What are some useful investigations for neuromuscular diseases?

A 
Electromyography (EMG)
Nerve conduction studies
Muscle enzymes (e.g CK)
Inflammatory markers (CRP, plasma viscosity)
Muscle biopsy
63
Q

What are some features of an EMG?

A

Electrodes detect presence of muscular activity, records frequency and amplitude of muscle fibres action potentials, findings help differentiate primary muscle disease from muscle weakness caused by neuralgic disease, findings not pathognomonic of specific disease

64
Q

What do nerve conduction studies help to do?

A

Determine the functional integrity of peripheral nerves (done at same time as EMG)

Musculoskeletal (57 decks)

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