Skeletal Muscle Physiology 2

Question 1

Gradation of skeletal muscle tension depends on which 2 primary factors?

Answer 1

No. of muscle fibres contracting within the muscle

Tension developed by each contracting muscle fibre

Question 2

What do motor units allow?

Answer 2

Motor units allow simultaneous contraction of a no. of muscle fibres

Question 3

What is motor unit recruitment?

Answer 3

Stronger contraction could be achieved by stimulation of more motor units

Question 4

How is muscle fatigue prevented?

Answer 4

ASYNCHRONOUS motor unit recruitment during sub-maximal contractions helps prevent muscle fatigue

Question 5

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

Answer 5

Frequency of stimulation and summation of contractions

Length of muscle fibre at the onset of contraction

Thickness of muscle fibre

Question 6

What can contractions be made stronger?

Answer 6

In skeletal muscle, duration of a.p is much shorter than duration of resulting muscle twitch

Thus, it is possible to SUMMATE TWITCHES to bring about a stronger contraction through repetitive fast stimulation of skeletal muscle

Question 7

Describe what happens to stimulate consecutive single muscle twitches

Answer 7

If a muscle fibre is re-stimulated after it has completely relaxed, 2nd twitch is the same magnitude as the 1st

Single twitches produce little tension and are not useful for purposeful muscle activity

Question 8

Describe what happens in twitch summation

Answer 8

If a muscle fibre is re-stimulated before it has completely relaxed, the 2nd twitch is added on the 1st

Results in summation
and the 2nd twitch causes a greater muscle tension to develop

Question 9

Describe what happens in tetanus

Answer 9

If a muscle fibre is stimulated so rapidly that it does not have an opportunity to relax at all between stimuli, a maximal sustained contraction occurs

This continues until either stimulation ceases or fatigue begins

Question 10

Unique feature of cardiac muscle with relation to contractions?

Answer 10

CANNOT be tetanised - long refractory period prevents generation of tetanic contraction

Question 11

How is tension in skeletal muscle increased?

Answer 11

Increase frequency of stimulation to modulate the force of contraction

Question 12

How is maximal tetanic contraction achieved?

Answer 12

When the muscle is at its OPTIMAL LENGTH (lo) BEFORE the onset of contraction

i.e: developed tension depends on the initial length of skeletal muscle fibre

Question 13

…………..

Answer 13

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Question 14

What is the optimal length of a skeletal muscle fibre?

Answer 14

Its resting length is approx. the same as its optimal length

Question 15

How can skeletal muscle length tension relationship be explained?

Answer 15

Sliding filament mechanism

Question 16

How is skeletal muscle tension transmitted to bone?

Answer 16

Skeletal muscle tension - as a result of cross-bridge cycling (contractile component)- is transmitted to bone via stretching & tightening of muscle connective tissue and tendon (elastic component)

Question 17

2 types of skeletal muscle contraction and what they are used for?

Answer 17

Isotonic contraction - muscle TENSION remains CONSTANT as muscle length changes; used for:

1. Body movements
2. Moving objects

Isometric contraction - muscle tension develops at CONSTANT MUSCLE LENGTH; used for:

1. Supporting objects in fixed positions
2. Maintaining body posture

Question 18

Similarity between both isotonic and isometric contractions?

Answer 18

Muscle tension is transmitted to bone via the elastic components of muscle

Question 19

Relationship between velocity of muscle shortening and load?

Answer 19

Velocity of muscle shortening decreases as the load increases

When there is maximum load, there is 0 velocity of shortening, i.e: isometric contraction

Question 20

Differences between different types of skeletal muscle fibres?

Answer 20

• Enzymatic pathways for ATP synthesis
• Resistance to fatigue - muscle fibre with greater capacity to synthesise ATP are more resistant to fatigue
• Activity of myosin ATPase - determines the speed at which energy is made available for cross-bridge formation

Question 21

……………..

Answer 21

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Question 22

3 metabolic pathways that supply ATP in muscle fibres include?

Answer 22

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

Question 23

3 types of skeletal muscle fibres?

Answer 23

• Slow-oxidative (type I)
• Fast-oxidative (type IIa)
• Fast-glycolytic (type IIx)

Question 24

Uses of slow-oxidative type I fibres?

Answer 24

AKA slow-twitch fibres; these mainly use oxidative phophorylation (lots of ATP) but have low myosin ATPase activity (speed is low)

Mainly for prolonged, relatively low work aerobic activities, e.g: maintenance of posture, walking

Question 25

Uses of fast-oxidative type IIa fibres?

Answer 25

AKA intermediate-twitch fibres; produce lots of ATP and have high speed

Use BOTH AEROBIC and ANAEROBIC metabolism and are useful in prolonged, relatively moderate work activities, e.g: jogging

Question 26

Uses of fast-glycolytic type IIx fibres?

Answer 26

AKA fast-twitch fibers; produce little ATP but speed is high

Use ANAEROBIC metabolism and are mainly used for short-term high intensity activities, e.g: jumping

Question 27

What input do the motor nerves to skeletal muscles receive?

Answer 27

Excitatory or inhibitory input from the brain and also from a variety of receptors

Question 28

What is a reflex action?

Answer 28

Stereotyped response to a specific stimulus; they are the simplest form of coordinated movement

Question 29

What is the simplest monosynaptic spinal reflex?

Answer 29

Stretch reflex

Question 30

What is the stretch reflex?

Answer 30

Negative feedback that resists passive change in muscle length to maintain optimal resting length of muscle; this helps to maintain posture, e.g: while walking

Question 31

Steps in the stretch reflex?

Answer 31

1. Sensory receptor is the muscle spindle and is activated by muscle stretch
2. Stretching the muscle spindle increases firing in the afferent neurons
3. Afferent neurons synapse in the spinal cord with the α-motor neurons (efferent limb of the stretch reflex) that innervate the stretched muscle
4. Activation of the reflex results in contraction of stretched muscle

Question 32

Coordination of the stretch reflex?

Answer 32

Simultaneous relaxation of antagonist muscle

Question 33

Elicitation of stretch reflex?

Answer 33

Tapping the muscle tendon with a rubber hammer; this rapidly stretches the muscle,

E.: knee jerk reflex
(AKA patellar tension reflex) stretches the quadriceps femoris, causing its contraction

Question 34

Spinal segment and peripheral nerve stimulated by knee jerk reflex?

Answer 34

Spinal segment - L3, L4

Peripheral nerve - femoral nerve

Question 35

Spinal segment and peripheral nerve stimulated by ankle jerk reflex?

Answer 35

Spinal segment - S1, S2

Peripheral nerve - tibial nerve

Question 36

Spinal segment and peripheral nerve stimulated by biceps jerk reflex?

Answer 36

Spinal segment - C5-C6

Peripheral nerve - musculocutaneous nerve

Question 37

Spinal segment and peripheral nerve stimulated by biceps brachioradialis reflex?

Answer 37

Spinal segment - C5-C6

Peripheral nerve - radial nerve

Question 38

Spinal segment and peripheral nerve stimulated by triceps jerk reflex?

Answer 38

Spinal segment - C6-C7

Peripheral nerve - radial nerve

Question 39

What are ordinary muscle fibres also known as?

Answer 39

AKA extrafusal fibres

Question 40

What are muscle spindles?

Answer 40

AKA intrafusal fibres (run parallel to extrafusal fibres)

Specialised muscle fibres that have a sensory function and nerve endings called annulospiral fibres

Discharge from the spindle sensory ending is increased as the muscle is stretched; contraction of these fibres does not contribute to overall strength of contraction

Question 41

Motor supply to muscle spindles?

Answer 41

These neurons are called γ motor neurons and DO NOT stimulate contraction; rather, they adjust tension in the muscle spindles to maintain their sensitivity during muscle contraction

Question 42

4 causes of impairment of skeletal muscle function?

Answer 42

1. Intrinsic disease of muscle
2. Disease of the neuromuscular junction
3. Disease of lower motor neurons
4. Disruption of input to motor nerves, e.g: upper MND

Question 43

Genetically determined myopathies that can cause intrinsic muscle disease?

Answer 43

• Congenital myopathies - characteristic microscopic changes lead to reduced contractile ability of muscles
• Chronic degeneration of contractile elements, e.g: muscular dystrophy
• Abnormalities in muscle membrane ion channels, e.g: myotonia

Question 44

Acquired myopathies that can cause intrinsic muscle disease?

Answer 44

• Inflammatory myopathies, e.g: polymyositis, inclusion body myositis
• Non-inflammatory myopathies, e.g: fibromyalgia
• Endocrine myopathies, e.g: Cushing syndrome, thyroid disease
• Toxic myopathies, e.g: alcohol, statins

Question 45

Symptoms of muscle disease?

Answer 45

Muscle weakness/fatigue

Delayed relaxation after voluntary contraction (myotonia)

Muscle pain (myalgia)

Muscle stiffness

Question 46

Ix for neuromuscular disease?

Answer 46

Electromyography - differentiate primary muscle disease from muscle weakness caused by neurological disease

Nerve conduction studies

Muscle enzyme tests, e.g: creatine kinase for muscle damage

Inflammatory markers, e.g: CRP, plasma viscosity

Muscle biopsy