MSK 4 - Skeletal Muscle

Question 1

What are the three types of muscle?

Answer 1

• skeletal (striated)
• cardiac (striated)
• smooth (non-striated)

Question 2

Give some characteristics of skeletal muscle?

Answer 2

• multinucleated
• fused cells
• attached to skeleton
• voluntary

Question 3

Give some characteristics of cardiac muscle

Answer 3

• branched uninucleated
• heart only
• intercalated discs
• involuntary

Question 4

Give some characteristics of smooth muscle

Answer 4

• distance cells
• spindle shaped
• wall of internal organs
• involuntary

Question 5

What are the functions of skeletal muscle?

Answer 5

• movement
• posture
• joint stability
• heat generation

Question 6

What is fasciculation?

Answer 6

Small, local, involuntary muscle contractions and relaxation. May be visible under skin.

Question 7

What attaches a muscle to a bone?

Answer 7

Tendon

Question 8

What is the function of circular muscles?

Answer 8

They act as sphincters to adjust opening.

Question 9

How are circular muscles arranged?

Answer 9

They have concentric fibres, and attach to skin, ligaments and fascia rather than bone. Eg. orbicularis oris, which is around the mouth

Question 10

What are the three main categories of parallel muscles?

Answer 10

• Strap (fibres run longitudinally to contraction direction) eg. Sartorius
• Fusiform (wider and cylindrical shaped at centre) eg. Biceps brachii
• Fan-shaped (fibres converge at one end and spread over) eg. Pectoralis major

Question 11

What is a pennate muscle?

Answer 11

One or more aponeuroses run through the muscle body from the tendon

Question 12

Give an example of a multipennate muscle

Answer 12

Deltoid (this has central tendon branches)

Question 13

What is the origin of a muscle?

Answer 13

A point on bone, typically proximal, which has greater mass and is more stable during contraction than the muscle’s insertion

Question 14

What is a muscle’s insertion?

Answer 14

This is the structure (bone, tendon or connective tissue) that the muscle attaches to. Usually distal and moved by contraction.

Question 15

What are compartments in limbs?

Answer 15

Limbs are divided into compartments by fascia - eg. The lower leg has four compartments

Question 16

What is compartment syndrome?

Answer 16

When trauma in one compartment causes internal bleeding which exerts pressure on blood vessels and nerves.

Question 17

What are the symptoms of compartment syndrome?

Answer 17

• deep constant, poorly localised pain
• aggravated by passive stretch of muscle group
• paresthesia (pins and needles)
• compartment feels tense/firm
• swollen shiny skin with bruising
• prolonged capillary refill time

Question 18

How is compartment syndrome treated?

Answer 18

Fasciotomy, which can be covered by a skin graft

Question 19

What are agonist muscles?

Answer 19

‘Prime movers’ - the main muscles responsible for a particular movement

Question 20

What are antagonist muscles?

Answer 20

These oppose prime movers

Question 21

What is the role of synergist muscles?

Answer 21

They assist prime movers. They cannot perform the movement alone, but their angle of pull assists

Question 22

What is the role of neutraliser muscles?

Answer 22

They prevent the unwanted actions of a joint that would otherwise occur with an agonist

Question 23

What is the role of fixators?

Answer 23

These act to hold a body part immobile whilst another body part is moving

Question 24

What is the difference between isotonic and isometric contraction?

Answer 24

Isotonic - constant tension, variable muscle length

Isometric - constant length, variable tension, eg. hand grip

Question 25

What is the difference between concentric and eccentric isotonic contraction?

Answer 25

Concentric - muscle shortens Eccentric - muscle exerts a force while being extended eg. walking downhill

Question 26

What are the three types of levers in the human body?

Answer 26

- first class lever "see-saw" (effort at one end, load at the other, fulcrum in the middle) eg. head - second class lever "wheelbarrow" (effort at one end, fulcrum at other, load in middle) eg. standing on tip toes - third class lever "fishing rod" (effort in middle, load at one end, fulcrum at other) eg. biceps in elbow flexion

Question 27

What is the name given to what happens when ATP is depleted after death, causing myosin heads to be unable to detach?

Answer 27

Rigor mortis

Question 28

What is a motor unit?

Answer 28

An alpha-motor neurone and the muscle fibres it innervates

Question 29

True or false - muscle with less fibres are usually better at fine control, while muscles with more fibres are usually more powerful?

Answer 29

True

Question 30

What are the three main muscle fibre types?

Answer 30

- slow type I - fast type IIA - fast type IIX There are also several intermediates. Classification is based on myosin heavy chain expression.

Question 31

Give some characteristics of Type I muscle fibres.

Answer 31

- slow oxidative - aerobic, many mitochondria - red colour - high myoglobin levels - fatigue resistant - rich capillary supply - first type to be recruited: standing, walking

Question 32

Give some characteristics of type IIA muscle fibres

Answer 32

- aerobic, many mitochondria - high myoglobin levels - red-pink colour - rich capillary supply - moderate fatigue resistance - second type to be recruited: walking, running

Question 33

Give some characteristics of type IIX muscle fibres

Answer 33

- anaerobic glycolysis, few mitochondria - low myoglobin levels - white/pale colour - poor capillary supply - rapidly fatigable - last type to be recruited: sprinting, jumping

Question 34

What do intrafusal muscle fibres do?

Answer 34

They facilitate proprioception, allowing us to touch things accurately without watching our limbs

Question 35

How are intrafusal muscle fibres innervated?

Answer 35

Two sensory neurones (type 1a and type 2), and gamma motor neurones

Question 36

What does large-fibre sensory neuropathy cause?

Answer 36

Patients are unable to perform accurate movements without watching the affected limb

Question 37

What is the size principle of muscle contraction?

Answer 37

Small motor neurones are recruited before large, so motor units mostly have fibres types: Slow type I -> fast IIa -> fast IIx

Question 38

What is it called when muscles cannot contact any further/faster?

Answer 38

Fused tetanus

Question 39

Why are healthy muscle never fully relaxed (except in REM sleep)?

Answer 39

Baseline tone due to motor neuron activity and muscle elasticity

Question 40

What is hypotonia?

Answer 40

A lack of skeletal muscle tone - this is a symptom rather than a condition. Most common in babies after birth, known as 'floppy baby syndrome'

Question 41

Give some examples of conditions that can cause hypotonia

Answer 41

- muscular dystrophies - spinal muscular atrophy - Charcot-Marie-Tooth disease

Question 42

Outline how the neuromuscular junction functions

Answer 42

- action potential opens voltage gated Ca2+ channels and triggers vesicle fusion and ACh release - nACh channels open, Na+ flows into cell - ACh rapidly broken down in synaptic cleft by AChE - depolarisation opens voltage-gated Na+ channels in muscle cell - action potential occurs

Question 43

How is Ca2+ released into the muscle?

Answer 43

- voltage gated Ca2+ channels (DHP receptors) in the T-tubules release some calcium - ryanodine receptors in SR are triggered by DHP receptors to allow rapid Ca2+ release

Question 44

What does Ca2+ do once it is released into the muscles?

Answer 44

- Ca2+ binds to troponin, tropomyosin reveals actin binding sites for myosin heads - relaxation is facilitated by Ca2+ being pumped back into the SR via Ca2+ pumps

Question 45

Why is the resting membrane potential lower for skeletal muscle than other tissues?

Answer 45

Skeletal muscle has a high concentration of Cl- leak channels, so it is close to Nernst potential for Cl-

Question 46

Give a disease caused by a channelopathy of the Cl- channel in skeletal muscle

Answer 46

Myotonia congenita

Question 47

Give a disease caused by a channelopathy of the Na+ channel (alpha subunit) in skeletal muscle

Answer 47

Potassium aggravated myotonia, paramyotonia congenita, hyperkalemic periodic paralysis

Question 48

Give a disease caused by a channelopathy of the Ca2+ channel (alpha 1s subunit) in skeletal muscle

Answer 48

Hypokalaemic periodic paralysis

Question 49

What is myotonia congenita?

Answer 49

Inherited muscle stiffness which particularly occurs in the leg muscles. May be made worse by cold and inactivity.

Question 50

What causes myotonia congenita?

Answer 50

Normally in skeletal muscle, Cl- ions help to bring the membrane potential back to baseline. In myotonia congenita this can't happen, so the muscles go into myotonia

Question 51

What are the short term sources of ATP in muscle?

Answer 51

- stores of ATP in muscle fibre (very short term) - creatine phosphate - glycolysis - oxidative phosphorylation