Skeletal muscle contraction

Question 1

Define muscle ‘contraction’.

Answer 1

The active cycling of cross-bridges between the actin thin filaments and the myosin thick filaments within the sarcomeres.
Does not necessarily involve shortening of the muscle.

Question 2

What is the difference between isotonic and isometric contraction?

Answer 2

Isotonic contraction (‘equal tension’) = tension within the muscle remains constant but muscle length changes.

Isometric contraction (‘equal length’) = muscle generates tension but is not shortening. Occurs when the load against the muscle equals the contractlle force being generated (e.g. holding a weight in a fixed position, pushing against brick wall).

Question 3

What are the 2 different types of isotonic contraction? How are these different?

Answer 3

1. Concentric contraction
   
   • muscle shortens
   • e.g. biceps shortens when flexing elbow to lift a load
2. Eccentric contraction
   
   • active contraction of muscles whilst they are lengthening
   • e.g. setting object down gently involves active arm flexors to control fall of object

Question 4

What is the effect of eccentric contraction on muscles?

Answer 4

Damaging to muscles and results in delayed onset muscle pain

Question 5

What is the ‘passive stretch’ of muscles?

Answer 5

• Muscle is being lengthened while in a passive state (i.e. not stimulated to contract), e.g. pull in hamstrings whilst touching toes.
• Structures responsible for this are outside the cross-bridge itself since muscle activating is not required.

Question 6

What is the role of titin in striated muscle contraction?

Answer 6

• Connects the Z line to the M line in the sarcomere.
• Contributes to force transmission at the Z line and resting tension in the I band region.
• Limits the range of motion of the sarcomere in tension, thus contributing to the passive stiffness of muscle.

Question 7

Which structure stimulates muscles to contract?

Answer 7

Motor neurons

Question 8

What type of motor neuron innervates skeletal muscle fibres? Describe the location and structure of these.

Answer 8

• alpha-motor neurons
• Cell bodies of these neurons are located either in the ventral horn of the spinal cord (for muscles of limbs and trunk) or in the motor nuclei of the brainstem (for muscles of head and face).
• Axons of alpha-motor neurons leave the CNS and form part of a peripheral nerve to supply muscle fibres.

Question 9

How are alpha-motor neurons and individual muscle fibres connected?

Answer 9

Via the neuromuscular junction: ACh is released from vesicles into the synapse and activates nicotinic ACh receptors on the muscle surface.

Question 10

What is a “motor unit”?

Answer 10

• An alpha-motor neuron and the group of individual muscle fibres it innervates.

Question 11

How many muscle fibres does a motor unit contain?

Answer 11

Depends on muscle function:

• muscles that perform precise fine movements (e.g. inferior rectus which moves the eyeball) have ~10 muscle fibres in each motor unit
• powerful muscles, where fine control is less important (e.g. gastrocnemius), may have several thousand muscle fibres in each motor unit

Question 12

What are the 2 main muscle fibre types and what is this distinction based on?

Answer 12

Based on myosin heavy chain (MHC) isoform expression; affects contraction speed, force generated and energy requirements/susceptibility to fatigue.

1. Slow Type I muscle fibres
   - type 1 MHC
   - relatively slow contraction
   - produce low amounts of force
   - extremely resistant to fatigue
2. Fast Type II muscle fibres
   - type 2 MHC (IIA or IIX)
   - relatively fast contraction
   - produce large amounts of force

Question 13

Why are type I muscle fibres so resistant to fatique?

Answer 13

1. High MT content

2. Use of oxidative metabolism to produce the ATP they consume

Question 14

What are the 2 types of fast type II muscle fibres?

Answer 14

1. IIX muscle fibres - fast glycolytic
2. IIA muscle fibres - fast oxidative/glycolytic. Intermediate form between oxidative type I fibres (produce low force but are fatigue resistant) and fast IIX fibres (rely on glycolytic metabolism, produce greater amounts of force and fatigue rapidly).

Question 15

Do muscle fibres in each motor unit and muscle tend to be of the same contractile type (I or II)?

Answer 15

• Each motor unit tends to involve muscle fibres of same contractile type. So each unit is either fast, intermediate or slow contracting.
• But most muscles made up of different fibre and motor unit types, and proportion of fast to slow fibres depends on typical function of the muscle.

Question 16

What are the characteristics of slow motor units and in which muscles are they typically found?

Answer 16

• Very well vascularised, with a high myoglobin content and can maintain contractions for long periods of time.
• So typically found in postural muscles and those used for low intensity long duration activities such as walking (e.g. soleus muscle in leg)

Question 17

Name 6 differences between the 3 extrafusal muscle fibre types.

Answer 17

Type I (slow oxidative)

1. aerobic
2. high myoglobin levels
3. red colour
4. many MT
5. rich capillary supply
6. fatigue resistant

Type IIA (fast oxidative)

1. aerobic
2. high myoglobin levels
3. red-pink colour
4. many MT
5. rich capillary supply
6. moderate fatigue resistance

Type IIX (fast glycolytic)

1. anaerobic glycolysis
2. low myoglobin levels
3. white colour
4. few MT
5. poorer capillary supply
6. rapidly fatigable

Question 18

Under which conditions are the different muscle fibre types recruited?

Answer 18

Type I: 1st to be recruited (standing, walking)

Type II: 2nd to be recruited (walking, running)

Type IIX: last to be recruited (running, sprinting, jumping)

Question 19

How does the CNS detect muscle stretch?

Answer 19

Muscle spindle/intrafusal fibre located in muscle belly senses muscle stretch. Innervated by 1 motor and 2 sensory axons:

• gamma motor neuron keeps fibres taught
• type Ia sensory neurons relay rate of change in muscle length back to CNS
• type II sensory neurons provide position sense

Spindle walled off from rest of muscle by collagen sheath.

Question 20

Which 2 factors does the contractile force produced by a muscle depend on?

Answer 20

1. Size principle
   
   • small motor units are recruited before large
   • i.e. motor units with slow type I fibres recruited 1st, followed by those containing mostly faste IIA fibres and then fast IIX fibres
2. Rate code
   
   • frequency at which muscle fibres are stimulated by their alpha-motor neuron (more APs = more force)
   • consecutive APs result in summation, giving a slightly larger force with each contraction

Question 21

What is tetany?

Answer 21

Maximum muscle tension at which no further force can be produced (fused APs).

Question 22

Why does all muscle have some degree of baseline motor tone?

Answer 22

Due to:

• elasticity of motor tissue
• low levels of motor neuron activity.

Question 23

What controls baseline skeletal muscle tone?

Answer 23

Motor control centres in the brainstem: locus coerulus (contains noradrenergic cells) projects ascending axons to spinal motor neurons, where it facilitates muscle tone.

Question 24

Under which normal condition is muscle tone lost?

Answer 24

In REM sleep when the locus coeruleus cells shut off.

Question 25

Describe events occuring at neuromuscular junctions resulting in muscle contraction.

Answer 25

1. AP at neuron synapse opens voltage-gated Ca2+ channels… increased [Ca2+]i… vesicle fusion and ACh release into synaptic cleft.
2. ACh activates nicotinic ACh Rs in muscle fibre membrane… Na+ influx… local depolarisation.
3. Opening of voltage-gated Na+ channels… generation of AP in muscle fibre.
4. ACh rapidly broken down in cleft by acetylcholinesterase.

Question 26

Which ion and channels are mainly responsible for excitation-contraction coupling?

Answer 26

• Voltage-gated L-type Ca2+ channels (opening trigger by AP generated by ACh release) are concentrated in T-tubules where they come into contact with sarcoplasmic reticulum at triads.
• Are closely associated with ryanodine receptors (other type of calcium channel) in SR.
• Opening of former channel causes opening of latter channel. Subsequent influx of intracellular Ca2+ binds to troponin… initiates muscle contraction.

Question 27

Why are APs in skeletal muscle cells longer than in motor neurons?

Answer 27

• Besides the leak K+ channels, skeletal muscle fibres also have a high concentration of Cl- leak channels… resting membrane potential usually close to Nernst potential for Cl-.
• High Cl- permeability important in repolarisation after AP - takes 5msec rather than 2msec.

Question 28

What are the main sources of ATP for muscle contraction? Why are these energy sources required?

Answer 28

Very little ATP stored in muscle fibres - sufficient only for a few seconds of contraction.

1. Creatine phosphate: rapidly replenishes ATP but only provides immediate energy for initial activity burst lasting a few secs.
2. Glycolysis (aerobic or anaerobic)
3. Oxidative metabolism (aerobic)

Question 29

What are the advantages and disadvantages of using anaerobic glycolysis to provide muscle energy?

Answer 29

Advantages
- function under anaerobic conditions so not reliant of blood supplying O2 at rate required for max contraction

Disadvantages

1) lactate produced from pyruvate by lactate dehydrogenase… lactate accumulation associated with acidification of muscle cell environment… leads to ‘cramps’ and underlies 1 of reasons why maximal activity (e.g. sprinting) rapidly fatigues muscles.
2) very inefficient - only produces 2 ATP

Question 30

What type of energy generation mechanism is used for muscle activity in aerobic conditions?

Answer 30

Oxidative phosphorylation: pyruvate produced by glycolysis under aerobic conditions is oxidised by TCA cycle in MT to produce high energy electron donors used by ETC… allows much greater ATP production per molecule of glucose.

Question 31

What type of energy generation mechanism is used for prolonged muscle activity ?

Answer 31

beta oxidation of fatty acids released from triaglycerol stored in adipose tissue