muscle and contraction

Question 1

• What does the innervation ratio indicate and what is its relationship with the level of control a muscle has?

Answer 1

It defines the number of muscle fibres innervated by a single motor neurone

The innervation ratio is inversely proportional to the level of control

Question 2

• What will the innervation ratio be for fine control/ delicate movements broadly speaking?
• What are the 3 motor fibre types and their respective colour?

Answer 2

Innervation ratio will tend to be small, enabling nuances of movement of the entire movement

Slow (S, type I) - Red

Fast, fatigue resistant (FR, type IIA) - Pink

Fast, fatiguable (FF, type IIB) - White

Question 3

• Which motor fibre type has the lowest myoglobin content?

- Which motor fibre type has the highest aerobic capacity?

Answer 3

FF type IIB

S type I (also has the lowest anaerobic capacity).

Question 4

• How are motor unit types classified?

- List the properties of type IIB, type IIA and type I motor units

Answer 4

By the amount of tension generated, speed of contraction and fatiguability of the motor unit

Type IIB - Fast twitch, high tension, high fatigue

Type IIA - Fast twitch, moderate tension, fatigue resistant

Type I - Slow twitch, low tension, fatigue resistant

Question 5

• What does recruitment mean?
• Which motor units are recruited first?
• What level of force is required for fine control?

Answer 5

one of the mechanisms by which the brain regulates the force a single muscle can produce.

order to how muscle fibres are recruited. if more force required, more units required, allowing for fine control.

Smaller units are recruited first (these are generally slow twitch units)

low levels

Question 6

• Explain what rate coding is and the relationship between the firing rate and the force produced

Answer 6

determining force by the frequency in which muscle fibres are stimulated by their innervating axon. mechanism which regulates the level of force a single muscle can produce.

Motor units fire a range of frequencies. Slow units fire at a lower frequency

Increasing the firing rate, increases the force produced by the unit

Question 7

• When does summation occur?

- What can muscle force be regulated by?

Answer 7

Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving APs

Regulated by the number of motor units required

Question 8

What are the 3 different muscle types, are they mono or multi nucleated and what control are they under? (smooth muscle)

Answer 8

• Smooth muscleMono nucleated (existing as electrically linked units contracting together as a single-unit or as a multi-unit)Muscle is under involuntary control from the autonomic NS.

Question 9

What are the 3 different muscle types, are they mono or multi nucleated and what control are they under? (cardiac muscle)

Answer 9

• Cardiac muscleMono nucleated (existing as electrically linked units contracting together as a single-unit or as a multi-unit)Muscle is under involuntary control from the autonomic NS.

Question 10

What are the 3 different muscle types, are they mono or multi nucleated and what control are they under? (skeletal muscle)

Answer 10

• Skeletal muscleMulti-nucleatedMuscles under voluntary control, attached to bones and contract to facilitate movement

Question 11

• what is the name of the connective tissue muscle is wrapped in and what does it do?

Answer 11

Wrapped in sheath of connective tissue (Epimysium)

enabling muscle to contract and move powerfully while maintaining structural integrity. It separates muscle from other tissues.

Question 12

what is each myofibre encased by?

what does it do?

Answer 12

Each myofibre is encased by a thin layer of collagen, and reticular fibres → the endomysium.

Endomysium surrounds extracellular matrix of cells and plays a role in transferring force produced by muscle fibres to tendons.

Question 13

how are muscle fibres arranged and what are they surrounded by?

how does its arrangement enable movement?

Answer 13

Muscle is surrounded by epimysium

Muscle is arranged in bundles of myofibres (muscle fibres) called fascicles, surrounded by an intermediate layer of connective tissue called the perimysium.

Myofibres are formed from bundles of myofibrils, covered by a connective tissue layer called endomysium

myofibres are made from bundles of myofilaments

Fascicular arrangement enables system to trigger specific movement of a muscle by activating a subset of muscle fibres within a fascicle of the muscle.

Question 14

• What is the sarcoplasmic reticulum?

- What is the cytoplasm of myofibres called and what does it contain?

Answer 14

Network of fluid-filled tubules, constitutes the main intracellular calcium store in striated muscle, cardinal role in the regulation of excitation-contraction coupling.

Sarcoplasm- contains Myoglobin and mitochondria

Question 15

• What are myofibres composed of?

- What is sarcolemma?

Answer 15

Repeating units of sarcomere

Plasma membrane of myofibres

Question 16

• Describe the structure of myosin

Answer 16

2 globular heads

Single tail formed by 2 alpha-helices

Tails of several hundred molecules form one filament

Question 17

• Describe the structure of actin

Answer 17

They’re twisted into a helix. Each molecule has a myosin binding site

Filaments also contain troponin and tropomyosin

Question 18

• Explain the sliding filament theory and what happens to lengths of the the bands

Answer 18

Signalled by motor neurones → Skeletal muscle contracts as thin actin filaments are pulled towards M-lines

proceeds when myosin-binding sites on actin filaments are exposed due to Ca2+ entry into the sarcoplasm.

Tropomyosin is wound around actin filaments, and encapsulates the myosin-binding sites, preventing actin binding to myosin until Ca2+ binds to the troponin complex and the tropomyosin exposes the myosin-binding sites

Cross bridges are formed between myosin heads and actin filaments and myosin pulls actin towards the M-line

Question 19

what happens to I band, A band and H zone during contraction?

Answer 19

During contraction: I-band shorter, A-band same length and H-zone narrows/disappears

Question 20

• Outline the initiation of muscle contraction at the NMJ

Answer 20

1. NMJ: Arrival of neuronal AP terminates at NMJ, pre-synaptic membrane of motor neurone. AP stimulates Ca2+ voltage-gated ion channels to open
   
   2. Influx of Ca2+ into pre-synaptic knob, results in vesicles fusing with the pre-synaptic membrane and releasing Ach into synaptic cleft by exocytosis
   3. ACh diffuses across synaptic cleft, binding ACh receptors on the post-synaptic membrane within the motor-end plate of the sarcolemma. Inducing an AP, voltage-gated Na+ channels open to depolarise membrane
   4. Sodium ions enter the muscle fibre, AP is triggered along the membrane, initiating excitation-contraction coupling
   5. ACh is hydrolysed in synaptic cleft by acetylcholinesterase, thereby preventing continued triggering of AP

Question 21

• Outline the activation steps of muscle contraction

Answer 21

1. Propagation of AP along sarcolemma into T-tubules
   
   2. Voltage-sensitive dihydropyridine receptors (DHPR) on sarcolemma are mechanically linked to calcium channels (ryanodine receptors) in adjacent SR membrane. AP stimulated conformational change of DHPR (Detects potential difference).
   3. Triggers opening of RyR, enabling Ca2+ efflux from the SR into the sarcoplasm.
   4. Ca2+ in the sarcoplasm initiates the contraction and shortening of sarcomeres, binds to TnC (Troponin complexes) on the actin filaments. This causes tropomyosin to expose the myosin binding sites.
   5. Myosin globular heads forms cross-bridge with actin-binding site via the myosin binding site.
   6. Ca2+ actively transported into the SR continuously while AP continue. ATP-driven pump.
   7. Muscle contraction stops when there is a signal termination from the motor neurone, re-polarisation of sarcolemma and T-tubules, ryanodine receptor close, and tropomyosin occupy the myosin binding sites on the actin filaments.

Question 22

• Explain excitation-contraction coupling

Answer 22

1. In the presence of Ca2+ there is movement of troponin from tropomyosin.
   exposing the myosin-binding site, on surface of actin.
   
   3. ‘Charged’ myosin head is attracted to actin, binding actin at its respective actin-binding site forming a cross-bridge. (Cross-bridge formation occurs when myosin-head attaches to actin, ADP and Pi bound to myosin.)
   4. ADP and Pi released, myosin head retracts towards the M-line, pulling the actin-filament into the A-band. Movement defined as ‘power stroke’
   5. ATP binding - Myosin head detaches from actin. ATP hydrolysed by the intrinsic ATPase activity of myosin into ADP and Pi.
   6. The energy released from ATP hydrolysis changes the angle of the myosin head into a position for further movement, enables cross-bridge cycle to recur. Recharge

Question 23

• Explain the neural control of muscle contraction

Answer 23

Upper motor neurones in brain (primary motor cortex) provide the descending corticospinal tract, decussating at the medullary pyramids within the medulla oblongata

Terminates with lower motor neurones (Spinal cord or brainstem)

Voluntary neural control from upper and lower motor neurones

Question 24

• What is a motor unit and what are all the motor units in a muscle called collectively?

Answer 24

Single motor neurone in conjunction with corresponding muscle fibres that it innervates. Smallest functional unit with which to produce force

Collection of motor neurones collectively coordinate together to stimulate contractions of a single muscle, all of the motor units in a muscle are considered motor pool

Question 25

• What are neurotrophic factors?

Answer 25

Factors (peptides/proteins) that support the growth, survival and differentiation of developing and mature neurones

Prevents neuronal death, and promotes growth post-injury

Question 26

• What is the force generated by a contracted muscle known as?
• What type of contraction is most likely to cause damage?

Answer 26

Muscle tension

Eccentric

Question 27

• Outline the effect of neurotrophic factors in terms of crossing-over

Answer 27

Fast and slow twitch muscles are cross innervated, slow muscles exhibits characteristic functions and properties of that to fast muscle

Muscle displays the properties of the innervating nerve

Motor neurone influences the properties of its innervated muscle fibres

Question 28

• Define muscle plasticity

Answer 28

Ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it

Question 29

• In what cases does muscle fibres type I get converted to type II typically?

Answer 29

Severe de-conditioning or spinal cord injury

Microgravity during spaceflight results in shift from slow to fast muscle fibre types

Question 30

• What is the most common conversion following training?

- What is ageing associated with in terms of muscle fibres?

Answer 30

Conversion from IIB → IIA

Associated with loss of type I and II fibres, preferential loss for type II fibres
Results in a larger proportion of type I fibres in aged muscle (Evidence from slower contraction times)

Question 31

what are the different arrangement of muscle fibres on tendon

Answer 31

unipennate
bipennate
multipennate
triangular 
fusiform 
parallel

Question 32

what are the features of slow, fast fatigue resistant and fast fatigable in terms of

• axon diameter
• dendritic trees
• diameter cell bodies
• conduction velocity

Answer 32

slow- smallest, small, thinnest, slowest

fast fatigue resistant- larger, large, thicker, faster

fast fatigable- larger, large, thicker, faster

Question 33

what are the features of slow, fast fatigue resistant and fast fatigable in terms of

• Myoglobin content
• Colour
• Aerobic capacity
• Anaerobic capacity

Answer 33

slow- high, red, high, low

fast FR - high, pink, moderate, high

fast FF- low, white, low, high