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Physiology > Lecture 14 > Flashcards

Lecture 14 Flashcards

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1
Q

What is the primary job of skeletal muscles?

A

To develop force for movement and postural control

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2
Q

What are fascicles?

A

Bundles of muscle fibers, which are further bundled to form muscles

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3
Q

What connective tissues are involved in muscle structure?

A

Connective tissues surround fibers, fascicles, and muscles, and they form tendons that connect muscles to bones

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4
Q

What is the sarcomere?

A

The basic contractile unit of muscle fibres where actin and myosin interact

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4
Q

What is a myofibril?

A

Bundles of contractile proteins within a muscle fibre, made up of myofilaments (actin and myosin)

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5
Q

What is the neuromuscular junction (NMJ)?

A

A specialised synapse between a motor neuron and a skeletal muscle fibre

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6
Q

What enzymes break down acetylcholine?

A

Acetylcholinesterase

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6
Q

What is the role of the sarcoplasmic reticulum (SR)?

A

It stores and releases Ca²⁺ in response to an action potential

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6
Q

What happens when an action potential arrives at the motor neuron terminal?

A

Voltage-gated Ca²⁺ channels open, causing vesicles to release acetylcholine (ACh)

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6
Q

How does acetylcholine (ACh) trigger muscle activation?

A

It binds to nicotinic ACh receptors, causing Na⁺ influx, depolarization, and an action potential in the muscle

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7
Q

What causes tropomyosin to move and expose binding sites?

A

Ca²⁺ binding to troponin-C

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8
Q

What structures conduct the action potential deep into the muscle fibre?

A

Transverse tubules (T-tubules)

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9
Q

What is the sequence of events in excitation-contraction coupling?

A

AP → T-tubules → Ca²⁺ release from SR → Ca²⁺ binds troponin → Cross-bridge formation

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9
Q

How do Botox and Novichok interfere at the NMJ?

A
  • Botox inhibits ACh release
  • Novichok inhibits ACh breakdown, causing overstimulation
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10
Q

What pumps Ca²⁺ back into the SR after contraction?

A

SERCA pumps

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11
Q

What blocks myosin-binding sites on actin at rest?

A

Tropomyosin

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12
Q

What are the steps of cross-bridge cycling?

A
  1. Myosin binds actin
  2. Pi release and power stroke
  3. ATP binds myosin to detach
  4. ATP hydrolysis re-cocks myosin head
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13
Q

What happens if no ATP is available?

A

Rigor mortis (permanent muscle contraction)

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14
Q

What is the length-tension relationship in muscle?

A

Optimal force generation requires optimal overlap between actin and myosin; too stretched or too compressed reduces force

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15
Q

How does stimulation frequency affect muscle tension?

A

Higher stimulation frequency increases Ca²⁺ availability and force generation (leading to summation and tetanus)

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16
Q

What is recruitment in muscle physiology?

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A

Activating more motor units to generate greater force

17
Q

How do motor unit sizes differ between muscles?

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A
  • Small motor units: Fine control (eye, hand muscles)
  • Large motor units: Gross movement (quadriceps)
18
Q

What distinguishes muscle fiber types in the jaw compared to limb muscles?

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A

Jaw muscles have hybrid fibres expressing multiple myosin isoforms and differ in shortening velocities

19
Q

Which muscles are involved in protrusion of the jaw?

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A

1) Lateral & medial pterygoid
2) Masseter

20
What are the thick and thin filaments made of?
* Thick: Myosin (heavy and light chains) * Thin: Actin wrapped with regulatory proteins (tropomyosin, troponin)
20
What are the secondary roles of skeletal muscles?
1) Protect soft organs 2) Generate heat for temperature regulation (e.g., shivering) 3) Serve as an energy/protein store
21
Which muscle depresses the mandible (opens the mouth)?
Lateral pterygoid
22
Why do jaw muscles have special features?
Pennate arrangement allows high force over short distances and varied force directions
23
What is a motor unit?
A motor neuron and all the muscle fibres it innervates
23
How does muscle contraction develop force?
By myosin heads flexing and sliding thick and thin filaments past each other, shortening sarcomeres
24
What ion influx triggers neurotransmitter vesicle release in the NMJ?
Calcium (Ca²⁺)
25
What happens if acetylcholinesterase is inhibited (e.g., by Novichok)?
ACh accumulates, causing continuous stimulation and muscle paralysis
26
What are T-tubules and what is their function?
Invaginations of the muscle cell membrane that conduct action potentials into the fibre’s interior
27
How is Ca²⁺ release triggered from the sarcoplasmic reticulum (SR)?
By depolarization of T-tubules after AP arrival
28
What happens to Ca²⁺ after muscle contraction?
It’s pumped back into the SR by SERCA pumps, leading to muscle relaxation
29
Name the three troponin subunits and their roles
* Troponin-I: Inhibits actin-myosin interaction * Troponin-T: Binds to tropomyosin * Troponin-C: Binds Ca²⁺
30
What is the "power stroke" in cross-bridge cycling?
The pivoting movement of the myosin head pulling actin filaments toward the sarcomere centre
31
Why is ATP necessary for muscle relaxation?
ATP binding causes myosin to release actin, ending the cross-bridge cycle
32
What causes a twitch?
A single action potential leading to a short-duration contraction
33
What is summation in muscle fibres?
Increased force generation when multiple stimuli occur before complete relaxation
34
What is fused tetanus?
Sustained contraction without relaxation due to high-frequency stimulation
35
What two factors determine whole muscle force generation?
1) Force per fibre 2) Number of fibres activated
36
In what order are motor units typically recruited?
Smaller motor units are activated first, then larger ones as more force is needed
37
How does fibre diameter relate to type I and type II fibres in jaw muscles?
Type II fibers can be smaller than type I in the jaw, unlike in the limbs
38
Which muscles retract the mandible?
Posterior temporalis and masseter
39
Which muscles elevate the mandible (close the mouth)?
Anterior/middle temporalis, medial pterygoid, masseter
40
Which muscles are involved in mandibular rotation (side-to-side)?
Temporalis, medial and lateral pterygoid
41
Why does the pennate arrangement of jaw muscles increase force production?
More fibers packed into a smaller area allow higher force over a short contraction distance
42
How does complex fiber arrangement benefit jaw muscles?
Allows muscles to perform multiple functions depending on which fibers are activated
43
What are 2 components that Optimal force generation requires?
1. Cross-bridge formation 2. Room to move
44
What factors that influence muscle tension generation?
- Length-Tension Relationship - Frequency modulation and Summation - Recruitment

Physiology (11 decks)

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