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Neuroscience > Lecture 7: Muscle Function > Flashcards

Lecture 7: Muscle Function Flashcards

1
Q

what is an alpha motor neurone

A

Synapses with multiple
muscle fibres

Provides excitation for muscle fibres to generate tension

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2
Q

what is a motor unit

A

The alpha motor neuron and the skeletal muscle fibres it innervates

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3
Q

what is the all or none principle of the motor unit

A

When a motor unit is stimulated, all muscle fibres in that motor unit will fire

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4
Q

what is a motor neuron pool

A

all the motor neurons that innervate a single muscle

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5
Q

what is a neuromuscular junction

A

synapse between neuron and a muscle fibre

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6
Q

what are the 3 parts of the neuromuscular junction

A

presynaptic terminal

synaptic cleft

postsynaptic muscle fiber (motor endplate)

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7
Q

what are the 5 steps of Neuromuscular Junction Synaptic Transmission

A
  1. depolarization of presynaptic membrane (influx of calcium)
  2. acetylcholine released into synaptic cleft
  3. acetylcholine binds to receptors on muscle membrane
  4. depolarization of motor endplate
  5. endplate potential can trigger action potential
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8
Q

why do Motor neurons release excess acetylcholine to depolarize the postsynaptic membrane

A

safety factor for transmission:

Increases chance that muscle will contract, even when fatigued

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9
Q

what is the clinical implication of the safety factor

A

Myasthenia Gravis results in a loss of acetylcholine receptors in the muscle fibre

Reduced membrane depolarization and smaller safety factor → weakness

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10
Q

what is the muscle structure (4 parts)

A

muscle > fascicle > muscle fibre > myofibril

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11
Q

what is a sarcolemma

A

Each muscle fibre is surrounded

allows depolarizing current to reach the sarcomeres

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12
Q

what is the Sarcoplasmic reticulum

A

stores, releases, and retrieves calcium

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13
Q

what are Sarcomeres

A

the smallest functional unit of the muscle fibre

Consist of Actin and Myosin

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14
Q

what is actin

A

the thin filament

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15
Q

what is myosin

A

the thick filament

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16
Q

what are the 4 steps to excitation-contraction coupling

A
  1. endplate potential triggers action potential
  2. action potential travels to muscle via t-tubule
  3. release of calcium from sarcoplasmic reticulum
  4. interaction of actin myosin
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17
Q

why is calcium needed for muscle contraction

A

because it converts an action potential into a muscle contraction

18
Q

what are the 5 steps of the sliding filament theory

A
  1. myosin head is in a cocked position with ATP present
  2. calcium reveals binding site on actin. myosin head binds with actin to make a cross-bridge
  3. ‘Powerstroke’ – myosin pulls actin towards the centre of the sarcomere
  4. When ATP is still present, Myosin dissociates from actin, its head is re-cocked, and it binds to the next active site
  5. cross-bridge cycling
19
Q

when does muscle relaxation happen

A

when stimulation of the motor neuron stops or ATP is no longer available

Calcium is pumped back into the sarcoplasmic reticulum, breaking the link between actin and myosin, causing the muscle to relax

20
Q

what are the 6 factors that influence the amount of force (tension) a muscle can generate

A

muscle length

muscle velocity and direction

innervation ratio

motor unit type

size principle

rate coding

21
Q

how does muscle length affect the force produced by muscles?

A

Active contraction and passive structures contribute to total tension in the muscle

22
Q

what is passive contribution of muscle length

A
  • Weak actin-myosin bonds act like an elastic
  • Increased tension with increased length
23
Q

what is active contraction of muscle length

A
  • Smaller force generated when moving away from optimal length during active contraction

(think of trying to contract wrist in extension vs neutral)

  • Less actin-myosin overlap
24
Q

what is total tension in muscle length

A

it gets higher as muscle is lengthened

25
Q

when lengthening a muscle is most of the tension coming from passive or active components

A

passive

26
Q

how does muscle velocity and direction affect the force produced by muscles?

A

Eccentric > isometric > concentric

Lengthening (eccentric)
* More tension with increasing velocity and greater passive contribution

Shortening (concentric)
* Less tension with increasing velocity

27
Q

what is innervation ratio

A

The number of muscle fibres innervated by a single motor neuron

28
Q

how does innervation ratio affect the force produced by muscles?

A

Innervation ratio differs by muscle type

  • Power/Force muscles = higher innervation ratio
  • Precision muscles = smaller innervation ratio

Collateral sprouting increases the innervation ratio

29
Q

how does motor unit/ muscle fibre type affect tension?

A

slow motor unit (type 1)
-Low threshold for activation (activated first) produce the least force

Fast fatigue-resistant motor units (type lla)
-contract less quickly then type llx

fast fatigable (type llx)
high activation threshold (activated last)
produce the most force

30
Q

how does the size principle of motor unit recruitment affect tension?

A

With increasing levels of motor activation, motor units are recruited from smallest to largest

(S→FR→FF)

31
Q

how does rate coding affect tension?

A

Motor unit firing rate (rate of recruitment)

  • Smaller muscles typically rely on rate coding while larger muscles rely more on the size principle for recruitment
32
Q

how does age impact neuromuscular function

A

action potential propagation is slowed

Reduced density of active areas in the presynaptic terminal

Loss of motor units (but increased size of remaining ones)

33
Q

how does ALS affect neuromuscular function

A

Rapid loss of fast fatigable neurons, followed by loss of fast fatigue-resistant neurons but not type 1 neurons

Loss of strength and power

34
Q

how does stroke affect neuromuscular function

A

Loss of motor units on paretic side

Smaller muscle cross-sectional area

Reduced ability to use size principle

35
Q

how does a spinal cord injury affect neuromuscular function

A

Extensive atrophy, mostly affects type II fibres

Loss of motor units

Transformation toward type IIx

36
Q

how does Parkinson disease affect neuromuscular function

A

loss of type II
motor units

type I muscle fibre hypertrophy

  • Loss of strength and power
37
Q

what can neuromuscular junction changes could be causally linked to as people age

A

decline in muscle mass and function

38
Q

How many alpha motor neurons can innervate a single muscle fibre?

A

1

39
Q

true or false:
The longer the length of the muscle, the greater the active contribution to total tension

A

false

40
Q

true or false
Concentric muscle contractions produce less force when moving faster

A

true

41
Q

Which of the following 3 muscle groups would you expect to have the larger innervation ratio?

Muscles controlling shoulder and back movements (e.g. latissimus dorsi)

Muscles controlling eye movement (extra-ocular muscles)

Muscles controlling the thumb (e.g. abductor pollicis brevis)

Both the muscles controlling the thumb and muscles controlling eye movement each have the greatest number per motor unit

A

Muscles controlling shoulder and back movements (e.g. latissimus dorsi)

42
Q

Place the following steps in the appropriate order

Graded potential is triggered

Actin and myosin form cross bridges

Current travels down the t-tubules

Sarcoplasmic reticulum releases calcium

Action potential is triggered

A

Graded potential is triggered

Action potential is triggered

Current travels down the t-tubules

Sarcoplasmic reticulum releases calcium

Actin and myosin form cross bridges

Neuroscience (20 decks)

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