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Neurorehab > Spinal Circuitry and Motor Control > Flashcards

Spinal Circuitry and Motor Control Flashcards

1
Q

What are intrafusal muscle fibres?

A

Are located within the muscle spindle of the muscle. Are composed of Dynamic Bag, Static bag and chain fibres

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2
Q

Define extrafusal fibres

A

Normal muscle fibres which are within the main muscle belly (i.e. not within the muscle spindle)
They surround the muscle spindle

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3
Q

Describe some features of muscle spindles (4)

A
  1. All human skeletal muscle contains muscle spindles
  2. The ends of the muscle spindle are attached to the striated muscle
  3. Whenever the muscle belly lengthens/shortens the muscle spindle lengthens/shortens
  4. The density and structure of the muscle spindles within the muscle vary depending on the function of the muscle e.g. higher number in hand muscles are they are needed to relay detailed information about length and speed in order for finely dexterous tasks to be completed.
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4
Q

What separates the muscle spindle form the striated muscle?

What is its function? (2)

A

Capsule - a thin connective tissue membrane

  1. Insulates the muscle spindle separating it form the ionic concentrations that are outside the spindle
  2. Has viscous fluid that protects the MS from mechanical pressure
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5
Q

What is another name for the dynamic bag?

What is its function?

A

Type 1 bag, D1 bag, adaptive bag
Adapts its shape to mechanical perturbation due to its low viscosity (amount of thin viscous fluid) in the central portion of the bag (this allows the fluid to move more easily)

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6
Q

What is another name for the static bag?

What is its function?

A

Type 2 bag, D2 bag, non-adaptive bag
has a central viscous bag in the middle of its fibre which contains thick viscous fluid that is less likely to change shape upon mechanical perturbation

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7
Q

What are chain fibres?

A

Have no viscous fluid in their centre so they cannot change shape. All fibres are arranged in a line

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8
Q

What are type 1a afferents?

How do they interact with the muscle spindle?

A

Large myelinated, fast-conducting axons. Sensory information that is picked up by these axons is transmitted at a rate of 150m/s
Wrap around the central part of the intrafusal fibre and are connected to the capsule

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9
Q

What are type 2a afferents?

How do they interact with the muscle spindle?

A

Smaller, slower conducting axons

They attach to the distal part of the membrane of the intrafusal fibre

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10
Q

What type of fibres are mechanoreceptors?

A

DB and SB fibres

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11
Q

Outline the process of sensory transduction in the muscle spindle. (9)

A

Mechanical deformation – puts tension on the intrafusal fibres
Deformation of nerve membrane
Increase in membrane permeability – this will cause ions to flow according to their electrochemical gradient
NA+ flows in
Membrane Depolarises
> -55mv = Voltage gated channels open
Influx of NA+
Action potential.
Mechanical Deformation = Freq Action Potentials

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12
Q

What are static gamma motor neurons?

A

Motor neurons that innervate the SB and chain fibres
They take action potentials from the spinal cord, the axon innervate the striated muscle at the end of the spindles (at the neuromuscular junctions - acetylecholine is released which causes contraction) which leads to contraction of the striated muscle which puts tension on the central component of the intrafusal fibres. It takes less of a stretch of the MS before AP’s are generated as a result. It brings the MS nearer to threshold

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13
Q

What are dynamic gamma motor neurons?

A

Only innervate the DB fibres. They are able to contract the intrafusal muscle fibres which would tension up the central component

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14
Q

What happens when a stretch is applied to a muscle?

A

DB is tensioned due to the stretch
Depolarisation occurs
Viscous fluid in the DB

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15
Q

What happen the DB when a stretch is applied?

A

No as once the viscosity has changed the stretch is taken off the DB but not the muscle (this is called adaptation)

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16
Q

What 4 neurons can cause contraction of the muscle spindle?

Which of these neurons makes the muscle spindle more sensitive to stretch?

A

Type 1a and 2 afferents
Static and dynamic gamma motor neurons
The 2 gamma motor neurons

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17
Q

What is the function of 2nd order neurons?

A

Take information from the spinal cord to the thalamus

18
Q

What is the function of 3rd order neurons?

A

Take information from the thalamus to the sensory cortex

19
Q

Where in the spinal cord are gamma motor neurons and alpha gamma motor neurons located?

A

Ventral horn

20
Q

What type of neurons are type 1a and type 2 afferents?

Where is the cell body located in sensory neurons?

A

Sensory neurons

Their cell body or soma is in the dorsal root ganglion.

21
Q

What is the function of gamma motor neurons in sensory transduction?

A

Gamma MN are located on the ventral horn of the spinal cord (this is also where the alpha-MN’s are located also). Axons of the gamma MN come out through the ventral root, travel down the femoral nerve, into the quads and into the muscle spindle. Gamma MN are slower conducting than alpha MN

22
Q

What occurs when a stretch is applied to an adaptive fibre within a muscle spindle?

A

Stretch causes tension on the dynamic bag. This causes depolarisation of the sensory endings
The viscous area then stretches taking the tension out of the Dynamic Bag Fibre (adaptation).
The sensory endings are no longer depolarised (no Na+ flowing in)
The sensory endings are only depolarised during lengthening of the muscle.
The degree of depolarisation is proportional to the rate/speed of lengthening

23
Q

At what point do action potentials fire within a DB fibre?

A

Action potentials fire only when there is lengthening. They do not fire when the muscle is held at a static length

24
Q

What occurs when a stretch is applied to a non- adaptive fibre within a muscle spindle?

A

Due to differences in viscosity and structure, static Bag fibres and chain Fibres do not stretch to reduce tension (Non-Adaptive).
Therefore the sensory endings on these fibres continue to be depolarised.
The rate of depolarisation will constantly reflect the length of these fibres.

25
Q

What is the key difference between DB and SB fibres?

A

Non-adaptive/static bag fibres have no mechanism in which they can take the tension off their membrane

Adaptive/DB bag fibres - the pattern of action potential generation is time-limited

26
Q

When do the frequency of action potential begin to reduce in a SB fibre?

A

When the muscle is brought back to its original position the freq of AP’s reduce

27
Q

In SB fibres what is true about the frequency of action potential generation?

A

The frequency of AP generation is directly proportionate to the length of the muscle and the frequency of AP’s will continue for as long as a muscle is held at a given length.

28
Q

What is true about the frequency of action potential generation in an adaptive fibre?

A

The frequency of the AP generation is directly proportionate to the speed of the stretch

29
Q

Which fibre types has 1A afferents receptors on?

A

Static and dynamic fibres

30
Q

What type of information does type 1A afferents carry?

A

Length and speed of lengthening of the muscle group

1a afferents also trigger stretch reflexes (spinal reflexes)

31
Q

When will more action potentials fire (overall)?

A

When the muscle is lengthening more APs will fire than when it is shortened

32
Q

Name 3 motor neurons that innervate skeletal muscle and describe their functions and the effect of applying tension to these MNs

A

Alpha motor neuron (MN) - supply the extrafusal fibre of the muscle

Static Gamma MN supply the intrafusal fibre of the muscle i.e. the muscle spindle (specifically SB and chain fibres). Tension will sensitise the muscle spindle to the length of the muscle

Dynamic Gamma MN supply the DB of the muscle spindle. Tension will sensitise the MS to the speed of stretch of the muscle

33
Q

What is alpha-gamma co-activation?

A

Alpha MN shortens the muscle and when this occurs gamma MNs shorten the muscle spindle accordingly so that the muscle spindle is always responsive to the length changes of the muscle.

The gamma MN can operate separately to alter the sensitive pf the spindle (i.e. without the activation of the alpha MN) but the alpha MN cannot operate without the activation of the gamma MN

34
Q

How does stroke affect alpha gamma co-activation?

A

Causes abnormal levels of gamma activation meaning the spindle is not tensioned appropriately

35
Q

What else could affect spindle activity?

A

Trauma to the muscle
Postural changes
Muscle contracture - extrafusal fibre is abnormally contracted
Botox

36
Q

What is a Golgi tendon organ (GTO)?

What is another name for the GTO?

A

A Golgi tendon organ (GTO) is a sensory receptor. It is located in the transition between the muscle and the tendon belly i.e. at the musculotendinous junction

Tension receptor organ

37
Q

Where within the muscle belly is the GTO situated?

A

Its intertwined amongst the collagen fibres (connective tissue) which is within a capsule

38
Q

What are the advantages of where the GTO is located in the muscle?

A

It can easily pick up mechanical perturbation/tension being exerted on the musculotendinous junction and it is close to sensory neurons (which are within the capsule and are intertwined with the collagen fibres)

39
Q

What is the purpose of the capsule surrounding the GTO?

A

GTO is encapsulated as it needs to be isolated from the ionic concentrations as they cannot affect the sensory neurons within the GTO. The only thing these neurons should pick up is the mechanical perturbations exerted on the GTO

40
Q

How does the GTO and the muscle spindle differ?

A

The GTO is a mechanoreceptor – it responds to the amount of tension going through the MTJ
The muscle spindle is also a mechanoreceptor that responds to the length of the muscle

GTO: its location in series with the tendon fibres. The MS is in parallel with the extrafusal contracting fibres of the muscle

Neurorehab (13 decks)

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