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Neuro > Lower motor neuron pools & muscle stretch reflex > Flashcards

Lower motor neuron pools & muscle stretch reflex Flashcards

1
Q

Where do motor neurons come from?

A

The ventral horn of the spinal cord

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2
Q

What types of motor neuron do we have?

A

Alpha and gamma

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3
Q

What does the dorso-ventral arrangement of motor neurons reflect?

A

The nature of the muscle

- flexors are located more dorsally and extensors more ventrally

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4
Q

What does the medal-lateral arrangement reflect?

A

More proximal muscles medially

More distal muscles laterally (as these will travel furthest in descent and will be the last to leave the spinal cord)

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5
Q

Target of alpha motor neurones

A

extrafusal fibres which mediate contraction

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6
Q

Target of gamma motor neurones

A

intrafusal fibres whcih are part of the muscle spindle and have a role in maintaining spindle sensitivity

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7
Q

What is the neurotransmitter of spinal motor neurones in the ventral horn?

A

ACh

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8
Q

What are the different types of skeletal muscle fibre? What are their different innervations?

A
  • slow contracting (type 1) → innervated by the smallest alpha neurons, recruited first and used in all forms of locomotion
  • fast contracting fatigue resistant (type 2a) → innervated by medium sized alpha motor neurons
  • fast contracting easy fatigued (type 2b) → innervated by the largest alpha motor neurons and used in short, powerful movements (e.g. sprinting)
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9
Q

What is a motor unit?

A

The one alpha motor neuron and the muscle fibres it innervates

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10
Q

What is the collection of alpha motor neurones which innervated a single muscle called?

A

Alpha motor neuron pool

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11
Q

What is the size principle?

A

Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons) as contraction increases

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12
Q

Describe reciprocal inhibition

A

It is the process of muscles on one side of a joint relaxing to accomodate contraction on the other side of that joint
it is accomplished by the actions of an inhibitory interneuron in the spinal cord

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13
Q

How does reciprocal inhibition work?

A

The afferent of the muscle spindle bifurcates in the spinal cord. One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the reflex. The other branch innervates the inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle. Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.

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14
Q

What are central pattern generators?

A

Biological neural circuits that produce rhythmic outputs in the absence of rhythmic input
- e.g. for walking, swimming, etc

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15
Q

What are the simplest central rhythm generators?

A

Individual neurons which allow pacemaker properties due to their membrane physiology

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16
Q

What is the main characteristic symptom of lower motor neuron lesion?

A

Flaccid paralysis

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17
Q

What are all the characteristics of lower motor neuron lesion?

A 
Muscle weakness or paralysis 
Hypotonia
Hyporeflexia
Initial 'fasciculation' (a brief spontaneous contraction affecting a small number of muscle fibres, often causing a flicker of movement under the skin)
Longer term muscle wasting
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18
Q

What receptors in muscles are used for proprioception?

A

muscle spindles

Golgi tendon organs

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19
Q

Describe muscle spindles

A

Situated in the fleshy part of muscles

in parallel with the extrafusal fibres and attached to muscle connective tissue

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20
Q

Describe Golgi tendon organs

A

situated in the tendons at the ends of the muscle

they are in series with the extrafusal fibres

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21
Q

What do muscle spindles signal?

A

The length and the rate of change of length of the muscle in which they are located

22
Q

What do Golgi tendon organs signal?

A

The tension in the tendon

23
Q

What does the muscle stretch reflex do?

A

Contracts the muscle after it has been stretched to prevent injury

24
Q

What type of fibres are muscle spindles?

A

Intrafusal fibres (these are fibres that serve as proprioceptors, where extrafusal fibres are contractile elements)

25
Q

Which way do the muscle spindles lie in relation to the muscle fibres?

A

In parallel so that the force acting on the muscle acts similarly on the spindle

26
Q

What is the structure of a muscle spindle?

A

Fluid-filled capsule of connective tissue
Contains 7 modified intrafusal fibres with contractile ends and a non-contractile centre, detecting length and rate changes
- the non-contractile centre is innervated by terminating sensory fibres
- the contractile ends are innervated by motor axons

27
Q

What happens to the muscle spindle when the muscle elongates?

A

the intrafusal fibres also elongate

sensory ending which wrap around the centre are deformed and increase their firing rate

28
Q

What are the two types of sensory afferent that innervate muscle spindles?

A

Ia - primary afferent

II - secondary afferent

29
Q

What are the two types of muscle spindle fibre?

A

nuclear bag fibres (b)

nuclear chain fibres (c)

30
Q

Discuss nuclear bag fibres (b)

A
  • swollen in centre

- may be dynamic (Ia primary afferent) or static (II secondary afferent, as well as Ia)

31
Q

Discuss nuclear chain fibres (c)

A
  • smaller than b
  • both Ia and II afferents
  • stiff in a similar way to static b fibres meaning they signal muscle length also
32
Q

What type of fibres does a muscle spindle typically containing?

A

one dynamic b1, one static b2 and 5c fibres

33
Q

How can the muscle spindle reflex be strengthened if required?

A

a single sensory afferent may form synapses with multiple motor neurons in order to strengthen the response

34
Q

How many neurons constitute the muscle spindle relfex?

A

2 - it is monosynaptic

35
Q

Describe the muscle spindle relfex

A
  • stretch imposed on muscle deforms the intrafusal muscle fibres of the muscle spindles, which initiate APs by activating mechanically-gated ion channels in afferent Ia. axons
  • Ia for synapse with alpha motor neurons that innervate same (homonymous) muslce and, via local circuits, form inhibitory connections with the alpha motor neurons of antagonistic (heternymous) muslces - this is called reciprocal innervation
36
Q

Give an example of reciprocal inhibition

A

Knee jerk reflex
quads contract and swing leg forwards
Hamstring relaxes, allowing extension of leg

37
Q

What do gamma neurons do in the muscle spindle reflex?

A

adjust sensitivity of the muscle spindles
their activation causes contraction of the polar regions of the intrafusal fibres
this stretches the central region from both ends, leading to an increase in firing rate of the sensory endings or greater likelihood that the sensory endings will fire in response to stretch of muscle

38
Q

Which motor neurons are therefore involved in the muscle spindle reflex?

A

Coactivation of alpha and gamma motor neurones - allows spindles to function at all muscle lengths during movements and postural adjusments

39
Q

What do Renshaw inhibitory interneurons do?

A

associate with alpha motor neuron and provide neg feedback to inhibit firing in order to prevent fatigue

40
Q

Discuss the latency components of the muscle spindle reflex

A
  • M1 - short latency, involved in proximal muscle control

- M2 - long latency component, slow and involved in fine voluntary distal limb movements

41
Q

Level of jaw jerk

A

trigeminal

42
Q

level of biceps reflex

A

C5-C6

43
Q

level of brachioradialis reflex

A

C5-C6

44
Q

Level of triceps reflex

A

C6-C7

45
Q

Level of finger jerk reflex

A

C8-T1

46
Q

Level of knee jerk reflex

A

L3-L4

47
Q

Level of ankle jerk reflex

A

S1

48
Q

Discuss flexor withdrawal reflex

A

Painful stimulus percieved and limb withdrawn by flexor action

49
Q

Discuss crossed-extensor reflex

A

A reflex in which the contralateral limb compensates for loss of support when the ipsilateral limb withdraws from painful stimulus in a withdrawal reflex
- opposite of the flexor withdrawal reflex

50
Q

What does the Golgi tendon reflex do?

A

Protective mechanism to control the tension of an active muscle by causing relaxation before the tension becomes high enough to cause damage

51
Q

How does the Golgi tendon reflex work?

A

Ib sensory fibres are sent through the dorsal root into the spinal cord to synapse on Ib inhibitory interneurones that in turn terminate directly on the motor neurons that innervate the same muscle
The fibres also make direct excitatory synapses onto motoneurons that innervate the antagonist muscle

52
Q

Structure of GTO

A

strands of collagen that are encapsulated

in series with a group of muscle fibres at one end and merge with the tendon proper at the other

Neuro (23 decks)

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