Neuromuscular Control Flashcards

1
Q

How does the grey and white matter look on a cross section of the spinal cord?

A

White matter on the outside

Butterfly shape of grey matter on the inside

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2
Q

What is the alpha motor neuron? Where is it located?

A

Occupy the anterior / ventral horn of the grey matter of the spinal cord
They are the lower motor neurons of the brainstem and spinal cord
Innervate the extrafusal muscle fibres (muscle fibres with contractal element in them) - activation = muscle contraction

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3
Q

What are intrafusal muscle fibres?

A

Contain sensory organs - responds to stretch and tension within the muscle
Communicates sensory info about the status of the muscle - brings about reflex activity

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4
Q

What is a motor neuron pool?

A

Contains all the alpha motor neurons innervating a single muscle

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5
Q

What is a motor unit?

How does this develop before birth?

A

A single motor neuron and all the muscle fibres it innervates
Smallest functional unit required to produce force

Before birth, there may be an overlap in the muscle fibres that receive innervation from multiple neurons, but one neuron wins eventually
So a muscle fibre is only innervated by one motor neuron
But one motor neuron may innervate many muscle fibres (avg. 600)

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6
Q

What are the 3 types of motor unit?

How are they distributed in muscles?

A

Slow (S, type 1) - smallest diameter cell bodies, small dendritic trees, thinnest axons, slowest conduction velocity

Fast, fatigue resistant (FR, type IIA) - larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity

Fast, fatiguable (FF, type IIB) - larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity

Randomly distributed, not congregated in one area

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7
Q

For the 3 different types of motor units, what is the: force generated, speed of contraction, and fatiguability?

A

Type I (S) = least force, continues to generate it’s own max force for over an hour

Type IIA (FR) = more force, continues to generate max force but dies after 10 minutes

Type IIB (FF) = greatest force, loses ability to generate it’s max force quickly

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8
Q

So how does the function of the muscle relate to motor unit?

A

Function = dependent on the type of motor unit that makes up most of the muscle e.g. for standing and posture = mainly S

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9
Q

What are the 2 main mechanisms by the CNS that can regulate muscle force? And how do they work?

A

Recruitment = turn on number of neurons required for muscle fibre contraction, so more force required = more neurons turned on. Governed by the ‘size principle’ = slow units recruited first, fast units recruited afterwards

Rate coding = motor neurons fire at range of frequencies, increased firing rate = increased force production by unit

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10
Q

What is meant by muscle wastage? How does it occur?

A

i.e. if a muscle still has it’s blood supply, but not it’s nerve supply, the muscles can no longer be contracted

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11
Q

What are neurotrophic factors?

A

Neurotrophic factors - growth factor, prevents neuronal death, promotes growth of neurons after injury

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12
Q

What are motor unit and fibre characteristics dependent on?

A

The nerve that innervates them
e.g. if the innervation to slow and fast muscles are swapped, the properties of the muscle also swap due to the swapped innervation

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13
Q

Is there plasticity of the different motor units? Which are common / uncommon?

A
Fibre types can change properties under many different conditions 
Type IIB (FF) to IIA (FR) most common following training 
Type I to II - only in cases of injury / severe deconditioning (e.g. in spaceflights)
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14
Q

Ageing is associated with which type of motor unit loss?

A

Loss of types I and II fibres, but preferential loss of II as larger propertion of type I fibres = slower contraction time

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15
Q

What is a reflex?

A

An involuntary, automatic response to a stimulus without reaching the level of consciousness
Magnitude and timing depends on intensity and onset of stimulus

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16
Q

How and why can reflex amplitude be altered?

Use the Jendrassik manoeuvre as an example:

A

Descending (supraspinal) control of reflexes:

CNS exerts inhibitory and excitatory regulation on reflexes. CNS has resting level of inhibition. Decerebration (separation of the cerebral cortex from the lower spinal cord) results in stronger, overactive and tonic reflex due to lack of inhibition

Knee-jerk reflex made more intense if tested whilst patient is clenching their teeth, making a fist and/or pulling against looked fingers
Removes descending inhibitor from the brain

17
Q

What are the 5 different pathways that produce descending (supraspinal) control of the reflexes?

A
  1. Activating alpha motor neurons - lower motor neurons = voluntary contraction
  2. Activating inhibitory interneurons
  3. Activating propriospinal neurons - activates nearby muscles of the spinal cord
  4. Activating gamma motor neurons (responsible for altering sensitivity of sensory receptors within muscles)
  5. Activating terminals of afferent fibres
18
Q

What is hyper-reflexia? A lesion in which neurons is it caused by?

How can the heightened reflex activity be seen clinically?

A

Overactive reflexes
Loss of descending inhibition
Associated with upper motor neuron lesions
(Loss of voluntary movement)

See involuntary and rhythmic muscle contractions when the muscle is stretched out (until there is inhibition of the reflexive contractions)

19
Q

What is Babinski’s sign in hyper-reflexia?

A

Sole stimulated with blunt instrument normally leads to toes curling downwards (particularly big toe) - normal plantar response

In an adult with an upper motor lesion - toe curls upwards instead of downwards
Indicative of corticospinal tract lesion

Upward curl = normal in healthy infants (as their corticospinal tract has not developed)

20
Q

What is hypo-reflexia? A lesion in which neurons is it caused by?

A

Below normal or absent reflexes

Associated with lower motor neuron lesions / diseases