Week 7 - voluntary movement - finished

Question 1

How many axons from the CNS is a muscle fibre innervate by?

Answer 1

Supplied by a single axon from the CNS

Question 2

Muscles can be classified by what features and list some examples of these categories

Answer 2

They can be classified by location and/or function.

This includes:

• Flexors/extensors
• Agonist/antagonist/synergist
• Girdle/proximal/distal

Question 3

What is the axon muscles are innervated by and where do these axons originate?

Answer 3

They are innervated by LMN’s which originate in the ventral horn of the spinal cord

Question 4

Generally, what is the organisation of flexors/extensors and proximal and distal neurons in the ventral horn of the spinal cord? What image can we use to help describe this?

Answer 4

Neurons for flexors are more dorsal, extensors are more ventral
Neurons for promixal muscles are more medial, and distal are more lateral.

We the image of a bicep curling arm.

Question 5

Why do we have cervical and lumbar enlargements of the ventral horn?

Answer 5

Because the distribution of muscles supplied by LMN’s is not even throughout the body, so we have variations in ventral horn size in areas of the spine that correlate to the areas that supply the upper and lower limbs, i.e. the lumbar and cervical enlargements

Question 6

What neurons are soley responsible for the generation of force by a muscle?

Answer 6

Alpha motor neurons

Question 7

Is a muscle fibre supplied by more than one alpha LMN? Can one alpha LMN supply more that one muscle fibre?

Answer 7

One muscle fibre is supplied by ONE alpha LMN only, however an alpha LMN can supply more than one muscle fibre.

Question 8

What is the name for one alpha LMN and the muscle fibres it supplies?

Answer 8

A motor unit

Question 9

What is the name for the alpha motor neurons required to supply an entire muscle?

Answer 9

A motor neuron pool

Question 10

Where and how to alpha motor neurons communicate with skeletal muscle fibres?

Answer 10

They communicate with them at a neuromuscular junction via the release of Ach

Question 11

What happens when an action potential is generated at an alpha motor neuron?

Answer 11

It produces an EPSP (excitatory post synaptic potential) which produces a twitch in the muscle fibres.

Question 12

What is a muscle twitch?

Answer 12

A rapid contraction followed by a rapid relaxation

Question 13

In terms of action potential being received by an alpha motor neuron and producing a twitch in a muscle fibre, how is a sustained contraction achieved?

Answer 13

By a continuous barrage of action potentials

Question 14

What does high frequency activity at the presynaptic terminal result in?

Answer 14

Temporal summation at the post synaptic terminal

Question 15

What does twitch summation cause in a muscle contraction?

Answer 15

It causes increased tension in the muscle fibre and smoothes the muscle fibre contraction.

Question 16

What is a way to grade muscle contraction in relation to action potential?

Answer 16

Looking at the frequency of neuron firing. This is because twitch summation causes increased tension in the muscle fibre and the increased rate or twitch leads to a smoother muscle contraction.

Question 17

What is the principle behind motor recruitment patterns?

Answer 17

Smaller motor units require a smaller alpha motor neuron which must be easier to excited, thus smaller motor units are recruited first.

Question 18

What does innervation ratio dictate?

Answer 18

Dexterity

Question 19

What dictates dexterity in movement?

Answer 19

Innervation ratio

Question 20

How do muscle spindles provide proprioceptive feedback?

Answer 20

They relay information about a muscles length back to the DRG via alpha motor neurons

Question 21

Where do alpha motor neurons receive their input from? Which of these gives the most input?

Answer 21

• UMN from the cerebrum
  AND
• interneurons of the spinal cord which give both inhibitory and excitatory signals.

The interneurons give the most input.

Question 22

What is a muscle spindle? How is it shaped?

Answer 22

A group of modified muscle fibres (intrafused fibres) within a fibrous capsule.

This complex is shaped like a spindle with a type A axon wrapped around it.

Question 23

What are intrafused fibres? What are extrafused fibres? Where do each of these receive their input from?

Answer 23

Intrafused fibres are a group of modified muscle fibres within a fibrous capsule (also called a muscle spindle).
They receive their input from gamma motor neurons, NOT alpha motor neurons.

Extrafused fibres are fibres not within this capsule. These receive their input from alpha motor neurons.

Question 24

What causes a muscle spindle neurons to fire?

Answer 24

When the modified muscle fibres wrapped with a type A axon are stretched.

Question 25

Where is the signal from a fired neuron in a muscle spindle sent? What then happens to this information?

Answer 25

To the DRG and cord. This information the synapses at spinal interneurons and alpha motor neurons.

Question 26

What makes muscle spindle input so powerful?

Answer 26

One muscle spindle may communicate with an alpha motor neurons for every muscle fibre within the muscle that the signal came from. I.e. if a single muscle spindle in a bicep muscle is activated, the muscle spindle can communicate that information to all alpha motor neurons that suuply every muscle fibre in the biceps muscle.

Question 27

What is the myotatic reflex? Give an example.

Answer 27

A theoretical monosynaptic response between the muscle spindle input and the alpha motor neuron output.

e.g. knee jerk
You pull on a muscle and it pulls back

Question 28

What is the key controller of posture?

Answer 28

Antigravity muscles are monitored and maintained by the myotatic response as the the myotatic response maintains muscle tone.

Question 29

Describe the UMN activation of gamma and alpha motor neurons.

Answer 29

UMN activate both alpha and gamma motor neurons, in order to make sure that the gamma motor neurons can always report muscle length.

Gamma motor neurons act ONLY on intrafused fibres (the modified fibres inside a muscle spindle) and therefore have control over the contraction of these fibres. Alpha fibres only act on the extrafusal fibres, which essentially are all muscle fibres outside of the muscle spindle fibres.

If the UMN contracted only the fibres supplied by the alpha motor neurons then the muscle length would shorten but the muscle spindle fibres would be the same length and would essentiall go “offline”, as due to their length being longer than the whole muscle length, they cant relay muscle length information. So to counter this, the UMN always contracts both the alpha and gamma fibres, so that the muscle length always shortens in relation to the muscle spindle length and visa versa, meaning the muscle spindle can always send muscle length information to the DRG.

Question 30

Where are golgi tendon organs located? What do they act like?

Answer 30

They sit in the junction of a muscle and its tendon. They act like a strain gauge.

Question 31

Do the nerve endings of muscle spindle fibres run in parallel to the muscle fibres or across the fibres?

Answer 31

In parallel. Hence they send information about the muscle length

Question 32

Do the nerve endings of golgi tendon organ fibres run in parallel to the muscle fibres, or across it?

Answer 32

They run across the muscle fibres. Hence they send information about tension within the muscle

Question 33

Where does the information from the golgi tendon organs synapse and with what?

Answer 33

In the ventral horn, and it synapses with the interneurons (unlike the muscle spindles which synapse with the alpha motor neurons). Some of these interneurons form inhibitory connections with the alpha motor neurons to fibres of the same muscle.

Question 34

Where do alpha motor neuron received the majority of their input from? (3)

Answer 34

Interneurons that relay primary sensory input

• Golgi tendon organs
• Muscle spindle fibres

Descending axons from the cerebrum

Collaterals of other LMN’s

Question 35

Hows do inhibitory interneurons play a role in normal motor function?

Answer 35

They use reciprocal inhibition to avoid an antagonistic myotatic reflex (say the muscle spindle of the patellar tendon fires and causes the quads to contract, if there were no inhibitory interneurons to reciprocally inhibit the hamstrings muscle spindles, the sudden contraction of the quads would cause the hamstrong muscle spindle fibres to fire…causing the hamstrings to also contract).

Question 36

Is there such a thing as excitatory interneurons?

Answer 36

Yes

Question 37

Describe the flexor reflex. Are both inhibitory and excitatory interneurons required for this to work?

Answer 37

A polysynaptic reflex to activate the flexors at multiple cord levels to withdraw a limb from a noxious stimuli.

Nociceptive fibres bring information into the cord. Collaterals communicate with interneurons over several levels. Interneurons excite the alpha motor neurons to the flexors of the affected limb. The limb is drawn away from the stimulus.

Excitatory interneurons excite the alpha motor neurons.
Inhibitory interneurons inhibit the extensor muscles from resisting the sudden change in length and tension of the flexor muscles.

Question 38

Which is faster, the flexor reflex or the myotatic reflex?

Answer 38

The myotatic

Question 39

What is the crossed extensor reflex and why do we get it?

Answer 39

It is a follow on from the flexor reflex, where the extensors on the other side of the body are activated while the flexors are inhibited.

This happens as a compensation for the shift in body mass which occurs with the flexor reflex.

Question 40

Information: spinal motor programs for walking:

Answer 40

The crossed extensor reflex appears to be the building block or starting point for locomotion. Walking is simply rhythmic repetition of alternating the withdrawal and activation of crossed extensor reflexes bilaterally. The only thing missing is the mechanism for the regulation of timing.

One theory is that we have descending control from the cerebrum and also, though this could be possible, we know there must also be some kind of spinal regulation.

Circuitry that produces rhythmic motor patterns is referred to in a generic sense as a central pattern generator.

We are yet to identify the pathways that form these central pattern generators.

Question 41

What are the 2 types of motor units?

Answer 41

Fast and slow

Question 42

What kind of muscle fibres do fast motor units contain? Describe their fatiguability

Answer 42

They contain white muscle fibres which fatigue easily

Question 43

Why do fast motor fibres appear white?

Answer 43

Because of their RELATIVE lack of mitochondria

Question 44

What are the subtypes of fast motor units?

Answer 44

IIa and IIb

Question 45

What kind of energy do IIa fast motor units use?

Answer 45

They use oxidative metabolism and glycolysis

Question 46

What kind of energy do IIb fast motor units use?

Answer 46

They use almost soley glycolysis

Question 47

What kind of muscle fibres do slow motor units contain? Describe these fibres in terms of their fatiguability

Answer 47

They contain red muscle fibres which are slower to fatigue than fast motor uni muscle fibres

Question 48

In terms of axon size, what is the difference between fast and slow motor units and how is this reflected in their function?

Answer 48

Fast motor units will have larger diameter axons for faster conduction and are able to generate periodic bursts of high frequency firing.

Slow motor units will have smaller diameter axons which hinder conduction velocity and thus makes it impossible to generate high frequency impulses

Question 49

What happens if we remove a fast neuron from a fast fibre and attach a slow neuron instead?

Answer 49

The function of the muscle fibre will change from a fast fibre to a slow fibre.

Question 50

What is Brodmanns area 4? Where is it located on the brain?

Answer 50

The primary motor cortex (M1)

Located on the precentral gyrus of the frontal lobe

Question 51

What is Brodmanns area 6? Where is it located on the brain?

Answer 51

The pre and supplementary motor cortices

Located just anterior to M1 on the frontal lobe. The SMA is located just medial to the PMA.

Question 52

What is normal goal related movement often referred to as? Why?

Answer 52

Sensorimotor function

This is due to the massive input from the sensory cortex

Question 53

List what is needed in terms of sensory input in order to be able to execute a normal goal related movement.

Answer 53

We need to know where in space and time we are
- proprioceptive, visual and vestibular input

We need to use prior experience to decide on the best way to execute the movement
- prefrontal input

Once the appropriate motor plan has been decided upon, we need to execute the plan
- prefrontal, thalamic and basal ganglia input

Question 54

What happens when an area of M1 is electrically stimulated? How does this differ from what happens when an are of the premotor cortex is electrically stimulated? What does this mean in terms of the PMA neural code?

Answer 54

When an area of the M1 is stimulated, a located group of muscles of the contralateral side of the body twitch

When the PMA is stimulated we see a more complex movement pattern elicited in the proximal limb muscles bilaterally.

This means that the PMA neurons code for not only the muscles to excited but also the direction of movement of the limbs. i.e. a specific PMA neuron would increase its activity if the limb moved to the right, but not to the left. We can assume that another PMA neuron would have the encoding for movement to the left.

Question 55

What is the difference in function between the premotor cortex and the supplementary motor cortex?

Answer 55

Their function appears to be similar but for the distal limb muscles instead of the proximal limb muscles (done by the PMA)

Question 56

When are the most pronounced deficits noted with lesions to the SMA/PMA?

Answer 56

They occur during activities that require coordination of both hands

Question 57

What is apraxia?

Answer 57

The inability to perform previously acquired complex motor skills

Question 58

Motor planning in the SMA info:

Answer 58

Its function appears to be quite similar to PMA but for the distal limb muscles

This is supported by electrical stimulatory tests

Again somatotopic arrangement was noted in these tests

More recent imaging on behaving animals show that the activity of SMA neurons about a second prior to voluntary movement of the hands
Supporting the theory that it acts as a motor planning centre
This increase in activity occurs with movement of either hand, indicating an intense interconnection via the corpus callosum.

The most pronounced deficits noted with lesions to either SMA occur during activities that require coordination of both hands

This inability to perform previously acquired complex motor skills is known as apraxia.

These imaging studies appear to support theories on a “motor hierarchy’ which emphasises the function of these motor cortices in calculating tactics to achieve goals

According to this hierarchy, accurate motor planning relies on sensory information about our position in space from cortical association areas

The decision making prefrontal cortex helps us decide how we might get from our current position to our desired position

The basal ganglia also play a role in this highest level of the hierarchy by determining which of any number of strategies is most feasible

The ‘feed forward’ or predictive functions of the cerebellum are also implicated in this function

Question 59

What is the posterior parietal cortex?

Answer 59

An advanced sensory integration area which allows us to form an accurate mental body image of our relationship to our environment.

Question 60

What is Brodmanns area 5? What is its function?

Answer 60

It is the superior parietal lobule

Receives details and partically pre-integrated information from the primary somatosensory cortex (S1)

The superior lobule is essential for the completion of self image and lesions of this region may lead to disorders in the recognition or visualisation of the contralateral side of the body.

Question 61

What is Brodmanns area 7? What is its function?

Answer 61

It is the inferior parietal lobule

Receives detailed and partially pre-integrated information from higher order visual cortex

The inferior lobule integrates diverse sensory information for speech and perception and is more concerned with creating a complete perception of our environment.

Question 62

What is the ‘motor hierachy’? What areas of the brain supply the highest input into this and how? What parts of the brain are implicated at which levels of this hierachy?

Answer 62

This hierachy states that accurate motor planning relies on sensory information about our position in space from cortical association areas.

The areas of that brain that contribute:

Prefrontal cortex: help us decide how we might get from our current position to our desired position

Basal ganglia: helps to determine which of any number of strategies identified by the prefrontal cortex is the most feasible. (also some input from the cerebellum in terms of ‘feed forward’ or predictive functions)

The posterior parietal cortex: allows us to form an accurate mental body image of our relationship to the environment.

The neurological connection and communication between the PPC and the prefrontal association cortex is imperative to the anticipation of the consequences of the actions that are to be performed and therefore it is the intercommunication between these 2 areas of the brain that help to determine what the potential action should be.

Therefore the PPC and the prefrontal cortex form the cortical portion of the highest level of the motor hierachy, and the basal ganglia forms the subcortical portion of this highest level of the motor hierachy.

Question 63

Information on the light up of areas of the brain associated with movement compared to the thought of movement.

Answer 63

When a patient is asked to perform a series of finger movements from memory all of the areas discussed above light up (3, 2, 1, 5, 7, 8, 6, 4 ).

When the patient is asked to only think about the motor activity but not actually perform it, area 6 remains active but area 4 does not.