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Motor Systems 21332 > Lectures 4, 12-14 (Riccardo Storchi) > Flashcards

Lectures 4, 12-14 (Riccardo Storchi) Flashcards

Spinal interneurons & spinal reflexes, Proprioception, Rhythmic movements

1
Q

What is the central nervous system composed of and what does it contain?

A

Brain and Spinal cord.

Relay Neurons (Interneurons)

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2
Q

What types of nerves does the peripheral nervous system contain?

A

Cranial nerves, spinal nerves and peripheral nerves.

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3
Q

What are the main 4 sections of the spinal cord? (5th is listed too)

A

4: top-down
- Cervical
- Thoracic
- Lumbar
- Sacral

(5th is coccygeal)

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4
Q

What does the white matter and grey matter of the spinal cord contain?

A

White matter: tracts
Grey matter: cell bodies

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5
Q

What is the grey matter of the spinal cord organised into? Briefly outline all 10 and their locations.

A

Into 10 layers = Rexed’s Laminae

In the dorsal horn:
- L1-L6: Sensory

Intermediate:
- L7-L8: Interneurons

Ventral horn:
- L9: Motor

Around central canal:
- L10: neuroglia

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6
Q

Outline the types of information related to: dorsal root ganglion, posterior root and anterior root.

A

Dorsal root ganglion:
- Contains cell bodies of the 1st order sensory neurons.

Posterior root:
- Where sensory information enters the spinal cord.

Anterior root (ventral horn):
- Where motor information projects from, to the muscles.

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7
Q

What are the dorsal and ventral horn and what do they contain?

A

They are parts of the spinal cord.

Dorsal (posterior) horn:
- contains rexed’s laminae 1-6 which is sensory information.

Ventral (anterior) horn:
- contains laminae 9, which is motor information

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8
Q

What are motor pools?

A

Organisations of motor neurons classed together anatomically.

(groups of similar function/location motor neurons)

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9
Q

What is the relationship between motor neurons and the body part they innervate?

A

Proximal/axial muscles (trunk/neck) have their motor neurons more medially.

Distal muscles have their motor neurons more laterally.

(These motor neurons are located along the mediolateral axis of ventral horn).

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10
Q

What is the simple experiment done to find the motor pools that innervate which muscles?

Think cat

A

Retrograde labelling via the soleus and gastrocnemius which labels back up to the motor pools of those muscles.

See lecture 4 slide 9 for picture

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11
Q

What are the two main methods that we classify interneurons in the spinal cord?

A

Electrophysiogical classification

Developmental genetics

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12
Q

What are the advantages and caveats of electrophysiological classification of interneurons in the spinal cord?

A

+
Can link to specific funtional motifs
-
Expression of functional motifs can be flexible depending on behavioural state

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13
Q

What are the advantages and caveats of developmental genetics as a means of classification of interneurons in the spinal cord?

A

+
Based on neurodevelopment so can be used to identify distinct cell types and deduce connectivity.
-
Cannot always map neuronal function to distinct genetic sub-types.

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14
Q

The majority of neurons in the spinal cord are…?

A

Interneurons

(We still don’t understand completely how the interneurons cuircuits work - lot’s of current research being done on them).

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15
Q

What are the main synaptic input and functional motif of Renshaw cells (RC)? How was this determined?

A

Motor Neuron collateral and Recurrent inhibition respectively.

Electrophysiological classification.

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16
Q

Describe how recurrent inhibition works, the cell involved and what is the overarching term that can be used to describe the type of process occuring?

A

Renshaw Cells (RC)

RC receive excitatory input from MN collateral axons.

In turn, RC cells inhibit the same MN motor pool (via glycine) which reduces the contraction of synergistic muscles.

Provides negative feedback control

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17
Q

What are the main synaptic input and functional motif of 1a inhibitory interneurons? How was this determined?

A

1a afferents and reciprocal inhibition respectively.

Electrophysiological classification

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18
Q

Describe how reciprocal inhibition works, the cell involved and what is the overarching aim of this process occuring?

A

1a inhibitory interneurons.

They receive excitation from muscle spindle receptors (sensing stretch).

In turn, 1a ii’s inhibit antagonistic MN motor pool which reduces contraction of antagonistic muscles.

Prevents synergistic and antagonistic muscles to work against each other

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19
Q

What are the main synaptic input and functional motif of 1b inhibitory interneurons? How was this determined?

A

1b afferents and non-reciprocal inhibition respectively.

Electrophysiological classification.

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20
Q

Describe how non-reciprocal inhibition works, the cell involved and what is the overarching aim of this process occuring?

A

1b inhibitory interneurons.

They receive excitation from the golgi tendon organs (sensing stretch).

In turn, 1b inhibitory interneurons inhibit the MN motor pool responsible for the associated muscle, which reduces the contraction of the synergistic muscles.

Prevents excessive muscle elongation

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21
Q

What did Goulding 2009 find in their developmental genetics study?

What did they find in reference to RC’s and 1a inhibitory interneurons

A

11 progenitor domains that give rise to the ‘‘cardinal’’ classes (dorsal dl1-6, ventral V0-3, and motor neurons) with unique transcription factors.

Renshaw cells belong to V1 class (5% of V1 population)

1a inhibitory interneurons derive from two genetically distinct pools, the V1 and V2b classes.
(V1 - derived preferentially inhibit flexors and V2b-derived interneurons preferentially inhibit extensors)

(progenitor = the parent or direct ancestor of a person, animal, or plant)

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22
Q

What is the role of ascending tracts and what are the two main types?

A

To convery sensory information to the brain.

Dorsal column and spinothalamic tracts.

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23
Q

Briefly describe Dorsal Column tracts.

A

Collect and transmit information from skin/muscle/golgi tendon receptors to the brain.

Important for providing the brain with information regarding: fine touch, tactile discrimination, control of fine movements.

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24
Q

Briefly describe Spinothalamic tracts and what they are responsible for.

A

Carries nociceptive, temperature, crude touch, and pressure from our skin to the somatosensory area of the thalamus.

Responsible for our quick withdraw reaction to a painful stimulus.

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25
Q

What is the term for descending tracts and what are the two main types of them?

Where does the information they carry originate from?

A

Corticospinal tracts.

Pyramidal and extrapyramidal tracts.

Originates in the upper motor neurons in motor and premotor cortex (Brodmann Area 4, 6)

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26
Q

What are the two pyramidal tracts? Why are they “pyramidal”?

A

Lateral corticalspinal tract and the ventral corticospinal tract.

They pass through the pyramids of the Medulla Oblongata.

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27
Q

What is the main difference between the Lateral and the Ventral corticospinal tracts structurally?

A

Lateral decussates at the level of the pyramids, whereas the Ventral/Anterior tract does this at the level of the spinal cord.

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28
Q

What does the lateral corticospinal tract control?

A

Extremity muscles: voluntary contralateral movements.

  • Limbs, fingers.
  • Upper motor neurons innervate single muscles or small sets of muscles.
  • Therefore, responsible for fine motor control.
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29
Q

What does the ventral corticospinal tract control?

A

Controls voluntary contralateral movements of the axial muscles (e.g., neck, trunk muscles).

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30
Q

Why are the extrapyramidal tracts named as such?

A

Johann Prus 1898 – could not stop epileptic motor activity by disturbing the pyramids. Thus, alternative descending pathways – extrapyramidal

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31
Q

What are the roles of the extrapyramidal tracts? (4)

A

Control of learned/automatic and/or involuntary movements
Control of muscle tone
Control posture, drive postural adjustments
Control reflexes and orienting responses to sensory stimulation

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32
Q

What are the names of the 4 extrapyramidal tracts?

A

Reticulospinal tract
Rubrospinal tract
Vestibulospinal tract
Tectospinal tract

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33
Q

What are the roles of the reticulospinal tract? (4) What is its neural organisation? (2)

A

Control of posture and gross movements - locomotion, reaching.

Neurons branch extensively contacting many pools of motor neurons controlling synergistic muscles.

Single axons may innervate muscles on both sides.

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34
Q

What is the role of the rubrospinal tract? What is its neural organisation?

A

Control of muscle tone, arm muscles.

Originates in Red Nucleus (pink due to iron in haemoglobin/ferritin).

Transmit signals from cerebellum and motor cortex.

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35
Q

What are the roles of the vestibulospinal tracts? What is their neural organisation?

Name the two types and their specific details.

A

Receive information from vestibulococlear cranial nerve about angular and linear head accelerations.

Originate from vestibular nuclei in the pons.

Medial:
- Stabilises head position by innervating neck muscles.
- Receives input from the medial vestibular nucleus

Lateral:
- Controls “antigravity” muscles – extensor muscles of the legs.
- Receives information from the lateral vestibular nucleus

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36
Q

What is the role of the tectospinal tract? What is its neural organisation?

A

Coordinated orientation of neck, head and eyes according to visual and auditory stimuli (Note eyes control via cranial nerves III,IV,VI not via spinal cord)

Originates in superior colliculus (in non-mammalian vertebrates called optic tectum).

(Superior colliculus is an important centre of multisensory & sensorimotor integration– combines information to detect salient stimuli and orient the animal towards them).

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37
Q

What does decerebrated mean?

Example in stretch reflex study

A

Surgical separation of the upper part of the brain from the lower part of the brain.

(transection between superior and inferior nucleus)

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38
Q

Give a brief description of Sir Charles Sherringtons 2 studies into cat reflexes.

A
  1. Decerebrated the cat and found the stretch reflex was still intact.

Therefore the reflex doesn’t depend on most of the brain.

  1. Cut the sensory afferent reaching the dorsal horn (so a cut at dorsal root) and found the reflex was abolished.

Shows that the reflex was reliant on this connection being intact.

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39
Q

What is the basic circuitry for stretch (moytatic) reflexes?

Reciprocal inhibition…

A

There is direct sensory innveration (by 1a afferents) of the motor neurons that stimulate homonymous muscle and synergistic muscles.

Step 1.
1a afferents are excited by muscle elongation so excite the motor neurons from the same and synergistic mucsles to contract.

Step 2.
1a afferents excited by muscle elongation, excite 1a inhibitory interneurons which inhibit muscle contraction in antagonist muscles.

This results in contraction of the agonist and relaxation of the antagonist.

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40
Q

Outline the basic circuitry for the withdrawal reflex on the ipsilateral side.

Ipsilateral…

A

This is a polysynaptic reflex.

Step 1.
Starts with sensory informaiton from the Adelta fibres (nociceptive) exciting the spinal sensory neurons (interneurons).

Step 2.
These interneurons then synapse two other interneurons, which are inhibitory and excitatory.

Inhibitory signal inhibits the extensor muscles and the excitatory signal excites the flexor muscle.

This results in a quick withdrawal away from the nociceptive stimulus (due to harsh unihibited flexion at the joint)

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41
Q

Outline the basic circuitry for the withdrawal reflex on the contralateral side.
Why is there a contralateral component?

A

This is a polysynaptic reflex.

Step 1.
Spinal interneurons that receive the nociceptive signal on the ipsilateral side stimulate contralateral commissural neurons - one inhibitory and one excitatory.

Step 2.
The inhibitory neuron will inhibit the flexor muscles.
The excitatory neuron will inhibit the extensor muscles.

This allows us to steady our posture and ensure we have balance, as the limb on the ipsilateral side will withdraw quickly and impact our balance (literally pulling your leg out from underneath you without conscious control).

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42
Q

How are Renshaw cells harnessed for efficient descending control?

A

They can be used to control reflexes and muscles in a very efficient way:

  • Renshaw cells produce recurrent inhibition of activated muscles through motor neuron collaterals.
  • They also inhibit 1a inhibitory interneurons, thus indirectly exciting antagonist muscles.

Descending pathways can control these above stated “double action” via a single projection onto Renshaw Cells = efficient for descending control

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43
Q

What is special about the tendon reflex in relation to behavioural states?

A

It can be repurposed due to current behavioural state - behavioural state performed at that moment.

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44
Q

Describe the experiment into the impact of behavioural state on the effects of stimulation to 1b sensory afferents. Then summarise the general finding.

A

The experiment has two conditions under which it stimulates 1b afferents.

AT REST:
- 1b inhibitory interneurons are more active
- stimulation induces non-reciprocal inhibition of muscle (inhibiting muscle)

WALKING:
- Excitatory interneurons are more active.
- stimulation induces excitation of muscle during walking.

SUMMARY:
The activity of 1b inhibitory interneurons and excitatory interneurons is controlled by descending pathways according to behavioural state.

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45
Q

Define proprioception.

A

The perception of joint and body movement as well as position of the body, or body segments, in space.

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46
Q

Define exteroception.

A

The perception of external/environmental stimuli acting on the body (e.g., touch, vision, sound, smell).

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47
Q

What did the noble prize winning paper reveal about how mechanical stimuli are converted into electrical impulses in the nervous system?

A

The existence of PIEZO1 and PIEZO2 mechanosensitive ion channels.

They open upon the delivery of a mechanical force on a cell membrane.

Specifically, Piezo2 is highly expressed in proprioceptive peripheral endings (both muscle spindles and golgi tendon organs).

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48
Q

What did a mice study reveal about the molecular basis of proprioception?

A

They used conditional KO mice in which PIEZO2 was not expressed in dorsal root ganglion proprioceptive neurons.

These animals could perform basic locomotion but express impaired limb coordination.

(Because there was little to no mechanotransduction to provide feedback about where their limbs are in space).

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49
Q

What are the two types of fibres in muscles?

A

Extrafusal and Intrafusal.

50
Q

What are extrafusal fibres (EF)?

A

Fibres that provide force to the muscle and are stimulated by alpha motor neurons.

51
Q

What are intrafusal fibres (IF)?

A

Fibres that lay in parallel to EF fibres, do not provide force, and are stimulated by gamma motor neurons.

52
Q

What are muscle spindles?

A

They are spindle-like structures that are made up of IF fibres.

Central muscle spindle regions are non-contractile and contain sensory endings.

Contractile regions of IF fibres used to adjust length and are important to modulate sensory signalling.

53
Q

What are the three types of Intrafusal fibres?

A

Dynamic Nuclear Bag Fibre

Static Nuclear Bag Fibre

Nuclear Chain Fibre

54
Q

What types of afferent nerve fibres stimulate which Intrafusal fibres?

A

Dynamic Nuclear Bag Fibre:
- Dynamic gamma MN’s
- 1a afferents

Static Nuclear Bag Fibre:
- Static gamma MN’s
- 1a afferents
- II afferents

Nuclear Chain Fibre:
- Static gamma MN’s
- 1a afferents
- II afferents

55
Q

What are the two types of afferent nerve fibres that stimulate intrafusal fibres, what are their roles?

A

Ia afferents: sense velocity and length (change)
- Innervates all 3 IF’s

II afferents: sense length
- Doesn’t innervate Dynamic nuclear bag fibres

56
Q

How do muscle spindles relay information about muscle length?

A

IN and EF fibres are positioned in parallel.

When the muscle is stretched it elicits a firing rate in the dorsal root ganglion interneurons that are associated with sensory endings.

The firing rate is related to the amount of stretch.

HIGH FIRING RATE = STRETCHED

57
Q

Outline briefly the structure and process of how Golgi tendon organs work.

A

They are arranged in series with the muscle and their sensory endings are 1b afferents, which are intertwined with collagen fibres.

When the muscle fibres contract, collagen fibres compress the axons which generates 1b signalling.

58
Q

What do Golgi tendon organs (GTOs) encode? What is the relationship between what they encode and their firing rate?

A

They encode the force in the muscle.

Their firing rate increases as a function of the force applied to their endings.

(The higher the force of the contracting muscles, the higher their firing rate)

59
Q

What are joint capsule receptors and where are they located?

How many types are there and what are their associated fibres?

A

They are proprioceptors found in the joints.

There are 4 types:

  • Type 1: ruffini endings.
  • Type 2: paciniform endings.
    ^^Both of these respond to deformation of joint capsule or ligaments
  • Type 3 and Type 4
    Both these respond to noxious mechanical stimuli.
60
Q

FIBRE TYPE Ia:

Summarise the receptor type, characteristics and what they’re sensitive to.

A

Muscle spindle-primary

Myelinated, large, diameter: 12-20um, conductance velocity: 72-120m/s

Muscle length and dynamic change in length

61
Q

FIBRE TYPE Ib:

Summarise the receptor type, characteristics and what they’re sensitive to.

A

Golgi tendon organ

Myelinated, large, diameter: 12-20um, conductance velocity: 72-120m/s

Muscle contraction/tension.

62
Q

FIBRE TYPE II:

Summarise the receptor type, characteristics and what they’re sensitive to.

A

Muscle spindle secondary, Joint capsule mechanoreceptor.

Myelinated, medium, diameter: 6-12um, conductance velocity: 36-72m/s

Muscle stretch and joint angle/deep pressure

63
Q

Define gain.

A

Ration between output from and (sensory) input to a given system.

64
Q

Outline the gain control of muscle spindles.

A

If you apply weight to a muscle then the muscle spindle length increases, therefore increasing the Ia fibres firing rate.

However, stimulation of EF fibres alone causes EF fibres to contract but not the IF fibres.

This INHIBITS feedback and therefore we don’t have control of muscle length - GAIN DROPS TO 0

So, not only do we need muscle control from alpha and gamma neurons, we also need gamma neurons to control the length of muscles spindles - to maintain sensory feedback GAIN.

65
Q

Outline the gain control by static and dynamic gamma fibres.

A

Muscle spindles signal both muscle length (static response) and changes in muscle length (dynamic response).

Static gamma fibres increase the gain of static response.
- Their firing rate is a relatively flat line as it signals the current length of muscles (current length will always be constant)

Dynamic gamma fibres increase the gain of dynamic response.
- Their firing rate is graded because they show that the stretch of the muscle is changing.

Differential modulation of both these fibre types changes sensory feedback by altering the balance between static and dynamic responses.

66
Q

Outline the type of gamma fibre associated with different types of movement.

A

Static gamma fibres:
- Preferentially active during static postures or slow movements.

Dynamic gamma fibres:
- Active during fast or unpredictable or complex movements requiring fast corrections

67
Q

Outline the monkey study into flexibility in gain control according to actions.

A

METHODS:
- Monkey trained to use a joystick to move a cursor on a screen in response to a visual cue.
- At the same time, an interneuron from the spinal cord was recorded from.
- It received sensory feedback from either muscle (proprioception) or cutaneous afferents (tactile)
- Each would be stimulated when the monkey was resting or was engaged with the joystick.

RESULTS:
- IF they stimulated the tactile feedback during rest, there was a strong response, therefore strong gain.
- IF the monkey was moving the joystick, this stimulation of tactile was reduced massively (therefore reduced gain).
- However, the proprioceptive feedback had much more gain because it now needed feedback from the muscles that it was using.

68
Q

Why is flexibility in gain control important?

A

So that the CNS can amplify the more relevant feedback and suppress the less relevant one depending on the situation.

69
Q

Describe the methods and results of the mice study into how sensory feedback from muscle spindles affect movements.

A

METHODS:
- Developed mutant mice in which the proprioceptive feedback from the dorsal root ganglion was abolished (Erg3 mutant mice).
- Used a gate analysis and electromyographic recordings from the flexor and extensor muscles to compare the mice.

RESULTS:

SIMPLE WALK:
- Mutant mice’s pattern was partially compromised - still able to walk and had alternation between extensor and flexor muscles.
- WT mouse was fine.

COMPLEX LADDER WALK:
- The mouse would either slip of fall through the ladder rungs and had irregular contractions of the flexor and extensor muscles.

CONCLUSION:
- Proprioception is more important in complex movements as it requires fine motor control.

70
Q

What were the two competing theories regarding the generation of rhytmic motor movements

Week 7 - Rhythmic movements 1

A

Chain of Reflexes vs Central Model (CPGs)

71
Q

What is the ‘Chain of reflexes’ theory of rhythmn motor pattern generation

Week 7 - Rhythmic movements 1

A
  • Rhythmic pattern arise through series of reflexes
  • One reflex after another generates the motor pattern
  • Know we have sensory afferents from tactile receptors in our feet, and have proprioception in our muscles → such tactile and proprioceptive sensory reafference is necessary for rhythmic movements
72
Q

What is the ‘Central Model’ theory of rhythmn motor pattern generation

Week 7 - Rhythmic movements 1

A
  • Rhythm is centrally generated not mediated by sensory reafference
  • I.e not provided by the interaction between the central and periphery nervous system
  • Sensory reafference is therefore not necessary → however sensory reafference is important to adjust motor patterns to external conditions
73
Q

How do the Central model and chain of reflexes theories of motor pattern generation differ

Week 7 - Rhythmic movements 1

A

In the chain of reflexes model, sensory reafference is necessary for the generation of rhytmic movements due to the sequential generation of rhytmic movements.

However for the central model, though important, sensory reafference is not necessary for rhytmic movement generation as the rhythmn is centrally generated

74
Q

Who performed the first demonstation of CPGs and how

Week 7 - Rhythmic movements 1

A
  • Graham Brown observed rhythmic movements in cats with removed cerebral hemispheres (decerebrate cats), showing that these movements can occur independently of higher brain centers. This suggested the presence of CPGs in the spinal cord, capable of generating rhytmic motor patterns w/o direct brain input
75
Q

What did Graham browns (1911) study using decerebrated cats do for our understanding of CPGs

Week 7 - Rhythmic movements 1

A

Brown’s study provided early evidence supporting the idea that spinal cord mechanisms can control rhythmic movements, contributing to our understanding of CPGs.

76
Q

What are fictive motor patterns?

Week 7 - Rhythmic movements 1

A

Fictive motor patterns are rhythmic firing patterns generated by nervous tissue in the spinal cord ex-vivo, resembling those observed in-vivo.

(ex-vivo = outside the living body, in-vivo = inside the living body)

77
Q

What do fictive motor patterns provide for research

Week 7 - Rhythmic movements 1

A

Further demonstration & thus further evidence of CPGs

78
Q

What was demonstrated by recording electroneurography traces in crustaceans both in-vivo and ex-vivo)

(Demonstration of CPGs)

Week 7 - Rhythmic movements 1

A

Sequential patterns of muscle activation in the stomatogastric ganglion region via recording electroneurography traces

This was seen both in-vivo and ex-vivo

(ex-vivo = outside the living body, in-vivo = inside the living body)

79
Q

What CPGs are thought to exist in the human body (2 locations x 3 examples)

Week 7 - Rhythmic movements 1

A

CGPs Located in
* Brainstem (e.g respiratory CPGs, chewing CPG, swallowing CPG)

  • Spinal cord (defecation CPG, ejaculation CPG, locomotion CPGs)
80
Q

What is a limitation in our understanding of CGPs in humans

Week 7 - Rhythmic movements 1

A

very few/none have been recorded in humans, thus the locations of CGPs in humans are estimations based off of recordings on animal models

81
Q

The expression of rhythmic patterns by CPGs depends on the interaction of two factors. What are they?

Week 7 - Rhythmic movements 1

A
  1. The circuitry - the connectivity among cells in the network (synapses and gap junctions)
  2. The circuit elements - the intrinsic dynamics of individual cells within the network

The rhythm is an emergent property of these interactions

82
Q

What is the circuitry of CPGs for locomotion?

Week 7 - Rhythmic movements 1

A

We do not have a definitive answer, but we do have several proposed models

(will be elaborated on in other flash cards)

83
Q

In reference to the circuitry of CPGs for locomotion

What model is proposed for the involvement of CPGs in individual limbs

Week 7 - Rhythmic movements 1

A

Distinct CPGs are associated w/ individual limbs (as we know single limbs can generate rhythmic patterns)

84
Q

In reference to the circuitry of CPGs for locomotion

What model is proposed for the involvement of CPGs in limb coordination and left-right coordination

Week 7 - Rhythmic movements 1

A

Limb coordination (e.g alternation in walking) arises from connectivity among the CPGs of individual limbs

Left-right coordination achieved by commissural interneurons crossing the spinal cord midline

commisural interneuron = cell w/ axonal projection that crosses midline

85
Q

In reference to the circuitry of CPGs for locomotion

How is forelimb-hindlimb coordination achieved in tetrapods (cats,mice, animals that stand on four legs)

Week 7 - Rhythmic movements 1

A
  • By long propriospinal descending neurons (LPDNs)
  • LPDNs connect cervial and upper thoracic segments w/ lumbar spine segments
86
Q

What are the three circuit models of CPGs

Week 7 - Rhythmic movements 1

A

Half centre model, multilevel model, multiple interacting CPG model

87
Q

What is the half centre CPG model

4 points

Week 7 - Rhythmic movements 1

A
  • Features both excitatory and inhibitory interneurons
  • The activation of an excitatory interneuron (for flex) ecites an inhibitory interneuron which inhibits the other excitatory interneuron (for extension)
  • Rhythm is generated by reciprocal inhibition & rhythm provides alternate excitation of extensor and flexor motor neurons
  • Downstream interneuron provide reciprocal inhibition of motor neurons
88
Q

What are some limitations of the Half centre CPG model

Week 7 - Rhythmic movements 1

A

the half centre CGP model is simple but does not account for many experimental observation

  • Does not explain the complexity of both natural and fictive motor patterns - There are other muscles that have activation patterns that does not follow strict extensor-flexor alternation
  • Does not explain the effects of sensory reafference on phase and/or duration of locomotor patterns - Changes in relative duration of flexor and extensor activation phase does not always cause an effect on the overall cycle duration
89
Q

What is the multilevel CPG model

(4 points)

Week 7 - Rhythmic movements 1

A
  • There is an intermediate level between the rhythm generator level (RG) and motorneurons called the Pattern formation level (PF)
  • while the RG provides the rhythm, the PF adjusts the specific pattern activations (e.g by interacting with Ia and Renshaw interneuron cells)
  • In this, the pattern formation level can adjust/control the phases of extensor and flexor activation indepndently according to sensory reafference, without affecting cycle duration
  • The rhythm generator level connects to multiple pattern formation levels, allowing it to achieve complex sequential muscle activations that we see in animals
90
Q

What is the multiple interacting CPGs model

Week 7 - Rhythmic movements 1

A
  • Within each limb there are separate CPGs for each set of synergistic muscles.
  • Interactions among CPGs generate complex patterns of muscle activation
  • still debated whether the multilevel or multiple CPG model apply to locomotion.
91
Q

In reference to individual cell properties

What is endogenous bursting?

Week 7 - Rhythmic movements 1

A
  • Refers to cells that can generate a rhythm w/o needing stimulation
  • also referred to as a pacemaker property due to myogenic rhythm generation
92
Q

In reference to individual cell properties

What are plateau potentials

Week 7 - Rhythmic movements 1

A
  • Refers to cells that have bistable properties
  • These cells can be turned on w/ a positive current and will start firing and keep firing until they are turned off w/ a negative current
  • these cells can be used by health centres to generate flexor or extensor muscle contraction
93
Q

In reference to individual cell properties

What is a postinhibitory rebound

Week 7 - Rhythmic movements 1

A

We think a large fraction of interneurons in the spinal cord are inhibitory, so when they communicate, a useful response would be to express this rebound in electrical activity when the inhibition is released

94
Q

In reference to individual cell properties

What is spike frequency adaptation

Week 7 - Rhythmic movements 1

A

An idea proposed originally by Graham brown in the half centre model, under the name of ‘fatigue’

(EXTRA CONTEXT: Two half-centre CPGs - one for each side.

When active, they inhibit the other side, allowing for the alternation.

Spike frequency adaptation is the slow decrease in firing rate due to adaptation.

As a result, the inhibition from to the other side decreases.

When this decreases, the other side increases and inhibits the originally firing side.

This repeats again and again to create alternating patterns of movement)

(this is all the info he tells us)

95
Q

Using walking as an example

What are the two main phases of locomotion

Week 7 - Rhythmic movements 1

A

Stance - the foot is in contact w/ the floor
Swing- there is no contact, the leg is swinging

96
Q

Using walking as an example

What would result in a decrease of stance duration

Week 7 - Rhythmic movements 1

A

Walking at a higher speed

97
Q

What controls the timing of stance to swing transition in locomotion

(two things)

Week 7 - Rhythmic movements 1

A
  • Proprioception
  • Sensory reafferrence also controls transition at the level of the spinal cord
98
Q

In reference to locomotion

What are decerebrated cats able to do when walking on a treadmill

Week 7 - Rhythmic movements 1

A

They are able to adjust their locomotor pattern according to the different speeds on a treadmill

99
Q

What is the ‘stance’ phase of locomotion associated with the activation of

Week 7 - Rhythmic movements 1

A

Extensor (anti-gravity) muscles

100
Q

What is the ‘swing’ phase of locomotion associated with the activation of

Week 7 - Rhythmic movements 1

A

Flexor muscles

101
Q

What is hindlimb extension in tetrapod animals

(tetrapod = 4 limbed vertebrate animals)

Week 7 - Rhythmic movements 1

A
  • Swing is initiated when the leg is extended
  • in spinalized cats extending the hip (but not knee or ankle) initiates flexor burst
  • This effect is elicited by muscle spindles in hip flexors

(Spinalised = surgical severing of spinal cord)

102
Q

What is hindlimb unloading in tetrapod animals

(tetrapod = 4 limbed vertebrate animals)

Week 7 - Rhythmic movements 1

A
  • swing is initiated when the leg is unloaded
  • in decrebrated cats,loading of ankle extensors inhibits flexor burst
  • this effect is mediated by 1b afferents
103
Q

Which region is the ‘interface’ for the brain and spinal cord in locomotion?

A

The brainstem

104
Q

What is the Mesencephalic Locomotor Region (MLR)?

A

It is a loosely defined region within the brainstem which was discovered using electrostimulation.

105
Q

What was does MLR stand for and what was the study by Shik, Severin, Orlovskii (1996) into its discovery?

A

Mesencephalic Locomotor Region:

METHODS:
- Used Cats
- Electrically stimulated it at constant frequencies.
- Observed the elicited behaviour.

RESULTS:
- Low intensity stimulation elicited walking.
- High intensity stimulation elicited running.

It has been found in fish, birds and other legged animals such as mice.

106
Q

What aspects of locomotion does MLR control? Outline how this was found.

A

MLR firing rates determine initiation and speed of locomotion.

Optogenetic stimulation of glutamatergic neurons at 10hz doesn’t evoke locomotion, whereas 20hz does.

SO the frequency of stimulation is important.

However, there is a large degree of heterogeneity among individual neurons in this region (some neurons negatively correlate with locomotion - slide 7 lecture 14)

107
Q

What are the structures within the MLR? What types of neurons do they express?

A

It is composed of two main anatomical regions:
- PPN (the pedunculopontine nucleus)
- CnF (the cuneiform nucleus)

Both PPN and CnF express excitatory glutamatergic and inhibitory GABAergic neurons.

PPN also expresses cholinergic neurons.

108
Q

What are the neuron types within the MLR and what is their role in locomotion?

A

Glutamatergic:
- Initiate and drive locomotion.

GABAergic:
- Terminate locomotion.

Cholinergic:
- Modulate locomotion.
- (they also have ascending pathways back to the brain so their involvement is a bit more complex)

109
Q

What are the roles of the CnF and PPN within the MLR, what types of neurons and firing rate relate to each modality? How was this discovered?

A

Optogenetic stimulation of glutamatergic neurons.

CnF:
- Evokes the full range of speed, from slow (walking) to fast locomotion (running, galloping, bounding).
- Firing rate of glutamatergic neurons are linearly related to speed.

PPN:
- ONLY evokes slow locomotion.
- Firing rate of glutamatergic neurons have more complex relationship with speed.

110
Q

What are some alternative roles that glutamatergic PPN neurons are selectively active during?

A

Rearing (neurons project to spinal cord)

Grooming and handling (neurons project to basal ganglia)

111
Q

What do neurons in the PPN - that are active during rearing - control? Why is this an important discovery?

A

Body extension.

To effectively move, we need to be in the correct posture.

Therefore, understanding the role of these neurons in posture is key to understanding how locomotion works.

(optogenetic inhibition occurs, the extension is reduced massively to flexion - video on slide 13 lecture 14).

112
Q

Briefly summarise the differences in synaptic inputs to both the CnF and PPN.

A

PPN neurons receive a wider range of synaptic inputs from other brain regions.

CnF more specific inputs from colliculi and periaqueductal grey - regions associated with orienting and defensive movements.

113
Q

What does the MLR use to control spinal circuits?

A

The Medullary Reticular Formation (MRF)

114
Q

What is the MRF?

A

Medullary Reticular Formation

115
Q

Summarise the role of the CnF in the MLR.

A

Sufficient to evoke fast locomotion via glutamatergic neurons.

Receives projections from limited set of regions including the Colliculi and the Periaqueductal Grey - areas mediating defensive behaviours.

Plays a dominant role in fast locomotion, escape.

116
Q

Summarise the role of the PPN in the MLR.

A

Only evokes slow locomotion via glutamatergic neurons.

Different sub-classes of glutamatergic neurons encode a wider variety of behaviours including rearing, locomotion, grooming and handling.

Receives a wider range of projections from cortex, basal ganglia and other brainstem nuclei.

Plays a dominant role in slow pace exploratory behaviours

117
Q

What happens to decerebrated cats in relation to locomotion?

A

They can walk but balance and muscle tone are impaired.

Their weight needs to be sustained.

118
Q

What is the role of the vestibular system in locomotion? How do we know this.

A

Patients with vestibular disorders suffer from:

  • Postural instability
  • Reduced ability to respond to unexpected perturbations
  • Increased variability of locomotor pattern (especially at low velocity).

Therefore it must play a role in all of these to some degree.

119
Q

What is the LVN?

A

The Lateral Vestibular Nucleus.

120
Q

Outline the role of the LVN in locomotion.

A

Neurons from the LVN project to the spinal cord via the lateral vestibulospinal tract.

Involved in the control of posture:
- LVN controls the tone of extensor muscles (antigravity).

“Decerebrated Rigidity”:
- Decerebrated cats expressed extended and stiff limbs, this was reduced after lesioning of the LVN>

LVN neurons fire more during locomotion then at rest, with highest firing during extensor muscle contraction.

121
Q

Outline the study into the LVN’s responses to balance perturbations.

A

METHODS:
- Got mice to walk along a balance beam and then perturbated the beam to test balance
- Tested this in control animals and animals with lesion/ablation of LVN.

RESULTS:

CONTROL mice:
- Can walk across easily with balance maintained.
- They track with tailbase it is mainly in line with where they walk.
- When perturbations occur:
- Fast EMG response to perturbation in extensor muscle (~20ms latency)
- Slower EMG co-activation of flexors and extensors (~50ms)

LVN lesion mice:
- Abolishes EMG fast and slow response.
- Increases postural instability.
- Even without perturbations, their tale tracking was off.

122
Q

Outline down regulation of vestibular control.

A

Vestibular reflexes (e.g., LVN excitation of extensor muscles) oppose movements to maintain stable balance.

This ensures that motion vestibular reflexes are transiently downregulated during inititation and termination of locomotion.

Motor Systems 21332 (7 decks)

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