6.1 Motor Control

Question 1

Define: Lower Motor Control

Answer 1

Lower Motor Control: If this is active, skeletal muscles innervates are active

Question 2

Define: Upper Motor Control

Answer 2

Upper Motor Control: Anything that controls the excitability of a lower motor neuron**

Question 3

What kind of neurons are motoneurons, Where are their cell bodies, What do they do?

Answer 3

Large cholinergic neurons, with cell bodies in ventral spinal cord and large fast-conducting myelinated axons, innervates muscles and cause contractions

Question 4

How are motor neurons organised?

Answer 4

• Motoneurons are organised into distinct, rod-shaped clusters called motor neuron pools

Motor Neuron Pools

• Motor neuron pools are clusters of motoneurons that innervate muscle fibres within a single muscle
• Motor neurons pools have little overlap and follow a broad topography: There is a relationship between where muscles are and where motor neurons are located in the spinal cord (i.e., neurons that are clustered together tend to innervate muscles which are adjacent to each other)

Question 5

Relationship between motor neurons and muscles - Organisational structure

Answer 5

Somatotropic Organisation

Proximal muscles are innervated by lower motor neurons whose cell bodies are located medially in the ventral spinal cord

Distal muscles are innervated by lower motor neurons whose cell bodies are located laterally in the ventral spinal cord

Question 6

Relationship between motor neurons and fibres.

Also, how are muscle fibres distributed and why?

Answer 6

• Each single motor neuron innervates a number of muscle fibres. This is called a motor unit
• Each muscle fibre is innervated by a single lower motor neuron
• Muscle fibres that are innervated by a single motor neuron are distributed widely (not located adjacently). This is to prevent muscle damage.

Question 7

How do motor units differ in number they innervate?

Answer 7

Some motor units innervate a small number of muscle fibres
Some motor units innervate a large number of muscle fibres

Question 8

How do motor units differ in type

Answer 8

1. Slow: Small motor neurons produce the least amount of force, and are recruited first

• Always recruited, can contract constantly, aerobic capacity
• Allows us to do fine, delicate things

2. Fast fatriguable-resistant
3. Fast faiguable: Large motor neurons produce the most amount of force, and are recruited last
   * Recruited less, anaerobic capacity

Maximum force = All motor neurons recruited

Question 9

What is the Size Principle

Answer 9

The size principle states that motor units will be recruited in order of size from smallest to largest depending upon the intensity

Question 10

Synaptic inputs to lower motoneurons includes

2 ++

Answer 10

1. Sensory input from muscle spindles
   - Key sensory input from muscles and comes through to dorsal spinal cord

• Either: (a) directly to alpha motor neurons or (b) indirectly through spinal interneurons to alpha motor neurons
  2. Inputs from upper motor neurons originating in the brain
• Either: (a) directly or; (b) more commonly indirectly through circuit neurons

Question 11

What do muscle spindles detect and what is their main role?

Do all muscles have muscle spindles?

Answer 11

• Stretch sensory receptors (Mechanoreceptors) embedded in the intrafusal muscle detects changes in muscle length or stretch
• Maintain joint and muscle position by detecting muscle length

Question 12

What are the properties of Group 1 and 2 afferents?

Answer 12

Group 1 and 2 afferents are the biggest, fastest conducting afferent axons

Question 13

Muscle spindle reflex: Activation

Answer 13

When muscle spindles are activated beyond set point, they generate a set of APs which travels to the spinal cord via sensory afferent

Question 14

Sensory afferent of muscle spindles (2). What does this act maintain

Answer 14

Disynapse:

• Directly excites (monosynapse) the (alpha) lower motor neurons that innervate that same muscle
• Indirectly inhibits antagonistic muscles via an inhibitory interneuron
• This acts to maintain joint position by correcting changes made to joint position : Negative feedback

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Question 15

Muscle Spindles: Reactions to muscle contracting or relaxing

Question 16

Overview and Main Role of Golgi Tendon Organs (GTO)

Answer 16

Detect the force of the muscle which is being transferred through the tendon

Maintain force generated by detecting and regulating tension coming through tendons

Question 17

GTO reflex: Activation

Answer 17

If force surpasses a set point, GTOs are activated and creates inhibitory reflexive circuits.

Question 18

Sensory afferent of GTO (2)

Answer 18

Disynapse

1. Synapse with an inhibitory interneuron (1b) in the spinal cord, which synapses with the alpha motor neuron innervating the same muscle
2. Synapse with an excitatory interneuron, which synapses with the alpha motor neuron innervating the antagonistic muscles

Question 19

Example: Stretch Reflex. When muscles contract, how do spindles and GTO respond and what is the implication?

Answer 19

• Reduced length decreases muscle spindle activity
• The increased force increases GTO activity
• Hence, they provide 2 independent information about muscles

Question 20

What are Gamma motor neurons (vs alpha motor neurons)

Answer 20

Gamma motor neurons innervates intrafusal fibres at 2 ends of the spindle while alpha motor neurons innervates extrafusal fibres. This is often done in synchrony (Tuning function)

Question 21

Flexion-crossed extension reflex (2)

Answer 21

Noxious stimulus activates nociceptor, and its sensory afferents called cutaneous afferent fibres (via. interneurons) will:

• Activate flexor muscles and inhibit extensor muscles on the same leg, allowing us to withdraw from the noxious stimulus
• Activate extensor muscles and inhibit flexor muscles on the opposite leg, allowing us to weight-bear
• Some of them also crossover

Ipsi = withdraw, Contral = Extend

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Question 22

Pattern Generation: Walking. Phases in humans and animals

Answer 22

Sequentially:

• Flexion phase: Activating flexors and inhibiting extensors (Swing in animals)
• Extension phase: Activating extensors and inhibiting flexors (Stance in animals)

Question 23

What controls locomotor pattern?

In cats, what are the findings?

Answer 23

Spinal motor pattern generator creates locomotor patterns

The transition between gait (i.e. walk to run) and pattern of gait is controlled by the spinal cord. When treadmill is sped up, the cat moves to match the treadmill speed

Question 24

Supra-spinal inputs to motoneurons: Types (2)

Answer 24

Lateral white matter tracts: Axons from motor cortex

Medial white matter tracts: Axons from brain stem

Question 25

Difference between complex: aspects of motor control vs motor tasks (2)

Answer 25

Spinal cord mediates complex aspects of motor control such as feedback control of muscle stretch and force, and integration of reflex and rhythmic activity.

More complex motor tasks - like maintaining body posture and formulating and executing voluntary movements - require the brain: the main sources of this input are the brainstem and the motor areas of the cerebral cortex..

Question 26

What forms the basis of alternative movements such as breathing and locomotion? (2)

Answer 26

Pattern generating circuits in the spinal cord

Basis of these pattern generators are intrinsically active neurons firing bursts of action potentials, and their reciprocal interconnections such that alternating output to flexors and extensors, and to the left and right limbs is produced.

Question 27

What are the ventral-medial tracts, specifically? What information do they carry?

Answer 27

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Ventral-medial: Brainstem to spinal cord

Ventral / medial tracts (tectospinal, vestibulospinal and reticulospinal) carry information related to maintenance of posture.

Their activity is influenced by visual, vestibular (ear,balance) and proprioceptive activity (proprioceptive = ““body position sense””: mainly muscle spindle activity).

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Question 28

Common features of ventral-medial tracts (2)

Answer 28

”

• Axons project ipsilaterally in medial white matter tracts
• Synapses with medially located interneurons on either side of the midline in cervical spinal cord

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Question 29

Descending Pathways from motor cortex (2)

Answer 29

1. Corticobulbar Pathways
2. Corticospinal Pathways

Question 30

Descending Pathways from motor cortex (2): Corticobulbar Pathways

Answer 30

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Synapses at brainstem

Reticular formation is often a target of the motor cortex neuron axons, which then descend down the reticulospinal tract

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Question 31

Descending Pathways from motor cortex (2): Corticospinal Pathways

Answer 31

”

Directly from motor cortex to spinal cord

• Internal capsule into brainstem
• ~90% will decussate in brainstem (medella), and continue in the lateral corticospinal tract’;

~10% will continue ipsilaterally and continue to the ventral corticospinal tract, decussating in spinal cord

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Question 32

Organisation of motor cortex

Answer 32

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Regions of functionally related movements (rather than a topographic map of the body), corresponds with their degree of motor innervation

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Question 33

What do motor maps represent?

Answer 33

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• Mapped on the M1 are relevant combinations of muscle action that produce movements
• Purposeful movements result from prolonged stimulation of M1

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Question 34

Role of Motor association cortex

Answer 34

Motor association cortex encodes more complex movements and more abstract representations of motor goals

Question 35

What are lower motoneuron signs

Answer 35

Without innervation from motoneurons, small regions within muscles (single fibre or groups of fibres - fascicles) may exhibit weak spontaneous contractile activity; prolonged denervation results in muscle atrophy.

Question 36

What are upper motoneuron signs

Answer 36

Upper motoneuron signs are characterised by over-excitability of muscles, due to a loss of the net inhibition the brain exerts over the spinal cord.

Question 37

Example of an upper motoneuron syndrome

Answer 37

“Babinski sign:

Brain unable to control spinal cord and tonically inhibit reflex, causing extensor plantar response

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Question 38

Feedforward/Feedback Mechanisms

Answer 38

Feedforward system from central command anticipates postural adjustments based on previous experience

Once the movement is executed, if we still have postural instability, the feedback system from postural reflexes will correct this by feedback for unanticipated postural instability