HNS23 Control Of Movement

Question 1

Motor unit

Answer 1

Each motor neuron innervates multiple muscle fibres
—> One motor unit
—> One motor neuron + All muscles fibres innervated

• Basic element which cause movement
• Controlled by the brain
• Size of motor neuron (no. of muscle fibres innervated by a single neuron) varies greatly among different muscles
  E.g.
  1. Slow fatigue-resistant muscles (Soleus) —> fewer muscle fibres per motor neuron —> smaller motor unit
  2. Fast fatigable muscles (Gastrocnemius) —> more muscle fibres per motor neuron —> larger motor unit
  3. Muscles that generate small contraction force + Require better control —> Eyeball —> fewer muscle fibres per motor neuron —> smaller motor unit

Question 2

Force of muscle contraction

Answer 2

Controlled by nervous system in 2 ways:

1. ***Frequency coding:
   - ↑/↓ frequency of action potential firing in motor neuron
   —> ↑ frequency of firing of motor neuron allow forces of successive muscles twitches to summation —> sustained (tetanic) contraction (larger force + smoother contraction)
2. ***Population coding:
   - ↑/↓ recruitment of motor unit number
   - 3 types of motor unit co-exist in a given muscle:
   —> Slow
   —> Fast fatigue-resistant
   —> Fast fatigable (generate larger force at a given AP firing rate)
   - Size principle: ↑ recruitment of motor units in order from weakest (slow fatigue-resistant) to strongest (fast fatigable) muscle

Question 3

***3 types of movement

Answer 3

1. Reflex
   - Stereotyped responses (e.g. stretch reflex)
   - Stereotyped motor coordination (e.g. flexion reflexes)
2. Rhythmic motor pattern
3. Voluntary movement

Question 4

Reflex

Answer 4

• Simple, rapid, stereotyped (predictable result), involuntary movement
• Controlled and elicited by stimulus (e.g. stretch reflex, flexion withdrawal reflex)
• Protective (withdrawal of limb from painful stimulus, sneezing, coughing)
• Essential for voluntary action

Question 5

Rhythmic motor pattern

Answer 5

• Repetitive movements (e.g. walking, running, swallowing)
• Combining features of **voluntary + **reflex acts
• Typically only ***initiation + termination are voluntary
• Once initiated —> sequence of movement is ***stereotyped, repetitive and may be automatic

Question 6

Voluntary movement

Answer 6

• Complex movement
• 2 features:
  1. ***Complex, goal-directed (intentional)

2. ***Learned (performance improves with practice) (e.g. playing the piano)
   - More the skilled movements are mastered with practice —> Less require conscious direction (e.g. signing your name)

Question 7

Control of movement

Answer 7

More than controlling muscle contraction and relaxation (on/off):

Also involves

1. Accurate time control of contractions of different muscles
2. Associated postural adjustment (associated muscles contract as well)
3. Adjusting mechanical properties of joints and muscles
4. Sensorimotor integration
5. Error detection, feedforward and feedback control

Question 8

Upper and Lower motor neurons

Answer 8

Upper motor neurons (modulate LMN):

1. Motor cortex (highest level: can also control LMN in brainstem)
2. Brainstem

Lower motor neurons (directly innervates muscle):

1. Brainstem (Motor nuclei of cranial nerves)
2. Spinal cord (Anterior horn cells)

Question 9

Organisation of motor control

Answer 9

3 levels of motor control:

• Spinal cord
• Brainstem
• Cerebral cortex
• Hierarchical organisation of 3 levels + Parallel organisation (independent effect)
• ALL 3 influenced by independent subcortical structures: Basal ganglion + Cerebellum
• ALL levels receive continuous flow of sensory information about environment, position, orientation of body and limbs, degree of contraction of muscles
• Most important:
  —> Vestibular input
  —> Joint afferents
  —> Muscle spindles

Question 10

Spinal cord

Answer 10

Ventral horn:
- contain LMN (final common pathways for motor execution)

Ventral horn neurons are Somatotopically arranged:

• Medial neurons (Anterior CST): **Axial + **Proximal musculature
• Lateral neurons (Lateral CST): ***Distal musculature (more ventral motor neurons: extensor)

Interneurons in spinal cord —> modulate motor neurons

Question 11

Spinal circuitry without input from Brain

Answer 11

1. Stereotyped responses (e.g. stretch reflex)
2. Stereotyped motor coordination (e.g. flexion reflexes)
3. Rhythmic locomotor pattern (e.g. walking)

Question 12

Stretch reflex (Stereotyped responses)

Answer 12

• Stereotyped reflex
• Triggered by muscle spindle (in response to muscle stretch) (Proprioceptor)
  —> 1a fibre afferent fibre
  —> α motor neuron innervating homonymous muscle directly activated (monosynaptic) —> contraction
  —> inhibitory interneurons in spinal circuitry inhibit motor neuron innervating antagonistic muscle (polysynaptic) —> relaxation
• Adapted for speed of operation —> allow muscle tone to be regulated quickly without intervention from higher centres

Question 13

Flexion reflex (Stereotyped motor coordination)

Answer 13

• Polysynaptic pathways + involve multiple interneurons (to both limbs) (Cutaneous afferent fibre from ***Nociceptor —> Interneuron —> Motor neuron)
• Contraction of flexor + Inhibition of extensor of stimulated limb
  —> but opposite response of contralateral limbs (額外)
• Descending control from brain can adjust ***strength of spinal reflexes to the requirements of specific motor acts (e.g. withstand hotness of cup for a while)
• More than protective response —> also coordinate voluntary movement (e.g. jumping, running)

Question 14

Rhythmic locomotor pattern

Answer 14

• Voluntary action but once initiated —> not require conscious direction (e.g. walking)
• Initiation: ***Mesencephalic locomotor region (MLR) (in Brainstem)
• Rhythmic locomotor pattern obtained when spinal cord disconnected from brain —> Rhythmic stepping produced by neural circuit (Central pattern generator) ***within spinal cord (無腦部connection都可以行路)

Question 15

Brainstem

Answer 15

• LMN: motor innervation to Head + Neck
• UMN: modulate activities of motor neurons and interneurons (within Spinal cord) via descending pathways
• Controls posture by integrating Visual, Vestibular with Somatosensory information
• Specific areas coordinate movements of Head + Eyes

Question 16

3 classes of Brainstem LMN

Answer 16

Brainstem LMN —> motor innervation to Head + Neck

1. General somatic motor neurons
   - tongue muscles
2. Special visceral (Branchial) motor neurons
   - striated muscles —> chewing, facial expression, larynx, pharynx
3. General visceral motor neurons
   - parasympathetic preganglionic neurons —> pupillary constriction, salivary secretion

Question 17

***Brainstem UMN: 4 Descending pathways (Extrapyramidal tracts)

Answer 17

Medial descending system
—> Modulate ventral motor neurons on Medial side (Anterior CST)
—> Axial + Proximal muscles:
1. Tectospinal tract
- Crossed
- Optic tectum / Superior colliculus (midbrain)
- ***Eye / Head orientation

2. Reticulospinal tract
   - Reticular formation (brainstem)
   - ***Control posture (input from motor cortex)
3. Vestibulospinal tract
   - Vestibular nuclei (medulla)
   - ***Control posture (input from vestibular nuclei)

Lateral descending system
—> Modulate ventral motor neurons on Lateral side (Lateral CST)
—> Distal muscles:
4. Rubrospinal tract
- Crossed
- Red nucleus (midbrain)
- ***Control goal-directed movement (distal muscles, manipulating objects)

ALL receives input from motor cortex

Question 18

3 parts of Motor cortex

Answer 18

1. Primary motor cortex (M1 / Area 4)
   - Precentral gyrus of frontal lobe
2. Premotor area (PMA, within Area 6)
   - immediately anterior to M1 cortex
3. Supplementary motor area (SMA, within Area 6)
   - medial part of Brodmann area 6 —> buried within longitudinal fissure

Question 19

4 Motor cortex output

Answer 19

1. Lateral corticospinal tract
   - Act on **Lateral spinal motor neurons **directly (bypass Brainstem)
   - Crossed in Pyramids of Medulla
   - ***Contralateral Distal muscle
2. Ventral corticospinal tract
   - act on **Medial spinal motor neurons **indirectly via Brainstem (Brainstem UMN)
   - Uncrossed —> affect **both side of Brainstem
   - **Bilateral Axial + Proximal muscle for posture
3. Corticobulbar tract
   - act on Brainstem LMN
   - control Facial muscle
4. To Striatum (Basal ganglia), Thalamus
   - form elaborate neural loops through Basal ganglia and Cerebellum
   - modulate motor performances

Question 20

Primary motor cortex (M1)

Answer 20

• Most important function: ***Initiate movement
• Precentral gyrus of frontal lobe from mid-sagittal sulcus to lateral sulcus
• Upon electrical stimulation produces twitch of
  —> Contralateral body muscle
  —> Bilateral upper face, tongue, jaw, larynx, pharynx
• Lowest stimulation threshold to induce movement (compared to PMA, SMA)
  —> M1 has strong connections with spinal cord
• Well-defined somatotopical organisation (homunculus) relates to degree of motor control
  —> larger representation
  —> precision and fine motor control (face, fingers)
• Direction of movement
  —> encoded by Population of neurons (rather than single neuron i.e. Population coding)
  —> a single neuron has different firing rates for different directions movement
  —> summation of vectors of all activities of M1 cells
  —> correlates well with direction of movement
• ***Input from:
  1. PMA, SMA
  2. Sensory areas within parietal lobe (Primary somatosensory cortex)
  3. Thalamus
  4. Cerebellum
  5. Basal ganglion
  —> collectively provide M1 with sets of instructions —> execution of skilled movements

Question 21

Premotor cortex (Area 6)

Answer 21

• Most important function: ***Respond to cues + planning / intention of goal-directed movements
• PMA neurons fire before movements are initiated
• Upon electrical stimulation —> Complex movement
• Higher stimulation threshold
  —> fewer connections with spinal cord
  —> still send corticospinal tract to spinal cord (but not as abundant as M1)
• Less defined motor homunculus
• Inputs from:
  1. **Posterior parietal cortex (PMA, SMA may respond to **visual / **somatosensory cues) —> then project to Prefrontal cortex
  2. **Prefrontal cortex (motivation, intention)

Question 22

Supplementary Motor Area

Answer 22

• Programming complex sequences of movements
• Coordinating ***bilateral movement
• ***Planning of self-initiated movement (rather than respond to cues), not directly with execution of movement

Question 23

***Lesion in motor cortex

Answer 23

Lesion of M1

• interfere with ***motor proficiency + manual dexterity
• independent control of ***fingers lost

Lesion of PMA
- slow execution of **complex motor sequence
- **visually guided movements are severely affected (apraxia)
(- weakness of axial and proximal muscles)

Lesion of SMA
- interferes with **goal-directed / self-initiated behaviours that require **planning and execution of complex motor sequence
- deficit in **bimanual coordination (both hands)
- deficiency in **initiation and suppression of movements
—> Utilisation behaviour: compulsively grab and use objects inappropriately

Question 24

***Functions of cerebellum

Answer 24

1. Feedback control
   - correct ongoing movements when deviate from intended movements
2. Feedforward control
   - modify central programs —> subsequent movements can fulfill their goal with less errors

Question 25

***Cerebellar lesion

Answer 25

1. Unsteady gait + Ataxia
2. Delay in initiating responses
3. Errors in range / force of movement (dysmetria, past-pointing, intention tremor)
4. Errors in rate / regularity of movement (dysdiadochokinesia, dyssynergia)

Question 26

Basal ganglia structures

Answer 26

5 interconnected nuclei

1. Caudate nucleus
2. Putamen
3. Globus pallidus
4. Subthalamic nucleus
5. Substantia nigra

Question 27

***Basal ganglia and motor control

Answer 27

Functions:

1. Processing of information needed for planning
2. Trigger self-initiated movements
3. Postural adjustment

• Receives inputs from Cortex
• Output back to Frontal cortex via Thalamus (motor planning + coordinating movement)
• Output of Basal ganglion = Balance between Direct vs Indirect pathway

1. Direct pathway (***Promote movement):
Striatum —(inhibit)—>
Globus pallidus (internal) —(dis-inhibit)—>
VA/VL of Thalamus —(more excite)—>
Frontal cortex

2. Indirect pathway (**Inhibit movement):
   Striatum —(inhibit)—>
   Globus pallidus (external) —(dis-inhibit)—>
   **Subthalamic nucleus —(more-excite)—>
   Globus pallidus (internal) —(more-inhibit)—>
   VA/VL of Thalamus —(less-excite)—>
   Frontal cortex

Substantia nigra

• Dopaminergic input
• D1 neuron: stimulate Direct pathway (D1 receptor) —> promote movement
• D2 neuron: inhibit Indirect pathway (D2 receptor) —> promote movement

Question 28

***Basal ganglion lesion

Answer 28

1. Parkinson’s disease (Hypokinetic)
   - degeneration in Substantia nigra (loss of Dopaminergic neurons)
   —> less promotion of direct pathway and less inhibition of indirect pathway
   —> inhibit movement
   - bradykinesia, rigidity, tremor, disturbance of posture and gait
   - L-DOPA: ↑ dopamine level —> alleviate symptoms
2. Huntington’s chorea (Hyperkinetic)
   - degeneration of neurons in Striatum selective in indirect pathway
   —> inhibition of indirect pathway
   —> less inhibition of movement
3. Hemiballismus (Hyperkinetic)
   - stroke involving Subthalamic nucleus (excites internal Globus pallidus —> inhibit Thalamus)
   —> less inhibition of Thalamus
   —> facilitates activity in Thalamus