HNS23 Control Of Movement Flashcards
1
Q
Motor unit
A
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
2
Q
Force of muscle contraction
A
Controlled by nervous system in 2 ways:
- ***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) - ***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
3
Q
***3 types of movement
A
- Reflex
- Stereotyped responses (e.g. stretch reflex)
- Stereotyped motor coordination (e.g. flexion reflexes) - Rhythmic motor pattern
- Voluntary movement
4
Q
Reflex
A
- 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
5
Q
Rhythmic motor pattern
A
- 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
6
Q
Voluntary movement
A
- Complex movement
- 2 features:
1. ***Complex, goal-directed (intentional)
- ***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)
7
Q
Control of movement
A
More than controlling muscle contraction and relaxation (on/off):
Also involves
- Accurate time control of contractions of different muscles
- Associated postural adjustment (associated muscles contract as well)
- Adjusting mechanical properties of joints and muscles
- Sensorimotor integration
- Error detection, feedforward and feedback control
8
Q
Upper and Lower motor neurons
A
Upper motor neurons (modulate LMN):
- Motor cortex (highest level: can also control LMN in brainstem)
- Brainstem
Lower motor neurons (directly innervates muscle):
- Brainstem (Motor nuclei of cranial nerves)
- Spinal cord (Anterior horn cells)
9
Q
Organisation of motor control
A
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
10
Q
Spinal cord
A
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
11
Q
Spinal circuitry without input from Brain
A
- Stereotyped responses (e.g. stretch reflex)
- Stereotyped motor coordination (e.g. flexion reflexes)
- Rhythmic locomotor pattern (e.g. walking)
12
Q
Stretch reflex (Stereotyped responses)
A
- 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
13
Q
Flexion reflex (Stereotyped motor coordination)
A
- 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)
14
Q
Rhythmic locomotor pattern
A
- 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)
15
Q
Brainstem
A
- 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
16
Q
3 classes of Brainstem LMN
A
Brainstem LMN —> motor innervation to Head + Neck
- General somatic motor neurons
- tongue muscles - Special visceral (Branchial) motor neurons
- striated muscles —> chewing, facial expression, larynx, pharynx - General visceral motor neurons
- parasympathetic preganglionic neurons —> pupillary constriction, salivary secretion
17
Q
***Brainstem UMN: 4 Descending pathways (Extrapyramidal tracts)
A
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
- Reticulospinal tract
- Reticular formation (brainstem)
- ***Control posture (input from motor cortex) - 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
18
Q
3 parts of Motor cortex
A
- Primary motor cortex (M1 / Area 4)
- Precentral gyrus of frontal lobe - Premotor area (PMA, within Area 6)
- immediately anterior to M1 cortex - Supplementary motor area (SMA, within Area 6)
- medial part of Brodmann area 6 —> buried within longitudinal fissure
19
Q
4 Motor cortex output
A
- Lateral corticospinal tract
- Act on **Lateral spinal motor neurons **directly (bypass Brainstem)
- Crossed in Pyramids of Medulla
- ***Contralateral Distal muscle - 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 - Corticobulbar tract
- act on Brainstem LMN
- control Facial muscle - To Striatum (Basal ganglia), Thalamus
- form elaborate neural loops through Basal ganglia and Cerebellum
- modulate motor performances
20
Q
Primary motor cortex (M1)
A
- 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
21
Q
Premotor cortex (Area 6)
A
- 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)
22
Q
Supplementary Motor Area
A
- Programming complex sequences of movements
- Coordinating ***bilateral movement
- ***Planning of self-initiated movement (rather than respond to cues), not directly with execution of movement
23
Q
***Lesion in motor cortex
A
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
24
Q
***Functions of cerebellum
A
- Feedback control
- correct ongoing movements when deviate from intended movements - Feedforward control
- modify central programs —> subsequent movements can fulfill their goal with less errors
25
Q
***Cerebellar lesion
A
- Unsteady gait + Ataxia
- Delay in initiating responses
- Errors in range / force of movement (dysmetria, past-pointing, intention tremor)
- Errors in rate / regularity of movement (dysdiadochokinesia, dyssynergia)
26
Q
Basal ganglia structures
A
5 interconnected nuclei
- Caudate nucleus
- Putamen
- Globus pallidus
- Subthalamic nucleus
- Substantia nigra
27
Q
***Basal ganglia and motor control
A
Functions:
- Processing of information needed for planning
- Trigger self-initiated movements
- 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
- 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
28
Q
***Basal ganglion lesion
A
- 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 - Huntington’s chorea (Hyperkinetic)
- degeneration of neurons in Striatum selective in indirect pathway
—> inhibition of indirect pathway
—> less inhibition of movement - Hemiballismus (Hyperkinetic)
- stroke involving Subthalamic nucleus (excites internal Globus pallidus —> inhibit Thalamus)
—> less inhibition of Thalamus
—> facilitates activity in Thalamus
