The Motor System

Question 1

What is motor control?

Answer 1

Motor control involves a dynamically changing mix of conscious and unconscious regulation of muscle force, informed by a continuous and complex sensory feedback, operating in a framework sculpted by evolutionary pressures

Question 2

What different types of motor control are there?

Answer 2

1. Voluntary = runnning, walking, talking etc
2. Goal-directed = conscious, explicit, controlled
3. Habit = unconscious, implicit, automatic
4. Involuntary = eye movements, facial expressions, diaphragm, cardiac etc

Question 3

What is the hierarchical control architecture of movement from an evolutionary perspective?

Answer 3

1. Pain = Spinal Cord
2. Loom = Sensorimotor
3. Learned threat = Cortex + Limbic System

All use motor, autonomic and endocrine systems to create a defence related output

Question 4

Describe an overview of the sensorimotor system

Answer 4

Association cortex
Secondary motor cortex
Primary motor cortex
Brainstem motor nuclei
Spinal motor units/motor units
Muscle

Descending circuits and continuous sensory feedback

Question 5

What modulates movement in this sensorimotor system?

Answer 5

Basal ganglia = WHAT to do
Cerebellum = HOW to do it

Question 6

What are the 4 main systems involved in the control of movement?

Answer 6

1. Descending systems (upper motor neurons) = Motor cortex (planning, initiating voluntary movement) and brainstem centres (rhythmic, stereotyped movements, postural control)
2. Basal ganglia = initiation of intended movement
3. Cerebellum = coordination of ongoing movement
4. Spinal cord and brainstem circuits = local circuit neurons (sensorimotor integration), motor neuron pools (lower motor neurons)

Question 7

What governs motor control?

Answer 7

Lower and upper motor neurons

Lower motor neurons begin in the brainstem or spinal cord and project to the muscles
Upper motor neurons originate in higher centres and project down to meet lower motor neurons

Question 8

What are the three types of muscle?

Answer 8

Skeletal muscle
Smooth muscle
Cardiac muscle

Question 9

Describe the structure of a muscle

Answer 9

Contains several muscle fasciculi
Muscle fasciculi comprises several muscle fibres/sarcolemmas
Muscle fibre is constituted of several myofibrils
Myofibrils made up of short units called sarcomeres
Sarcomeres contain 2 protein filaments = actin and myosin myofilaments

Question 10

How do muscles contract?

Answer 10

1. Nerve impulse reaches the neuromuscular junction
2. Acetylcholine released by the motor neuron
3. Ach binds to the muscle receptor site
4. Sodium channels in the muscle open causing sodium ions to rush into the cell initiating an action potential
5. Action potential impulse causes the sarcoplasmic reticulum to release Ca+ into the cell and around the sarcomere contraction units
6. Ca+ binds to troponin releasing tropomyosin out of the actin-myosin binding site = causes myosin heads to fit into actin filaments
7. Myosin heads pull actin filaments toward the centre allowing filaments to slide past each other and shorten the sarcomere
   ATP also binds

Question 11

What causes rigor mortis?

Answer 11

ATP is required to break the bond between the myosin head and actin filament
So the muscle becomes contracted and remain that way until enzymes begin to disrupt the actin/myosin
Therefore, if there is no ATP i.e., if the person is dead, the bond between the myosin head and actin filament cannot be broken so the muscles stay contracted

Question 12

What is the motor unit?

Answer 12

Single alpha motor neuron (lower motor neuron) + all the muscle fibres it innervates

The less fibres means the greater movement resolution

Activation of an alpha motor neuron depolarises and causes contraction of all muscle fibres in that unit (all or none)

Question 13

What determines the average number of muscle fibres innervated by a single motor neuron?

Answer 13

1. Level of control
2. Strength

Size principle = units are recruited in order of size (smallest first)
Fine control typically required at lower forces

Question 14

What are the different types of muscle fibre?

Answer 14

Fast and slow muscle fibres
- Slow (sustained muscular contraction e.g., maintaining posture)
- Fast fatigue resistant (generate twice the force of slow motor unit)
- Fast fatiguable (brief exertions that require large forces e.g., runnning, jumping)

Question 15

Can the thickness of muscle fibres and the proportion of different muscle fibre types change?

Answer 15

Yes with training and exercise

Question 16

What is the motor pool?

Answer 16

All the lower motor neurons that innervate a single muscle
The motor pool contains both the alpha and gamma motor neurons
Motor pools are often arranged in a rod like shape within the ventral horn of the spinal column

Question 17

What 2 types of lower motor neurons are found in the motor neuron pools of the ventral horn?

Answer 17

Alpha motor neurons = control muscle contraction involved in voluntary movement

Gamma motor neurons = control muscle contraction in response to external forces acting on the muscle - in response to external forces the gamma motor neurons produce the involuntary reflexive movement

Question 18

Where does the sensory input and motor output occur in the spinal cord?

Answer 18

Sensory input = dorsal root
Motor output = ventral root

Question 19

What information is needed by the control system allowing muscles to contract and relax - a key part of proprioception?

Answer 19

1. How much tension is on the muscle = Golgi tendon organs sense tension
2. What is the length (stretch) of the muscle = muscle spindles sense stretch

Question 20

What is the role of the Golgi tendon organs?

Answer 20

Found within the tendons
Sends ascending sensory information to the brain via the spinal cord about how much force there is in the muscle
Critical for proprioception

Under conditions of extreme tension, it is possible that GTOs act to inhibit muscle fibres to prevent damage

Question 21

What are muscle spindles important for?

Answer 21

Sense the length of muscles i.e., the amount of stretch - this information forms a key part of reflex circuits

Question 22

What is a monosynaptic reflex?

Answer 22

Most simple reflex e.g., patellar tendon reflex
A reflex arc that provides direct communication between sensory and motor neurons innervating the muscle

Question 23

Describe the reflex circuit in movement

Answer 23

Motor pool in the ventral root of the spinal cord connected to the muscle fibres via alpha motor neurons
Within the muscle there are 2 types of muscle fibres
- Extrafusal muscle fibres (innervated by gamma motor neurons)
- Intrafusal muscle fibres which contain the muscle spindles (innervated by alpha motor neurons)

Question 24

Explain the stretch reflex

Answer 24

1. Muscle spindle is stretched
2. Once detected it causes action potentials to be fired by sensory afferent fibres
3. These synapse within the spinal cord with alpha motor neurons which innervate extrafusal fibres
4. The antagonistic muscle is inhibited and the agonist muscle contracts

Question 25

Why are intrafusal fibres innervated separately to extrafusal fibres and what innervates both types?

Answer 25

Intrafusal fibres = gamma motor neurons Extrafusal fibres = alpha motor neurons Need a system to detect stretch regardless of the current muscle length If both muscle fibres are controlled by the same motor neurons, when a muscle is slack the system wont be sensitive to slight changes Gamma motor neurons keep intrafusal fibres set at a length that optimises muscle stretch detection

Question 26

What is the vestibular righting reflex or the righting reflex?

Answer 26

Vestibular system detects that the body is not upright as well as any acceleration due to gravity Information from the vestibular system is combined with visual, somatosensory and proprioceptive sensory input in order to specify a pattern of motor activity that will restore uprightedness and a safe landing The cerebellum which compares the intended motor plan with the actual situation is critical for computing the desired motor activity

Question 27

Why is it so hard to build robots based off of these simple circuits?

Answer 27

Control of gross movement patterns can be devolved to simple spinal circuitry, but constant modulation based on sensory feedback is required to account for the unexpected Higher CNS centres constantly adjust ongoing activity to resolve conflicting demands on the motor system and direct it towards goals

Question 28

What is the organisation of descending motor control?

Answer 28

The most medial part of the ventral horn contains lower motor neuron pools that innervate axial muscles or proximal muscles of the limbs The most lateral parts contain lower motor neurons that innervate the distal muscles of the limbs Motor command originate in the motor cortex pyramidal cells = upper motor neurons Pyramidal cell axons project directly or indirectly to spinal cord where they synapse with lower motor neurons

Question 29

What is the main descending motor pathway?

Answer 29

The pyramidal tracts = most cortical projections innervate contralateral motor units

Question 30

What are the 2 tracts within the pyramidal tracts?

Answer 30

Dorsolateral corticospinal tract/ corticuorubrospinal tract Ventromedial corticospinal tract/ ventral cortico-brainstem tract

Question 31

Explain the corticospinal and corticobulbar tracts

Answer 31

Neurons in the motor cortex give rise to axons that travel through the internal capsule and come together on the ventral surface of the midbrain within the cerebral peduncle Axons continue through the pons and come to lie on the ventral surface of the medulla giving rise to the medullary pyramids The components of the corticobulbar tract innervating cranial nerve nuclei, the reticular formation and the red nucleus leave the pathway at the appropriate levels of the brainstem Lateral corticospinal tract decussates at the base of the pyramids = 85% (controls movement of the limbs) Anterior corticospinal tract 15% do not decussate and enter the spinal cord ipsilateral (control of axial muscles e.g., trunk)

Question 32

What are the main differences in the dorsolateral and ventromedial (anterior) tracts?

Answer 32

Dorsolateral tracts - Innervate contralateral side of one segment of spinal cord - Sometimes project directly to alpha motor neurons - Project to distal muscles Ventromedial tracts - Diffuse innervation projecting to both sides and multiple segments of spinal cord - Project to proximal muscles of trunk and limbs

Question 33

How does the basal ganglia modulate movement?

Answer 33

Receives excitatory input from many areas of cortex (glutamate) Output goes back to cortex via the thalamus Output is mainly inhibitory (GABA) Complex internal connectivity involving 5 principle nuclei - Substantia nigra - Caudate & Putamen - Globus pallidus - Subthalamic nucleus

Question 34

Explain disinhibitory gating of motor cortex output

Answer 34

At rest the caudate/putamen are quiet which means the globus pallidus is tonically active as the caudate and putamen is not inhibiting it Tonic activation of the globus pallidus inhibits the thalamus causing reduced excitation of the motor cortex When excited, a dopamine input from the substantia nigra causes the caudate and putamen are transiently excited, transiently inhibiting the globus pallius. This disinhibits (excites) the thalamus causing increased excitation in the motor cortex

Question 35

What is the cerebellum's role in modulating movement?

Answer 35

Plays a major role in adapting and fine-tuning motor programs to make accurate movements through a trial-and-error process The cerebellum is a large brain structure that acts as a parallel processor, enabling smooth, controlled movement No direct projection to the lower motor neurons, instead modulate activity of the upper motor neurons

Question 36

What are the inputs and outputs of the cerebellum?

Answer 36

CORTICAL - Mostly from motor cortex - Also somatosensory and visual areas of the parietal cortex SPINAL - Proprioceptive information about limb position and movement (muscle spindles, other mechanoreceptors) VESTIBULAR - Rotational and acceleratory head movement (semicircular canals)

Question 37

How can this cerebellar function be used in robots and brain machine interfaces?

Answer 37

Record and decode brain activity Specify desired movement sequence Bypass the motor cortex and lower motor neurons or damaged spinal pathways Move limbs

Question 38

What are BMI systems and explain their design?

Answer 38

Brain machine interface systems 1. The acquisition of brain-generated signals that reflect information processing and the encoding of action goals 2. The processing and decoding of brain signals using artificial neural networks to extract salient features and translate them into pragmatic control signals 3. The implementation of control signals for the operation of digital and mechanical systems 4. The generation of sensory-based feedback signals to promote adaptive plasticity and improved brain control of BMI technology 1. Acquisition of brain-derived signals/BOLD signals e.g., fMRI, EEG, NIRS 2. BOLD signal processing - feature extraction, machine learning, pattern classification, translation algorithm 3. Device commands - simple = yes or no communication, intermediate = control of external devices, complex = limbs controlled by electrodes