Motor systems

Question 1

What are the functions of the motor system?

Answer 1

1. To move in and manipulate elements of the external environment
2. To maintain internal and external equilibrium (homeostasis, balance)
3. Autonomic functions (e.g. respiration, gut motility…)
4. Communication (speech, hand gestures, facial expression, writing…)
5. Sensation

Question 2

What are the types of basic movements?

Answer 2

1. Reflex (e.g. knee-jerk, coughing…): Pre-programmed, simple movements that usually don’t require any CNS input and are automated. They are also rigid, stereotyped and occur in the same way each time in response to the same stimuli. They are usually a graded function of eliciting stimulus.
2. Rhythmic (e.g. walking, chewing…): On-going motor actions in a set pattern repeated in set intervals. They are stereotyped and consist of a mixture of voluntary and reflex movements, but can be influenced by voluntary control.
3. Voluntary (e.g. playing piano, catching ball…): Complex movements determined by particular external stimuli or internal motivation. They are usually goal driven and can be modified voluntarily to suit particular circumstances.

Question 3

What factors make the motor system difficult to study?

Answer 3

1. The motor system is non-linear as a result of convergence/divergence. This means no simple assumption can me made about its function (i.e. one cannot simply presume that if A occurs, B must also occur).
2. Joints have many degrees of freedom, giving an endless combination of possible movements that are impossible to predict.
3. Motor systems usually comprise a large networks containing thousands of neurones with thousands of integrating synapses. Understanding these networks require large amounts of information.
4. Repetitive stimulation of a muscle causes gradual change in muscle response despite motor input remaining constant (thixotropy). This implies that motor output is also dependent on changes intrinsic to the muscles and effectors themselves.

Question 4

What are the 2 rules concerned with organisation of motor neurones in the spinal cord?

Answer 4

1. Proximal-distal rule: Motor neurones innervating proximal muscles (trunk, shoulder, hip) are more medial compared to those innervating distal muscles (fingers, toes…).
2. Flexor-extensor rule: Motor neurones responsible for extension are more ventral compared to those responsible for flexion.

Question 5

What are the types of motor units present in muscles?

Answer 5

1. Slow motor units
2. Fast motor units
3. Fast (fatigue-resistant) motor units

Question 6

What are the characteristics of slow motor units?

Answer 6

• Few small fibres per unit.
• Highly vascularised.
• Generates small amount of tension.
• Contraction can be sustained.

Question 7

What are the characterstics of fast motor units?

Answer 7

• Many large fibres per unit.
• Low vascularisation.
• Generates large amount of tension.
• Contraction is only transient as fibres fatigue.

Question 8

What are the characteristics of fast (fatigue-resistant) motor units?

Answer 8

• Generates intermediate amount of tension.
• Contraction can be sustained for longer than fast motor fibres.

Question 9

What are the types of intrafusal fibres present in muscle spindles?

Answer 9

1. Nuclear bag fibres
2. Nuclear chain fibres

Question 10

What are the afferent innervations of the various intrafusal fibres?

Answer 10

• Nuclear bag: Ia (dynamic) + II (static)
• Nuclear chain: II

Question 11

What are the properties and functions of Ia afferents that innervate spindle fibres?

Answer 11

• Adapting
• Detection of muscle rate of change

Question 12

What are the properties and functions of II afferents that innervate spindle fibres?

Answer 12

• Non-adapting
• Measures muscle length

Question 13

What are the efferent γ fibres that innervate the muscle spindle fibres?

Answer 13

• γ_S: Innervates nuclear chain fibres and increases sensitivity of II fibres
• γ_D: Innervates nuclear bag fibres and increases sensitivity of Ia and II fibres

Question 14

What are the contents of typical muscle spindle?

Answer 14

• 1 Nuclear bag fibres (static)
• 1 Nuclear bag fibre (dynamic)
• Several nuclear chain fibres

Question 15

What are the functions of muscle spindles?

Answer 15

• Detects absolute muscle length or rate of change in muscle length
• In parallel with muscle fibres
• Used in feedback control of muscle length through “Servo hypothesis”

Question 16

What are the functions of Golgi tendon organs?

Answer 16

• Detects muscle tension
• In series with muscle fibres
• Causes negative feedback inhibition of associated muscles and stimulation of activity of antagonists to prevent damage by excess contraction
• Causes positive feedback stimulation of associated muscles to provide powerful movements

Question 17

What is the afferent innervation of Golgi tendon organs?

Answer 17

Ib fibres

Question 18

What are the types of descending inputs into the spinal cord?

Answer 18

1. Fast pathway: Uses NTs to exert fast control on transient and specific aspects of motor activity.
2. Slow pathway: Uses neuromodulators to alter the long-term activity of spinal cord motor centres.

Question 19

What are the components of the fast descending motor pathways?

Answer 19

• Ventromedial pathways (controls movement of the proximal muscles of the trunk):
  1. Reticulospinal tract: Orientation and CPG activaton.
  2. Vestibulospinal tract: Postural control.
  3. Tectospinal tract: Orientation to sounds and objects.
  4. Corticospinal tract
• Dorsolateral pathways (controls movement of distal muscles of the extremities):
  1. Rubospinal tract
  2. Corticospinal tract

Question 20

What are the parts of the corticospinal tract?

Answer 20

1. Decussating (~80%): Descends in the lateral corticospinal tract.
2. Non-decussating (~20%): Descends in the ventral corticospinal tract.

Question 21

How do vestibular nuclei distinguish between vestibular stimulation due to intended movement compared to loss of balance?

Answer 21

• Internal models are able to predict vestibular changes due to intended movement.
• When movement occurs, efferent copy sent back to CNS where vestibular response is predicted.
• The predicted vestibular response is subtracted from actual response, any discrepancies are corrected for.

Question 22

Which part of the reticular formation seems to be involved in mediating repetitive patterns?

Answer 22

Mesencephalic locomotor region

Question 23

What is the location of the primary motor cortex (M1)?

Answer 23

• Pre-central gyrus
• Brodmann’s area 4

Question 24

Where does M1 receive inputs from?

Answer 24

1. Periphery and spinal cord
2. Primary somatosensory cortex
3. Sensory association areas
4. Cerebellum
5. Basal ganglia

Question 25

What are the areas of the thalamus associated with the motor cortex?

Answer 25

1. Ventrolateral nucleus: Projects into M1 2. Ventroanterior nucleus: Projects into secondary motor areas 3. Centromedial nuclei: Projects into both M1 and secondary motor areas

Question 26

Why does sensory information need to project into the cortex to be integrated with motor responses and not at spinal cord level?

Answer 26

1. Spinal cord reflexes are very much restricted segmentally, in the sense that sensory information useful for particular motor activity may not synapse in dorsal horn of same spinal segment. M1 brings all sensory and motor endings into proximity, allowing appropriate signals to ‘jump’ across as necessary. 2. The spinal cord is not intelligent enough to determine what appropriate motor responses are needed for given sensory input, and which motor fibres should be selectively activated, especially for movements that involve multiple muscle groups. Motor learning mediates this, but requires synaptic plasticity, which M1 offers.

Question 27

In what ways do somatosensation depend on motor activity?

Answer 27

1. It is well established that many cutaneous mechanoreceptors (e.g. Pacinian corpuscules) are fully adapting, so constant movement is needed to sustain sensory input. This explains why the texture of a surface can only be successfully determined by moving across it, as opposed to placing hand in stationary on it. 2. In order to accurately judge the sensation of an object, knowledge of amount of force being applied to object needs to be known.

Question 28

What is the organiation of M1 and its functional significance?

Answer 28

- Fractured somatotopic arrangement - Allows for functional representations (i.e. one area of cortex controls all muscles involved in particular movements instead of in area of body)

Question 29

What locations does the cerebellum output to?

Answer 29

1. Motor cortex 2. Premotor areas 3. Reticulospinal tract 4. Rubrospinal tract 5. Vestibulospinal tract

Question 30

What types of inputs are received by the cerebellum and where do they originate?

Answer 30

1. Climbing fibres: Inferior olive 2. Mossy fibres: Various peripheral and central sources

Question 31

What type of synapse is the climbing fibre-Purkinje cell synapse?

Answer 31

Aspartate (strong excitatory)

Question 32

What type of synapse is the granule cell-Purkinje cell synapse?

Answer 32

Glutamate (excitatory)

Question 33

What are the sources of the information carried by the climbing fibres?

Answer 33

Cerebral cortex (mainly)

Question 34

What are the sources of the information carried by the mossy fibres?

Answer 34

1. Vestibular system 2. Muscle proprioceptors 3. Special senses 4. Reticular fromation 5. Cerebral cortex (via pons)

Question 35

Which sides of the body do each cerebellar hemisphere control?

Answer 35

Ipsilateral

Question 36

What are the different parts of the cerebellum and their associated functions?

Answer 36

1. Vestibulocerebellum: Balance and posture 2. Spinocerebellum: Internal feedback regulation of movements (using internal models) 3. Cerebrocerebellum: Planning and initiation of voluntary movements

Question 37

What are the inputs and outputs of the vestibulocerebellum?

Answer 37

Vestibular nuclei ↓ Vestibulocerebellum ↓ Vestibular nuclei

Question 38

What are the inputs and outputs of the spinocerebellum?

Answer 38

Peripheral sensory infromation ↓ Spinocerebellum ↓ 1. Reticulospinal tract 2. Vestibulospinal tract

Question 39

What are the inputs and outputs of the cerebrocerebellum?

Answer 39

1. Sensory cortex 2. Motor and premotor cortices 3. Posterior parietal cortex ↓ Cerebrocerebellum ↓ 1. Premotor cortex 2. Motor cortex

Question 40

What are the smaller lobules of the cerebellum called?

Answer 40

Folia

Question 41

What are the types of cells in the cerebellum?

Answer 41

Molecular layer: 1. Basket cells 2. Stellate cells Purkinje layer: Purkinje cells Granular layer: 1. Granule cells 2. Golgi cells

Question 42

What is the rule regarding nature of inputs into the cerebellum?

Answer 42

- Climbing, mossy fibres are excitatory glutaminergic - Granule cells are excitatory glutaminergic - All other cells are inhibitory

Question 43

What types of action potentials are caused in Purkinje cells by the different input fibres to cerebellum?

Answer 43

- Climbing fibres = Complex spike - Mossy fibres = Simple spike

Question 44

What are the types of circuits in the cerebellum?

Answer 44

1. Direct circuits: Excitatory inputs of climbing and mossy fibres into deep nuclei. 2. indirect circuits: Inhibitory inputs of Purkinje fibres to the deep nuclei.

Question 45

What evidence is there to suggest that motor learning is mediated by the cerebellum?

Answer 45

1. Cerebellar activity changes during motor learning 2. Motor learning is abolished following lesion of the cerebellum 3. Lesion of the motor cortex abolishes subsequent motor learning but allows pre-learned actions to still be performed

Question 46

What are the effects of simultaneous climbing fibre and paralel fibre input into Purkinje cell?

Answer 46

Long term depression

Question 47

What are the symptoms of cerebellar lesion?

Answer 47

1. Dysmetria: Over-reaching, resulting in intention tremors 2. Hypotonia: Loss of muscle tone 3. Dysdiadochokinesis: Inability to make rapid, repetitive movements 4. Decomposition of movement: Complex movements are carried out in simple steps 5. Scanning speech: Speech difficulties due to lack of coordination of lips and tongue

Question 48

What are the components of the basal ganglia?

Answer 48

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

Question 49

What are the components of the striatum?

Answer 49

- Putamen - Caudate nucleus

Question 50

What are the sources of inputs into the basal ganglia?

Answer 50

1. Motor cortex 2. Sensory cortices 3. Association cortices (main) 5. Limbic system

Question 51

Which areas of the thalamus does the basal ganglia output into?

Answer 51

- Ventroanterior - Ventrolateral

Question 52

What is the structure of the direct pathway in basal ganglia?

Answer 52

Question 53

What is the structure of the indirect pathway in basal ganglia?

Answer 53

Question 54

Why is the indirect basal ganglia pathway 'indirect'?

Answer 54

It feeds into and modulates activity of the direct pathway.

Question 55

What are the effects of dopamine on individual areas of the basal ganglia?

Answer 55

1. Promotes activity of striatum via D1 receptors 2. Inhibits activity of globus pallidus (external segment) via D2 receptors

Question 56

What are the overall effects of dopamine on the basal ganglia?

Answer 56

Supports the direct pathway and inhibits indirect pathway,

Question 57

What are the symptoms of Parkinson's disease?

Answer 57

"Classic triad": 1. Resting tremor (trembling of limbs only present in absence of movement 2. Muscle stiffness 3. Bradykinesia (slowness of movement and 'shuffling gait')

Question 58

What is Parkinson's disease thought to be caused by?

Answer 58

Deficiency in dopamine from substantia nigra pars compacta

Question 59

What is the mechanism of pathogenesis for Parkinson's disease?

Answer 59

- Dopamine has the effect of promoting motor activity by promoting the direct pathway in basal ganglia and inhibiting indirect pathway. - Reduction in dopamine removes this motor activity-promoting effect and causes inhibition to dominate, resulting in hypokinesia in Parkinson's.

Question 60

What are the treatments available for Parkinson's disease?

Answer 60

1. L-DOPA 2. Foetal grafts 3. Lesions 4. Deep brain stimulation (DBS) of subthalamic nucleus

Question 61

What is Huntington's disease thought to be caused by?

Answer 61

Degeneration of striatal GABAinergic neurones

Question 62

What is the mechanism of pathogenesis for Huntington's disease?

Answer 62

- Decreased output from striatum decreases disinhibition of globus pallidus external segment. - This in turn inhibits indirect pathway and promotes thalamus and increases motor activity, causing hyperkinesia.