Motor pathways: cortical motor function, basal ganglia and cerebellum

Question 1

How is motor control hierarchically organised?

Answer 1

High order areas of hierarchy are involved in more complex tasks (programme and decide on movements, coordinate muscle activity).

Lower level areas of hierarchy perform lower level tasks (execution of movement).

Primary motor cortex and non-primary motor cortex to cerebellum and basal ganglia, to thalamus, and to the brainstem from cerebellum, to spinal cord and muscles of the face, head and neck, and from spinal cord to muscles of the body.

Question 2

What is the role of the cerebellum and basal ganglia in the motor system?

Answer 2

Adjust the commands received from other parts of the motor control system.

Question 3

Where is the primary motor cortex?

Answer 3

Precentral gyrus in the frontal lobe, anterior to the central sulcus.

Question 4

What is the function of the primary motor cortex (MI)?

Answer 4

Control fine, discrete, precise voluntary movement.

Provide descending signals to execute movement- low down in hierarchy of motor system.

Upper motor neurons project down spinal cord- long axon, large pyramidal cells in layer 5 of the primary motor cortex.

Question 5

What are the descending motor pathways (start to end)?

Answer 5

CORTICOSPINAL TRACT:

Begins in the primary motor cortex.

Motor neurons send their axons down through subcortical structures (internal capsule).

The internal capsule becomes the cerebral peduncles in the midbrain of the brainstem.

At this point the fibres are ipsilateral still.

Fibres then pass through pons (covered by transverse fibres), and the tract reemerges in the medulla as pyramids.

At the base of the medulla 90-95% of fibres cross over in the pyramidal decussation.

Fibres then descend in the lateral corticospinal tract in the dorsal spinal cord, and synapse with alpha motor neurons in ventral horn of spinal cord at the appropriate level.

The axons exit via the ventral root to spinal nerve and out to appropriate musculature.

5-10% of fibres don’t cross over in the pyramidal decussation, but form the anterior corticospinal tract in the spinal cord.

These fibres cross over in the spinal cord at the appropriate level to supply axial musculature.

Question 6

What are the corticobulbar pathways?

Answer 6

Primary motor cortex projections to motor nuclei within brainstem.

Question 7

Where is the premotor cortex?

Answer 7

Frontal lobe, anterior to primary motor cortex (MI).

Question 8

What are the functions of the premotor cortex?

Answer 8

Planning of movements.

Regulates externally cued movements.

Question 9

Where is the supplementary motor area?

Answer 9

Frontal lobe, anterior to primary motor cortex (MI), medially.

Question 10

What are the functions of the supplementary motor area?

Answer 10

Planning complex movements, programming sequencing of movements.

Regulates internally driven movements (e.g. speech).

SMA becomes active when thinking about a movement before executing it.

Question 11

What is the association cortex?

Answer 11

Brain areas not strictly motor areas as their activity does not correlate with motor output/act.

Posterior parietal cortex: ensures movements are targeted accurately to objects in external space.

Prefrontal cortex: involved in selection of appropriate movements for a particular course of action.

Question 12

What is a lower motor neuron?

Answer 12

In ventral horn of spinal cord with processes out to musculature, and in brainstem.

Question 13

What is an upper motor neuron?

Answer 13

In primary motor cortex, has corticospinal or corticobilbar fibres projecting to next motor neuron in the chain.

Question 14

What is ‘pyramidal’?

Answer 14

Lateral corticospinal tract.

Question 15

What is ‘extrapyramidal’?

Answer 15

Basal ganglia and cerebellum.

Question 16

What are the negative signs of upper motor neuron lesion?

Answer 16

Loss of function.

Paresis: graded weakness of movements.

Paralysis (plegia): complete loss of muscle activity.

Question 17

What are the positive signs of upper motor neuron lesion?

Answer 17

Increased abnormal motor function due to loss of inhibitory descending inputs.

Spasticity: increased muscle tone.

Hyperreflexia: exaggerated reflexes.

Clonus: abnormal oscillatory muscle contraction.

Babinski’s sign.

Question 18

What is apraxia?

Answer 18

A disorder of skilled movement.

Patients are not paretic but have lost information about how to perform skilled movements.

Lesion of inferior parietal lobe, frontal lobe (premotor cortex, supplementary motor area).

Any disease of these areas can cause apraxia, although stroke and dementia are the most common causes.

Question 19

What is the typical presentation of a lower motor neuron lesion?

Answer 19

Weakness.

Hypotonia (reduced muscle tone).

Hyporeflexia (reduced reflexes).

Muscle atrophy.

Fasciculations: damaged motor units produce spontaneous action potentials, resulting in a visible twitch.

Fibrillations: spontaneous twitching of individual muscle fibres, recorded during needle electromyography examination.

Question 20

What are fasciculations?

Answer 20

Damaged motor units produce spontaneous action potentials, resulting in a visible twitch.

Question 21

What are fibrillations?

Answer 21

Spontaneous twitching of individual muscle fibres, recorded during needle electromyography examination.

Question 22

What is motor neuron disease (MND)?

Answer 22

Progressive neurodegenerative disorder of the motor system.

Spectrum of disorders with similar clinical presentations.

Disease of both upper and lower motor neurons.

Amyotrophic lateral sclerosis (ALS).

Question 23

What are the upper motor neuron signs of motor neuron disease (MND)?

Answer 23

Increased muscle tone (spasticity of limbs and tongue).

Brisk limbs and jaw reflexes.

Babinski’s sign.

Loss of dexterity.

Dysarthria.

Dysphagia.

Question 24

What are the lower motor neuron signs of motor neuron disease (MND)?

Answer 24

Weakness.

Muscle wasting.

Tongue fasciculations and wasting.

Nasal speech.

Dysphagia.

Question 25

What are the key structures included within the basal ganglia?

Answer 25

Caudate nucleus.

Lentiform nucleus (putamen + external globus pallidus).

Subthalamic nucleus.

Substantia nigra.

Ventral pallidum, claustrum, nucleus accumbens, nucleus basalis of Meynert.

Question 26

What is the striatum?

Answer 26

Caudate nucleus + lentiform nucleus (putamen + external globus pallidus) in the basal ganglia.

Question 27

What structure bisects the caudate nucleus from the putamen in the basal ganglia?

Answer 27

Internal capsule.

Question 28

Label the following diagram:

Answer 28

Question 29

Label the following diagram:

Answer 29

Question 30

What are the functions of the basal ganglia?

Answer 30

Elaborating associated movements (e.g. swinging arms when walking, changing facial expression to match emotions).

Moderating and coordinating movement (suppressing unwanted movements).

Performing movements in order.

Question 31

What aspect of the basal ganglia circuitry is interrupted in Parkinson’s disease?

Answer 31

Nigrostriatal connectivity breakdown.

Question 32

What aspect of the basal ganglia circuitry is interrupted in Huntington’s disease?

Answer 32

Affects neurons within striatum and local circuitry within striatum.

Affects connections with globus pallidus (internal segment).

Question 33

What is ballism?

Answer 33

Uncontrolled arm movements (ballistic) due to diruption of subthalamic nucleus of the basal ganglia.

Question 34

What is Parkinson’s disease?

Answer 34

Common degenerative disease

Generalised atrophy of the cortex and Lewy bodies in substantia nigra may be seen.

Shaking palsy (paralysis agitans).

Involuntary motion, with lessened muscular power, in parts not in action even when supported, with propensity to bend the trunk forward, and to pass from a walking to a running pace.

Question 35

Describe the neuropathology of Parkinson’s disease.

Answer 35

Classically the primary pathology involves the neurodegeneration of the dopaminergic neurons that originate in the substantia nigra and project to the striatum.

‘Locus classicus’ in the midbrain- less neuromelanin produced by dopaminergic neurons.

Question 36

What are the main motor signs of Parkinson’s disease?

Answer 36

Bradykinesia- slowness of (small) movements (doing up buttons, handling a knife).

Hypomimic face- expressionless, mask-like (absence of movements that animate the face).

Akinesia- difficulty in the initiation of movements because cannot initiate movements internally.

Rigidity- muscle tone increase, causing resistance to externally imposed joint movements.

Tremor at rest- 4-7Hz, starts in one hand (‘pill-rolling tremor’); with time, spreads to other parts of the body.

Question 37

What is Huntington’s disease?

Answer 37

Genetic neurodegenerative disorder.

Chromosome 4, Huntingtin protein, autosomal dominant.

CAG repeat.

Degeneration of GABAergic neurons in the striatum, caudate and then putamen.

Atrophy of caudate nucleus and putamen. Graded on degree of atrophy.

Question 38

What are the motor signs of Huntington’s disease?

Answer 38

Choreic movements (chorea)- rapid jerky involuntary movements of the body; hands and face affected first, then legs and rest of body.

Speech impairment.

Difficulty swallowing.

Unsteady gait.

Later stages: cognitive decline and dementia.

Question 39

Where is the cerebellum?

Answer 39

Sits in the posterior cranial fossa, covered by the tentorium cerebelli fold of the dura, which keeps it in place.

Question 40

What are the three main pathways to the cerebellum?

Answer 40

Cerebellar peduncles:

Inferior- input from the spinal cord (spinocerebellar tracts)

Middle- largest, transverse fibres seen anteriorly connect 2 halves of cerebellum

Superior- main output pathway up to basal ganglia and thalamus etc.

Question 41

Describe the internal structure of the cerebellum.

Answer 41

Trilaminar structure.

Outer molecular layer- glial cells but few neurons, dendritic trees of Purkinje cells.

Middle piriform layer- Purkinje cell layer.

Inner granular layer- neuronal granule cells.

Inferior olive projects to Purkine cells via climbing fibres.

All other input to granule cells via mossy fibres and then onwards via parallel fibres.

All output from Purkinje cells via deep white matter nuclei.

Question 42

What are the divisions of the cerebellum?

Answer 42

Vestibulocerebellum

Spinocerebellum

Cerebrocerebellum

Question 43

What is the vestibulocerebellum responsible for?

Answer 43

Regulation of gait, posture and equilibrium.

Coordination of head movements with eye movements- connections with superior colliculus.

Question 44

What is the spinocerebellum responsible for?

Answer 44

Coordination of speech.

Adjustment of muscle tone.

Coordination of limb movements.

Question 45

What is the cerebrocerebellum responsible for?

Answer 45

Coordination of skilled movements.

Cognitive function, attention, processing of language.

Emotional control.

Motor learning.

Question 46

What is vestibulocerebellar syndrome?

Answer 46

Damage (tumour) causes syndrome similar to vestibular disease leading to gait ataxia and tendency to fall (even when patient is sitting with eyes open).

Question 47

What is spinocerebellar syndrome?

Answer 47

Damage (degeneration and atrophy associated with chronic alcoholism) affects mainly legs, causes abnormal gait and stance (wide-based).

Question 48

What is cerebrocerebellar or lateral cerebellar syndrome?

Answer 48

Damage affects mainly arms/skilled coordinated movements (tremor) and speech.

Question 49

What are the main motor signs of cerebellar problems?

Answer 49

Deficits apparent only upon movement.

Ataxia- general impairments in movement coordination and accuracy; disturbances of posture or gait: wide-based, staggering (‘drunken’) gait.

Dysmetria- inappropriate force and distance for target-directed movements (knocking over a cup rather than grabbing it).

Intention tremor- increasingly oscillatory trajectory of a limb in a target-directed movement (nose-finger tracking).

Dysdiadochokinesia- inability to perform rapidly alternating movements (rapidly pronating and supinating hands and forearm).

Scanning speech- staccato, due to impaired coordination of speech muscles.