Motor Pathways: Basal Ganglia and Cerebellum

Question 1

What is the collective term given to the basal ganglia and cerebellum?

Answer 1

Extrapyramidal system

Question 2

Broadly speaking, what is the role of this system?

(basal ganglia and cerebellum

Answer 2

It checks that the movement selected by the motor cortex is correct

Both parts of the extrapyramidal system has functions aside from modulating motor control

Question 3

Where is the basal ganglion located?

Answer 3

White matter in the middle of the brain

Question 4

What are the different parts of the basal ganglia?

Answer 4

Striatum = caudate + putamen

Globus Pallidus = external segment (GPext) + internal segment (GPint)

Substantia Nigra = pars compacta (SNc) + pars reticulata (SNr) Subthalamic Nucleus

Question 5

What is the function of the basal ganglia?

Answer 5

It is involved in planning and coordinating movement

It elaborates associated movements (e.g. swinging arms when walking)

Contributes to the smoothness of complex action

Involved in moderating and coordinating movements-(movemens done in right order)

Question 6

Describe the structure of the direct pathway in the basal ganglia.

Answer 6

Fibres initially come from the motor cortex to the striatum (caudate and putamen) Direct Pathway: putamen –> globus pallidus internal segment + substantia nigra pars reticulata

ie no prjection to STN

Question 7

Describe the structure of the indirect pathway in the basal ganglia.

Answer 7

Fibres initially come from the motor cortex to the striatum (caudate and putamen)

Indirect Pathway: putamen –> globus pallidus external segment –> subthalamic nucleus –> globus pallidus internal segment

ie projection to STN

Question 8

What is the difference in the function of the direct and indirect pathways?

Answer 8

Direct Pathway = excitatory on the motor cortex Indirect Pathway = inhibitory on the motor cortex

Question 9

What extra component modulates the function of the direct and indirect pathways?

Answer 9

Nigro-striatal pathway

Question 10

Where do the projections go after leaving the basal ganglion structures?

Answer 10

They go to the thalamus

From the thalamus they go to the cortex (supplementary motor area and primary motor area = two regions involved in movement preparation and planning)

Question 11

Describe how the basal ganglia are involved in choosing correctmotor programmes to carry out particular functions.

Answer 11

The basal ganglia and cortex form a processing loop.

The basal ganglia enable proper motor programmes (stored in the cortex) via the direct pathway (exicitatory)

The basal ganglia inhibit the competing motor programmes via the indirect pathway

In summary, the basal ganglia and its direct and indirect pathways make sure that appropriate motor commands get transmitted down the hierarchy.

Question 12

Connections with which parts of the brain allow the basal ganglia to have a role in enabling various cognitive, executive and emotional programmes?

Answer 12

Prefrontal association cortex

Limbic cortex

Question 13

What causes Parkinson’s disease?

Answer 13

Parkinson’s disease is the neuronal degeneration of dopaminergic neurones in the substantia nigra pars compacta It is caused by the progressive depletion of dopaminergic neurones NOTE: symptoms only appear hen 80% of the dopamine cells in the substantia nigra have died

Question 14

What are the consequences of this loss of dopamineric neurones in SNc with regards to the circuitry of the basal ganglia?

Answer 14

The loss of nigro-striatal dopaminergic axons in the caudate and putamen mean that the connection between the striatum (caudate and putamen) and the substantia nigra pars compacta is lost. This means that the direct pathway is reduced and so the excitation of the motor cortex is reduced. The lack of excitatory input interferes with the ability of the motor cortex to generate commands for voluntary movement, resulting in poverty of movement.

Question 15

State the main signs of Parkinson’s disease.

Answer 15

Bradykinesia – slow movement

Akinesia – difficulty in the initiation of movements

Hypomimic face – expressionless face

Tremor

Rigidity – increase in muscle tone

Parkinsonian gait.

Question 16

Describe the Parkinsonian gait.

Answer 16

Walking slowly, small steps, shuffling feet, reduced arm swing

Stooped posture with head and body bent forwards and downwards

Question 17

What is Huntington’s disease caused by?

Answer 17

Abnormality on chromosome 4 (autosomal dominant) Caused by the degeneration of GABAergic neurones in the striatum (firstly the caudate and then the putamen)

disrupts fine balance as motor cortex gets excessive excitatory input–>continuous involuntary commands to muscles

Question 18

What are the consequences of this (huntingtons) with regards to the circuitry of the basal ganglia?

Answer 18

The inhibitory effect of the indirect pathway no longer keeps the direct pathway under control so the cortex will be hyperexcitable. Patients will continuously have abnormal movements because the cortex is continuously sending involuntary commands for movements.

Question 19

What are the main signs of Huntington’s disease and how does the disease progress?

Answer 19

Choreic movements – rapid, jerky, involuntary movements of the body

The hands and face are affected first

Chorea gradually increases over time until the patients are totally incapacitated by it

Later on the patients will develop cognitive decline and dementia.

also affected: speech, dysphagia, unsteady gait

Question 20

State the 3 lobes of the cerebellum.

Answer 20

Anterior Posterior Flocculonodular

Question 21

The cerebellum is divided sagitally into 3 zones. What are these zones?

Answer 21

Vermis (midline) Intermediate hemisphere (closest to vermis) Lateral hemisphere

Question 22

State the 3 layers of the cerebellar cortex.

Answer 22

Granular layer

Pirifor layer (contains purkinje cells)

Molecular layer

Question 23

What are the 2 sources of input into the cerebellum and what do they connect with?

Answer 23

Mossy Fibres – from the cortex and pons (corticopontine fibres) and from the spinocerebellar tract

Climbing Fibres – from the inferior olive

Question 24

Functionally, the cerebellum can be divided in three. What are these three divisions?

Answer 24

Vestibulocerbellum Spinocerebellum Cerebrocerebellum

Question 25

What is the role of the vestibulocerebellum?

Answer 25

Vestibulocerbellum is the flocculonodular lobe It is connected to vestibular nuclei Involved in balance, posture and regulation of gait

It is also involved in coordination of head movements with eye movements

Question 26

Which parts of the cerebellum are part of the spinocerebellum?

Answer 26

Vermis and intermediate hemisphere

Question 27

From where does the spinocerebellum receive inputs?

Answer 27

Afferents from axial portions of the body, visual, auditory and trigeminal inputs –> vermis

Afferents from limbs –> intermediate hemisphere

Question 28

What are the roles of the spinocerebellum?

Answer 28

Coordination of speech

Adjustment of muscle tone

Coordination of limb movement

Question 29

Which part of the cerebellum is part of the cerebrocerebellum?

Answer 29

Lateral hemisphere

Question 30

From where does the cerebrocerebellum receive inputs and what are its main functions?

Answer 30

It receives projections from the cortex Main functions are:

• Coordination of skilled movements
• Cognitive function
• Attention
• Processing of language
• Emotional control

Question 31

In summary, what are the four main functions of the cerebellum?

Answer 31

Maintenance of balance and posture

Coordination of voluntary movements

Motor learning

Cognitive functions

Question 32

State three syndromes caused by dysfunction of different parts of the cerebellum.

Answer 32

Vestibulocerebellar syndrome/Flocculonodular lobe syndrome - (from tumour)- gait ataxia

Spinocerebellar syndrome- (from degeneration/atrophy)-abnormal gait + wide base stance

Cerberocerebellar syndrome-(damage) arms/skilled movement+ speech

Question 33

What are the symptoms of vestibulocerebellar syndrome?

Answer 33

This syndrome is similar to vestibular disease Patients tend to lose their balance with gait ataxia and a tendency to fall.

Question 34

What behavioural habit is spinocerebellar syndrome associated with?

Answer 34

Chronic alcoholism

Question 35

Describe the symptoms of spinocerebellar syndrome.

Answer 35

Mainly affects the legs It causes abnormal gait and a wide-based stance

Question 36

Describe the symptoms of cerebrocerebellar syndrome.

Answer 36

Damage mainly affects the arms It affects coordinated movements Speech becomes very hesitant and slow (staccato)

Question 37

What are the main signs of cerebellar disorders?

Answer 37

Ataxia– general impairments in movement coordination and accuracy

Dysmetria– inappropriate force and distance for target-directed movements

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

Dysdiadochokinesia– inability to perform rapid alternating movements

Scanning speech– staccato, due to impaired coordination of speech muscles

Question 38

State a hereditary and acquired cause of the symptoms listed above.

Answer 38

Hereditary – Friedreich’s Ataxia

Acquired – Multiple Sclerosis