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?

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) Contribute to the smoothness of complex action Involved in moderating and coordinating movements

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

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

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 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

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)

Question 18

What are the consequences of this 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.

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

Granule cell layer Purkinje cells Molecular layer

Question 23

Name the 3 deep nuclei that are involved in the connections of the cerebellum with other parts of the body. Include their function.

Answer 23

• Fastigial = involved in control of balance and connected with vestibular nuclei
• Interposed + Dentate = both involved in voluntary movement and are connected to the thalamus and the red nucleus

Question 24

What are the 3 sources of input into the cerebellum and what do they connect with?

Answer 24

• Mossy Fibres – from the cortex and pons (corticopontine fibres)
• Mossy Fibres – from the spinocerebellar tract
• Climbing Fibres – from the inferior oliviary nucleus

Question 25

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

Answer 25

Vestibulocerbellum Spinocerebellum Cerebrocerebellum

Question 26

What is the role of the vestibulocerebellum?

Answer 26

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 27

Which parts of the cerebellum are part of the spinocerebellum?

Answer 27

Vermis and intermediate hemisphere

Question 28

From where does the spinocerebellum receive inputs?

Answer 28

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

Question 29

What are the roles of the spinocerebellum?

Answer 29

Coordination of speech Adjustment of muscle tone Coordination of limb movement

Question 30

Which part of the cerebellum is part of the cerebrocerebellum?

Answer 30

Lateral hemisphere

Question 31

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

Answer 31

It receives projections from the cortex Main functions are:  Coordination of skilled movements  Cognitive function  Attention  Processing of language  Emotional control

Question 32

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

Answer 32

Maintenance of balance and posture Coordination of voluntary movements Motor learning Cognitive functions

Question 33

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

Answer 33

Vestibulocerebellar syndrome/Flocculonodular lobe syndrome Spinocerebellar syndrome Cerberocerebellar syndrome

Question 34

What are the symptoms of vestibulocerebellar syndrome?

Answer 34

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

Question 35

What behavioural habit is spinocerebellar syndrome associated with?

Answer 35

Chronic alcoholism

Question 36

Describe the symptoms of spinocerebellar syndrome.

Answer 36

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

Question 37

Describe the symptoms of cerebrocerebellar syndrome.

Answer 37

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

Question 38

What are the main signs of cerebellar disorders?

Answer 38

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 39

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

Answer 39

Hereditary – Friedreich’s Ataxia Acquired – Multiple Sclerosis