Motor Functions of the Basal Ganglia

Question 1

What are the 5 pairs of nuclear masses of the basal ganglia?

Answer 1

1. caudate nucleus
2. putamen
   > make up dorsal striatum
3. globus pallidus
   > consist of internal segment and external segment
4. subthalamic nucleus
5. substantia nigra
   > a midbrain structure reciprocally connected with the basal ganglia of the forebrain)

Question 2

What are the functions of the basal ganglia?

Answer 2

1. to fine-tune voluntary movements.
   - They receive the impulses for the upcoming movement from the cerebral cortex, which they process and adjust.
   - They convey their instructions to the thalamus, which then relays this information back to the cortex.
   - Ultimately, the fine-tuned movement instruction is sent to the skeletal muscles through the corticospinal tract
2. Basal ganglia mediate some other higher cortical functions e.g. planning and modulation of movement, memory, eye movements, reward processing, and motivation

Question 3

Describe the motor loop in the basal ganglia?

Answer 3

Starts in the sensorimotor cortex and returns there via the striatum, thalamus and supplementary motor area (SMA)
2 pathways:
1. Direct: Promotes movement (hyperkinetic)
2. Indirect: Inhibits movement (hypokinetic)
- Balance of these two circuits underlies regulation of movements

Question 3

Describe the motor loop in the basal ganglia?

Answer 3

Starts in the sensorimotor cortex and returns there via the striatum, thalamus and supplementary motor area (SMA)
2 pathways:
Direct: Promotes movement (hyperkinetic)
Indirect: Inhibits movement (hypokinetic)
Balance of these two circuits underlies regulation of movements

Question 4

Describe the direct pathway?

Answer 4

• Synapses from cortical cells excite cells in the putamen, which make inhibitory synapses on neurons in the globus pallidus.
• These in turn make inhibitory connections with the cells in VLo.
• The thalamocortical connection (from VLo to SMA) is excitatory and facilitates the discharge of movement-related cells in the SMA

Question 5

Describe the indirect pathway?

Answer 5

• The most unique features of the indirect pathway are the GPe and the subthalamic nucleus (STN).
• Striatal neurons inhibit cells of the GPe, which then inhibit cells of both the GPi and STN.
• The STN is also excited by axons from the cortex, and its projections excite the neurons of the GPi which inhibit thalamic neurons

Question 6

Name clinical disorders that affect the direct pathway of the basal ganglia?

Answer 6

Dyskinesia
Tremor
Dystonia
Athetosis
Chorea
Ballismus
Tics
- Imbalance in favour of DIRECT pathway will result in HYPERKINETIC disorders
> decreased basal ganglia output

Question 7

What is chorea?

Answer 7

Characterized by involuntary non-repetitive jerky movements of the distal extremities, with spasmodic contortions of facial muscles i.e. flicking movements in the hands, face and other body parts.
- Is associated with damage (small lesions) to the corpus striatum
> mainly the caudate nucleus and putamen

Question 8

What are the potential causes of chorea?

Answer 8

The cause may be hereditary (Huntington’s chorea/disease) or infective (Sydenham’s chorea)

Question 9

What is the treatment for chorea?

Answer 9

Treatment is with dopamine receptor antagonists
e.g. phenothiazines or GABA-enhancing agents such as sodium valproate

Question 10

What is athetosis?

Answer 10

Is a disorder which presents as frequent slow, writhing (sinuous) movement of the limb, with forced axial rotations of the trunk and neck
- Associated with damage to the striatum
> Associated with several neurological disorders e.g. cerebral palsy, Rett syndrome

Question 11

Name the clinical disorders that affect the indirect pathway of the basal ganglia?

Answer 11

Hypokinesia
Bradykinesia
Akinesia
- Imbalance in favour of INDIRECT pathway will result in HYPOKINETIC disorders
> increased inhibition of the thalamus by the basal ganglia

Question 12

Describe Parkinsons disease?

Answer 12

The most common disorder of the basal ganglia
First described by James Parkinson in 1817
Characterized by a triad of rigidity, hypokinesia, or bradykinesia and tremor
Most patients with this disease are over 50 years

Question 13

What are the causes of Parkinsons disease?

Answer 13

1. In over 75% of cases it is idiopathic (no known cause)
2. drug toxicity, notably phenothiazines which block dopamine receptors
3. viral encephalitis
4. head injury especially repeated trauma as in boxers
5. cerebral tumours

Question 14

Describe the pathophysiology of Parkinsons disease?

Answer 14

Cellular loss and depigmentation of the substantia nigra with a degeneration of its dopaminergic projection to the striatum
There is also atrophy of the globus pallidus and a patchy cortical atrophy
Parkinsonism also leads to depletion of dopamine content of the striatum and the substantia nigra

Question 15

Describe tremor as a clinical feature of Parkinsonism?

Answer 15

An early symptom is tremor of the fingers spreading to the proximal parts of the arm, the tongue, and later the legs
The tremor is present at rest and often lessened during purposeful activity
It is regular in rhythm about 4-8 cycles per second
It consists of alternation between agonist and antagonist muscles
The tremor commonly reduces as rigidity progresses and is absent during sleep

Question 16

Describe muscular rigidity as a clinical feature of parkinsonism?

Answer 16

There is increased resistance to passive movement in both agonist and antagonist muscles
Facial muscles are immobile giving a mask-like facial appearance
The eyes stare with little blinking
Emotional expressions of the face are slow in onset, limited and prolonged
Speech is slow in starting, of low volume and difficult in execution (dysarthria)
When writing, the letters become smaller and illegible (micrographia)
When rigidity if fully developed, the patient assumes a fixed hunched posture with the hips, elbows and knees flexed

Question 17

Describe hypokinesia (poverty of movement) as a clinical feature of Parkinsonism?

Answer 17

Movements are infrequent, slow in initiation and execution are lacking precision
The gait is reduced to short shuffling steps, with uncontrolled acceleration when walking down-hill, which leads to frequent falls
Loss of arm swinging when walking
Fine movements e.g. fastening buttons are difficult

Question 18

Name motor impairments caused by Parkinsons?

Answer 18

Bradykinesia
Hyper salivation
Postural imbalance
Tremor
Muscular rigidity
Walking difficulties
Freezing movements

Question 19

Name non-motor impairments causes by Parkinsons?

Answer 19

Cognitive impairments
Sleeping disorders
Mood disturbances
GIT issues
Sweat and olfactory disorders
Anxiety and depression
Pain