Basal Ganglia

🧠 DETAILED MULTI-PARAGRAPH SUMMARY

The basal ganglia and cerebellum are two major motor control systems that function together as “co-directors” of movement. While both are essential for smooth and purposeful motor behaviour, they have distinct but complementary roles. The basal ganglia are primarily responsible for internally generated action selection, meaning they decide whether to perform an action and which action to execute. In contrast, the cerebellum is responsible for externally guided execution, ensuring that once a movement is selected, it is carried out with proper timing, coordination, and accuracy. These systems do not directly control muscles; instead, their outputs are integrated in the thalamus, which relays the final motor command to the motor cortex.

The basal ganglia consist of four principal nuclei: the striatum (caudate nucleus and putamen), globus pallidus (internal GPi and external GPe), substantia nigra (pars compacta SNpc and pars reticulata SNpr), and the subthalamic nucleus (STN). The striatum is the main input region, receiving excitatory (glutamatergic) signals from the cortex. Approximately 90% of its neurons are medium spiny neurons (MSNs), which are inhibitory (GABAergic) and normally inactive until stimulated. The GPi and SNpr serve as the main output nuclei and are tonically active, constantly inhibiting the thalamus and preventing unwanted movements.

Movement control within the basal ganglia relies on two major pathways: the direct pathway and the indirect pathway. The direct pathway facilitates movement by disinhibiting the thalamus. When activated, MSNs inhibit the GPi/SNpr, which reduces their inhibitory output to the thalamus, allowing it to excite the motor cortex and initiate movement. In contrast, the indirect pathway suppresses unwanted movements. It involves inhibition of the GPe, leading to activation of the STN, which then excites the GPi/SNpr, increasing inhibition of the thalamus and reducing cortical activation. A third pathway, the hyperdirect pathway, acts as a rapid “emergency brake,” quickly inhibiting movement via direct cortical input to the STN.

Dopamine, released from neurons in the SNpc, plays a crucial modulatory role in these pathways. It acts on two receptor types in the striatum: D1 receptors (direct pathway) and D2 receptors (indirect pathway). Dopamine binding to D1 receptors enhances excitability (depolarization), facilitating movement, while binding to D2 receptors reduces excitability (hyperpolarization), suppressing the indirect pathway. Thus, dopamine overall promotes movement by strengthening desired actions and suppressing competing ones.

Disorders of the basal ganglia arise when this balance is disrupted. In Parkinson’s disease, degeneration of dopaminergic neurons in the SNpc leads to reduced dopamine levels. This results in decreased activity in the direct pathway and increased activity in the indirect pathway, causing excessive inhibition of the thalamus. Clinically, this manifests as hypokinetic symptoms such as bradykinesia (slow movement), akinesia (difficulty initiating movement), rigidity, and resting tremor. Treatments include L-DOPA, a dopamine precursor, and deep-brain stimulation, which modulates abnormal circuitry activity.

In contrast, Huntington’s disease is a genetic disorder characterised by degeneration of MSNs, particularly those in the indirect pathway. This leads to reduced inhibition of unwanted movements, resulting in hyperkinetic symptoms such as involuntary jerking movements (chorea). Thus, while Parkinson’s disease reflects too little movement due to excessive inhibition, Huntington’s disease reflects excessive movement due to insufficient inhibition.

📌 BULLET POINT SUMMARY

🔹 Basal Ganglia vs Cerebellum

• Basal ganglia: action selection (internal decisions)

• Cerebellum: execution (timing & coordination)

• Both send output via the (thalamus

🔹 Functions of Basal Ganglia

• Initiate voluntary movement

• Select appropriate motor programs

• Inhibit competing movements

• Roles in cognition, emotion, learning

🔹 Main Structures

• Striatum (input; GABAergic MSNs)

• Globus pallidus (GPe, GPi)

• Substantia nigra (SNpc dopamine, SNpr output)

• Subthalamic nucleus (excitatory)

🔹 Key Properties

• GPi & SNpr: tonically inhibitory

• Striatum: tonically inactive

• STN: only excitatory nucleus

🔹 Pathways

Direct pathway

• Inhibits GPi → disinhibits thalamus → ↑ movement

Indirect pathway

• Activates GPi via STN → inhibits thalamus → ↓ movement

Hyperdirect pathway

• Rapid inhibition (emergency brake)

🔹 Dopamine Effects

• D1 receptors → excitation → promote movement

• D2 receptors → inhibition → suppress indirect pathway

• Net effect: facilitates movement

🔹 Disorders

Parkinson’s disease

• Loss of SNpc dopamine

• ↓ direct pathway, ↑ indirect pathway

• Symptoms: bradykinesia, rigidity, tremor

Huntington’s disease

• Loss of MSNs (indirect pathway) medium spiny neruons

• Excess movement (chorea)

🧩 FILL-IN-THE-BLANK SUMMARY

Section 1: Functional Roles

1. The basal ganglia are responsible for _action_________ selection.

2. The cerebellum controls movement _excutation_________.

3. Both systems communicate via the __thalamus________.

Section 2: Structures

4. The main input nucleus is the _straitum_________.

5. The main output nuclei include _GPi______ and SNPr______.

6. Dopamine is produced in the _SNPc________.

Section 3: Pathways

7. The __direct________ pathway facilitates movement.

8. The __indirects_______ pathway inhibits unwanted movement.

9. The STN is the only __excitatory___ nucleus.

Section 4: Dopamine

10. D1 receptors cause _depolarisation______ (depolarization/hyperpolarization).

11. D2 receptors cause ___hyperpolarisation_______.

12. Dopamine overall _facillates_________ movement.

Section 5: Disease

13. Parkinson’s disease involves loss of __dopaminergic_____ neurons.

14. Huntington’s disease involves loss of __GABAerigic Medium Spiny Neurons_____ neurons.

15. Parkinson’s results in _hypo_____ (hypo/hyper)kinesia.

✅ ANSWERS (Fill in the blanks)

1. action

2. execution

3. thalamus

4. striatum

5. GPi, SNpr

6. SNpc

7. direct

8. indirect

9. excitatory

10. depolarization

11. hyperpolarization

12. facilitates

13. dopaminergic

14. GABAergic MSNs

15. hypokinesia

📝 40 MEDIUM-LEVEL EXAM MCQs

Questions

1. Basal ganglia primarily control:
   A. Reflexes
   B. Action selection
   C. Vision
   D. Hearing

2. Cerebellum primarily controls:
   A. Decision-making
   B. Execution of movement
   C. Memory
   D. Emotion

3. Output from BG goes through:
   A. Cortex
   B. Thalamus
   C. Spinal cord
   D. Cerebellum

4. Striatum consists of:
   A. GPi + GPe
   B. Caudate + putamen
   C. SNpc + SNpr
   D. STN + GPi

5. GPi neurons are:
   A. Excitatory
   B. Inhibitory
   C. Sensory
   D. Motor

6. Striatal MSNs are:
   A. Excitatory
   B. Inhibitory
   C. Sensory
   D. Reflex

7. GPi is:
   A. Input nucleus
   B. Output nucleus
   C. Sensory nucleus
   D. Relay

8. SNpc contains:
   A. GABA neurons
   B. Dopamine neurons
   C. Glutamate neurons
   D. Sensory neurons

9. STN is:
   A. Inhibitory
   B. Excitatory
   C. Neutral
   D. Dopaminergic

10. Direct pathway results in:
    A. Inhibition of movement
    B. Facilitation of movement
    C. No change
    D. Reflex

11. Indirect pathway results in:
    A. Facilitation
    B. Inhibition
    C. Excitation
    D. Reflex

12. Hyperdirect pathway:
    A. Slow
    B. Emergency brake
    C. Excitatory
    D. Sensory

13. Dopamine acts on:
    A. Cortex only
    B. Striatum
    C. Cerebellum
    D. Brainstem

14. D1 receptors:
    A. Inhibit neurons
    B. Excite neurons
    C. Neutral
    D. Block signals

15. D2 receptors:
    A. Excite
    B. Inhibit
    C. Neutral
    D. Activate reflex

16. Dopamine overall:
    A. Inhibits movement
    B. Facilitates movement
    C. No effect
    D. Stops movement

17. Parkinson’s disease involves loss of:
    A. GABA
    B. Dopamine
    C. Glutamate
    D. Serotonin

18. Parkinson’s symptoms include:
    A. Hyperactivity
    B. Bradykinesia
    C. Chorea
    D. Seizures

19. Huntington’s disease causes:
    A. Reduced movement
    B. Excess movement
    C. Paralysis
    D. Blindness

20. Huntington’s affects:
    A. Direct pathway
    B. Indirect pathway
    C. Cortex
    D. Cerebellum

21. Thalamus function:
    A. Sensory input
    B. Relay motor signals
    C. Vision
    D. Hearing

22. SNpr is:
    A. Input nucleus
    B. Output nucleus
    C. Sensory
    D. Motor

23. GPe function:
    A. Output
    B. Internal regulation
    C. Sensory
    D. Motor

24. Direct pathway inhibits:
    A. Cortex
    B. GPi
    C. STN
    D. GPe

25. Indirect pathway activates:
    A. Cortex
    B. GPi
    C. GPe
    D. Striatum

26. STN excites:
    A. Cortex
    B. GPi
    C. Striatum
    D. Cerebellum

27. MSNs are:
    A. Tonically active
    B. Tonically inactive
    C. Sensory
    D. Reflex

28. GPi is:
    A. Tonically inactive
    B. Tonically active
    C. Neutral
    D. Reflex

29. BG output is:
    A. Excitatory
    B. Inhibitory
    C. Neutral
    D. Sensory

30. Dopamine loss leads to:
    A. Increased movement
    B. Decreased movement
    C. No change
    D. Reflex

31. Chorea is seen in:
    A. Parkinson’s
    B. Huntington’s
    C. Stroke
    D. Epilepsy

32. Bradykinesia means:
    A. Fast movement
    B. Slow movement
    C. No movement
    D. Jerky movement

33. Akinesia means:
    A. Excess movement
    B. No movement
    C. Fast movement
    D. Reflex

34. Deep brain stimulation targets:
    A. Cortex
    B. STN
    C. Eye
    D. Ear

35. L-DOPA:
    A. Blocks dopamine
    B. Increases dopamine
    C. Reduces dopamine
    D. Neutral

36. BG and cerebellum both:
    A. Directly control muscles
    B. Use thalamus
    C. Control vision
    D. Control hearing

37. BG loop involves:
    A. Cortex only
    B. Cortex–BG–thalamus
    C. Cerebellum only
    D. Spinal cord

38. Direct pathway uses:
    A. D2
    B. D1
    C. Both
    D. None

39. Indirect pathway uses:
    A. D1
    B. D2
    C. Both
    D. None

40. Net BG function:
    A. Stop movement
    B. Select appropriate movement
    C. Control vision
    D. Control hearing

✅ MCQ ANSWERS

1. B

2. B

3. B

4. B

5. B

6. B

7. B

8. B

9. B

10. B

11. B

12. B

13. B

14. B

15. B

16. B

17. B

18. B

19. B

20. B

21. B

22. B

23. B

24. B

25. B

26. B

27. B

28. B

29. B

30. B

31. B

32. B

33. B

34. B

35. B

36. B

37. B

38. B

39. B

40. B