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Basal ganglia 5 marks physiology. 1 year mbbs

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Here is a complete, exam-ready 5-mark answer on the Basal Ganglia for 1st year MBBS physiology:

Basal Ganglia - Physiology (5 Marks)

Definition / Introduction

The basal ganglia are a group of deep nuclei of the telencephalon that play a key role in the planning and execution of smooth, voluntary movements. They form part of the highest level of the motor control hierarchy - concerned with strategy (choosing the type of movement).

Components (Nuclei)

Core nuclei:
  • Caudate nucleus
  • Putamen (caudate + putamen = Striatum - the main input station)
  • Globus pallidus - internal segment (GPi) and external segment (GPe) - the main output station
Associated nuclei:
  • Subthalamic nucleus (diencephalon)
  • Substantia nigra - pars compacta (SNc) and pars reticulata (SNr) (midbrain)
  • Ventral anterior (VA) and Ventral lateral (VL) nuclei of thalamus

Functions of Basal Ganglia

  1. Aid in planning and execution of smooth voluntary movements
  2. Suppress unwanted competing motor programs
  3. Facilitate initiation of willed movements
  4. Contribute to affective, cognitive, and motivational functions
  5. Working memory and decision-making

Circuits / Pathways

Almost all areas of the cerebral cortex send excitatory (glutamatergic) input topographically to the striatum. Information then flows via two major parallel pathways:
Basal Ganglia Pathways - Direct and Indirect
Fig. Pathways in the basal ganglia showing direct (excitatory overall) and indirect (inhibitory overall) pathways. (Costanzo Physiology, 7th Ed.)

1. Direct Pathway (Overall EXCITATORY to cortex)

Cortex (+) → Striatum → GPi/SNr (-) → Thalamus → Cortex (+)
Step by step:
  • Cortex sends excitatory input to striatum (putamen)
  • Striatum sends inhibitory (GABA) input to GPi/SNr
  • GPi/SNr is therefore inhibited (disinhibited thalamus)
  • Thalamus sends excitatory input back to motor cortex
  • Net result: EXCITATION of motor cortex → facilitates movement initiation ("Go signal")
  • Sign chain: (+)(-)(-) = excitatory

2. Indirect Pathway (Overall INHIBITORY to cortex)

Cortex (+) → Striatum → GPe (-) → Subthalamic nucleus (+) → GPi/SNr (-) → Thalamus → Cortex (+)
Step by step:
  • Cortex sends excitatory input to striatum
  • Striatum sends inhibitory (GABA) input to GPe
  • GPe (now less inhibited) ... actually GPe disinhibits STN - STN is released
  • Subthalamic nucleus sends excitatory (glutamate) input to GPi/SNr
  • GPi/SNr is MORE active, sending MORE inhibitory (GABA) to thalamus
  • Thalamus is inhibited → cortex is less activated
  • Net result: INHIBITION of motor cortex → suppresses unwanted competing movements
  • Sign chain: (+)(-)(-)(+)(-) = inhibitory
Key principle: The direct and indirect pathways are equal and opposite - their balance determines smooth motor control. Disruption causes disease.

Role of Dopamine (Nigrostriatal Connection)

  • Substantia nigra pars compacta (SNc) sends dopaminergic projections back to the striatum
  • Dopamine acts on:
    • D1 receptors on direct pathway neurons → excites (facilitates direct pathway)
    • D2 receptors on indirect pathway neurons → inhibits (suppresses indirect pathway)
  • Net effect: dopamine promotes movement by boosting direct and dampening indirect pathway

Diseases of the Basal Ganglia

DiseaseLesionPathway EffectClinical Features
Parkinson's diseaseDegeneration of SNc (dopaminergic neurons lost)Indirect pathway overactive; Direct pathway underactiveHypokinesia, bradykinesia, resting tremor ("pill-rolling"), rigidity, shuffling gait
Huntington's diseaseDegeneration of striatal (caudate + putamen) GABAergic and cholinergic neuronsIndirect pathway underactiveHyperkinesia, choreiform (writhing) movements, dementia
  • Increased basal ganglia output (inhibition of thalamus) → Hypokinesia (Parkinson's)
  • Decreased basal ganglia output → Hyperkinesia (Huntington's, hemiballismus)

Treatment Note (For Parkinson's)

  • L-Dopa (precursor to dopamine) - crosses blood-brain barrier, boosts dopamine synthesis
  • Dopamine agonists (e.g., bromocriptine)
  • Deep brain stimulation (DBS) of subthalamic nucleus

Summary Mnemonics

  • Striatum = INPUT | Globus pallidus (GPi) = OUTPUT
  • Direct = Disinhibition = movement facilitation ("Do it")
  • Indirect = Inhibition = movement suppression ("Don't do it")
  • Dopamine = "Do move" (facilitates movement via D1/D2)
Sources: Costanzo Physiology 7th Ed., p. 115-116 | Neuroscience: Exploring the Brain 5th Ed., p. 1351-1356
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