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from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 10 mark long answer on Basal Ganglia -connections, functions; Parkinsonism- clinical features

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Basal Ganglia and Parkinsonism

(Guyton and Hall Textbook of Medical Physiology, 4th South Asia Edition)

Introduction

The basal ganglia form an "accessory motor system" that, like the cerebellum, does not act independently but works in close association with the cerebral cortex and the corticospinal system to control movement. They receive most of their input from the cerebral cortex and send almost all their output back to the cortex - Guyton and Hall Textbook of Medical Physiology.

Anatomical Components

On each side of the brain the basal ganglia consist of four major structures:
  1. Striatum - composed of the caudate nucleus and putamen, separated by the internal capsule
  2. Globus pallidus (internal and external segments)
  3. Substantia nigra
  4. Subthalamic nucleus
These nuclei lie lateral to and around the thalamus. The internal capsule, carrying almost all motor and sensory fibers between the cerebral cortex and spinal cord, passes between the caudate nucleus and putamen - reflecting the intimate anatomical relationship between the basal ganglia and the corticospinal system.

Neuronal Circuitry (Connections)

Two major circuits dominate basal ganglia function:
1. The Putamen Circuit (execution of learned motor patterns) Signals originate mainly in the premotor and supplementary motor cortex and somatosensory cortex, pass to the putamen (bypassing the caudate), then to the internal globus pallidus, next to the ventroanterior and ventrolateral relay nuclei of the thalamus, and finally return to the primary motor cortex and adjoining premotor/supplementary areas. Ancillary loops also pass from the putamen through the external globus pallidus, subthalamus, and substantia nigra, before returning via the thalamus to the motor cortex.
2. The Caudate Circuit (cognitive planning of movement) Input arises from the association areas of the cerebral cortex (which the C-shaped caudate nucleus underlies in the frontal, parietal, occipital, and temporal lobes). Signals pass to the caudate nucleus, then the internal globus pallidus, then the ventroanterior/ventrolateral thalamus, and finally back mainly to the prefrontal, premotor, and supplementary motor cortex (not the primary motor cortex directly).
A separate nigrostriatal dopaminergic pathway runs from the substantia nigra to the caudate nucleus and putamen; dopamine here acts as an inhibitory transmitter that keeps this circuitry in balance - this pathway is central to Parkinson disease.

Functions of the Basal Ganglia

  1. Execution of complex learned patterns of movement (putamen circuit) - e.g., writing letters, cutting with scissors, hammering nails, shooting a basketball, throwing a ball, vocalization, and controlled eye movements. Damage causes writing and other skilled movements to become crude, as though relearning the skill.
  2. Cognitive control of sequential motor patterns (caudate circuit) - the caudate integrates sensory input and stored memory to select which patterns of movement should be used together to achieve a complex goal within seconds, largely subconsciously (e.g., the instinctive sequence of turning, running, and climbing when confronted by danger).
  3. Timing and scaling of movements - the basal ganglia help determine how rapidly a movement is performed and how large it is (e.g., writing a small versus large letter "A" while keeping proportions the same). Basal ganglia lesions impair this timing/scaling function.
  4. General inhibitory/regulatory role - suppressing unwanted movements and providing background muscle tone appropriate for posture, allowing smooth voluntary movement by the corticospinal system.

Clinical Syndromes of Basal Ganglia Damage

  • Athetosis - lesion of the globus pallidus causing continuous, slow writhing movements of the hand, arm, neck, or face.
  • Hemiballismus - lesion of the subthalamic nucleus causing sudden, violent flailing movements of an entire limb.
  • Chorea - widespread basal ganglia/cortical lesions causing brief, rapid, uncoordinated, involuntary movements of different body parts.
  • Parkinson disease - degeneration of the substantia nigra, discussed below.

Parkinsonism (Parkinson Disease)

Pathophysiology: Parkinson disease results from destruction of the dopamine-secreting neurons of the substantia nigra, which normally send inhibitory dopaminergic fibers to the caudate nucleus and putamen. Loss of this dopamine allows the caudate and putamen to become excessively active, producing continuous excitatory output to the corticospinal motor system.
Clinical Features (Motor):
  • Rigidity - due to excessive, continuous excitatory signals from the overactive striatum to muscles throughout the body.
  • Tremor - occurs during all waking hours (an involuntary, resting tremor), unlike cerebellar tremor which appears only with intentional movement. It results from oscillation of feedback circuits whose gain has increased after loss of normal dopaminergic inhibition.
  • Akinesia/Bradykinesia - often the most distressing feature; the patient must exert intense concentration and mental effort even for simple movements, which then appear stiff and staccato rather than smooth. This is attributed partly to decreased dopamine secretion in the limbic system (nucleus accumbens), reducing the psychic drive for motor activity.
Nonmotor Features: sleep disturbances, depression and anxiety, autonomic dysfunction, and cognitive impairment in advanced disease.
Treatment (brief):
  • L-dopa - crosses the blood-brain barrier (unlike dopamine itself) and is converted to dopamine in the brain, restoring the balance between inhibition and excitation in the caudate and putamen; it improves rigidity and akinesia but has little effect on nonmotor symptoms.
  • Monoamine oxidase (MAO) inhibitors - prevent breakdown of released dopamine, prolonging its action and possibly slowing further destruction of substantia nigra neurons; often combined with L-dopa for better effect.
  • Fetal dopaminergic cell transplantation into the caudate and putamen has shown some short-term success experimentally.
  • Guyton and Hall Textbook of Medical Physiology, 4th South Asia Edition, p. 709-712
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