Basal Ganglia | Medical Science Optional Notes for UPSC PDF Download

Introduction

The basal ganglia are dispersed clusters of gray matter immersed in the subcortical substance of the cerebral hemisphere. They comprise the striatum (caudate nucleus and putamen), globus pallidus, subthalamic nuclei, and substantia nigra. The basal ganglia modulate thalamic outflow to the motor cortex, facilitating the planning and execution of smooth movements.

Basal ganglia-Connections

Numerous synaptic connections exert inhibitory effects and utilize GABA as their neurotransmitter. The connections linking the striatum and the substantia nigra employ dopamine as their neurotransmitter. In general, the action of dopamine is excitatory.

Basal ganglia-Functions

Instructions for voluntary movement arise from cortical association areas. The cortex, basal ganglia, and cerebellum collaborate in the planning of movements. Movements initiated by the cortex are transmitted through the corticospinal tracts and corticobulbar tracts to motor neurons. The cerebellum offers feedback to refine and ensure the smoothness of movement.

Damage to the globus pallidus leads to an incapacity to sustain postural support. Lesions affecting the subthalamic nucleus result in uncontrolled and abrupt movements, as seen in conditions like hemiballismus. Lesions impacting the striatum give rise to rapid, continuous, and uncontrollable movements, particularly evident in patients diagnosed with Huntington's disease.

Descending Tracts

• Routes through which motor signals are transmitted from the brain to lower motor neurons
• Categorized into pyramidal and extrapyramidal tracts

Extrapyramidal Tracts

Pyramidal vs Extrapyramidal Tracts

Syringomyelia

• Formation of a cyst or cavity (SYRINX) in the spinal cord
• Causes
  (a) Congenital - Arnold-Chiari malformation
  (b) Acquired - Trauma, meningitis, hemorrhage, tumor, arachnoiditis
• Symptoms include chronic pain, abnormal sensations, and paralysis
• Syringobulbia involves the brainstem, leading to vocal cord paralysis, ipsilateral tongue wasting, and trigeminal nerve sensory loss.
• Classically, syringomyelia spares the DCML (dorsal column-medial lemniscus) of the spinal cord, preserving pressure, vibration, touch, and proprioception in the upper extremities.

Consciousnes

William James characterized consciousness as the recognition of oneself (or one's own cognitive experience) and the surrounding environment. Despite its seemingly straightforward definition, this concept conceals intricate brain functions that encompass numerous distinct yet interconnected qualities and components.

Consciousness Components: Consciousness is a dynamic process comprising multiple elements.

• Alertness: The ascending reticular activating system, with various anatomical and neurochemical components in the rostral brainstem tegmentum, thalamus, and cortex, is responsible for alertness, a crucial prerequisite for heightened awareness.
• Sensations: After the initial reception in the cortex, sensations undergo further selection and processing in interconnected regions.
• Perception: Involves the abstraction of chosen sensory information, providing a limited understanding of external events and internal states.
• Attention: Directs and selects specific information while excluding others.
• Memory: Information is temporarily stored in working memory to facilitate immediate actions and decisions. Certain forms of memory are associated with conscious awareness, deemed essential for maintaining a sense of continuity in daily life.
• Motivation: Involves prioritizing and choosing behavior.

Disorders of Consciousness

Medical conditions that inhibit consciousness are considered disorders of consciousness

Basal ganglia-Repeats

Explain the roles of the basal ganglia in voluntary actions. Contrast the functions of the basal ganglia with those of the primary cortex. (2017)

Descending Pathways-Repeats

• Identify the lateral descending tracts. Enumerate the effects of their lesions. Discuss the impact on sensory loss in lesions around the central canal of the spinal cord. (2014)
• Corticospinal Tracts: Provide details on the origin, course, and function(s) of the corticospinal tracts. Include a note on the effects of damage to these tracts and the resulting paralysis. (2012)
• Extrapyramidal Tracts: Define extra-pyramidal tracts and delineate their functions. Highlight the distinctions between extrapyramidal and pyramidal tracts. (2018)