Final answer:
Huntington's Disease primarily affects the GABAergic medium-sized spiny neurons in the cortex and striatum, with disrupted extrasynaptic NMDA receptor activity contributing to cell death and symptomatic neurological decline.
Step-by-step explanation:
The type of neurons primarily affected by Huntington's Disease (HD) is the cortical and striatal neurons, specifically certain GABA-releasing neurons known as GABAergic medium-sized spiny neurons. The mutation in the Huntington gene leads to a production of a huntingtin protein with an abnormal length, causing a dysregulation of NMDA receptor expression. The disrupted extrasynaptic NMDA receptor activity increases intracellular calcium concentrations, which may trigger mitochondrial dysfunction and cell death, thus contributing to neurological symptoms like spastic involuntary movements, cognitive decline, and difficulties with voluntary movement control.