Question

Activation of D1 receptors and D2 receptors induce opposite effects on their respective striatal medium spiny neurons (MSNs). The former, found in the Direct Pathway, increase the likelihood of action potential generation, while the latter, found in the Indirect Pathway, decrease the likelihood of action potential generation. The functioning of these pathways is in a delicate balance, and alterations in the phasic and/or tonic release of dopamine into the system can have major effects. The current, most well-supported dopamine hypothesis of Tourette's Syndrome is one of a hyperdopaminergic state, in which both tonic and phasic release of dopamine are elevated. Excess phasic release onto D1 receptors aberrantly increases the reinforcement of motor actions, and leads to habit formation and tics. Since this normally occurs in the striatum to connect actions to received reward, it is hypothesized that alleviation of premonitory urge-derived distress further acts as a driver for habit formation and tic learning. Conversely, excess tonic release and activation of D2 receptors increases the likelihood of previously-learned movements. This is due to the reduced inhibitory tone on the thalamus.

Answer 1

Parkinson's disease is characterized by the loss of dopaminergic neurons in the SNc, disrupting the balance of direct and indirect pathways in the basal ganglia and leading to motor symptoms like tremors and rigidity.

Step-by-step explanation:

In Parkinson's disease (PD), the loss of dopaminergic neurons in the Substantia Nigra pars compacta (SNc) disrupts normal function of the basal ganglia circuitry. This causes an imbalance in glutamate and GABA neurotransmission in the nigrostriatal pathway, leading to the disease's characteristic tremors and rigidity. The direct pathway, facilitated by D1 receptors, is responsible for increasing cortical movement commands, and its inactivity leads to a hypokinetic disorder. Conversely, the indirect pathway operates through D2 receptors and when overactive due to loss of inhibitory dopamine signaling, it further enhances the symptoms of Parkinson's.

Understanding these pathways and neurotransmitter dynamics is important for grasping the effects of various neurological disorders on motor function. Disruptions in dopamine release can lead to either hypokinetic or hyperkinetic movement disorders, depending on which pathway and receptors are involved. Treatment strategies for PD often aim to balance the dopamine system in order to reduce symptoms