A patient with Parkinson's disease has progressive loss of dopaminergic neurons in the substantia nigra. Which dopamine pathway is most directly affected, and what is the primary symptom cluster?
AMesolimbic pathway — hallucinations and delusions
BNigrostriatal pathway — motor deficits including tremor and rigidity
CMesocortical pathway — cognitive and motivational deficits
The nigrostriatal pathway runs from the substantia nigra to the striatum and is the pathway that controls voluntary motor coordination. Its degeneration causes the classic Parkinson's triad of tremor, rigidity, and bradykinesia. The mesolimbic pathway is implicated in reward and psychosis; the mesocortical pathway in cognition and negative symptoms of schizophrenia.
Question 2 True / False
Serotonin functions as a uniform 'happiness chemical' that increases mood when levels rise throughout the brain.
TTrue
FFalse
Answer: False
Serotonin's effects depend heavily on which receptor subtype is activated and in which brain region. For example, 5-HT1A receptor activation tends to have anxiolytic and antidepressant effects, while 5-HT2A activation in the cortex can produce psychedelic-like perceptual changes. The same neurotransmitter can have opposing effects depending on local circuit context, which is why 'serotonin = happiness' is a significant oversimplification.
Question 3 Short Answer
Why do SSRIs (selective serotonin reuptake inhibitors) typically take 2–4 weeks to produce antidepressant effects, even though they block reuptake within hours?
Think about your answer, then reveal below.
Model answer: Blocking reuptake raises synaptic serotonin rapidly, but this initially triggers presynaptic autoreceptors (especially 5-HT1A on cell bodies) that reduce neuron firing and serotonin release, partially offsetting the effect. Over weeks, these autoreceptors desensitize, allowing net synaptic serotonin to rise. Downstream changes in receptor expression and neural circuit remodeling — not the initial reuptake block itself — appear to drive the therapeutic effect.
This illustrates why neurotransmitter systems cannot be understood as simple on/off switches. The brain compensates dynamically for changes in synaptic chemistry. Understanding autoreceptor desensitization and adaptive plasticity is essential for predicting pharmacological effects across time.