Final answer:
Neurotransmitters can be both excitatory and inhibitory due to the different types of receptors present on postsynaptic cells. Acetylcholine can trigger excitation while GABA can cause inhibition, depending on their interaction with specific receptors. The complexity of receptor types and neurotransmitter actions within the CNS allows for a range of effects beyond just membrane depolarization, including gene transcription and metabolic processes.
Step-by-step explanation:
Why can some neurotransmitters be both excitatory and inhibitory? The effect a neurotransmitter has on the postsynaptic cell is predominantly determined by the type of receptors that it activates. This versatility allows neurotransmitters to have varying effects on different cells. For example, acetylcholine at the neuromuscular junction causes an excitatory postsynaptic potential (EPSP) by opening postsynaptic Na+ channels, leading to depolarization. Conversely, the neurotransmitter GABA at inhibitory synapses triggers a hyperpolarization by opening Cl- channels, resulting in an inhibitory postsynaptic potential (IPSP).
The biogenic amines, such as dopamine, can have both excitatory and inhibitory effects within the same neurotransmitter system. For instance, dopamine can bind to D1-type receptors causing excitation or to D2-type receptors resulting in inhibition. Additionally, neurotransmitter systems like the glutamate receptors have various effects. Metabotropic glutamate receptors (mGluRs) affect second messenger signaling pathways, while ionotropic glutamate receptors (iGluRs) elicit fast responses to glutamate release. These complexities allow neurotransmitters to facilitate nuanced communication within the central nervous system (CNS), affecting everything from cell membrane potential to gene transcription.