**Question:** Inhibitory neurotransmitter of central nervous system is:

A. Acetylcholine
B. Glutamate
C. Gamma-aminobutyric acid (GABA)
D. Norepinephrine

**Core Concept:**
The central nervous system (CNS) is composed of the brain and spinal cord, where neurons communicate with each other through the release of neurotransmitters. Neurotransmitters are chemical messengers that facilitate or inhibit the transmission of signals across a synapse, the small gap between neurons. Inhibitory neurotransmitters are responsible for reducing neuronal activity in the post-synaptic neuron.

**Why the Correct Answer is C:**
GABA (Gamma-aminobutyric acid) is a primary inhibitory neurotransmitter in the CNS. It works by binding to GABAA or GABAB receptors on the post-synaptic neuron, leading to the opening of chloride channels that allow chloride ions to flow into the neuron, hyperpolarizing the cell membrane and reducing neuronal excitability.

**Why each wrong option is incorrect:**
A. Acetylcholine (ACh) is primarily involved in the parasympathetic and sympathetic nervous systems, not exclusively in the CNS. It is an excitatory neurotransmitter, promoting neuronal activity.
B. Glutamate is an excitatory neurotransmitter that plays a crucial role in synaptic transmission, especially in the cerebral cortex and hippocampus, but not exclusively in the CNS.
D. Norepinephrine (NE) is an excitatory neurotransmitter involved in the sympathetic nervous system, contributing to the "fight-or-flight" response, and is not primarily responsible for inhibition in the CNS.

**Clinical Pearl:**
Understanding the role of inhibitory neurotransmitters, such as GABA, is essential for appreciating the delicate balance between excitation and inhibition in maintaining neuronal homeostasis and overall CNS function. A disruption in this balance can lead to various neurological disorders, including epilepsy, anxiety disorders, and sleep disorders like insomnia.