#synapses

A nervous system cannot work by excitation alone. Neurons need ways to encourage other neurons to fire, but they also need ways to restrain them, sharpen their timing, filter competing signals, and prevent activity from spreading indiscriminately through a network. In the vertebrate brain, much of that inhibitory work depends on a small molecule called GABA, short for gamma-aminobutyric acid. GABA is commonly described as the brain's main inhibitory neurotransmitter, and that is a useful starting point. But it can also make inhibition sound simpler than it really is. GABA can make a neuron less likely to fire, change how strongly it responds to incoming signals, influence when it fires, or even increase activity elsewhere in a circuit by inhibiting another inhibitory neuron. The deeper story is not simply that GABA acts as a brake. It is that nervous systems use inhibition as one of the basic tools for organizing activity.

The chemistry begins with an interesting relationship. GABA is synthesized from glutamate, another neurotransmitter that usually plays the opposite role in the vertebrate central nervous system. An enzyme called glutamic acid decarboxylase modifies glutamate to produce GABA, which can then be packaged into tiny membrane-bound vesicles inside a neuron and released at synapses. After release, GABA can be taken back up by specialized transport proteins and recycled or broken down. Two of the most important neurotransmitters in the brain, one usually associated with excitation and the other with inhibition, are therefore separated by only a short biochemical step. Their very different effects arise not because the molecules carry instructions like "excite" or "inhibit," but because different receptors, ion gradients, cells, and circuits respond to them differently.

Descriptions of the nervous system can sound contradictory.

Neurons are often said to carry electrical signals. But we also hear constantly about neurotransmitters like dopamine, serotonin, glutamate, and GABA: chemicals that neurons use to communicate with one another.

So which is it?

The useful answer is: both.

A typical neural signal repeatedly changes form. Within a neuron, information can travel as an electrical change in the cell membrane. At most synapses, the junctions between neurons, that electrical signal is converted into a chemical one. The receiving neuron then converts the chemical message back into an electrical change of its own.

The nervous system is therefore not purely electrical or purely chemical. It is an enormous network built around translating between the two.

Within a Neuron and Between Neurons

A useful first approximation is:

Within a neuron, signaling is electrical. Between neurons, signaling is usually chemical.

That is not a perfect rule. Some neurons communicate through electrical synapses, and the electrical activity inside a neuron is itself inseparable from chemistry. But as a mental model, it gets surprisingly far.

Imagine one neuron receiving signals from many others. Neurotransmitter molecules bind to receptors on its surface and alter what electrically charged particles, called ions, can move through the neuron's membrane.

Those changes affect the electrical voltage across the membrane.