Gamma-aminobutyric acid (GABA) is a neurotransmitter, a chemical that sends signals between cells in the brain. The GABA neurotransmitter is mainly responsible for inhibitory signals between neurons. It also controls muscle tone, inhibiting muscle contractions. Early in development, GABA plays a different role, acting as an excitatory neurotransmitter.
Neuron cells look like trees, with branches called dendrites coming out of one end and a long axon on the other side. Signals travel from the dendrites to the cell body, where they are added together. An electrical signal called an action potential travels down the axon if the signals in the cell body are strong enough.
Neurons communicate with each other by sending chemical signals called neurotransmitters across a gap between cells. The gap between one neuron’s axon and another neuron’s dendrite is called a synapse. Neurotransmitters such as GABA bind to receptors on the receiving neuron and then are reabsorbed, in a process called reuptake, by the transmitting neuron.
When the GABA neurotransmitter binds to a receptor in the central nervous system, it has an inhibitory effect on the neuron. The receptors receiving GABA will send signals that open gated ion channels in the cell. These ion channels either allow positively charged potassium ions to leave the cell or negatively charged chloride ions to enter the cell. Either way, the cell becomes hyperpolarized, meaning it is more negatively charged than the area around it. Hyperpolarization makes the cell less likely to create an action potential which will send signals to other cells.
In humans, the GABA neurotransmitter is found not only in the central nervous system but in the peripheral nervous system as well. In the peripheral nervous system, GABA controls muscle tone. GABA signals are inhibitory in this function as well.
The GABA neurotransmitter does not always act as an inhibitor, however. In early human development, GABA is actually the chief excitatory neurotransmitter, sending signals within a cell and to other cells. GABA regulates the growth of some types of stem cells.
Spastic diplegia, a type of cerebral palsy, is caused by problems with the GABA neurotransmitter signaling in the spinal cord and lower body. Spastic diplegia is characterized by spasticity and constant muscle tension in the lower body. These symptoms are caused when the nerve receptors that control this part of the body cannot absorb GABA. Without GABA signals, the muscles constantly contract, because there is no inhibiting signal telling them not to.