The five protein subunits of the GABAa receptor are arranged in circular formation to form a pore which transverses the membrane of the post-synaptic neuron. When GABA is not present, this pore is shaped somewhat like an hourglass. The openings at either end are relatively large, reaching a diameter of about 3 nm, yet the inside of the channel narrows to about .5 nm. The narrowing of the channel is due to a kink in the three dimensional structure of the five protein units which form the receptor. It is this narrow region which prevents chloride ions from entering the neuron when GABA is not present. However, when GABA binds to the recognition site, it induces several conformational changes in the receptor molecule. One of these changes involves the rotation of the five protein subunits such that the diameter of the kink is widened. A wider channel makes it possible for choride ions to pass freely into the neuron.