The nerve cells and their processes form a network (A) that is not a continuum of nerve fibers (continuity theory) but consists of countless individual elements, the neurons (neuron theory). As the basic building block of the nervous system, the neuron repre-sents a structural, genetic, trophic, and functional unit.
The neurons in the network are intercon-nected in a specific way (neuronal circuits). The connections for the inhibition of excita-tion are as important as those for the trans-mission of excitation, for they are the ones through which the continuous influx of im-pulses is restricted and selected: important signals are transmitted, unimportant ones are suppressed.
Postsynaptic inhibition does not inhibitsynaptic
transmission but subsequent dis-charge of the postsynaptic neuron.
Inhibitory GABAergic neurons can be inte-grated into the neuronal circuit in different ways. In the case of recurrent (feedback) inhi-bition (B), an axon collateral of the exci-tatory projection neuron (green) activates the inhibitory cell (red), which in turn in-hibits the projection neuron via a recurrent collateral. In the case of feed-forward inhibi-tion (C), the inhibitory interneuron is not ac-tivated by a recurrent collateral of the exci-tatory cell but by excitatory afferents from another brain region. The effect on the pro-jection neuron, however, is the same; acti-vation of the inhibitory GABAergic neurons leads to inhibition of the projection neu-rons. In case of disinhibition (D), an inhibi-tory interneuron is again activated by an ex-citatory afferent. The target cell of this inter-neuron, however, is another inhibitory in-terneuron. Activation of the first inter-neuron through the excitatory afferent therefore means an increased inhibition of the second interneuron, which now cannot exert an inhibitory effect on the next projec-tion neuron. The inhibitory effect is re-moved (disinhibition).
A single nerve cell in the brain receives a large number of connections (convergence).One pyramidal cell of the cerebral cortex can establish over 10 000 synaptic connec-tions with other nerve cells. In turn, such a cell itself creates numerous connections with many other nerve cells by numerous axon collaterals (divergence). The spatial and temporal summation of excitatory and in-hibitory inputs of a cell decides at a given moment whether the cell is depolarized and generates an action potential, which then runs along the axon and leads to excitation of subsequent neurons in the series. If the inhibitory inputs predominate, the dis-charge of neurons does not take place.