In the central nervous system, billions of interconnected neurons are organized into specialized functional groups called neuronal pools. Within each pool, neurons are arranged in five distinct patterns of neural circuits. In a simple series circuit, one presynaptic neuron stimulates another single postsynaptic neuron, which then stimulates another neuron, and so on. For a diverging circuit, one presynaptic neuron may synapse with several postsynaptic neurons or cells, such as muscle fibers and gland cells, to stimulate them simultaneously. In a converging circuit, several presynaptic neurons synapse with a single postsynaptic neuron. For example, different sensory stimuli may trigger the same memory. While in a reverberating circuit, the signal travels in a series and back to the source or previous neuron, creating a feedback loop. Such circuits are involved in short-term memory or maintaining consciousness. The parallel processing circuit, or parallel after-discharge circuit, permits one presynaptic cell to stimulate a parallelly arranged neurons synapse with a common postsynaptic cell. Such circuits are thought to be involved in complex activities like mathematical calculations.