There are inhibitory and excitatory synapses between neurons. Axons transmit signals to other neurons by means of specialized junctions called synapses. BDNF enhances the growth and survival of cortical neurons and synapses. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Instead, the sensory neuron synapses directly on a motor neuron in the spinal cord. The GABA receptor is a protein complex located in the synapses between neurons. There is also evidence that microglia can refine synaptic circuitry by engulfing and eliminating synapses. In a diverging circuit, one neuron synapses with a number of postsynaptic cells. In 1962, Bernard Katz modeled neurotransmission across the space between neurons known as synapses. This increases acetylcholine concentrations at cholinergic synapses. Furthermore, acetylcholine release from the presynaptic membranes of muscarinic nerve synapses is blocked. This tract ascends all the way to the VPLN, where it synapses on tertiary neurons. This leakage causes a tonic release of neurotransmitter to the synapses. At chemical synapses, glutamate is stored in vesicles. Certain viruses can replicate in brain cells and cross synapses.