<P> The two most common neurotransmitters in the brain, glutamate and GABA, have actions that are largely consistent . Glutamate acts on several different types of receptors, and have effects that are excitatory at ionotropic receptors and a modulatory effect at metabotropic receptors . Similarly, GABA acts on several different types of receptors, but all of them have effects (in adult animals, at least) that are inhibitory . Because of this consistency, it is common for neuroscientists to simplify the terminology by referring to cells that release glutamate as "excitatory neurons", and cells that release GABA as "inhibitory neurons". Since over 90% of the neurons in the brain release either glutamate or GABA, these labels encompass the great majority of neurons . There are also other types of neurons that have consistent effects on their targets, for example, "excitatory" motor neurons in the spinal cord that release acetylcholine, and "inhibitory" spinal neurons that release glycine . </P> <P> The distinction between excitatory and inhibitory neurotransmitters is not absolute, however . Rather, it depends on the class of chemical receptors present on the postsynaptic neuron . In principle, a single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still . For example, photoreceptor cells in the retina constantly release the neurotransmitter glutamate in the absence of light . So - called OFF bipolar cells are, like most neurons, excited by the released glutamate . However, neighboring target neurons called ON bipolar cells are instead inhibited by glutamate, because they lack the typical ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors . When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them . </P> <P> It is possible to identify the type of inhibitory effect a presynaptic neuron will have on a postsynaptic neuron, based on the proteins the presynaptic neuron expresses . Parvalbumin - expressing neurons typically dampen the output signal of the postsynaptic neuron in the visual cortex, whereas somatostatin - expressing neurons typically block dendritic inputs to the postsynaptic neuron . </P> <P> Neurons have intrinsic electroresponsive properties like intrinsic transmembrane voltage oscillatory patterns . So neurons can be classified according to their electrophysiological characteristics: </P>

The process by which neurons increase in number is called neuronal