<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> <Ul> <Li> Tonic or regular spiking . Some neurons are typically constantly (or tonically) active . Example: interneurons in neurostriatum . </Li> <Li> Phasic or bursting . Neurons that fire in bursts are called phasic . </Li> <Li> Fast spiking . Some neurons are notable for their high firing rates, for example some types of cortical inhibitory interneurons, cells in globus pallidus, retinal ganglion cells . </Li> </Ul>

Which neurone is not part of the central nervous system