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Figure 9-5

system is a neuronal pathway that originates in the brain stem and spreads to influence wide areas of the brain.) E. Reverberatory circuits help to sustain a continuing discharge along a neural pathway. Relatively prolonged discharge can also occur through increased numbers of synapses in a pathway, as each synapse entails a certain delay in the passage of information.

Fig. 9-6. Cone sensitivities in the retina. The visual system provides an excellent example of more complex levels of organization of neural pathway connections. The human visual system can perceive hundreds of gradations of colors of the spectrum. Yet it accomplishes this with only 3 kinds of cone photoreceptors in the retina, termed "red", "green", and "blue" photoreceptors. Each receptor type is most sensitive to stimulation at a specific wavelength of light. E.g. light of 650nm wavelength, in the red spectrum, will stimulate red cones. Light at 550 nm, corresponding to yellow, will stimulate both red and green cones. Light at 490nm in the green

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