Second-order neurones and receptor mechanisms in visual- and olfactory-information processing.

The retina and olfactory bulb are relatively simple in their synaptic operation, and provide insight into the fundamental synaptic mechanisms of brain functions. In the visual system, bipolar cells receive glutamate input from photoreceptors, and segregate visual inputs into parallel ON and OFF responses to light exposure and termination. In the olfactory system, the mitral and tufted cells respond to excitatory inputs from olfactory receptor neurones, and undergo reciprocal regulation through dendrodendritic synapses with their associated granule cells. Recent studies of the synaptic operation and regulation of the bipolar, mitral and tufted cells, at the molecular level, have revealed the detailed synaptic mechanisms of the second-order neurones in the segregation and discrimination of sensory information, as well as the modulatory synaptic mechanism that is involved in olfactory-recognition memory formation.