Relationships between physiological and morphological properties of retinocollicular axons in the hamster

Intracellular recording and HRP injection were used to characterize retinocollicular axons in the hamster. Eighteen retinotectal axons were filled with HRP. The type of axon recovered most often (N = 10) had a receptive field with center-surround organization, gave sustained responses to flashed spots, had linear spatial summation, and responded only to more slowly moving stimuli. These fibers had a mean axonal diameter of 0.58 microns (SD = 0.21) and an average conduction latency of 3.4 msec (SD = 1.2) to optic chiasm (OX) stimulation. They gave rise to circular or vertically elongated arbors with an average cross-sectional area of 33,238 microns2 (SD = 12,763) and were almost completely restricted to the stratum griseum superficiale (SGS). We recovered 6 fibers with very different structural and functional properties. The receptive fields of these axons also had center-surround organization, but gave phasic responses to flashed stimuli, had nonlinear spatial summation, and responded to rapidly moving stimuli. These axons had an average diameter of 0.84 micron (SD = 0.09) and an average latency of 2.1 msec (SD = 0.40) to OX stimulation. They had elongated arbors with an average cross-sectional area of 78,045 microns2 (SD = 14,252) and innervated the SGS, the stratum opticum (SO), and upper stratum griseum intermediale (SGI). We also recovered 2 fibers that had thin (0.3 and 0.6 micron) axons and latencies of 4.2 and 3.6 msec to OX stimulation. They both gave rise to horizontally oriented arbors in the stratum zonale (SZ) and upper SGS.