| 1. |
Positive potentials with the waveform of a smoothed retinula receptor potential are recorded from dragonfly lamina. All potentials of this type are calledlamina positive potentials, unless their origin is certain. To establish their origin and to see how information is processed in the lamina their sensitivity characteristics are examined in detail and compared with retinula cell somata.
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| 2. |
By comparing response noise at low intensities (discrete potentials), polarised light sensitivity, angular sensitivity, spectral sensitivity (Fig. 1) and intensity/ response functions it becomes clear that not all lamina positive potentials originate from retinula cell axons. The potentials are divided into two groups,axon responses andlamina depolarisationa.
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| 3. |
Axon responses have sensitivities and characteristics that resemble closely retinula cell somata (Fig. 2, 3, 5). In some cases recordings are correlated with a definite resting potential. It is concluded that axon responses probably originate intracellularly from retinula axons in the lamina. Some axon responses show small light induced action potentials (Fig. 6).
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| 4. |
Lamina depolarisations show the properties of a summed response from several retinula cell somata, i.e. high signal:noise ratio at low intensities, no polarised light sensitivity (Fig. 1), broad or distorted spectral sensitivities (Fig. 4). On the basis of this evidence, together with their broader angular sensitivity functions and unique intensity/response functions (Figs. 2, 3) it is proposed that lamina depolarisations are extracellular in origin.
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| 5. |
Previous studies also suggest an extracellular origin for the lamina depolarisation. It is concluded that this extracellular signal may act as a negative feed-back within the lamina.
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| 6. |
No pre-synaptic summation of retinula axon signals can be found in dragonfly lamina. Voltage amplification and improved signal: noise ratio result from a summation of inputs upon second order neurons. The unique individual properties of retinula cells are maintained in the lamina and they may function as inputs to other systems.
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