Generation of graded potential signals in the second order cells of locust ocellus

Abstract

1. The electric properties of the large second order neurons (L neurons) of locust ocellus have been examined with two independently controlled intracellular microelectrodes placed in the same cell. 2. L neurons showed delayed rectification and could produce action potentials in response both to current and on termination of light stimulation. Action potentials are generated at a site in the brain and are conducted passively, with considerable attenuation along L neuron axons. 3. Severe hyperpolarisation of an L neuron induced by extrinsic current turned on a slowly developing conductance. The time taken for full development of this conductance was strongly voltage-dependent but on the order of 1 s. After termination of the extrinsic current the conductance increase typically persisted for several hundred ms. 4. Saturating light intensities were always associated with an increase in input conductance in L neurons. Lower intensities produced either a small increase or decrease in conductance. Conductance increase at the postsynaptic membrane is thought to be opposed by a voltage-sensitive conductance decrease elsewhere in the cell. 5. The mean reversal potential for the peak hyperpolarisation during light stimulation was 41 mV negative to dark resting potential. In the majority of cells the plateau phase of the light response reversed at the same potential. 6. Following termination of the light pulse the input conductance of the cell was shown to return briefly to its dark value probably as a consequence of the turning off of receptor transmitter release. The input conductance of the cell, probably to Na+ or Ca++ ions, was then seen to rise transiently to produce a small depolarising “off” potential. 7. Hyperpolarisation of cells by extrinsic current, within their normal voltage range, failed to decrease their sensitivity to light. Severe hyperpolarisation, outside a cell's normal voltage range could decrease it's sensitivity by up to 1.26 log units. This effect is thought to be due to the decrease in the cell's input resistance produced by severe hyperpolarisation. 8. Hyperpolarising signals produced in the subretinal neuropile show decrement along L neuron axons. Data from ocellar L neurons are consistent in principle with passive propagation of graded potentials.