File:Dendritic integration.jpg

Consequences of dendritic integration on the representation of visual motion: Schematic of a fly LPTC with two branches of its dendrite, the axon and the axon terminal. The LPTC receives retinotopically organised input from local motion detecotrs (vertical lines terminating with ´synapses´ red dots (excitatory synapses) and turquoise dots (inhibitory synapses) on the dendrite). (A) As a consequence of this input the cell is excited by motion in its preferred direction (upper diagram) and inhibited by motion in the null-direction (lower diagram). (B) Even when the velocity of motion is constant, the activity of the local input elements of an LPTC is modulated depending on the texture of the surround in the receptive fields of the local elements. Traces on the right indicate the time-dependent signals of three local input elements of the LPTC. By dendritic pooling of many local elements this pattern dependence in the timecourse of the responses is reduced (left trace). (C) Gain control in the TC makes its responses relatively independent of the number of activated input elements and, thus, of pattern size, while the response amplitude still depends on pattern velocity. Left: The enlargement illustrates that each point in visual space is subserved by a pair of input elements of the LPTCs, one of them being cholinergic and excitatory, the other GABAergic and inhibitory. Right: Even during motion in the preferred direction both types of local input elements are activated, though to a different extent depending on the velocity of motion (red and turquoise columns). As a consequence, the membrane potential approaches different saturation levels for different velocities when the number of activated local input elements increases.