Sequential pictorial presentation of neural interaction in the retina. 2. The depolarizing and hyperpolarizing bipolar cells at rod terminals.
Each rod is connected to one depolarizing and one hyperpolarizing bipolar cell. The synaptic connections of cone processes to each bipolar cell and presynaptically to the two rod-bipolar cell synapses establishes conditions for lateral interaction at this level. Thus, the cones raise the threshold for bipolar cell depolarization which is the basis for spatial brightness contrast enhancement and consequently for high visual acuity (Sjöstrand, 2001a). The cones facilitate ganglion cell depolarization by the bipolar cells and cone input prevents horizontal cell blocking of depolarization of the depolarizing bipolar cell, extending rod vision to low illumination. The combination of reduced cone input and transient hyperpolarization of the hyperpolarizing bipolar cell at onset of a light stimulus facilitates ganglion cell depolarization extensively at onset of the stimulus while no corresponding enhancement applies to the ganglion cell response at cessation of the stimulus, possibly establishing conditions for discrimination between on- vs. off-signals in the visual centre. Reduced cone input and hyperpolarization of the hyperpolarizing bipolar cell at onset of a light stimulus accounts for Granit's (1941) 'preexcitatory inhibition'. Presynaptic inhibition maintains transmitter concentration low in the synaptic gap at rod-bipolar cell and bipolar cell-ganglion cell synapses, securing proportional and amplified postsynaptic responses at these synapses. Perfect timing of variations in facilitatory and inhibitory input to the ganglion cell confines the duration of ganglion cell depolarization at onset and at cessation of a light stimulus to that of a single synaptic transmission.
Sjöstrand
F S
FS
Department of Biology, University of California, Los Angeles, USA. fsjostra@ucla.edu
D025042:Amacrine Cells / Q000033:anatomy & histology / Q000502:physiology; D021621:Imaging, Three-Dimensional; D008027:Light; D009431:Neural Conduction / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology; D012160:Retina / Q000033:anatomy & histology / Q000502:physiology*; D017949:Retinal Cone Photoreceptor Cells / Q000033:anatomy & histology / Q000502:physiology; D017948:Retinal Rod Photoreceptor Cells / Q000033:anatomy & histology / Q000502:physiology*; D013569:Synapses / Q000502:physiology*