Bipolar cells of the retina
R: chopsticks, C: pin,
H: Horizontal cell Bi: bipolar cell ,
A: amacrine cell, G: ganglion cell,
GC: ganglion cell layer.
(The light comes in from below.)
The bipolar cells of the retina are nerve cells of the retina with a bipolar structure and are also called bipolar cells for short .
The light-sensitive sensory cells ( rod cells and cone cells ) form synapses with bipolar cells as the second neuron of the visual pathway . Their task is to collect information from the photoreceptors , to weight it and to transmit it to the ganglion cells of the retina (3rd neuron).
The extensions of bipolar cells form synapses on the one hand in the outer plexiform layer (OPL) and on the other hand in the inner plexiform layer (IPL) of the retina. The cell bodies of the bipolar cells lie between them in the inner granular layer (INL). In the retina of mammals, one finds, in addition to one type of rod- controlled, eight to eleven types of cone- controlled bipolar cells, depending on the species .
Transmission of excitation
Similar to the photoreceptors from which the bipolar cells receive signals, they have a peculiarity of the conduction of excitation: In contrast to most other neurons, they encode information through graduated changes in potential , which then lead to changes in the amount of neurotransmitters released (see also the detailed description of this excitation transmission for rods ).
Contrary to the earlier assumption that bipolar cells encode information exclusively in this analogous way, recent results indicate that some particular types of bipolar cells in mice and fish also produce clear all-or-nothing signals that regulate the flow of information the retinal ganglion cells are divided into temporal blocks.
species
A distinction is made between four main types of bipolar cells according to the type and duration of their transmission of excitation. The term on-cells and off-cells refers to the differentiation of their behavior when light falls on the upstream receptor cells. The neurotransmitter glutamate released by the photoreceptors has opposite effects on on and off bipolar cells. The on-cells and off-cells are also classified according to the duration of their reaction as tonic (sustained = permanent) or phasic (transient = short-term). This division into four main types has existed since 1969. It is now known that there are mixed types and the total number of types of bipolar cells is about 12.
In the case of an off- bipolar, the reduced release of transmitters when the light is incident on the now hyperpolarized sensory cells (rods and cones as 1st afferent neurons) and the lower number of membrane receptors of the bipolar cell to which the transmitters are bound also lead to the hyperpolarization of this neuron. The cellular signal of these (2nd afferent) neurons, which is reduced (under exposure), consequently means a reduced release of the transmitter of these bipolar cells at their synapses with retinal ganglion cells (3rd afferent neurons).
In contrast, the reaction is reversed in the case of On bipolar cells. Depolarization occurs and more transmitters are then released. The transmitter of bipolar cells at their synapses with the ganglion cells is glutamate in both cases, sometimes with glycine as a cotransmitter .
The classification as phasic or tonic gives information about the temporal response of the cell. While phasic cells only respond briefly to a change in light intensity, tonic cells respond to a light stimulus with a permanent change in their signal transmission as long as this light stimulus exists.
Ganglion cells
The axons of ganglion cells form the innermost layer of the retina as a nerve fiber layer and, after leaving the eye, the optic nerve. Each of these ganglion cells has been assigned a specific area of the retina as a receptive field due to the interconnection via bipolar cells.
On-center cells increase their activity when light falls on the center of their receptive field, but reduce their activity when the periphery of their receptive field surrounding the center is illuminated.
Off-center cells react exactly the other way around: if light falls on the center, they reduce their activity; if light falls on the periphery, they increase their activity.
Lateral interconnections
The bipolar cells transmit processed signals from the photoreceptors to the ganglion cells. However, there are also a large number of lateral connections through other types of nerve cells in the retina, which significantly influence this transmission of activity. At the level of the synapses between the photoreceptors and the bipolar cells, these lateral connections and influences are made by horizontal cells - at the level of the synapses between the bipolar cells and the ganglion cells, on the other hand, by amacrine cells .
literature
- Eric R. Kandel , James H. Schwartz, Thomas M. Jessell : Neurosciences. Spectrum, Akademischer Verlag, Heidelberg et al. 1996, ISBN 3-86025-391-3 .
Individual evidence
- ↑ Tom Baden, Philipp Berens, Matthias Bethge, Thomas Euler: Spikes in Mammalian Bipolar Cells Support Temporal Layering of the Inner Retina. In: Current Biology . Vol. 23, No. 1, January 7, 2013, pp. 48-52, doi : 10.1016 / j.cub.2012.11.006 .
- ↑ Tom Baden, Federico Esposti, Anton Nikolaev, Leon Lagnado: Spikes in Retinal Bipolar Cells Phase-Lock to Visual Stimuli with Millisecond Precision. In: Current Biology. Vol. 21, No. 22, November 3, 2011, pp. 1859-1869, doi : 10.1016 / j.cub.2011.09.042 .
- ^ RH Masland: The neuronal organization of the retina. In: Neuron. Volume 76, number 2, October 2012, ISSN 1097-4199 , pp. 266-280, doi : 10.1016 / j.neuron.2012.10.002 , PMID 23083731 , PMC 3714606 (free full text) (review).