Inner Nuclear Layer

Inner Nuclear Layer

The inner Nuclear Layer consists of the following:

1.    Bipolar Cells

2.    Horizontal Cells

3.    Amacrine Cells

Bipolar Cells

Bipolar cells are the direct path that signals pass through from the photoreceptors to the ganglion cells. Therefore, a single bipolar cell is connected to one or many photoreceptors through a single dendrite and many synapses. It is also responsible for converting the chemical response from the rods to electrical impulses as input for the ganglion cells. Some signals pass through from the photoreceptor to the horizontal cells and to the bipolar cell. There are 2 types of bipolar cells and they are the on and off center-surround bipolar cells. Figure 1 shows the center and surround representation of the bipolar cells. More elaboration on the center-surround structure and its effects will be discussed under ganglion cells section. [4]

On Bipolar cells (Invaginating Bipolar)

This class of bipolar cells are called On bipolar is because they react to light spots or stimulus. Because they respond to light therefore they are connected to the photoreceptors through the inhibitory synapses. The reason is that as mention earlier, when photoreceptors are expose to light, they are inactive and thus closing the sodium channels. When this happens, the neurotransmissions that the on bipolar receives when the photoreceptors are active (no light) are suppressed. As the on bipolar cells are connected through the inhibitory synapses therefore the inhabitation signals are removed causing the on bipolar to be active/depolarized with the excitatory signals present. [4]

Off Bipolar cells (Flat Bipolar)

This next class of bipolar is the off bipolar. It is termed off bipolar because it reacts to dark spots. Thus they are connected to the photoreceptors in the direct opposite of the on bipolar which is through the excitatory synapses. Due to the fact that off bipolar are active in the dark therefore when there is light stimulus, the photoreceptors are hyperpolarized closing sodium channels and that will cause the removal of the excitatory signals to the off bipolar which in turn deactivating/hyperpolarizing them with the presence of inhibitory signals. Figure 2 further illustrates the synapses paths of the both types of cells. [4]

Figure 1:Diagram of a general representation of a bipolar cell

Figure 2:Synapses path of on/off bipolar cell

Horizontal Cells

This is a class of cells that does not show any signs of having an axon channel. Therefore they are able to feed signal back to the photoreceptors or to the bipolar cells through the dendrite. The photoreceptors are connected through the excitatory synapses because the horizontal cells are also hyperpolarized by light. It is also found that the photoreceptors that are connected to the horizontal cells are spread out over a wide area and as they are connected to the excitatory synapses of the on bipolar and to the inhibitory of the off bipolar cells. Thus making them responsible for the receptive field surrounds of the bipolar cells. [4]

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