Quick Facts
An amacrine cell is a type of multipolar neuron, located in the inner nuclear layer of the retina, that lacks an axon (Dorland, 2011).
Structure and/or Key Features
Amacrine cells are local circuit neurons, or interneurons. Interneurons, a specific type of neuron that act as an intermediary, affecting the transfer of information between two projection neurons. Amacrine cells are classified as inhibitory interneurons (Standring, 2016) (Standring, 2016).
They tend to have large cell bodies and lobulated nuclei. Their cellular morphology and pharmacology are quite variable. Most do not have a typical axon; instead they possess what resembles a field of dendrites that acts as both axons and dendrites, i.e., they can be both pre- and postsynaptic. These cell processes extend into the inner plexiform layer where they are presynaptic to bipolar cells, and postsynaptic to retinal ganglionic and other amacrine cells. Thus, they modify signal transfer between the intermediate bipolar cells and the output neurons of the retina, the ganglionic cells (Standring, 2016; Remington and Goodwin, 2011). Other synaptic connections are also found.
It is currently estimated that there are approximately 30–40 different amacrine cell types. Amacrine cells may be categorized depending on their process morphology and stratification (stratified or diffuse). They may be further classified depending on the spread or reach of their processes (narrow-field, small-field, medium-field, and large-field). Additionally, they may be classified by the type of inhibitory neurotransmitter they release. The diversity of amacrine cell types is reflected in the complexity and scale of their signaling role within the retina (Remington and Goodwin, 2011; Balasubramanian and Gan, 2014).
Anatomical Relations
The cell bodies of amacrine cells are generally located in the inner nuclear layer, close to the border of the inner plexiform layer, where they are mostly surrounded by other amacrine cells, bipolar cells and Müller cell bodies. Their processes extend into the inner plexiform layer. Some classes of amacrine cell have their bodies located in the ganglionic layer and these are known as displaced amacrine cells. The processes of displaced amacrine cells also extend into the inner plexiform layer (Standring, 2016).
Function
Amacrine cells are largely responsible for the detection of directional motion and for regulating our eyes’ ability to adapt to light. They also play a role in establishing the body’s circadian rhythm (Balasubramanian and Gan, 2014).
Like horizontal neurons, amacrine cells are known as inhibitory neurons. Amacrine cells receive their excitatory input from bipolar cells. They subsequently integrate and regulate the information that reaches ganglion cells via feedback synapses onto bipolar terminals or via feedforward synapses onto ganglion cell dendrites. Amacrine cells may also communicate, via lateral inhibitory synapses, with other amacrine cells (Diamond, 2017).
Amacrine cells play an important role in shaping the visual signaling transmitted through the retina. For example, inhibitory feedback signals from amacrine cells to bipolar cells sharpen the timing of bipolar cell responses and additionally sharpen the edges of their center-surround receptive fields. Feedforward signals, on the other hand, ensure there is spatial and temporal detail in the receptive field (Diamond, 2017).
References
Balasubramanian, R. and Gan, L. (2014) 'Development of Retinal Amacrine Cells and Their Dendritic Stratification', Curr Ophthalmol Rep, 2(3), pp. 100-106.
Diamond, J. S. (2017) 'Inhibitory Interneurons in the Retina: Types, Circuitry, and Function', Annu Rev Vis Sci, 3, pp. 1-24.
Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.
Remington, L. A. and Goodwin, D. (2011) Clinical Anatomy of the Visual System E-Book. Elsevier Health Sciences.
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.
