Retina and Its Functions – The Role of 150 Million Receptors and 1 Million Optic Nerve Fibres
11th July 2019
In our previous post, we have introduced the main similarities between the eye and the camera. Now we are going to focus on the retina and its functions. The impressive layers of tissue that form the retina are more complex than any camera sensor on the market, be it an analogue film or digital CCD/CMOS sensor. However, the retina and the camera sensor, both play the same role in transforming light photons into electrical impulses.
Anatomy of the Retina
The retina in human adults is 0.5mm thick and 22mm in diameter and is considered to be the most important part of the eye. It’s made of neuron layers that can be divided into 4 main groups based on the stages of processing the light that enters the eye, as such:
Reception of light rays, transmission to the bipolar cell layer and ganglion cell layer and finally transmission through the optic nerve. These retinal layers are connected by synapses and supported by the outer layer of pigment epithelium which provides nutrients to the retina.
The retina contains approx. 7 million cones located predominantly in the central part. The part with a high concentration of cones is called the macula, which is around 5.5mm. Whereas, the most important one part of the macula is its centre called the fovea, where the highest concentration of cones are located. This allows the human eye to achieve a sharp and visually detailed colour vision, crucial for reading for example.
The peripheral part of the retina is composed of approx. 100 million rods. For this reason, looking at an object in the dark, it’s easier to focus if looking slightly to the side of the object and not directly at it.
The bipolar cells are a type of neuron that connects the rods and cones with the ganglion cells, this is called synaptic connection. At this stage the ganglion cell, another type of neuron, once the information has been received from the photoreceptors, it transmits it to the brain through the optical nerve. Based on their location in the retina, these cells can receive inputs from 100 rods and cones. One of the nerve fibres in the ganglion cells called axon, together with glial cells forms the optical nerve, which transmits the visual signals to the brain.
There are a few ways to examine the retina, by using different devices such as an ophthalmoscope, a slit lamp, a retinal camera or an OCT, for a distinct view of the retina’s layers. Ophthalmologists and optometrists examine the retina in order to detect signs of damage or abnormalities, which could cause retinal diseases. But we will talk more on the most common retinal diseases in our future posts. In this video, we can observe another way of visualising the retina, with an operating microscope. The video showcases the incredible retina, just before a vitreoretinal surgery procedure, where the optic disc that connects to the optical nerve and the blood vessels are visible. We are able to observe the retina thanks to the cutting-edge Takagi OM-19 Operating Microscope, that uses exceptional quality objective lenses and eyepieces made in Japan. In addition to high-end quality lenses, an inverter and a BIOM, are used for fundus observation.
For fundus observation devices adaptations, OM-19 is compatible with Oculus BIOM and Haag Streit EIBOS.
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