rod-cells

New function for rods in daylight

Vision – so crucial to human health and well-being – depends on job-sharing by just a few cell types, the rod cells and cone cells, in our retina. Botond Roska and his group have identified a novel function for rod photoreceptor cells in the retina in daylight. Driven by cones and mediated by horizontal cells, rods help to increase contrast information at times when they are not directly sensing light. The retina thus repurposes its cells in different light conditions to increase the amount of visual information about the environment.

(Caption: Horizontal cells in the retina)

Task sharing in the retina seemed clear: Two different kinds of photoreceptor cells take on two different visual tasks. Rods allow us to see at night, cones operate during the day and enable color vision. However, the question as to why there are about 20 times more rods than cones in a human retina, when daytime vision is much more relevant for us, has usually led to a shrug of shoulders. It seemed a waste of resources.

Botond Roska and his group at the Friedrich Miescher Institute for Biomedical Research, could now show in a study published recently in Nature Neuroscience that the rods in mouse take on an important function during daytime vision as well.

The scientists showed that in bright light, the rods mediate a so called surround inhibition. Surround inhibition is an important feature in the retina because it allows not only to transmit information about whether a photoreceptor is exposed to light, but also about contrast. While the cone cells hyperpolarize in bright light and thus send a visual signal to the inner retina, the rods depolarize, inversely matching the activity pattern of the cone cells. The response in the rods is driven by cone cells and mediated through horizontal cells. These horizontal cells connect rods and cones through their dendrites and long axons, and at the same time form a mesh of connections among each other. The hyperpolarization of one cone thus leads to the depolarization of many surrounding rods.

During bright light conditions, the cells of the inner retina receive therefore information through two pathways: First through the well-established cone pathway, and second through this newly identified rod pathway. “We think that the surround information relayed to the inner retina through the rod pathway has different functional properties than the information obtained through the cone pathway,” comments Roska. “In any case it is fascinating to see how the retina repurposes the rod cells during bright light conditions to increase contrast information, at times when they are not directly sensing light.”

After all, these large numbers of rods don’t seem to be present in the retina in vain.

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Brainbeauty: Did you know chickens ‘one-up’ humans in ability to see color?

The retina contains two types of cells, rods and cones. Rods handle low light vision where as cones handle color vision and detail. A series of complex chemical reactions occurs when light contacts these two types of cells. The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal.

Birds have five types of cones including four single cones and a double cone thought to mediate achromatic motion perception. Birds have a cone photoreceptor for violet/ultraviolet light in addition to the red, blue and green single cone cells that humans share. Much however is still unknown about the spatial organization of avian cones and the adaptive significance. 

The vibrancy of avian color vision is currently thought to be a result of not experiencing an evolutionary nocturnal period. In contrast, mammals spent millennia as nocturnal organisms and developed a high density of rod photoreceptors. Since chickens possess minimal rods, mammals still have the ‘one-up’ in the dark.

anonymous asked:

(your last anon reminded me of this) but I swear I sometimes see shadows following me. Out of the corner of my eye most times but sometimes in plain sight. I've see them fully developed at night and it is a lot like sleep paralysis but I am fully awake and able to move when I see them. It hasn't happened in a few months but there are stretches of time where I see things. The worst part is that I don't feel like I'm going crazy.. which is a sign that you're going crazy.

To be honest I’m not much for paranormal activity (I mean aliens, yeah sure, the universe is massive and it would be extremely close-minded of me to believe that we are the only superior species in existence) and so I’m one of those people who believe everything can be explained scientifically so. From this article:

Rods are more sensitive to light and movement than cones but cannot see colour. Multiple rod cells converge into a single interneuron cell. This reduces overall resolution, like merging several pixels into one on a TV picture, but improves sensitivity to movement. This poor resolution and high sensitivity to movement is what causes everyday ‘corner of the eye’ phenomena, where you are vaguely aware of something in your peripheral vision but cannot see it properly.

Rods insensitivity to colour and poor resolution means that corner of the eye phenomena are typically black and white and their shapes vague. If you turn to look at an object seen in the ‘corner of your eye’, you will start to see it in much greater detail and in colour. As a consequence, it will look completely different. What was a vague dark shadow in the corner of your eye will turn into a differently shaped object in full colour. Sometimes the difference will be so pronounced that the original object will appear to vanish!

Due to the in-built human propensity to see faces and figures in random shapes, it is inevitable that some corner of the eye phenomena include such shapes. Once we turn to look at them properly, they will no longer make sense as figures or faces. Our brains will then ‘rationalise’ that the face or figure has vanished! It is no wonder that witnesses think they have seem a ghost or 'shadow person’.

Furthermore, our brains may 'substitute’ objects that are not seen well, such as those seen in the peripheral vision, with similar things from our visual memory. These ’visual substitutions’, which occur before we are consciously aware of what we’re seeing, can appear strikingly real. Such 'substitutions’ are not limited to just faces and figures, though these are common.

Distance is difficult to judge in peripheral vision, partly due to a lack of details and also because some objects may only be visible in one eye (no stereopsis) due to the nose intruding! The distance of objects seen in peripheral vision may thus often be wrongly reported. Something nearby may appear further away or vice versa. In particular, a nearby small object can appear bigger and further away. Naturally, the effect disappears when you turn to look directly at the object. So a 'large object’ (seen as a 'ghost’) in the middle distance 'vanishes’, leaving only a small bush nearby!

Agnatha:

Agnatha are jawless fish. Lampreys and hagfish are in this class. Members of the agnatha class are probably the earliest vertebrates. Scientists have found fossils of agnathan species from the late Cambrian Period that occurred 500 million years ago.

Members of this class of fish don’t have paired fins or a stomach. Adults and larvae have a notochord. A notochord is a flexible rod-like cord of cells that provides the main support for the body of an organism during its embryonic stage. A notochord is found in all chordates.

Most agnathans have a skeleton made of cartilage and seven or more paired gill pockets. They have a light sensitive pineal eye. A pineal eye is a third eye in front of the pineal gland. Fertilization of eggs takes place outside the body.

The lamprey looks like an eel, but it has a jawless sucking mouth that it attaches to a fish. It is a parasite and sucks tissue and fluids out of the fish it is attached to. The lamprey’s mouth has a ring of cartilage that supports it and rows of horny teeth that it uses to latch on to a fish.

Lampreys are found in temperate rivers and coastal seas and can range in size from 5 to 40 inches. Lampreys begin their lives as freshwater larvae. In the larval stage, lamprey usually are found on muddy river and lake bottoms where they filter feed on microorganisms.

The larval stage can last as long as seven years! At the end of the larval state, the lamprey changes into an eel-like creature that swims and usually attaches itself to a fish. There are around 50 living species of lampreys.

The hagfish is also know as the slime fish. It is eel-like and pinkish in color. It has glands along its sides that produce a thick, sticky slime that it uses as a defense mechanism. The hagfish can also twist its body into knots! It may do this to clean off slime or escape predators. The hagfish may also sneeze to clear its nostrils of slime.

The hagfish is almost completely blind, but it has a good sense of touch and smell. It has a ring of tentacles around its mouth that it uses to feel for food. It has a tongue-like projection that comes out of its jawless mouth. At the end of the projection are tooth-like rasps that close when the “tongue” is pulled back into the hagfish’s mouth.

The hagfish eats marine worms and other invertebrates. It has a very low metabolism and can go for as long as seven months without eating. Newly hatched hagfish are miniature copies of the adult hagfish. The hagfish is found in cold ocean waters in the Northern and Southern Hemispheres. It is found on muddy sea floors and may live in very large groups of up to 15,000 individuals. There are about 60 species of hagfish.

Tunicates, Lancelets and Vertebrates

At some point, all of the organisms in this phylum have a structure called a notochord. A notochord is a flexible rod-like cord of cells that provides the main support for the organism’s body during its embryonic stage. In some organisms, like the tunicates, the notochord disappears in the organism’s adult stage. In other organisms, like the vertebrates, the notochord is replaced or surrounded by the backbone in the organism’s adult stage.

Chordates also have pharyngeal slits. These are openings that connect the pharynx or throat to the outside of the neck. In some primitive species, the slits are used to filter food out of the water. In other species, like fish, the pharyngeal slits have gills. In other species, like mammals, the pharyngeal slits are only present during the embryonic stage.

Chordates also have a dorsal nerve cord that runs down the length of the organism. The dorsal nerve cord has pairs of nerves that connect to the organism’s muscles. In some organisms, the dorsal nerve cord expands into a brain at the top.

All chordates have a post-anal tail. A post-anal tail is an extension of the body that runs past the anal opening. In some species, like humans, this feature is only present during the embryonic stage.

The chordata phylum is divided into three groups or subphylums: lancelets, tunicats, and vertebrates.