Chemical Senses and the Eye

Chemical Senses

The two most familiar chemical senses to humans are gustation (taste) and olfaction (smell). Based on the textbook reading, olfaction seems to be significantly more complex than gustation. Smell seems to contribute much more to taste than the taste receptor cells on one’s tongue.  When people get sick, they will often get stuffy noses and lose their sense of smell. A loss of smell also results in a significant loss of taste. I have also learned to pinch my nose when I need to consume something that tastes awful to reduce my sense of taste.

Gustation can detect five main types of senses. These include salty, sour, sweet, bitter, and Unami. All other senses of taste come from olfaction. Certain areas of the tongue have papillae which contain taste receptor cells. These receptor cells transduce in response to chemical stimulus. There are three main types of transduction, ions may directly pass through ion channels (salty and sour), chemicals may block ion channels (sour), or chemicals may bind to G-protein-coupled receptors in the membrane and activate second messengers to open ion channels (bitter, sweet, and unami).

Eating something salty results in sodium diffusing into a receptor cell, depolarizing the membrane, activating voltage-gated sodium and calcium channels, which consequently release vesicle filled with serotonin to the gustatory afferent neuron. Eating something sour will result in H+ ions blocking K+ channels depolarizing the membrane, which opens voltage-gated sodium and calcium channels signaling vesicles filled with serotonin to undergo exocytosis. Sweet, bitter, and unami type chemicals undergo a G-protein-coupled receptor transduction pathway. The ability to taste bitter was vital to human survival because many poisons contain bitter taste. However, senses of taste can adapt as many people enjoy coffee, which is bitter. All of these tastes are processed in the brain. The primary gustatory axons connect to the brain stem, which leads to the thalamus (ventral posterior nucleus), and ends up in the gustatory cortex.

Olfaction begins at the olfactory epithelium. The olfactory epithelium contains three types of cells, receptor cells, supporting cells which produce mucus, and basal cells which are the source of new receptor cells. An interesting fact we learned this week is that olfactory receptor cells have a four to eight-week life cycle. The mucus layer allows chemicals to dissolve and attach to cilia. This results in transduction of the receptor cells. The signal converges at the olfactory bulb and can lead two to two different pathways to the brain. The path to the orbitofrontal cortex is longer and belongs to conscious thought. The olfactory bulb can also pass its signal straight to the olfactory cortex. The olfactory bulb is very complex and intricately maps receptor cell types in its glomeruli. Smells are encoded both spatially and temporally. There is a lot of research currently on population coding and olfactory maps.

The eye

The eye is a specialized organ that has the ability to detect, localize, and analyze light. 

The cornea is the layer of the eye through which light first travels into the eye. Then, the light enters the pupil, the black part in the middle of the eye. The colored iris is in charge of controlling how much light can enter the eye by enlarging or dilating the pupil. This phenomenon  is similar to how a camera lens works. Behind the pupil is the lens. The lens also changes shape in  order to focus the light onto the retina. The lens thickens when focusing on things that are close and spreads flatter when focusing on this further away. The retina is the last layer of the eye onto which the image is formed. Photoreceptors are cells that are specialized to sense light and they are tightly packed in the retina. There are two types of photoreceptors: cones and rods. Cones sense details and colors and they are dense in an area of the retina called the fovea. Rods have the ability to control night vision as well as peripheral vision. There are more rods than cones and they are mostly located in the peripheral area. Therefore, the two types of photoreceptors differ in the kind of images they sense. The photoreceptors are each connected to a nerve fiber. The nerve fibers bundle together to form the optic nerve. 

  

Figure 1: Structure of the human eye

The way that light energy is transformed into an image on the retina is through membrane potential changes and a subsequent chemical signal. Electromagnetic radiation is absorbed by photopigment called rhodopsin in outer segments of the photoreceptor. This triggers G-proteins, which the cause effector enzymes to set forth a chain of events that causes ion channels to close and membrane potential thus changes. Membrane potential in the dark is more positive than in other neurons and contrary to most action potentials, membrane potential hyperpolarizes in response to light triggers. Special channels let in sodium ions in a flow called the dark current. These sodium channels are stimulated to close by cyclic guanosine monophosphate (cGMP), an enzyme that is reduced by light. When the sodium channels close, membrane hyperpolarizes. An interesting thing noted in  the reading is that the transduction of signals in photoreceptors is similar to that of olfactory cells. The neurotransmitter involved in the chemical signal is glutamate. 

The only source of output from the retina is from the action potentials from ganglion cells. Each photoreceptor is linked with two retinal neurons, bipolar cells and horizontal cells. Bipolar connect photoreceptors directly to ganglion cells and horizontal cells send information “horizontally” and also affect other bipolar cells and photoreceptors. Information from about 97 million photoreceptors is condensed and sent through the pathways to ganglion cells as action potentials. The mapping of images into optic nerve fibers is not event; instead in some areas, such as the fovea, less photoreceptors send information to each ganglion cell. This means that the images are crisper in the fovea.

The central visual system

Previously we saw the many ways how eyes are quite similar to cameras in their functioning. However, after the visual information leaves the retina and enters neural pathways there are much more complex structures that are capable of doing so much more compared to a simple camera. Central visual system is responsible for, for instance, combining the visual fields of both eyes, extracting information about differences in brightness and color in addition to detecting motion.

Each eye sees a part of the visual space that defines its visual field. The central portion of both hemifields are viewed by both retinas, creating a binocular visual field. The axons of ganglion cells exit the eyes via retinofugal projection which is a neural pathway consisting of the optic nerve, the optic chiasm where axons from nasal retinas cross and the two optic tracts. Because the optic nerves cross, the left visual hemifield is viewed by the right hemisphere and the right hemifield is viewed by the left hemisphere.

The optic tracts project to four subcortical regions of the brain: (1) the lateral geniculate nucleus (LGN) that gives rise to axons that project to primary visual cortex via optic radiation, (2) the superior colliculus, that controls the orienting eye and head movements, (3) the hypothalamus that regulates the circadian rhythms, and (4) the pretectum, that controls the pupillary light reflect.

 

Figure 2. The structures of retinofugal projection and the processing of visual fields.

 

LGN is the main target of retinal ganglion cells. It receives input from both eyes and relays the information to the primary visual cortex. The retinal M-type and P-type ganglion cells respectively project to two ventral magnocellular layers and four dorsal parvocellular layers of the LGN. Each of the six LGN layers receives inputs from either the ipsilateral or contralateral eye i.e. the axons from the left eye project to layers 1, 4 and 6 of the right LGN and the axons from the right eye project to the layers 2, 3 and 5. There are also six koniocellular layers K1-K6 that lie ventral to each “main” layer of the LGN. This is where all nonM—nonP-type ganglion cells project.

Magnocellular and Parvocellular layers each contain specific visual functions. Magnocellular layers are involved in detecting fast moving stimuli whereas parvocellular layers are  responsible for processing of color and fine detail.  Therefore, selective lesions in these areas cause lesions in only some visual functions without impairing the entire vision.

The primary visual cortex (V1) has an organization where neighboring cells in the retina feed information to neighboring places in the target structure i.e. the 2D structure of the retina is mapped onto the 2D surface of subsequent structures in V1. This phenomenon is called retinotopy. However, this mapping is often distorted because the visual space is not uniformly sampled in retina (high density of ganglion cells in fovea) and discrete point of light activates many cells in retina due to overlapping receptive fields. As was seen in LGN, V1 also has a layered structure which divides the labor in the cortex and keeps the inputs from two eyes separate.

In order for the brain to make sense of the visual world and to combine all the visual input, there are three pathways within V1 that work in parallel: (1) magnocellular pathway, (2) parvo-interblob pathway, and (3) the blob pathway. These pathways are briefly modeled in Figure 3.

Figure 3. A hypothetical model of parallel pathways in the primary visual cortex.

Via these pathways, the brain forms the images we know, with color, shape and motion. However, these pathways are just hypothetical and further research shows that the signals from each pathway mix and and the receptive field properties overlap making the understanding of visual information processing even more difficult.

The striate cortex is only one area of the cortex that processes visual information. Beyond V1 are located another two dozen distinct extrastriate areas of the cortex whose full functions are still under debate. However, there appears to be at least a dorsal stream and a ventral stream which are responsible for observing movement and recognizing objects, respectively. When inspecting the central visual system as a whole, we can clearly see a pattern throughout: there is a division of labor principle, i.e. a group of cells or cortical areas may contribute to perception, some dealing more with color or shape, others more with motion. However, the detailed principle on how this information is perceived as coherent images, is still quite unknown.

 

Figure 1. Available (accessed on 4.10.2020):

https://www.thoughtco.com/how-the-human-eye-works-4155646 

Figure 2. Available (accessed on 2.10.20): http://brain.phgy.queensu.ca/pare/assets/Central%20Pathways%20handout.pdf

Figure 3. Bear, M., Connors, B., PAradiso, M. Neuroscience: Exploring the brain, Fourth edition. 2015, pp. 355.

 

Posted by Veera Sairanen

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