During the previous week we saw how information propagates from origin to the brain. This week we are going to describe the senses, particularly related to olfaction, taste and sight.
Taste and Olfactory Senses
Humans evolved as omnivores feasting on both plants as well as on meat. Accordingly, our sense of taste has evolved in a such a manner that it could distinguish between new sources of food and toxins. An intrinsic fondness for sweetness increased while poisons taste bitter and hence are impulsively rejected. However, some experience has modified human instincts as some people are quite fond of coffee. The taste also reflects a deficiency of nutrient, for e.g. when body needs sugar, we have a craving for sugary delicacies.
Current studies have shown that we perceive only 5 tastes; sweetness, saltiness, bitterness, sourness and fifth taste called umami meaning delicious in Japanese. However, it is possible to experience numerous flavors including sweet chocolate that tastes different to sweet mango. This is due to the fact that every food activates a unique combination of elementary flavors. Also, smell is vital for the taste, smell in conjunction with taste provides a distinctive flavor.
Traditionally it was assumed that only tongue is responsible for taste, however other areas like palate, pharynx and epiglottis also play a vital part in taste.The odor from the food can pass from pharynx into the nasal cavity where they can be detected by olfactory receptors. The tip of tongue is most sensitive to sweetness, the back to bitterness but that doesn’t imply that we can only taste sweetness with the tip. Major part of tongue is sensitive to all basic tastes. There are small projections on the surface of the tongue known as papillae (Latin for “bumps”). Each papilla has taste buds ranging from one to several hundreds, which in turn has 50-150 taste receptor cells. A person typically has 2000-5000 taste buds, although there can be as few as 500 or as many as 20,000.
Apical end near the surface of tongue is the part which is chemically sensitive part of taste receptor cells. They are extended into thin extensions called microvilli that project into the taste pore, the opening at the surface which is exposed to the contents of the mouth. According to traditional standards the taste receptor cells are not neurons, but they do form synapses with the endings of the gustatory afferent axons near the taste buds. Taste receptor cells make both electrical and chemical synapses onto some of the basal cells.
Olfaction or simply smell has been historically crucial for humans. As much as we enjoy pleasant fragrances it can also warn us of potentially harmful substances. It is also a mode of communication. Body releases chemical called pheromones that are important signals for reproductive behaviors, and are also used to mark territories, identify individuals and indicate aggression or submission.
We have always thought nose to be the organ of smell. But we actually smell with a thin sheet of cells high up in the nasal cavity called olfactory epithelium.
The olfactory epithelium has three main cell types.
Olfactory receptor cells where transduction takes place. They fit in the traditional definition of neuron with axons that penetrate into central nervous system.
Supporting cells these are similar to glia, they help in mucus production.
Basal cells these are the source of new olfactory receptor cells as olfactory receptors continually grow, die and regenerate.
Structure of the eye and image formation
The gross anatomy of the eye can be divided into pupil, iris, cornea, sclera and extraocular muscles, presented in the figure 1. Pupil is the opening through which light enters into the eye and reaches retina and iris is the colored area of the eye which is responsible for controlling the size of the pupil. Sclera is the “white of the eye,” which forms the tough wall of the eyeball and it is continuous with cornea. There are three pairs of extraocular muscles and they control the movement of the eyeball in its orbit.
When viewing the eye through ophthalmoscope, seen in figure 2, we can see the retina. At the middle of each retina lies a macula, a darker-colored region, which is responsible for central vision and we can also notice fovea, a dark spot, which marks the center of the retina. Lastly, we can note a blind spot or optic disk. Optic disk is the spot in the retina where the retinal vessels originate from and where the optic nerve fibers exit the retina. When it comes to the image formation of the eyes, objects are brought into focus by the combined refractive powers of the cornea and lens. as light strikes the cornea, it passes from the air into the aqueous humor and it speed slows down. Because of this it bends toward a line that is perpendicular to the cornea and this means that all the rays of light which arrive perpendicular to the cornea will pass straight to the retina, but the curved rays with refract into the back of the retina. 
Abnormalities in the eye structure will cause various disorders, such as strabismus. It is a disorder where eyes will point in different directions i.e. there is, in general, misalignment or lack of coordination between the two eyes. Among older adults, cataract, also called as the clouding of the lens, is rather common.  Its symptoms include i.a. faded colors, blurry vision, halos around light and difficulties with being able to see at night. The risk for it increases not only with older age but also with e.g. smoking, alcohol overconsumption and other health problems, such as diabetes.  A third disorder worth mentioning is glaucoma, a progressive loss of vision due to the elevated intraocular pressure and it is a leading cause of blindness. The pressure inside the eye and in the aqueous humor have an important role in maintaining the normal shape of the eye. When the pressure inside the eye increases, it eventually damages the point where the optic nerve leaves the eye, and this damage causes the loss of vision. 
Photoreceptor structure and color perception
There are two types of photoreceptors in the retina: rods and cones. Rods have a cylindrical outer segment, containing many disks whereas cones have a shorter outer segment with fewer membranous disks. Because of the greater number of the disks, rods are in general over 1000 times more sensitive to the light that cones. Rods mainly work in scotopic conditions, i.e. during nighttime or when there’s less light and cones work in photopic conditions, i.e. in daylight or when there’s enough light coming from the environment. However, both rods and cones are responsible for vision in conditions where there are intermediate amounts of light, e.g. outdoor traffic lighting at night.
The perception of colors happens in cones. The retina contains three cone types and each of these is maximally sensitive to a certain, different spectrum of wavelengths, i.e. green, red and blue. When all three types of cones are equally activated, white light is perceived and the colors we have used to, will arise as mixtures from the three base colors. For example, yellow is a mixture of the base colors red and green whereas orange is a mixture of yellow and red. However, no color will look simultaneously same as their “opponent” color (see figure 3). This means that no color will simultaneously seem red and green or blue and yellow, for example.Visual pathway
Retinofugal projection is a neural pathway that exits the eye and it has three parts. The retinofugal projection starts from the optic nerves towards the optic chiasm where the two nerves combine. The nerves from nasal retinas cross to the opposite side of the brain, which is called a decussation. After the optic chiasm the nerves from the optic tracts. Most of the axons of the optic tract innervate the lateral geniculate nucleus (LGN). (Some of the axons connect to the hypothalamus and the midbrain.)
Optic radiation is a projection from the LGN to the primary visual cortex. This pathway mediates conscious visual perception. Lesions in the retinofugal projection or the optic radiation lead to blindness in parts or the whole visual field. The visual cortex can be separated into multiple visual fields that process the different aspects of visual information. The fields are not strictly differentiated but a progression from one to another.
The visual field can be divided into left and right visual hemifields. These hemifields overlap in the center forming a binocular visual field. Because nerves from the nasal retinas cross to the opposite side of the brain but nerves from the temporal retinas remain in the same side, all optic information from the left visual hemifield is processed in the right hemisphere of the brain etc. The reason for this decussation is unknown.
- NBE-E4210 Course Material
- National Eye Institute, USA. “Facts About Cataract”. September 2009. Archived from the original on 24 May 2015. Retrieved 24 May 2015.
- Hofer et al. 2010.Reconstruction and dissection of the entire human visual pathway using diffusion tensor MRI. Frontiers in Neuroanatomy. Available:https://doi.org/10.3389/fnana.2010.00015