Alright, finally we are getting to talk about the special senses. This is the last chapter helping us understand the nervous system.
20.1. In this chapter, we discuss the sensory nervous system in more detail. We will spend most time covering the special senses, but start with some general concepts. One of them is the fact, that we differentiate between awareness of a sense delivered to the brain, and having it be interpreted for meaning. This is actually often discussed in modern psychology when we try to understand suffering; there is a significant portion of suffering that comes from the interpretationof a stimulus such as pain, and not simply it’s awareness. What terminology is used to describe that difference?
20.2. Sensory receptors are found throughout the body. Can you list some examples?
20.3. We classify receptors by what they code for: temperature, touch or pressure, pain, light, sound waves, odors…Some are simple such as free nerve endings, or ‘onion-layer’ looking encapsulated one’s, others are way more complicated. What kind of senses do complex receptors transduce?
20.4. Many nerve stimuli don’t need to excite our brain at all time. It often is more important to register a change, and not continuously be made aware of a specific temperature for example; often the water temperature one likes at the beginning of a shower is lower then at it’s end because one gets ‘used’ to an ongoing stimulus. Other senses such as touch & pressure, or the brightness of light also adapt. Can you name two receptor types that do not adapt?
20.5. When we discussed the primary sensory cortex in the brain chapter, we mentioned that the distribution of nerve receptors is not equal for all body areas. For example the lips or fingertips are much more densely innervated with sensory nerve receptors then a back. We can feel a coin in our pocket and know what size it is; try doing that with your back! How big an area is that one receptor innervates is called a receptive field. What test can we use to determine the size of a receptive field?
20.6. In trying to understand vision, we have to mostly study the eyeball. List some supportive accessory organs that help make vision possible.
20.7. The eyeball is a sphere that transforms the visual world we see into nerve impulses (electricity), which is the language our brain can understand; it has light coming in on one end, and a nerve (cable) leaving the other. Inside, a clear picture of what we see has to project onto a receptor filled tiny screen at the back of the eyeball. There, a gazillion receptors get activated when projected on. They then send a signal to the brain, where it appears like a pixel projected onto the primary visual cortex of the occipital lobe, sort of re-creating the visual image. Let’s now consider the parts of this incredible organ. First, can you list the different layers, or tunics, and its subcomponents that make up the eyeball. Can you also briefly explain the function of each of them.
20.8. If we look at a cross section of an eyeball, we can separate an area behind, and one in front of the lens/iris. The larger space behind the lens, known as the posterior chamber consists of a gello-like, somewhat firm, but wobbly substance called vitreous humor/body. It helps the eyeball’s structural integrity by pressing outward against it’s walls. In the front chamber, a more liquid, aqueous humor provides internal pressure, but also brings nutrients to the eyeball. Nutrients get used up, so this fluid has to be secreted from the blood stream and drained in order to not accumulate and exert excessive pressure onto the eyeball. Can you name the canal the aqueous humor drains from?
20.9. The eye ‘sees’ a picture best when projected to a specific area on the retina that is most densely populated with receptors. In order to do that, the eyeball has to be able to move freely around it’s own axes. It does that by having 6 muscles attach onto it and pull it in all possible directions. The four recti muscles make us be able to look up, down, and to the sides. What muscle pulls the eyeball up and out, and which one down and out?
20.10. Let’s now better understand the transformation from a light stimulus into a nerve impulse. The whole process boils down to molecules stacked up in round disks changing shape when activated by light-waves. There are 2 types of photoreceptors, one of them is so sensitive, that it picks up dim light. Those get overwhelmed when the light is too bright and they stop firing; we feel the process of them ‘bleaching’ out when we go from a darkness to bright light quickly. The other receptor type is activated when it is light. They come in three different subtypes that help us see color. Can you name the two different receptor types?
20.11. There is a part in our peripheral vision that projects onto the retina, where the optic nerve leaves the eyeball. That area, called the optic disc has no photoreceptors and cannot transduce a picture; the brain actually just ‘imagines’ that part of the visual field. On the other hand, the area of the retina coding for the center of our visual field is most densely filled with receptors giving us the best, most acute vision. Can you name that area of the retina?
20.12. The light rays entering the eye have to bend (refract) in a perfect way so that they reunite on the retina sharply; if not, we ‘see’ blurry. The lens can change shape, which changes the angle the light rays bend; it can accommodate for images from different distances, the rounder it is, the steeper the angle it bends the light rays. Does a round lens best focus images that are close or far away? Why?
20.13. We need reading glasses as we get older because the lens becomes less flexible, which makes it bend less. Some of us also need glasses, because it is very delicate to project a perfect image onto the retina. In some of us, the light rays reunite not quite on it, but either a little in front of it or behind it. Can you explain how one can suffer from farsightedness and how it can be corrected?
20.14. We can understand the ’sharpness’ (acuity) of one’s vision by measuring how well they can differentiate two points at a distance. What tool do we commonly use to test visual acuity?
20.15. Interestingly, we can see 3D! This is made possible because we have 2 eyeballs that mostly pick up the same picture, just from a slightly different perspective (or angle). If we overlay those pictures, voila, we can see three dimensional. It is also just cool to have 2 eyes. In case one doesn’t work, we can still see! In order to be able to overlay the information from the 2 different eyes, both stimuli have to be processed in the same area of the primary visual cortex (located in the occipital lobe). This is accomplished by having half of the optic nerve cross over to the other side and join that side’s optic nerve. Now, both impressions of a visual field can be brought to the occipital lobe for processing. Smart, right? What is the place called where half of the nerve crosses?
20.16. Give me two main reasons for us to have eyebrows.
20.17. What is conjunctivitis?
20.18. Tears are great, they clean and nourish our eye’s cornea. They also help us process emotionally; crying stimulates the release of endorphins for example. What do we call the ‘tear’ gland and where is it located?
20.19. When we see an ear, we likely associate it with the sense of hearing. We also have the sense of balance and equilibrium in that area. In which part of the skull do we find the inner ear?
20.20. Our ears pick up sound waves that travel through the funnel shaped external, visible ear. They then vibrate the eardrum, or tympanic membrane, and pass the vibration on to 3 small bones, which conduct the waves to the organ of hearing, the cochlea. Can you name the 3 little ossicles of the middle ear?
20.21. The inner ear is basically a fluid filled channel system carved out of bone. The snail looking cochlea makes electrical impulses out of sound waves. The three half circles (semi-circular canals) and their base (vestibule) encode for starting and stopping motions in different directions, which helps us with balance. Within the ’bony’ channels, we find membranous one’s, filled with a different fluid. In the cochlea, the bony ’labyrinth’ together with the membranous one creates three such channels; the top and bottom one connect at the ‘snail’s’ apex as they narrow. Can you name them?
20.22. The middle of the three channels houses the organ of corti, or the organ that makes nerve impulses out of sound waves. It’s basic structure consists of a bottom membrane, which gives rise to specialized ‘hair’ cells that have cilia ‘sticking’ up touching another membrane. When sound waves, which are pressure waves, travel through that area within the cochlea, those ‘hairs’/cilia vibrate and fire a nerve impulse. Thousands of sensory cells are spread along the entire cochlea. The ducts within the cochlea narrow as they circle towards the apex. What do we call the place at the apex where the ducts connect?
20.23. The hair cell’s place along the cochlea determines the pitch we hear. The fact that the ducts narrow as it reaches the apex, makes sound waves of differing pitches (=differing heights) touch the bony wall at different locations along the snail. Wherever a wave touches the bony wall is where it crosses over to the organ of corti. This then vibrates the hair cells at that location. Do high pitches cross over close to the base or apex?
20.24. The part of the inner ear that helps us with balance essentially picks up any motion our head does in gravity and translates it into a nerve impulse. Any motion starts and stops, even if very tiny, the head accelerates and then decelerates. In total we have five areas in each inner ear picking up that kind of motion. Two deal with up and down, as well as sideways movements; you feel those activate when in an elevator (especially a long one) or speeding up in a car. The other three encode rotational movements through all the three planes; you feel those when spinning in a circle or doing a cartwheel. In the organ of equilibrium, hair cells don’t bend due to sound waves displacing them, they bend because their held in place by a weighted membrane that ‘lags’ behind so to speak when our head moves. What part of this organ registers horizontal movements for us?
20.25. Taste and smell help us appreciate the world of odors. The mouth (especially the tongue) and upper nose have receptors that detect chemicals. Tiny cilia, or ’hairs’ in this sense don’t react to movements of sound waves, but to chemicals that attach to receptors in their membrane. Our sense of smell is limited compared to other animals, but even though, we pick up thousands of different fragrance combinations. Our mouth, however, only responds to five basic tastes. Can you name them?
Sorry, that was a lot of questions. I hope though they help you understand the material better. Please read them critically and give me input to where my wording needs editing. Thank you.
Please send me the answers you came up with to those questions before the class meets to [email protected] Thank you.