Mosensory System Lab Report

We are able to visualize, feel, smell, and hear the world around us due to a system called the sensory system. All around the surface of our body we have detectors that are known as receptors. These receptors, in the form of cells, are specialized to capture specific forms of energy- whether heat, light, chemical, or mechanical (1). The environmental cues that are detected by our receptors on the surface of our body are then transformed into electrical signals, or nerve impulses, that can be sent to the brain for processing. This ensures communication between the environment and our internal system, specifically, our brain. For instance, if we touch a very hot surface, the receptors on that area of our body recognize that we are touching a
dangerous surface. Within seconds, a message is sent to our brain and back to the area of the body to say, “Hey, move away. That hot surface is dangerous.”
In between the receptor on the surface of our skin and the message that is being sent to the brain (neural impulse), we have a receptor potential. The receptor potential is a “relatively long lasting potential change across the receptor cell membrane (1).” In other words, in order for the environmental cue to enter our body, there must be a change in potential that triggers that the channels to open and let the message in. This is known as a nerve impulse that sends the message toward our central nervous system (1).
One specific sensory system that we looked closely into was the eye which passes along visual messages to the brain (2). The eye is made of several layers that each serve a specific function. On the outside of the eye, we have a thin protective layer called the cornea. This transparent layer helps protect the eye. Beneath that, the sclera, a tough connective tissue, helps to protect the eye against injury as well as attach to the muscles that help move the eye. Beneath the sclera lies the choroid, a layer that absorbs excess light. Beneath the choroid is the retina. The retina is another thin layer that consists of rods and cones- the photoreceptors of our eye. On the one hand, rods are light sensitive and contribute to our night vision. On the other hand, cones are not light sensitive and therefore contribute to our vision of color. The lens sits behind the pupil and cornea and serves to focus the light by changing its shape. It is the portion of our eye that expands and contracts in different light environments. As light passes these layers, it focuses directly on the fovea, a region where most photoreceptors are located. Because this area of our eye has the most photoreceptors, it is the sharpest region of our eye. Then, the visual message is sent to the brain via the optic nerve.
In this lab, we dissected the eye of a cow and carried out an experiment on our own eye to understand the anatomy of the visual sensory system.
In addition, we also tested the sensitivities of touch to various regions of our body to understand the somatosensory system.
Methods:
The procedures on pages 32-38 of the lab manual (1) were followed.
Results
Sketch of the retina:

Calculating the distance from the center of the blind spot to the fovea of my eye: Hi Di Hi 17mm Hi = 4.85mm Ho Do 87mm 305mm
Do is 305mm (12inches)
Di is a fixed distance of 17mm (distance of the lens to the retina)
Solve for Hi (distance from blind sport to fovea)
Ho is the distance from the center of the points to the cross

Labeled diagram:

Diameter of cow eye: ~ 5mm
Diameter of the human blind spot: 1.11 mm
Distance corresponding to the separation of the images of 2 lines on the fovea: 0.008 mm
Distance corresponding to the separation of the images of 2 lines in an off-foveal region: 0.0151

1 We expected the distance of separation to be more in the fovea region than in the off-foveal region because of the density of photoreceptors in the fovea region than that of the off-foveal
region.

Table 1: Touch (Somatosensory System and Tactile Acuity)

Partner 1:
Partner 2:
Forearm
Average 2-point threshold
2.40 cm
1.00 cm
Palm
Average 2-point threshold
1.15 cm
0.50 cm
Index Finger
Average 2-point threshold
0.25 cm
0.30 cm
Lower Back
Average 2-point threshold
1.05 cm
2.50 cm
Table 1 Legend: The table above shows the sensitivity of various regions of the body. Each region was tested using two sharp pencil tips. If the person felt two pencil tips, the person would say, “2” whereas if they felt 1, they would say, “1.” The average of the descending and ascending pencil tips was taken. We see that for partner 1, the index finger is most sensitive, indicating that the density of receptors in that area are greatest. Partner 2’s conclusions were misleading- more trials could have made more conclusive results.

Discussion
1. According to our results, we see that humans have a much smaller blind spot than that of a cow. We know that a blind spot is “the area of retina where the retinal ganglion cell axons come together to exit as the optic nerve…[it] is devoid of any rods of cones (1).” This means that in this region, there are no photoreceptors because the cell axons are coming together to leave the eye, into the optic nerve, and into the brain. The FINISH THIS

2. Based on the visual acuity results, we can conclude that the density of the receptor cells in the fovea is higher than the density of the receptor cells in the off-fovea region. We can affirm this because we know that the fovea region is the eye sharpest processing area. I would expect there to be more receptor cells that are closer together in the fovea region than the off-fovea region because this is where light is directly targeted in a normal functioning eye.

3. According to our results, we can conclude that the lower back is the least sensitive to touch. This is because the density of receptor cells are much lower in this region compared to the other regions that we tested. The most sensitive is the index finger and that is because the receptor cells are much denser in that area that any of the other areas that we tested.

Sources of error: Overall this lab was very well conducted. However, we did have a few sources of error: for the touch (Somatosensory system) test, partner 1’s results were not aligned with partner 2’s in that the least sensitive regions of the body were not the same. Partner 2’s least sensitive area is the lower back whereas Partner 1’s is the forearm. If more trials were conducted, the results would have been more conclusive. Otherwise, our lab was very well conducted with minimal human error and the rest of the results aligned with our hypotheses.
Real-life application: Scientists are working on testing sensory substitution devices for the blind, other than vision as a means of communication (3). For instance, in this study done by scientists from California, blind people were given verbal and vibrotactile commands when attempting to walk through a complex path. Those who used the feedback system (sensory substitution device) reported that the usability of the device was “above average (3).”

References
1. Murolo, M.A. Sensory Systems. In Principles of Biology Lab Manual. Wesleyan, 2017.
2. Murolo, M.A. Sensory Systems. Lecture Handout. In Principles of Biology pre-lab lecture. Wesleyan, 2017.
3. Adebiti A, et al. 2017. Assessment of feedback modalities for wearable visual aids in blind mobility. PubMed. 1, 1.