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Stimuli are detected by a number of receptors or sense organs: different receptors being sensitive to different stimuli. The sense receptors consist of a group of sensory cells which are connected to the brain or spinal cord through the nerve fibers.
The particular sensation impulses will be transmitted to the brain and identified appropriately. Repeated stimulation of the same receptors soon stops the sending of impulses. This is due to sensory adaptation. Adaptation can be defined as a change in the relationship between a stimulus and a response. Receptors can be mechanoreceptors, chemoreceptors, photoreceptors, thermoreceptors and nociceptors based on the function they perform. The response to the stimuli occurs through the motor nerves which transmits the information from the brain to the peripheral muscles.
Experiment 1: When the index fingers were rubbed on the coarse paper, it was noticed that the paper was coarse and it could be graded around 6 or 7 on the scale. After a minute or two, on again touching the paper, the feel appeared less coarse and the grading came down to 5 or 6. The touch receptors or mechanoreceptors on the skin first sent impulses to the brain through the appropriate sensory nerve fibers by responding to the mechanical energy or stimulus of touch. Then the feel became less coarse because the number of impulses sent by the touch receptors were less as the number of receptors stimulated were reduced due to some receptors ceasing to respond. This change of response was caused by sensory adaptation (Roberts and Mitchelmore, 2000).
The touch receptors for crude touch and fine touch are found in the cutaneous region of skin. The information for crude touch or coarse touch is carried along the anterior spinothalamic tract to the thalamic nucleus in the brain stem, and then relayed to the primary somato-sensory receptive area of the neo-cortex (Shepherd, 2007). The information for light touch is transmitted by the dorsal columns.
Experiment 3: Fifteen index cards and a flashlight were taken into a dark room. When all the cards were arranged over the beam of light, the light was not seen. One card was removed at a time till the light was barely detected. It was found that six cards were over the light when it was first detected. After about 15 minutes, the light appeared to become brighter. One card was added (now seven). The light was just barely seen. After some time, the light became brighter and one more card was added. Slowly I could add all the remaining cards and still see the light. This phenomenon can be explained by sensory adaptation.
The retina has the receptors, rods and cones which are photoreceptors and sensitive to light. The cones function in daylight and permit color vision and the rods in dim light. When I entered the dark room first, my pupils dilated to allow maximum light to enter to stimulate the retina. In the dark, the rods which perceived low levels of light allowed the adaptation to the dark. The light concentration of the rods increased and I became fully adapted to the dark in half an hour. The cones meanwhile did not respond to the light levels and thereby color was not appreciated in the dark. The sensory adaptation of the retinal receptors explains how all the 15 cards could be used over the light and the light still appeared bright in the dark room after some time.
The information from the retina is carried through the optic nerve, optic chiasma, optic tract, lateral geniculate body in the thalamus in an orderly manner and then relayed through the optic radiations to the primary visual receptive area in the neocortex or striate cortex (Shepherd, 2007).
Experiment 4: In this experiment three medium-sized bowls were taken. One had hot water, the second had lukewarm water and the third had cold water. I placed my right hand in the cold water and my left hand in the hot water. After three minutes, I transferred both hands into the lukewarm water. I found that the hand earlier placed in the hot water appeared colder when placed subsequently in the lukewarm water than the hand placed originally in the cold water.
Thermoreceptors which detect heat and cold are found on the skin. These do not detect temperature but are only able to detect differences in temperature. This is why the hand originally in the hot water felt colder than the hand originally in the cold water even though the temperature in the lukewarm water was constant and should have made the hands feel the same. This is due to the adaptation of the cutaneous thermoreceptors which could only detect how quickly the temperature was lost or gained (Roberts and Mitchelmore, 2000). The mechanism of detecting the changes in temperature is similar to the touch sensation pathway.
The impulse is carried from the receptors which respond to differences in temperature through the lateral spinothalamic tracts of the spinal cord to the thalamic nucleus in the brain stem, and then relayed to the primary somato-sensory receptive area of the neo-cortex sub-serving temperature (Shepherd, 2007). Sensory adaptation and evolution.
Frequent stimulus to a particular sensation could evoke a lesser response with time. This could result in the evolution of a new response to a particular stimulus due to the sensory adaptation. The adaptation could change from organism to organism and the final stimulus-response could be an evolutionary product based on the changes caused by necessity and experience. The variations in retinae and the nervous systems in different animals have all been subject to evolutionary adaptation.
Roberts, M.B.V. and Mitchelmore, (2000). “Biology for CXC”. Published by Nelson Thornes.
Shepherd, G.M. (2007). “The Major Senses: Sight, Hearing, Taste, Smell, and Touch”: The Dana guide. The Dana Foundation.