Human beings are not capable recognising all bands of spectrum which is coming to this living planet. Some specialised instruments are there to trap non-visible bands of such spectrum. The case is similar regarding some other senses for which our body design makes some arrangement. A quick reference is advanced in this section to develop some basic understanding related to functioning of our senses. Steps involved in transmission of sensory information in living body involves four prominent mechanisms: Stimulation[1], transduction[2], transmission[3] and interpretation[4].
1: All sensory receptors can be classified as either interoceptors or exteroceptors. The four steps by which information is conveyed to the CNS are stimulation, transduction, transmission, and interpretation in the CNS. Gated ion channels open or close in response to stimuli, altering membrane potential; if this change exceeds a threshold, an action potential is generated. Why is the relationship between intensity of stimulus and frequency of action potentials said to be logarithmic?[5]
2: Do snakes see heat? By what process they recognise movement of their prey even in darkness?[6]
3: Receptor classes include, but are not limited to, thermoreceptors, nociceptors, and electromagnetic receptors. Thermoreceptors detect changes in temperature inside and outside of an animal body. If thermoreceptors are overstimulated by extreme changes in temperature, nociceptors can be activated, resulting in pain. Detection of heat can also aid predators in locating prey. Nociceptors carry signals perceived as pain. Electromagnetic receptors afford some animals the ability to detect electric currents and magnetic fields. Would a heat-sensing organ be useful for hunting ectothermic prey?[7]
4: Mechanical distortion of the plasma membrane of mechanoreceptors produces nerve impulses. Proprioceptors monitor muscle length and tension. Baroreceptors monitor blood pressure in arteries. Why is it important to detect stretching of muscles?[8]
5: Sound waves cause middle ear ossicles to vibrate; fluid in the inner ear is vibrated in turn, bending hair cells and causing action potentials. In terrestrial animals, sound waves in air must transition to the fluid in the inner ear. Hair cells in the vestibular apparatus of terrestrial vertebrates provide a sense of acceleration and balance. Why is a lateral line system not useful to adult amphibians?[9]
6: The five tastes humans perceive are sweet, sour, salty, bitter, and umami (amino acids). Taste and smell chemoreceptors detect chemicals from outside the body; olfactory receptors can identify thousands of different odors. Internal chemoreceptors monitor acid–base balance within the body and help regulate breathing. What are the advantages of insects having taste receptors on their feet?[10]
7: Many invertebrate groups have eyespots that detect light without forming images. Annelids, mollusks, arthropods, and chordates have independently evolved image-forming eyes. The vertebrate eye admits light through a pupil and then focuses it with an adjustable lens onto the retina, which contains photoreceptors. Photoreceptor rods and cones contain the photopigment cis-retinal, which indirectly activates bipolar neurons and then ganglion cells. The latter then transmit action potentials that ultimately reach the occipital lobe of the brain. Can an individual with red-green color blindness learn to distinguish these two colors? Why or why not?
Individuals with complete red-green color blindness (those who have no red cones or no green cones, as opposed to those who lack only some red or green cones) would be highly unlikely to be able to learn to distinguish these colors. In order for an individual to perceive the colors in the red-green area of the spectrum, both red and green cones are required; without both cones there is no reference point by which individuals could compare the signals between the retina and the brain. If there are other cues available, such as color saturation or object shape and size, individuals with a less severe form of color blindness may be able to learn to distinguish these colors. In the absence of other references, however, it would be very difficult for individuals with even partial red-green color blindness to distinguish these colors.
- Multidisciplinary approaches can clarify the evolutionary history of the world’s biological diversity. The Pax6 gene and its many homologues indicate that eye development, although highly diverse in outcome, may have a single evolutionary origin. Why would mutations leading to defective Pax6 persist in cavefish? If these fish were introduced into a habitat with light, what would you expect to occur?[11]
9: How would the lateral line system of a shark detect an injured and thrashing fish?[12]
10: Humans detect smells by means of olfactory neurons (receptor cells) located in the lining of the nasal passages. The axons of these neurons transmit impulses directly to the brain via the olfactory nerve. Basal cells regenerate new olfactory neurons to replace dead or damaged cells. Olfactory neurons typically live about a month. Inquiry question In what ways do the senses of taste ? and smell share similarities? How are they different?[13]
11: For animals that can detect near- ultraviolet light, where would the maximum light absorption peak for this receptor be in relation to the other photoreceptors? [14]
12: Additional kinds of cones can enhance color vision in two ways. First, they may be sensitive to regions of the color spectrum not covered by other cones, such as those that detect ultraviolet light in some animals. In addition, they may allow enhanced discrimination of color even in regions to which other cones are sensitive. Humans have three cones, each of which is sensitive to a varying extent to spectra with a wavelength of approximately 520 nm. The human brain integrates the response of the three cones and interprets the color as yellow. Imagine, however, if we had a fourth cone whose peak sensitivity was in the region of 520 nm. Output from that cone would provide additional discrimination not possible based on the other three cones, and thus would allow an individual to identify shades of color not distinguishable by individuals lacking that cone.
- Mechanoreceptors: Different receptors detect touch, depending on intensity. Various receptors in the skin respond to mechanical distortion of the membrane to convey touch. Muscle length and tension are monitored by proprioceptors. Proprioceptors provide information about the relative position or movement of body parts and the degree of muscle stretching. Baroreceptors detect blood pressure.
- When blood pH falls too low, a potentially fatal condition known as acidosis results. Among the variety of responses to this condition, the body changes the breathing rate. How does the body sense this change? How does the breathing rate change? How does this increase pH?[15]
- The function of the vertebrate eye is unusual compared with other processes found within the body. For example, the direction in which sensory information flows is actually opposite to the path that light takes through the retina. Explain the sequence of events involved in the movement of light and information through the structures of the eye, and explain why they move in opposite directions. Consider, also, how this sequence of events compares to the functioning of the mollusk eye. [16]
- How would the otolith organs of an astronaut respond to zero gravity? Would the astronaut still have a subjective impression of motion? Would the semicircular canals detect angular acceleration equally well at zero gravity?[17].
All the sense organs have their own limitations; our optical sensors cannot recognise all the radiations; our audio instrument is tuned to work in between a selected range of frequencies; we rarely make ourselves fit to get bands of all sensations implied on our skin.
[1] A physical stimulus impinges on a sensory neuron or an associated, but separate, sensory receptor.
[2] The stimulus energy is transformed into graded potentials in the dendrites of the sensory neuron.
[3] Action potentials develop in the axon of the sensory neuron and are conducted to the CNS along an afferent nerve pathway
[4] The brain creates a sensory perception from the electrochemical events produced by afferent.
[5] When the log values of the intensity of the stimulus and the frequency of the resulting action potentials are plotted against each other, a straight line results; this is referred to as a logarithmic relationship.
[6] The depression between the nostril and the eye of this rattlesnake opens into the pit organ. In the cutaway portion of the diagram, you can see that the organ is composed of two chambers separated by a membrane. Snakes known as pit vipers have the ability to sense infrared radiation (heat).
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[7] The body temperature of an ectothermic organism is not necessarily the same as the ambient temperature; for example, other reptiles may bask in the sun, wherein on a chilly day the sun may warm the animal’s body temperature above the ambient temperature. In this situation, the heat-sensing organs of, for example, a pit viper would still be effective in hunting because it would be able to distinguish the differences between the environment and its ectothermic prey.
[8] Proprioceptors detect the stretching of muscles and subsequently relay information about the relative position and movement of different parts of the organism’s body to the central nervous system. This knowledge is critical for the central nervous system; it must be able to respond to these data by signaling the appropriate muscular responses, allowing for balance, coordinated locomotion, and reflexive responses.
[9] The lateral line system supplements the sense of hearing in fish and amphibian larvae by allowing the organism to detect minute changes in the pressure and vibrations of its environment. This is facilitated by the density of water; without an aquatic environment, the adult, terrestrial amphibian will no longer be able to make use of this system. On land, sound waves are more easily detectable by the sense of hearing than are vibratory or pressure waves by the similar structures of a lateral line.
[10] Many insects, such as the housefly, have chemoreceptors on their feet with which they can detect the presence of edible materials as they move through their environment. These insects can thus “taste” what they are walking on, and when they encounter an edible substrate they can then descend their proboscis and consume the food.
[11] There is no need for eyes in the dark. Perhaps the fish expend less energy when eyes are not produced and that offered a selective advantage in cavefish. In a habitat with light, a mutation that resulted in a functional Pax6 would likely be selected for, and over time more of the fish would have eyes. Keep in mind that the probability of a mutation restoring Pax6 function is very low, but real.
[12] As the injured fish thrashed around, it would produce vibrations—rapid changes in the pressure of the water. The lateral line system in fish consists of canals filled with sensory cells that send a signal to the brain in response to the changes in water pressure.
[13] Both taste and smell utilize chemoreceptors as sensory receptors, wherein the binding of specific proteins to the receptor induces an action potential, which is sent as a sensory signal to the brain. The chemicals detected by both systems must first be dissolved in extracellular fluid before they can be detected. One major difference between the two systems is that the olfactory system does not route a signal through the thalamus; instead, action potentials are routed directly to the olfactory cortex. Another difference is that olfactory receptors occur in larger numbers—tens of millions, as opposed to tens or hundreds of thousands for taste receptors.
[14] Ultraviolet light has shorter wavelengths than the blue light, and so the peak absorption would fall to the left of the blue cone’s peak absorption of 420 nm. The UV portion of the electromagnetic spectrum is between 10 nm and 400 nm, which is the end of the visual spectrum. The peak would fall in that range, probably closer to the 400 nm to be considered “near-ultraviolet.”
[15] When blood pH becomes acidic, chemoreceptors in the circulatory and the nervous systems notify the brain and the body responds by increasing the breathing rate. This causes an increase in the release of carbon dioxide through the lungs. Decreased carbon dioxide levels in the blood cause a decrease in carbonic acid, which, in turn, causes the pH to rise.
[16] In order to reach the retina and generate action potentials on the optic nerve, light must first pass through the ganglion and bipolar cells to reach the rods and cones that synapse with the bipolar cells. The bipolar cells then synapse with ganglion cells. These in turn send action potentials to the brain. Because the retina comprises three layers, with the rods and cones located farthest from the pupil, light must travel to the deepest level to set off reactions that move up through the more superficial levels and result in optic signals.
[17] Without gravity to force the otoliths down toward the hair cells, the otolith organ will not function properly. The otolith membrane would not rest on the hair cells and would not move in response to movement of the body parallel or perpendicular to the pull of gravity. Consequently, the hair cell would not bend and so would not produce receptor potentials. Because the astronauts can see, they would have an impression of motion—they can see themselves move in relation to objects around them—but with their eyes closed, they would not know if they were moving in relation to their surroundings. Because their proprioceptors would still function, they would be able to sense when they moved their arms or legs, but they would not have the sensation of their body moving through space.
The semicircular canals would not function equally well in zerogravity conditions. Although the fluid in the semicircular canals is still able to move around, some sensation of angular movement would most likely occur, but the full function of the semicircular canals requires the force of gravity to aid in the directional movement of the fluid in the canals. The latter view represents our current understanding. There are many examples of small gene families whose apparent role in generating phenotypic diversity among major groupings of organisms is in altering the expression of other genes. An example evolution of an image-forming eye. Recent studies suggest, in contrast to the view that eyes across the animal kingdom evolved independently multiple times, that image-forming eyes from very distantly related taxa (e.g., insects and vertebrates) may trace back to the common origin of the Pax6 gene. If that view is correct, then genes controlling major developmental patterns would seem to be highly conserved across long periods of time, with expression being the major form of variation.
