Ten key points: #1) Nerve impulses are the same for both sensory and motor nerves. #2) Intensity is varied by sending more impulse, or fewer. (NOT bigger ones). There is no way to make the individual action potentials bigger or smaller. #3) Some animals can sense signals to which humans are blind: Rattlesnakes, Copperheads and their near relatives (Pit Vipers), have special "Pit" organs between their nose and eyes which "sees" infrared light, and thereby sees mammal (food). The eyes of bees and many other insects can see ultraviolet light, in addition to the longer wave-lengths of light that we can see. Normal human eyes filter out UV light, because our lens system cannot focus it to the same locations as other wave-length. Bees can also see the plane of polarization of light in the sky Which supposedly humans can learn to do!?? But I can't. Many fish can detect weak electric fields in water around them, And use these fields to find food, either detecting electric fields from other animals, or generating their own fields as "radar"! And in many species using electric fields for mating signals! [These sensory uses of electric fields seems to have been an evolutionary stepping stone toward independent development in 6 or 7 groups of fish of shocking ability to stun food. Electric Eels; Electric Catfish, etc. etc. Darwin worried that electric fish might be a disproof of evolution, in that no ability can evolve by natural selection unless there is some advantage to having just small amounts. Bats reflect high frequency sound off of obstacles and food. (using high frequency sound to see moths, mosquitoes, etc.) #4) How do our eyes detect light? Millions of light-detecting cone cells and rod cells, along the inner surface of each eyeball, where the cornea and the lens focus light. The membranes of rod and cone cells contain rhodopsin = a combination of a protein "opsin" with retinal (a chemical related to vitamin A), which has a long series of alternating single and double bonds in a chain of carbon atoms. One of these double bonds is in an unstable "cis" orientation. The oscillating electric field of a light ray allows this cis isomer of retinal to isomerize to the (preferred) trans isomer. This un-bending of a retinal molecule changes the conformation of the opsin protein to which it is bound; and that changes the permeability of that cell's membrane; eventually causing nerve impulses to be sent to the brain. #5) How do we distinguish colors? Normal humans have 3 different kinds of cone cells; each has a slightly different opsin protein, that absorbs a different range of wave-lengths. Blue-sensitive, Green-sensitive and Red-sensitive For example, if light from a source causes retinals to unbend mostly in the blue-sensitive rhodopsins, and not in the other 2 then it will look blue. If light caused unbending of retinals only in the red-sensitive rhodopsins, then what color must that light be? #6) What causes color blindness? Mutation in the gene for one or other of the 3 rhodopsins results in reduced ability to discriminate between colors. NOTE: Color blind people DO distinguish colors, and many do not realize that they are color blind until their 20s or later. Certain pairs of colors look the same to them that do not look the same to people who have all 3 kinds of rhodopsins. #7) Complementary colors (that when added together = white or gray) And color after-images. If you stare steadily at a red object for 10 or 20 seconds, and then instead stare at something white, what do you see? What if you stare steadily at something green, then look away? Try it! #8) Compare how we distinguish colors with how we distinguish tastes and smells. Membrane receptors for various specific chemicals, which detect these chemicals by binding to them, with this changing that cell's ion permeabilities, thereby initiating action potentials. #9) Compare how we distinguish colors with how we distinguish sounds of different frequencies. Inner ear; Hair cells; Cochlea; Basilar membrane & tectorial membrane The concept of resonant frequencies #10) Muscles contract by active sliding of myosin fibers relative to actinfibers; Using ATP energy. In striated muscles, actin and myosin are lined up in spatial ranks called "sarcomeres" Contraction of muscles is stimulated by action potentials in the plasma membrane of the muscle cells (and also in some cytoplasmic membrane systems) and by release of calcium ions into the muscle cell cytoplasm. ----- Questions that might be on an exam a) Are there any differences between motor nerves and sensory nerves in the mechanism of propagation of action potentials? b) Can you figure out whether sensory or motor nerves have acetylcholine gated sodium channels in the part of their membrane that is located inside the spinal cord? c) Which of these kind of nerve cells would secrete vesicles of acetylcholine next to the surfaces of muscle cells? d) When a pain-sensing sensory nerves signals a larger amount of pain, instead of a smaller amount, then does it do this by making each action potential larger [hint: no], or what? e) What wave-length of light can rattlesnakes sense that most other kind of animals are blind to? f) Why are rattlesnakes called "pit vipers"? Do they live down in pits, or something? [hint: no] g) Can any kinds of animals sense ultraviolet light? What is an example? *h) Why did it seem to Darwin and many others that it should have been impossible for electric eels to evolve by natural selection? i) What seems to be the true explanation for this paradox? (i.e. that electric eels, torpedo rays, and other electric fish HAVE evolved, somehow? j) Can weak electric fields be detected by these or other kinds of fish? *k) For what different kinds of functions to different kinds of fish generate electric fields? (at least 3 different functions) l) How does the function of rhodopsin in the eye differ from the function of the kind of rhodopsin found in the membranes of some bacteria? m) Do both kinds cause changes in movements of ions across plasma membranes of cells? [hint: yes] *n) What part of the rhodopsin molecule actually absorbs the light? o) How is the detection of light by the human eye related to the shift of an electric field along a sequence of alternating double and single covalent bonds connecting carbon atoms in a long chain? p) How many different kinds of cone cells are there in the eyes of normal people? q) When a person has one of the usual forms of "color blindness", then how many different kinds of cone cells do they have? r) How do the different kinds of cone cells differ from each other? s) If you stare steadily at a red object for a while, and then look away at something white, what color "after-image" will you see? t) Why do after-images always have the exact complementary color to the color of whatever object you have previously been staring at? u) Do color blind people see no colors at all, but only black and white? v) In general terms, how does the ear distinguish between sounds of different frequencies? Is this basically the same as the method by which the eye distinguishes between light of different frequencies? (Red having a lower frequency = longer wave length than blue, etc.) *w) Imagine some kind of science fiction organism that had prisms in its eyes in place of lenses! Explain how that would be much more comparable to the system by which the ear distinguishes between sounds of different frequencies (or wave lengths). x) What is meant by hair cells in the inner ear? y) What do you sense by means of membrane receptor proteins that bind specifically to different shaped chemicals, and open sodium channels in response to binding to those chemicals? z) What are sarcomeres, what two key proteins are they made of and in what differentiated cell type.

 

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