Answers to Last issue's Do You Know?
1. Can the brain feel pain?
Ans: The pain experience for all of us begins when unpleasant stimuli activate sensory nerve fibers called nociceptors (see Box). These specialized fibers — which are located in skin, muscles, joints, and some organs — transmit pain signals from the periphery to the brain, where the message of pain is ultimately perceived. 
BOX: Sensory Neurons
Sensory neurons, also known as afferent neurons, are nerve cells or neurons in the nervous system. They convert sensory information (like touch, heat) to signals that travel on the sensory nerve fibers in a sensory nerve, to the brain via the spinal cord.
Nociceptors are special sensory neurons that respond to potentially damaging stimuli. This process, called nociception, usually causes the perception of pain. For instance, if you touch a hot object, nociceptors detect that this stimulus can cause damage and then tells the brain to signal the hand to withdraw. Nociceptors are found in internal organs as well as on the surface of the body to "detect and protect".
The picture shows four types of sensory neurons and their receptor cells. Nociceptors shown as free nerve endings type A.
END OF BOX
There are no pain receptors in the brain itself. But the meninges (coverings around the brain), periosteum (coverings on the bones), and the scalp all have pain receptors. Surgery can be done on the brain and technically the brain does not feel that pain.
With that said, the brain is the tool we use to detect pain. Let’s say you’re on the beach and you step on a sharp shell. Special pain receptors in your skin activate whenever there has been an injury, or even a potential injury, such as breaking the skin or causing a large indentation. Now, an impulse is heading through the nerve into the spinal cord, and eventually all the way to your brain. This happens within fractions of a second.
Your spinal cord is a complex array of nerves, transmitting all kinds of signals to and from the brain at any given time. The spinal cord is also in charge of your reflexes. The brain does not have to tell your foot to move away from the shell, because the spinal cord has already sent that message. The pain signal continues to the brain. This is because pain involves more than a simple stimulus and response. Your brain needs to make sense of what has happened.
Pain gets catalogued in your brain’s library, and emotions become associated with stepping on that shell. When the pain signal reaches the brain it goes to the thalamus, which directs it to a few different areas for interpretations. Some areas in the cortex figure out where the pain came from and compare it to other kinds of pain with which is it familiar. Was it sharp? Did it hurt more than stepping on a tack? Have you ever stepped on a shell before, and if so was more or less painful?
Signals are also sent from the thalamus to the limbic system, which is the emotional center of the brain. Feelings are associated with every sensation you encounter, and each feeling generates a response. For example, your heart rate may increase, and you may break out into a sweat.
If your brain doesn't feel pain, how do you get headaches? Headaches, however, are a different story. Though your brain does not have nociceptors, there are nociceptors in layers of tissue known as the dura and pia that serve as a protective shield between the brain and the skull. In some situations, chemicals released from blood vessels near the dura and pia can activate nociceptors, resulting in headaches, such as migraines. Increased blood flow can also trigger a migraine, which is why migraines are considered vascular headaches. Migraine headaches are often throbbing and are accompanied by hypersensitivity to light, sound, and touch.
Sources: https://www.brainline.org/author/brian-greenwald/qa/can-brain-itself-feel-pain, https://www.brainfacts.org/ask-an-expert/if-the-brain-cant-feel-pain-why-do-i-get-headaches, Wikipedia

2. Many animals (like snakes) periodically shed their skin and grow a new one. Are there any animals that replace their internal organs?
Ans: It would be wonderful if we could regenerate a missing limb or damaged organ. Imagine never having to get dentures because you could grow new teeth throughout your lifetime! Although these abilities might sound like a futuristic science fiction movie, they exist in the animal kingdom. While most animals do not have these abilities, there are a few that do. However, those few creatures may provide us with insight into how regeneration occurs in their species.

Salamander
The salamander is an amphibian with a tail and short legs. There are over 700 species of salamander. Although all salamanders have a certain degree of regeneration, some species have a higher regenerative capacity than others. Some salamanders can regrow their tails in a few weeks, after dropping the old one to distract predators. The new appendage is as fully functional as the original.
Axolotl Salamander
Axolotls are an aquatic species of salamander with extraordinary regenerative ability. These little salamanders can regenerate organs, skin, limbs, or practically any body part.
Great White Shark
Although sharks can’t regenerate organs or other body parts, they are certainly ahead in dental regeneration. Sharks can regrow teeth throughout their lifetimes. The length of time it takes for a shark to regrow a tooth varies from a few days to a few months. If scientific researchers can determine how this process works, it could revolutionize dentistry.
Starfish
Starfish are remarkable regenerative animals. Not only can starfish grow a new limb, but these creatures can also grow a whole new body from the lost limb. Several new starfish can grow from pieces of the original one. Now, that’s amazing!
Mexican Tetra
The river-dwelling Mexican tetra is a fish that can regenerate heart tissue. Research shows that this fish can generate tissue with no scarring. Scientists hope that studying the Mexican tetra could lead to breakthroughs in the field of cardiovascular disease. However, the Mexican tetra isn’t the only fish that can regenerate heart tissue. The zebrafish can also generate its heart with little or no scarring.
Chameleon
Although chameleons are best known for their color-changing ability, they are also able to regenerate. Chameleons can regrow their tails and limbs. They can also heal damaged nerves and skin during the regenerative process.
These are just a few animals with the ability to regrow body parts. There are many more. Some of these creatures have limited regenerative abilities that allow the growth of only specific limbs or organs. However, some animals can grow an entire body from a severed limb. In some animals, their regenerative abilities decline as they age. We can learn a lot from these creatures. Imagine how the medical industry would change if we could harness these capabilities. Perhaps one day, we will be able to regrow limbs and regenerate organs. Such discoveries could end suffering and increase overall health, thanks to many of these miraculous creatures that we are just beginning to understand.
Source: https://scitechdaily.com/

3. The air in the Earth's atmosphere exerts tremendous pressure. Why don't we feel this air pressing down on our heads?
Ans: Air does not crush you down. As a fluid, air flows around you and tries to crush you in. Fortunately, there is typically just as much pressure inside your body pressing outward as there is air pressure outside your body pushing inward. In general the atmospheric forces typically cancel out, meaning that there is no overall force on you and you don't get crushed. Even when the internal and external pressure don't exactly cancel each other out, your skin, muscles, and other tissues are usually strong enough and flexible enough to not be damaged by the force.
Stand outside in a field and look straight up. You are looking at a one- hundred kilometer column of air that is being pulled down toward you by gravity. Although air is very tenuous compared to other materials, it is composed of atoms and molecules like nitrogen and oxygen and has mass. As such, air is pulled down by gravity just like everything else that has mass. A handful of air may not have much mass, but one hundred kilometers of air is a different story. If you draw a one-meter by one-meter square on the ground, then all of the air directly over that square has a total mass of about twenty thousand kilograms! Because of earth's gravity, this mass pushes down on the square meter with a force of about ten thousand kilos, which is the same as the weight of a bus.
Now draw a one-cm by one-cm square on your shoulder. The one hundred kilometers of air directly above that square has a total mass of 1 kilogram. Under the influence of earth's gravity, this mass pushes down on each square cm of your shoulder with a force of about 1 kg. Why don't you feel it? The answer is that air is a fluid.
As a fluid, air is able to flow in every direction and take any shape. As it does so, air transmits the crushing force of its weight in all directions, even up. Hold out your hand with the palm facing up. While it's true that the air is pushing down on your palm, at the same time the air is pushing up on the back of your hand with the same force. As a result, the total force on your entire hand is zero. The individual surfaces (the top and bottom of your hand) feel the air pressure, but the hand as a whole experiences no net force from the atmosphere. That is why one hundred kilometers of air above you does not pin you or your hand to the ground.
This canceling of forces on your hand only happens if atmospheric air is able to reach both the palm and the back of your hand. If you remove the air that is pushing up on the back of your hand, there will no longer be a canceling of forces and you will experience atmospheric pressure on your hand overall.
Even when the force of the air on the back of your hand cancels the force of the air on the palm of your hand, your hand is still getting crushed in the middle between these two opposing forces. Fortunately, there is also pressure inside your hand directed outwards which cancels the inward force of air pressure. As a result, there is no net force on the surface of your hand. The internal pressure is not caused by air but is caused by trapped water. Internal body pressure is created and maintained by having semi-rigid cells that are pumped up with water using the chemical attractive forces between water and ions such as sodium. Each cell is a bit like a water balloon. If you pump water into a balloon, the internal water pressure can cancel out the external air pressure and the balloon holds its shape without being crushed.
Note that even if the internal body pressure and the external air pressure are not exactly equal and therefore don't cancel each other out, most tissues in your body are strong enough to withstand the resulting net force. For instance, if you place a human in the vacuum of space without a space suit, there will be the regular internal body pressure but no external air pressure to cancel it out. Despite this difference, such a person will not explode since the skin is strong enough to withstand the pressure.
Source: https://wtamu.edu/~cbaird/

4. Why does the ocean look a different colour in different places?
Ans: Ocean color is the branch of ocean optics that specifically studies the color of the water and information that can be gained from looking at variations in color. The color of the ocean, while mainly blue, actually varies from blue to green or even yellow, brown or red in some cases.
Most of the ocean is blue in color, but in some places the ocean is blue-green, green, or even yellow to brown. Blue ocean color is a result of several factors. First, water preferentially absorbs red light, which means that blue light remains and is reflected back out of the water. Red light is most easily absorbed and thus does not reach great depths, usually to less than 50 meters. Blue light, in comparison, can penetrate up to 200 meters. Second, water molecules and very tiny particles in ocean water preferentially scatter blue light more than light of other colors. Blue light scattering by water and tiny particles happens even in the very clearest ocean water, and is similar to blue light scattering in the sky.
The main substances that affect the color of the ocean include dissolved organic matter, living phytoplankton with chlorophyll pigments, and non-living particles like marine snow and mineral sediments. Chlorophyll can be measured by satellite observations. Regions with high ocean productivity show up in yellow and green colors because they contain more (green) phytoplankton, whereas areas of low productivity show up in blue.
Ocean color depends on how light interacts with the materials in the water. When light enters water, it can either be absorbed (light gets used up, the water gets "darker"), scattered (light gets bounced around in different directions, the water remains "bright"), or a combination of both. Underwater absorption and scattering depend on the colour, and so the relative amount of each determines the water colour.
The picture shows the blue ocean in Moonlight State Beach, Encinitas, United States, the blue-green ocean, and a NASA satellite view of Southern Ocean phytoplankton bloom, with the different swirls of colour due to different levels of chlorophyll activity.
Source: Wikipedia