Nervous System and Senses

The nervous system consists of two types of cells. Nerve cells are called neurons. Various support cells are associated with the neurons, most typically, Schwann cells. The parts of a neuron include the dendrite which receives the impulse (from another nerve cell or from a sensory organ), the cell body (numbers of which side-by-side form gray matter) where the nucleus is found, and the axon which carries the impulse away from the cell. Wrapped around the axon are the Schwann cells, and the spaces/junctions between Schwann cells are called nodes of Ranvier. Collectively, the Schwann cells make up the myelin sheath (numbers of which side-by-side form white matter).

Schwann cells wrap around the axon (like the camp food, “pigs in a blanket”). Having an intact myelin sheath and nodes of Ranvier are critical to proper travel of the nerve impulse. Diseases which destroy the myelin sheath (demyelinating disorders) can cause paralysis or other problems. Schwann cells are analogous to the insulation on electrical wires, and just as electrical wires short out if there’s a problem with the insulation, so also, neurons cannot function properly without intact myelin sheaths.

The nervous system has three basic functions:

1. sensory neurons receive information from the sensory receptors,
2. interneurons transfer and interpret impulses, and
3. motor neurons send appropriate impulses/instructions to the muscles and glands.

A nerve impulse is an electrical charge that travels down the cell membrane of a neuron’s dendrite and/or axon through the action of the Na-K pump. Ordinarily, the inside of a neuron’s cell membrane is negatively-charged while the outside is positively-charged. When sodium and potassium ions change places, this reverses the inner and outer charges causing the nerve impulse to travel down the membrane. A nerve impulse is “all-or-none:” it either goes or not, and there’s no halfway. However, a neuron needs a threshold stimulus, the minimum level of stimulus needed, to trigger the Na-K pump to go and the impulse to travel. A neuron cannot immediately fire again; it needs time for the sodium and potassium to return to their places and everything to return to normal. This time is called the refractory period.

A junction between two nerve cells or a nerve and a muscle cell is called a synapse. In a synapse, various chemicals are used to transfer the impulse across the gap to the next cell. These are collectively known as neurotransmitters, and include such chemicals as dopamine (brain levels of which are low in Parkinson’s disease), serotonin, and acetylcholine (levels of which are low in myasthenia gravis).

The nervous system can be subdivided several ways depending on if one is looking at function or location:

Most body organs/systems are enervated by both sympathetic and parasympathetic nerves, and these have opposite effects on the various organs. For example, the sympathetic NS prepares for action by increasing heart and respiration rates. by telling the liver to release stored glycogen as sugar, and by decreasing digestive processes. Conversely, the parasympathetic NS stores energy by slowing heart and respiration rates, by telling the liver to store up sugar as glycogen, and by increasing digestive processes.

The brain consists of the cerebrum which is the large, anterior portion; the cerebellum which is the wrinkled-looking, posterior part; the pons which is the closest, larger bulge at the top of the spinal cord; the medulla which is the farther, smaller bulge between the pons and the top of the spinal cord; then the spinal cord starts after the medulla. Also note under the cerebrum, the optic chiasma, the place where the optic nerves cross to the other side of the brain. The cerebellum, medulla, and pons are collectively referred to as the hindbrain. Many of their functions are involved in homeostasis, coordination of movement, and maintenance/control of breathing and heart rate. While a stroke in the cerebrum might result in partial paralysis, a stroke in the hind brain is actually, potentially more dangerous becuase it could knock out coordination of the cerebrum’s activities, or worse yet, automatic control of breathing and/or heart beat. The midbrain is responsible for receiving and integrating of information and sending/routing that information to other appropriate parts of the brain. The forebrain is composed of the cerebrum and related parts, and functions in pattern and image formation, memory, learning, emotion, and motor control. In addition, the right side functions more in artistic and spatial concepts, while the left side controls speech, language, and calculations. Keep in mind that motor skills are controlled by the opposite half of the brain, thus a left-brain stroke would cause paralysis on the right side of the body. Also, a left-brain stroke might cause problems with speech while a right-brain stroke is more likely to cause abnormal/inappropriate emotional responses.

Senses

We say we have five senses. Can you name them? Here’s the list of the five senses.

Sensory adaptation is a decrease in sensitivity during continued stimulation. For example, can you hear the heating/cooling system moving air? Are you aware of any rings you may be wearing?

Mechanoreceptors, are stimulated by physical means such as touch, pressure, motion, or stretching. Many of these are in the skin. Note that pressure-sensitivity also includes sound receptors or ears, our sense of hearing, which is actually a sensitivity to changes in air pressure.

To hear a sound, the outer ear collects ripples or waves of compressed air that we call sound, and passes them to the tympanum. Vibrations of the tympanum are transferred through three tiny bones in the middle ear: the malleus, the incus, and the stapes to the inner ear, which contains a coiled organ called the cochlea where the actual receptors or nerve endings are located. These receptors are fragile enough that exposure to very loud sounds can irreversibly damage them, and the more loud noises to which a person is exposed, the greater the damage. People who frequently participate in rock music concerts have noticeably reduced hearing ability. The inner ear also has a balance sensor, which is composed of three loops at right angles to each other called the semicircular canals.
(clipart edited from Corel Presentations 8)

Fish “hear” via their lateral lines, a line of pressure sensors running along each side of the fish that pick up pressure waves (= sound) in water. When someone pounds on an aquarium, that creates waves of pressure in the water that, to the fish, would be analogous to cupping your hands and pounding on your ears--NEVER POUND ON A FISH TANK! This, by the way, is the same principle used when explosives are detonated in lakes to “stun” (or kill) the fish so they’ll float to the surface and can be more easily collected for whatever purpose, the human(s) involved had in mind (and they probably were wearing ear/hearing protection).

Thermoreceptors are temperature sensitive. Most of these are in our skin.

There are several kinds of pain receptors. Some are sensitive to too much heat, others to too much pressure, etc. Sensitivity of these (and other receptors) can be increased or reduced by certain drugs. Painkillers are supposed to decrease the sensitivity of the pain receptors. Our bodies’ natural endorphins function in this manner, and the tendency to rub an injury stimulates the release of endorphins in that location, lessening the pain. The stress of overexertion when doing strenuous exercise also triggers the release of endorphins. Interestingly, endorphins belong to the category of chemicals known as opiates, thus are chemically related to opium, and also may potentially be addicting! It is thought that a number of people who “have” to frequently do strenuous exercise to feel good may actually be addicted to the endorphins their bodies release under those circumstances--they exercise to “get high.” In general, it may not be a good idea to attempt to deaden any/all pain we feel. Pain is a message from our bodies that something is wrong, thus can be “good” at times when it reminds us to not do something we shouldn’t. For example, if a person with a back injury is on pain medication, the tendency is for that person to overexert him/herself because it doesn’t feel bad, and perhaps injure the back further. If (s)he would not have been on painkillers, (s)he would have gotten the message. “Stop, it’s too much!”

Chemoreceptors include chemical senstivities like smell and taste. Interestingly, many insects taste/smell with their feet and/or antennae. For example, if a butterfly’s (or fly’s) feet are dipped in sugar water, it extends its tongue (if it’s hungry). In humans, the senses of taste and smell are very complex. There are both genetic and learned components to our sense of taste. One “famous” demonstration frequently done in genetics classes is PTC paper. This is a tissue paper impregnated with a chemical called phenylthiocarbamide. About 70% of the people in the U.S. can taste this substance, which has a horrible, bitter taste. About 30% of people who taste this test paper, cannot taste the chemical and it just tastes like “paper”. Preferences for certain tastes can also be acquired: people from other countries are frequently repulsed by the amount of sugar in many foods eaten here in the U.S. Perhaps tied in with that, it appears that tastes change as a person matures. Strong tastes like mustard, onions, and radishes are often repulsive to small children, yet many children who won’t eat cooked vegetables love the taste of raw vegetables fresh out of the garden. The sense of taste is also influenced by the adequacy of one’s diet, and people who have a zinc deficiency tend to have taste buds that are considerably less sensitive (a common complaint is “I can’t taste my food”). Smoking also tends to obliterate the unique tastes of foods, and people who quit are often amazed at how different, how much better their food tastes. Similarly, people who unthinkly add salt to everything are so used to everything tasting like salt that when they have to or choose to reduce their salt consumption, they frequently are amazed at how different all their foods taste.

Different areas of the human tongue have sensitivities to different tastes. Each of these areas contains proportionately more of certain chemoreceptors. Typically, the middle-front of the tongue is more sensitive to sweet tastes, the sides to salty tastes, the center-back to sour tastes, and the very back to bitter tastes. One old herbal remedy for sore throat is tea made from licorice root. I have noticed, when I drink this tea, when it comes into contact with most of the taste sensors on my tongue, it just tastes like water, but as I swallow it, it has a fairly strong, sweet taste very far back on my tongue, down in my throat, where nothing else I’ve ever eaten triggers a response. I have never seen any discussion of this in the literature.
(clipart edited from Corel Presentations 8)

Electromagnetic receptors include sensitivities to light, including light we humans cannot see, as well as things like electric and magnetic fields. Many animals can see colors of light we can’t (infrared, ultraviolet). Some animals, like whales, can sense “gravity”, variations in the Earth’s magnetic field, and use that in navigation.

Our eyes need vitamin A as the precursor to our visual pigment. This pigment absorbs light energy and changes it to chemical energy, then transfers an electrical impulse to the appropriate nerve endings. This pigment is destroyed in the process and must be regenerated. When a person spends time in the dark, part of the acclimation process is synthesizing more visual pigment to increase the eyes’ sensitivity. Therefore, if you get up in the middle of the night for a snack, you can probably see better if you don’t turn on more than just a night light to navigate safely. If you turn on a lot of bright lights, much of the visual pigment accumulated in your eyes will be destroyed, and when you turn out the lights to go back to bed, you won’t be able to see in the dark.

The parts of the human eye include the cornea covering the front, the pupil which is the opening in the center of the eye, the size of which is controlled by the iris, and the lens, which focuses light onto the retina, which contains the photoreceptors. The white of the eye is the sclera.
(clipart edited from Corel Presentations 8)

The eyes of a person who is nearsighted (has myopia) are out of round such that they are too long front-to-back, thus an image is in focus somewhere in the middle of the fluid in the eye. The eyes of a person who is farsighted (has presbyopia) are out of round such that they are too short front-to-back, and the image is in focus somewhere behind the eyeball. Note that the lens flips the image over upside down, and as our brains process the information, the image is flipped back, right-side up. Experiments were done in which people were asked to wear special glasses that made everything look upside down, and after a time, their brains learned to compensate and things, once again, looked right-side up.

An animal that is potential prey for another animal has its eyes on the sides of its head and the eyes operate independently, giving the animal nearly 360° vision to better watch for danger. A predator has its eyes on the front of its face, giving it excellent binocular vision for depth perception and judging distance to prey. An interesting combination of these traits can be found in a chameleon (not an anole). Chameleons eat insects, so need binocular vision to capture dinner, but are also potentially dinner for someone else. They have their eyes on the sides of their heads, but the eyes stick out and can swivel around. Chameleons can use their eyes independently to watch for predators, yet when a potential meal hops into sight, can focus both eyes on the insect to judge the distance before flicking out a sticky tongue to catch it. Interestingly, because of the location and mobility of a chameleon’s eyes, it can rotate its eyes backwards, and have binocular vision behind its head!

Another light-sensitive organ that we are only beginning to understand is the pineal gland. This organ manufactures melatonin in response to darkness, thus the shorter the day (like in winter) the more melatonin is secreted. In many animals, the pineal gland is located just under the skin somewhere on the head, and is directly stimulated by light. Some lizards even have a third eye! In humans, the pineal gland is inside the skull and it is thought that it receives it stimuli from nerves from the eyes. Some people make too much melatonin in the winter, making them sleepy and/or depressed. This is called seasonal affective disorder (SAD) and is treated by having the person spend a certain number of hours each day in front of bright lights. There is also a drop in melatonin production at puberty, and it is thought that these may be related. Studies have been done on blind girls (with a form of blindness in which no impulses can travel down the optic nerve and reach the brain and pineal gland), which showed that these girls tended to have higher levels of melatonin for a longer time, resulting in a delay in the onset of puberty. While some older people, who don’t make very much melatonin, thus don’t sleep well, might benefit from a melatonin supplement, I’m leery of the recent melatonin craze in this country. When so many people apparently are suffering from SAD, I question the wisdom of purposly ingesting more melatonin.

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Copyright © 1996 by J. Stein Carter. All rights reserved.
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