The nervous system is a network of specialized cells that communicate information about an animal’s surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glia, that aid in the function of the neurons. The nervous system is divided broadly into two categories; the peripheral nervous system and the central nervous system. Neurons generate and conduct impulses between and within the two systems. The peripheral nervous system is composed of sensory neurons and the neurons that connect them to the nerve cord, spinal cord and brain, which make up the central nervous system. In response to stimuli, sensory neurons generate and propagate signals to the central nervous system which then processes and conducts signals back to the muscles and glands. The neurons of the nervous systems of animals are interconnected in complex arrangements and use electrochemical signals and neurotransmitters to transmit impulses from one neuron to the next. The interaction of the different neurons form neural circuits that regulate an organism’s perception of the world and what is going on with its body, thus regulating its behavior. Nervous systems are found in many multicellular animals but differ greatly in complexity between species.
Rhinencephalon, Amygdala, Hippocampus, Neocortex, Lateral ventricles
Epithalamus, Thalamus, Hypothalamus, Subthalamus, Pituitary gland, Pineal gland, Third ventricle
Tectum, Cerebral peduncle, Pretectum, Mesencephalic duct
Nervous system in humans
The human Nervous system can be described both by gross anatomy, (which describes the parts that are large enough to be seen with the naked eye,) and by microanatomy, (which describes the system at a cellular level.) In gross anatomy, the nervous system can be divided into distinct organs, these being stations through which the neural pathways cross. These organs can be divided into two systems: the central nervous system (CNS) and the peripheral nervous system (PNS).
Central nervous system
The central nervous system (CNS) is the largest part of the nervous system, and includes the brain and spinal cord. The spinal cavity holds and protects the spinal cord, while the head contains and protects the brain. The CNS is covered by the meninges, a three layered protective coat. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae.
Peripheral nervous system
The PNS is a regional term for the collective nervous structures that do not lie in the CNS. The bodies of the nerve cells lie in the CNS, either in the brain or the spinal cord, and the longer of the cellular processes of these cells, known as axons, extend through the limbs and the flesh of the torso. The large majority the axons which are commonly called nerves, are considered to be PNS.
The cell bodies of afferent PNS nerves lie in the dorsal root ganglia.
The nervous system is, on a small scale, primarily made up of neurons. However, glial cells also play a major role.
Neurons are electrically excitable cells in the nervous system that process and transmit information. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves. A number of different types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli effecting sensory organs and send signals to the spinal cord and brain, motor neurons receive signals from the brain and spinal cord and cause muscle contractions and affect glands, Interneurons connect neurons to other neurons within the brain and spinal cord.
Glial cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, glia are estimated to outnumber neurons by about 10 to 1.
Glial cells provide support and protection for neurons. They are thus known as the “glue” of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons.
A less anatomical but much more functional way of dividing of the human nervous system is classification according to the role that the different neural pathways play, regardless of whether or not they cross through the CNS/PNS:
The somatic nervous system is responsible for coordinating voluntary body movements (i.e. activities that are under conscious control).
The autonomic nervous system is responsible for coordinating involuntary functions, such as breathing and digestion.
In turn, these divisions of the nervous system can be further divided according to the direction in which they conduct nerve impulses:
Afferent system by sensory neurons, which carries impulses from a somatic receptor to the CNS
Efferent system by motor neurons, which carries impulses from the CNS to an effector
Relay system by interneurons (also called “relay neurons”), which transmit impulses between the sensory and motor neurons (both in the CNS and PNS).
The junction between two neurons is called a synapse. There is a very narrow gap (about 20nm in width) between the neurons called the synaptic cleft. This is where an action potential (the “message” being carried by the neurons, also known as the nerve impulse) is transmitted from one neuron to the next. This is achieved by relaying the message across the synaptic cleft using neurotransmitters, which diffuse across the gap. The neurotransmitters then bind to receptor sites on the neighboring (postsynaptic) neuron, which in turn produces its own electrical/nerve impulse. This impulse is sent to the next synapse, and the cycle repeats itself.
Nerve impulses are a change in ion balance between the inside and outside of a neuron. Because the nervous system uses a combination of electrical and chemical signals, it is incredibly fast. Although the chemical aspect of signaling is much slower than the electrical aspect, a nerve impulse is still fast enough for the reaction time to be negligent in day to day situations. Speed is a necessary characteristic in order for an organism to quickly identify the presence of danger, and thus avoid injury/death. For example, a hand touching a hot stove. If the nervous system was only comprised of chemical signals, the nervous system would not be able to signal the arm to move fast enough to escape dangerous burns. Thus, the speed of the nervous system is evolutionarily valuable, and is in fact a necessity for life.
Some landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.
The evolution of a complex nervous system makes it possible for various animal species to have advanced perception abilities like sight, complex social interactions, rapid coordination of other organ systems, and integrated processing of many concurrent signals. In humans, the advanced development of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other outcomes of human society that would not be possible without our brains.
Many people have lost basic motor skills and other skills because of spinal cord injuries. If this portion is damaged, the biggest nerve and the most important one get damaged. This leads to paralysis or other permanent damage. Physical lesions or genetic abnormalities of the brain can also lead to major harm.
The nervous system is able to make basic motor skills and other skills possible. The basic 5 senses of texture, taste, sight, smell, and hearing are powered by the nervous system. If disabled, basic motor skills may be lost.
Taking Care of the Lymphatic System
It is important to keep the lymphatic system healthy as it is a vital part of body immunity and overall health. Since the system is closely allied with the cardiovascular system, approaches to keeping that system healthy are recommended for the lymphatic system, also.
The following all play a part in keeping the lymphatic system operating at peak efficiency: proper nutrition, healthy amounts of good-quality drinking water, adequate rest, regular exercise, and stress reduction.
If left unchecked, infection can quickly weaken the body’s immune response, leading to serious health problems. It is best to avoid sources of disease, infection, pollution, and other unsanitary substances. Caring for the body by practicing good hygiene will reduce the threat of infection from ever-present bacteria and viruses in the environment. Injuries such as scrapes, cuts, and wounds should be properly cleansed and cared for to prevent infection or the spread of infection. Serious injuries should be treated immediately by qualified medical personnel
Lymphatic System Disorders
AIDS: Acquired immune deficiency syndrome, a disorder caused by a virus (HIV) that infects helper T cells and weakens immune responses.
Allergy (AL-er-jee): An abnormal immune reaction to an otherwise harmless substance.
Autoimmune disease (au-toe-i-MYOON): Condition in which the body produces antibodies that attack and destroy the body’s own tissues.
Graves’ disease: Disorder in which an antibody binds to specific cells in the thyroid gland, forcing them to secrete excess thyroid hormone.
HIV: Human immunodeficiency virus, which infects helper T cells and weakens immune responses, leading to the severe AIDS disorder.
Lymphadenitis (lim-fad-e-NIE-tis): Inflammation of lymph nodes.
Lymphangitis (lim-fan-JIE-tis): Inflammation of lymphatic vessels.
Lymphoma (lim-FOE-mah): General term applied to cancers of the lymphatic system, which include Hodgkin’s lymphoma and non-Hodgkin’s lymphomas.
Multiple sclerosis (skle-ROW-sis): Disorder in which immune cells attack and destroy the insulation covering nerve fibers in the central nervous system, causing muscular weakness and loss of coordination.
Systemic lupus erythematosus (sis-TEM-ick LOU-pus er-i-the-mah-TOE-sis): Also called lupus or SLE, disorder in which antibodies attack the body’s own tissues as if they were foreign.
Tonsillitis (tahn-si-LIE-tis): Infection and swelling of the tonsils.
As animals perform their various metabolic processes, protein and nucleic acid, both of which contain nitrogen, are broken down. While some of the nitrogen is used to manufacture new nitrogen-containing molecules, much of it cannot be used for this purpose and must be disposed of as waste. Typically, the first nitrogen-containing molecule that forms is ammonia (NH3, which is very water-soluble, forming NH4OH, a strong base. In some way, this ammonia must be gotten rid of before it raises the pH of the body fluids. Because ammonia is so water-soluble, aquatic animals often can get rid of it just by diffusion into the surrounding water. That’s one reason why the water in your aquarium gets “bad” and needs to be changed, and why not changing the water could kill the fish. However, ammonia doesn’t readily go from body fluids into air, so terrestrial animals need other ways of getting rid of nitrogenous wastes.
The two most common substances used by terrestrial animals to get rid of excess nitrogen are urea and uric acid. Many animal species that aren’t terribly concerned about water-loss, including humans, convert the ammonia to urea, which is water-soluble and excreted in a water-based solution. Other organisms such as birds, insects, or lizards, especially if they live in an arid area, must conserve water whenever possible, thus convert the NH3 to uric acid. Uric acid is not water-soluble, thus can be excreted with little, if any, water with it. This is the white goo in bird droppings. While the major portion of human nitrogenous waste is in the form of urea, humans typically excrete some uric acid, too. Uric acid is another kind of purine like the adenine and guanine in our DNA (structure to the right).
Gout is a disorder in which humans start to accumulate more than the usual amount of uric acid (caused by either the body manufacturing excess uric acid or the kidneys not excreting enough of it) and since it’s not water-soluble, it gets stored in the body, frequently in toe joints, causing pain and deformation of the joints involved as well as the formation of kidney stones. Traditionally, people who had gout were put on diets low in purines to try to help alleviate the condition, but according to the Merck Manual, now these people are doped up with drugs rather than given nutritional counseling: [“Drugs are so effective in lowering the serum urate concentration that rigid restrictions of the purine content of the diet usually is unnecessary.”]. Typically, gout is treated with colchicine, a deadly poison (see further notes below)! Caffeine and its relatives, theobromine (in cocoa), and theophylline (in tea) are classified as xanthines (a subgroup within the purines), thus it would make sense that people with gout should be counseled to avoid coffee, tea, and chocolate.
Some insects, notably blowfly larvae (larvae of those shiny green or blue flies) excrete their nitrogenous wastes as allantoin, another purine. Allantoin is known to be a “cell-proliferant,” thus is used to help wounds to heal. For hundreds of years, people have recognized that the presence of blowfly larvae in a gangrenous wound actually helped it to heal better. From about the turn of the century until the invention of a lot of synthetic drugs, blowfly larvae were raised aseptically, and used to treat severe wounds. With the increase in availability of chemicals after World War II, the use of blowfly larvae declined, but I’ve heard of several cases lately where, for some reason, this treatment was necessary and/or preferred over synthetic drugs. It has been found that the fly larvae only eat dead, gangrenous tissue, leaving the live, healthy tissue, and since their nitrogenous waste is allantoin, that stimulates the wound to heal, usually with less scaring. In this procedure, small, sterile larvae are introduced into the wound and, if needed, traded for other small ones when they get big.
We excrete nitrogenous wastes via our kidneys. Our kidneys are located on either side of the spine, just up under the bottom ribs. They are well supplied with blood via the renal artery and renal vein. Urine made in the kidney collects in the renal pelvis within the kidney, then flows down the ureter to the bladder where it is stored until voided. From the bladder, the urine flows to the outside via the urethra, (which in the male also serves as part of the reproductory tract).
The kidney is composed of an outer layer, the cortex, and an inner core, the medulla. The kidney consists of repeating units (tubules) called nephrons. The “tops” of the nephrons make up or are in the cortex, while their long tubule portions make up the medulla. To the right is a diagram of an individual nephron. Each nephron has a closely associated blood supply. Blood comes in at the glomerulus and transfers water and solutes to the nephron at Bowman’s capsule. In the proximal tubule, water and some “good” molecules are absorbed back into the body, while a few other, unwanted molecules/ions are added to the urine. Then, the filtrate goes down the loop of Henle (in the medulla) where more water is removed (back into the bloodstream) on the way “down”, but the “up” side is impervious to water. Some NaCl (salt) is removed from the filtrate at this point to adjust the amount in the fluid which surrounds the tubule. Capillaries wind around and exchange materials with the tubule. In the distal tubule, more water and some “good” solutes are removed from the urine, while some more unwanted molecules are put in. From there, the urine flows down a collecting duct which gathers urine from several nephrons. As the collecting duct goes back through the medulla, more water is removed from the urine. The collecting ducts eventually end up at the renal pelvis which collects the urine from all of them. The area where the collecting ducts enter the renal pelvis is a common area for formation of kidney stones, often giving them a “staghorn” shape.
(clipart edited from Corel Presentations 8)
Antidiuretic hormone (ADH) from the pituitary is one factor influencing urine production. ADH promotes water retention by the kidneys, and its secretion is regulated by a negative feedback loop involving blood water and salt balances. ADH helps the kidney tubules reabsorb water to concentrate the urine. When the blood water level is too high (when you’ve been drinking a lot of liquids), this acts as a negative feedback to inhibit the secretion of ADH so more water is released. Ethanol also inhibits secretion of ADH, so a person who consumes a lot of alcoholic beverages could excrete too much water (and maybe even become dehydrated). Many diuretics work by interfering with ADH production, thus increasing the volume of urine produced. These diuretic effects are one reason why a person drinking beer (alcohol) or coffee (caffeine) needs to urinate more frequently.
When a person’s kidneys cease functioning, due to illness or other causes, renal dialysis can be used on a short-term basis to filter the person’s blood. This is not a perfect process; it can’t do everything a person’s kidneys can. Typically a person is put on renal dialysis as a temporary measure to extend the person’s life until a kidney transplant can be found. While life-saving, this procedure is often very inconvenient and stressful for the person. It requires spending long periods of time, several days a week, hooked up to the dialysis machine: the person’s blood must actually pass into the dialysis machine so the wastes can be filtered out, and then the blood is returned to the person’s body. This, combined with symptoms caused by the renal failure (the inability of the person’s kidneys to function) often preclude working at a job to earn the money to pay for the treatment. People can get by with one kidney, and the closest tissue match for a kidney transplant is often a sibling. However, as one former student who was a kidney-transplant recipient pointed out, even kidney transplants don’t last “forever”. Besides the constant workings of the person’s immune system to reject this foreign tissue, whatever disease caused the problem in the first place will probably eventually also affect the transplanted kidney. Since the same donor can’t provide another new kidney, this may mean going back on dialysis and hoping a matching donor (accident victim) can be found before it’s too late.
Some diseases and disorders of the excretory system include:
Nephritis is an inflammation of the glomeruli, due to a number of possible causes, including things like strep throat. Symptoms include bloody urine, scant urine output, and edema (swelling/puffliness). Another, more severe form, is due to an autoimmune attack on the glomeruli. Other types of nephritis affect the tubules.
Nephrosis also affects the glomeruli, and is characterized by excretion of abnormally large amounts of protein (often causing “foamy” urine) and generalized edema (water retension/swelling) throughout the whole body, especially noted as “puffy” eyelids. Because these people’s kidneys often do not handle sodium properly, a low-salt diet is usually prescribed. My younger brother developed nephrosis at age 4, and to control it, had to stay on a no-added-salt diet and take prednisone on a regular basis from then until age 16, at which point, his body finally responded positively to being weaned off the drug.
Most urinary tract infections (UTIs) are caused by Gram negative bacteria such as E. coli. If there is an obstruction of the urethra, catheterization may be needed, but as a general rule, catheterization in cases of UTI is contraindicated because it can actually introduce pathogens and make the infection worse. Women tend to acquire more urethral and bladder infections than men, perhaps because the opening of the urethra is closer to the anus. The way a woman cleans the area after relieving herself can influence her chances of contracting a UTI and/or vaginal infection. When parents are toilet-training toddlers, the common mistake is to wipe young girls from back to front. The toddlers get used to this feeling, and when they start to wipe themselves, they also go from back to front. This technique wipes bacteria from the anal area towards or into the ends of the vagina and urethra. Rather, young girls should be trained to wipe from front to back, and women who were not trained this way should make a conscious effort to change their habits.
There are a variety of types of kidney stones depending on what conditions caused their formation. According to the Merck Manual, in the United States, about 80% are calcium oxalate (and/or other calcium-based stones), 5% are uric acid, 2% are cystine, and the other 13% due to magnesium ammonium phosphate or other causes. Stones may be microscopic to large “staghorn” stones that fill the whole renal pelvis. Often, as the stone is passed down the ureter, the person experiences much pain, and the affected kidney may even temporarily become nonfunctional. Stones may be broken up by ultrasound so they can be passed more easily, but large stones may have to be surgically removed. If possible, the underlying cause of the stone(s) should be identified and alleviated. For example, calcium stones might be caused by anything from a parathyroid gland problem to too much vitamin D to some forms of cancer to a genetic predisposition.
Taking Care of the Excretory System
It is well known that aging taxes the urinary system. However, the many problems than can arise are not the inevitable consequence of aging and can be prevented or at least minimized. A person can lessen the effects of aging on the urinary system (like every other system in the body) by following a healthy lifestyle. This includes getting adequate rest, reducing stress, drinking healthy amounts of good-quality drinking water, not smoking, drinking moderate amounts of alcohol (or not drinking at all), following a proper diet, and exercising regularly.
A healthy diet is important in maintaining the health of the urinary system. A poor diet—one high in fats and meats—can cause kidney stones to develop. People who are obese or overweight place undue pressure on the organs of the urinary system, which can lead to further medical problems.
Drinking plenty of water is a necessity in keeping the urinary system healthy. The amount of water in the body helps the urinary system determine how many mineral ions, such as sodium, should be eliminated. The urinary bladder should be emptied every few hours during the day, and drinking enough water should allow an individual to produce a large enough amount of urine to accomplish this. Producing large amounts of urine also helps to flush the urinary system, washing bacteria out of the normally sterile urinary tract.
To further prevent infection of the urinary system by bacteria, it is important to practice good hygiene by keeping the genital area clean. This is vital in both sexes, but is of special concern to women because of the structure of their anatomy. In women, the urethral opening is very close to the vaginal opening and the anus. After urinating or defecating, women should wipe their genital area from the front to the back to avoid introducing fecal matter into the urethral opening.
Good hygiene after sexual intercourse is also important. During intercourse, bacteria from the vagina or from a man’s penis may be introduced into a woman’s urethra. If left unchecked, the bacteria may spread and create conditions such as urethritis and cystitis. Washing the genital area and urinating after intercourse can help flush out any bacteria from the urethra.