How Drugs Work

CONTENTS OF CURRICULUM UNIT 12.05.04

  1. Unit Guide
  1. Introduction
  2. Rationale
  3. Objectives
  4. Background
  5. Strategies
  6. Lesson Activities
  7. Bibliography
  8. Appendix
  9. Notes

What Is Going On Inside My Body? An Introduction to the Central Nervous System and the Digestive System

Tracy Lewis

Published September 2012

Tools for this Unit:

Background

There are the 11 systems that make up the human body: skeletal (bones), muscular (muscles), nervous (nerves), integumentary (hair, skin and nail), lymphatic and immune (hormones and white blood cells), endocrine (hormones), cardiovascular (blood), respiratory (oxygen), digestive (nutrients), urinary (waste), and reproductive (babies) systems. These systems work cooperatively while fulfilling their own vital functions in an effort to maintain health 1. While all of these systems work together, one can be viewed as the conductor of sorts. This is the nervous system.

Nervous System - "The Fact, Just the Facts"

Research on how the body functions shows that the human body is made up of millions of small structures that perform a huge number of different tasks. These actions are organized and integrated into a smooth harmonious system. 2 The two main communication systems in the human body are the nervous system and the endocrine systems. These systems send messages and impulses to all of the structures in the body. In other words, when the body is working optimally the brain knows what is happening in all regions of the body, even when we are unaware. First, we will look at the nervous system.

The nervous system is made up of three parts, the central nervous system, peripheral nervous system and autonomic nervous systems. Each system plays a key role in how communication is orchestrated in the body. The function of the central nervous system (CNS) is to communicate and coordinate the other systems in the body, meaning this is the system responsible for receiving messages, interpreting them, responding and carrying out the corresponding action. Its second function is to provide a safe site that will house our intellect, the ability to reason and understand. The brain and spinal cord make up the CNS. 3 This complex, highly organized system uses special cells in the body to perform this task, nerve cells.

Nerves

Nerve cells or neurons are the agents that carry out the function of transmitting messages from one to another 4 5. The brain alone has over 100 billion nerve cells and the body contains many more. Similar to all other cells in the body, the neuron has a cell body with a nucleus that sits inside of it toward the middle. However, nerve cells are different in that they have extensions or processes called dendrites and axons. On any nerve cell there may be several different dendrites, but there is only one axon. These dendrites tend to be shorter in length and branch out in many directions, gathering information. Axons on the other hand are usually longer with fewer branches. Along the line of some axons fatty wrappings called myelin sheaths are found. They look like beads on a chain of a necklace. This special covering insulates the axon to prevent it from short-circuiting and it also speeds up transmission of the signal being delivered. Dendrites and axons form a pathway for communication. Each one has a specific job in this unique communication process. Dendrites receive messages from other cells and carry that message into the cell body 6 7.

Neurons like all other cells in the body need provisions; therefore, supporting cells called glial cells protect and nourish the neurons. These "protectors" guard the environment of the neurons by destroying foreign particles and clear away cell debris from cut off fibers. They are sometimes called the "nerve glue". Neurons and glial together create nervous tissue. These structures are vital to ensuring the message gets to its destination.

Information travels through the body by way of tiny electrical currents called nerve impulses or actions potentials. The current is created when sodium, which is rich in the (extracellular) fluid outside of the neuron, passes across the membrane of the cell, creating an electrical change. With delicate precision, potassium, which is rich in the fluid inside the membrane (intracellular fluid) of the neuron, also passes across the membrane in an effort to recover the normal electrical state. This action happens again and again through the neuron until it reaches the end of the neuron. The rate at which these impulses complete the entire process is faster than a blinking eye (300 - 400 milliseconds), a few milliseconds. Electrical waves like this happen all day long, all throughout the body, millions of times for an entire lifetime 8.

Next up is the big "jump" across the synapse. When an electrical process reaches the end of the axon it changes into a chemical form to jump across to the next dendrite to continue the message through the body. The reason it needs to make a jump is due to how dendrites and axons communicate. While they are connected, they never actually touch. The space between the axon and receiving dendrite is called the synaptic cleft. This gap fills with a liquid as molecules (neurotransmitters) glide across this extremely small space to pass on the impulse.

Brain - Three pounds of power!

The human brain is a soft tissue organ colored white and gray that works closely with the spinal cord to manage processes in the body. Some of these processes we are conscious of, yet others we are not 9. For example, you do not have to think about making your heart beat or keeping track of its rhythm. Breathing is another function of the body the brain controls without a conscious effort on the part of the human in which it resides. The structure is unlike any other organ in the body.

The tissues of the brain have unique grooves due to the folding of the surface called the cerebral cortex10. This precious organ has three levels of protection. (1) The meninges are reported to be a tough, dense membrane of connective tissue that has a great quantity of blood vessels. It looks like a "cobweb" with fluid filling the spaces. The function of the meninges is to provide nutrients to the organ and act as a shock absorber. (2) Cerebrospinal fluid like meninges supplies nutrients to the brain, and absorbs shock waves from daily activities. Contained within a unique network of ventricles and linings it establishes a filtration system to remove and carry away waste products. The shock absorbing qualities are key, without them the brain organ would knock up against the skull bone repeatedly. This would give a new definition the term "headache". (3) Finally there is the bony cranial cavity call the skull. Simply put it is the bone's lightweight structure that houses brain matter. Inside of this protection is the brain, which has four parts the cerebrum, diencephalon, cerebellum and the brain stem. Each part has a special function and responsibility11 12.

Cerebrum

The largest section of the brain is the cerebrum. It takes up "more then four-fifths" of the entire tissue 13. Deep folds in the surface tissue form four key sections in the cerebrum. The greatest of these sections is the frontal lobe, as it is located in the front of the actual brain. This lobe is responsible for speech production, beginning the movement process, and characteristics of personality are associated with this lobe. On the top back portion of the brain is the parietal lobe. All the body's sensory data is reported and synthesized here. An example would be sensations like pressure, pain and temperature. Moving toward the back of the brain is the occipital lobe. This area has important sensory function to sight. It is the processing point in the brain for the information collected in the eyes. Finally, on the sides of the brain is the temporal lobe. This region is responsible for recognizing sounds, tones and loudness. It also plays a role in storing memories 14.

Diencephalon

Located between the cerebrum and the mid brain is the diencephalon. It is comprised of two main structures, the thalamus and the hypothalamus. The first sits on top of the brain stem and is shaped somewhat like an egg. This control center serves as a receptionist for the cerebrum by monitoring and processing incoming and outgoing nerve impulses. The later structure called the hypothalamus has been thought of as, "the brain" of the brain. It is the center where all parts are connected. The diencephalon performs but is not limited to the following vital functions: breathing, heart rate, temperature, appetite control, signals thirst, release of oxytocin (hormone released that contracts the uterus & produces milk), digestion, display of feelings, sleep cycles and "mind-over-body" experiences15 16.

Cerebellum

The smallest section of the brain is structured some like the cerebrum. It is made up of gray matter on the outside and white matter on the inside. The cerebellum is responsible for the control of all body functions dealing with skeletal muscles. It controls the accuracy and timing of skilled movements such as balance, posture, coordination and muscle tone17.

Brain Stem

The brain stem is structured in three parts: the midbrain, pons and medulla. These centers regulate several vital survival functions in the body such as heartbeat, blood pressure, breathing, and reflex actions of swallowing and vomiting. It also provides a pathway for messages to flow to and from the cerebrum. It extends and connects with the second part of the CNS, the spinal cord18.

Spinal Cord

The spinal cord is an intricate collection of nerve fibers that extend 16 - 18 inches long. Starting at the base of the brain it extends to the lower part of the body. Its shape is like that of a flattened cylinder, and no wider than a pencil for the most part. As it extends down the spinal column it trails off into a threadlike tail at the base. There are 31 spinal nerves that branch out from the spine. These nerves connect to the skin, muscles, chest, abdomen and the limbs. They carry messages to the brain about the conditions in their perspective areas in the body. One special function of the spinal cord is that in some cases it can bypass the brain to give a motor response to certain sensed nerve impulses, in the form of a spinal reflex 19 20.

Peripheral Nervous System - Let us make "sense" of it all.

The nerves in this system connect the brain and spinal cord with sensory receptors, muscles and glands in all parts of the body. There are 12 pairs of cranial nerves directly connected to the brain rather than the spinal cord, and 31 pairs of spinal nerves. Each of these nerves is connected to a specific segment along the spinal cord by two roots. The posterior roots give sensory information, while anterior roots give motor information. Medical research has revealed that specific branches are associated with specific organs in the body 21.

Autonomic Nervous System - Checks and Balances

The autonomic nervous system (ANS) has the distinctive job of maintaining constant and consistent conditions in the body. The name for this function is homeostasis. The ANS accomplishes most of it work independent of the conscious mind, therefore we are hardly aware of it work. Some of the nerves in this system are shared with the CNS and PNS. It provides involuntary responses due to the task completed are generally "automatic", immediate and long term.

Here is quick snap shot of how it works. Sensory nerves send data about an organ and internal activities like the lungs and breathing. The data is integrated in to the hypothalamus, brainstem or spinal cord. It is the ANS that then send out the commands to the organ, as a motor nerve signal. The response command is given to all the key participants in the process of breathing within fractions of a millisecond 22.

Digestive System - Give Me What I Need!

In order for the human body to survive, every small region in the body must have nutrients. As discussed before, nutrients are carried throughout the body by the circulatory system and cells of all types. The digestive system can be described as a long passage way distinctively designed to breakdown food substances into usable forms for the cells that support life. While this sounds like a simple process of eating, mixing and pushing out, the system is much more complex than that. Digestion begins at the mouth and ends at the anus 23. By definition digestion is "The process of changing complex solid foods into simpler soluble forms which can be absorbed by the body cells 24." How exactly does this happen?

The entire gastrointestinal tract (GI) is an unbroken tube of about thirty feet in length. The structure is made up of the mouth, throat, esophagus, stomach, small intestine, large intestine, and the anus. Its job is to break down food into smaller parts, and extract useful nutrients such as fats, carbohydrates and proteins. It must also absorb the nutrients into the body for use and get rid of all unusable products. The process from start to finish is a voyage that can take up to one whole day (24 hours). Like other systems in the body it too has helpers to complete its process called glands. Glands produce juices that aid in different ways to get nutrients into the systems of the body 25 26.

Mouth and Throat

When food enters the mouth, four classes of teeth (incisors, canine, premolars and molars) work together to crush and grind foods. Once your "mouth full" or "bite size" amount of food is ground up it is now called a bolus. This bolus can be swallowed. Swallowing is an involuntary reflex where the muscles in the throat contract to push food (bolus) back and down to the esophagus (food pipe). Here another structure lends a simple but key role. The epiglottis is a flap of tissue that blocks the connection to the windpipe and guides food to toward the esophagus. This simple function keeps food from "going down the wrong way 27".

Stomach, Accessory Organs and Small Intestine

The esophagus carries the bolus into the next section along the digestive trail, the stomach. Normally, the stomach can hold 1.5 - 4.0 liters of food from a meal. The organ itself is muscular and shaped like the letter "J". Inside this sac, food is broken down physically and chemically. After food enters the stomach waves of contractions churn and mix the food with gastric juices containing digestive enzymes and hydrochloric acid. This breaks the food down and also kills harmful bacteria. In the process food becomes liquefied and small amounts, about a teaspoon full, gets spewed into the small intestine.

Final arrangements for nutrient to be absorbed into the body happen in the small intestine. The small intestine that coils in the abdominal section of the body can be as long as twenty feet. It is also divided into three sections (duodenum, ileum, jejunum). Chyme, a mixture of food and stomach acid, is ejected into the small intestine. As it travels along the three sections it receives digestive juice from some accessory organs, the liver, pancreas, and gallbladder 28. These and other secretions break food down to its useable parts. Accessory organs are defined as organs that assist in the digestion of food but are not apart of the digestive tract. The tongue, saliva glands, pancreas, liver and gallbladder are a few examples 29.

Absorption of nutrients takes place in the lining of the small intestine. The lining contains many tiny projections, called villi. This is the structure that takes the nourishment up into the blood stream for cells to use. The portions of chyme in the small intestine that remain unusable are passed on to the large intestine 30.

Large Intestine -Last Stop!

The large intestine is the last stop before elimination from the body. The organ is about five feet long and about two inches in diameter. Chyme enters from the small intestine passing through a valve into a small pocket called the cecum. This valve is important as it stops back flow. Attached at the bottom is the appendix, which has no digestive function. The function of the large intestines is to take the remaining liquid chyme and translate it into semisolid feces for disposal. As the water is absorbed from the liquid like substance stool is formed 31.

Cellulose is one of the substances found in stool. This fibrous part of plants is indigestible by humans. It makes up a great portion of the feces. This bulk of cellulose encourages the muscles to activate in the colon, producing defecation. Getting rid of unusable substances is good for the body. Scientists report that having a diet rich in foods that contain cellulose is desirable as it promotes digestive health. Foods rich in cellulose include fruit, vegetables, and whole-grain cereals 32.

The rectum and anus are the exits from the body. The rectum is normally empty until just before an exiting movement. Just below the rectum is the anus canal, which has two of the strongest muscles in the body. As movement happens in the colon, the muscles relax allowing feces to exit.

Nutrients - The Good Stuff

All systems in the body need nutrients. So what exactly is a nutrient anyway? Nutrients, are substances found in food that humans need for their cells to function properly. Therefore, in order for foods to be considered nutritious they must contain one or more of these essential substances: water, carbohydrates, lipids (fats), proteins, minerals and vitamins. These substances can be used to support one or all systems in the body because they are involved in many processes.

Water is listed first because 55 - 65% of a humans total body weight comes from water. Everyday the body loses water through excretion, evaporation and respiration. This water must be replaced: therefore it is recommended that every one should drink eight 8 oz glasses per day. Exercising, sweating or vomiting increases the need for water. In fact, experts report that by the time a person recognizes they are thirsty, mild dehydration has already set in.

Carbohydrates in food function as the main energy source in the body. They are classified by their chemical make up into four groups. Smaller molecules of sugar are more easily absorbed into the body. When there is an excess of carbohydrates in the body they are converted into fats and stored in fat tissues 33. It is recommended that 50% - 60 % of the daily intake of food should come from carbohydrates.

Ideally, the key is to get the most from the foods you eat by selecting options that give you the most nutrients. This helps the body to function well. In addition to providing energy, these foods also provide minerals, roughage (fiber), and vitamins. Roughage is the portion of foods that are indigestible. They are vital to maintaining a healthy digestive system. Fibers are credited with many health benefits such as fighting off constipation, and lowering the risk of cardiovascular disease by combining with cholesterol to prevent its over-absorption in the body. Foods that fit into this group are whole-grain breads, cereals, fruit, potatoes, vegetables, and brown rice.

Lipids are next on the list of essential nutrients needed in the body. These are compounds of fatty acids and alcohol, or simply "fats". Like carbohydrates, they also provide the body with an energy source. Lipids are different in that they have twice as many calories as the same amount of protein or carbohydrates. Their stored location is inside of tissues for the purpose of an emergency, like sudden illness or starvation. Working in the body, lipids have the following functions: cushion internal organs, insulate against cold, make up a part of cell membranes, and contribute to the making of bile for digestion 34. It is recommended that 25 -35% of lipids should be in your diet 35.

A word on cholesterol is appropriate here. Cholesterol is a white, waxy like substance that is sold at room temperature. This substance can be found in two forms. Cholesterol can come from foods eaten such as meat or it can come from the liver. The liver produces cholesterol to build cells and make hormones. Now, there is "good" cholesterol that takes this fat away from the tissues in your body to the liver for processing. On the other hand there is also "bad" cholesterol, this type carries the waxy like substance to other tissues in the body. This can cause substantial problems once there is blockage. Foods that support the body getting rid of the "bad" cholesterol are garlic, fresh fruit, vegetables, prunes, and oat or wheat bran.

Proteins are another vital nutrient for the body. Proteins are broken down by the digestive system into amino acids. These amino acids can then be used by cells in the body to build proteins that are needed for body functions. Proteins function in the body as a regulator of chemical reactions, and provide tissue growth and repair. They too can be use as an energy source. All cells and organ systems in the body depend on the use of protein to function properly. Systems include but are not limited to the circulatory, muscular, endocrine, nervous, and immune all use proteins in one way or another.

There are two ways humans can attain the essential proteins need for life, by eating animal products or vegetables. There is a slight difference in the two sources. First, animal proteins like milk, eggs, and meat are considered "complete" proteins. They have all you need in one item. On the other hand should you choose to get protein from vegetables it must be done by mixing and matching, because vegetable are considered "incomplete" proteins. Eating one type alone is not sufficient to supply what the body needs. An example would be eating whole wheat bread alone. However, almond butter and whole wheat bread eaten together will get the job done. Finally, the human body cannot store extra amino acid. Instead, when excess is presents, the compound is further broken down by the liver, after which some portions are use as immediate energy or they are stored as fat.

The final two nutrients the body gains from food intake are minerals and vitamins. Four percent of the body is made up from minerals. Some like sodium, potassium, calcium, iron, phosphorus, and zinc are used to help in various functions. Their work in the body is directly related to growth and maintenance. For example, minerals help to form bone and teeth structures, maintain heart rhythm, and produce muscle contractions.

Vitamins are active compounds needed by the body to work properly 36. As stated in the digestion section, when coming from food, they are absorbed from the digestive system and carried by the blood to all parts of the body. When there is extra or excess in the body it is not stored but excreted out. Nutritionists do not have a firm suggestion on the intake vitamins because intake greatly depends on the individuals needs at the time. If pregnancy, disease, emotional stress and or old age are present then the recommendations will change.

After looking at two systems of the body closely in addition to nutrition, the essential question still remains. "What should I eat to be healthy?" As it relates to the two systems discussed in the background section, experts have a lot to say on this matter. In order to keep your central nervous system stay healthy, researchers suggest eating a variety of nutrients in order to keep the mind cognitively sharp, boost memory and rid the system of free radicals.

Vitamin A, B9 (folic acid), B12, B6, E, and lipids were found to be the best to support the CNS. Vitamin A promoted higher learning and improved short-term memory by affecting brain cells in the hippocampus. Folic acid or vitamin B9 is essential to CNS development in the embryo. It also has antioxidant that protects and nourishes the blood vessels of the CNS. Vitamin B12 & B6 also support thinking and short-term memory. B12 is needed to produce myelin, the protective covering along the axon. This is key for protecting the brain against age-related degeneration. A diet with varied proteins and fat sources is also suggested as omega-3 lipids make up a portion of nerve cells 37 38.

The digestive system is the site of absorption of all nutrients. In order for this system to work efficiently several nutrients are needed, specifically, vitamin B-complex, vitamin C (absorbs iron), and Folic acid, which helps in the production of vitamin K 39. These complex B vitamins function to helps break down proteins and get rid of waste products in the body 40.

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