Biology-INTEG3

UNIVERSIDAD AUTÓNOMA DE NUEVO LEÓN – PREPARATORIA NO. 9

BIOLOGY II Integrative Activity GROUP 220 TEAM: NATALIA DAENNA GONZÁLEZ VIERA LUIS ROBERTO GONZÁLEZ GUAJARDO JORGE ALBERTO CANTÚ REYES NATIVIDAD ARON DE LEÓN RAMÍREZ ANDRÉS EDUARDO REYES GÓMEZ

Circulatory System The circulatory system is a vast network of organs and vessels that is responsible for the flow of blood, nutrients, hormones, oxygen and other gases to and from cells. Without the circulatory system, the body would not be able to fight disease or maintain a stable internal environment — such as proper temperature and pH — known as homeostasis.

FUNCTION The main function of the circulatory system is to move blood and lymph through the body. Doing this transports nutrients and oxygen to the cells of the body and removes waste products such as carbon dioxide.

Along the way, the blood picks up nutrients, attacks diseases and gathers waste for eventual elimination.

The circulatory system is necessary to regulate temperature and pH balance and protect the body from diseases.

The heart The heart is a muscular pumping organ located medial to the lungs along the body’s midline in the thoracic region. The bottom tip of the heart, known as its apex, is turned to the left, so that about 2/3 of the heart is located on the body’s left side with the other 1/3 on right. The top of the heart, known as the heart’s base, connects to the great blood vessels of the body: the aorta, vena cava, pulmonary trunk, and pulmonary veins.

Blood Vessels Blood vessels are the body’s highways that allow blood to flow quickly and efficiently from the heart to every region of the body and back again. The size of blood vessels corresponds with the amount of blood that passes through the vessel. All blood vessels contain a hollow area called the lumen through which blood is able to flow. Around the lumen is the wall of the vessel, which may be thin in the case of capillaries or very thick in the case of arteries.

There are three major types of blood vessels: arteries, capillaries and veins.

Blood vessels are often named after either the region of the body through which they carry blood or for nearby structures.

Arteries and Arterioles: Arteries are blood vessels that carry blood away from the heart. Blood carried by arteries is usually highly oxygenated, having just left the

lungs on its way to the body’s tissues. The pulmonary trunk and arteries of the pulmonary circulation loop provide an exception to this rule – these arteries carry deoxygenated blood from the heart to the lungs to be oxygenated.

Capillaries: Capillaries are the smallest and thinnest of the blood vessels in the body and also the most common. They can be found running throughout almost every tissue of the body and border the edges of the body’s avascular tissues.

Capillaries connect to arterioles on one end and venules on the other.

Veins and Venules: Veins are the large return vessels of the body and act as the blood return counterparts of arteries.

Because the arteries, arterioles, and capillaries absorb most of the force of the heart’s contractions, veins and venules are subjected to very low blood pressures.

This lack of pressure allows the walls of veins to be much thinner, less elastic, and less muscular than the walls of arteries.

Diseases Atherosclerosis– Literally, “hardening of the fatty stuff.” High fat diets can lead to formation of fatty plaques lining blood vessels. These fatty areas can become calcified and hard leading to arteriosclerosis, hardening of the arteries. When blood vessels become less stretchable, blood pressure rises and can result in heart and kidney damage and strokes. Double cheese bacon burger, anybody?

Myocardial infarction (MI)– You know we are talking about heart muscle, right, myocardial? An infarction is blockage of blood flow resulting in death of muscle tissue.

Layman’s language for this is a “heart attack.”

The blockage occurs in one of the arteries of the heart muscle itself, a coronary artery.

Depending upon how much tissue dies, a victim of an MI may survive and undergo cardiac rehabilitation, strengthening the remaining heart muscle, or may die if too much muscle tissue is destroyed. Did you exercise at the gym this week?

Mitral prolapse, stenosis, regurgitation– Blood flows through four chambers in the heart separated by one-way valves. A major valve is the one separating the upper and lower chambers on the left side of the heart. The left side is especially important because freshly oxygenated blood returning from the lungs is circulated out of the heart to the rest of the body. The left valve, called atrioventricular, for the chambers it separates, is also called the mitral valve, because it is shaped like an upside down Bishop’s hat, a miter. If the flaps of this valve tear away due to disease, the process is called prolapse, “a falling forward.” This results in leakage and backward flow called “regurgitation” (get the picture?). Sometimes a valve is abnormally narrow causing partial obstruction constricting flow. Stenosis means “a narrowing.”

Angina pectoris– Literally, “pain in the chest.” But, this is a special kind of pain associated with the heart and is distinctive as “crushing, vise-like”, and often accompanied by shortness of breath, fatigue and nausea. Anginal pain indicates not enough blood is getting to the heart muscle, and the heart is protesting and begging for more. People with a history of angina often take nitroglycerine tablets to

relieve the pain by increasing blood flow to the heart muscle.

Arrhythmia/dysrhythmia– Abnormal heart rates and rhythms all have special names like ventricular tachycardia, fibrillation, but generically are termed arrhythmias or dysrhythmia, meaning “no rhythm” and “abnormal rhythm.” There are fine distinctions between the two, but they are often used interchangeably.

Ischemia– Sometimes the heart muscle is not getting enough blood flow, more importantly, the oxygen the blood carries is insufficient to sustain muscle which has a very high metabolic rate, and oxygen demand. The term loosely means “not quite enough blood.” Typically, the patient suffers angina pain (see above) and they may think they are having a heart attack. And, they may be!

Lymphatic System

The lymphatic system is a network of tissues and organs that help rid the body of toxins, waste and other unwanted materials. The primary function of the lymphatic system is to transport lymph, a fluid containing infectionfighting white blood cells, throughout the body.

Blood Blood is composed of a solid portion called plasma. Blood cells make up 45% of the total blood volume, and plasma makes up the other 55%.

The solid portion of blood is composed of three different types of cells: • • •

Erythrocytes - also called red blood cells (RBCs).

Leukocytes - also called white blood cells (ABCs).

Thrombocytes - also called clotting cells, cell fragments, or platelets.

Diseases and disorders of the lymphatic system Diseases and disorders of the lymphatic system are typically treated by immunologists.

Vascular surgeons, dermatologists, oncologists and physiatrists also get involved in treatment of various lymphatic ailments. There are also lymphedema therapists who specialize in the manual drainage of the lymphatic system.

The most common diseases of the lymphatic system are enlargement of the lymph nodes (also known as lymphadenopathy), swelling due to lymph node blockage (also known as lymphedema) and cancers involving the lymphatic system, according to Dr. James Hamrick, chief of medical oncology and hematology at Kaiser Permanente in Atlanta.

When bacteria are recognized in the lymph fluid, the lymph nodes make more infectionfighting white blood cells, which can cause

swelling. The swollen nodes can sometimes be felt in the neck, underarms and groin, according to the NLM. Lymphadenopathy is usually caused by infection, inflammation, or cancer. Infections that cause lymphadenopathy include bacterial infections such as strep throat, locally infected skin wounds, or viral infections such as mononucleosis or HIV infection, Hamrick stated.

Lymphoma is cancer of the lymph nodes. It occurs when lymphocytes grow and multiply uncontrollably. There are a number of different types of lymphoma, according to Dr. Jeffrey P. Sharman, director of research at Willamette Valley Cancer Institute and medical director of hematology research for the U.S. Oncology Network.

Castleman disease is a group of inflammatory disorders that cause lymph node enlargement and can result in multiple-organ dysfunction, according to the Castleman Disease Cooperative Network. While not specifically a cancer, it is a similar to a lymphoma and is often treated with chemotherapy. It can be unicentric (one lymph node) or multicentric, involving multiple lymph nodes.

Lymphangiomatosis is a disease involving multiple cysts or lesions formed from lymphatic vessels, according to the Lymphangiomatosis & Gorham's Disease Alliance. It is thought to be the result of a genetic mutation.

Respiratory System

The respiratory system is the system in the human body that enables us to breathe.

The act of breathing includes: inhaling and exhaling air in the body; the absorption of oxygen from the air in order to produce energy; the discharge of carbon dioxide, which is the byproduct of the process.

Nose Pharynx

STRUCTURES

Epiglotis

Bronchus

Lungs

Bronchioles

Larynx

Alveolis

Trachea Diaphragm

Exchange and transportation of gases in the lungs The transportation of gases is a very efficient process. Oxygen gets carried by haemoglobin of the red blood cells since it has a great affinity for oxygen. Each hemoglobin molecule binds to four molecules of oxygen. This oxygen that is picked up by hemoglobin gets transported with the blood to various tissues. As carbon dioxide is more soluble in water than oxygen, they are transported in the dissolved form in our blood, while some are also transported by haemoglobin. Not all of the carbon dioxide formed is expelled from the body as some of it react with water to form compounds useful for life processes.

Breathing

Breathing starts at the nose and mouth. You inhale air into your nose or mouth, and it travels down the back of your throat and into your windpipe, or trachea. Your trachea then divides into air passages called bronchial tubes. For your lungs to perform their best, these airways need to be open during inhalation and exhalation and free from inflammation or swelling and excess or abnormal amounts of mucus.

As the bronchial tubes pass through the lungs, they divide into smaller air passages called bronchioles. The bronchioles end in tiny balloon-like air sacs called alveoli. Your body has over 300 million alveoli. The alveoli are surrounded by a mesh of tiny blood vessels called capillaries. Here, oxygen from the inhaled air

passes through the alveoli walls and into the blood.

After absorbing oxygen, the blood leaves the lungs and is carried to your heart. Your heart then pumps it through your body to provide oxygen to the cells of your tissues and organs. As the cells use the oxygen, carbon dioxide is produced and absorbed into the blood. Your blood then carries the carbon dioxide back to your lungs, where it is removed from the body when you exhale.

Hairs in your nose help filter out large particles.

Microscopic hairs, called cilia, are found along your air passages and move in a sweeping motion to keep the air passages clean. But if harmful substances, such as cigarette smoke, are inhaled, the cilia stop functioning properly, causing health problems like bronchitis. Mucus produced by cells in the trachea and bronchial tubes keeps air passages moist and aids in stopping dust, bacteria and viruses, allergy-causing substances, and other substances from entering the lungs.

Smoking is bad for your health. Smoking harms nearly every organ of the body.

Cigarette smoking causes 87 percent of lung cancer deaths. It is also responsible for many other cancers and health problems. These include lung disease, heart and blood vessel disease, stroke and cataracts. Women who smoke have a greater chance of certain pregnancy problems or having a baby die from sudden infant death syndrome (SIDS). Your smoke is also bad for other people - they breathe in your smoke secondhand and can get many of the same problems as smokers do.

Damage to the respiratory system from cigarette smoking is slow, progressive, and deadly. A healthy respiratory system is continuously cleansed. The mucus produced by the respiratory tubules traps dirt and disease-causing organisms, which cilia sweep toward the mouth, where it can

be eliminated. Smoking greatly impairs this housekeeping. With the very first inhalation of smoke, the beating of the cilia slows.

With time, the cilia become paralyzed and, eventually, disappear altogether. The loss of cilia leads to the development of smoker's cough. The cilia no longer effectively remove mucus, so the individual must cough it up. Coughing is usually worse in the morning because mucus has accumulated during sleep.

Tobacco Consumption Consequences Cigarette smoking harms nearly every organ in the body. It has been conclusively linked to cataracts and pneumonia, and accounts for about one-third of all cancer deaths. The overall rates of death from cancer are twice as high among smokers as nonsmokers, with heavy smokers having rates that are four times greater than those of nonsmokers. Foremost among the cancers caused by tobacco use is lung cancer—cigarette smoking has been linked to about 90 percent of all cases of lung cancer, the number one cancer killer of both men and women. Smoking is also associated with cancers of the mouth, pharynx, larynx, esophagus, stomach, pancreas, cervix, kidney, bladder, and acute myeloid leukemia.

In addition to cancer, smoking causes lung diseases such as chronic bronchitis and emphysema, and it has been found to exacerbate asthma symptoms in adults and children.

Exposure to high doses of nicotine, such as those found in some insecticide sprays, can be extremely toxic as well, causing vomiting, tremors, convulsions, and death.

In fact, one drop of pure nicotine can kill a person. Nicotine poisoning has been reported from accidental ingestion of

insecticides by adults and ingestion of tobacco products by children and pets.

Death usually results in a few minutes from respiratory failure caused by paralysis.

Cocaine Consumption Consequences

When short-term use crosses the line into long-term use, the risks increase for new and exaggerated negative results. These lasting health risks illustrate the drastic impact cocaine has on the abuser's physical health.

The potential health consequences of longterm use include: •Chronic, extreme fatigue.

•Unrelenting headaches.

•Abdominal pain.

•Nosebleeds.

•Significant weight loss.

•Bloodborne diseases such as HIV and hepatitis from unsafe injection use.

•Heart arrhythmias and heart attack.

•Cardiac arrest.

•Widespread ischemic vascular disease.

•Stroke.

•Seizures.

•Respiratory arrest.

•Death.

Not only does the method of ingesting cocaine alter the immediate side effects, it also leads to different long-term effects:

•Snorting cocaine can lead to losing your sense of smell, irritation of the nasal septum, sniffling, nosebleeds, and hoarseness. •Injecting cocaine can lead to puncture marks, collapsed veins, localized and systemic infection and allergic reactions.

Cocaine use over a long time period can also lead to addiction, depression, isolation from family and friends, psychosis, paranoia, and severe respiratory infections.

Alcohol Consumption Effects Drinking too much – on a single occasion or over time – can take a serious toll on your health. Here’s how alcohol can affect your body: Brain: Alcohol interferes with the brain’s communication pathways, and can affect the way the brain looks and works. These disruptions can change mood and behavior, and make it harder to think clearly and move with coordination.

Heart: Drinking a lot over a long time or too much on a single occasion can damage the heart, causing problems including: • • • •

Cardiomyopathy – Stretching and drooping of heart muscle Arrhythmias – Irregular heart beat Stroke High blood pressure

Research also shows that drinking moderate amounts of alcohol may protect healthy adults from developing coronary heart disease.

Liver: Heavy drinking takes a toll on the liver, and can lead to a variety of problems and liver inflammations including: • Steatosis, or fatty liver • Alcoholic hepatitis • Fibrosis • Cirrhosis Pancreas: Alcohol causes the pancreas to produce toxic substances that can eventually lead to pancreatitis, a dangerous inflammation and swelling of the blood vessels in the pancreas that prevents proper digestion.

Cancer: Drinking too much alcohol can increase your risk of developing certain cancers, including cancers of the: • •

Mouth Esophagus

• • •

Throat Liver Breast

Immune System: Drinking too much can weaken your immune system, making your body a much easier target for disease. Chronic drinkers are more liable to contract diseases like pneumonia and tuberculosis than people who do not drink too much. Drinking a lot on a single occasion slows your body’s ability to ward off infections – even up to 24 hours after getting drunk.

Hallucinogens Immediate effects The effects of hallucinogens can last several hours and vary considerably, depending on the specific type of hallucinogen. Some of the typical effects of hallucinogens are: •feelings of euphoria; •blurred vision; •sense of relaxation and well-being; •hallucinations and distorted perception, including visual, auditory, body, time and space; •disorganized thoughts, confusion and difficulty concentrating, thinking or maintaining attention; •anxiety, agitation, paranoia and feelings of panic; •dizziness; •blurred vision; •loss of coordination; •increased breathing rate; •increased heart rate and blood pressure; •irregular heartbeat, palpitations; •nausea and vomiting; •increased body temperature and sweating, may alternate with chills and shivering;

•numbness.

Long-term effects The most common long-term effect of hallucinogen is the 'flashback'. Flashbacks are a re-experience of the drug and can occur days, weeks, months and even years later.

Flashbacks can be triggered by the use of other drugs, or by stress, fatigue or physical exercise. The flashback experience can range from being pleasant to causing severe feelings of anxiety. They are usually visual and last for a minute or two.

Stimulants Short-term Effects The short-term effects of stimulants include exhaustion, apathy and depression—the “down” that follows the “up.” It is this immediate and lasting exhaustion that quickly leads the stimulant user to want the drug again. Soon he is not trying to get “high,” he is only trying to get “well”—to feel any energy at all.

Long-term Effects Stimulants can be addictive. Repeated high doses of some stimulants over a short period can lead to feelings of hostility or paranoia. Such doses may also result in dangerously high body temperatures and an irregular heartbeat.

Depressants Short-term Effects •Slow brain function •Slowed pulse and breathing •Lowered blood pressure •Poor concentration •Confusion •Fatigue •Dizziness •Slurred speech •Fever •Sluggishness •Visual disturbances •Dilated pupils •Disorientation, lack of coordination •Depression •Difficulty or inability to urinate •Addiction Higher doses can cause impairment of memory, judgment and coordination, irritability, paranoia,3 and suicidal thoughts. Some people experience the opposite of the intended effect, such as agitation or aggression.

Using sedatives (drugs used to calm or soothe) and tranquilizers with other

substances, particularly alcohol, can slow breathing and the heart rate and even lead to death.

Long-term Effects Long-term use of depressants can produce depression, chronic fatigue, breathing difficulties, sexual problems and sleep problems. As a dependency on the drug increases, cravings, anxiety or panic are common if the user is unable to get more.

Withdrawal symptoms include insomnia, weakness and nausea. For continual and high-dose users, agitation, high body temperature, delirium, hallucinations and convulsions can occur. Unlike withdrawal from most drugs, withdrawal from depressants can be life-threatening.

These drugs can also increase the risk of high blood sugar, diabetes, and weight gain.

The Nervous System

The nervous system is a complex collection of nerves and specialized cells known as neurons that transmit signals between different parts of the body. It is essentially the body’s electrical wiring.

Structurally, the nervous system has two components: the central nervous system and the peripheral nervous system.

Functions The nervous system allows us to perceive, comprehend, and respond to the world around us. The nervous system also operates the body’s essential physiologic functions, such as breathing and digestion.

The nervous system has two major parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The central system is the primary command

center for the body, and is comprised of the brain and spinal cord. The peripheral nervous system consists of a network of nerves that connects the rest of the body to the CNS. The two systems work together to collect information from inside the body and from the environment outside it. The systems process the collected information and then dispatch instructions to the rest of the body, facilitating an appropriate response.

Neurons

Neurons come in many different shapes and sizes. Some of the smallest neurons have cell bodies that are only 4 microns wide. Some of the biggest neurons have cell bodies that are 100 microns wide. (Remember that 1 micron is equal to one thousandth of a millimeter!).

Neurons are similar to other cells in the body because: 1. Neurons are surrounded by a cell membrane.

2. Neurons have a nucleus that contains genes.

3. Neurons contain cytoplasm, mitochondria and other organelles.

4. Neurons carry out basic cellular processes such as protein synthesis and energy production.

However, neurons differ from other cells in the body because: 1. Neurons have specialize cell parts called dendrites and axons. Dendrites bring electrical signals to the cell body and axons take information away from the cell body.

2. Neurons communicate with each other through an electrochemical process.

3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

The Neuron

One way to classify neurons is by the number of extensions that extend from the neuron's cell body (soma).

Bipolar neurons have two processes extending from the cell body (examples: retinal cells, olfactory epithelium cells).

Pseudounipolar cells (example: dorsal root

ganglion cells). Actually, these cells have 2 axons rather than an axon and dendrite. One axon extends centrally toward the spinal cord, the other axon extends toward the skin or muscle.

Multipolar neurons have many processes that extend from the cell body. However, each neuron has only one axon (examples: spinal motor neurons, pyramidal neurons, Purkinje cells).

Neurons can also be classified by the direction that they send information.

•

• •

Sensory (or afferent) neurons: send information from sensory receptors (e.g., in skin, eyes, nose, tongue, ears) TOWARD the central nervous system.

Motor (or efferent) neurons: send information AWAY from the central nervous system to muscles or glands.

Interneurons: send information between sensory neurons and motor neurons. Most

interneurons are located in the central nervous system.

There are several differences between axons and dendrites:

Axons •

• • • • •

Take information away from the cell body Smooth Surface Generally only 1 axon per cell No ribosomes Can have myelin Branch further from the cell body

Dendrites •

•

•

• • •

Bring information to the cell body Rough Surface (dendritic spines) Usually many dendrites per cell Have ribosomes No myelin insulation Branch near the cell body

What is inside of a neuron? A neuron has many of the same organelles such as mitochondria, cytoplasm and a nucleus, as other cells in the body. •

Nucleus - contains genetic material (chromosomes) including information for cell development and synthesis of proteins

• • •

•

• •

necessary for cell maintenance and survival. Covered by a membrane.

Nucleolus - produces ribosomes necessary for translation of genetic information into proteins Nissl Bodies - groups of ribosomes used for protein synthesis.

Endoplasmic reticulum (ER) - system of tubes for transport of materials within cytoplasm. Can have ribosomes (rough ER) or no ribosomes (smooth ER). With ribosomes, the ER is important for protein synthesis.

Golgi Apparatus - membrane-bound structure important in packaging peptides and proteins (including neurotransmitters) into vesicles.

Microfilaments/Neurotubules - system of transport for materials within a neuron and may be used for structural support.

Mitochondria - produce energy to fuel cellular activities.

Central Nervous System

The central nervous system (CNS) is made up of the brain, the spinal cord, and the optic nerves. The central nervous system controls thought processes, guides movement, and registers sensations throughout the body.

The spinal cord is a single continuous structure that goes from the brain through the base of the skull and down the spinal column. Individual paired spinal nerves continue down to the tailbone. The optic nerves are at the back of the eye and carry visual information from the eye to the brain.

CENTRAL NERVOUS SYSTEM:

BRAIN The brain is the control center of the body. It has a wrinkled appearance due to bulges and depressions known as gyri and sulci. There are three main brain divisions: the forebrain, the brainstem, and the hindbrain. The forebrain is responsible for a variety of functions including receiving and processing sensory information, thinking, perceiving, producing and understanding language, and controlling motor function. The forebrain contains structures such as the thalamus and hypothalamus which are responsible for such functions as motor control,

relaying sensory information, and controlling autonomic functions. It also contains the largest part of the brain, the cerebrum. Most of the actual information processing in the brain takes place in the cerebral cortex. The cerebral cortex is the thin layer of gray matter that covers the brain. It lies just beneath the meninges. Below the cortex is the brain's white matter, which is composed of nerve cell axons that extend from the neuron cell bodies of gray matter. White matter fiber tracts connect the cerebrum with different areas of the brain and spinal cord.

The midbrain and the hindbrain together make up the brainstem. The midbrain is the portion of the brainstem that connects the hindbrain and the forebrain. This region of the brain is involved in auditory and visual responses as well as motor function.

The hindbrain extends from the spinal cord and contains structures such as the pons and cerebellum. These regions assist in maintaining balance and equilibrium, movement coordination, and the conduction of sensory information. The hindbrain also contains the medulla oblongata which is responsible for controlling such autonomic functions as breathing, heart rate, and digestion.

CENTRAL NERVOUS SYSTEM:

SPINAL CORD

The spinal cord is a cylindrical shaped bundle of nerve fibers that is connected to the brain.

The spinal cord runs down the center of the protective spinal column extending from the neck to the lower back. Spinal cord nerves transmit information from body organs and external stimuli to the brain and send information from the brain to other areas of the body. The nerves of the spinal cord are grouped into bundles of nerve fibers that travel in two pathways. Ascending nerve tracts carry sensory information from the body to the brain. Descending nerve tracts send information pertaining to motor function from the brain to the rest of the body.

Peripheral Nervous System

The peripheral nervous system (PNS) is one of the two major components of the body's nervous system. In conjunction with the central nervous system (CNS), the PNS coordinates action and responses by sending signals from one part of the body to another. The CNS includes the brain, brain stem, and spinal cord, while the PNS includes all other sensory neurons, clusters of neurons called ganglia, and connector neurons that attach to the CNS and other neurons.

Divisions of the Peripheral Nervous System The PNS can also be divided into two separate systems: the autonomic nervous system and the somatic nervous system.

Autonomic Nervous System The autonomic nervous system regulates involuntary and unconscious actions, such as internal-organ function, breathing, digestion, and heartbeat. This system consists of two complementary parts: the sympathetic and parasympathetic systems. Both divisions work without conscious effort and have similar nerve pathways, but they generally have opposite effects on target tissues.

The sympathetic nervous system activates the "fight or flight" response under sudden or stressful circumstances, such as taking an exam or seeing a bear. It increases physical arousal levels, raising the heart and breathing rates and dilating the pupils, as it prepares the body to run or confront danger.

These are not the only two options; "fight or flight" is perhaps better phrased as "fight or flight or freeze," where in the third option the body stiffens

and action cannot be taken. This is an autonomic response that occurs in animals and humans; it is a survival mechanism thought to be related to playing dead when attacked by a predator. Posttraumatic stress disorder (PTSD) can result when a human experiences this "fight or flight or freeze" mode with great intensity or for large amounts of time.

The parasympathetic nervous system activates a "rest and digest" or "feed and breed" response after these stressful events, which conserves energy and replenishes the system. It reduces bodily arousal, slowing the heartbeat and breathing rate. Together, these two systems maintain homeostasis within the body: one priming the body for action, and the other repairing the body afterward.

Somatic Nervous System The somatic nervous system keeps the body adept and coordinated, both through reflexes and voluntary action. The somatic nervous system controls systems in areas as diverse as the skin, bones, joints, and skeletal muscles. Afferent fibers, or nerves that receive information from external stimuli, carry sensory information through pathways that connect the skin and skeletal muscles to the CNS for processing. The information is then sent back via efferent nerves, or nerves that carry instructions from the CNS, back

through the somatic system. These instructions go to neuromuscular junctions—the interfaces between neurons and muscles—for motor output.

The somatic system also provides us with reflexes, which are automatic and do not require input or integration from the brain to perform. Reflexes can be categorized as either monosynaptic or polysynaptic based on the reflex arc used to perform the function. Monosynaptic reflex arcs, such as the knee-jerk reflex, have only a single synapse between the sensory neuron that receives the information and the motor neuron that responds. Polysynaptic reflex arcs, by contrast, have at least one interneuron between the sensory neuron and the motor neuron. An example of a polysynaptic reflex arc is seen when a person steps on a tack—in response, their body must pull that foot up while simultaneously transferring balance to the other leg.