DNA Replication

DNA replication is the process of creating an exact copy of a molecule of DNA. This replication is said to be semi-conservative, because each new molecule of DNA contain one strand from the original molecule, and one new parent strand.
Replication starts at a specific nucleotide sequence, called replication origin. An enzyme called helicase, binds to the DNA molecule and unravel it. An enzyme called primase construct a short stand called primer. This primer will be the starting point for the attachment of new nuclotides, which is a job of the enzyme called  DNA polymerase, in a process called elongation.
However, the DNA is anti parallel. The replication can occur continuously in one strand called leading strand and in other one lagging strand is replicated in short segments called Okazaki fragment. To end this process DNA ligase, spliced all this together.

DNA vs RNA

• Sugar: The RNA sugar component is ribose, instead of deoxyribose.
• RNA does not have the nucleotide thymine (T), however, in its place is the nucleotide uracil (U).
• RNA is single stranded.

Structure of DNA

DNA is formed by long chains of nucleotides. Each nucleotide is formed by a phosphate group, a five-carbon sugar (deoxyribose), and a base, which can be adenine (A), guanine (G), cytosine (C), and thymine (T).

Chargaff's rule affirms that the amount of Adenine and Thymine, as well as guanine and cytosine, are always going to be similar.

The chains of nucleotides that form DNA are bound together in spiral shape. Giving it the name of double helix.

In DNA, there is a pattern called complementary base pair, which says that adenine will always bound with thymine, while guanine will always bound with cytosine. 

The two strands of DNA are antiparallel, which means they run in different directions.

Meiosis

Meiosis is the process that produces haploid cells from diploid cells. Meiosis have two key outcomes:

• Reduction Division: It produces daughter cells with fewer chromosomes than the parent cells.
• Recombination: The products of meiosis have different combinations of genes. Genetic recombination give rises to genetically distinct offspring. 

Meiosis is divided into two phases: Meiosis 1 and meiosis 2.

Interphase: Interphase is the same for both meiosis and mitosis. There is the growth, genetic duplication, and formation of structures necessaries for duplication.

Prophase 1: In prophase 1, each pair of homologous chromosomes, in this case non-sister chromatids, align side by side, forming the synapsis. non-sister chromatids exchange pieces of chromosomes, in a process of crossing over.

Metaphase 1: The spindle fibre guide the tetrads to the center of the cell. There they line up in homologous pairs. Each homologous of the pair is positioned in each side of the equator.

Anaphase 1: Homologous pairs are divided from one another, generating lonely chromosomes. 

Telophase 1: DNA is not duplicated in this part.

Meiosis 2: Identical process as mitosis.

Nondisjunction

Sometimes, chromosomes or chromatids do not separate as they should during anaphase 1 or anaphase 2.

• In Anaphase 1, nondisjunction occurs when homologous chromosomes pairs do not separate to opposites poles, instead they all go to they same pole.
• In anaphase 2, nondisjunction occurs when sisterchromatids, fail to be divided, and they go to the same pole together. 

Mitosis

The process of cell division have important functions, which are growth of the body, replacement, and repair. To accomplish each of these function, each daughter cell must have exactly the same genetic information as the parents. For this to happen, the genetic material of the parent cell must be correctly duplicated, chromatin must be condensed into chromosomes, and one set of chromosomes must be divided into each of two nuclei.

• Prophase: This is the first phase of the four of mitosis. during prophase, the chromatin condenses into chromosome. The nuclear membrane breaks down, as well as the nucleolus. Centrioles start to move to the poles, and spindles fibres start to be formed. 
• Metaphase: The spindle fibres guides the chromosome to the center of the cell, the equator line. Each sister chromatid faces one pole.
• Anaphase: centromeres are splits apart and the sister chromatids separate from each other.
• Telophase: This is the final phase of mitosis. Chromatids starts to transform into chromatids. Nuclear membrane and nucleolus are back. Centrioles divids.
• Cytokinesis: The cytoplasm divides. 

Cell Cycle

Cell cycle is the life cycle of a cell. Their duration vary among the different types of cells. A single cell cycle is the set of events before each division. It relates a parent cell, which is the original cell and the daughter cells, which are the new cells that a are formed in each cell division.
The cell cycle is divided into two phases: The growth phase and the division phase. Each of them are divided in other phases.

The Growth Phase


The growth phases is also called the interphase, and it is divided into 4 distinct phases.

• G1 (gap 1) phase: The G1 phase is the period where the cell grows.
• G0 phase: This only happens in certain cells. G0 phase is basically a phase where the cell stop growing and doesn't look forward to divide.
• S (synthesis) phase: In this phases the DNA is copied
• G2 (gap 2) phase: Period at which there is more growth of the cell, as well as other structures are formed.

The Division Phase

The division phase is divided into two phases:

• Mitosis: The division of genetic material and the contents of the nucleus into two different sets.
• Cytokinesis: The division of the the cytoplasm and the other organelles. 

Technologies

• in vitro fertilization: This process is also called, fertilization in test tube.Which means that fertilization occurs in  a solution that is not inside the mother. Once cleavage happens, the embryo is introduced to the uterus for implantation.
• Ultrasound: High frequency waves are passed over the fetus to determine fetal position or size.
• Amniocentesis: Needle is inserted into the uterus, and some cells from the amniotic fluid are remove. This process is used to identify any type of abnormalities.
• CVS: Chorionic cells are analyzed.
• GIFT: Solution with sperm and ovum is mix and then transferred to the fallopian tubule.

Lactation

Lactation is controlled by hormone. Prolactin, the hormone needed for milk production, is not secreted during pregnancy, due to the high levels of estrogen and progesterone. After birth, however, the pituitary of the mother starts to produce prolactin. Thus, milk is made.
When the baby sucks the mother's nipples, sensory neurons send the information to the hypothalamus, that will produce oxytocin, and then send to the pituitary, whom will release it. The oxytocin stimulates the movement of the milk, by causing the mammary lobules to contract.

Labour

Labour is also known as parturition. This event typically begins with uterine contractions.
Uterine contraction occurs during the whole pregnancy, however, as close as it gets to the big moment, the contractions become stronger and more frequently.
The onset of labour includes hormonal and neural components. Everything start by the stretch of the cervix, which starts the release of the hormone oxytocin, from the posterior pituitary gland. The oxytocin stimulates the contraction, that will bring the baby downwards, stretching the cervix even more, repeating the cycle.

Teratogens

As well as the mother is able to pass nutrients and other beneficial substance, It is possible for the mother to pass harmful substances that can be very bad for the fetus.
Almost all the substances that the mother ingest or inhales can be passed through the placenta. This is specially significant in the first nine weeks, when the embryo is very sensitive to environmental factors.
Teratogens is the name given to any substance that causes structural abnormality to the baby. Cigarette smoke, for example, prevents enough oxygen to reach the baby.
One of the most damaging teratogens is called alcohol. Alcohol can affect the fetus brain, nervous system and physical development. The term that is used to describe all alcoholic disorders is called FASD. This includes  the fetal alcohol syndrome (FAS).
Some prescriptions of medicines are consider teratogens.

Structures of Support

• Amnion: Transparent disk that form from cells of the embryonic disk. It surrounds the embryo completely and it is only penetrated by the umbilical cord. The amnion is filled with the amniotic fluid that protects the baby from injures and drastic temperature changes. 
• Yolk sac: Yolk sac in humans is responsible to provide the embryo blood cell, while it can produce by its own. It also contributes for the formation of the digestive tract.
• Allantois: Part of the umbilical cord. It is used to clean the waste of the baby.
• Chorion: The membrane that is part of the placenta.
• Placenta: disk shaped organ that is rich in blood-vessels. It transport nutrients to the fetus, cleans its waste. transport oxygen, secretes hormone, and transport antibodies from the mother into the fetus.

Embryonic Development

In medicine, the prebirth period is divided into three trimesters. They are divided into two periods:

• Embryonic period of development: This period of development takes place in the first two third of the first trimester, which means the first 8 weeks. During this time, cells are been divided and tissues and organs are been formed, as well as structures that support and nourish the developing embryo. 
• Fetal period of development: This period starts in the ninth month until the birth. During the fetal period, the body growth rapidly and organs start to work, formatting organ systems.

Fertilization

Fertilization is the starting point of the human development. It involves the joining of male and female gametes to form a single cell that have 46 chromosomes, 23 from each parent.

In the female body, after the egg is release of the follicle, thanks to the hormone LH, it start its journey to the uterus in the oviduct, with the help of muscular contractions and the wavelike movements of the scilia, that lines in the walls of the oviduct. It takes about four days to reach the uterus, so the egg needs to be fertilized in this period of time.

Millions of sperms get into the vagina, they need to survive a long path until reach the ovum. Some die due to acid in the vagina, or because of the female's immune system, some however, take the wrong way.

The ovum is protected by a membrane that also provides it energy. In order to penetrate this membrane, the sperm cells come with the acrosome, that have enzymes. Hundreds of sperm are necessary to make a pathway through the membrane, that's why the first sperm to reach the ovum, is not always the first one to get inside it. Once one sperm cells get inside, the membrane of the ovum depolarize and prevent the other perms to get in.

As time passes by the genetic material of the gametes mix and his result in the formation of a zygote.

Cleavage

As the egg is being fertilized, it keeps moving in the oviduct. During the journey to the uterus, many events can be observed in the zygote. 

In short period of time, the zygote start to divide into different numbers of cells. It divides so fast that the size of the zygote remains the same. The process of division without the enlargement of the cells is called cleavage.

When the number of cells reach 16, the zygote is now called morula. When the morula reaches the uterus it starts to be filled by liquid from uterus. As the fluids gets in the embryo, it start a formation of two different group of cells. 

The entire structure is called blastocyst. One group of cells called trophoblast forms the outer layer of the blastocyst. The trophoblast will form a membrane called chorion, that will become part of the placenta.

The other group of cells are called the inner cell mass, or embryoblast. They will develop in the embryo.

The balstocyst will attach to the endometrium, with the inner cell mass against it. The trophoblast will secrete enzymes that digest some of the tissues and blood vessels of the endometrium. In a process called implatation, the blastocystslowly sinks into the endometrium. After this process is done, the woman is said to be pregnant.

About the time that the implantation occurs, the trophoblast starts to secret the hormone hCG. This hormone has the same function as the LH. It maintains the corpus luteum alive, that will keep the level of estrogen and progesterone high, which will prevent the endometrium to be broken down. After a couple of months, the corpus luteum can be degenerated, since the placenta can secret enough estrogen and progesterone by its own.

Tissue Formation

When the process of implantation is completed, the inner mass cell starts to change. A space starts to be formed between the trophobalst and the inner mass cells. This space is called amniotic cavity, and will soon be filled with fluid, which the embryo will be floated in.

As the amniotic cavity forms, the inner mass cells start to from a disk, called the embryonic disk. The embryonic disk is formed by three layer: ectoderm, endoderm, and mesoderm. These are called the primary germ layers, and the process of their formation is called gastrulation. The disk is naw called gastrula.

The ectoderm will form our skin, nervous tissue, teeth, eye lens, and etc. The mesoderm will form the blood vessels, muscles tissues, bones, heart and etc. The endoderm will form the respiratory and digestive tract, among other structures.

Gastrulation marks the initiation of the morphogenesis, that is the series of events that form distinct structures of the developing organism. Differentiation is the cellular process which enables the cells to develops in a certain shape and perform a certain function.

Organ Formation 

During the thrid week, a group of cells starts to be developed along the back of the embryonic disk. This cells are gonna be transformed in the baby's back and also a structure called notochord. The notochord will be the basic framework of the skeleton. The nervous system develops from the ectoderm that is just aboce the notochord. The cells above the notochord begin to thick, than they will form a tube, this tube will form the brain and the spinal cord. This process of the formation of this tube is called neurulation, and starts the organ formation. Soon after the neurulation, the heart starts to be formed.

After the 8 week, 90% of the organs are formed, and the embryo can now be called fetus.   

Female Reproductive System - Hormonal Regulation

Human females follow a cyclical pattern called the menstrual cycle. The menstrual cycle is responsible to release the ovum in the same period as the uterus is most receptive to a fertilized egg.

The menstrual cycle usually takes 28 days to be completely, however it may very from women to women. The cycle is said to begin with menstruation and end with the start if the next menstrual period.

The menstrual cycle is said to be divided in two different, but interconnect, set of events. One takes place in the ovaries and its called the ovarian cycle, while the other takes place in the uterus, and its known as the uterine cycle.

The Ovarian Cycle

Each follicle contains a immature ovum inside. Many follicles degenerate during a girl's life time. In a single ovarian cycle, only on follicle will release a ovum, and then it will turn to a yellowish structure called corpus luteum, which will eventually degenerate. 

The ovarian cycle can be divided into two stages:

•  Follicular Stage: It all begins with a increase in the level of FSH in the body. This FSH will stimulate the development of one follicle. As the follicle matures, It releases some of the hormone called estrogen and some progesterone. As the level of estrogen increases, the production of FSH decreases and increases the production of GnRH which will stimulate the production of LH. The LH will trigger the release of ovum from the follicle (ovulation). 
• Luteal Stage: Once the ovum is released, the LH causes the follicle to develop into a corpus luteum. The corpus luteum will secrete some progesterone and estrogen, which will inhibit the production of LH. When the copus luteum degenerates, there is a decrease in the level of estrogen and progesterone. The low levels of these hormones will cause the production of FSH, which will trigger the cycle all over again. 

If the ovum is fertilized and implanted in the endometrium, the level of estrogen and progesterone will remain high in order of the hormone hCG, secreted by the embryo-supporting membrane. Progesterone will maintain the endometrium to support the developing fetus, while estrogen prevents other follicle to mature. 

The Uterine Cycle

If fertilization occurs, it take a few days until the zygote gets to the uterus. Thus, the uterus and endometrium must be prepared. There is a build up of tissues and blood vessels in the endometrium. If fertilization doesn't occur, the endometrium will disintegrates and menstrual cycle will begin all over again.

The uterine cycle start in the same day as the ovarian cycle. As a new follicle starts to be developed and the levels of estrogen and progesterone start to increase, which will cause the endometrium to begin thickening. If the corpus luteum degenerates and fertilization does not occur, the levels of estrogen and progesteron decreases causing the endometrium to break down.

Male Reproductive System - Hormonal Regulation

During a life time, a male individual is able to produce millions of sperm cells. The hormones that triggers this production, are also responsible for the regulation of the male reproductive system.

The release of the hormone GnRH, by the hypothalamus, is able to stimulate the pituitary gland to produce and secrete FSH and LH.

FSH is a hormone that stimulate the testes to produce sperm cells and also to secret the hormone called inhibin, that will act in the negative feedback loop to inhibit the production of FSH. In another hand, LH is not affected by the inhibin. It continuously stimulates the production of testosterone, which will work at the negative feedback loop to inhibit the production of LH. 

Male Reproductive System - Maturation

Puberty is  the period where the reproductive system is fully developed and it is now able to be used. During the period, the boy goes under several physical changes.

Puberty begins when the hypothalamus secretes GnRH. This hormone acts on the anterioor pituitary gland, causing it to release two different sex hormones: FSH and LH. These hormones causes the sperm to testes to produce sperm and testosterone.

Most men experience a condition called andropause, which is the gradual decline of testosterone due age. The symptoms are fatigue, depression, loss of muscle.  

Sexually Transmitted Infection (STI)

HIV/AIDS


AIDS is a disease caused by a group of virus that are called human immunodeficiency virus, or HIV. This virus attacks a particular type of white cell, the helper T cells, which form part of the immune system.  As the level of white cells decrease, the individual becomes more vulnerable to infections that may lead to sickness and death.
HIV is transmitted through sexual contact with a infected individual, shared needles or from mothers that are infect that infect their children during birth or breast-feeding.
There are no cure for HIV. Treatments can alleviate the symptoms of some diseases. The best way to combat HIV is to prevent transmission.

Hepatitis

Hepatitis is actually a group of diseases: hepatitis A, B and C. They are all viral infections. Hepatitis A is acquired by drinking contaminated water. Hepatitis B is spread sexually, or by contact with infected blood. Hepatitis C is transmitted by the contact with infected blood.
The symptoms of Hepatitis B are fever, headache, nausea, loss of appetite, and abdominal pain. If propagated it can cause liver failure, liver cancer, and even death. Some people can recovery completely, however, other becomes asymptomatic, which doesn't show any symptom but still can transmitted the disease.
There are vaccine to prevent Hepatitis.

Genital Herpes

Genital herpes is a very common viral STI. ONE OUT OF THREE SEXUALLY ACTIVE PEOPLE IN CANADA HAS GENITAL HERPES!
Genital herpes is caused by two different viruses: HSV 1, commonly causes infection of the mouth, and HSV 2 that caused genital herpes and is transmitted by genital contact.
The most common symptom is the appearance of blisters in infected areas. It can cause to the individual flu-like symptoms. There is no cure for herpes!

Human Papilloma Virus (HPV)

A group of virus responsible for the genital warts. It is transmitted skin-to-skin contact, which means, condoms cannot prevent this disease. This disease may lead to cancer.

Chlamydia

Chlamydia is a sexual disease caused by a bacterium. It may cause fever and burning pain while urinating. When the woman doesn't treat it, it can be passed to the cervix or the oviduct, which can lead to pelvic inflammatory disease, blocking the oviducts.
Treatments with antibiotics can cure completely Chlamydia.

Gonorrhea

Bacteria that transmits gonorrhea 

It is a bacterial disease.. It causes infections that can lead to painful urination. It can lead to PID and affect the heart and brain. Antibiotic are the cure.

Syphilis

Another bacterial disease. It format rashes in the skin. It can also affect the cardiovascular and nervous system. A infected person can become mentally ill, blind, or acquire heart disease. It can be cured with antibiotics.

Sperm Cell vs Egg Cell

• Size: The sperms cell are very small compared to the ovum.
• Energy Reserves: Before ejaculation the sperm is provided with fat, and after ejaculation the sperm use the fructose provide by the fluid secreted by the seminal vesicles, thus, the sperm cell can live for three to five days. In another hand, the egg can only live for one day if not fertilized. If fertilized, it will be implant in the endometrium, which will provide enough energy.
• numbers produced: Sperm are continuously produced, while eggs are not as much.
• Motility: The sperm are motile, however the eggs cannot move by themselves.
• Outer structures: The sperm head has a cap called acrosome, that contains enzymes that will help the sperm to enter the egg. The egg has a outer coating, which in most cases can only be penetrated by the sperm of the same specie. 

Female Reproductive System

The female reproductive system does not mass-produce gonads. The ovaries only produce one egg, or ova.
The ovaries contain the follicles, structures that the ovum develops from. When the ovum is released from the follicle, it is called ovulation.
Structures called fimbriae continually sweep over the ovary and send the ovum to the oviduct, which will direct the ovum to the uterus. The oviduct is the site of fertilization

Male Reproductive System

The males reproductive system produce a special type of cell called sperm cell. The sperm cells are produced by the seminiferous tubules inside the testes.
The scrotum is a structure responsible for holding the testes. It regulates the temperature of the gonads, thus, allowing the production of sperm.
Interstitial cells are the cells responsible for the production of the male hormone testosterone. It lies right next to the seminiferous tubules.
The Sertoli cells are a the ones responsible for nourishing and support the developing sperm cells.
The sperm cells are going to develop fully at a structure called the epididymis, from there it goes on the ductus deferens which will take them to the ejaculatory duct and then the penis.

Seminal Fluid

As the sperm cells passes through the ducts deferens, it is mixed with different fluids.
Seminal vesicles produce a fluid that provide sugar (fructose) therefore energy for the sperm cells. The prostate gland and Cowper's gland produce a alkaline fluid that will protect the sperm from the acid in the urethra.

Reproductive System

Both males and females have gonads, which are structures that produce reproductive cells -- sperm and and eggs. These cells are called gametes.
The gonads also produce sex hormones, which are chemical messengers that control the development and function of the reproductive system.
The structures that play a direct role in reproduction are called the primary sex characteristics. The ones that don't play a directly role are called secondary sex characteristics.

Pancreas - Endocrine Function

Pancreas is formed by islets of Langerhans, which are clusters of specials cells.
There are two type of  these special cells beta cells and alpha cells.
Beta cells secrete Insulin, which takes action when the blood glucose level rises. Insulin makes the target cells more permeable to glucose and stimulate them to convert glucose into glycogen and store it.
Alpha cells secrete Glucagon. Stimulates the liver to produce glycogen back to glucose. It is used when the level of sugar is low.

Disorders 

Diabetes: There are two types of diabetes, type 1 (juvenile) which is the attack to the beta cells from the immune system, and type 2 (adult -onset) which can be the malfunction of the beta cells, or the body is not responded well to the insulin as it should.
Hyperglycemia: Condition generated when there is high level of blood sugar in the body.

Adrenal Glands

Our body is composed of two adrenals glands. They are located just above our kidney.
Each gland is composed of two parts: the inner layer, called the adrenal medulla, and the outer layer, called adrenal cortex.

The Adrenal Medulla


The adrenal medulla produces two very related hormones: epinephrine (adrenaline) and norepinephrine (noradrenaline). These hormones are responsible for short-term responses, which is also referred to as the fight-or-flight response. These hormones increase the breathing rate, heart rate, blood pressure, blood flow to the heart, dilation of the pupils and etc.

The Adrenal Cortex

• Cortisol: When brain interpreters a situation as dangerous, the hypothalamus secretes the hormone ACTH that targets the adrenal cortex, which cause the release of the stress hormone cortisol. It breaks down the protein of the muscles into amino acids, and also lipids.
• Aldosterone: It stimulates the kidney to reabsorb sodium into the bloodstream, which will eventually raise the blood pressure. If the Aldosterone be damaged, it will cause the Addison's disease that will cause weight loss, sodium-potassium  irregular and etc. 

Parathyroid Gland

Parathyroid glands are four small structure located over the thyroid gland. They produce the hormone parathyroid hormone (PTH). This hormone works when the concentration of calcium in the blood is too low. It stimulates the bones to release calcium and kidney to reabsorb Ca from the urine.

Thyroid Gland - Thyroxine and Calcitonin

Thyroid Gland is responsible for secreting and produce the hormones responsible for our metabolism rate. One of these hormones is called thyroxine (T4).
The thyroxine will increase the rate of metabolism of  fats, proteins and carbohydrates for energy. It stimulates the cells of several tissues through out the body. It also play an important role in the growth and development of children.

Disorders

If the thyroid fails to develop properly during childhood, the children will develop a condition called cretinism which will result in shorter individual with metal problems. Other condition called hypothyroidism is caused by the insufficient   production of thyroxine. Over production will lead to hyperthyroidism which will lead the individual to weight loss, heat intolerance, and irregular heart beat. Graves'disease is a severe state of hyperthyroidism that is result from the immune system attacking the thyroid gland.
The thyroxine production is controlled by a negative feedback loop. The anterior pituitary gland release the TSH which will cause the thyroid gland to secrete thyroxine. The thing is that thyroxine needs iodine to be produced. If there is no iodine, thyroxine level in the blood will never be reaches, therefore TSH will always be produced, generating the condition called goitre.

Calcitonin

Calcitonin is essential hormone used to control the calcium concentration in the blood. If it is to high, it will stimulates the bones to not release calcium and the kidney to not reabsorb calcium form urine.

Human Growth Hormone (hGH)

Molecule of hGH

The human growth hormone affects the tissues all around the body. It can work as a direct hormones or, most of the times, as a tropic hormone.  This hormone is very important to the body development. It is responsible for the protein synthesis, cell division and growth of cartilage, and metabolic breakdown.  It also stimulates the growth of bones and muscles.

Disorders

If the pituitary gland secretes too much of hGH during childhood, the individual will have a condition called gigantism. If the secretion of hGH doesn't stop after the skeletal growth is completed, the individual will have a condition called acromegaly. The bones cannot develop any taller, but they get larger, thus the face widen, the ribs ticked, and feet and hand enlarge. This condition can also cause cardiovascular diseases, sugar intolerance, breathing problems and muscle weakness.
The insufficient hGH production during childhood results in dwarfism which the stature of the individual will be very small compared to the average.

Pituitary Gland

Pituitary gland is a very small gland located in our brain. Despite of its small size, the pituitary gland release important hormones for the body, many of them are tropic hormones, giving itself the nickname of the "master gland"

The pituitary gland is divided in two lobes, posterior and anterior pituitary.

The posterior lobe is not part of the endocrine system. It is actually a apart of the nervous system. It does not produce any hormones, however it stores and secrete some very important hormones, that are produced by the hypothalamus, ADH and oxytocin.

The anterior gland belongs to the endocrine system. It produces and secret six major hormones, like hGH and LH.

Endocrine System

As we learned before, all the systems within our body works in order to maintain homeostasis. The endocrine system is responsible specially for the distribution of hormones, which are chemical messengers. The endocrine system has a slower, however, longer effects in the body than the nervous system. Nevertheless, these two systems has their similarities:

• Some nervous system tissues actually secrete hormones, for example the hypothalamus.
• Several chemicals works as both neurotransmitters and hormones, depending on their location in the body.
• Both systems are regulated by negative feed back loops.
• The regulation of several process inside our body involves the conjunction of these two systems.

Hormone Action

The cells that are effected by the hormones are called target cells, they have what is called receptors proteins that will bind with hormones. The result of this bind is the start of several chemical reaction within the cells.

There are two types of hormones: lipid-soluble hormone and water-soluble hormone 

Lipid-soluble hormones, also called as lipid-based hormone or steroid hormones, can diffuse through the lipid bilayer of cell membrane. The receptors proteins for these hormones are located inside the cell. 

Water-soluble hormone, also known as amino-acid based hormones are unable to diffuse through the cell membrane, so their receptors proteins are located out side the cell.

The action of hormones is regulated by negative feedback loops, that works when a certain blood concentration of hormone is reached, or when target cells have responded to specif hormones, the endocrine gland will inhibit the releasing of the hormone.

The Ear - Balance and Coordination

Three very important structures -- the semicircular canal, utricle, and saccule-- found at the inner ear are responsible for our balance.
The semicircular canal is responsible to detect body and head rotation (rotational equilibrium). For such a task, these structures are filled with liquid and bulge. Each bulge is filled with stereiocilia. When the head or the body rotates, the fluid inside the semicircular canals move and bends the stereocilia, causing the hair cells to send rotational information to the brain.
Utricle and Saccule, which together make up the fluid-filled vestibule. These structures are responsible for the balance required while moving the head forward and backward. These structures are filled with calcium carbonate granules, called  otoliths. When the head move up or down, the gravitation brings down the otoliths which will make the stereocilias to bend.
Proprioceptors are another type of mechanoreceptor involved in coordination. They are found in our muscles and joints throughout our body, and they send information of our position to the brain. That is why we can dress in the dark.

The Ear - Hearing

The ear is divided into three divisions: Outer ear, middle ear, and inner ear. The outer ear consists by the pinna and the auditory canal. Pinna is the flap of cartilage that focus the sounds vibrations into the ear. The auditory canal is a pathway that amplifies the sound, as well as prevent the ear from infections due to its hair.
The middle ear is formed by the tympanum, ossicles and the oval window. The tympanum, also called eardrum, transforms the sound waves into mechanical energy, and passed to tiny bones called ossicle. This bones amplifies the sound a lot. They are: Hammer, anvil and stirrup.
The ear also count with a tube that connects the tongue and  the ear. The Eustachian tube works to equalize the pressure in the ears.
The inner ear consists of three structures: the semicircular canals, the vestibule, and the cochlea.
The cochlea is used for hearing. It is within the cochlea that mechanical energy turns into electochemical impulses and are translated by the nervous system. The cochlea is filled with fluid. Wen the oval window vibrates, waves a made.
In the middle of Cochlea, there is the organ of Corti, which is known as the organ for hearing. Along the base of the organ of Corti, there is the basilar membrane, which is the location of the hair cells, that have structures called stereocilia. The end of the stereocilia are attached to the tectorial membrane. When the waves hit the basilar membrane, it goes up and down, bending the streocillia, that will be felt by the hair cells that will send a neural impulse to the brain.

The Eye

The eyes is divide in several parts, each of them with different functions. The sclera is a tough and white membrane that is responsible for protection of the eye. The sclera becomes the cornea, a transparent membrane, which bend the light rays through a set of muscle called the iris, and passes through the opening called pupil, to finally reach into the lens, flexible structure that is used to focus the light into the retina, thin and inner membrane, which is formed by the photoreceptors rods and cones.
The eye is also formed by the choroid, which is a membrane filled with blood vessel that will nourish the cells of the eyes, and also the choroid will absorb the light that is not absorbed by the photorecepetors. Ciliary muscles, which will help to shape the lens. Vitreous and aqueous humour, they will maintain the shape of the eyes, as well as nourish them.


Photoreceptors

The photoreceptors found in the eye are called rods and cones. The rods are extremely sensitivity to intensity of the light, it doesn't distinguish color, but it does enable us to determine shapes in the dark.  The rods contain a light-absorbing pigment called rhodopsin. When the rods absorb light, rhodopsin is broken down which will release a neural transmission to the brain.
Cones  are the color detecting of the eye. They are found mostly at the fovea centralis, however, few of them can be found around the retina (peripheral vision). They require a relatively intense light to stimulate the,. There are three tyoes of cones, red, blue and green. A similar process of neural transmission happens with the cones, however, the pigments is different and only respond to certain wavelengths of light.

Transmission

The rods and cones are found in the layer that is closest to the choroid.  They synapse with bipolar cells in the middle layer. When the pigments of rods and cones are broken down they stop sending inhibitory neurotransmitters, the bipolar cells then send a neural impulse to the ganglion cells, that form the optic nerve, and will transmit visual images to the occipetal lobe of the brain.  

Disorders

• Glaucoma: This condition is formed if the ducts that provide the eye with aqueous humour plugged. This will build up pressure, and it can harm the dedicated blood vessels in the eye to rupture, which will affect the cells in the eyes, causing blindness.
• Astigmatism: A condition generated to an uneven curvature of part of the cornea.
• Myopia: Also known and nearsighted, it is caused by the eye balls that are too short, thus, every time an individual with myopia view a distant object, the rays focused in front of the retina. The use of concave lens will help.
• Hyperopia: Farsighted, short eyeball, rays focus behind the retina when seeing close object. The use of convex lens will help.
• Color blindness: disorder caused by the lack or deficiency of particular cones, specially red or green.

Sensory Reception

In our organism, special cells, and nerve endings, called sensory receptors, are stimulated for certain types of stimulus and send them to the brain in form of electrochemical impulses. Summarizing, sensory receptors start the neural impulse in our body.
Sensation is the feeling we have when the neural impulse reach our brain -- feeling cold in the face -- however the interpretation of the feeling is known as perception -- Jack feels colder than Juan. Sensory adaptation is the process by which our brain chose to ignore information from sensory receptors, for example our noses are seem by the eye, but our brain choose to ignore it. 

Sensory Receptors 


able to transform the stimuli to electrochemical energy, which can be translated by our nervous system
Our sensory receptors are specialized cells that are able to detect certain types of stimuli. In humans, sensory receptors are divide into four categories: photoreceptors, chemoreceptors, mechanoreceptors, and thermoreceptors. Each type is able to convert the stimuli they receive into electrochemical energy, so it can be translated by our nervous system.

• Photoreceptors are stimulates by light. They are important for our vision, since the most important photoreceptors are found in our eyes (they are called rods and cones).
• Chemoreceptors are stimulated by chemicals, the tongue and nose contain certain types of chemoreceptors and they are responsible to recognize taste and smell. Other chemoreceptors are found inside our body. 
• Mechanoreceptors respond to mechanical force in form of pressure. Our skin is full of these receptors. It is also responsible for the hearing, balance and body position.
• Thermoreceptors respond to the changes in temperature.  

Peripheral Nervous System

Peripheral Nervous System

The Peripheral nervous system is composed by the nerves that forms the nervous system. It is divided into two categories: Somatic System and Autonomic System 

Somatic System

The somatic system is related to voluntary control, it includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. These nerves send information from and to the CNS to and from the body related to voluntary movements -- for example, when we are done reading a page of a book and we plan to turn to the next page, our somatic system is being used in this situation.

Autonomic System

The autonomic system is responsible for the involuntary control. It helps to maintain homeostatic by adjusting the body to variations in the external and internal environments. There are two divisions for the autonomic system:

• Sympathetic Nervous System: This system is used  in stressful situations and is often referred as "fight-or-flight" response.  When activated it decreases the energy consume of other system that are not going to be used in a stressful situation, for example the digestion. Blood pressure and heart beat increase.
• Parasympathetic Nervous System: System that is activated when the body is calm and at rest. Restores energy. Referred as rest-and-digest response. Slows heart rate, reduces blood pressure, promotes the digestion of food. It is also responsible for stimulating genital organs.

The Cerebrum

The cerebrum is divided into grey and white mass. The grey mass is known to be cerebral cortex, and that's where our neural information are processed and analyzed. The cerebral cortex is highly convulated, which gives the brain a greater surface area.

The cerebrum is divided into two sides, called cerebral hemisphere. They are connected by a bundle of white matter called corpus callosum, which is responsible to send information from one hemisphere to another.

It is known that each side of the brain is responsible for a certain activities. The right hemisphere is related to artistic skills -- drawing, painting , intuitive thinking and etc -- while the left side is related with the logical thinking -- math, solving problems, writing, and etc --.

Divisions of the Cerebrum

The cerebrum is divided into four lobes:

• Occipital lobe: Receive and processes visual information.
• Parietal lobe: receive sensory information from skin, and process information about the body position.
• Temporal lobe: involved in auditory reception
• Frontal lobe: associated with conscious thought, intelligence, memory and personality.

Broca's Area and Wernicke's Area

Both of these areas are found in the left side of the brain. Broca's Area is responsible for coordinating the muscles relating to speaking, and also translate thought into speech, while wernicke's area is responsible for the comprehesion of speech. 

The Brain - Divisions

The brain is divided into different regions that are each responsible for a series of of processes. These parts are:

• Cerebellum: Not a part of the brain, however, it's very related to it. It is responsible for equilibrium, unconscious coordination of posture, reflexes and motor skills -- when you kick a ball for example, your cerebellum is working so that your body find a posture whether you won't fall and still kick the ball.
• Medulla Oblongata: Medulla oblongata is related specially with automatic, involuntary responses, such as heart rate, constriction of the blood vessels, blood pressure, rate and depth of breathing, swallowing and coughing.
• Pons: The pons is known to be a very important relay centre.
• Thalamus: Thalamus is known as the greater relay center of the brain.
• Hypothalamus: Hypothalamus is a very important part of the brain. It participates in the coordination some involuntary response, such as blood pressure, heart rate, body temperature, thirsty, hunger, and emotions. The hypothalamus is also the major link that connect the nervous system with the endocrine system.
• Cerebrum: The cerebrum is the largest part of the brain. It is responsible for our intellect, memory, consciousness, and language. 

The Brain - Protection

Skull

The skull forms a protective bony armour around the brain that will prevent the brain to be hit by any external object.

Meninges

Meninges are three layers of tough, elastic tissues that surround the brain, providing it nourishment and protection. There are three types of meninges in mammals: the outer layer is known as the dura mater, than comes the arachnoid layer, and the inner layer is called pia mater. 

Blood-brain barrier 

The blood that reaches the brain is not the same as the blood that passes through the body. There is a barrier, created by the meninges, that prevent this contact between the brain and the blood. These blood-brain barrier prevents the entrance of infectious particles, however, is permeable to necessary substances like oxygen and other nutrients. 

Some harmful substances are able to pass through the blood-brain barrier, like alcohol and other molecules generated by the consumption of drugs.

Cerebrospinal Fluid

Between the arachnois layer and the pia mater, a special fluid floats around, nourishing the brain and being able to absorb the impact, therefore protecting the brain. This fluid is called Cerebrospinal fluid.

The Spinal Cord

The spinal cord is a column of nerves that is considered to be a vital link between the brain and the peripheral nervous system. Nevertheless, the spinal cord is also known to be the primary reflex center, coordinating incoming and outgoing information.
The spinal cord is formed by white matter and grey matter.
The delicate tissues of the spinal cord is protects by the cerebrospinal fluid, a series of soft tissues, and a set of backbones known as vertebrae. Injury to this area can cause paralysis.

Signal Trasmission

Signal Transmission

At any signal transmission between neurons, there is the participation of the presynaptic neuron, which will send the information through the synaptic cleft, and the postsynaptic neuron that will receive this information.
Chemical messengers called neurotransmitters  carry the information form the presynaptic neuron to the postsynaptic neuron. They can also carry the signal from one neuron to an effector.
When the action potential reaches the end of the axon of the presynaptic neuron, it makes sacs that contain neurotransmitter -- synaptic vesicles -- to fuse with the membrane of the neuron. Then they release their neurotransmitter through exocytosis.
These neurotransmitter will reach the receptors of the postsynaptic neuron, which will start a action potential.
Neurotransmitter have either excitatory or inhibitory effect on the neuron. The difference is which channels they will open (excitatory will open the sodium gates, while inhibitory will open potassium gates.)
Acetylcholine is an example of excitatory neurotransmitter. It will cross the neuromuscular junction and will make the muscle contract. To breakdown the neurotransmitter, enzyme are necessary. That is the job for cholinesterase.

Action Potential

Resting Membrane Potential

In a resting neuron, the charge inside its membrane is relatively negative compared to the exterior. This charge separation across the membrane is called the membrane potential.
The membrane potential across a membrane of a resting neuron is called the resting membrane potential, which is around -70 mV. The resting membrane potential provides energy for the generation of a nervous impulse due to any kind of stimuli.
The process of generation a resting membrane potential is called polarization. Neurons become polarized do to several process that happen at the same time.
Large protein molecules that are negatively charge are found in the intracellular fluid, but not outside. These proteins cannot pass through the cell membrane. The neural membrane is permeable to small negatively ions, such as chlorine.
The biggest contributor to the resting membrane potential is called sodium-potassium exchange pump. This system, along with the use of ATP, is able to exchange sodium ions (negative) with potassium ions (positive).
The sodium-potassium exchange pump exchanges three sodium ions for two potassium ions. As a result, the outside becomes positive. However, these ions can leak slowly by diffusion, messing up the whole system, that why there is a constant exchange, keeping the -70 mV.

Action Potential 

To a nerve impulse is composed by a series of action potentials.
Action potential is divided into three main phases:

1. Depolarization: when the membrane potential is reduced to less the -70 mV, this membrane is said to be depolarized. If the membrane potential is reduced to -55 mV or less, a dramatic change occur in the neuron, called action potential. Action potential is said to be "all-or-none" effect because if the membrane potential become between -55 mV and -70mV, nothing really happens. That is why -55 mV is such a important mark to the  Action potential. Thus, we call -55 mV, the Thershold potential. During this process, voltage-gated sodium channels open in a way that let a lot of sodium ions in the membrane, creating an extreme positive membrane potential.
2. Repolarization: As a result the voltage-gated potassium gates open and let potassium ions in, which will take out the cell the positive gradient, creating a less negative environment inside the neuron. Therefore, there is a return to its previous polarization.
3. Refractory Period: Period at witch the neuron cannot receive any order action potential.

Neurons

Neurons are formed by dendrites, structures that receive information from other neurons or sensory receptors, cell body, region where metabolic reaction happens and nucleus is found, axon, longest part of the cell, take the information away from the cell body and lead to the axon terminal, region of transmitting information.

Some neurons have their axons surrounded by a special layer of glial cells called Schawann cells, these layers are called Myelin Sheath. The space between Myelin Sheath are called node of Ranvier. Myelin sheath will nourish and protect the neuron, as well will speed up the nervous impulse transmission.
Myelinated neurons ( neurons that have myelin sheath) are known to compose the white matter, while Unmyelinated neurons form the grey  matter.

The Nervous System

 Nervous System

The Nervous System is equipped with several structures that allows it to sense and respond to stimulus from within the body and from the external environment. The Nervous System is very important for the maintenance of homeostasis, since it regulates the body structures and processes by monitoring and controlling them.
The human Nervous System is one of the most complex systems in the environment. It is composed by the brain, spinal cord, and the nerves.
The Nervous System is divided in two different systems: Central Nervous System (brain and spinal cord, receive, processes, and send information) and Peripheral Nervous System (nerves, carry sensory information to the CNS, and send information from the CNS to muscles and glands). The PNS is also divided into different divisions.
The Nervous System is composed by two  special cells: Neurons and Glial cells. Neurons are the basic structural and functional units of the nervous system. They are responsible to respond to any physical or chemical stimuli, conduct electrochemical signals, and release chemicals.
The activity of the neuron is supported by Glial cells, that are responsible for nourishing the neurons, cleaning the waste and protect them from infections.

Causes of Gene Pool Changes

Mutations

HIV virus in action.

Mutations is a change the occur in the DNA of an organism. If this defect is able to be passed through the generations that come, inheritable mutation, it can cause a change in the gene pool of a population. There are three types of mutation, some are neutral, some are beneficial, and most of them are harmful for the individual that has it.
Mutation increase the diversity of a gene pool.  The more genetic variation, caused by mutation, the greater the chances of selective advantage, which will change the environment at some degree.
A good example of mutation in humans is the presence of some individuals that are resistant to the HIV virus, due to a homozygous mutation, which will cause the lack of the functioning receptor of the virus in the cell, which will prevent the individual to be infected.

Gene Flow 

Gene flow describes the change on gene pools due to migration of individuals. It may increase the genetic variation in one population, however, it can decrease in another population -- the one that emigration is occurring.

Wolves are usually the best example of gene flow, since they migrate several times during their life spam. Thus, some wolves in certain region share some genetic traits with wolves from a different region.

Non-Random Mating

Random mating in a population means that there is no prediction of who is going to mate with who, thus leading to a greater genetic variation. However, in the many animal populations, some individual choose to mate with other individuals based on their physical and mental traits. This leads to a non-random mating because it prevents some individuals to breed. Conclusion of this is that some animals will not contribute with the gene pool of the next generation of their specie.
Another example of non-random mating is inbreeding. This is a process which animals that are closely related breed together.

In many population of lions, only the alpha-male is allowed to breed with all the females.

Genetic Drift

Genetic drift is the change in allele frequencies due to chance events in small breeding populations. When only few individuals reproduce in small population it may cause the extinction of certain alleles. A good example of Genetic Drift would be if we took a very small population of five roses ( 3 AA, 1 Aa, and 1 aa). if only the AA type reproduce, in a couple of generation, the allele A will be the only one present in the genetic code of individuals.
Genetic Drift can also occur when the population decreases due to climate changes, disease, or fragmentation of habitat. However, in stable and large population, Genetic Drift are unusual to happen.

The Founder Effect

The founder effect is the process of which few individual start a new and isolated population, limiting the genetic variation. This often occurs on islands. When a really small number of birds start a new population in a isolated island, their offspring will carry genetic information from the parents and other birds from the same species, but that live in different regions. However, as time passes by, the newest generation of the birds on the isolated island are more likely to be very different genetically compared to the birds that are found in other non-isolated regions.

The Bottleneck Effect

Bottleneck Effect is the change in Gene Pool that is a result from a rapid decrease in population size. It usually occurs in species that are close to extinction. this effect can be related  to the Genetic Drift.

Natural Selection and Human Activities

These are very common factors that influence the gene pools in many species. While natural selection "eliminate" weaker alleles, human activities are able to vanish alleles even if they are helpful for the specie.