Human Nervous, Endocrine, and Reproductive Systems
The Human Nervous System
The nervous system in humans is, from a human point of view, the most perfect machine known. It consists of a central nervous system and a peripheral nervous system. The central nervous system consists of the brain and spinal cord.
Brain Development
The brain consists of three initial vesicles: the forebrain, midbrain, and hindbrain. Later, in embryological development, five vesicles originate by subdivision:
- The forebrain gives rise to the telencephalon and diencephalon.
- The midbrain is not divided.
- The hindbrain gives rise to the metencephalon and myelencephalon.
Telencephalon
The telencephalon is subdivided into two hemispheres and contains two olfactory lobes, which are highly developed in fish but gradually reduce in size in more advanced vertebrates. They form the cerebral hemispheres. The mammal brain reaches its highest development. The brain can control movements of the senses to gather information, store memories, and develop complex responses, even using those memories to modulate the final response.
Diencephalon
The diencephalon is a small section that is part of the brain. In the upper epithalamus, which in fish, amphibians, and reptiles has a photoreceptor function. There is also the thalamus, where sensory stimuli are regulated. In the lower region is the hypothalamus, which regulates the activity of the pituitary hormone and body temperature.
Midbrain
The midbrain contains optic lobes in lower vertebrates and the inferior colliculus in mammals.
Metencephalon
The metencephalon gives rise to the cerebellum. This body posture and movement control coordinates the movement. This gall is in the pons, which is a crossing area of nerve pathways in which the fibers from the right side of the body are directed to the left side of the brain. The opposite occurs with fibers from the right side.
Myelencephalon
The myelencephalon originates the brainstem or medulla oblongata. It also regulates visceral activities such as swallowing, heart rate, and respiratory rate.
Spinal Cord
The spinal cord is located inside the spine. Two areas are distinguished by their color and composition: white matter on the outside, formed by the axons of neurons, and gray matter, more internal and butterfly-shaped, formed by cell bodies of neurons and with a hole called the interior ependyma. The gray matter is shaped like a butterfly. You see in the image it consists of dorsal horns (small butterfly wings) where sensory fibers enter and ventral horns (large butterfly wings) from where motor fibers exit. The functions of the spinal cord consist of transmitting information from the sensitive area of the brain and the motor areas. It also performs reflexes, which are quick answers, without the intervention of the brain.
Peripheral Nervous System
The peripheral nervous system consists of nerves that enter and leave the central nervous system and nerve ganglia. It can send information to voluntary muscle contraction or regulate vegetative functions. The set of ganglia and nerves that control these functions form the autonomic nervous system.
Autonomic Nervous System
The autonomic nervous system consists of two subsystems: the sympathetic nervous system and the parasympathetic nervous system. Both systems are, for most of their functions, antagonistic to each other; that is, what one system activates, the other inhibits. The sympathetic nervous system activates functions such as cardiac or respiratory motion. The parasympathetic nervous system acts as an inhibitor in these two tasks. However, the parasympathetic nervous system accelerates intestinal activity, and the sympathetic nervous system inhibits it.
The Endocrine System
The endocrine system is a system of coordination. It receives signals, processes the information received, and produces the appropriate response to be made by hormone receptor bodies. The endocrine system produces slow responses that spread through chemicals called hormones that circulate in the blood and act on bodies that recognize these substances. These organs, called target organs, produce responses consistent with the concentration of hormone detected in the blood. The existence of a hormone can lead to the emergence of structures that would not appear without its presence. Examples are the crest of the rooster or the female sexual tissue of the chimpanzee.
Hormone Secretion
The hormones secreted by cells are usually grouped into organs called glands. Sometimes they are secreted by neurons. In this case, the hormones are called neurohormones. The endocrine system is regulated by negative feedback or feedback inhibition (feed-back):
- The gland receives information for the secretion of the hormone.
- The gland releases the hormone.
- The hormone acts on the organ or target cell, resulting in a change in the internal environment.
- The change in the internal environment is sensed by the secreting gland, and it inhibits the secretion of the hormone until it receives a new order of discharge.
Hormones in Invertebrates
Authentic glands do not appear in invertebrates. Hormones are secreted by nerve cells, so hormones are neurohormones. These hormones are responsible for regulating animal growth and sexual maturation. They can also control color changes, which allow the animal to blend with the environment. The stimulus that produces the hormone secretion is visual. The light changes are detected by the eye. In arthropods, animal growth means that the exoskeleton is changed by a new, larger one. This process is called molting or ecdysis. Molting is controlled by hormonal mechanisms.
Crustaceans
Crustaceans possess neurosecretory cells in the organs called X and Y. Neurohormone secretion by the body X, located in the eyestalks, inhibits molting. Neurohormone secretion by the body Y, located on the antennae, activates molting.
Insects
In insects, a neurohormone secreted by the protocerebrum called neotenin promotes the formation of larval structures and inhibits sexual structures. Also in the protocerebrum, in the body called the heart, there is another neurohormone called ecdysiotropin, acting on a true gland, the prothoracic gland, and induces the release of ecdysone. Ecdysone stimulates the formation of the pupal molt and the appearance of adult characters.
Vertebrate Endocrine System
In vertebrates, the areas of most important hormonal secretion are the hypothalamus, pituitary, thyroid, parathyroid glands, pancreas, adrenal glands, gonads, and placenta. There are also hormone-producing cells scattered throughout the gastrointestinal tract, which produce gastrin in the stomach, secretin, and cholecystokinin in the duodenum and jejunum. The kidney produces renin, which acts by regulating blood pressure. Angiotensin I and angiotensin II are produced in the lung. The mechanism of action is basically the principle of negative feedback.
Hypothalamus and Pituitary
The hypothalamus gland is the coordinator of the entire system. In addition, as part of the nervous system, it has functions of nervous control on body temperature or the state of wakefulness or sleep, in the case of mammals. The pituitary, together with the hypothalamus, is the hypothalamic-pituitary axis, which is the control center of hormone production. The hypothalamus, upon receiving information from the organism, releases a neurohormone called a releasing factor, which acts on the pituitary gland, promoting the secretion of a particular pituitary hormone. Pituitary hormones act on target tissues or organs. The result is a metabolic change in the tissue or organ of the hormone receptor. In the event that the target organ is a gland, the effect will be the production of another hormone. The change in the internal environment is detected by the hypothalamus, and it inhibits the production of neurohormones, which blocks the secretion of hormones from the pituitary. The conditions in the internal environment return to the original situation that triggered the whole process, so the hypothalamus will produce neurohormone again.
Reproduction
The beginning of a new life can occur in different ways, but in any case, sooner or later it leads to the death of the individual. If in that time interval, we call life, that being does not reproduce, the continuity of individuals of the species may be affected. Therefore, reproduction is a mechanism to perpetuate the species. Life is not important to follow the parents; the important thing is the offspring and the genetic information they possess.
Asexual Reproduction
Animals have two possible types of reproduction: asexual reproduction and sexual reproduction. Asexual reproduction is typical of single-celled organisms, algae, fungi, and plants, but few animals are the reproductive tract. In this type of reproduction, one parent results in an individual genetically identical to it, so it is a clone of the parent. The different types of asexual reproduction in animals are budding and fragmentation.
Budding
In budding, some parental individual cells actively divide, forming a bud. This structure may end up separating from the parent to form a single individual or be joined by a structure as part of a colony. This type of reproduction is exhibited by sponges and polyps, solitary or colonial, for example, corals.
Division or Fragmentation
In division or fragmentation, the parent is broken spontaneously (longitudinal or transverse), causing a subsidiary stock. This type of reproduction occurs in polyps, jellyfish, and Platyhelminthes. A special case of fragmentation occurs in polyembryony. This process occurs when, from an early developing embryo, there is a separation of cell groups. Each of these groups creates a complete individual. In this case, the resulting litter will have the same genotype. Polyembryony is typical in the armadillo (mammal toothless). It is the process by which human identical twins arise.
Regeneration
Regeneration is not usually a process of reproduction of the whole individual. It is rather a defense mechanism used by many animals. It consists of disregarding any part of the body with the aim of not being caught by a predator. Later, the lost part of the body is regenerated. This is the case of the tail of the lizard, internal structures, as part of the digestive tract of sea cucumbers, or arms of the starfish. In the latter case, sometimes a new star can emerge from the severed arm. This only occurs if the arm section of the disk is started oral animal. If so, this is true asexual reproduction.
Sexual Reproduction
Sexual reproduction is the most common mode of reproduction made by the animals. It is characterized by the presence of specialized cells called gametes and causes different things to the parents. The male gametes are called sperm and female gametes, eggs. These cells are produced in specialized organs called gonads. The testes are the gonads that produce sperm. The ovaries are the gonads that produce eggs. Gamete formation is triggered by a mechanism called gametogenesis. The process of sperm formation is called spermatogenesis, and the process of egg formation is called oogenesis. Both processes involve a phase of maturity and cell division by meiosis. At the end of the process, the gametes formed have had their chromosome number reduced to allow fertilization and the formation of a new being with the same number of chromosomes as their parents. In the species, there may be individuals of different sexes, males and females, and have the ability to produce male and female gametes, hermaphrodites. In the case of hermaphrodites, such as worms or garden snails, it encourages cross-fertilization. Typically, the gametes come together in a process called fertilization. Other times, no fertilization occurs; from unfertilized female germ cells, the new animal originates. In this case, we have parthenogenesis.
Fertilization and Development
There is not always the union of gametes in sexual reproduction, although the mechanism of reproduction is more common. Fertilization is the union of a sperm and an egg. The cell formed after fertilization will undergo a process called embryogenesis, which is the formation of the embryo. Depending on where you perform, it can be external or internal.
External Fertilization
In external fertilization, sperm and egg are joined outside the animal. Sperm cells are very sensitive to their environment. They must be in an environment with plenty of water to travel to the egg, so this type of fertilization should be done in water or in a very humid environment, as in the case of earthworms.
Internal Fertilization
Internal fertilization occurs within the animal, which is the female in species with separate sexes. To do this, sperm must enter the oviduct. The way to do this can be through a copulatory organ, like the penis, by close contact between oviduct and spermiduct, as in copulation in birds, or the production of spermatophores that are introduced into the oviduct.
Embryogenesis
Fertilization results in an egg or zygote cell. Through a complex process of mitotic divisions, called embryonic development or embryogenesis, the new offspring will form. Embryogenesis is the formation of the embryo from the zygote formed at fertilization. The process is divided into the following phases:
Segmentation
The zygote divides several times, forming a structure called a morula. The process of formation of the morula is performed by successive mitotic divisions. Formed cells are totipotent and are called blastomeres.
Blastulation
The cells of the morula continue to divide and migrate outward, forming a single cell layer that surrounds an inner cavity called the blastocoel. The structure formed is called a blastocyst.
Gastrulation
The blastula cells continue dividing. At a certain point, the cells divide at different rates, leading to a cavity inside the blastocyst. The structure formed is called the gastrula, and the inner cavity is called the archenteron, which opens to the outside through an opening called the blastopore. Thus, cells lining the archenteron belong to the so-called germ layer and endoderm cells were belong to the ectoderm. The gastrula arises in different ways, depending on the type of animal. In triblastic animals, still in the process of gastrulation, a new embryonic worksheet called the mesoderm is created, located between the endoderm and ectoderm. The way to lead the mesoderm varies by type of animal. Sometimes the mesoderm contains an inner cavity called the coelom. Animals with this cavity are called coelomates.
Organogenesis
Organogenesis is the stage where the various tissues and organs that make up the animal will form. Depending on the animal, this phase can be very complex.
Animal Classification Based on Embryonic Development
Depending on where embryonic development occurs, animals are classified as:
- Oviparous: Animals that develop inside an egg.
- Ovoviviparous: Animals that develop inside the egg that is inside the mother’s body, but it can be directly contacted.
- Viviparous: Animals that develop inside the mother, establishing intimate contact with her.
Postembryonic Development
After embryonic development and birth, the development of the animal remains. The postembryonic development may be direct or indirect.
Direct Development
Direct development is to reach sexual maturity without apparent morphological changes, but an increase in size.
Indirect Development
In indirect development, the animal comes from an egg and larval stage; to move to the adult stage, it must undergo marked changes in morphology. Sometimes, there are different larval stages. Sometimes, the development may even go through a phase in which the larva does not feed and is wrapped in a protective structure, forming a cocoon or pupa, built by her as she reached adulthood. In this case, it is said that the development is indirect and complex.
Cloning
A clone is a genetically identical unit to the predecessor unit, which is cloned. The unit can be molecular, cloning a gene, a group of genes, the complete DNA, a cell, tissue, organ, or a whole individual. The clones are produced naturally by asexual division. Cloning raises a number of problems that are still unresolved.
Molecular Cloning
Cloning of molecules can be performed by two processes: acellular cloning or cellular cloning.
Acellular Cloning
Acellular cloning, also known as a mechanism of amplification of DNA or RNA (PCR). This cloning may have two objectives: obtaining a large amount of DNA for different purposes or determining the sequence of a small portion of DNA in solution. Its application is varied. It is used to detect DNA sequences, for DNA sequencing, to scan for mutations, disease diagnosis (parental or otherwise), for evolutionary studies, detection of tumor cells, amplification of DNA for cell cloning, etc.
Cellular Cloning
This system uses cells to clone DNA fragments; it is not a clone of cells. To this end, previously had to be amplified (i.e., get many cloned copies) the DNA to be cloned. Then insert the DNA into vehicles, called vectors, which is transported and introduced into the cells. The name given to these cells is the host, and the cells to be cultivated, that is, get multiplied in a culture medium. The cells then multiply, replicate the DNA itself and the fragment you want to clone. In this way, you get a large number of cells containing the DNA you want to clone, called recombinant. The aim of this type of cloning can be cloned DNA amplification in order to study its sequence, structure, for phylogenetic studies, or for the identification of mutations. This method is also used to study the mechanism of gene regulation, transcription, and translation. Another application is in obtaining the protein encoded by the cloned DNA sequence, either to analyze the structure of the protein, to alter, or sell it according to their properties. This is the technique used to obtain insulin.
Cloning of Cells, Tissues, or Organs
Cloning of cells, tissues, or organs using stem cells are able to form other differentiated cells, which can lead to tissue or organs may be due to their totipotency. With this type of cloning, obtain compatible with the adult cells that could differentiate into various cell types, forming a tissue or put back. Even an organ. This technique can be used for burns, to generate epithelial cells from stem cells and reduce the rejection of skin grafts. We have tried, in cases of diabetes, stem cells enter the pancreas, a gene “normal” producer of insulin. These cells differentiate to form cells of the pancreas and are grafted into the patient. Also, this technique has been used in patients who have suffered heart attacks. Stem cells mature to form makes cardiac muscle cells, are grafted onto the heart and supplement the dead cells, regenerating the damaged area by the stroke.
Cloning of Plant Bodies
Cloning plant bodies has been made in agriculture and gardening for a long time. The method of planting “by cutting” is to take one serving (one branch) of a plant and put it on the ground, waiting to grab and build a root. Thus, clonal plants are obtained from the mother plant with the same characteristics.
Cloning of Animals
For centuries we have sought to improve livestock breeds, selecting individuals with more meat and better producers of milk and wool. In agriculture, the most attractive crop varieties, more resistant and larger, have been selected. The way to get these new individuals have been by crossing parents with desirable characteristics, although this has not always been achieved. The use of hormones to produce an increase in size and faster growth in animals intended for human consumption is currently prohibited by the negative consequences that appear on the health of consumers. It is currently used for cloning of organisms genetically modified organisms, known to non-scientists as GM. It seeks to achieve isolate the genes responsible for fattening, milk production, resistance to infection, resistance to herbicides. The technique consists in obtaining animal cells containing the gene responsible for the desired characteristic. Subsequently, the nucleus of that cell is inserted into an egg, which was previously removed its nucleus. Then you have to get that diploid cell divides to form a new individual, as would a zygote formed by fertilization of egg by sperm. Finally, to develop the embryo must be implanted in the uterus of a female. You can genetically modify the cell nucleus of an animal for subsequent cloning. In this case, the egg that has been extracted from the nucleus is injected into the nucleus with genetic information modified. In the case of plants used cells capable of division. To these are added to the selected genes. Then, the cells are cultured in the laboratory and obtained from them, plants with different characteristics to the initial plant. Plants are cloned.
Problems Arising from Cloning
The cloning problems arise from molecular cloning, cell cloning, and cloning of individuals.
Molecular Cloning Problems
Molecular cloning creates different kinds of problems, depending on the method used:
- In order to clone acellular molecules, it is necessary to know the sequence of DNA that we amplified. If this sequence is not known, one cannot perform the cloning.
- To perform cell cloning of molecules, it is necessary to perform the following steps:
- Knowing the sequence of DNA to be cloned.
- Choose a vector efficiently enough to insert the DNA sequence.
- Find cells that have been recombined by the vector.
- Collect the product encoded by being cloned.
Cell Cloning Problems
Cell cloning also has drawbacks. You have to get stem cells. Stem cells are obtained from:
- Cells generating specific tissue, known as multipotent cells committed or, as the cells that produce blood cells in the bone marrow.
- Totipotent cells, capable of generating any tissue in any cell line. Stem cells can only be obtained in the early stages of cell division in embryonic development. The cells, which we are referring to are the blastomeres displayed by segmentation of the zygote.
To use them you need to work with embryos. This work can be done with animal cells, but not humans, as current law does not allow the use of human embryos. The law wants, with this ban, which does not lead to develop the technology for cloning human individuals. This prohibition includes not being able to decipher the mechanisms by which a totipotent cell becomes a committed, capable of generating only one type of cell line. The ban also prevents investigate therapies for hitherto impossible to cure, such as quadriplegia, or create custom-designed bodies for each patient, without the appearance of rejection in transplantation.
Cloning of Genetically Modified Organisms Problems
The cloning of genetically modified organisms poses two problems:
- A social problem: it has encouraged a rejection of the agencies “transgenic”, since it ignores the influence that genetic change can result in that being in the environment and consumer.
- Technical problems: the technique is still too recent. These technical problems are greater in the cloning of animals in the vegetable.