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The endocrine system is the collection of glands that produce hormones that regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood, among other things.
The endocrine system refers to the collection of glands of an organism that secrete hormones directly into the circulatory system to be carried towards a distant target organ. The major endocrine glands include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, gastrointestinal tract and adrenal glands. The endocrine system is in contrast to the exocrine system, which secretes its hormones using ducts. The endocrine system is an information signal system like the nervous system, yet its effects and mechanism are classifiably different. The endocrine system's effects are slow to initiate, and prolonged in their response, lasting from a few hours up to weeks. The nervous system sends information very quickly, and responses are generally short lived. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. The field of study dealing with the endocrine system and its disorders is endocrinology, a branch of internal medicine.
The field of Endocrinology, the study of hormones.
Hormones are chemical messengers secreted into the blood by specialized cells.
- Hormones act on their target cells in one of basic ways
- 1. controlling the rates of enzymatic reaction
- 2. controlling the the transport of ions or molecules across cell membranes
- 3. controlling gene expression and synthesis of protein.
Autocrine signaling is a form of cell signaling in which a cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on that same cell, leading to changes in the cell. This can be contrasted with paracrine signaling, intracrine signaling, or classical endocrine signaling.
An example of an autocrine agent is the cytokine interleukin-1 in monocytes. When interleukin-1 is produced in response to external stimuli, it can bind to cell-surface receptors on the same cell that produced it.
Paracrine signaling is a form of cell-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behavior or differentiation of those cells. Signaling molecules known as paracrine factors diffuse over a relatively short distance (local action), as opposed to endocrine factors (hormones which travel considerably longer distances via the circulatory system), juxtacrine interactions, and autocrine signaling. Cells that produce paracrine factors secrete them into the immediate extracellular environment. Factors then travel to nearby cells in which the gradient of factor received determines the outcome. However, the exact distance that paracrine factors can travel is not certain.
Cytokines are a broad and loose category of small proteins (~5–20 kDa) that are important in cell signaling. They are released by cells and affect the behavior of other cells, and sometimes the releasing cell itself. Cytokines include chemokines, interferons, interleukins, lymphokines, tumour necrosis factor but generally not hormones or growth factors (despite some terminologic overlap). Cytokines are produced by a broad range of cells, including immune cells like macrophages, B lymphocytes, T lymphocytes and mast cells, as well as endothelial cells, fibroblasts, and various stromal cells; a given cytokine may be produced by more than one type of cell.They act through receptors, and are especially important in the immune system; cytokines modulate the balance between humoral and cell-based immune responses, and they regulate the maturation, growth, and responsiveness of particular cell populations. Some cytokines enhance or inhibit the action of other cytokines in complex ways.They are different from hormones, which are also important cell signaling molecules, in that hormones circulate in much lower concentrations and hormones tend to be made by specific kinds of cells.They are important in health and disease, specifically in host responses to infection, immune responses, inflammation, trauma, sepsis, cancer, and reproduction.
Hormones Half Life
The rate of hormones breakdown is indicated by a hormones half life in the circulation, the amount of time required to reduce the conscentration of hormones by one half.
A Hormone that controls the secretion of another hormone is known as tropic hormone.
Tropic hormones often have names that end with the suffix - tropin- as in gonadotropin. The gonadotropin are hormones that are tropic to the gonads.
3 Types of Hormone Interaction
1. Synergism - two or more hormones interact at their target so that the combination yields result that is greater than the additive. In other words, the combine effect of 2 hormones is greater than the sum of the effects of the 2 hormone individually.
2.Permissive - one hormone cannot fully exert its effect unless a second hormone is present.
3. Antagonistic - two molecule work against each other, one diminishing the effectiveness of the other. Antagonism may result when two molecules complete for the same receptor.
A gland is an organ in an animal's body that synthesizes a substance such as hormones for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).
1. Hormone excess - if a hormone is present in excessive amount, the normal effects of the hormone is exaggerated. Most instances of hormone excesses are due to hypersecretion.
2- Hormone deficiency - occurs when too little hormone is secreted (hyposecretion).
- 3. Abnormal responsiveness of a target tissue to a hormone.
- Down Regulation - if hormone secretion high for an extended period of time, target cells my down regulate (decrease the number) of receptors in an effort to diminish their responsiveness to excess hormone.
Receptor and Signal transduction Abnormalities - if a mutation alter the sequence of the receptor, the cellular response to receptor-hormone binding maybe altered. In other mutation, the receptor may be absent or completely non-functional.
Neurons or Nerve Cells carry electrical signals rapidly and in some cases over long distances. They are uniquely shaped cells and most have long thin extension, or processes that can extend up to a meter in length.
These processes are usually classified as either;
- a. dendrites-which receives incoming signal
- b. axons - which carry outgoing information
In most pathways, neurons release chemical signals, called neurotransmitters, into the extracellular fluid to communicate with the neighboring cells.
The nervous system can be divided into 2 parts.
1. Central Nervous Systmem (CNS) consist of brain and spinal cord.
2. Peripheral Nervous System (PNS) consist of sensory neurons and efferent neurons.
The region where the axom terminal meets its target cell is called a synapse. The neuron that delivers a signal to the synapse is known as the presynaptic cell, and the cell that receive the signal is called postsynaptic cell.
Glial cells provides physical support to the neurons and provide important biochemical support.
The peripheral nervous system (PNS) has two types of Glial cells;
- a. Schwann cells - support and insulate axons by forming myelin. In addition to providing support, the myelin acts as insulation around axons and speeds up their signal transmission.
- b. Satellite Cell - is a nonmylinating Schwann cell. Satellite cell form supportive capsule around nerve cell bodies located in the ganglia.
The Central Nervous System (CNS) has four types;
a. Oligodendrocytes - is a myelin forming glial in the CNS, it support and insulate several axon by forming myelin.
b. Astrocytes- has multiple roles , synapses, where they take up and release chemicals, provides neurons with substrates for ATP production, maintain homeostasis in the CNS extracellular fluid by taking K+ and water, act as blood-brain barrier that regulate the movement of materials between blood and extracellular fluid.
c. Microglia - are specialized immune cells that reside permanently in the CNS. When activated they remove damaged cell and foreign invaders.
d. Ependymal Cells- that create a selectively permeable epithelial layer that separates the fluid compartments of the CNS. The ependyma is one source of neural stem cell, immature cells that can differentiate into neurons and glial cells.