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Hormones - Body’s little messengers Hormones play a very important role in our life
Hormones and Endocrine System

Hormones {horma: to stir up/to excite/to put into action} play a very important role in our life right from our birth by regulating growth, development and reproduction in many ways and help maintain homeostasis. The moodiness of teenagers, obesity of humans and even the transformation of caterpillar into a butterfly are regulated by hormones.

Millions of people with diabetes all over the world take the hormone insulin. Hormones are also used in cosmetics to keep the skin smooth and in livestock feed to fatten cattle. So our life style, moods and emotions are also influenced by the hormones. The activities in our body are highly complex and they need to be so regulated that every activity takes place at a proper time and in a correct sequence. For example, the gastric juice, bile and pancreatic juice should be poured into the food canal only when there is food in it. Though this kind of regulation is done to some extent by the nervous system but it also brought about by hormones.

Multitude hormones work in the body to keep it balanced! A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism.
How hormones work?

These are the steps below, how hormones come into play

  • Biosynthesis of a particular hormone in a particular tissue
  • Storage and secretion of the hormone
  • Transport of the hormone to the target cells
  • Recognition of the hormone by an associated cell membrane or intracellular receptor protein
  • Relay and amplification of the received hormonal signal via a signal transduction process: This then leads to a cellular response. The reaction of the target cells may then be recognized by the original hormone–producing cells, leading to a down–regulation in hormone production. This is an example of a homeostatic negative feedback loop.
  • Degradation of the hormone.

Hormone cells are typically of a specialized cell type, residing within a particular endocrine gland, such as thyroid gland, ovaries, and testes. Hormones exit their cell of origin via exocytosis or another means of membrane transport.

The hierarchical model is an oversimplification of the hormonal signaling process. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal.

Hormones are responsible for chemical coordination in the body Endocrine system consists of several glands/glandular cells which bring about the overall common function of chemical coordination in the body.
Hormonal Secretions

Hormones are secretions from specific cells or glands in the body and are carried to all parts by the blood and act as chemical signals to regulatory messages within the body. Most hormones are secreted by special glands, like thyroid and pituitary glands called the endocrine glands (endo: inside, crine: secrete) meaning “secrete internally”. They are also called ductless glands because their secretions are poured directly into the blood and not through any special duct.

Certain hormones are also produced from some such glands or other body parts which otherwise have a different primary function; for example, the stomach and duodenum. Hormones are also produced from living cells in an organ such as the beta cells of pancreas which produce insulin and Leydig cells of testis which produce testosterone. Hormone–secreting cells are present in many organs including the heart, thymus, liver, stomach, small intestine, kidney and placenta.

Hormone is a product of living cell that circulates in the body fluids to all parts away from its point of origin but produces a specific effect on the activity of the cells of the target organs only. A hormone may reach all parts of the body, but only certain types of cells, the target cells, are equipped to respond. Thus, a given hormone traveling in the bloodstream elicits specific responses such as a change in metabolism from its target cells, while other cell types are unaffected by that particular hormone. Endocrine glands act in a coordinated manner, activate each other and work as a system of organs called endocrine system.

A system is defined as a complex of organs performing an overall common function. Endocrine system consists of several glands/glandular cells which bring about the overall common function of chemical coordination in the body. The nervous system and endocrine system work in conjunction and some times it is difficult to distinguish them. A part of the brain called the hypothalamus contains neurosecretory cells which release hormones into the blood. The hormones produced by neurosecretory cells are sometimes called neurohormones to distinguish them from the “classic” hormones released by endocrine glands .

Some of the important hormones of the human body Endocrine glands release hormones, chemicals that act as signals telling different parts of the body what to do. The body makes over 20 hormones, each with a different job to do. The blood carries hormones around the body until reaching the target organ, the body part needing it. Hormones can affect the way a person feels. As a person ages, the body makes less of some hormones.
Important Hormones

Three major classes of molecules function as hormones in vertebrates: proteins and peptides (small polypeptides containing up to 30 amino acids); amines derived from amino acids; and steroids.

Most protein/peptide and amine hormones are water–soluble, whereas steroid hormones are not. For ex: Adrenalin is an amino acid, Glucagon is a Peptide, Insulin is a Protein, Thyroid stimulating hormone(TSH) is a Glycoprotein and Testosterone is a Steroid.

Hormones are secreted to regulate the physiological processes by chemical means, affect the enzyme systems of the body and act on target organs or cells usually away from their source. Hormones produced in one species usually show similar influence in other species.

Hormones are produced in very small quantities and are biologically very active. For example, adrenalin is active even in a concentration of 1 part in 300,000,000 parts. Chemically some hormones are peptides (proteins) which are water soluble, some are amines (derived from amino acids) again water – soluble, some are steroids lipid – soluble. Their excess (hypersecretion/ oversecretion) or deficiency (hyposecretion/undersecretion), both may lead to serious consequences. Hormones are not stored in the body and are excreted from the system.

Simple endocrine pathway A simple endocrine pathway a receptor or sensor detects a stimulus and sends the information to a control center.
Hormonal Control and Regulation

The endocrine system and the nervous system act individually and together for internal communication, messaging, control and regulation. The nervous system is made up of specialized cells called neurons, which convey high–speed electrical signals, where as endocrine system is a collection of all hormone secreting cells.

The signals communicated through neurons control the movement of body parts in response to sudden environmental changes, such as occur when you jerk your hand away from a hot pan or when your pupils dilate as you enter a dark room.

Hormones coordinate slower but longer–acting responses to stimuli such as stress, dehydration and low blood glucose levels. Hormones also regulate long–term developmental processes by informing different parts of the body how fast to grow or when to develop the characteristics that distinguish male from female or juvenile from adult. Hormone–secreting organs, called endocrine glands, are referred to as ductless glands because they secrete their chemical messengers directly into extracellular fluid and from there, these chemicals diffuse into the circulation through blood.

Simple neuro hormone pathway Stimulus binds to a sensory neuron, which causes the neuro secretory cell in the hypothalamus to release a substance which is transferred from the blood to target effectors and causes a response.
Chemical Signals

A few chemicals serve both as hormones in the endocrine system and as chemical signals in the nervous system. Epinephrine, for example, functions in the vertebrate body as the so called "fight–or–flight hormone" (produced by the adrenal medulla, an endocrine gland) and as a neurotransmitter, a local chemical signal that conveys messages between neurons in the nervous system.

In addition, the nervous system plays a role in certain sustained responses for example, controlling day/night cycles and reproductive cycles in many animals often by increasing or decreasing secretion from endocrine glands.

Thus, although the endocrine and nervous systems are anatomically distinct, they interact functionally in regulating a number of physiological processes.

Simple neuro endocrine pathway In each pathway, a receptor/sensor (blue) detects a change in some internal or external variable – the stimulus and informs the control center (green). The control center sends out an efferent signal, either a hormone (red circles) or neurohormone (red squares). An endocrine cell carries out both the receptor and control center functions.
Control and regulation

The control/regulation is achieved through positive and negative feedback mechanisms through feedback loops connecting the response to the initial stimulus. In negative feedback, the effector response reduces the initial stimulus, and eventually the response ceases.

This feedback mechanism prevents overreaction by the system and wild fluctuations in the variable being regulated. Negative feedback operates in many endocrine and nervous pathways, especially those involved in maintaining homeostasis. Negative feedback contributes to the hormonal control of blood calcium and glucose levels.

In contrast to negative feedback, which dampens the stimulus, positive feedback reinforces the stimulus and leads to an even greater response. The neurohormone pathway that regulates the release of milk by a nursing mother is an example of positive feedback. Suckling stimulates sensory nerve cells in the nipples, which send nervous signals that eventually reach the hypothalamus, the control center. An outgoing signal from the hypothalamus triggers the release of the neurohormone oxytocin from the posterior pituitary gland. Oxytocin then causes the mammary glands to secrete milk. The release of milk in turn leads to more suckling and stimulation of the pathway, until the baby is satisfied.

Integration between endocrine and nervous system Brain – The ventricles (pink) circulate the cerebrospinal fluid (CSF), which cushions the brain. Two lateral ventricles lie either side of the mid–line of the brain, one in each cerebral hemisphere. They communicate with the fourth ventricle (bottom centre). Beneath the lateral ventricles lie the sensory – processing thalami (orange) and the hypothalamus (green, centre), which controls emotion and body temperature, and releases chemicals that regulate hormone release from the pituitary gland (green, above fourth ventricle, which is pink).
Integration of Endocrine and Nervous Systems

The integration of endocrine and nervous systems results in regulation of the body's activities to adjust to shifting environmental and developmental conditions.

Let us start with hypothalamus and pituitary gland, which control much of the endocrine system. The hypothalamus plays an important role in integrating the vertebrate endocrine and nervous systems. This region of the lower brain receives information from nerves throughout the body and from other parts of the brain, then initiates endocrine signals appropriate to environmental conditions.

In many vertebrates, for example, the brain passes sensory information about seasonal changes and the availability of a mate to the hypothalamus by means of nerve signals; the hypothalamus then triggers the release of reproductive hormones required for breeding.

The hypothalamus contains two sets of neurosecretory cells whose hormonal secretions are stored in or regulate the activity of the pituitary gland, a lima bean–sized organ located at the base of the hypothalamus. No organ illustrates the close structural, functional and developmental relationship between the endocrine and nervous systems better than the pituitary gland.

It has discrete posterior and anterior parts, which are actually two fused glands that develop from separate regions of the embryo and perform very different functions. The posterior pituitary or neurohypophysis, is an extension of the hypothalamus that grows downward toward the mouth during embryonic development. It stores and secretes two hormones that are made by certain neurosecretory cells located in the hypothalamus; the long processes (axons) of these cells carry the hormones to the posterior pituitary.

The hypothalamus has a central neuro-endocrine function The hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland (hypophysis).
Differences between Hormonal control and Nervous Control

In order to protect itself the body has developed ways react to changes in its environment. For example, if you get too hot you will sweat to cool down. If light is too bright, your pupils will constrict so that your eyes are not damaged. In order to do this, the body needs to be able to detect internal and external changes (called stimuli) and make the appropriate response.

To function effectively, there needs to be good communication inside the body. Hormones are just one of the tools used to send messages to the various parts of the body and nerve impulses are used to send messages within the body. Hence both the systems coordinate to perform the stimulatory reactions in our body.

Cell signaling Artwork of cell signaling molecules (blue) traveling via the blood stream between cells. Signaling molecules allow cells to respond to changes in their environment and to coordinate this response. They can trigger a number of reactions, including changes in metabolism and gene expression. Cell communication over long distances via the bloodstream, as shown here, is known as endocrine (hormone) signaling.
Hormonal Signaling

An endocrine cell carries out both the receptor and control center functions. Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses.

Signaling by hormones involves three key events: reception, signal transduction and response. Reception of the signal occurs when the signal molecule binds to a specific receptor protein in or on the target cell. Each signal molecule has a specific shape that can be recognized by that signal's receptors. Receptors may be located in the plasma membrane of a target cell or inside the cell. The binding of a signal molecule to a receptor protein triggers events within the target cell signal transduction that result in a response, a change in the cell's behavior.

Cells that lack receptors for a particular chemical signal are unresponsive to that signal. The receptors for most water–soluble hormones are embedded in the plasma membrane, projecting outward from the cell surface. Binding of a hormone to its receptor initiates a signal transduction pathway, a series of changes in cellular proteins that converts an extracellular chemical signal to a specific intracellular response.

Depending on the hormone and target cell, the response may be the activation of an enzyme, a change in the uptake or secretion of specific molecules or rearrangement of the cytoskeleton. Signal transduction from some cell–surface receptors activates proteins in the cytoplasm that then move into the nucleus and directly or indirectly regulate transcription of specific genes.

Steps involved in hormonal signaling Activation of receptor molecule is always the initial step (the cause) leading to the cell's ultimate responses (effect) to the messenger.
Steps involved in Hormonal Signaling

Water – soluble hormone binds to a receptor protein on the surface of a target cell. This interaction triggers a signal transduction pathway that leads to a change in a cytoplasmic function or a change in gene transcription in the nucleus. A lipid – soluble hormone penetrates the target cell's plasma membrane and binds to an intracellular receptor, either in the cytoplasm or in the nucleus. The signal – receptor complex acts as a transcription factor, typically activating gene expression. Early evidence for the role of cell – surface receptors in triggering signal transduction pathways came from studies on how the hormone epinephrine stimulates breakdown of glycogen to glucose.

Another demonstration of the role of cell – surface receptors involves changes in a frog's skin color, an adaptation that helps camouflage the frog in changing light. Skin cells called melanocytes contain the dark brown pigment melanin in cytoplasmic organelles called melanosomes. The frog's skin appears light when melanosomes cluster tightly around the cell nuclei and darker when melanosomes spread throughout the cytoplasm.

A peptide hormone called melanocyte – stimulating hormone controls the arrangement of melanosomes and thus the frog's skin color. Adding melanocyte – stimulating hormone to the interstitial fluid surrounding the pigment – containing cells causes the melanosomes to disperse. However, direct microinjection of melanocyte – stimulating hormone into individual melanocytes does not induce melanosome dispersion – evidence that interaction between the hormone and a surface receptor is required for hormone action. Epinephrine, the primary fight – or – flight–hormone, produces different responses in different target cells. Responses of target cells may differ if they have different receptors for a hormone. Target cells with the same receptor exhibit different responses if they have different signal transduction pathways and/or effector proteins.

Epinephrine, commonly known as adrenaline secreted by the medulla of the adrenal glands. Epinephrine (also known as adrenaline or adrenalin) is a hormone and a neurotransmitter. Epinephrine has many functions in the body, regulating heart rate, blood vessel and air passage diameters, and metabolic shifts; epinephrine release is a crucial component of the fight–or–flight response of the sympathetic nervous system.
Epinephrine

The first indication that the receptors for some hormones are located inside target cells came from studying the vertebrate hormones estrogen and progesterone which are necessary for the normal development and function of the female reproductive system hormones.

The chemical signal of hormones activates the receptor, which then directly triggers the target cell's response by way of a change in gene expression.

Intracellular receptors located in the nucleus bind hormone molecules, which have diffused in from the bloodstream. The resulting hormone – receptor complexes bind, in turn, to specific sites in the cell's DNA and stimulate the transcription of specific genes.

Some steroid hormone receptors, however, are trapped in the cytoplasm and binding of a steroid hormone to its cytoplasmic receptor forms a hormone – receptor complex that can move into the nucleus to stimulate transcription of specific genes. mRNA produced in response to hormone stimulation is translated into new protein in the cytoplasm.

Paracrine signaling Artwork of neurotransmitter molecules traveling between a neuron (nerve cell, green) and an effector cell (orange). The junction between the two cells is called a synapse. Signaling molecules, such as neurotransmitters, allow cells to respond to changes in their environment and to coordinate this response. They can trigger a number of reactions, including changes in metabolism and gene expression. Cell communication over short distances, as shown here, is known as paracrine signaling.
Local Regulators

Local regulators secreted by cells that make them convey messages between neighboring cells a process referred to as paracrine signaling. Local regulators act on nearby target cells within seconds or even milliseconds, eliciting cell responses more quickly than hormones can. Some local regulators have cell–surface receptors; others have intracellular receptors. Binding of local regulators to their specific receptors triggers events within target cells similar to those elicited by hormones.

Several types of chemical compounds function as local regulators. Many neurotransmitters, the key local regulators in the nervous system, are amino acid derivatives. Among peptide/ protein local regulators are cytokines, which play a role in immune responses, and most growth factors, which stimulate cell proliferation and differentiation. Growth factors must be present in the extracellular environment in order for many types of cells to grow, divide and develop normally.

The nitric oxide (NO) gas is another example of a local regulator. When the blood oxygen level falls, endothelial cells in blood vessel walls synthesize and release NO. Nitric oxide activates an enzyme that relaxes the neighboring smooth muscle cells, which in turn dilates the vessels and improves blood flow to tissues. Nitric oxide has other functions as well: In the nervous system, it can function as a neurotransmitter and NO secreted by certain white blood cells can kill bacteria and cancer cells in body fluids.

Prostaglandin A molecule of prostaglandin, a hormone–like chemical. It is an unsaturated fatty acid or lipid which has a broad range of physiological activity. The molecule's atoms are color–coded: carbon is yellow, hydrogen is blue & oxygen is red. There are many types of prostaglandins, each with 20 carbon atoms, that are continually produced in mammal tissues. Their effects include lowering blood pressure, causing contractions of smooth muscle, regulating cell function & fertility, promoting inflammation, & affecting the platelet cells that form blood clots. They have localized effects because they are rapidly broken down once released by cells.
Examples of local regulators

A group of local regulators called prostaglandins (PGs) are modified fatty acids, often derived from lipids in the plasma membrane. They are so named because they were first discovered in prostate gland secretions that contribute to semen.

Released from most types of cells into interstitial fluid, prostaglandins regulate nearby cells in various ways, depending on the tissue. In semen that reaches the reproductive tract of a female, prostaglandins stimulate smooth muscles of the female‘s uterine wall to contract, helping sperm reach an egg. During childbirth, prostaglandins secreted by cells of the female‘s placenta cause the nearby muscles of the uterus to become more excitable, helping to induce labor.

In the immune system, various prostaglandins help induce fever and inflammation and also intensify the sensation of pain. These responses contribute to the body‘s defense by sounding an alarm that something harmful is occurring. The anti – inflammatory effects of aspirin and ibuprofen are due to the drugs‘ inhibition of prostaglandin synthesis. Prostaglandins also help regulate the aggregation of platelets, an early step in the formation of blood clots. This is why some physicians recommend that people who are at risk for a heart attack take aspirin on a regular basis.

In the respiratory system, two prostaglandins with very similar molecular structures have opposite effects on the smooth muscle cells in the walls of blood vessels serving the lungs. Prostaglandin E signals the muscle cells to relax, which dilates the blood vessels and promotes oxygenation of the blood. Prostaglandin F signals the muscle cells to contract, which constricts the vessels and reduces blood flow through the lungs. Shifts in the relative concentrations of these two antagonistic (opposing) signals help maintain homeostasis in changing circumstances.