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HOMEOSTASIS – refers to the maintenance of a relatively stable internal environment, achieved by the regulation of numerous factors including temperature, cell pH, the concentration of ions, water balance and reproductive cycles. In order to maintain the internal environment within the certain acceptable limits, the organism must respond to external factors. GENERAL MODEL FOR HOMEOSTATIC REGULATION STIMULUS – refers to a detectable change in the internal environment (which may be caused by a change in the external environment) i.e. when a factor under regulation falls above or below a point at which regulation is triggered. SENSOR/RECEPTOR – specialized cells or tissues, capable of detecting a change in factors under regulation and sending a message via the nervous system or endocrine system (responsible for the production of hormones) to an effector site. TYPES OF RECEPTORS – there are a number of different types of specialized cells in the body, capable of detecting different types of stimuli. CHEMORECEPTORS – capable of detecting chemical changes e.g. concentration of ions, blood glucose level, water concentration, CO2 level. MECHANORECEPTORS – capable of detecting pressure and movement e.g. vibrations in the ear, muscle stretching, blood pressure, inflation/deflation of lung. PHOTORECEPTORS – capable of detecting light stimuli e.g. in the eye. THERMORECEPTORS – capable of detecting changes in temperature e.g. thermoreceptors in skin, capable of detecting internal changes, hypothalamus capable of detecting changes in core body temperature. EFFECTOR SITE – refers to the site where a response to the stimulus occurs, activated by neural impulse of hormone. RESPONSE – refers to the change instigated in response to the stimulus and will, ideally, bring the factor under regulation back within stable limits. If the response that occurs is in the opposite (negative) direction to the stimulus, then NEGATIVE FEEDBACK is said to have occurred. NEGATIVE FEEDBACK- maintenance of a stable internal environment relies on negative feedback mechanisms. In order to maintain a stable environment, the body must possess mechanisms by which it may counteract external stimuli by producing a response in the opposite direction of the stimulus, to reduce the effects of the stimulus. POSITIVE FEEDBACK – whilst positive feedback systems to occur in nature, it is important to note that they are not mechanisms of homeostatic regulation. Generally, positive feedback occurs in response to abnormally stressful conditions e.g. during childbirth. During childbirth pressure on the cervix stimulates the production of oxytocin, a hormone, which produces further contractions and increased pressure on the cervix. The process of blood clotting is another process in which positive feedback occurs. The injured tissue releases a chemical which activates platelets in the blood. The activated platelets then release more of the chemical, activating more platelets, leading to the formation of a blood clot. ENDOCRINE SYSTEM – refers to a system of glands which produce hormones, a means of regulating some bodily processes. EXAMPLES HYPOTHALAMUS – located in the brain, the hypothalamus acts as both a receptor and producer of hormones. Osmoreceptors, responsible for the detection of water concentration in the blood and thermoreceptors, responsible for detecting changes in core body temperature are located in the hypothalamus. The hypothalamus also produces hormones which act directly on effector sites, as well as inhibitory and releasing hormones which control the production of other hormones in the body. PANCREAS – secretes insulin and glucagons, hormones which regulate the concentration of glucose molecules in the blood. HORMONE – in animals, refers to chemical compounds produced (usually) in the cells of endocrine organs that may act on the same cell, be released into the surrounding extra cellular fluid to exert an effect on a nearby cell or be released into to the bloodstream, via lymph, in order to exert an effect on a specific (target or effector) site. Some hormones have relatively immediate effects e.g. regulation of glucose, ions etc., whilst others are involved in the long term development of an organism e.g. growth hormones, hormones involved in reproductive cycles and sexual development. hormones may be amino acid derivatives, polypeptide/protein hormones or steroid based hormones. AMINO ACID DERIVATIVE AND POLYPEPTIDE/PROTEIN HORMONES – are made/modified and packaged by golgi into vesicles are secreted form the cell via exocytosis. Amino acid and peptide hormones have a relatively short life span (they are broken down by the organism). These hormones are water soluble (polar) and therefore able to travel in the bloodstream, dissolved in plasma. This, however means they are unable to pass through hydrophobic cell membranes via simple diffusion. They must, instead, bind with receptor molecules embedded in the cell membrane, which may be carbohydrates or proteins, and exert their effects on the contents of the cell via a process referred to as signal transduction. SIGNAL TRANSDUCTION – refers to the process by which hydrophilic hormones are able to exert an effect on the internal environment of cells, despite being unable to pass through their membranes. Having attached to a receptor protein of carbohydrates on the outer side of the cell membrane, a g-protein on the inner side of the membrane is activated. The activation of the g-protein triggers a response, being the creation of a signaling or relay molecule which triggers a second response and so on, until the final response occurs. The final response may be the activation of a gene resulting in the creation of mRNA in order to synthesize a protein, which may then act as an enzyme for a chemical reaction in the cell. STERIOD HORMONES – are lipid based hormones, formed from precursors within cells, and being hydrophobic, are able to pass through the phospholipid bi-layer membrane of cells and attach to a cytosol receptor, a receptor molecule floating in the cytosol, forming a hormone-receptor complex. The hormone-receptor complex then enters the nucleus to activate a gene, inducing production of mRNA coding for the synthesis of a protein, which may act as an enzyme for cell reactions. Generally, steroid hormones regulate long term development such as growth and sexual development. PLANT REGULATION – hormones are also involved in the regulation of plants and are generally transported via xylem or phloem and sometimes both, however particularly via phloem. Plant hormones may produce different effects in different types of plant tissues, differeing form animal hormones which are generally very specific in their functions. AUXINS – are a type of plant hormone, of which indoleacetic acid (IAA) is one. A major function of auxins is to control enlargement and elongation of cells. In stems and leaves of plants, auxins stimulate cell elongations, however may inhibit cell elongation in the roots of plants. TROPISMS – the term tropism refers plant growth in response to a stimulus such as water of light. Tropisms are terms to be positive if the plant grows towards the stimulus and negative if the plant grows away from the stimulus. PHOTOTROPISM – a positive tropism referring to the tendency for plants to grow towards a light stimulus. This tendency occurs due to the presence of indoleacetic acid (IAA), an auxin, which occurs in higher concentrations in cells that have less exposure to light. If a plant is exposed to an overall even intensity of light, the plant will grow straight, due to even concentrations of IAA. If a light source is of greater intensity on one side of the plant, auxin will become concentrated in cells with less exposure to light. In plant stems, IAA stimulates elongation of cells, thus, cell with higher concentrations of IAA will become more elongated than those exposed to light, causing the plant to bend towards the light stimulus. GEOTROPISM – refers to a tendency for plants to grow in the vertical direction i.e. if a seed is planted so that both the roots and the shoot emerge horizontally, after a period of time, the roots will turn to grow downwards and the shoot upwards. This tropism is due to the effects of auxin, which accumulates on the lower sides of horizontally growing shoots and roots. In the shoots of plants, auxin stimulates cell elongation, causing cells on the lower side of the shoot to become more elongated. This causes the shoot to turn upwards. Auxins, however, have an inverse effect when present in the roots of plants. Accumulated auxins on the lower side of horizontally growing roots inhibit the elongation of cells, causing the root to turn downwards. CYTOKININS – a type of plant hormone which stimulates cell growth and cell reproduction. GIBBERELLINS – a class of plant hormones which stimulate cell growth and reproduction. Gibberellins also promote seed germination and flowering. Gibberellins promote cell growth by stimulating the synthesis of the enzyme amylase, a catalyst in the breakdown of starch. In breaking down starch molecules, glucose, a reactant in cellular respiration, is made available to the cell. ABSCISIC ACID – may inhibit germination of seeds, leading to seed dormancy and seasonal dormancy. Abscisic acid may also influence the opening and closure of stomata. ETHYLENE – also known as ethene, is secreted as a gas and promotes ripening of fruits and the abscission (shedding) of leaves through stimulating the formation of a protective layer referred to as an abscission layer, at the site of abscission. FLORIGEN – a hormone believed to stimulate the flowering of plants, its precise identity is as of yet unknown. PHEROMONES – are chemicals produced in the exocrine glands of animals, especially insects, which are then secreted to the external environment to influence the behaviour of other members of that species. NERVOUS SYSTEM – refers to the system of the brain and nerves, which is responsible for detecting, transmitting, processing and responding to information which an organism receives. The nervous system can divided into two main parts: the central nervous system (CNS) consisting of the brain and spinal cord and the peripheral nervous system (consisting of all other nerve tissue within the organism.) PERIPHERAL NERVOUS SYSTEM – can be furthered divided into parts, based on the various functions it carries out. SOMATIC NERVOUS SYSTEM – consists of sensory (afferent) neurones and motor (efferent) neurones. Sensory neurones carry sensory information from the sensory organs (e.g. nose, touch receptors in the skin etc.) to the spinal cord, which will then carry the neural messages to the brain where they may be processed. Motor neurones carry neural messages to skeletal muscles, in order to control voluntary movement. AUTONOMIC NERVOUS SYSTEM – regulates involuntary processes within the body e.g. digestion, heart rate, breathing etc. and can divided into two further categories: the sympathetic nervous system and the parasympathetic nervous system. The function of the parasympathetic nervous system is to heighten the body’s state of arousal (e.g. by increasing heart rate, release of adrenalin etc.) in order to prepare the organism for action. The function of a parasympathetic is to calm the organism after having been in a state of arousal, and increase or decrease the rate of process under its regulation as part of maintaining homeostasis. Note: whilst the general function the parasympathetic nervous system is an inhibitory one, the parasympathetic nervous system may also increase factors such as heart rate, as part of maintain a relatively constant internal environment of the organism. NEURONS – refer to the type of cell which makes up the nerve tissue of an organism. Their function is to relay neural impulses, and thus, they have specialised features which enable them to do so. DENDRITES – are the receptors of the neuron; they receive neural impulses from other neurons and sensory information from receptor cells. AXON – transmits the neural impulse down the nerve cell, to the terminal buds, and may be protected by a layer of Schwann cells, referred to as the myelin sheath. AXON TERMINALS/TERMINAL BUDS – are found at the end of the axon, and contain neurotransmitters, which are released in order to transmit impulses to surrounding nerve cells. SYNAPSE – refers to the junction of two nerve cells. The space between the two, containing tissue fluid, is referred to as the synaptic gap. NEUROTRANSMITTERS – are chemicals secreted from axon terminal buds which pass across the synaptic gap (containing tissue fluid) to stimulate the cell membrane of an adjacent neuron. NEUROHORMONES – are chemicals secreted from nerve cells, however they differ from neurotransmitters in that they can be released into the bloodstream to exert an effect on a target organ. Essentially, neurohormones function in the same way as other animal hormones. NEURONE POLARITY – when not in a state of conducting a neural impulse, the neurone is resting, however it must also maintain a charge difference between the outside of the cell membrane which is positively charged, and the inside of the membrane which is negatively charged. This charge difference is maintained via the active transport of sodium (Na+) ions. NEURAL IMPULSE – when a neural impulse is transmitted down the axon of a nerve cell, a rapid change in the polarity of the cell occurs. As the impulse travels down the axon, a change in the permeability of the membrane allows positively charged ions to pass across the membrane. This changes the overall charge of the outside of the nerve cell from positive to negative. This difference in charge is then reversed once the neural impulse has been transmitted. The process occurs in a period of milliseconds. SOME DETAILED EXAMPLES OF HOMEOSTATIC REGULATION FRIGHT STIMULUS – the body’s response to a frightening stimulus is a good example of nervous and endocrine systems working together. Photoreceptors in the eyes or mechanoreceptors in the ears (capable of detecting vibrations) will detect a frightening stimulus and send a message via sensory (afferent) nerves to the brain. The sympathetic nervous system will then send neural messages to the heart to increase heart rate, to the lungs to increase breathing rate etc. Hormones (adrenalin) are released from the adrenal medulla, increasing the blood flow to muscles. A hormone is released from the hypothalamus to stimulate the production of another hormone, which in turn stimulates the production of another hormone, cortisol, which acts on the liver to release glucose into the blood. The combined effects of the hormonal and nervous systems prepare the organism to respond to a frightening stimulus. BLOOD GLUCOSE – RISE IN BLOOD GLUCOSE Concentrations of blood glucose greater than 6.8 mmol/L(stimulus) are detected by the pancreas(receptor). Alpha cells in the pancreas decrease production of glucagon, a hormone (message) which stimulates the breakdown of glycogen energy stores, stored in the liver and skeletal muscles (effectors). As a result, the level of glucose in the blood lowers, due to a decrease in the amount of glycogen being broken down into glucose and released into the blood (response). Beta cells, also located in the pancreas, increase production of insulin. Insulin, a hormone (message) stimulates uptake of glucose by cells (effectors). As glucose is absorbed by the cells and converted into glycogen stores, the level of glucose in the blood decreases (response). BLOOD GLUCOSE – FALL IN BLOOD GLUCOSE Concentrations of blood glucose less than 3.6 mmol/L(stimulus) are detected by the pancreas(receptor). Beta cells in the pancreas decrease production of insulin (message) which stimulates the uptake of glucose by cells (effectors). As a result, the level of glucose in the blood increases, due to a decrease in the amount of glucose being absorbed by cells (response). Alpha cells increase production of glucagons which stimulates the breakdown of glycogen stored in the liver and skeletal muscles (effectors) into glucose, which is then released into the blood. As glucose is released into the blood, the level of glucose in the blood increases (response). WATER BALANCE – the water concentration within an organism may be detected via two different stimuli. Firstly, the hypothalamus is capable of detecting a change in osmotic pressure, due to changes in the concentrations of salts in the body. High osmotic pressure due to an increase in salt concentration (decreased water concentration) is detected by the hypothalamus which stimulates the release of vasopressin, a hormone, from the pituitary gland. Vasopressin causes the distal tubules of the nephrons in the kidney to become more permeable to water, thus allowing more water to be reabsorbed from the filtrate into the bloodstream. Secondly, blood volume, which affects blood pressure, may be detected by pressure receptors in the kidney. The kidneys then increase production of renin, an enzyme, which stimulates the production of a hormone aldosterone, in the adrenal cortex. Aldosterone stimulates active transport of sodium ions back into the blood from the kidney filtrate, which leads to increased reabsorption of water from the nephron tubules. These factors combined will increase the concentration of water within the body, by decreasing the concentration of water excreted in the urine of the organism. The increase in water concentration will decrease the concentration of salts and therefore osmotic pressure, and increase blood volume and therefore pressure. CORE BODY TEMPERATURE –a stable core body temperature will be maintained at the expense of factors under regulation. In extreme heat, an organism may sweat to the point of dehydration, in extreme cold, circulation to extremities such as fingers and toes ceases in order to reduce heat loss and maintain a stable core temperature. FALL IN CORE BODY TEMPERATURE – is detected by the hypothalamus, which sends messages via neurohormones to the thyroid gland, which stimulate the production of thyroxin. Thyroxin is then released into the bloodstream where it increases the organism’s metabolic rate. The process of cellular respiration generates heat, thus negative feedback occurs. Messages are also sent via neural impulses to sweat glands stimulating a decrease in sweat production to minimize heat loss due to evaporation of sweat, and to blood vessels, which constrict peripheral circulation, to minimize heat transfer from blood to the cooler air. RISE IN CORE BODY TEMPERATURE – is detected by the hypothalamus, which decreases production of neurohormones responsible for stimulating the production of thyroxin. Production of thyroxin is therefore decreased which decreases the organism’s metabolic rate. The process of cellular respiration generates heat, thus a decrease in cellular respiration would decrease the amount of heat produced. Messages are also sent via neural impulses to sweat glands stimulating an increase in sweat production to maximise heat loss due to evaporation of sweat, and to blood vessels, which dilate aterioles to increase peripheral circulation, to maximise heat transfer from blood to the cooler air. BLOOD PRESSURE – a decrease in blood pressure is detected by receptors in blood vessel walls. The drop in pressure is then conveyed to the central nervous system which sends neural impulses to the heart to increase the heart rate. Neurohormones are also released by the CNS which stimulates the constriction of arterioles which increases resistance to the blood flow, thus increasing the pressure.