02 Nervous and Hormonal Communication

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02 Nervous and Hormonal Communication

  1. 1. Nervous and hormonal communication ALBIO9700/2012JK
  2. 2. Nervous Hormonal control controlType of Electrical & Chemicaltransmission chemical (hormones)Speed of Rapid Slower (excepttransmission adrenaline)Response Immediate Slow actingArea of Very localised Widespread,response e.g.: growthChanges Short-term Long-termPathway Specific Through blood, but (nerve cells) specific target ALBIO9700/2012JK
  3. 3. Hormonal communicationExocrine and endocrine glands• Hormones are made in endocrine glands• A gland is a groups of cells which produces and releases one or more substances (secretion by secretory cells directly into blood)• ‘Endocrine’ means ‘secreting to the inside’ – endocrine glands secrete hormones into blood capillaries inside the gland• ‘Exocrine ’ glands mean ‘secreting to the outside’ – secrete substance (not hormones) into tube or duct, along which the secretion flows (e.g. salivary glands secrete saliva into salivary ducts which carry the saliva into the mouth) ALBIO9700/2012JK
  4. 4. Endocrine glands ALBIO9700/2012JK
  5. 5. Hormones• Relatively small molecules• Polypeptides or proteins (insulin); steroids (testosterone)• Transported in blood plasma in small concentrations (< few micrograms per cm3 of blood)• Rate of secretion is also low (few micrograms or milligrams/day)• Most endocrine glands can secrete hormones very quickly when an appropriate stimulus arrives (e.g. adrenaline)• Many hormones have a very short life in the body – they are broken down by enzymes in the blood or cells (e.g. insulin – 10 to 15 minutes/adrenalin – 1 to 3 minutes)• Each hormone has a particular group of cells which it affects (target cells )• These cells contain receptors specific to the hormone• The receptor for protein hormones are on the plasma membrane• The receptors for steroid hormones are inside the cell, in the cytoplasm ALBIO9700/2012JK
  6. 6. The pancreas• Parts function as an exocrine gland while other parts function as an endocrine gland• Exocrine: secretion of pancreatic juice which flows along the pancreatic duct into the duodenum where it helps digestion• Endocrine: carried out by groups of cells ( islets of Langerhans )• Islets contain 2 types of cells (hormones involved in control of blood glucose levels): – α cells secrete glucagon – β cells secrete insulin ALBIO9700/2012JK
  7. 7. • The control of blood glucose – Carbohydrate is transported through the human bloodstream in the form of glucose, in solution in the blood plasma – In healthy humans, each 100cm3 of blood normally contains between 80 and 120mg of glucose. – If blood glucose level drops below this, then cells may run short of glucose for respiration and be unable to carry out their normal activities – Very high blood glucose levels can also cause major problems – As blood with glucose flows through the pancreas, the α and β cells detect the raised glucose levels – α cells stop secreting glucagon; β cells respond by secreting insulin into blood plasma – Insulin affects many cells, especially those in liver and muscles: • An increased absorption of glucose from the blood into the cells • An increase in the rate of use of glucose in respiration • An increase in the rate at which glucose is converted into the storage polysaccharide glycogen – All these processes take glucose out of the blood, so lowering blood glucose levels ALBIO9700/2012JK
  8. 8. • A drop in blood glucose: α cells secrete glucagon and β cells stop secretion of insulin• Lack of insulin – stop increased uptake and usage of glucose by liver and muscle cells• Presence of glucagon – affects the activities of the liver cells: – Breakdown of glycogen to glucose – Use of fatty acids instead of glucose as the main fuel in respiration – Production of glucose from other compounds such as fats• Liver releases glucose into blood• Time delays in control systems results in oscillation, where things do not stay absolutely constant, but sometimes rise slightly above and sometimes drop slightly below the ‘required’ level ALBIO9700/2012JK
  9. 9. ALBIO9700/2012JK
  10. 10. Control of insulin secretion ALBIO9700/2012JK
  11. 11. • Diabetes mellitus (sugar diabetes) – 2 forms: • Juvenile-onset diabetes /insulin-dependent diabetes – pancreas incapable of secreting sufficient insulin, possibly due to a deficiency in the gene which codes for the production of insulin, or because of an attack on the β cells by the person’s own immune system • Non-insulin-dependent diabetes – pancreas does secrete insulin but the liver and muscle cells do not respond properly to it (associated with obesity) – Symptoms: • Blood glucose levels rise and stay high • Glucose in urine because kidney cannot reabsorb all the glucose • Extra water and salts accompany glucose so the person feels extremely hungry and thirsty • The combination of dehydration, salt loss and low blood pH can cause coma in extreme situations. Build-up of substances called keto-acids in the blood, due to metabolism of fats and proteins as an alternative energy source, lowers the blood pH. • Coma may also result because of lack of glucose for respiration due to the lack of glycogen to be mobilised. Therefore, blood glucose levels of a person with untreated diabetes may plummet. ALBIO9700/2012JK
  12. 12. • In insulin-dependent diabetes, regular injections of insulin, together with controlled diet, are used to keep blood glucose levels near normal• In non-insulin-dependent diabetes, insulin injections are not normally needed but control is by diet alone• Advantages of using genetically engineered human insulin: – More rapid response – Shorter duration of response – Less chance of an immune response to the insulin developing – Effective in people who have developed a tolerance for animal-derived insulin – More acceptable to people who feel it is unethical to use pig or cattle insulin ALBIO9700/2012JK
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  14. 14. Nervous communicationNeurones• aka nerve cells: cells which are specialised for the conduction of action potentials• Motor neurone : transmits messages from the brain or spinal cord to a muscle or gland. Cell body of a motor neurone lies within the spinal cord or brain.• Dendrite /dendron : a short/long cytoplasmic process of neurone, that conducts action potential towards the cell body• Axon : a long cytoplasmic process of a neurone, that conducts action potentials away from the cell body (may be extremely long); contains all usual organelles, large number of mitchondria at the tips of terminal branches of the axon and vesicles containing chemicals called transmitter substances• Schwann cells : cells which is in close association with a neurone, whose plasma membrane wraps round and round the axon or dendron of the neurone to form a myelin sheath (made largely of lipid and protein)• Not all axons have myelin sheaths• The sheath affects the speed of conduction of the nerve impulse• Node of Ranvier : a short gap in the myelin sheath surrounding an axon (every 1-3 mm in human neurones, 2-3 μm long)• Sensory neurone : bring impulses from receptors to the brain or spinal cord (one long dendron and an axon shorter than the dendron) ALBIO9700/2012JK
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  17. 17. A reflex arc• A pathway along which impulses are carried from a receptor to an effector, without involving ‘conscious’ regions of the brain• Reflex action : a fast, automatic response to a stimulus; may be innate (inborn) or learned (conditioned)Transmission of nerve impulses• Neurones transmit impulses as electrical signals• Signals travel very rapidly along the plasma membrane from one end of the cell to the other and are not a flow of electrons like an electric current• Signals are brief changes in the distribution of electrical charge across the plasma membrane, caused by the very rapid movement of sodium and potassium ions into and out of the axon ALBIO9700/2012JK
  18. 18. ALBIO9700/2012JK
  19. 19. Resting potential• In a resting axon, inside of the axon always has a slightly negative electrical potential compared with the outside• The difference between these potentials (potential difference ) is around -65mV (inside lower than outside)• Resting potential is produced and maintained by the sodium-potassium pump in the plasma membrane of the axon• The process involves moving the ions against their concentration gradients, and so use energy from the hydrolysis of ATP• The sodium-potassium pump removes 3 sodium ions from the cell for every 2 potassium ions it brings into the cell (K+ diffuses back faster than Na+)• The result is an overall excess of positive ions outside the membrane compared with the inside ALBIO9700/2012JK
  20. 20. • Action potentials – A fleeting reversal of the resting potential across the plasma membrane of a neurone, which rapidly travels along its length, caused by changes in permeability of the plasma membrane to Na+ and K+ – Voltage-gated channels : a protein channel through a cell membrane that opens or closes in response to changes in electrical potential across membrane, that allow Na+ and K+ to pass through – The electric current used to stimulate the axon causes the opening of the channels in the plasma membrane which allow Na+ to pass through (they flood through the open channels) – The high concentration of positively charged Na+ inside the axon makes it less negative inside than it was before. The membrane is said to be depolarised – As Na+ continue to flood in, the inside of the axon swiftly continues to build up positive charge, until it reaches a potential of +40mV compared with the ouside – At this point Na+ channels close and K+ channels open – K+ diffuses out of the axon down concentration gradient – The outward movement of K+ removes positive charge from inside the axon to the outside, thus beginning to return the potential difference to normal (repolarisation ) – So many K+ leave the axon that the potential difference across the membrane briefly becomes even more negative than the normal resting potential – The potassium channels then close, and the sodium-potassium pump begins to acts again, restoring the normal distribution of Na+ and K+ across the membrane and restoring the resting potential ALBIO9700/2012JK
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  23. 23. • Transmission of action potentials – The function of a neurone is to transmit information along itself – An action potential at any point in an axon’s plasma membrane triggers the production of an action potential in the membrane on either side of it – The temporary depolarisation of the membrane where the action potential is causes a ‘local circuit’ to be set up between the depolarised region and the resting regions on either side of it – Na+ flow sideways inside the axon (away from positively charged region towards the negatively charged regions on either sides). This depolarises these adjoining regions and so generates an action potential in them – Refractory period : a period of time during which a neurone is recovering from an action potential, and during which another action potential cannot be generated ALBIO9700/2012JK
  24. 24. ALBIO9700/2012JK
  25. 25. • How action potentials carry information – Action potentials do not change in size (+40mV) nor speed at which it travels (the intensity of the stimulus which orginally generated the action potential has absolutely no effect on the size of the action potential) – The action potential frequency differs between a strong and a weak stimulus (strong stimulus produces a rapid succession of action potential and vice versa) – A strong stimulus is likely to stimulate more neurones than a weak stimulus – The brain can interpret the frequency of action potentials arriving along the axon of a sensory neurone, and the number of neurones carrying action potentials, to get information about the strength of the stimulus being detected by that receptor – The nature of the stimulus (light, heat, touch) is deduced from the position of the sensory neurone bringing the information (e.g. retina) ALBIO9700/2012JK
  26. 26. • Speed of conduction – Myelinated human neurone: 100ms-1 – Nonmyelinated neurones: 0.5ms-1 – Myelin speeds up rate by insulating axon membrane – Na+ and K+ cannot flow through myelin sheath (not possible for depolarisation or action potentials to occur in parts surrounded by it – can only occur at nodes of Ranvier) – Saltatory conduction : conduction of an action potential along a myelinated axon or dendron, in which the action potential jumps from one node of Ranvier to the next (can increase speed of transmission by up to 50 times) – Diameter also affects speed of transmission – thick axons transmit action potentials faster than thin ones ALBIO9700/2012JK
  27. 27. ALBIO9700/2012JK
  28. 28. • What starts off an action potential? – Wide variety of initial stimulus: electric current (light, touch, sound, temperature or chemicals) – Receptor cell : a cell (in sense organs) which is sensitive to a change in the environment that may generate an action potential as a result of a stimulus – they convert energy in one form (light, heat or sound) into energy in an electrical impulse in a neurone – Pacinian corpuscle : one type of receptor found in the dermis of the skin containing an ending of a sensory neurone, surrounded by several layers of connective tissue ( capsule ) – ending of sensory neurone inside capsule has no myelin – Pressure applied – capsule pressed out of shape – nerve ending deformed – Na+ and K+ channels open – Na+ flood in/K+ flow out – membrane depolarised – increased positive charge inside axon (receptor potential ) – if pressure great enough, receptor potential becomes large enough to trigger an action potential – Below a certain threshold, the pressure stimulus only causes local depolarisation (not action potential) and therefore no information is transmitted to the brain ALBIO9700/2012JK
  29. 29. Pacinian corpuscle ALBIO9700/2012JK
  30. 30. Synapses• Synaptic cleft : a very small gap between two neurones at a synapse• Synapse : a point at which two neurones meet but do not touch; made up of the end of the presynaptic neurone, the synaptic cleft and the end of the postsynaptic neurone• The mechanism of synaptic transmission – Transmitter substance : a chemical that is released from a presynaptic neurone when an action potential arrives that then diffuses across the synaptic cleft and may initiate an action potential in the postsynaptic neurone – Action potential arrive along plasma membrane of presynaptic neurone – release transmitter substance into cleft – transmitter substance molecules diffuse across cleft (< a millisecond as distance is so small) – sets up an action potential in the plasma membrane of the postsynaptic neurone ALBIO9700/2012JK
  31. 31. ALBIO9700/2012JK
  32. 32. – The cytoplasm of the presynaptic neurone contains vesicles of transmitter substance (>40 are known: noradrenaline and acetylcholine – throughout nervous system; dopamine and glutamic acid – only in the brain)– Cholinergic synapses : a synapse at which the transmitter substance is acetylcholine– In the part of the membrane of the presynaptic neurone which is next to the synaptic cleft, the arrival of the action potential also causes calcium channels to open– The action potential causes calcium ions to rush in to the cytoplasm of the presynaptic neurone– This influx of calcium ions causes vesicles of ACh to move to the presynaptic membrane and fuse with it, emptying their contents into the synaptic cleft (each vesicle contains up to 10 000 molecules of ACh which diffuses across the synaptic cleft in <0.5ms) ALBIO9700/2012JK
  33. 33. – The plasma membrane of the postsynaptic neurone contains receptor proteins which has a complementary shape to part of the ACh molecule – ACh temporarily binds with the receptors – shape of protein changes – channels open and sodium ions can pass – sodium ions rush into the cytoplasm of the postsynaptic neurone, depolarising the membrane and starting off action potential– The synaptic cleft contains acetylcholinesterase which splits each ACh into acetate and choline to stop action potential and avoid wasting ACh– Choline is taken back into the presynaptic neurone, where it is combines with acetyl coenzyme A to form ACh which is transported into the presynaptic vesicles– Whole process takes about 5-10ms– Between motor neurone and a muscle, the nerve forms motor end plates and the synapse is called a neuromuscular junction ALBIO9700/2012JK
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  35. 35. • The effects of other chemicals at synapses – Nicotine • Part of the molecule is similar in shape to ACh and will fit into the ACh receptors on postsynaptic membranes • Unlike ACh, nicotine is not rapidly broken down by enzymes and so remains in the receptors fro longer than ACh • A large dose of nicotine can be fatal – Botulinum toxin • Produced by an anaerobic bacterium which occasionally breeds in contaminated canned food • Prevents the release of ACh (can be fatal) • Injections of tiny amounts of the botulinum toxin into the muscles of eyelids that contract permenantly (cannot open) can cause them to relax, so allowing the lids to be raised – Organophosphorous insecticides • Inhibits the action of acetylcholinesterase, thus allowing ACh to cause continuous production of action potentials in the postsynaptic membrane • Found in flea sprays and collars for cats and dogs, organophosphorous sheep-dip (used to combat infections by ticks), several nerve gases also acts the same way ALBIO9700/2012JK
  36. 36. • The roles of synapses – Synapses slow down the rate of transmission of a nerve impulse. So why have synapses? • Synapses ensure one-way transmission – Allows signals to be directed towards specific goals rather than spreading at random • Synapses increase the possible range of actions in response to a stimulus – Action potential arriving at some of these synapses will stimulate an action potential while arriving at others will cause the release of transmitter substances which will actually make it more difficult to depolarise plasma membrane and so inhibit production of action potential – The loss of speed is more than compensated for in the possible variety of responses which can be made but we do have very rapid responses called reflex actions • Synapses are involved in memory and learning – Picturing a face from the voice ALBIO9700/2012JK

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