Nervous and hormonal Nervous and hormonal communicationcommunication

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Nervous control

Hormonal control

Type of transmission

Speed of transmission

Response

Area of response

Changes

Pathway

Electrical & chemical

Chemical (hormones)

Widespread, e.g.: growth

Short-term

Very localised

Slow actingImmediate

Slower (except adrenaline)

Rapid

Long-term

Specific (nerve cells)

Through blood, but specific target

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Hormonal communicationHormonal communicationExocrine and endocrine glandsExocrine and endocrine glands• HormonesHormones are made in are made in endocrine glandsendocrine glands• A A glandgland is a groups of cells which produces is a groups of cells which produces

and releases one or more substances and releases one or more substances ((secretionsecretion by secretory cells directly into by secretory cells directly into blood)blood)

• ‘‘Endocrine’ means ‘secreting to the inside’ – Endocrine’ means ‘secreting to the inside’ – endocrine glands secrete hormones into endocrine glands secrete hormones into blood capillaries inside the glandblood capillaries inside the gland

• ‘‘ExocrineExocrine’ glands mean ‘secreting to the ’ glands mean ‘secreting to the outside’ – secrete substance (not hormones) outside’ – secrete substance (not hormones) into tube or duct, along which the secretion into tube or duct, along which the secretion flows (e.g. salivary glands secrete saliva into flows (e.g. salivary glands secrete saliva into salivary ducts which carry the saliva into the salivary ducts which carry the saliva into the mouth)mouth)

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Endocrine glandsEndocrine glands

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HormonesHormones• Relatively small moleculesRelatively small molecules• Polypeptides or proteins (insulin); steroids (testosterone)Polypeptides or proteins (insulin); steroids (testosterone)• Transported in blood plasma in small concentrations (< Transported in blood plasma in small concentrations (<

few micrograms per cmfew micrograms per cm33 of blood) of blood)• Rate of secretion is also low (few micrograms or Rate of secretion is also low (few micrograms or

milligrams/day)milligrams/day)• Most endocrine glands can secrete hormones very Most endocrine glands can secrete hormones very

quickly when an appropriate stimulus arrives (e.g. quickly when an appropriate stimulus arrives (e.g. adrenaline)adrenaline)

• Many hormones have a very short life in the body – they Many hormones have a very short life in the body – they are broken down by enzymes in the blood or cells (e.g. are broken down by enzymes in the blood or cells (e.g. insulin – 10 to 15 minutes/adrenalin – 1 to 3 minutes)insulin – 10 to 15 minutes/adrenalin – 1 to 3 minutes)

• Each hormone has a particular group of cells which it Each hormone has a particular group of cells which it affects (affects (target cellstarget cells))

• These cells contain These cells contain receptorsreceptors specific to the hormone specific to the hormone• The receptor for protein hormones are on the plasma The receptor for protein hormones are on the plasma

membranemembrane• The receptors for steroid hormones are inside the cell, in The receptors for steroid hormones are inside the cell, in

the cytoplasmthe cytoplasm

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The pancreasThe pancreas• Parts function as an exocrine gland while Parts function as an exocrine gland while

other parts function as an endocrine glandother parts function as an endocrine gland• Exocrine: secretion of pancreatic juice Exocrine: secretion of pancreatic juice

which flows along the pancreatic duct into which flows along the pancreatic duct into the duodenum where it helps digestionthe duodenum where it helps digestion

• Endocrine: carried out by groups of cells Endocrine: carried out by groups of cells ((islets of Langerhansislets of Langerhans) )

• Islets contain 2 types of cells (hormones Islets contain 2 types of cells (hormones involved in control of blood glucose levels):involved in control of blood glucose levels):– αα cells cells secrete secrete glucagonglucagon– ββ cells cells secrete secrete insulininsulin

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• The control of blood glucoseThe control of blood glucose– Carbohydrate is transported through the human bloodstream Carbohydrate is transported through the human bloodstream

in the form of glucose, in solution in the blood plasmain the form of glucose, in solution in the blood plasma– In healthy humans, each 100cmIn healthy humans, each 100cm33 of blood normally contains of blood normally contains

between 80 and 120mg of glucose. between 80 and 120mg of glucose. – If blood glucose level drops below this, then cells may run If blood glucose level drops below this, then cells may run

short of glucose for respiration and be unable to carry out short of glucose for respiration and be unable to carry out their normal activitiestheir normal activities

– Very high blood glucose levels can also cause major problemsVery high blood glucose levels can also cause major problems– As blood with glucose flows through the pancreas, the As blood with glucose flows through the pancreas, the αα and and

ββ cells detect the raised glucose levels cells detect the raised glucose levels– αα cells stop secreting glucagon; cells stop secreting glucagon; ββ cells respond by secreting cells respond by secreting

insulin into blood plasmainsulin into blood plasma– Insulin affects many cells, especially those in liver and Insulin affects many cells, especially those in liver and

muscles:muscles:• An increased absorption of glucose from the blood into the cellsAn increased absorption of glucose from the blood into the cells• An increase in the rate of use of glucose in respirationAn increase in the rate of use of glucose in respiration• An increase in the rate at which glucose is converted into the An increase in the rate at which glucose is converted into the

storage polysaccharide glycogenstorage polysaccharide glycogen– All these processes take glucose out of the blood, so lowering All these processes take glucose out of the blood, so lowering

blood glucose levelsblood glucose levels

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• A drop in blood glucose: A drop in blood glucose: αα cells secrete cells secrete glucagon and glucagon and ββ cells stop secretion of cells stop secretion of insulininsulin

• Lack of insulin – stop increased uptake and Lack of insulin – stop increased uptake and usage of glucose by liver and muscle cellsusage of glucose by liver and muscle cells

• Presence of glucagon – affects the activities Presence of glucagon – affects the activities of the liver cells:of the liver cells:– Breakdown of glycogen to glucoseBreakdown of glycogen to glucose– Use of fatty acids instead of glucose as the Use of fatty acids instead of glucose as the

main fuel in respirationmain fuel in respiration– Production of glucose from other compounds Production of glucose from other compounds

such as fatssuch as fats• Liver releases glucose into bloodLiver releases glucose into blood• Time delays in control systems results in Time delays in control systems results in

oscillation, where things do not stay oscillation, where things do not stay absolutely constant, but sometimes rise absolutely constant, but sometimes rise slightly above and sometimes drop slightly slightly above and sometimes drop slightly below the ‘required’ level below the ‘required’ level

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Control of insulin secretionControl of insulin secretion

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• Diabetes mellitus (sugar diabetes)Diabetes mellitus (sugar diabetes)– 2 forms:2 forms:

• Juvenile-onset diabetesJuvenile-onset diabetes//insulin-dependent diabetesinsulin-dependent diabetes – – pancreas incapable of secreting sufficient insulin, possibly due pancreas incapable of secreting sufficient insulin, possibly due to a deficiency in the gene which codes for the production of to a deficiency in the gene which codes for the production of insulin, or because of an attack on the insulin, or because of an attack on the ββ cells by the person’s cells by the person’s own immune systemown immune system

• Non-insulin-dependent diabetesNon-insulin-dependent diabetes – pancreas does secrete – pancreas does secrete insulin but the liver and muscle cells do not respond properly insulin but the liver and muscle cells do not respond properly to it (associated with obesity)to it (associated with obesity)

– Symptoms:Symptoms:• Blood glucose levels rise and stay highBlood glucose levels rise and stay high• Glucose in urine because kidney cannot reabsorb all the Glucose in urine because kidney cannot reabsorb all the

glucoseglucose• Extra water and salts accompany glucose so the person feels Extra water and salts accompany glucose so the person feels

extremely hungry and thirstyextremely hungry and thirsty• The combination of dehydration, salt loss and low blood pH The combination of dehydration, salt loss and low blood pH

can cause coma in extreme situations. Build-up of substances can cause coma in extreme situations. Build-up of substances called keto-acids in the blood, due to metabolism of fats and called keto-acids in the blood, due to metabolism of fats and proteins as an alternative energy source, lowers the blood pH.proteins as an alternative energy source, lowers the blood pH.

• Coma may also result because of lack of glucose for Coma may also result because of lack of glucose for respiration due to the lack of glycogen to be mobilised. respiration due to the lack of glycogen to be mobilised. Therefore, blood glucose levels of a person with untreated Therefore, blood glucose levels of a person with untreated diabetes may plummet.diabetes may plummet.

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• In insulin-dependent diabetes, regular In insulin-dependent diabetes, regular injections of insulin, together with injections of insulin, together with controlled diet, are used to keep blood controlled diet, are used to keep blood glucose levels near normalglucose levels near normal

• In non-insulin-dependent diabetes, insulin In non-insulin-dependent diabetes, insulin injections are not normally needed but injections are not normally needed but control is by diet alonecontrol is by diet alone

• Advantages of using genetically engineered Advantages of using genetically engineered human insulin:human insulin:– More rapid responseMore rapid response– Shorter duration of responseShorter duration of response– Less chance of an immune response to the Less chance of an immune response to the

insulin developinginsulin developing– Effective in people who have developed a Effective in people who have developed a

tolerance for animal-derived insulintolerance for animal-derived insulin– More acceptable to people who feel it is More acceptable to people who feel it is

unethical to use pig or cattle insulinunethical to use pig or cattle insulin

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Nervous communicationNervous communicationNeuronesNeurones • aka nerve cells: cells which are specialised for the conduction of aka nerve cells: cells which are specialised for the conduction of

action potentialsaction potentials• Motor neuroneMotor neurone: transmits messages from the brain or spinal cord : transmits messages from the brain or spinal cord

to a muscle or gland. Cell body of a motor neurone lies within the to a muscle or gland. Cell body of a motor neurone lies within the spinal cord or brain.spinal cord or brain.

• DendriteDendrite//dendrondendron: a short/long cytoplasmic process of neurone, : a short/long cytoplasmic process of neurone, that conducts action potential towards the cell bodythat conducts action potential towards the cell body

• AxonAxon: a long cytoplasmic process of a neurone, that conducts action : a long cytoplasmic process of a neurone, that conducts action potentials away from the cell body (may be extremely long); potentials away from the cell body (may be extremely long); contains all usual organelles, large number of mitchondria at the contains all usual organelles, large number of mitchondria at the tips of terminal branches of the axon and vesicles containing tips of terminal branches of the axon and vesicles containing chemicals called transmitter substanceschemicals called transmitter substances

• Schwann cellsSchwann cells: cells which is in close association with a neurone, : cells which is in close association with a neurone, whose plasma membrane wraps round and round the axon or whose plasma membrane wraps round and round the axon or dendron of the neurone to form a dendron of the neurone to form a myelin sheathmyelin sheath (made largely of (made largely of lipid and protein)lipid and protein)

• Not all axons have myelin sheathsNot all axons have myelin sheaths• The sheath affects the speed of conduction of the nerve impulseThe sheath affects the speed of conduction of the nerve impulse• Node of RanvierNode of Ranvier: a short gap in the myelin sheath surrounding an : a short gap in the myelin sheath surrounding an

axon (every 1-3 mm in human neurones, 2-3 axon (every 1-3 mm in human neurones, 2-3 μμm long)m long)• Sensory neuroneSensory neurone: bring impulses from receptors to the brain or : bring impulses from receptors to the brain or

spinal cord (one long dendron and an axon shorter than the spinal cord (one long dendron and an axon shorter than the dendron)dendron)

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A reflex arcA reflex arc• A pathway along which impulses are carried from A pathway along which impulses are carried from

a receptor to an effector, without involving a receptor to an effector, without involving ‘conscious’ regions of the brain‘conscious’ regions of the brain

• Reflex actionReflex action: a fast, automatic response to a : a fast, automatic response to a stimulus; may be innate (inborn) or learned stimulus; may be innate (inborn) or learned (conditioned)(conditioned)

Transmission of nerve impulsesTransmission of nerve impulses• Neurones transmit impulses as electrical signalsNeurones transmit impulses as electrical signals• Signals travel very rapidly along the plasma Signals travel very rapidly along the plasma

membrane from one end of the cell to the other membrane from one end of the cell to the other and are and are notnot a flow of electrons like an electric a flow of electrons like an electric currentcurrent

• Signals are brief changes in the distribution of Signals are brief changes in the distribution of electrical charge across the plasma membrane, electrical charge across the plasma membrane, caused by the very rapid movement of sodium caused by the very rapid movement of sodium and potassium ions into and out of the axonand potassium ions into and out of the axon

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Resting potentialResting potential • In a resting axon, inside of the axon always has a In a resting axon, inside of the axon always has a

slightly negative electrical potential compared with slightly negative electrical potential compared with the outsidethe outside

• The difference between these potentials (The difference between these potentials (potential potential differencedifference) is around -65mV (inside lower than ) is around -65mV (inside lower than outside)outside)

• Resting potential is produced and maintained by the Resting potential is produced and maintained by the sodium-potassium pumpsodium-potassium pump in the plasma in the plasma membrane of the axonmembrane of the axon

• The process involves moving the ions against their The process involves moving the ions against their concentration gradients, and so use energy from concentration gradients, and so use energy from the hydrolysis of ATPthe hydrolysis of ATP

• The sodium-potassium pump removes 3 sodium The sodium-potassium pump removes 3 sodium ions from the cell for every 2 potassium ions it ions from the cell for every 2 potassium ions it brings into the cell (Kbrings into the cell (K++ diffuses back faster than Na diffuses back faster than Na++))

• The result is an overall excess of positive ions The result is an overall excess of positive ions outside the membrane compared with the insideoutside the membrane compared with the inside

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• Action potentialsAction potentials– A fleeting reversal of the resting potential across the plasma A fleeting reversal of the resting potential across the plasma

membrane of a neurone, which rapidly travels along its length, membrane of a neurone, which rapidly travels along its length, caused by changes in permeability of the plasma membrane to Nacaused by changes in permeability of the plasma membrane to Na++ and Kand K++

– Voltage-gated channelsVoltage-gated channels: a protein channel through a cell : a protein channel through a cell membrane that opens or closes in response to changes in electrical membrane that opens or closes in response to changes in electrical potential across membrane, that allow Napotential across membrane, that allow Na++ and K and K++ to pass through to pass through

– The electric current used to stimulate the axon causes the opening The electric current used to stimulate the axon causes the opening of the channels in the plasma membrane which allow Naof the channels in the plasma membrane which allow Na++ to pass to pass through (they flood through the open channels)through (they flood through the open channels)

– The high concentration of positively charged NaThe high concentration of positively charged Na++ inside the axon inside the axon makes it less negative makes it less negative insideinside than it was before. The membrane is than it was before. The membrane is said to be said to be depolariseddepolarised

– As NaAs Na++ continue to flood in, the inside of the axon swiftly continues continue to flood in, the inside of the axon swiftly continues to build up positive charge, until it reaches a potential of +40mV to build up positive charge, until it reaches a potential of +40mV compared with the ousidecompared with the ouside

– At this point NaAt this point Na++ channels close and K channels close and K++ channels open channels open – KK++ diffuses diffuses outout of the axon down concentration gradient of the axon down concentration gradient– The outward movement of KThe outward movement of K++ removes positive charge from inside removes positive charge from inside

the axon to the outside, thus beginning to return the potential the axon to the outside, thus beginning to return the potential difference to normal (difference to normal (repolarisationrepolarisation))

– So many KSo many K+ + leave the axon that the potential difference across the leave the axon that the potential difference across the membrane briefly becomes even more negative than the normal membrane briefly becomes even more negative than the normal resting potentialresting potential

– The potassium channels then close, and the sodium-potassium The potassium channels then close, and the sodium-potassium pump begins to acts again, restoring the normal distribution of Napump begins to acts again, restoring the normal distribution of Na++ and Kand K++ across the membrane and restoring the resting potential across the membrane and restoring the resting potential

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• Transmission of action potentialsTransmission of action potentials– The function of a neurone is to transmit The function of a neurone is to transmit

information information alongalong itself itself– An action potential at any point in an axon’s An action potential at any point in an axon’s

plasma membrane triggers the production of an plasma membrane triggers the production of an action potential in the membrane on either side action potential in the membrane on either side of itof it

– The temporary depolarisation of the membrane The temporary depolarisation of the membrane where the action potential is causes a ‘local where the action potential is causes a ‘local circuit’ to be set up between the depolarised circuit’ to be set up between the depolarised region and the resting regions on either side of itregion and the resting regions on either side of it

– NaNa++ flow sideways inside the axon (away from flow sideways inside the axon (away from positively charged region towards the negatively positively charged region towards the negatively charged regions on either sides). This charged regions on either sides). This depolarises these adjoining regions and so depolarises these adjoining regions and so generates an action potential in themgenerates an action potential in them

– Refractory periodRefractory period: a period of time during : a period of time during which a neurone is recovering from an action which a neurone is recovering from an action potential, and during which another action potential, and during which another action potential cannot be generatedpotential cannot be generated

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• How action potentials carry informationHow action potentials carry information– Action potentials do not change in size (+40mV) Action potentials do not change in size (+40mV)

nor speed at which it travels (the intensity of the nor speed at which it travels (the intensity of the stimulus which orginally generated the action stimulus which orginally generated the action potential has absolutely no effect on the size of potential has absolutely no effect on the size of the action potential)the action potential)

– The action potential The action potential frequencyfrequency differs between a differs between a strong and a weak stimulus (strong stimulus strong and a weak stimulus (strong stimulus produces a rapid succession of action potential produces a rapid succession of action potential and vice versa)and vice versa)

– A strong stimulus is likely to stimulate more A strong stimulus is likely to stimulate more neurones than a weak stimulusneurones than a weak stimulus

– The brain can interpret the The brain can interpret the frequencyfrequency of action of action potentials arriving along the axon of a sensory potentials arriving along the axon of a sensory neurone, and the neurone, and the numbernumber of neurones carrying of neurones carrying action potentials, to get information about the action potentials, to get information about the strengthstrength of the stimulus being detected by that of the stimulus being detected by that receptorreceptor

– The The naturenature of the stimulus (light, heat, touch) is of the stimulus (light, heat, touch) is deduced from the deduced from the positionposition of the sensory neurone of the sensory neurone bringing the information (e.g. retina)bringing the information (e.g. retina)

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• Speed of conductionSpeed of conduction– Myelinated human neurone: 100msMyelinated human neurone: 100ms-1-1

– Nonmyelinated neurones: 0.5msNonmyelinated neurones: 0.5ms-1-1

– Myelin speeds up rate by insulating axon Myelin speeds up rate by insulating axon membranemembrane

– NaNa++ and K and K++ cannot flow through myelin sheath cannot flow through myelin sheath (not possible for depolarisation or action potentials (not possible for depolarisation or action potentials to occur in parts surrounded by it – can only occur to occur in parts surrounded by it – can only occur at nodes of Ranvier)at nodes of Ranvier)

– Saltatory conductionSaltatory conduction: conduction of an action : conduction of an action potential along a myelinated axon or dendron, in potential along a myelinated axon or dendron, in which the action potential jumps from one node of which the action potential jumps from one node of Ranvier to the next (can increase speed of Ranvier to the next (can increase speed of transmission by up to 50 times)transmission by up to 50 times)

– Diameter also affects speed of transmission – thick Diameter also affects speed of transmission – thick axons transmit action potentials faster than thin axons transmit action potentials faster than thin onesones

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• What starts off an action potential?What starts off an action potential?– Wide variety of initial stimulus: electric current (light, Wide variety of initial stimulus: electric current (light,

touch, sound, temperature or chemicals)touch, sound, temperature or chemicals)– Receptor cellReceptor cell: a cell (in sense organs) which is sensitive : a cell (in sense organs) which is sensitive

to a change in the environment that may generate an to a change in the environment that may generate an action potential as a result of a stimulus – they convert action potential as a result of a stimulus – they convert energy in one form (light, heat or sound) into energy in an energy in one form (light, heat or sound) into energy in an electrical impulse in a neuroneelectrical impulse in a neurone

– Pacinian corpusclePacinian corpuscle: one type of receptor found in the : one type of receptor found in the dermis of the skin containing an ending of a sensory dermis of the skin containing an ending of a sensory neurone, surrounded by several layers of connective tissue neurone, surrounded by several layers of connective tissue ((capsulecapsule) – ending of sensory neurone inside capsule has ) – ending of sensory neurone inside capsule has no myelinno myelin

– Pressure applied – capsule pressed out of shape – nerve Pressure applied – capsule pressed out of shape – nerve ending deformed – Naending deformed – Na++ and K and K++ channels open – Na channels open – Na++ flood flood in/Kin/K++ flow out – membrane depolarised – increased positive flow out – membrane depolarised – increased positive charge inside axon (charge inside axon (receptor potentialreceptor potential) – if pressure ) – if pressure great enough, receptor potential becomes large enough to great enough, receptor potential becomes large enough to trigger an action potentialtrigger an action potential

– Below a certain threshold, the pressure stimulus only Below a certain threshold, the pressure stimulus only causes local depolarisation (not action potential) and causes local depolarisation (not action potential) and therefore no information is transmitted to the brain therefore no information is transmitted to the brain

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Pacinian corpusclePacinian corpuscle

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SynapsesSynapses• Synaptic cleftSynaptic cleft: a very small gap between : a very small gap between

two neurones at a synapsetwo neurones at a synapse• SynapseSynapse: a point at which two neurones : a point at which two neurones

meet but do not touch; made up of the end meet but do not touch; made up of the end of the presynaptic neurone, the synaptic cleft of the presynaptic neurone, the synaptic cleft and the end of the postsynaptic neuroneand the end of the postsynaptic neurone

• The mechanism of synaptic The mechanism of synaptic transmissiontransmission– Transmitter substanceTransmitter substance: a chemical that is : a chemical that is

released from a presynaptic neurone when an released from a presynaptic neurone when an action potential arrives that then diffuses across action potential arrives that then diffuses across the synaptic cleft and may initiate an action the synaptic cleft and may initiate an action potential in the postsynaptic neuronepotential in the postsynaptic neurone

– Action potential arrive along plasma membrane of Action potential arrive along plasma membrane of presynapticpresynaptic neurone – release transmitter neurone – release transmitter substance into cleft – transmitter substance substance into cleft – transmitter substance molecules diffuse across cleft (< a millisecond as molecules diffuse across cleft (< a millisecond as distance is so small) – sets up an action potential distance is so small) – sets up an action potential in the plasma membrane of the in the plasma membrane of the postsynapticpostsynaptic neurone neurone ALBIO9700/2012JK

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– The cytoplasm of the presynaptic neurone The cytoplasm of the presynaptic neurone contains vesicles of transmitter substance (>40 contains vesicles of transmitter substance (>40 are known: are known: noradrenalinenoradrenaline and and acetylcholineacetylcholine – – throughout nervous system; throughout nervous system; dopaminedopamine and and glutamic acidglutamic acid – only in the brain) – only in the brain)

– Cholinergic synapsesCholinergic synapses: a synapse at which the : a synapse at which the transmitter substance is acetylcholinetransmitter substance is acetylcholine

– In the part of the membrane of the presynaptic In the part of the membrane of the presynaptic neurone which is next to the synaptic cleft, the neurone which is next to the synaptic cleft, the arrival of the action potential also causes arrival of the action potential also causes calcium channelscalcium channels to open to open

– The action potential causes calcium ions to rush The action potential causes calcium ions to rush in to the cytoplasm of the presynaptic neuronein to the cytoplasm of the presynaptic neurone

– This influx of calcium ions causes vesicles of ACh This influx of calcium ions causes vesicles of ACh to move to the presynaptic membrane and fuse to move to the presynaptic membrane and fuse with it, emptying their contents into the synaptic with it, emptying their contents into the synaptic cleft (each vesicle contains up to 10 000 cleft (each vesicle contains up to 10 000 molecules of ACh which diffuses across the molecules of ACh which diffuses across the synaptic cleft in <0.5ms)synaptic cleft in <0.5ms)

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– The plasma membrane of the postsynaptic The plasma membrane of the postsynaptic neurone contains neurone contains receptor proteinsreceptor proteins which has a which has a complementary shape to part of the ACh molecule complementary shape to part of the ACh molecule – ACh temporarily binds with the receptors – shape – ACh temporarily binds with the receptors – shape of protein changes – channels open and sodium of protein changes – channels open and sodium ions can pass – sodium ions rush into the ions can pass – sodium ions rush into the cytoplasm of the postsynaptic neurone, cytoplasm of the postsynaptic neurone, depolarising the membrane and starting off action depolarising the membrane and starting off action potentialpotential

– The synaptic cleft contains acetylcholinesterase The synaptic cleft contains acetylcholinesterase which splits each ACh into acetate and choline to which splits each ACh into acetate and choline to stop action potential and avoid wasting AChstop action potential and avoid wasting ACh

– Choline is taken back into the presynaptic Choline is taken back into the presynaptic neurone, where it is combines with acetyl neurone, where it is combines with acetyl coenzyme A to form ACh which is transported into coenzyme A to form ACh which is transported into the presynaptic vesicles the presynaptic vesicles

– Whole process takes about 5-10msWhole process takes about 5-10ms– Between motor neurone and a muscle, the nerve Between motor neurone and a muscle, the nerve

forms forms motor end platesmotor end plates and the synapse is and the synapse is called a called a neuromuscular junctionneuromuscular junction

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• The effects of other chemicals at synapsesThe effects of other chemicals at synapses– NicotineNicotine

• Part of the molecule is similar in shape to ACh and will fit Part of the molecule is similar in shape to ACh and will fit into the ACh receptors on postsynaptic membranesinto the ACh receptors on postsynaptic membranes

• Unlike ACh, nicotine is not rapidly broken down by Unlike ACh, nicotine is not rapidly broken down by enzymes and so remains in the receptors fro longer than enzymes and so remains in the receptors fro longer than AChACh

• A large dose of nicotine can be fatalA large dose of nicotine can be fatal– Botulinum toxinBotulinum toxin

• Produced by an anaerobic bacterium which occasionally Produced by an anaerobic bacterium which occasionally breeds in contaminated canned foodbreeds in contaminated canned food

• Prevents the release of ACh (can be fatal)Prevents the release of ACh (can be fatal)• Injections of tiny amounts of the botulinum toxin into the Injections of tiny amounts of the botulinum toxin into the

muscles of eyelids that contract permenantly (cannot muscles of eyelids that contract permenantly (cannot open) can cause them to relax, so allowing the lids to be open) can cause them to relax, so allowing the lids to be raised raised

– Organophosphorous insecticidesOrganophosphorous insecticides• Inhibits the action of acetylcholinesterase, thus allowing Inhibits the action of acetylcholinesterase, thus allowing

ACh to cause continuous production of action potentials ACh to cause continuous production of action potentials in the postsynaptic membranein the postsynaptic membrane

• Found in flea sprays and collars for cats and dogs, Found in flea sprays and collars for cats and dogs, organophosphorous sheep-dip (used to combat infections organophosphorous sheep-dip (used to combat infections by ticks), several by ticks), several nerve gasesnerve gases also acts the same way also acts the same way

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• The roles of synapsesThe roles of synapses– Synapses slow down the rate of transmission Synapses slow down the rate of transmission

of a nerve impulse. So why have synapses?of a nerve impulse. So why have synapses?• Synapses ensure one-way transmissionSynapses ensure one-way transmission

– Allows signals to be directed towards specific goals Allows signals to be directed towards specific goals rather than spreading at randomrather than spreading at random

• Synapses increase the possible range of actions Synapses increase the possible range of actions in response to a stimulusin response to a stimulus

– Action potential arriving at some of these synapses will Action potential arriving at some of these synapses will stimulatestimulate an action potential while arriving at others an action potential while arriving at others will cause the release of transmitter substances which will cause the release of transmitter substances which will actually make it more difficult to depolarise plasma will actually make it more difficult to depolarise plasma membrane and so membrane and so inhibitinhibit production of action production of action potentialpotential

– The loss of The loss of speedspeed is more than compensated for in the is more than compensated for in the possible possible varietyvariety of responses which can be made but of responses which can be made but we do have very rapid responses called we do have very rapid responses called reflex actionsreflex actions

• Synapses are involved in memory and learningSynapses are involved in memory and learning– Picturing a face from the voicePicturing a face from the voice

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