2 The nervous system is made of: Interconnected neurones, specialised for rapid transmission of impulsesThese carry impulses from receptor cellsNeurones also carry impulses to effector cells which carryout appropriate responses
3 Simplest nervous system is made up of a receptor, neurone and nerve endings associated with an effectorIt can be much more complexMany receptors working together make up sensory organs eg the eyeComplex nerve pathways exist.Sensory neurones only carry information from receptors to processing areas of the nervous system
4 As animals increase in complexity, so do their nervous systems They develop specialised concentrations of nerves – the Central Nervous System(brain and spinal cord in us)Incoming information is processed hereImpulses are then sent out via motor neurones
5 Neurones are individual cells each one having a nerve fibre that carries impulses Nerves are bundles of nerve fibres(axons or dendrons the name relates to direction of impulse)Some nerves are exclusively motor nerves, some sensory, some are a mix
6 Simple organisms Hydra etc have simple nerve net They respond to limited stimuliThey have limited effectorsThe nerve net is made of simple nerve cells with extensions that branch out connect up to others in various directions.
11 Structure and Function of Neurones Neurones are the basic building blocks of a nervous systemThey have a cell body containing the nucleus etc along with Nissl’s granules – groups of RER and ribosomes needed to make neurotransmittersDendrites are finger like processes that connect to neighbouring neurones
12 The nerve fibre itself is a slender fibre that carries the impulses Fibres that carry impulses away from the cell body are called axonsThose that carry impulses toward the cell body are dendronsRelay neurones are found in the CNS linking sensory and motor neurones
13 Myelinated Nerve Fibres Vertebrate neurones are associated with specialist cells called Schwann cellsIt is a membrane that wraps round the nerve fibre many timesIt forms a myelin sheathThere are gaps between Schwann cells called nodes of RanvierThe myelin sheath protects the nerve from damage and speeds up impulse transmission
15 Speedy Nerve Impulses The thicker the fibre, the quicker the impulse Myelinated nerves are faster than unmyelinatedInvertebrates have no myelin sheaths and their fibres are thin… they are slow at around 0.5ms-1So how do they avoid danger?Some invertebrates have giant axons upto 1mm in diameterThis allows impulses to move at 100ms-1
16 Vertebrates have both myelinated and unmylinated nerves. Voluntary motor neurones are myelinatedAutonomic motor neurones eg those controlling digestive system muscles are notMyelinated nerves means you don’t need giant axonsThis saves roomIt also means that can have a more versatile network carrying impulses at up to 120 ms-1
17 Investigating Nerve Impulses Best way is to measure the (small) electrical changes taking place.Needs apparatus sensitive to these changesUses micro-electrodes to record and displayed on a screen (oscilloscope)Most work done using motor neurones (axons)Can you think why?????
20 Nerve ImpulsesThe basis of nerve impulses is different levels of Na+ and K+ on the inside and outside of axonsRemember membranes are partially permeableIt has different permeability to the 2 ionsAt rest the axon is impermeable to Na ions, but permeable to K ions.It also has a very active sodium/potassium pumpThis uses ATP to move Na+ out of the axon, K+ in
21 End up with less Na+ on the inside, pumped out but can’t get back in At same time, K+ gets moved in, but it diffuses back out along the concentration gradientEventually the movement of K down the conc. gradient is stopped by the electrochemical gradientThe inside of the axon is left slightly –ve compared with the outside: polarisedThis resting potential is around -70mV
23 Action PotentialWhen a neurone is stimulated there is a rapid change in membrane permeability to Na+Specific Na+ channels (sodium gates) open allowing movement to rapidly diffuse down their conc. gradientThis means the potential across the membrane is briefly reversedCell becomes +ve on inside (compared to outside)This depolarisation last 1ms, and the difference is about +40mVThis is the Action Potential
24 At the end of this brief period, Na+ channels close again Sodium pumps removed the excess ions(requires ATP)the membrane becomes hyperpolarised as voltage dependant K+ channels also openSo more K+ move out than shouldThis is soon reversed when they shut
25 All or NothingThere is a threshold amount of Na channels needed to be open before the rush of Na+ in, is more than K+ outWhen this has been reached, an action potential will occurThe size of the action potential is always the same.This is the ‘all or nothing’ law
26 Weak Stimulus Some Sodium gates opened Some Depolarisation Does NOT reach ThresholdSo NO Action potential
27 Strong Stimulus Many sodium gates open Enough depolarisation to reach thresholdAction Potential produced
28 Very Strong Stimulus Depolarisation reaches threshold Action potential producedBUT the action potential is no bigger than before!
29 The recovery time of an axon is called its refractory period This depends on the sodium/potassium pump and membrane permeability to K +For the first ms or so, you cant send another impulse down the fibreThis is the absolute refractory periodAfter this there is a few ms where it can be re- stimulated, but it requires a much higher stimulusThis is the relative refractory periodDuring this time, the voltage-dependent K+ channels are still open, resting potential cant be restored until they are shut.
30 Refractory periods are important in the nervous system It limits rate of impulses to each secondIt also ensures impulses flow in only one direction down a nerveUntil resting potential is restored, that section of a fibre cannot conduct an impulseThis means the impulse can only go forward, never in reverse…
35 In myelinated neurones it is more complex Ions can only pass through membranes at nodes of RanvierThese occur every 1mmThis means Action Potentials can only occur at nodesThey appear to jump from node to nodeThe effect of this is to speed up transmissionIt is called saltatory conduction(from the Latin saltare – to jump)
38 Synapses Neurones need to intercommunicate Receptors pass on to sensory nerves, they relay to the CNS. The CNS processes and pass on to effectors via motor neurones.Where 2 neurones meet they are linked by a synapseEvery cell in the CNS is covered with synaptic knobs from other cellsNeurones don’t touch each other, there is a gap between
39 Synapses rely on movement of Ca ions When an impulse reaches the synaptic knob, it increases the presynaptic membrane’s permeability to Ca ions (Ca ion channels open)Ca ions move inThe influx makes synaptic vesicles to move to the membraneThey fuse and release their contentsThey contain neurotransmitter (~ molecules)
40 The molecules diffuse across the synaptic cleft and bond with receptors on the post-synaptic membraneThis opens Na channels so there is an influx intp the post-synaptic neuroneThis creates an excitatory post-synaptic potential (EPSP)With sufficient EPSPs, the positive charge exceeds the threshold and an Action Potential is set up.
41 Once the transmitter has had its effect, it is broken down by enzymes in the cleft so it can react with new impulses.In some cases the transmitter can have opposite effectsOther ion channels open so the inside becomes even more negativeInhibitory post-synaptic potential is set upIt makes it less likely an AP will be set upIPSPs are important, for example, in how we hear sounds
42 Transmitter Substances One of the most common neurotransmitters found in most synapses is acetylcholine (ACh)It is made in the synaptic knob using the ATP made by all the mitochondria presentNerves which use ACh are called cholinergic nervesCholinesterase breaks it down in the cleftThe products are reabsorbed and recycledNot all nerves use ACh, some (symapthetic nervous system) use noradrenaline – adrenergic nervesDopamine is used in the CNS
43 Interactions Between Neurones Neurones interact in a variety of complex ways…
44 Summation and Facilitation Often one synaptic knob wont release enough transmitter to set up an APIf 2 or more knobs are stimulated at the same time releasing on the same membrane, the effects add together and an AP may be generatedThis is spatial summation
45 Sometimes, if one knob isnt enough to stimulate a response, but a second impulse from the same one arrives soon after, it may trigger an APThis is called temporal summationThis also involves facilitationThe first impulse doesn’t trigger an AP, but it makes it easier for (facilitates) the next one
47 AccommodationIf your senses are continually triggered, you eventually get used to it and no longer notice it.This is accommodationEssentially, what has happened is all the neurotransmitter in a synaptic knob has been releasedIt can no longer function, it is fatiguedA short rest and they regenerate
48 Sensory SystemsSensory receptors play a vital role in providing an animal with information about both its internal and external environment.Simple receptors are just neurones with a dendrite sensitive to a stimulusThis type of cell is a primary receptorA secondary receptor is more complicated
49 Secondary receptors are made up of one or more completely specialised cells (NOT neurones) These cells then synapse with a normal sensory neuroneA good example is retinal cells in the eyeAs animals become more complex, so do their sensory systemsIn higher animals, many sensory receptors come together to make sensory organs.
51 The Brain’s Chemical Balance The brain uses a number of different neurotransmittersAn imbalance in these transmitters can result in mental and physical symptomsTreating these imbalances means getting drugs across the blood-brain barrierThis makes it difficult
52 Drugs that affect the brain usually work at the synapses So there are a number of stages in transmission which they can target
55 Illegal Drugs and the Brain L-dopa and SSRIs can have clear benefitsThey are therapeutic and legalOthers are legal and enjoyable eg caffeineCaffeine crosses the blood brain barrier and affects the brain in several wayseg slows down the rate of dopamine reabsorption in synapses
56 Some drugs are used because of the impact they have on the brain They are often illegalEcstasy (3,4-methylenedioxy-N- methylamphetamine or MDMA) is oneEcstasy acts as a stimulant raising heart rateIt is also psychotropicShort term affects make people happy, sociable, full of energy, warm and empatheticAll by affecting serotonin synapses
57 MDMA blocks serotonin reuptake so the synapses are flooded with serotonin There is some evidence it makes the transport system work in reverse so serotonin in the pre- synaptic knob is moved into the cleft, flooding itIt may also affect dopamine systems leading to the pleasure sensation
58 Ecstasy causes physical changes like increased heart rate It can cause problems with thermoregulationThere may be no desire to drink so you overheat (hyperthermia) – can lead to deathIt affects the hypothalmus so you secrete more antidiuretic hormone.This stops production of urineIf you keep drinking water to stay hydrated and stop overheating, you can retain so much water, osmosis destroys your cells
60 Communication in Plants Specific chemicals released by plant cells are used to carry messagesPlants rely on these chemicals to carry messages to different parts of their structureThey help respond to factors like light and gravityThey move cell to cell and through the transport system
61 Stimuli that affect plants LightDirection, intensity and length of time of exposureGravityWaterTemperatureTouch (in some cases)Chemicals (in some cases)
62 Plant ResponsesResponse to stimuli is by making or destroying chemical messages – plant hormonesThey are produced in one part of a plant, travel elsewhere and have an effect there.In animals hormones have a variety of effectsIn plants it is usually just a growth response - tropism
63 How Plants Grow Define growth… It is brought about by: Cell divisionAssimilationCell expansionCell expansion most noticeable, takes up water by osmosisMeristems are where most growth occur (just behind stem & root tips).Meristems are also the area most sensitive to hormones
65 The effect of Light on plants Day length determines bud development, flowering, fruit ripening and leaf fall. Chlorophyll formation needs light. Without light plants die.
66 Sensory Systems in Plants Seeds of many plants will not germinate without exposure to lightHas been shown red light ( nm) is most effective (in lettuce)Far red light ( nm) actually inhibits germinationIt was hypothesised a plant pigment that reacts with diff. types of light was responsibleIn 1960, it was found and isolated, phytochrome
68 Phytochrome is blue-green pigment It exists in 2 forms P660 (PR) and P730 (PFR)When one form absorbs light it is converted into the other form.PR is more stable but it is PFR that is biologically active
69 Phytochromes enable plants to respond to cues like changes in day length Sometimes they are excitatoryOther times they are inhibitoryHow phytochromes influence plant responses are not fully understood!
70 Phytochromes and Etiolation In the dark plants become etiolated:Grow rapidly using up food in an attempt to reach the lightPlants become tall and thin, little chlorophyllWhen they reach light, growth slows and they make more chlorophyll – survival mechanismIn the dark there is plenty of PR but little PFR.PFR inhibits the lengthening of internodes and stimulates both chlorophyll production and leaf growth
71 Photoperiodism In the UK, daylight can vary from 9 – 15hrs It acts as an important cue for organismsOne of the most obvious affected activities is flowering in plantsIn the 1920s Garner and Allard studied a tobacco plant (in USA)Most tobacco plants flower in summer, they looked at one that flowered in December.
72 They realised they were responding to day length as a cue With more than 14hrs light – no flowersWhen less than 14hrs – it flowered.Called critical day length
73 It has been found that plants respond differently: Short day plants (SDPs) flower when days are shortLong day plants (LDPs) flower when days are longDay Neutral Plants (DNPs) don’t care!Since then it has been shown it is length of darkness, not light, that is important!!!
74 How the length of the dark signal is received The detection of photoperiod takes place in the leavesThe presence of a hormone, florigen, was hypothesised in the 1930sIt was thought florigen was made in response to changing levels of phytochromes and carried in the transport system to buds.
75 If the whole plant is kept in the dark but one leaf is given usual levels of light and dark – flowering occurs as normal.If the whole plant is kept in the dark, it doesn’t.Using the same set up, if the leaf that gets normal light is removed straight away after stimulus, plant still does not flower
76 If a number of plants are grafted together, but only one is exposed to correct lighting regime, all flower!You can even graft a leaf that has been exposed correctly to a plant which hasn’t been exposed correctly and it will flower.
77 Nobody has ever managed to isolate ‘florigen’ It fell out of favour and was thought not to exist.Lately though, a particular form of mRNA has been found to be produced in the leaf associated to the Flowering Locus T gene.FTmRNA was thought not to be able to leave the cell and therefore it could not be florigenBut it has been shown to move via plasmodesmata and does go to the shoot apex activating flowering genes!
79 Plants move in response to various cues: Light from one direction (Phototropism)Gravity (Geotropism)Chemicals (chemotropism)Touch (Thigmotropism)They all require chemical signals.
80 When a seed starts to germinate, shoot and root need to grow. Shoot grows up to the lightRoot grows down into the soil where water and nutrients are.
81 Phototropism In bright, all round light, plants will grow straight In unilateral light (from one side) they will bend towards it, roots move away from itShoots are positively phototropic, roots are negatively phototropicHow does this work?!
87 The growth substance (hormone) involved in phototropism is called auxin The first auxin discovered was indoleacetic acid (IAA)Auxins are now made commercially to help gardeners make cuttings root quickly.
89 The side of a plant exposed to light has less auxin in than the darker side Light seems to make auxins move laterally across the shootThis movement means the tip is acting as a photoreceptorMore hormone diffuses down the dark side of the plant to the zone of elongationSo the dark side grows moreSo the plant bends towards the light