The Nervous System

The nervous system is made of: Interconnected neurones, specialised for rapid transmission of impulses These carry impulses from receptor cells Neurones also carry impulses to effector cells which carryout appropriate responses

Simplest nervous system is made up of a receptor, neurone and nerve endings associated with an effector It can be much more complex Many receptors working together make up sensory organs eg the eye Complex nerve pathways exist. Sensory neurones only carry information from receptors to processing areas of the nervous system

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 here Impulses are then sent out via motor neurones

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

Simple organisms Hydra etc have simple nerve net They respond to limited stimuli They have limited effectors The nerve net is made of simple nerve cells with extensions that branch out connect up to others in various directions.

Reflex arc

Sensory Neurone

Motor Neurone

Relay neurone

Structure and Function of Neurones Neurones are the basic building blocks of a nervous system They have a cell body containing the nucleus etc along with Nissl’s granules – groups of RER and ribosomes needed to make neurotransmitters Dendrites are finger like processes that connect to neighbouring neurones

The nerve fibre itself is a slender fibre that carries the impulses Fibres that carry impulses away from the cell body are called axons Those that carry impulses toward the cell body are dendrons Relay neurones are found in the CNS linking sensory and motor neurones

Myelinated Nerve Fibres Vertebrate neurones are associated with specialist cells called Schwann cells It is a membrane that wraps round the nerve fibre many times It forms a myelin sheath There are gaps between Schwann cells called nodes of Ranvier The myelin sheath protects the nerve from damage and speeds up impulse transmission

Myelin Sheath

Speedy Nerve Impulses The thicker the fibre, the quicker the impulse Myelinated nerves are faster than unmyelinated Invertebrates have no myelin sheaths and their fibres are thin… they are slow at around 0.5ms-1 So how do they avoid danger? Some invertebrates have giant axons upto 1mm in diameter This allows impulses to move at 100ms-1

Vertebrates have both myelinated and unmylinated nerves. Voluntary motor neurones are myelinated Autonomic motor neurones eg those controlling digestive system muscles are not Myelinated nerves means you don’t need giant axons This saves room It also means that can have a more versatile network carrying impulses at up to 120 ms-1

Investigating Nerve Impulses Best way is to measure the (small) electrical changes taking place. Needs apparatus sensitive to these changes Uses micro-electrodes to record and displayed on a screen (oscilloscope) Most work done using motor neurones (axons) Can you think why?????

Nerve Impulses The basis of nerve impulses is different levels of Na+ and K+ on the inside and outside of axons Remember membranes are partially permeable It has different permeability to the 2 ions At rest the axon is impermeable to Na ions, but permeable to K ions. It also has a very active sodium/potassium pump This uses ATP to move Na+ out of the axon, K+ in

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 gradient Eventually the movement of K down the conc. gradient is stopped by the electrochemical gradient The inside of the axon is left slightly –ve compared with the outside: polarised This resting potential is around -70mV

Action Potential When 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. gradient This means the potential across the membrane is briefly reversed Cell becomes +ve on inside (compared to outside) This depolarisation last 1ms, and the difference is about +40mV This is the Action Potential

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 open So more K+ move out than should This is soon reversed when they shut

All or Nothing There is a threshold amount of Na channels needed to be open before the rush of Na+ in, is more than K+ out When this has been reached, an action potential will occur The size of the action potential is always the same. This is the ‘all or nothing’ law

Weak Stimulus Some Sodium gates opened Some Depolarisation Does NOT reach Threshold So NO Action potential

Strong Stimulus Many sodium gates open Enough depolarisation to reach threshold Action Potential produced

Very Strong Stimulus Depolarisation reaches threshold Action potential produced BUT the action potential is no bigger than before!

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 fibre This is the absolute refractory period After this there is a few ms where it can be re- stimulated, but it requires a much higher stimulus This is the relative refractory period During this time, the voltage-dependent K+ channels are still open, resting potential cant be restored until they are shut.

Refractory periods are important in the nervous system It limits rate of impulses to 500-1000 each second It also ensures impulses flow in only one direction down a nerve Until resting potential is restored, that section of a fibre cannot conduct an impulse This means the impulse can only go forward, never in reverse…

Neurones in Action

In myelinated neurones it is more complex Ions can only pass through membranes at nodes of Ranvier These occur every 1mm This means Action Potentials can only occur at nodes They appear to jump from node to node The effect of this is to speed up transmission It is called saltatory conduction (from the Latin saltare – to jump)

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 synapse Every cell in the CNS is covered with synaptic knobs from other cells Neurones don’t touch each other, there is a gap between

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 in The influx makes synaptic vesicles to move to the membrane They fuse and release their contents They contain neurotransmitter (~ 3000 molecules)

The molecules diffuse across the synaptic cleft and bond with receptors on the post-synaptic membrane This opens Na channels so there is an influx intp the post-synaptic neurone This creates an excitatory post-synaptic potential (EPSP) With sufficient EPSPs, the positive charge exceeds the threshold and an Action Potential is set up.

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 effects Other ion channels open so the inside becomes even more negative Inhibitory post-synaptic potential is set up It makes it less likely an AP will be set up IPSPs are important, for example, in how we hear sounds

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 present Nerves which use ACh are called cholinergic nerves Cholinesterase breaks it down in the cleft The products are reabsorbed and recycled Not all nerves use ACh, some (symapthetic nervous system) use noradrenaline – adrenergic nerves Dopamine is used in the CNS

Interactions Between Neurones Neurones interact in a variety of complex ways…

Summation and Facilitation Often one synaptic knob wont release enough transmitter to set up an AP If 2 or more knobs are stimulated at the same time releasing on the same membrane, the effects add together and an AP may be generated This is spatial summation

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 AP This is called temporal summation This also involves facilitation The first impulse doesn’t trigger an AP, but it makes it easier for (facilitates) the next one

Accommodation If your senses are continually triggered, you eventually get used to it and no longer notice it. This is accommodation Essentially, what has happened is all the neurotransmitter in a synaptic knob has been released It can no longer function, it is fatigued A short rest and they regenerate

Sensory Systems Sensory 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 stimulus This type of cell is a primary receptor A secondary receptor is more complicated

Secondary receptors are made up of one or more completely specialised cells (NOT neurones) These cells then synapse with a normal sensory neurone A good example is retinal cells in the eye As animals become more complex, so do their sensory systems In higher animals, many sensory receptors come together to make sensory organs.

Nerves and chemicals

The Brain’s Chemical Balance The brain uses a number of different neurotransmitters An imbalance in these transmitters can result in mental and physical symptoms Treating these imbalances means getting drugs across the blood-brain barrier This makes it difficult

Drugs that affect the brain usually work at the synapses So there are a number of stages in transmission which they can target

Illegal Drugs and the Brain

Illegal Drugs and the Brain L-dopa and SSRIs can have clear benefits They are therapeutic and legal Others are legal and enjoyable eg caffeine Caffeine crosses the blood brain barrier and affects the brain in several ways eg slows down the rate of dopamine reabsorption in synapses

Some drugs are used because of the impact they have on the brain They are often illegal Ecstasy (3,4-methylenedioxy-N- methylamphetamine or MDMA) is one Ecstasy acts as a stimulant raising heart rate It is also psychotropic Short term affects make people happy, sociable, full of energy, warm and empathetic All by affecting serotonin synapses

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 it It may also affect dopamine systems leading to the pleasure sensation

Ecstasy causes physical changes like increased heart rate It can cause problems with thermoregulation There may be no desire to drink so you overheat (hyperthermia) – can lead to death It affects the hypothalmus so you secrete more antidiuretic hormone. This stops production of urine If you keep drinking water to stay hydrated and stop overheating, you can retain so much water, osmosis destroys your cells

Sensitivity in Plants

Communication in Plants Specific chemicals released by plant cells are used to carry messages Plants rely on these chemicals to carry messages to different parts of their structure They help respond to factors like light and gravity They move cell to cell and through the transport system

Stimuli that affect plants Light Direction, intensity and length of time of exposure Gravity Water Temperature Touch (in some cases) Chemicals (in some cases)

Plant Responses Response to stimuli is by making or destroying chemical messages – plant hormones They are produced in one part of a plant, travel elsewhere and have an effect there. In animals hormones have a variety of effects In plants it is usually just a growth response - tropism

How Plants Grow Define growth… It is brought about by: Cell division Assimilation Cell expansion Cell expansion most noticeable, takes up water by osmosis Meristems are where most growth occur (just behind stem & root tips). Meristems are also the area most sensitive to hormones

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.

Sensory Systems in Plants Seeds of many plants will not germinate without exposure to light Has been shown red light (580-660nm) is most effective (in lettuce) Far red light (700-730nm) actually inhibits germination It was hypothesised a plant pigment that reacts with diff. types of light was responsible In 1960, it was found and isolated, phytochrome

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

Phytochromes enable plants to respond to cues like changes in day length Sometimes they are excitatory Other times they are inhibitory How phytochromes influence plant responses are not fully understood!

Phytochromes and Etiolation In the dark plants become etiolated: Grow rapidly using up food in an attempt to reach the light Plants become tall and thin, little chlorophyll When they reach light, growth slows and they make more chlorophyll – survival mechanism In the dark there is plenty of PR but little PFR. PFR inhibits the lengthening of internodes and stimulates both chlorophyll production and leaf growth

Photoperiodism In the UK, daylight can vary from 9 – 15hrs It acts as an important cue for organisms One of the most obvious affected activities is flowering in plants In 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.

They realised they were responding to day length as a cue With more than 14hrs light – no flowers When less than 14hrs – it flowered. Called critical day length

It has been found that plants respond differently: Short day plants (SDPs) flower when days are short Long day plants (LDPs) flower when days are long Day Neutral Plants (DNPs) don’t care! Since then it has been shown it is length of darkness, not light, that is important!!!

How the length of the dark signal is received The detection of photoperiod takes place in the leaves The presence of a hormone, florigen, was hypothesised in the 1930s It was thought florigen was made in response to changing levels of phytochromes and carried in the transport system to buds.

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

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.

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 florigen But it has been shown to move via plasmodesmata and does go to the shoot apex activating flowering genes!

Tropic responses

Plants move in response to various cues: Light from one direction (Phototropism) Gravity (Geotropism) Chemicals (chemotropism) Touch (Thigmotropism) They all require chemical signals.

When a seed starts to germinate, shoot and root need to grow. Shoot grows up to the light Root grows down into the soil where water and nutrients are.

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 it Shoots are positively phototropic, roots are negatively phototropic How does this work?!

Went’s experiments of the 1920s

Unilateral Light and Phototropisms

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.

How Do Auxins Work in a Plant?

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 shoot This movement means the tip is acting as a photoreceptor More hormone diffuses down the dark side of the plant to the zone of elongation So the dark side grows more So the plant bends towards the light