1. The Nervous System

2. 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

3. 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

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 here
Impulses 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 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.

7. Reflex arc

8. Sensory Neurone

9. Motor Neurone

10. Relay neurone

11. 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

12. 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

13. 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

14. Myelin Sheath

15. 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

16. 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

17. 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?????

20. 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

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 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

23. 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

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

25. 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

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

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

28. Very Strong Stimulus Depolarisation reaches threshold
Action potential produced
BUT 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 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.

30. Refractory periods are important in the nervous system
It limits rate of impulses to 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…

31. Neurones in Action

35. 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)

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 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

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

40. 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.

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 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

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 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

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 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

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 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

47. 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

48. 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

49. 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.

50. Nerves and chemicals

51. 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

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

53. Illegal Drugs and the Brain

55. 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

56. 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

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

58. 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

59. Sensitivity in Plants

60. 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

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

62. 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

63. 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

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 light
Has been shown red light ( nm) is most effective (in lettuce)
Far red light ( nm) 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

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 excitatory
Other times they are inhibitory
How 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 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

71. 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.

72. 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

73. 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!!!

74. 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.

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

78. Tropic responses

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

82. Went’s experiments of the 1920s

86. Unilateral Light and Phototropisms

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.

88. How Do Auxins Work in a Plant?

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 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