The Nervous System

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 organseg 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 ordendronsthe 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 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…

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(~ 3000 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