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The Nervous System

The Nervous System. J. Gilbert March 2004. BiologyMad.com. Overview. Nerve Impulses (completed12/03/04) Resting Membrane Potential (completed12/03/04) How do nerve impulses start? (completed 19/03/04) Action Potential (completed 19/03/04) How Fast are Nerve Impulses? Synapses.

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The Nervous System

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  1. The Nervous System J. Gilbert March 2004 BiologyMad.com

  2. Overview Nerve Impulses (completed12/03/04) Resting Membrane Potential (completed12/03/04) How do nerve impulses start? (completed 19/03/04) Action Potential (completed 19/03/04) How Fast are Nerve Impulses? Synapses BiologyMad.com

  3. Nerve Impulses

  4. Menu Nerve Impulses • Neurones send messages electrochemically– this means that chemicals cause an electrical impulse. • Chemicals in the body are ‘electrically charged’ when they have an electrical charge, they are called ions. BiologyMad.com

  5. Resting Membrane Potential

  6. Menu Resting Membrane Potential • When a neurone is not sending a signal, it is at ‘rest’. • The inside of the neurone is negative relative to the outside. • K+ can cross through the membrane easily • Cl- and Na+ have a more difficult time crossing • Negatively charged protein molecules inside the neurone cannot cross the membrane. BiologyMad.com

  7. Menu Resting Membrane Potential • The membranes contain sodium-potassium pumps (Na+K+ATPase). • Uses ATP to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in. BiologyMad.com

  8. Menu Resting Membrane Potential • There are also sodium and potassium ion channels in the membrane. • These channels are normally closed, but even when closed, they ‘leak’, allowing sodium ions to leak in and potassium ions leak out – down their concentration gradients. BiologyMad.com

  9. Menu Resting Membrane Potential • The imbalance of ions causes a potential difference (or voltage) between the inside of the neurone and its surroundings • The resting membrane potential is –70mV

  10. Menu Resting Membrane Potential • Overall: • K+ pass easily into the cell • Cl- and Na+ have a more difficult time crossing • Negatively charged protein molecules (A-) inside the neurone cannot pass the membrane. • The Na+K+ATPase pump uses energy to move 3 Na+ out for every 2K+ in to neurone • This imbalance in voltage causes a potential difference across the cell membrane – called the resting membrane potential. BiologyMad.com

  11. Menu Resting Membrane Potential • Membrane potential is always negative inside the cell. • The Na+K+ATPase is thought to have evolved as an osmoregulator to keep the internal water potential high and so stop water entering animal cells and bursting them. • Plant cells don’t need this as they have strong cells walls to prevent bursting. BiologyMad.com

  12. How do Nerve Impulses Start?

  13. Menu How do Nerve Impulses Start? • Neurones are stimulated by receptor cells • These contain special sodium channels that are not voltage-gated, but are gated by the appropriate stimulus. • stimulus causes the sodium channel to open • Causes sodium ions to flow into the cell • Causes a depolarisation of the membrane potential  affects the voltage-gated sodium channels nearby and starts an action potential.

  14. Menu How do Nerve Impulses Start? • Some examples: • chemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them • mechanically-gated ion channels in the hair cells of the inner ear open when they are distorted by sound vibrations; and so on.

  15. Menu How do Nerve Impulses Start? In each case the correct stimulus causes the sodium channel to open (reaches the threshold value) ↓ causes sodium ions to flow into the cell ↓ causes a depolarisation of the membrane potential ↓ affects the voltage-gated sodium channels nearby and starts an action potential.

  16. Action Potential

  17. Menu Action Potential (AP) • The resting potential tells about what happens when a neurone is at rest. • An action potential occurs when a neurone sends information down an axon. • Is an explosion of electrical activity • The resting membrane potential changes BiologyMad.com

  18. Menu AP - Depolarisation • Resting potential is –70mv (inside the axon). When stimulated, the membrane potential is briefly depolarised • Stimulus causes the membrane at one part of the neurone to increase in permeability to Na+ ions • Na+ channels open. This causes resting potential to move towards 0mV BiologyMad.com

  19. Menu AP - Depolarisation • When depolarisation reaches –30mV more Na+ channels open for 0.5ms • Causes Na+ to rush in  cell becomes more positive BiologyMad.com

  20. Menu AP - Repolarisation • At a certain point, the depolarisation of the membrane causes the Na+ channels to close • This causes K+ channels open BiologyMad.com

  21. Menu AP - Repolarisation • K+ rush out  making inside the cell more negative. • Since this restores the original polarity, it is called repolarisation • There is a slight ‘overshoot’ in the movement of K+ (called hyperpolarisation). • Resting membrane potential is restored by the Na+K+ATPase pump

  22. Menu AP - Overview (Click here for animation) BiologyMad.com

  23. Menu AP – All or nothing • AP only happens if the stimulus reaches a threshold value • Stimulus is strong enough to cause an AP • It is an ‘all or nothing event’ because once it starts, it travels to the synapse. • AP is always the same size • Frequency of the impulse carries information  strong stimulus = high frequency

  24. Menu Action Potential • At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron. BiologyMad.com

  25. Menu Action Potential • When stimulated past the threshold, sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon. BiologyMad.com

  26. Menu Action Potential • The region of positive charge causes nearby sodium channels to open. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon. BiologyMad.com

  27. Menu Action Potential • This process continues as a chain-reaction along the axon. The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon. BiologyMad.com

  28. Menu Action Potential • The sodium/potassium pump restores the resting concentrations of sodium and potassium ions BiologyMad.com

  29. Menu Action Potential BiologyMad.com BiologyMad.com BiologyMad.com

  30. Menu AP – Refractory Period • There is a time after depolarisation where no new AP can start – called the refractory period. • Time is needed to restore the proteins of voltage sensitive ion channels to their original resting conditions • NA+ channels cannot be opened, as it can’t be depolarised again • Therefore impulses travel in one direction • Can last up to 10 milliseconds – this limits the frequency of impulses BiologyMad.com

  31. BiologyMad.com

  32. AP - Refractory Period • Absolute refractory period = During the action potential, a second stimulus will not cause a new AP • Exception: There is an interval in which a second AP can be produced but only if the stimulus is considerably greater than the threshold = relative refractory period • Refractory period can limit the number of AP in a given time. • Average = about 100 action potentials/s BiologyMad.com

  33. How Fast are Nerve Impulses? BiologyMad.com

  34. Menu How fast are impulses? • AP can travel 0.1-100m/s along axons • Allows for fast responses to stimuli • Speed is affected by: • Temperature • Axon diameter • Myelin sheath BiologyMad.com

  35. Menu Na+ Na+ Na+ Sodium channel Nodes of Ranvier Myelinated Neurones • The axons of many neurones are encased in a fatty myelin sheath (schwann cells). • Where the sheath of one Schwann cell meets the next, the axon is unprotected. • The voltage-gated sodium channels of myelinated neurons are confined to these spots (called nodes of Ranvier). BiologyMad.com

  36. Menu Na+ Na+ Na+ Sodium channel Nodes of Ranvier Myelinated Neurones • The in rush of sodium ions at one node creates just enough depolarisation to reach the threshold of the next. • In this way, the action potential jumps from one node to the next (1mm) – called saltatory propagation(click here for animation) • Results in much faster propagation of the nerve impulse than is possible in nonmyelinated neurons. BiologyMad.com

  37. Facts about Propagation • Nerve impulse conduction is really the bumping of positive charge down the axon • AP initiated at one end of the axon is only propagate in one direction. • The AP doesn’t turn back because the membrane just behind is in its refractory period i.e. voltage gated Na+ channels are inactivated

  38. Facts about propagation • To increase conduction velocity: • Increase the axonal diameter • Myelin of the axon facilitates current flow down the inside of the axon. • Breaks in the myelin wrapping occur at the Nodes of Ranvier, which have increased concentrations of voltage gated Na+ channels. Regeneration of the AP occurs at the nodes • Saltatory conduction– propagation and regeneration of an AP down myelinated axon • E.g. Local anaesthesia temporarily blocks AP generation by binding the interior of voltage gated Na+ channels

  39. Synapses BiologyMad.com

  40. Menu Synapses • Junction between two neurones is called a synapse • An AP cannot cross the synaptic cleft • Impulse is carried by chemicals called neurotransmitters BiologyMad.com

  41. Menu Synapses - Neurotransmitters • Neurotransmitters are made by the cell sending the impulse (the pre-synaptic neurone) and stored in synaptic vesicles at the end of the axon • The cell receiving the impulse (post-synaptic neurone) has chemical gated ion channels called neuroreceptors BiologyMad.com

  42. Menu Synapses Click here for animation BiologyMad.com

  43. Menu Synapses • At the end of the pre-synaptic neurone there are voltage gated calcium channels. • When AP reaches the synapse, the channels open • Calcium ions flow into the cell BiologyMad.com

  44. Menu Synapses • Calcium ions cause synaptic vesicles to fuse with the cell membrane • Neurotransmitters diffuse across the synaptic cleft BiologyMad.com

  45. Menu Synapses • Neurotransmitter binds to neuroreceptors in the post-synaptic membrane • Channels open, Na+ flow in • Causes depolarisation • AP initiated in post-synaptic neurone BiologyMad.com

  46. Synapses • Function: • Prevents impulses travelling in the wrong direction. • An impulse can pass along an axon in either direction, but can only cross a synapse in one direction because the synaptic vesicles are only found in the synaptic knobs and end plates • A vast number of synaptic connections allow for great flexibility. They are equivalent to the switchboard in an elaborate telephone exchange enabling messages to be diverted from one line to another and so on

  47. Menu Integrating Signals • If the diffusion of ions reaches a threshold value, it will cause the AP in the postsynaptic membrane. BiologyMad.com

  48. Menu Neurotransmitters • Neurotransmitters are broken down by a specific enzyme in the synaptic cleft. • Breakdown products are absorbed by the pre-synaptic neurone • Used to re-synthesise more neurotransmitter BiologyMad.com

  49. Menu Neurotransmitters • Acetylcholine (ACh) • Released by motor neurones onto skeletal muscle cells • Released by neurones in the parasympathetic nervous system • Cholinergic synapses • Ach is removed from the synapse by acetylcholinesterase • Nerve gasses used in warfare (e.g. sarin) and the organophosphate insecticides (e.g. parathion) achieve their effects by inhibiting acetylcholinesterase this allowing Ach to remain active. • Atropine is used as an antidote because it blocks ACh receptors BiologyMad.com

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