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

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Presentation on theme: "The Nervous System J. Gilbert March 2004 BiologyMad.com."— Presentation transcript:

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

2 Overview Nerve Impulses Resting Membrane Potential
(completed12/03/04) Resting Membrane Potential How do nerve impulses start? (completed 19/03/04) Action Potential 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. Important ions in the nervous system are Na+, K+, Ca2+ and Cl- There are also negatively charged protein molecules. Important to remember - Nerve cells are surrounded by a membrane that allow some ions to pass through and blocks the passage of other ions – the membrane is semi-permeable BiologyMad.com

5 Resting Membrane Potential

6 Resting Membrane Potential
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 Resting Membrane Potential
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. Three sodium ions from inside the cell first bind to the transport protein. Then a phosphate group is transferred from ATP to the transport protein causing it to change shape and release the sodium ions outside the cell. Two potassium ions from outside the cell then bind to the transport protein and as the phosphate is removed, the protein assumes its original shape and releases the potassium ions inside the cell. BiologyMad.com

8 Resting Membrane Potential
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 Resting Membrane Potential
Menu Resting Membrane Potential Ion Concentration inside cell/mmol dm-3 Concentration outside cell/mmol dm-3 K+ 150.0 2.5 Na+ 15.0 145.0 Cl- 9.0 101.0 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 K+ ions do not move out of the neurone down their concentration gradient due to a build up of positive charges outside the membrane. This repels the movement of any more K+ ions out of the cell. Cl- ions do not move into they cytoplasm due to negatively charged protein molecules that cannot cross the surface membrane, and repel incoming Cl- ions, preventing movement into the cell.

10 Resting Membrane Potential
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 At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane. In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron. This imbalance in voltage causes a potential difference across the cell membrane – called the resting membrane potential. BiologyMad.com

11 Resting Membrane Potential
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 How do Nerve Impulses Start?
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. eg 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. In each case the correct stimulus causes the sodium channel to open; which causes sodium ions to flow into the cell; which causes a depolarisation of the membrane potential, which affects the voltage-gated sodium channels nearby and starts an action potential.

14 How do Nerve Impulses Start?
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 How do Nerve Impulses Start?
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 This phase is referred to as a depolarisation since the normal voltage polarity (negative inside) is reversed (becomes positive inside). 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 Action Potential Menu BiologyMad.com BiologyMad.com BiologyMad.com
Notice how the action potential involves an influx of  Na+ which causes a region of positive charge that then opens nearby Na+ channels. This "chain reaction" is how the action potential moves down the axon. Just behind the axtion potential, K+ channels open and K+ ions leave the axon taking a positive charge with them. This brings the axon back to resting 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 AP can only depolarise the membrane ‘in front’ as the membrane ‘behind’ is in its refractory period and cannot be depolarised again 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 How fast are impulses? AP can travel 0.1-100m/s along axons
Menu How fast are impulses? AP can travel m/s along axons Allows for fast responses to stimuli Speed is affected by: Temperature Axon diameter Myelin sheath BiologyMad.com

35 Menu 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). Na+ Sodium channel Nodes of Ranvier BiologyMad.com

36 Menu 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. Na+ Sodium channel Nodes of Ranvier BiologyMad.com

37

38 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

39 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

40 Synapses BiologyMad.com

41 Synapses Junction between two neurones is called a synapse
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

42 Synapses - Neurotransmitters
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

43 Synapses Menu Click here for animation BiologyMad.com
At the end of the pre-synaptic neurone there are voltage-gated calcium channels. When an action potential reaches the synapse these channels open, causing calcium ions to flow into the cell. These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis. The neurotransmitters diffuse across the synaptic cleft. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the channels to open. In the example shown these are sodium channels, so sodium ions flow in. This causes a depolarisation of the post-synaptic cell membrane, which may initiate an action potential. The neurotransmitter is broken down by a specific enzyme in the synaptic cleft; for example the enzyme acetylcholinesterase breaks down the neurotransmitter acetylcholine. The breakdown products are absorbed by the pre-synaptic neurone by endocytosis and used to re-synthesise more neurotransmitter, using energy from the mitochondria. This stops the synapse being permanently on. Click here for animation BiologyMad.com

44 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

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

46 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

47 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

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

49 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

50 Neurotransmitters Acetylcholine (ACh)
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

51 Neurotransmitters Noradrenaline
Released by neurones in the sympathetic nervous system Adrenergic synapses


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