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6.5 (part 1)The nervous system

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1 6.5 (part 1)The nervous system

2 6.5.1The nervous system consists of the: C entral N ervous S ystem: The brain and spinal cord P eripheral N ervous S ystem: The sensory and ‘motor’ neurones

3 A nerve is an organ containing a bundle of nerve cells called neurons. Neurons carry electrical messages called impulses throughout the body. The picture shows a nerve containing hundreds of severed neuron axons

4 6.5.2 Draw and label a diagram of the structure of a motor neuron. Include dendrites, cell body with nucleus, axon, myelin sheath, nodes of Ranvier and motor end plates. Motor End plate

5 6.5.3 State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons. Sensory Motor Relay Central nervous system – Spinal cord

6 6.5.4 Define resting potential and action potential (depolarization and repolarization). Impulses that travel down neurons are electrical – so how does this work in living things? Lets start with some definitions: “The resting potential is the electrical potential across a membrane of a cell that is not conducting an impulse” – Andrew Allott (Biology – IB study Guide page 53) “An action potential is the reversal and restoration of the electrical potential across a membrane of a cell, as an electrical impulse passes along it. This is called Depolarisation and repolarisation. – Andrew Allott (Biology – IB study Guide page 53)

7 6.5.5 Explain how a nerve impulse passes along a non- myelinated neuron.

8 There are gated channels that can open and close that can allow Sodium (Na + ) to flow through. There are also gated channels that can open and close that can allow Potassium (K + ) to flow through. Now we need to know the significance of action and resting potentials and the neuron membrane to see how a neuron works…. There are protein pumps in the membrane that can pump Sodium (Na + ) and Potasium (K + ) ions This is the axon of a motor neuron.

9 Sodium potassium Pump Sodium channel Potassium channel This is a neuron in the un- ready state. The sodium and the potassium are evenly distributed. The cell is neutrally charged by chlorine ions

10 Sodium potassium Pump Sodium channel Potassium channel The sodium potassium pumps 3 sodium ions out of the axon for every 2 potassium ions it pumps in. This leaves the axon with more positively charged ions pumped out than pumped in. So outside the membrane becomes positive and inside becomes negative. The axon is at Resting Potential

11 So now we have a neuron ready to work, lets look at how it passes an impulse from one end to the other. The gated channels are at work here. There is a higher concentration of sodium outside the cell and potassium inside and the inside is ‘-ve’ charged with the outside ‘+ve’. +-

12 If a weak ‘+ve’ charge opens the sodium channel the sodium ions flood into the cell to balance the diffusion gradient. This makes the inside of the axon near the sodium channel very ‘+ve’ and a bit further along a little more positive The weak ‘+ve’ charge opens the next sodium channel. The strong ‘+ve’ charge opens up the potassium channel. Potassium floods out. This depolarizes the axon ready for the sodium potassium pump to repolarize it. The gated sodium channels are opened by weak positive charges on the inside of the membrane The process continues along the axon taking the impulse with it

13 Some really good animations: Sodium potassium pump: Sodium potassium exchange Voltage gated channels Propagation of an action potential

14 When an action potential reaches the end of one neuron there must be a way to start an action potential in the next neuron. The two neurons will not be in direct contact and action potentials cannot jump across the gap, called a synapse (or synaptic cleft), so another method is employed Synapses

15 A synapse Post synapse Synapse Direction of impulse Pre synapse

16 The electrical impulse cannot cross the synaptic cleft, so a chemical called a neurotransmitter is released at the end of the first neurone out of the pre-synaptic membrane. It can diffuse across the synapse, bind with the second neurone on the post- synaptic membrane and generate an action potential.

17 Post synapse Synapse Direction of impulse Pre synapse Vesicles containing Neurotransmitter

18 Two examples of neurotransmitters are acetylcholine and noradrenaline. They are synthesised in vesicles, which requires energy, so the synaptic knobs have many ATP-producing mitochondria in them.

19 Synapse Direction of impulse Ca 2+ Na + Impulse continues on its way Pre synapse Post synapse Vessicle containing neurotransmitter Arrival of impulse opens Ca 2+ Channels Na + Ca 2+ Post synaptic receptor

20 As the action potential reaches the end of the first neurone, Ca2+ channels are also opened. Ca2+ flows into the cell and this induces several hundred vesicles containing the neurotransmitter to fuse with the presynaptic membrane. The neurotransmitter is released into the synaptic cleft. The molecules of neurotransmitter bind with complementary receptors (similar to an enzyme and substrate fitting together) in the postsynaptic membrane. This makes the Na+ channels open and depolarisation occurs in the postsynaptic membrane thus starting an action potential

21 To stop the neurotransmitter continually generating action potentials either the neurotransmitter is actively absorbed back into the presynaptic neurone or an enzyme is released to break it down before reabsorption.

22 Synapses break up the flow of action potentials and so slow down the transmission of impulses but they are useful... they ensure that the impulses travel only in one direction. they allow neurons to connect via neurotransmitters with many, many other neurons. This increases the range of possible responses to any particular stimulus or group of stimuli. Many drugs act by affecting the events at synapses


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