2Central Nervous System: Peripheral Nervous System: 6.5.1The nervous system consists of the:Central Nervous System:The brain and spinal cordPeripheral Nervous System:The sensory and ‘motor’ neurones
3What is a Nerve and what is a neuron? 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
46.5.2 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
56.5.3 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.RelayCentral nervous system – Spinal cordSensoryMotor
66.5.4 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)
76.5.5 Explain how a nerve impulse passes along a non-myelinated neuron.
8This is the axon of a motor neuron. Now we need to know the significance of action and resting potentials and the neuron membrane to see how a neuron works….This is the axon of a motor neuron.There are protein pumps in the membrane that can pump Sodium (Na+) and Potasium (K+) ionsThere 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.
9This is a neuron in the un-ready state This is a neuron in the un-ready state. The sodium and the potassium are evenly distributed. The cell is neutrally charged by chlorine ionsSodium potassium PumpSodium channelPotassium channel
10Sodium potassium PumpSo outside the membrane becomes positive and inside becomes negative. The axon is at Resting PotentialThe 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.Sodium channelPotassium channel
11So 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’.-+
12The gated sodium channels are opened by weak positive charges on the inside of the membrane. 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 process continues along the axon taking the impulse with itThis makes the inside of the axon near the sodium channel very ‘+ve’ and a bit further along a little more positiveIf a weak ‘+ve’ charge opens the sodium channel the sodium ions flood into the cell to balance the diffusion gradient.The weak ‘+ve’ charge opens the next sodium channel.+-++
13Some really good animations: Sodium potassium pump:Sodium potassium exchangeVoltage gated channelsPropagation of an action potential
14SynapsesWhen 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...
15A synapsePost synapseDirection of impulseSynapsePre synapse
16The 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.
17Post synapse Pre synapse Synapse Direction of impulse Vesicles containing Neurotransmitter
18Two examples of neurotransmitters are acetylcholine and noradrenaline 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.
19Post synapse Pre synapse Synapse Impulse continues on its way Post synaptic receptorDirection of impulseNa+Na+SynapseVessicle containing neurotransmitterCa2+Arrival of impulse opens Ca2+ ChannelsPre synapseCa2+
20As 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
21To 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.
22Synapses 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