IBDP Biology - Neurons

In this IBDP Biology topic, you will learn about neuron transmission.

• The other method (except hormones) to pass on a message is an electrical signal system along pathways, and the cells that make these paths and carry the signals are known as neurones.

• The cell body of this neuron always lies within the spinal cord or brain.
• The dendrites are processes and conduct impulses towards the cell body.
• The long process is known as an axon, and may stretch from the brain/spinal cord all the way to your foot, so can be extremely long.
• Large numbers of mitochondria are found at the terminal branches, with vesicles containing transmitter substances.
• The Schwann cells (they are not in every neuron) wrap themselves round the axon all the way along, enclosing the axon in a myelin sheath, which affects the speed of conduction.
• The small uncovered areas of axon between Schwann cells are known as nodes of Ranvier, which together with the myelin sheath massively increase the speed of conduction.
• A sensory neuron is similar to a motor neuron except it has one long dendron and is designed to bring impulses to the brain.

• Transmission of nerve impulses is controlled by rapid movement of sodium/potassium ions into/out of the axon.
• In a resting axon, the inside has a slightly negative electrical potential when compared with the outside - this is known as the potential difference (usually -70 mV).
• This resting potential is maintained by the sodium-potassium pump of the axon, which serves to carry sodium ions to the outside and potassium to the inside utilising active transport against their concentration gradients.

• Membranes are polarized or, in other words, exhibit a RESTING MEMBRANE POTENTIAL. This means that there is an unequal distribution of ions on the two sides of the nerve cell membrane. 
• This potential generally measures about 70 millivolts (with the INSIDE of the membrane negative with respect to the outside).
• So, the resting membrane potential is expressed as -70 mV, and the minus means that the inside is negative relative to the outside. It is called a resting potential because it occurs when a membrane is not being stimulated or conducting impulses.

• Action potentials occur only when the membrane in stimulated (depolarized) enough so that sodium channels open completely.
• The minimum stimulus needed to achieve an action potential is called the threshold stimulus.
• The threshold stimulus causes the membrane potential to become less negative (because a stimulus, no matter how small, causes a few sodium channels to open and allows some positively-charged sodium ions to diffuse in).
• If the membrane potential reaches the threshold potential (generally 5 - 15 mV less negative than the resting potential), the voltage-regulated sodium channels all open. Sodium ions rapidly diffuse inward, & depolarization occurs.

• Action potentials occur maximally or not at all. In other words, there's no such thing as a partial or weak action potential. Either the threshold potential is reached and an action potential occurs, or it isn't reached and no action potential occurs. 

An action potential is the rapid fleeting change in potential difference across the membrane of an axon when an electric current is applied.

This electric current controls voltage-gated channels for sodium and potassium in the axons plasma membrane.

1. First, the electric current stimulates the axon to open channels in the plasma membrane for sodium to come through
2. Sodium floods through, depolarizing that section of the axon, until it reaches +40 mV
3. At this point, Na+ channels close and K+ channels open and K+ ions diffuse out of the axon down their concentration gradient, known as re-polarisation.
4. So many K+ ions flood out that there is a brief overshoot in negativity of the potential difference, but this is corrected by the Na/K pump - restoring the action potential.

• These action potentials are just for one area of the axon
• The depolarization of the membrane of any part of the axon sets up a 'local circuit' between the depolarized region and the resting regions either side - sodium ions flow sideways inside the axon towards the resting regions, depolarizing them and setting off an action potential.
• Information can be sent one way and does not keep coming back on itself because after an action potential, (whilst the sodium-potassium pump is returning things to normal) the region cannot create a new action potential – it is having the refractory period.

• A synapse is where two neurones meet, and includes the parts of the two neurones next to each other and the small gap between them known as the synaptic cleft (cleft means gap).

1.  An action potential arrives at the presynaptic membrane.
2.  The action potential causes calcium channels to open, thus uptake of calcium ions occurs (similar to in muscle).
3.  The calcium ions cause vesicles containing acetylcholine or noradrenaline to fuse with the presynaptic membrane
4.  The transmitter is then released and diffuses across the synaptic cleft, (process known as exocytosis).
5.  The neurotransmitter molecules then bind with the postsynaptic membranes receptors, opening sodium channels.
6.  The sodium ions that consequently flood in and depolarize the membrane and initiate an action potential.
7.  Acetylcholine is hydrolyzed and diffuses back across the synaptic into the presynaptic neuron.

Synapses increase the range of reactions in response to a stimulus - the action potentials created by neurones come together at synapses, and your reaction can be determined by this - if you have to decide whether to pull out at a junction for example, action potentials from your eyes and ears telling your brain there is a car coming will make it more difficult for the action potentials from your brain thinking 'I'm late'.

Action potentials from your eyes and ears outweigh. However, reflex reactions are quick and automatic, and the value of a rapid response is greater than a considered one if it saves your life.

• Synapses are involved in memory and learning.
• Synapses ensure one-way transmission, allowing signals to be directed and not spread at random.

note: Synapses travel in one direction because only the presynaptic neurone has vesicles containing neurotransmitter and receptors are only found found on the postsynaptic neurone.

• To prevent acetylcholine (Ach) keeping the sodium channels open and creating continuous action potential
• the synaptic cleft contains acetylcholinesterase
• an enzyme that splits each Ach molecule into acetate and choline.
• The choline is then taken back to the presynaptic neuron, and combined with acetyl co-enzyme A to form ACh once more, and prepares in vesicles for the next action potential.

• This is between a motor neuron and a muscle where the nerve forms motor end plates, and the synapse is called the neuromuscular junction.

Low frequency impulses that produce insufficient amounts of neurotransmitter to trigger a new action potential in the postsynaptic neuron, can be made to do so by the process of summation. This entails a build-up of neurotransmitter in the synapse by either:

1.  a number of different presynaptic neurones together releasing enough neurotransmitter to trigger a new action potential or
2.  a single presynaptic neuron releases neurotransmitter many times over a short period and if neurotransmitter exceeds threshold value a new action potential is triggered.

1. Nicotine: Similar in shape to ACh molecules and will fit into the receptors on the postsynaptic membranes, initiating action potentials were there are none, and is not broken down by enzymes and so stays for a longer time.
2. Organophosphorous insecticides: inhibit the enzyme that breaks down ACh, allowing continuous production of action potentials, and several nerve gases work in this way.
3. Botulinum: A toxin produced by bacterium that prevents the release of ACh, and is usually fatal to consume.

• A reflex arc is the pathway along which impulses are carried from a receptor to an effector, without conscious thought - reflexes.
• The impulse comes from the receptor, up the dorsal root of the spinal nerve, crosses into an intermediate neuron, and then reaches the cell body of motor neuron.
• It then immediately leaves to the effector via the ventral root of the spinal nerve.

• Signal does not go through the brain, this spinal response is faster

This is the end of the topic

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Photo refernences:

1. https://www.merckmanuals.com/home/brain,-spinal-cord,-and-nerve-disorders/biology-of-the-nervous-system/nerves
2. https://en.wikipedia.org/wiki/Myelin
3. https://www.cambridge.org/core/books/basic-physiology-for-anaesthetists/nerve-action-potential-and-propagation/31B6C11A49B20CC69D3EB1E5CCCFB208
4. http://hiscience.pbworks.com/w/page/102819976/2%20ESO%20NOVEMBER%202015
5. https://www.oist.jp/news-center/photos/diagram-synaptic-transmission
6. https://ib.bioninja.com.au/options/option-a-neurobiology-and/a4-innate-and-learned-behav/reflex-arcs.html