1. 6.5 Neurons and synapses
Essential idea: Neurons transmit the message, synapses modulate the message.
The image shows a tiny segment of a human brain the lines show neurons and the dots show synapses. The image is intended to illustrate both the how complex even a small mammal's brain is and additionally how important the synapses between neurons are; it is the synapses that drive communication and conscious thought. With the exception of the memory centre the number of cells in the human brain does not increase after birth, what increase is the number of connections and hence synapses between neurons.

2. 6.5.U1 Neurons transmit electrical impulses.
Schwan cell

3. 6.5.U5 Nerve impulses are action potentials propagated along the axons of neurons.

4. Investigate how neurons generate electrical impulses
6.5.S1 Analysis of oscilloscope traces showing resting potentials and action potentials.
Investigate how neurons generate electrical impulses
Use the PhET simulation to build an understanding of resting and action potentials and how they relate to the voltage changes in the axon membrane.
The neuron lab worksheet activity acts as a guide for the investigation:

5. -70mV Plasma membrane is 50 times more permeable to K+ ions than Na+
6.5.U3 Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
-70mV
Plasma membrane is 50 times more permeable to K+ ions than Na+
n.b. proteins inside the nerve fiber are negatively charged which increases the charge imbalance.

6. 6.5.U4 An action potential consists of depolarization and repolarization of the neuron.
is the reversal (depolarization) and restoration (repolarization) of the membrane potential as an impulse travels along it. 1 2
The sodium-potassium pump (Na+/K+ pump) maintains the electrochemical gradient of the resting potential. Some K+ leaks out of the neuron (making the membrane potential negative, -70mv).
In response to a stimulus (e.g. change in membrane potential) in an adjacent section of the neuron some voltage gated Na+ channels open and sodium enters the neuron by diffusion. If a sufficient change in membrane potential is achieved (threshold potential) all the voltage gated Na+ channels open. The entry of Na+ causes the membrane potential to become positive (depolarisation)

7. 6.5.U4 An action potential consists of depolarization and repolarization of the neuron.
is the reversal (depolarization) and restoration (repolarization) of the membrane potential as an impulse travels along it. 3 4
The depolarisation of the membrane potential causes the voltage gated Na+ channels to close and the voltage gated K+ channels open. K+ diffuses out of the neuron rapidly and the membrane potential becomes negative again (repolarisation)
Before the neuron is ready to propagate another impulse the distribution of Na+ (out) and K+ (in) needs to be reset by the Na+/K+ pump, returning the neuron to resting potential. This enforced rest (refractory period) ensures impulses can only travel in a single direction.

8. 6.5.U4 An action potential consists of depolarization and repolarization of the neuron.
is the reversal (depolarization) and restoration (repolarization) of the membrane potential as an impulse travels along it.
From McGraw Hill:

9. Propagation of a nerve impulse in un-myelinated axons
6.5.U6 Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.
Propagation of a nerve impulse in un-myelinated axons
Cell body

10. More action potential resources:
6.5.U4 An action potential consists of depolarization and repolarization of the neuron.
More action potential resources:

11. myelination and saltatory conduction
6.5.U2 The myelination of nerve fibres allows for saltatory conduction.
myelination and saltatory conduction
As myelin acts as an insulator myelinated axons only allow action potentials to occur at the unmyelinated nodes of Ranvier.
This forces the the action potential to jump* from node to node (saltatory conduction).
*The jump along the axon is actually just the very rapid conduction inside the myelinated portion of the axon.
The result of this is that the impulse travels much more quickly (up to 200 m/s) along myelinated axons compared to unmyelinated axons (2 m/s).
Saltatory conduction from node to node also reduces degradation of the impulse and hence allows the impulse to travel longer distances than impulses in unmyelinated axons.
The myelin sheath also reduces energy expenditure over the axon as the quantity of sodium and potassium ions that need to be pumped to restore resting potential is less than that of a un-myelintated axon

14. 6.5.U7 Synapses are junctions between neurons and between neurons and receptor or effector cells.
To function the nervous system needs to receive input/stimuli and then to coordinate a response to it.
For this to happen impulses need to travel from sensory receptor cells via a series of nerve cells to effectors, which are commonly muscles and glands.
There are junctions between each cell called synapses across which impulses cannot travel.
A special group of molecules called neurotransmitters move across the synapse to effect an impulse in the adjacent cell.

15. 6.5.U7 Synapses are junctions between neurons and between neurons and receptor or effector cells.

16. 6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9 A nerve impulse is only initiated if the threshold potential is reached.

17. 6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9 A nerve impulse is only initiated if the threshold potential is reached.

18. 6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9 A nerve impulse is only initiated if the threshold potential is reached.

19. 6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9 A nerve impulse is only initiated if the threshold potential is reached.

20. 6.5.U8 When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. AND 6.5.U9 A nerve impulse is only initiated if the threshold potential is reached.

23. 6.5.A1 Secretion and reabsorption of acetylcholine by neurons at synapses.
Acetylcholine is a neurotransmitter used in many synapses through the nervous system
One use is at the neuromuscular junction, i.e. it is the molecule that motor neurons release to activate muscles. Interfering with the action of acetylcholine can cause a range of effect from paralysis to convulsions.

24. 6.5.A2 Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

25. Nature of science: Cooperation and collaboration between groups of scientists - biologists are contributing to research into memory and learning. (4.3)
Nowadays scientists often work in multidisciplinary teams for example the Centre for Neural Circuits and Behaviour (CNCB)
The aim of the CNCB is to understand how intelligence emerges from the physical interaction of nerve cells.
Studying the brain from this top-down approach to answer such fundamental questions requires techniques and understanding from a range of disciplines.
Gero Miesenböck FRS Waynflete Professor of Physiology, Wellcome Investigator
Martin Booth
Professor of Engineering Science
Tim Vogels
Sir Henry Dale Fellow (physicsist)
Scott Waddell
Professor of Neurobiology, Wellcome Trust Senior Research Fellow in Basic Biomedical Sciences
Stephen Goodwin
Professor of Neurogenetics, Wellcome Investigator
Korneel Hens
Group Leader (Biochemist)

26. Bibliography / Acknowledgments
Bob Smullen