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. Understandings Statement Guidance 6.5.U1
Neurons transmit electrical impulses.
The details of structure of different types of neuron are not needed.
6.5.U2
The myelination of nerve fibres allows for saltatory conduction.
6.5.U3
Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
6.5.U4
An action potential consists of depolarization and repolarization of the neuron.
6.5.U5
Nerve impulses are action potentials propagated along the axons of neurons.
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.
6.5.U7
Synapses are junctions between neurons and between neurons and receptor or effector cells.
Only chemical synapses are required, not electrical, and they can simply be referred to as synapses.
6.5.U8
When presynaptic neurons are depolarized they release a neurotransmitter into the synapse.
6.5.U9
A nerve impulse is only initiated if the threshold potential is reached.

3. Applications and Skills
Statement
Guidance
6.5.A1
Secretion and reabsorption of acetylcholine by neurons at synapses.
6.5.A2
Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.
6.5.S1
Analysis of oscilloscope traces showing resting potentials and action potentials.

4. A little background

5. 6.5.U1 Neurons transmit electrical impulses: Terms to know
Dendrites
branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project
Soma (cell body)
the bulbous end of a neuron, containing the cell nucleus.
Axon
also known as a nerve fibre; is a long, slender projection of a nerve cell, or neuron, that typically conducts electrical impulses away from the neuron's cell body.
Schwann cells
the principal glia (non-neuronal cells) of the peripheral nervous system (PNS)
Nodes of Ranvier
periodic gap in the insulating sheath (myelin) on the axon of certain neurons that serves to facilitate the rapid conduction of nerve impulses.
Synaptic knob/terminal end plate
change the action potential to a chemical message to interact with the neuron or effector. The process is known as synaptic transmission.
Myelin sheath—increases the speed at which the impulses can move down the neuron.

6. 6.5.U1 Neurons transmit electrical impulses.
Schwan cell

7. A little background information….
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.

9. 6.5.U5 Nerve impulses are action potentials propagated along the axons of neurons.

10. 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:

11. -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.

12. 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)

13. 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.

14. 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:

15. 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

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

17. 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

19. Sodium is found in greater concentrations outside of the cell while potassium is found in greater concentrations inside the cell. Sodium-potassium pumps exist in the plasma membrane to maintain the the concentration gradients and the membrane potential. Nerve impulses have a domino effect. An action potential in one part of the neuron causes another action potential in the adjacent part and so on. This is due to the diffusion of sodium ions between the region of the action potential and the resting potential. It is the movement of sodium and potassium that reduce the resting potential.
If the resting potential rises above the threshold level, voltage gated channels open. Voltage gated sodium channels open very fast so that sodium can diffuse into the cell down its concentration gradient. This reduces the membrane potential and results in more sodium channels opening. Sodium ions are positively charged and so the inside of the cell develops a net positive charge compared to the outside of the cell. This results in depolarization as the potential across the membrane is reversed.
A short while after this, voltage gated potassium channels open and potassium ions flow out of the cell down the concentration gradient. Since potassium ions are positively charged, their diffusion out of the cell causes a net negative charge to develop again inside the cell compared to the outside. The potential across the membrane is restored. This is called repolarization. 
Finally, the concentration gradients of both ions are restored by the sodium-potassium pump. Sodium is pumped out of the cell while potassium is pumped in. The resting potential is restored and the neuron is ready to conduct another nerve impulse. 

20. Explain how a nerve impulse passes along a non-myelinated neuron.
Summary: 
Resting potential rises above threshold level.
Voltage gated sodium channels open.
Sodium ions flow into the cell, more sodium channels open.
Inside of cell develops a net positive charge compared to the outside and results in depolarization.
Voltage gated potassium channels open. 
Potassium ions flow out of the cell.
Cell develops a net negative charge compared to the outside and results in repolarization. 
Concentration gradients restored by sodium-potassium pumps. 
Resting potential is restored.

22. 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.

23. 6.5.U7 Synapses are junctions between neurons and between neurons and receptor or effector cells.

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.
A synapse is a junction that permits a neuron to pass an electrical or chemical signal to another cell. At a synapse, the plasma membrane of the signal passing neuron (presynaptic neuron) is closely related to the plasma membrane of the target cell (postsynaptic neuron). Between the two there is a narrow fluid filled space called the synaptic cleft.  Chemical signals called neurotransmitters pass from the presynaptic neuron to the post synaptic neuron. 
This is how a synaptic transmission occurs:  An action potential travels along the neuron and reaches the end of the pre-synaptic neuron. The depolarization of the pre-synaptic membrane results in the opening of voltage gated calcium channels. Calcium ions flow into the presynaptic neuron and cause vesicles with neurotransmitters inside the neuron to fuse with the plasma membrane and release the neurotransmitters into the synaptic cleft via exocytosis. These neurotransmitters then diffuse within the synaptic cleft and some will bind to specific receptors located on the postsynaptic plasma membrane. The receptors are transmitted-gated ion channels which open and let sodium and other positively charged ions into the postsynaptic neuron when the neurotransmitters bind. As these positively charged ions enter the postsynaptic neuron they cause its membrane to depolarize.  This depolarization results in an action potential which passes down the postsynaptic neuron. The neurotransmitters in the synaptic cleft are then quickly degraded and the calcium ions are pumped back into the synaptic cleft from inside the presynaptic neuron.

29. 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.

30. Explain the principles of synaptic transmission
Explain the principles of synaptic transmission. …said a little differently
Action potential reaches the end of a presynaptic neuron.
V--Voltage gated calcium channels open.
Calcium ions flow into the presynaptic neuron.
--Vesicles with neurotransmitters Acetylcholine inside the presynaptic neuron fuse with the plasma membrane.
Neurotransmitters diffuse in the synaptic cleft and bind to receptors on the postsynaptic neuron.
The receptors are channels which open and let sodium ions into the postsynaptic neuron. 
The sodium ions cause the postsynaptic membrane to depolarize.
This causes an action potential which passes down the postsynaptic neuron. 
Neurotransmitters in the synaptic cleft are degraded and the calcium ions are pumped back into the synaptic cleft.

33. 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.

34. 6.5.A2 Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

35. 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)

36. Bibliography / Acknowledgments
Bob Smullen
Chris Paine