1. 6.5 Neurons and synapses
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 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 center 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
Essential idea: Neurons transmit the message, synapses modulate the message.

3. What would it be like to feel no pain?

4. 6.5.U1 Neurons transmit electrical impulses.
The Nervous System allows organisms to respond to external and internal stimuli
It consists of:
Brain and spinal cord – Central Nervous System (CNS)
Sensory Receptors & Peripheral Nerves – Peripheral Nervous System (PNS)
Neurons – functional unit of the nervous system; specialized cells for transmitting electrical and chemical signals

5. https://upload. wikimedia

6. Anatomy of a Nerve Cell:
Cell body – contains nucleus, most of the cytoplasm and most of the organelles
Dendrites and axon extend from the cell body
Dendrites – short and highly branched
Receive stimulus and send to cell body
Dendrites
Cell body

7. Axon – conducts impulses away from the cell body to another neuron or to a muscle or gland
Microscopic in diameter but may extend a meter or more in length
Divides at the end to form terminal branches that end in synaptic terminals – in motor neurons these are called motor end plates and send messages to muscles
Synaptic terminals release neurotransmitters (chemicals) that transmit impulse across the synapse (gap between neurons)

8. Myelin Sheath – fatty material surrounding the axons of neurons of the PNS; speeds up transmission of impulse
Composed of Schwann cells that form insulation
Nodes of Ranvier – gaps between Schwann cells

9. In Motor neurons these are called motor end plates
Include dendrites, cell body with nucleus, axon, myelin sheath, nodes of Ranvier and motor end plates
In Motor neurons these are called motor end plates

10. In the PNS:
Nerves consist of hundreds or thousands of axons wrapped together in connective tissue
Cell bodies are usually grouped together in masses called ganglia
In the CNS:
Bundles of axons are called tracts or pathways instead of nerves
Collection of cell bodies are called nuclei

11. Types of Neurons:
Sensory (afferent) neurons – conduct impulses into CNS from the periphery (sensory impulses)
Pick up stimulus from sensory receptors – mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors

12. Interneurons (associated neurons/relay neurons) – afferent neurons usually transmit impulses to interneurons
Located within CNS
Neurons that integrate input and output
Integration involves sorting and interpreting incoming sensory information and determining the appropriate response
Forms connecting lines between sensory and motor neurons
Brain & Spinal cord

13. Motor (efferent) neurons – transmit messages from CNS to effectors (muscle or gland)
Sensory receptors, afferent and efferent neurons are part of the Peripheral Nervous System
Afferent (input – “inform” CNS of changing conditions)
Efferent (output – transmit the “decisions” of the CNS)

14. Organization of the Nervous System

15. PNS
Motor Division
Autonomic NS :

16. 6.5.U3 Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
Membrane potential is the difference in electrical charge across the plasma membrane
Resting potential is the potential difference across a nerve cell membrane when it is not being stimulated.
Slight excess of positive ions outside the membrane and slight excess of negative ions inside the membrane
More negatively charged inside the cell compared to the interstitial fluid outside
Resting potential is normally about -70 millivolts (mV)
Membrane of neuron is polarized due to unequal distribution of ions – as a result, the cell can produce an action potential (nerve impulse)
Resting Potential

17. Sodium-potassium pumps in the membrane pump Na+ out and K+ in
6.5.U3 Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
Na+ concentration is 10x greater outside the cell and K+ concentration is 10x greater inside the cell
Sodium-potassium pumps in the membrane pump Na+ out and K+ in
K+ tends to leak out by diffusion through ion channels causing further negative charge inside as compared to outside of cell
Ion channels that allow the passage of Na+ are closed at resting potential
Include the movement of Na+ and K+ ions to create a resting potential and an action potential

18. 6.5.U4 An action potential consists of depolarization and repolarization of the neuron.
Depolarization is a change from the negative resting potential to the positive action potential
Action potential (nerve impulse) is the potential difference produced across the plasma membrane of the nerve cell when stimulated, reversing the resting potential from about -70 mV to +40 mV.
Repolarization is when the resting potential is re-established after a nerve impulse has been transmitted.

19. Action Potential Stimulation – all or none response
6.5.U5 Nerve impulses are action potentials propagated along the axons of neurons. AND 6.5.U9 A nerve impulse is only initiated if the threshold potential is reached.
Action Potential
Stimulation – all or none response
Threshold stimulation – minimum amount needed for depolarization to occur
Causes Na+ ion channels to open allowing Na+ to rush into interior of cell (depolarization)
Disturbs adjacent areas – Na+ channels open causing a depolarization wave  action potential (nerve impulse)
Polarity across membrane is momentarily reversed
K+ channels also open but more slowly allowing repolarization
Include the movement of Na+ and K+ ions to create a resting potential and an action potential

20. Impulse conduction video
6.5.U4 An action potential consists of depolarization and repolarization of the neuron.
Repolarization – after action potential passes, membrane begins to repolarize
Na+ channels close and membrane becomes impermeable to Na+
Open K+ channels allow K+ to leak out of the neuron repolarizing the membrane
Impulse is actually a series of depolarization and repolarization waves sweeping down the axon (takes place in less than 1 millisecond)
Then K+ channels close and sodium-potassium pumps are needed to reestablish resting conditions
For every three sodium ions pumped out, two potassium ions are pumped in
Include the movement of Na+ and K+ ions to create a resting potential and an action potential
Impulse conduction video

21. 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

22. 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

23. 6.5.S1 Analysis of oscilloscope traces showing resting potentials and action potentials.
Na+/K+ pump
Na+ & K+ channels closed
Na+ channels open
Inside of cell becomes positive
Na+ channels close
K+ channels open
Negative charge restored inside
Na+/K+ pump
Na+ & K+ channels closed

24. Myelinated vs. Non-myelinated
6.5.U2 The myelination of nerve fibers allows for saltatory conduction.
Myelinated vs. Non-myelinated
Impulse conduction is slower in unmyelinated axons
Here there is continuous conduction, the entire axon must depolarize
Vertebrate neurons are myelinated – speeds up transmission
Depolarization occurs only at the Nodes of Ranvier – Action Potential “jumps” from one node to the next  Saltatory conduction (“to leap”)
Degradation of the impulse is reduced and allows the impulse to travel longer distances
Energy expenditure is reduced 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
Diameter of axon also affects speed of transmission
Larger diameters transmit faster
*The jump along the axon is actually just the very rapid conduction inside the myelinated portion of the axon.
Include the movement of Na+ and K+ ions to create a resting potential and an action potential
Myelinated vs. non-myelinated animation:

25. 6.5.U7 Synapses are junctions between neurons and between neurons and receptor or effector cells.
Synapse (synaptic cleft or gap) – gap between axon of one neuron and dendrites of the next or between a neuron and an effector
Synapse between neuron and muscle cell is called a neuromuscular junction or motor end plate
Include the movement of Na+ and K+ ions to create a resting potential and an action potential

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. Include the release, diffusion and binding of the neurotransmitter, initiation of an action potential in the post-synaptic membrane, and subsequent removal of the neurotransmitter.

29. 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.

30. 6.5.A2 Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

31. 6.5.A2 Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

32. 6.5.A2 Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

33. 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)