1. 6.5 Neurons & Synapses

2. Understanding: Neurons transmit electrical impulses.
Two systems are used by internal communication:
Nervous system (neurons)
Endocrine system (glands & hormones)
A nerve impulse is an electrical signal.
Electricity is flow of electrons through a wire. Nerve impulse is flow of ions through a neuron.

3. Structure of a NEURON Draw & label on your whiteboard!

4. Structure of a NEURON

5. Motor Neuron

6. Schwann cell membrane = myelin (fatty substance)

7. Understanding: The myelination of nerve fibres allows for saltatory conduction.
From the Latin saltāre meaning to jump, leap or spring!
Nerve impulses “jump” from node to node,
allowing for a faster transmission (100m/s)
Na+ gates are only found at the nodes.

8. Propagation of a nerve impulse

9. Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
Membrane potential = difference in electrical charges across a membrane
Resting potential = difference in charges when neuron is not transmitting a signal
About -70mV

10. The purpose of the Na/K pump is to restore RESTING potential!

11. Understanding: An action potential consists of depolarization & repolarization of neuron.
Action potential = rapid rise and fall of membrane potential
= depolarization followed by repolarization
Depolarization = reversal of charges (from – to +)
Repolarization = restoration of charges (from + to -)

12. Action Potential

13. Depolarization and Repolarization
Depolarization = Na+ gates open, Na+ rushes into axon
Repolarization = K+ gates open, K+ rushes out of axon

14. Understanding: Nerve impulses are action potentials propagated along axons of neurons.
Propagate = to spread
Movement of ions that depolarize one part of axon trigger depolarization in neighboring parts of axon

15. Why can’t nerve impulses go backwards?
Impulse can only be initiated in dendrites of one neuron and can only be passed on to other neurons at axon terminals
There is a refractive period after a depolarization that prevents propagation of an action potential backwards along an axon
Nerve transmission skit!

16. Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.
Threshold potential = the critical level to which the membrane potential must be polarized in order to initiate an action potential
About -50 mV

17. Skill: Analysis of oscilloscope traces showing resting potentials and action potentials.
DBQ p 324

19. Understanding: Synapses are junctions between neurons and between neurons and receptor or effector cells.
Synapse = fluid-filled gap between 2 neurons (or between neuron and muscle, neuron and gland, or neuron and receptor cells)
Electrical impulse cannot pass across fluid
Electrical signal must turn into a chemical signal in the synapse

20. Synapse between neuron & muscle
= neuromuscular junction

21. Synapse between neuron & receptor

22. Understanding restated: Synapses are junctions between neurons and between neurons and receptor or effector cells.

23. Steps involved in Synaptic Transmission

24. Synaptic Transmission = chemical signal

25. Steps of Synaptic transmission
Nerve impulse reaches end of pre-synaptic membrane.
Depolarization of pre-synaptic membrane cause Ca2+ to diffuse thru channels in membrane
Influx of Ca2+ causes vesicles containing neurotransmitter to move to pre-synaptic membrane & fuse with it.
Neurotransmitter released into synapse by exocytosis.
Neurotransmitter diffuses across synaptic cleft and binds to receptors on post-synaptic membrane.
Binding of neurotransmitter causes adjacent sodium ion channels to open.
Na+ diffuse into post-synaptic neuron, causing membrane to reach threshold potential.
Action potential triggered in post-synaptic membrane and propagated along post-synaptic neuron.
Neurotransmitter rapidly broken down and removed from synapse.

26. Description of nerve transmission
Electrical signal  chemical signal  electrical signal
Animation: mission_across_a_synapse.html

27. Understanding: Secretion and reabsorption of acetylcholine by neurons at synapses.
Acetylcholine (Ach) = common neurotransmitter
Loaded into vesicles then released into synapse
Ach only binds to receptor for short time, so only 1 action potential initiated
Acetylcholinesterase (enzyme) breaks down Ach
Choline reabsorbed by pre-synaptic neuron and recycled/reused
A receptor is ”cholinergic” if it uses Ach as its neurotransmitter. A synapse is ”cholinergic” if it uses Ach as its neurotransmitter.

28. Where do we get choline? Also: Wheat germ Brussel sprouts Broccoli
Salmon
Milk
Peanut butter
Milk chocolate

29. Nicotine mimics Ach! Nicotine shaped like Ach
Ach involved in parasympathetic nervous system (muscle movement, breathing, heart rate, learning, memory)
Ach causes release of other neurotransmitters and hormones that affect your mood, appetite, memory
When nicotine gets into the brain, it attaches to Ach receptors and mimics Ach
Nicotine raises the levels of dopamine in the parts of the brain that produce feelings of pleasure and reward
Dopamine (the pleasure molecule) is the same neurotransmitter that is involved in addictions to other drugs like cocaine and heroin, which may explain why it is so hard for people to stop smoking.

30. Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to Ach receptors
Neo-nicotin-oid = pesticide = kills insects
Neo similar to nicotine: mimic Ach and bind to Ach receptors
Acetylcholinesterase can’t break down neo, so it stays bound to Ach
receptors so Ach can’t bind  insects become paralyzed & die.
imidacloprid = most widely used neo insecticide
Advantage: not toxic to mammals =)
Disadvantage: harm honeybees & other beneficial insects =(

31. Beneficial insects

32. Understanding: A nerve impulse is only initiated if the threshold potential is reached.
Nerve impulses follow an ALL OR NOTHING principle!
POSITIVE FEEDBACK: Stimulus  some Na+ gates open  diffusion of some Na+ increases membrane potential  causing more Na+ gates to open
If threshold potential reached, there will always be full depolarization
Threshold potential also has to be reached at the synapse.

33. How your brain makes decisions!
Post-synaptic neurons in brain & spinal cord synapse with many pre-synaptic neurons
If your brain is processing information from different sources in the body, the SUMMATION of neurotransmitters released will determine whether the nerve impulse is EXCITED or INHIBITED

34. Nature of Science: cooperation between groups of scientists: Biologists are contributing to research into memory & learning.
Research into memory & learning has typically been just an area of psychology, but now:
Molecular biology
Biochemistry
Biophysics
Medicine
Pharmacology
Computer science
Engineering
Optics
Genetics

35. Nature of Science
Centre for Neural Circuits & Behaviour at Oxford University:
Professor Miesenbock – medicine & physiology
Dr. Booth – engineering & optical microscopy
Dr. Hens – chem & biochem
Professor Waddell – genetics, molecular bio, neurobio
Research methods: optogenetics  neurons genetically engineered to emit light when firing, making brain activity visible; also made to respond to light signals
Research involves both competition and collaboration.
Success involves many different scientists in many different countries!