1. Neurons transmit the message, synapses modulate the message.
Topic 6.5
IB Biology
Miss Werba

2. TOPIC 6 – HUMAN PHYSIOLOGY
6.1
DIGESTION & ABSORPTION
6.2
THE BLOOD SYSTEM
6.3
DEFENCE AGAINST INFECTIOUS DISEASE
6.4
GAS ECHANGE
6.5
NEURONS & SYNAPSES
6.6
HORMONES, HOMEOSTASIS & REPRODUCTION

3. THINGS TO COVER U.1 U.2 U.3 U.4 U.5 U.6 U.7 U.8 U.9 Statement Guidance
Neurons transmit electrical impulses.
The details of structure of different types of neuron are not needed.
U.2
The myelination of nerve fibres allows for saltatory conduction.
U.3
Neurons pump sodium and potassium ions across their membranes to generate a resting potential.
U.4
An action potential consists of depolarization and repolarization of the neuron.
U.5
Nerve impulses are action potentials propagated along the axons of neurons.
U.6
Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.
U.7
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.
U.8
When presynaptic neurons are depolarized they release a neurotransmitter into the synapse.
U.9
A nerve impulse is only initiated if the threshold potential is reached.

4. THINGS TO COVER A.1 A.2 S.1 Statement Guidance NOS 4.3
Secretion and reabsorption of acetylcholine by neurons at synapses.
A.2
Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.
S.1
Analysis of oscilloscope traces showing resting potentials and action potentials.
NOS
4.3
Cooperation and collaboration between groups of scientists — biologists are contributing to research into memory and learning.

5. THE NERVOUS SYSTEM
The nervous system is composed of cells called neurons that can carry rapid electrical impulses.
Peripheral Nervous System
Central Nervous System
Brain and Spinal Cord
Sensory Neurons
Motor Neurons

6. NERVE IMPULSES Neurons transmit electrical impulses:
from receptors to the CNS by sensory neurons
within the CNS by relay neurons (interneurons)
from the CNS to the effectors by motor neurons
SENSORY NEURON
MOTOR NEURON
SKIN (RECEPTOR)
MUSCLE (EFFECTOR)
INTERNEURON

7. MYELINATION
U.2
Neurons are covered by special cells (Schwann cells) which act as insulation.
These cells form the myelin sheath.
There are small gaps between these cells called nodes of Ranvier.

8. MYELINATION
U.2

9. NEURON POTENTIALS
U.3-U.6
Nerve impulses result from the movement of charged ions across an exposed membrane.
This creates an electrical difference between the regions inside and outside the membrane.
This is called the potential difference and is measured in volts (V) and millivolts (mV).
Voltage-gated ion channels allow the facilitated diffusion of ions across the membrane.
These change shape (“open” and “close”) in response to voltage changes in the neuron.

10. NEURON POTENTIALS Resting potential:
U.3-U.6
Resting potential:
The potential difference across a neuron when it is not transmitting an impulse
-70mV
Action potential:
Has 2 parts:
Depolarisation
Repolarisation
Nerve impulses are action potentials propagated along the axons of neurons

11. THE ACTION POTENTIAL
U.3-U.6

12. THE ACTION POTENTIAL Resting potential:
U.3-U.6
Resting potential:
Difference in charge across the membrane when neuron is at rest
Maintained by Na+/K+ pump (3Na+ out, 2K+ in)
Inside becomes negative relative to outside (-70mV) 12

13. THE ACTION POTENTIAL Change in potential:
U.3-U.6
Change in potential:
Na+ ions “trickle down” the axon from a neighbouring region
This changes the potential difference across the membrane
Makes the inside less negative

14. THE ACTION POTENTIAL Threshold:
U.3-U.6
U.9
Threshold:
If the potential rises to mV (threshold), an action potential will be initiated.
Voltage-gated sodium channels open

15. THE ACTION POTENTIAL Depolarisation:
U.3-U.6
Depolarisation:
Because Na+ channels are open, Na+ enters along a concentration gradient
This changes the membrane potential and causes even more Na+ channels to open.
Potential rises to approx mV

16. THE ACTION POTENTIAL Repolarisation:
U.3-U.6
Repolarisation:
K+ channels open to try to restore resting potential
K+ exits the neuron
Potential drops to approx. -70 mV

17. THE ACTION POTENTIAL Hyperpolarisation:
U.3-U.6
Hyperpolarisation:
The neuron needs energy to fine-tune the balance of Na+/K+ to return to its resting potential.
A sodium-potassium pump actively transports (ie. ATP needed) the ions across the membrane and restores the resting potential (3Na+ out, 2K+ in)

18. OSCILLOSCOPES
S.1

19. SALTATORY CONDUCTION Regions of the axon are all side by side
Means that Na+ ions can passively trickle down and stimulate an action potential in the next region
The impulse can only travel in one direction along the neuron
b/c the Na+/K+ pumps are actively restoring resting potential in the previous regions

20. SALTATORY CONDUCTION
U.2
Action potentials need an accessible membrane so it can move ions in and out of the cell
The myelin sheath covers the membrane at all points other than the Nodes of Ranvier
This is how the myelin sheath “speeds up” an impulse.
The Na+ ions will continue to trickle along the axon into the next region with available membrane channels.
This is called saltatory conduction.

21. SYNAPSES
U.7
Synapses are junctions between neurons, and between neurons and receptor or effector cells.
The electrical impulse needs to change to a chemical signal so that it can cross the synaptic gap.
Neurotransmitters are the chemicals used to cross the synapse.

22. SYNAPSES
U.7

23. SYNAPSES Action potential arrives at the pre-synaptic terminal
U.7-U.8 1
Action potential arrives at the pre-synaptic terminal
It opens voltage-gated Ca2+ channels in axon terminal of the pre-synaptic neuron  influx of Ca2+
Ca2+ causes vesicles to release neurotransmitters (NTs) into the synaptic cleft
NTs diffuse across the synapse.
NTs bind to receptors on the post-synaptic membrane and either excite (depolarise) or inhibit (hyperpolarise)
If excited enough to reach threshold, an action potential will be propagated in the post-synaptic neuron.
Then enzymes breakdown neurotransmitters to prevent continuous firing; Ca2+ is pumped back out. 2 3 4 5 6

24. ACETYLCHOLINE
A.1
Acetylcholine (ACh) is a neurotransmitter made from choline and acetyl CoA
It is released from cholinergic neurons in vesicles in response to the arrival of an action potential at the axon terminal
ACh binds to cholinergic receptors on the post-synaptic membrane to trigger an action potential in the post- synaptic neuron
ACh is hydrolysed by the acetylcholinesterase enzyme back into acetate and choline.
The choline is reabsorbed into the pre-synpatic terminal.

25. ACETYLCHOLINE
A.1
Synapses are junctions between neurons, and between neurons and receptor or effector cells.
The electrical impulse needs to change to a chemical signal so that it can cross the synaptic gap.
Neurotransmitters are the chemicals used to cross the synapse.

26. BLOCKING SYNAPTIC TRANSMISSION
Some natural and synthetic chemicals can mimic neurotransmitters.
Others inhibit transmission by blocking neurotransmitter action.
Transmission at cholinergic synapses can be blocked in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

27. BLOCKING SYNAPTIC TRANSMISSION
How do neonicotinoids work?
Neonicotinoids bind to nicotinic receptors
These are cholinergic receptors in the insect CNS
Leads to overstimulation and blocking of the receptors
This results in paralysis and eventual death.
Environmental concerns:
Can accumulate in soil
Linked to colony collapse disorder (CCD) in honey bee colonies
Linked to decline of Monarch butterflies in the USA

28. BLOCKING SYNAPTIC TRANSMISSION

29. Cooperation and collaboration between groups of scientists.
LEARNING & MEMORY
NOS 4.3
Cooperation and collaboration between groups of scientists.
Biologists are contributing to research into memory and learning.

30. LEARNING & MEMORY
NOS 4.3
The field of neuroscience is just beginning to understand some of the psychological and physiological foundations of how we learn.
Click to read the related article

31. NEURONS & SYNAPSES
Q1.
Opening of sodium channels in the membrane of a neuron results in
depolarization
repolarization
hyperpolarization
increased negative charge inside the membrane
reestablishing the resting potential
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32. NEURONS & SYNAPSES
Q2.
What is the role of active transport in the transmission of nerve impulses by neurons?
Propagates an action potential by pumping sodium ions across the membrane out of the neuron.
Propagates an action potential by pumping sodium ions across the membrane into the neuron.
Initiates the action potential needed for the transmission of an impulse by pumping calcium ions out of the endoplasmic reticulum.
Establishes the resting potential needed for the transmission of an impulse by pumping sodium and potassium ions across the membrane.
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33. NEURONS & SYNAPSES
Q3.
The diagram shows the results obtained with an oscilloscope attached to a neuron.
Why is the change in the oscilloscope
occurring between X and Y?
Hyperpolarization
Hypopolarization
Repolarization
Depolarization
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34. NEURONS & SYNAPSES
Q4. Explain the principles of synaptic transmission. [8]
J WERBA – IB BIOLOGY 34

35. NEURONS & SYNAPSES
A1. A A2. D A3. D
J WERBA – IB BIOLOGY 35

36. NEURONS & SYNAPSES
A4.
nerve impulse travels to end of presynaptic neuron;
triggers influx of Ca2+ ;
causes synaptic vesicles to fuse with membrane;
release neurotransmitter molecules into synaptic cleft;
(neurotransmitter) crosses / diffuses across channel;
(neurotransmitter) binds to receptors on next / postsynaptic neuron;
causes ion channels to open on neuron;
eg Na+ diffuse into postsynaptic neuron;
can inhibit / excite;
by hyperpolarizing / depolarizing;
neurotransmitter degraded;
Ca2+ pumped back into the synaptic cleft;
acetylcholine / GABA / dopamine / other examples of neurotransmitter
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