Download 6.5 Neurons and Synapses

6.5 Neurons and Synapses
Understanding:
- Neurons transmit electrical impulses
- The myelination of nerve fibers allows for
salutatory conduction
- Neurons pump sodium and potassium ions
across their membranes to generate a resting
potential
- An action potential consists of depolarisation
and repolarisation of the neurons
- Propagation of nerve impulses is the result of
local crrents that cause each successive part
of the axon to reach the threshold potential
- Synapses are junctions between neurons and
between neurons and receptor on effector
cells
- When pre-synaptic neurons are depolarised
they release a neurotransmitter into the
synapse
- A nerve impulse is only initiated if the
threshold potential is reached
Applications:
- Secretion and reabsorption of
acetylcholine by neurons at synapses
- Blocking of synaptic transmission at
cholinergic synapses in insects by
binding of neonicotinoid pesticides to
acetylcholine receptors
Nature of science:
- Cooperation and collaboration
between groups of scientists:
biologists are contributing to research
into memory and learning.
Skills:
- Analysis of oscilloscope traces
showing resting potentials and action
potentials.
Communication in the body
Two systems:
1. Endocrine system
2. Nervous system
Make a brief summary of the organs involved in
each and what each organ does.
Endocrine
Endocrine
Consists of
glands,
pancreas and
reproductive
organs
Releases
hormones
Nervous
Nervous
Consists of
nerve cells
called neurons
Central and
peripheral
85 billion
neurons in
humans
Neuron
Label the
following:
Cell body
Axon
Dendrite
Axon terminal
Nucleus
Myelin sheath
Neuron
Non-myelinated
Nerve fibres are cylindrical
Plasma membrane enclosing cytoplasm
1um diameter
Nerve impulse 1 metre/second
Myelinated Sheath
Many layers of phospholipids bilayers
Schwann cells deposit myelin when it grows
round the axon
Node of Ranvier = gap between myelin
deposited by adjacent Schwann cells.
Myelinated Sheath
Nerve impulse jumps from one node of Ranvier
to the next.
Saltatory conduction
100 metres/second
Non Myelinated
Myelinated
Non Myelinated
Myelinated
No myelin layers
Schwann Cells make myelin
that wraps round axon
Impulse is 1m/s
Impulse jumps between
Nodes of Ranvier 100m/s
Multiple Sclerosis
What are the
symptoms?
What causes
it?
Is there a cure?
Resting Potential (-70 mV)
No transmission = resting potential
There IS a potential difference across the
membrane
Due to an imbalance of positive and negative
charges across the membrane
Resting Potential (-70 mV)
Continuous sodium potassium pumps transfer
sodium and potassium ions across the
membrane
3 sodium ions out, 2 potassium ions in
Concentration gradients created for both
Resting Potential (-70 mV)
Membrane is 50 times more permeable to
potassium ions
Potassium ions leak back across the membrane
faster than sodium ions
Sodium concentration gradient very steep
Resting Potential (-70 mV)
Overall there is a charge imbalance
Negative charge on the inside
Positive charge on the outside
Action Potential
Rapid change in membrane potential
1. Depolarisation – negative to positive
2. Repolarisation – positive to negative
(inside membrane of axon)
Depolarisation (+30mV)
Due to opening of
sodium channels
Allows sodium ions to
diffuse into neuron
down concentration
gradient
Reverses the charge
imbalance
Inside becomes positive
Repolarisation (-70mV)
Rapidly after
depolarisation
Sodium channels close
Potassium channels open
Potassium ions diffuse out
of ion down their
concentration gradient
Inside of cell becomes
negative again
Resting Potential (-70 mV)
Returns to resting potential before another
impulse can be sent
Sodium potassium pump moving sodium ions
out, and potassium ions in
Label the diagrams
Summarise what happens during each stage
Include the following on the diagrams:
1. Label pumps/proteins
2. What are pumps/proteins doing at each point
3. K+ and Na+ movement
4. Overall charges inside and outside
5. The potential inside the neuron (+30/-70mV)
Action Potential
Depolarisation
Sodium channels
Sodium ions
Potassium
channels
Potassium ions
Charge change
inside neuron
Overall charge in
neuron after
Repolarisation
Action Potential
Depolarisation
Repolarisation
Sodium channels
Open
Close
Sodium ions
Stay inside neuron
Potassium
channels
Potassium ions
Diffuse into
neuron
Closed
Stay inside neuron
Charge change
inside neuron
Negative to
positive
Diffuse out of
neuron
Positive to
negative
Overall charge in
neuron after
Positive (+30mV)
Negative (-70mV)
Open
Nerve impulses
Start at one end of a
neuron
Propagated along axon
to other end of neuron
Only move in one
direction as they are
only initiated at one end
of the neuron
Ion movements that
depolarise one part of
the neuron trigger
depolarisation of the
neighbouring part
Local Currents
The propagation of an action potential along the
axon is due to sodium ion movements
Sodium ion concentration increases inside the axon
during depolarisation
Sodium ions then diffuse inside the axon to the area
that has not yet been depolarised
The same happens outside of the axon, however
sodium ions move from the area that has not been
depolarised to the area that has
Local Currents
This causes the membrane potential in the
unpolarised area to rise from -70mV to -50mV
The Sodium channels are voltage-gated and open
when a membrane potential of -50mV has been
reached
This is known as the threshold potential
Oscilloscope traces
Membrane potentials can be measured
Displayed in an oscilloscope
Time on x-axis
Membrane potential on y-axis
Resting potential = -70mV
Local currents = rise to -50mV
Action potential = narrow spike to +30mV
Returns to resting potential = -70mV
Oscilloscope traces
Membrane potentials can be measured
Displayed in an oscilloscope
Time on x-axis
Membrane potential on y-axis
Resting potential = -70mV
Local currents = rise to -50mV
Action potential = narrow spike to +30mV
Returns to resting potential = -70mV
Synapses
Junctions between cells in the nervous system
Chemicals (neurotransmitters) send signals across
synapses
Synaptic Transmission
1. Nerve impulse propagated along pre-synaptic
neuron until it reaches the end of the neuron
and the pre-synaptic membrane
Synaptic Transmission
2. Depolarisation of pre-synaptic membrane
causes calcium ions (Ca2+) to diffuse through
channels in the membrane into the neuron
Synaptic Transmission
3. Influx of Ca2+ causes vesicles containing
neurotransmitter to move to the pre-synaptic
membrane and fuse with it
Synaptic Transmission
4. Neurotransmitter is released into the synaptic
cleft by exocytosis
Synaptic Transmission
5. Neurotransmitter diffuses across the synaptic
cleft and binds to receptors on the post synaptic
membrane
Synaptic Transmission
6. The binding of the neurotransmitter to the
receptors causes adjacent sodium channels to
open
Synaptic Transmission
7. Sodium ions diffuse down their concentration
gradient into the post-synaptic neuron, causing it
to reach its threshold potential
Synaptic Transmission
8. An action potential is triggered in the postsynaptic membrane and is propagated along the
neuron
Synaptic Transmission
9. The neurotransmitter is rapidly broken down
and removed from the synaptic cleft
Acetylcholine
Used as a neurotransmitter at many synapses
Produced from choline (diet) and acetyl (aerobic
respiration)
Loaded into vesicles and released into synaptic
cleft
Binding sites are specific
Acetylcholinesterase
Breaks acetylcholine into chlorine and acetate
Choline reabsorbed back into pre-synaptic neuron
where it is converted back into a neurotransmitter
Threshold Potentials
Action potential generated only when threshold
potential reached
Only at this potential do voltage gated sodium
channels open causing depolarisation
Threshold Potentials
At a synapse – the amount of neurotransmitter
secreted may not be enough to cause the threshold
potential to be reached in the post-synaptic membrane
It will then not depolarise
Sodium ions now in post-synaptic neuron are pumped
back out again and resting potential is resumed
Post-synaptic neurons can have many pre-synaptic
neurons
Research
Neonicotinoids
- What are they?
- What do they do?
- Advantages?
- Arguments against?