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Lecture 2

HTHSCI 1H06 Lecture Notes - Lecture 2: Neurotransmission, Histamine

Health Sciences
Course Code
Peter Helli

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- The transmission of nerve impulses across a synapse
Nerve Conduction
- The movement of nerve impulses down neurons
- Sometimes called propagation
- Brings messages to muscles and other nerves (dendrites)
- Conducts message out (axon)
- A substance that transmits signals across the synapse
- Released from axons (presynaptic membrane)
oSometimes from postsynaptic membrane
- Some can change voltage (generation of EPSPs or IPSPs)
- Can be removed by inactivation
oEnzyme breaks down the neurotransmitter in the synapse
- Can be removed by reuptake
oReuptake into presynaptic neuron
oCan be blocked by cocaine, SSRI’s and tranylcypromine
- Can be removed by diffusion
oReleased neurotransmitter molecules diffuse away from the synaptic cleft
- endocanabanoids – we make our self
Types of Neurotransmitters
oBiogenic amines
Contain amine group
Sleep, dreaming,
oAmino acids
Only found in the CNS
Substance P – pain neurotransmitter
Enkephalins – anti pain receptors
Catecholamines Synthesis
- LDOPA is given to people with Parkinsons Disease to enhance dopamine
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- Parkinson’s disease is marked by a decrease in the number and activity of
dopamine in the brain so enhanced neurological function will result in increased
dopamine synthesis
Conduction of Nerve Impulses
* In an axon, the potential difference is exploited to send a signal
1) Plasma membrane of axon separates negative cytoplasm from positive
extracellular fluid
2) Ions are lined up along membrane (more positives on the outside than inside)
EMG (electromyography)
- Measures electrical activity in the muscles
- Know when nerve is stimulated and when muscle reacts
Depolarization Along the Axon
- In excitable tissues like axons and muscle fibers, a wave of positive potential
moves away form the point where the stimulus is initiated.
- In an axon, its initiated at the axon hillock
1) Stimulus (increase in voltage) triggers an action potential, which depolarizes the
local membrane
2) The inside of the axon membrane becomes briefly positive. The increase in
voltage triggers the next membrane area to become positive because of the
opening of Na channels in the membrane
3) Voltage increase is terminated after it begins (closure of the Na channels and
opening of the K channels) so that only a short section of axon is depolarized at
any one time but the action potential moves along the axon.
a. The area behind the depolarized membrane is in its refractory period so
reverse conduction will not occur.
Propagation and Termination of Depolarization
- Depends on positive feedback
- Propagation: conduction of an action potential along the membrane
- Graded potentials are not propagated
- Affected by
oAmount of myelination
Faster with myelinated axons
oAxon Diameter
Faster with larger diameter axons (large surface area)
Higher temp = faster
Continuous Conduction:
1) Voltage increase causes
2) Voltage gated sodium channels open, sodium rushes in
3) Voltage gated sodium channels continue to open thus a wave of depolarization
moves down the excitable membrane
4) Voltage gated potassium channels open bringing the potential back down to below
the RMP (counteracts)
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