Synapses and drugs Flashcards
How is the speed of impulse propagation increased?
by myellin
What is saltatory conduction?
action potentials are generated at the nodes of ranvier
What is synaptic transmission
pass on the action potential
Where is the action potential?
at the presynaptic neurone
What are the 2 alternatives to explain the process of synaptic transmission?
electrical- potential goes from one cell to next
chemical- chemical is released by pre-synaptic neurone
What is the evidence for chemical synaptic transmission?
Synaptic cleft- as its the section between the 2 neurones and if neurones are not touching each other then it cant be electrical
Synaptic delay- there is a delay of 0.5-0.8msec which means that a chemical must be involved unlike an electrical signal which is fast
Classic Otto Loewi experiment- 1 heart attached to a vagus nerve, whenn stimulating vagus nerve the heart would slow down. Then he attached 2 hearts with the vagus nerve and they both slow down. This could only happen from a chemical being released from a vagus nerve
What is the length of the synaptic cleft
30-40nm
What are electical synapses?
adjacent cells joined via gap junctions, thus ionic continunity between cells
fast but inflexible
What do gap junctions act as?
electrical synapse
What does it mean if the electrical synapse is inflexible?
you can transmitt action potential but cant modify action potential
Why are electrical synapses fast?
No neurotransmitter is released
action potential potential will go from neurone directly to the post synaptic neurone
Where are electrical synapse?
You have them in your heart and muscles of eye
What is the structure of a chemical synapse?
mitochondria
vesicles ( containing neurotransmitter)
synaptic cleft
post synaptic receptors
Why does the presynaptic area has lots of mitochondria?
its very energy intense
What are the inital stages of chemical synaptic transmission?
- the arrival of an action potential causes the influx of ca2+
- this causes the vesicle to migrate towards the presynaptic membrane
- here it fuses with the meembrane and releases the neurotransmitter via exocytosis
- the transmitter diffuses down the concentration gradient across the synaptic cleft and binds to post synaptic receptors changing the shape of the protein and ion channels open and diffuse
What can a single post synaptic cell receive their synaptic input from?
from several thousand presynaptic neurons
What is motor endplate?
junction between nerve cell and muscle cell
a neuromuscular junction
What is the experiment of recording from 2 points on a muscle fibre following neural stimulation?
1, A motor neurone is stimulated
- 2 electrodes put on muscle cell
- action potential recorded
What is an endplate?
when recording the action potential close to where the stimulus was, you have a positive bump
What did the drug curare do?
Used a drug curarae
It stopped action potential in muscle cells- paraylsis/not able to move
Ended in endplate potential
What is an endplate potential?
when the action potential on a muscle fibre can be abolished by curare, revealing the endplate potential (triggering the action potential)
What is a neurotransmitter?
ACH
how does ACH trigger action potential at neuromuscular junction ?
bind to receptors on the sarcolemma, opening sodium channels
leading to influx of sodium ions and an EPP
If threshold is reached, an action potential is generated
What happens when ACH is on the muscle cell?
it will contract as ACH is released and triggered EPP, initiating depolarisation which triggers action potential on the muscle
What was the experiment between 2 neurones?
stimulated motor neurons in the spinal cord of a cat
then recorded from post synaptic neuron
What are the 2 neurons he used?
EPSP and IPSP
What is inhibitory?
When taking it further away from threshold (getting more negative)
What does the post synaptic cell do?
takes a decision whether fire or not depending on the potentials added together in the cell body
What is EPSP?
excitatory postsynaptic potential
What is IPSP?
Inhibitory postsynaptic potential
How do you make postsynaptic cell negative IPSP?
caused by neurotransmitter opening CL channels and CL go in to neurone or
K+ open and K+ go out the cell
Both would hyperpolarise the cell and cause IPSP
How do you make the inside of a neurone positive EPSP?
Small depolarisation caused by neurotransmitter released binding to receptors opening Na+ ion channels Na+ go into the neurone causes EPSP (depolarisation)
What can neurotransmitters do?
alter ionic permability by binding onto postsynaptic membrane protein receptors (activation of receptors)
Only some neurotransmitters affect
What are the 2 mode of action of transmitters?
Direct activation of an ionic channel (na+ etc)
Use of a secondary messenger
What is a secondary messenger?
neurotransmitter released
binds to receptor which is not the ion channel, but it releases chemicals which act on ion channel and open or shut them.
What are the 3 mechanisms of transmitter inacitvation (get rid of neurotransmitter such as ACH)?
Diffusion
Reputake- presynaptic neurone sucks neurotransmitter
Enzymatic breakdown (ACH-esterase)- breakdown of neurotransmitter
Summary of events at a chemical synapse
- arrival of action potential causes influx of calcium ions
- calcium causes vesicles contraning neurotransmitter to migrate to presynaptic membrane
- transmitter is released via exocytosis
- transmitter migrates to postsynaptic receptors
- activation of receptors causes change in postsynaptic ionic permability
- transmitter inacitvation via reputake,diffusion and enzyme breakdown
- IPSP, EPSP (EPP)
What is an example of another neurotransmitter?
nitric oxide
What is dayls law
one neurone contains one neurotransmitter
What are the 2 basic types of acetylcholine receptor?
nicotinic
muscarinic
What does nicotinic receptor do?
it occurs at neuromuscular junction- ACH- open Na+ channels which therefore means its excitatory- muscles contract
What does the muscarinic receptor do?
occur on heart-ACH- open K+ and Cl- channels- are therefore inhibitory
What is the affect of ACH?
same neurotransmitter but 2 different effects depending on the receptor
What is Agonists
drug that enhances effectiveness of a neuron- turn the neurone on
What are antagonists
drug that supresses specific neurons- turn neuron off
What do drugs do?
either enhance neurotransmission or depress
What are the examples of the mode of action of a selection of therapeutic drugs?
L-dopa
Botox
Atropine
Prozak
