Synapses and synaptic transmission Flashcards
What are pre and post synaptic neurons separated by?
gap (synaptic cleft)
what does synaptic transmission across a cleft involve?
chemical messengers
what are gap junction?
holes between cells allowing communication and transfer of substance
Where are the synaptic vesicles contained?
In the presynaptic nerve.
What would you find on the postsynaptic membrane?
Receptors which act in a lock and key mechanism with the chemical structure of the neurotransmitters released from the presynaptic terminal.
What causes the vesicles to bind to presynaptic membrane?
SNAP snare complex. It is a process which docks these vesicle membranes, cleaves them and docks them to the presynaptic membrane allowing the release of neurotransmitter.
how does botox work
Botox cleaves the snap snare complex resulting in paralysis of the neuromuscular junction because no neurotransmitter can be released.
What are the events in synaptic transmission?
- Presynaptic action potential
- Depolarisation of synaptic terminal
- Opening of voltage gated Ca channels in the presynaptic membrane
- Allows for the entry of Ca and this results in calcium dependent exocytosis (activates snap snare complex).
- This binds vesicle to membrane and docks it to membrane of neuron and allows release of neurotransmitter across cleft.
- This diffuses across and acts upon a specific postsynaptic receptor of choice depending on the neurotransmitter that neuron releases.
What receptor is usually for excitatory neurotransmitters?
Glutamate (allows Na- in)
What receptor is usually for inhibitory neurotransmitters?
GABA (allows cl- in)
Reduced Vesicle Release can occur due to what 4 things
Low calcium
High magnesium
Curare (poison)
Botox
What are end plates?
postsynaptic region of single axon in muscle fibres, always excitatory
What is an end plate potential?
Graded potential
causes depolarisation of skeletal muscle fibres due to neurotransmitters binding to post synaptic membrane in neuromuscular junction
If this above the threshold potential, then it will cause an Action potential
What does the end plate potential depend on?
its size depends on number of vesicles that are released onto post synaptic site
what happened if there is some threshold signal on an end plate?
If there is some threshold signal (not enough for AP) then it will simply dissipate the further the signal travels away from that synapse (along the muscle)
What are EPSPs
EPSP’s add to generate depolarisation (more likely to fire an action potential).
what are IPSP’s
IPSP’s add to generate hyperpolarisation (less likely to fire an action potential).
what are EPSPs and IPSPs
EPSP’s and IPSP’s are sub threshold events which determine whether a cell will reach threshold to fire an action potential – or not.
EPSP’s and IPSP’s act to cancel each other out.
Temporal Summation
slow accumulation of neurotransmitter over time to exceed threshold
what is spatial summation
multiple pre-synaptic neurones synapse to one post-synaptic neurone
The closer the synapse is to the cell body, the more likely it will fire a A
Outline the knee jerk reflex.
Hitting the patella tendon causes a stretch in the quadriceps muscle, that is sensed by the sensing organ, if it is strong enough it will send an action potential back into the spinal cord.
- That action potential will activate 2 types of muscles.
- Motor neuron to quadriceps muscle causing contraction.
- The other one via an inhibitory neuron, will inhibit the efferent pathway causing the relaxation of the hamstring muscle.
- This results in the knee jerk reflex which is an antagonistic action.
Explain the pathways that occur in the knee jerk reflex.
- When you trigger the reflex you generate an action potential in the sensory neuron that action potential propagates down the purple branch.
- It will cause neurotransmitter release in the ventral horn of the spinal cord which acts on the extensor motor neuron.
- Which travels to the extensor muscle which contracts and causes the flick.
- This is an excitatory pathway so we get an excitatory postsynaptic potential (EPSP) and it is a graded potential.
- In the other neuron the exact same input will trigger an action potential on an inhibitory interneuron which signals with chloride and releases GABA (- neg charged).
- This will activate GABA channels on the flexor motor neuron, they flux chloride ions which are negatively charged therefore hyper polarising this flexor motor neuron.
- Making it an inhibitory postsynaptic potential (IPSP).
- Summary: It is the summation of miniature end plate potentials to form end plate potentials to reach the threshold which then allows for an action potential to be fired in the postsynaptic muscle cell.
What are the 3 termination mechanisms?
Main principle mechanism is re-uptake.
The second method (less common and less effective) is breakdown.
The last method is if the neurotransmitter simply diffuses away from the synapse where there is no receptor
What can occur if the neurotransmitter release is too strong?
If the neurotransmitter release is too strong, and the nerve becomes too excitable and becomes too stable - that can be toxic to the nerve because it can drive it too fast and cause damage.
