Synaptic Transmission I Flashcards
What does the basis for the computational power of the brain lie in ?
The capability of the neurons to communicate at synapses and the way in which this signalling can be adjusted in a flexible manner
Synapses connect neurons to form ?
Bigger circuits
What did Sherrington predict?
Sherrington predicted the need for synapses on functional grounds but it took much longer to definitively establish their presence using EM methods
What did John Eccles argue and what did Henry Dale argue ?
- John Eccles argued that synaptic transmission was electrical, while Henry Dale supported a chemical mechanism
- Both were right - although chemical synapses are predominate
Current always flows along the path of ?
Least electrical resistance
Gap junctions are low resistance pathways that allow?
Sufficient current flow to change the postsynaptic potential and therefore propagate signal
Electrical ‘transmission’ only occurs at ?
Electrical synapses
Each gap junction is comprised of ?
Many gap junction channels that bridge the membranes of both neurons and ‘connect’ their cytoplasm
What kind of channels do gap junctions have ? and what are they called ?
A gap junction channel is comprised of two ‘hemi-channels’ called connexons, one on each neuron with a channel pore
What can a connexon be subdivided into ?
A connexon can be subdivided into six subunits - each one called a connexin. Finally each connexin protein is comprised of four membrane-spanning domains
What are properties of electrical synapses ?
- Faithful ‘graded’ transmission of sub-threshold input: +ve or -ve changes
- High-speed transmission: useful in fast circuits
- Bidirectional signalling current can flow in both directions between neurons: reinforces synchrony
- Limited flexibility: because they are faithful conveyors of signal, they have limited usefulness for some purposes, They cannot, for example amplify signal
What does evidence now suggest about electrical synapses ?
That they are capable of plasticity
How is chemical signalling different to hormonal release?
Synaptic transmission is fast and precisely targeted to very local substrates, whereas endocrine signalling is slow and non-targeted
What does the directed signalling rely on ?
Direct signalling relies on specialised release machinery at presynaptic terminals.
NOTE: some neurons in autonomic signalling that target smooth muscle do not have these structures
What does the action of a transmitter depend on ?
It depends on the properties of the receptor not the transmitter itself; for example, acetylcholine has both excitatory and inhibitory effects, depending on the receptor
What two key properties do receptors share ?
- They are membrane spanning proteins that bind transmitter
- They have an effector function influencing the opening or closing of ion channels
What is the neuromuscular junction ? and what does it use?
- It is actually a neuron-muscle junction but the presynaptic and postsynaptic structures are very similar to a conventional neuron-neuron synapse
- It uses acetylcholine (ACh) as its transmitter
What does ACh produces ?
ACh produces an excitatory postsynaptic potential (EPSP) called an endplate potential (EPP) which is a very large postsynpatic response - driving muscle contractions - and therefore making measurements relatively easy
What happens to allow ACh receptors to produce this excitatory response?
- Conformational change leading to ion flow
2. Desensitisation: slower structural change making receptor ACh-insensitive
The ‘driving force’ for movement is defined by ?
The ‘equilibrium potential’ for each ion. This is a value determined by the the concentrations of ions on either side of the membrane and formalised by the Nernst equation
Ions driving force is determined by ?
How far away its equilibrium potential is from the membrane potential
What is a precise determination of ion flow ?
It can be achieved by determining the reversal potential
In central inhibitory synapses, the typical postsynaptic response is ?
- A membrane hyperpolarisation, termed an inhibitory postsynaptic potential (IPSP)
