Synaptic Transmission I

Question 1

What does the basis for the computational power of the brain lie in ?

Answer 1

The capability of the neurons to communicate at synapses and the way in which this signalling can be adjusted in a flexible manner

Question 2

Synapses connect neurons to form ?

Answer 2

Bigger circuits

Question 3

What did Sherrington predict?

Answer 3

Sherrington predicted the need for synapses on functional grounds but it took much longer to definitively establish their presence using EM methods

Question 4

What did John Eccles argue and what did Henry Dale argue ?

Answer 4

• John Eccles argued that synaptic transmission was electrical, while Henry Dale supported a chemical mechanism
• Both were right - although chemical synapses are predominate

Question 5

Current always flows along the path of ?

Answer 5

Least electrical resistance

Question 6

Gap junctions are low resistance pathways that allow?

Answer 6

Sufficient current flow to change the postsynaptic potential and therefore propagate signal

Question 7

Electrical ‘transmission’ only occurs at ?

Answer 7

Electrical synapses

Question 8

Each gap junction is comprised of ?

Answer 8

Many gap junction channels that bridge the membranes of both neurons and ‘connect’ their cytoplasm

Question 9

What kind of channels do gap junctions have ? and what are they called ?

Answer 9

A gap junction channel is comprised of two ‘hemi-channels’ called connexons, one on each neuron with a channel pore

Question 10

What can a connexon be subdivided into ?

Answer 10

A connexon can be subdivided into six subunits - each one called a connexin. Finally each connexin protein is comprised of four membrane-spanning domains

Question 11

What are properties of electrical synapses ?

Answer 11

• 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

Question 12

What does evidence now suggest about electrical synapses ?

Answer 12

That they are capable of plasticity

Question 13

How is chemical signalling different to hormonal release?

Answer 13

Synaptic transmission is fast and precisely targeted to very local substrates, whereas endocrine signalling is slow and non-targeted

Question 14

What does the directed signalling rely on ?

Answer 14

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

Question 15

What does the action of a transmitter depend on ?

Answer 15

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

Question 16

What two key properties do receptors share ?

Answer 16

• They are membrane spanning proteins that bind transmitter

- They have an effector function influencing the opening or closing of ion channels

Question 17

What is the neuromuscular junction ? and what does it use?

Answer 17

• 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

Question 18

What does ACh produces ?

Answer 18

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

Question 19

What happens to allow ACh receptors to produce this excitatory response?

Answer 19

1. Conformational change leading to ion flow

2. Desensitisation: slower structural change making receptor ACh-insensitive

Question 20

The ‘driving force’ for movement is defined by ?

Answer 20

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

Question 21

Ions driving force is determined by ?

Answer 21

How far away its equilibrium potential is from the membrane potential

Question 22

What is a precise determination of ion flow ?

Answer 22

It can be achieved by determining the reversal potential

Question 23

In central inhibitory synapses, the typical postsynaptic response is ?

Answer 23

• A membrane hyperpolarisation, termed an inhibitory postsynaptic potential (IPSP)