Lecture 29: Synaptic Transmission

Question 1

How is a message transmitted from one neuron to another neuron (or to a muscle fibre)?

Answer 1

Across synapses via chemical signals

Question 2

How does transmission of signals occur at neuromuscular junctions?

Answer 2

1. Action potential arrives at pre-synaptic neuron
2. Activates opening of voltage-gated calcium ion channels
3. Calcium ions diffuse into the neuron and cause vesicles containing acetylcholine neurotransmitters to fuse with membrane
4. Acetylcholine neurotransmitters are released into the synaptic cleft
5. They bind to receptors on the postsynaptic neuron causing ligand-gated ion channels to open
6. Sodium and potassium ions flow into the postsynaptic causing depolarisation -> action potential created

Question 3

What are two types of chemical synapses in the CNS?

Answer 3

Excitatory synapse

Inhibitory synapse

Question 4

What is an excitatory synapse?

Answer 4

Depolarisation of postsynaptic membrane called excitatory postsynaptic potential

Question 5

What is an inhibitory synapse?

Answer 5

Hyperpolarisation of postsynaptic membrane called inhibitory postsynaptic potential

Question 6

How do EPSP (excitatory postsynaptic potential) occur?

Answer 6

Involve mainly glutamate or acetylcholine neurotransmitters -> cause transient opening of channels permeable to sodium, potassium and calcium ions

Question 7

How do IPSP (inhibitory postsynaptic potential) occur?

Answer 7

Involve gamma-aminobutyric acid (GABA) or glycine neurotransmitters -> cause transient opening of K+ channels (K+ ions will flow out therefore membrane potential becomes more negative

Question 8

What is GABA?

Answer 8

Gamma-aminobutyric acid -> type of neurotransmitter used in inhibitory synapses to generate IPSPs

Question 9

What is a neuropeptide?

Answer 9

Neuropeptides also called neuromodulators

• > large molecule chemicals that are slow
• > indirect action on postsynaptic receptors or modulatory action on the effects of other neurotransmitters

Question 10

What are the factors that determine synaptic action?

Answer 10

1. Type of neurotransmitter/neuromodulator
2. Type of neurotransmitter receptor/channel complex expressed in the postsynaptic membrane
3. The amount of neurotransmitter receptors present in the postsynaptic membrane

Question 11

What is synpatic plasticity?

Answer 11

Idea that synapses could change allowing long term potentiation (LTP) or long term depression (LTD)

Question 12

What is long term potentiation?

Answer 12

Repeated stimulation of synapses cause an increase in the number of receptors expressed in the postsynaptic membrane -> hence, these synaptic pathways are more easily activated

Question 13

How can neurotransmitters be inactivated?

Answer 13

1) Diffusion of neurotransmitters away from the synapse
2) Enzymatic breakdown of neurotransmitters in the synaptic cleft
3) Re-uptake and recycling by neurotransmitter transport proteins in the presynaptic neuron

Question 14

How does the integration of multiple synaptic inputs occur?

Answer 14

At the dendrite of a single axon, there are thousands of synaptic pathways

Each synapse is only able to produce very small postsynaptic potentials

These small potentials combine at the axon initial segment causing depolarisation of the membrane

Question 15

What is temporal summation?

Answer 15

When a synaptic pathway exists between two neurons, the frequency of depolarisation must increase in order to cause depolarisation for the activation of a action potential

Question 16

What is spatial summation?

Answer 16

When synaptic pathways exist between two presynaptic neurons and a single postsynaptic neuron the amount of depolarisation by each neuron combines to form a larger depolarisation which is above threshold potential

Question 17

What is excitotoxicity?

Answer 17

Increased release of neurotransmitters like glutamate causes build up of calcium ions. Too high calcium content causes activation of enzymes which kill the cell

Question 18

What would happen if the post-synaptic neuron lacks GABA receptors?

Answer 18

GABA receptors are involved in inhibitory synapses

Hence, less hyperpolarisation would occur meaning membrane potential increases

This will lead to more action potentials caused by this neuron

Overactivation of this synaptic pathway can cause conditions like schizophrenia -> experience hallucinations