M6 L4 : Synaptic transmission

Question 1

What is synaptic transmission

Answer 1

the process of transferring information between neurons (through axodendritic synapse) or between neurons and muscle fibres (neuromuscular junction=endplate)

Question 2

What are the two ways synaptic transmission occurs and where are they most likely

Answer 2

Chemical synapses (neurons in the brain as well as in endplates) and electrical synapses (via pores called gap junctions)

Question 3

What are the three key features of chemical synapses

Answer 3

• SPECIFICITY (specific neurotransmitters have specific effects on the post synaptic membrane.
• COMPLEXITY: type, time course, strength, location
• PLASTICITY : changes in synaptic structure and function associated with development aging and learning

Question 4

Describe the stages of excitatory synaptic transmission in neuromuscular junction

Answer 4

1. Presynaptic action potential triggers voltage gated Ca+ channels to open, Pca+ increases with Ca+ influx.
2. Transmitter : ACh is released by exocytosis which diffuse over the synaptic cleft
3. ACh binds to post synaptic receptors which activate ligand gated ion channels- non selective to both NA+ and K+ sometimes Ca2+
4. This always shifts RMP from -65 to toward threshold for AP (-55mV). This is End plate potential.
5. Once triggered AP is transmitted along muscle fibre by passive spread.

Question 5

How long is the synaptic delay between the first AP in presynaptic neuron and second AP in postsynaptic neuron

Answer 5

half a millisecond

Question 6

Why does the EPP always depolarise and trigger Excitatory Post Synaptic Potential (EPSP)

Answer 6

non selective cationic channel means that membrane potential will be somewhere between equilibrium potential for both (-80mV for K+ and +60 mV for Na+ ) so it will be closer to 0, definitely higher than 55

Question 7

What are the two main types of chemical synpases in the CNS and how do they effect the Post synaptic membrane

Answer 7

Excitatory Post synaptic potentials (EPSP) which DEPOLARISE the psm and Inhibitory postsynaptic potentials (IPSP) which HYPERPOLARISE the psm.

Question 8

What are the main neurotransmitters related to ESPS and what is the ionic mechanism

Answer 8

Glutamic acid (glutamate) in the brain or Acetylcholine. 
ionic mechanism is transient opening of Na+K+ and sometimes Ca2+ channels

Question 9

What are the main neurotransmitters related to ISPS and what is the ionic mechanism

Answer 9

GABA (gamma-aminobutyric acid) or glycine
Ionic mechanism: transient opening of ligand gated K+ channels. This shifts RMP towards equilibrium potential of K+ (-80mV) and therefore away from threshold.

Question 10

What are the two classifications of neurotransmitters (based on chemical structure)

Answer 10

Small molecule neurotransmitters (classical) and Neuropeptides (neuromodulators stored in presynaptic vesicles)

Question 11

What are examples of small molecule transmitters and general speed and direction

Answer 11

Usually fast action and direct on postsynaptic receptors

Eg: Amino acids (glutamate, GABA, glycine), ACh, Biogenic Amines: (serotonin, noradenaline and dopamine)

Question 12

What are neuropeptides, their general speed and direction as well as examples

Answer 12

Large molecule chemicals that have an indirect (metabotropic) action on post synaptic receptors or modulatory action on the effects of other neurotransmitters.
Slow (seconds to minutes) , direction is more diffusing rather than direct.
Dozens of neuropeptides identified Eg. Neuropeptide Y, Substance P, Kisspeptin, Enkephaln.

Question 13

What are the three factors determining synaptic action relating to neurotransmitter and channel action

Answer 13

1. the type of neurotransmitter released
2. The type of neurotransmitter receptor/channel complex expressed in the post synaptic membrane
3. The amount of neurotransmitter receptor present in the post synaptic membrane (synaptic plasticity : Long term potention or depression.

Question 14

Define synaptic plasticity

Answer 14

synaptic basis for memory formation: more receptors or less for long term potention or long term depression

Question 15

Why can one receptor initiate more than one type of response

Answer 15

many transmitters bind to more than one type of receptor (eg. Glutamate binds to AMPA, NMDA, and Kainate) and the response varies depending on the type of receptor that transmitter binds to,increasing the information handling capacity of neurons

Question 16

What are the two types of mechanism used to gate ion channels by transmitters and which class of neurotransmitter are they related to

Answer 16

Direct gating: fast and short lasting direct binding to ion channel is for classical neurotransmitter

Indirect gating: neuropeptidestransmitter binds to receptors : GPCR that activate G protein. which bind GTP. 2nd messengers are produced which activate protein kinase which phosphorylate ion channels and this causes them to open close slower and longer lasting.

Question 17

How are neurotransmitters inactivated and why is this important

Answer 17

1. diffusion away from the synapse (little)
2. enzymatic degradation in the synaptic cleft (main)
3. Re uptake and recycling which involves specific active neurotransmitter transporters in the presynaptic membrane that

Question 18

What are the enzymes that break down ACh, biogenic amines and neuropeptides

Answer 18

Acetylcholinesterase; monoamine oxidase for biogenic amines; peptidases for neuropeptides

Question 19

Why is integration of synaptic neurons needed

Answer 19

each neuron receives thousands of synapses E and I but each only produces a small 0.1mV at the axon initial segment and decay when they are passively conducted from dendrites.
In order to depolarise the intial segment ot threshold (15mV) EPSPS need to be enhanced

Question 20

What are the two types of temporal and spatial summation of post synaptic potentials at axon initial segment that result in threshold

Answer 20

2 post synaptic potentials occur at one excitatory synapse, 2nd coming before the first has finished. (temporal summation) and two coming at the same time (spatial summation) from two excitatory synapses

Question 21

What are the two types of temporal and spatial summation of post synaptic potentials at axon initial segment that result in the threshold not being reached

Answer 21

two ESPS coming in one after the other has finished and spatial summation of EPSP and IPSP which balances out the depolarisatoin with a hyperpolarisation.

Question 22

What is excitotoxicty

Answer 22

Too much glutamate release or insufficient reuptake leads to an excessive activation of neurons through the NMDA and AMPA/Kainate receptors. Large Ca2+ influx assocated with over activation of NMDA receptors causes neuron damage= excitoxicity. Seen in stroke, epilepsy, brain injury.

Question 23

What are the four types of recpetors that glutamate binds to which causes EPSP

Answer 23

3 directly gated : AMPA, NMDA and Kainate and 1 indirectly gated (Metabotropic glutamate receptor)