(06) Synaptic Transmission

Question 1

synaptic transmission occurs between neurons through either…

Answer 1

chemical synapses or electrical synapses (via pores called gap junctions)
in the developed mammalian brain, occurs almost entirely through CHEMICAL synapses

Question 2

structure allowing transmission of a message from one neuron to another

Answer 2

axo-dendritic synapse
like a long axon extending from the pre-synaptic neuron to a post-synaptic neuron

Question 3

structure for transmission from motorneuron to muscle fibre

Answer 3

neuromuscular junction = “end plate”

Question 4

what type of transmission happens at a neuromuscular junction?

Answer 4

excitatory synaptic transmission

Question 5

what is the neurotransmitter in a neuromuscular junction?

Answer 5

acetylcholine (ACh)

Question 6

process occurring at a chemical synapse

Answer 6

depolarisation at the presynaptic terminal –> release of neurotransmitter, diffuses across synaptic cleft, binds to receptors in post-synaptic membrane
–> opening of a channel / current

Question 7

three features of chemical synapses

Answer 7

specificity - specific neurotransitters have specific effects on postsynaptic membrane
complexity
plasticity - changes in synaptic structure/function with development / age / learning etc

Question 8

process occurring at a neuromuscular junction

Answer 8

AP down pre-syn terminal
increased pre-syn Ca2+ permeability –> influx
release of neurotransmitter by exocytosis
reacts w/ post-syn receptors
activation of ligand-gated non-selective cationic channels (permeable to BOTH Na+ and K+) –> post-syn End Plate Potentials (EPP) + AP in both directions down muscle fibre

Question 9

define suprathreshold

Answer 9

always triggers an AP (above threshold)
End Plate Potentials (EPP) always suprathreshold

Question 10

name two types of chemical synapses

Answer 10

Excitatory
Inhibitory

Question 11

what is an EPSP

Answer 11

Excitatory Postsynaptic Potential
= DEpolarisation of the postsyn membrane (so less negative)

Question 12

what is an IPSP

Answer 12

Inhibitory Postsynaptic Potential
= HYPERpolarisation of postsynaptic membrane (so more negative)

Question 13

neurotransmitters for EPSP

Answer 13

Glutamatic acid (glutamate), ACh

Question 14

neurotransmitters for IPSP

Answer 14

gamma-aminobutyric acid (GABA) or glycine

Question 15

ionic mechanism of EPSP

Answer 15

transient opening of ion channels for Na+, K+, sometimes Ca2+

Question 16

ionic mechanism for IPSP

Answer 16

transient opening of K+ channels

Question 17

name two classes of neurotransmitters

Answer 17

small molecule / “classical”
neuropeptides / modulators

Question 18

how do classical neurotransmitters work?

Answer 18

Fast action (milliseconds), direct on postsyn receptors

Question 19

examples of classical neurotransmitters

Answer 19

Amino acids: Glycine, GABA, glutamate
ACh
Amines: serotonin, noradrenalin, dopamine

Question 20

how do neuropeptides work?

Answer 20

large molecules, slow (seconds-mins)
usually more diffuse action
indirect (metabotropic) action on postsyn receptors and modulatory action on the effects of other neurotransmitters

Question 21

what three main factors affect synaptic action?

Answer 21

1. type of neurotransmitter
2. receptor / channel complex
3. synaptic plasticity: amount of NT receptor in postsyn membrane

Question 22

what are the four main types of glutamate receptors and how do they affect synaptic action?

Answer 22

Three are DIRECTLY gated ion channels:
AMPA receptor: fast response
NMDA receptor: slow
Kainate receptor

Second-messenger mediated: metabotropic glutamate receptor

Question 23

three paths to neurotransmitter inactivation

Answer 23

1. Diffusion - away from synaptic cleft. occurs in all NT to some extent
2. Enzymatic degradation - eg. ACh esterase
3. Re-uptake + recycling - for amino acids / amines. most common.

Question 24

why is NT inactivation important?

Answer 24

to allow a new signal to follow rapidly

Question 25

what must happen to depolarise the initial segment?

Answer 25

each neuron receives thousands of synapses, each producing only very small postsyn potentials
to reach threshold, EPSPs must be ENHANCED
= temporal / spatial SUMMATION at the axon initial segment

Question 26

how does temporal summation of EPSPs work?

Answer 26

multiple accumulation of stimulus at presynaptic neuron causes the membrane potential to exceed threshold level, generating AP

Question 27

how does spatial summation of postsynaptic potentials work?

Answer 27

multiple presyn attached to one postsyn neuron
EPSP + EPSP same time, causes large enough increase of MP to cause AP
EPSP + IPSP causes a tiny increase, no threshold reached

Question 28

what is excitotoxicity?

Answer 28

overactivation of excitatory NT (eg. glutamate), causes Ca2+ accumulation, activates enzymes which kills the cell / neuron damage

Question 29

what might happen if post-synaptic neurons are lacking GABA receptors?

Answer 29

GABA = inhibitory NT receptors
Post-syn potential increases
more firing of AP –> affects CNS w/ more activity than usual

a possible theory of schizophrenia

Question 30

name two mechanisms NTs are involved in which gate ion channels

Answer 30

Direct gating
Indirect gating

Question 31

describe direct gating by NTs

Answer 31

NT binds to receptor / ion channel
causes opening / passing of ions –> de/hyperpolarise membrane, making it more / less likely to generate an AP

typically FAST onset (<1ms), short-lasting effects

Question 32

describe indirect gating by NTs

Answer 32

NT binds to receptors (eg. G-protein-coupled, known as metabotropic (indirect) receptors)
activates biochemical pathway involving G-prot
–> activates second messengers (eg. cAMP/ IP3)
–> activates protein kinases which phosphorylate specific ion channels causing them to open/close

SLOWER onset, longer lasting (sec-mins)

Question 33

what is the main excitatory NT in the CNS?

Answer 33

Glutamate (L-glutamic acid)