synapses and neurotransmitters

Question 1

what is the criteria for neurotransmitters?

Answer 1

• be present in presynaptic terminals
• be released in response to stimulation
• act on the postsynaptic neuron
• blocking the neurotransmitter should prevent synaptic transmission

Question 2

what are the main classes of neurotransmitters?

Answer 2

• amino acids
• amines
• peptides (made up of many amino acids)

Question 3

evidence for neurons

Answer 3

• golgi stain
• physiological evidence from study of reflexes
• final evidence from electron microscopy

Question 4

why are synapses important?

Answer 4

they enable flexible processing and allow for specialization and integration

Question 5

what are electrical synapses?

Answer 5

gap junctions that allow current to pass directly between neurons
they directly connect the cytoplasm of two neurons
both depolarization and hyperpolarization are transmitted

Question 6

how to tell if neurons are connected by gap junctions?

Answer 6

physical technique - dyes can be inserted into neurons using electrodes
dyes diffuse from one neuron to the other demonstrating presence of gap junctions

electrical technique - hyperpolarizing and depolarizing stimuli are passed from one neuron to the other

Question 7

what are electrical synapses good for?

Answer 7

• fast communication
• synchronizing neurons

Question 8

what are the steps in chemical synaptic transmission?

Answer 8

1. package neurotransmitters in vesicles and put them in pre-synaptic terminal
2. action potential arrives - voltage-gated ca2+ channels open
3. Ca2+ influx - vesicles fuse to membrane, neurotransmitters released
4. neurotransmitters diffuse across the synaptic cleft, activate receptors on postsynaptic cell
5. neurotransmitters removed from cleft

Question 9

what are the two types of synaptic vesicles?

Answer 9

synaptic vesicles - clear, recycled by endocytosis, small molecule transmitters
dense-core secretory granules - dense, ‘one and done’, peptide neurotransmitters

Question 10

how do vesicles fuse with the membrane?

Answer 10

v-SNAREs on the vesicle have partner t-SNAREs on the membrane
calcium binds to synaptotagmin on v-SNAREs, causing a conformational change that makes the SNAREs ‘zipper’ together

Question 11

what are SNAREs commonly used for?

Answer 11

they are targets for toxins which cause paralyses but small amounts can be used to get rid of wrinkled (botox)

Question 12

what are the two types of neurotransmitter receptors?

Answer 12

ligand-gated ion channels (ionotropic receptors)
G-protein coupled receptors (metabotropic receptors)

Question 13

what are ionotropic receptors?

Answer 13

ligand-gated ion channels
binding of neurotransmitter triggers a conformational change on ion channel allowing ions to pass through

Question 14

what are metabotropic receptors?

Answer 14

G-protein-coupled receptors (not ion channels)
binding of neurotransmitter triggers conformational change of G-protein which has more complex effects

Question 15

how are neurotransmitters removed from synaptic cleft?

Answer 15

• they diffuse away
• actively taken up by transporters for recycling (into presynaptic neuron/glia)
• destroyed in synaptic cleft by enzymes

Question 16

differences between electrical and chemical synapses

Answer 16

• electrical signals pass in both directions, chemical only one direction
• electrical signals passed directly, chemical signals can be transformed (inverted, amplified etc)
• electrical synapses are fast, chemicals are slow

Question 17

similarities between electrical and chemical synapses

Answer 17

• both are plastic (can be modified), but chemical more so
• allow summing up inputs by post-synaptic neuron

Question 18

features of neuromuscular junction

Answer 18

• motor neuron action potentials always cause muscle cell action potentials
• uses neurotransmitter acetlycholine
• fast and reliable neurotransmission

Question 19

how does the neuromuscular junction achieve such efficient transmission?

Answer 19

• presynaptic membrane has large number of active zones
• postsynaptic membrane contains junctional folds densely filled with neurotransmitter receptors

active zones and junctional folds are precisely aligned

Question 20

how do neurotransmitters allow for convergence and divergence?

Answer 20

• each transmitter can activate multiple different receptors (convergence)
• each receptor can activate different downstream effectors
• different transmitters or receptors can activate the same downstream effector

Question 21

how does dendritic integration work?

Answer 21

EPSPs from different neurons and dendrites summate to reach threshold, so AP generated at axon hillock, which is actively regenerated along axon

Question 22

why does it matter how excitatory and inhibitory synapses are spatially arranged?

Answer 22

• an inhibitory synapse can block propagation of EPSP towards soma
• GABAa receptors can decrease membrane resistance so current leaks out
• they act by shunting inhibition

Question 23

how can inhibition act presynaptically?

Answer 23

GABA produced by 3rd party neuron can activate GABAb receptors which can close calcium channels so less neurotransmitter can be released

Question 24

how does cocaine and amphetamines affect monoamines?

Answer 24

block reuptake of dopamine, norepinephrine (noradrenaline)

Question 25

how do antipsychotics affect monoamines?

Answer 25

block dopamine receptors

Question 26

what are the main monoamines?

Answer 26

dopamine, serotonin, norepinephrine (noradrenaline), epinephrine (adrenaline)

Question 27

what are the modulatory neurotransmitters?

Answer 27

acetylcholine, monoamines

Question 28

how do antidepressants affect monoamines?

Answer 28

increasing serotonin selective serotonin reuptake inhibitors (SSRIs) MAO-A inhibitors (stop breakdown of serotonin)