synaptic transmission

Question 1

what happens when the action potential reaches the end of a neuron?

Answer 1

information is transferred from one neuron to another, at specialised points of contact called synapses.

Question 2

synapse: define

Answer 2

the specialised junction where a neuron contacts and communicates with another neuron or cell.

Question 3

2 types of synapses:

Answer 3

1. electrical: current (ions) flows through gap junctions

2. chemical: neurotransmitters pass through synaptic cleft

Question 4

which type of synaptic transmission is faster: electrical or chemical?

Answer 4

electrical.

Question 5

what is the main function of electrical synapses? in comparison, what is the main function of chemical synapses?

Answer 5

electrical synapses mainly transmit information. chemical synapses mainly amplify information.

Question 6

_____ synapses serve neuron to neuron, and glia to neuron transmission. ______ synapses can serve neuron to neuron, and neuron to cell transmission.

Answer 6

electrical; chemical.

Question 7

what does it mean for gap junctions to be electrically coupled?

Answer 7

one channel on a cell opens, the corresponding channel on another cell opens with a brief delay.

Question 8

features of electrical synapse

Answer 8

transfer is very fast, fail-safe, almost instantaneous

Question 9

is electrical synapse common in the mammalian brain?

Answer 9

yes.

Question 10

locations of electrical synapses and examples

Answer 10

often found where normal function requires the activity of neighboring neurons to be highly synchronised.
eg. CNS, heart

Question 11

what are glial cells (glia)?

Answer 11

are non-neuronal cells in the CNS and the PNS that do not produce electrical impulses. They maintain homeostasis, form myelin, and provide support and protection for neurons.

Question 12

synaptic integration

Answer 12

most neurons in the CNS make connections with many neurons. If several postsynaptic potentials (PSPs) occur simultaneously, the neuron integrates them and produce an output.

Question 13

______ synapses form the main mechanism of synaptic transmission

Answer 13

chemical

Question 14

synaptic cleft

Answer 14

the space between neurons at a nerve synapse, across which a nerve impulse is transmitted by a neurotransmitter.
The two cells are not connected across the synaptic cleft.

Question 15

how wide is the synaptic cleft?

Answer 15

20-50nm.

Question 16

what is in the synaptic cleft?

Answer 16

the synaptic cleft is filled with matrix of extracellular protein, which makes pre- and post-synaptic membrane adhere to each other.

Question 17

what are synaptic vesicles?

Answer 17

membrane enclosed spheres, each about 50nm in diameter, which store various neurotransmitters.

Question 18

what are active zones in the presynaptic axon?

Answer 18

where synaptic vesicles cluster

Question 19

what is the zone called postsynaptic density (motor endplate for skeletal muscle)?

Answer 19

a zone in the postsynaptic cell where neurotransmitter receptors are present.

Question 20

process of information transfer across chemical synapse

Answer 20

1. action potential reaches axon terminal. cell membranes become polarised.
2. voltage gated Ca2+ channels open.
3. an influx of Ca2+ enters axon terminal at the active zone, down the concentration gradient.
4. this triggers neurotransmitters to be released and diffuse into the cleft.
5. neurotransmitter binds to postsynaptic receptors, causing different effects.
6. neurotransmitters removed from synaptic cleft.

Question 21

how does Ca2+ entry cause neurotransmitter release?

Answer 21

Ca2+ surge acts as an intracellular messenger, resulting in the fusion of synaptic vesicles with axon membrane. They empty their contents by exocytosis.
Ca2+ is then removed from the terminal. It is taken up by mitochondria, or is removed via active Ca2+ pump. This resets voltage for another depolarisation.

Question 22

what is the effect of the binding of neurotransmitters?

Answer 22

the binding of neurotransmitters opens ion channels, resulting in graded potentials. The effect is transient and usually last a few milliseconds before the NT is removed.

Question 23

How are NTs removed?

Answer 23

1. reuptake (active): by astrocytes/glial cells, or the presynaptic terminal.
2. degradation (active): broken down by enzymes associated with the presynaptic membrane. then they can be transported back to the synapse.
3. diffusion away from synapse (passive).

Question 24

synaptic delay: define

Answer 24

the time for NT release, diffusion across the cleft, and binding to receptors.
Synaptic delay is the rate-limiting step of (chemical) neural transmission.

Question 25

neurotransmitter release is ___ dependent.

Answer 25

Ca2+

Question 26

there are more than ___ substances that act as neurotransmitters

Answer 26

100

Question 27

to reduce transmission time, NT is synthesised ______.

Answer 27

in advance.

Question 28

how are peptide NT synthesised?

Answer 28

1. a precursor peptide is synthesised in the rough ER. 2. the precursor peptide is split in the Golgi apparatus to yield the active neurotransmitter. 3. secretory vesicles containing the peptide bud off from the golgi apparatus 4. the secretory granules are transported down the axon to the terminal where the peptide is stored.

Question 29

how are amine and amino acid NT synthesised?

Answer 29

1. enzyme converts precursor molecules into NT in the cytosol. 2. transporter proteins load the NT into synaptic vesicles in the terminal, where they are stored.

Question 30

what are glutamate and glycine, and where are they found?

Answer 30

they are amino acids, and are NT in the CNS. | they are readily available in the cells.

Question 31

what is GABA and where are they found?

Answer 31

GABA is an amine that is an inhibitory NT in the CNS. it is made only by the neurons that secrete them.

Question 32

what is excitatory postsynaptic potential (EPSP)?

Answer 32

EPSP is a graded potential that depolarises the neuron and brings its membrane potential closer to the threshold.

Question 33

where and how does EPSP occur?

Answer 33

in excitatory chemical synapses. | caused by opening Na+ and K+ channels.

Question 34

what does EPSP cause?

Answer 34

it causes a sodium influx and increases the likelihood of AP firing.

Question 35

what is inhibitory postsynaptic potential (IPSP)?

Answer 35

IPSP is a graded potential that hyperpolarises the postsynaptic neuron and brings its membrane potential further from the threshold.

Question 36

where and how does IPSP occur?

Answer 36

in inhibitory chemical synapses. | caused by opening of Cl- channels.

Question 37

what does IPSP cause?

Answer 37

it lowers the likelihood of reaching the threshold and AP firing.

Question 38

what are the 2 types of NT receptors?

Answer 38

1. ionotropic: receptor itself is an ion channel | 2. metabotropic: receptor and ion channel are separate molecules

Question 39

how do metabotropic receptors affect ion channels?

Answer 39

1. NT binds to metabotropic receptors 2. intermediate molecules called G-proteins are activated 3. a series of intracellular events are triggered 4. ion channels open due to a range of second messenger chemicals

Question 40

what is the function of metabotropic receptors?

Answer 40

they typically amplify the response of the initial NT.

Question 41

metabotropic receptors lead to _____ responses.

Answer 41

slower

Question 42

SSRIs (selective serotonin reuptake inhibitors) are a type of antidepressant. How does it work?

Answer 42

it prevents serotonin from being reuptaken back into the presynaptic neuron, thus increasing serotonin levels in the synaptic cleft.

Question 43

examples of excitatory neurotransmitters

Answer 43

- ACh - norepinephrine - epinephrine - dopamine - glutamate - serotonin

Question 44

examples of inhibitory neurotransmitters

Answer 44

- GABA | - glycine

Question 45

_______ is the most prevalent excitatory neurotransmitter in the brain.

Answer 45

glutamate

Question 46

afferent neurons convey information _______ the CNS; efferent neurons convey information ________ the CNS. interneurons ____________ within the CNS.

Answer 46

towards; away from; connect neurons

Question 47

why is action potential unidirectional?

Answer 47

graded potential is caused by external stimulus, while action potential is caused by membrane depolarisation. when an AP occurs, the absolute refractory period prevents the initiation of an AP in a region of membrane that has just produced an AP.

Question 48

______ is the main inhibitory neurotransmitter in the adult vertebrate brain. ______ is the main inhibitory neurotransmitter in the spinal cord.

Answer 48

GABA; glycine

Question 49

the direction of information flow for electrical synapse is ______, while for chemical synapse it is _______.

Answer 49

bidirectional; unidirectional.