Lecture 5: Synaptic Transmission

Question 1

What is synaptic modulation?

Answer 1

Changes in synaptic function, allowing both robust function and higher complexity (learning, memory, etc.)

Question 2

Gap junction structure

Answer 2

6 connexin transmembrane proteins form a connexon hemi-channel. Pairs of hemi-channels cross both cell membranes. Arrays of these pairs form a gap junction.

Question 3

Properties of gap junctions

Answer 3

-Channels can be modulated open/closed
-Post-synaptic response is very fast, <1 ms
-Transmitted signal depends on relative size/electrical properties of pre/post synaptic neurons + gap junctions
-No amplification or polarity reversal possible

Question 4

How do botulinum toxins paralyze?

Answer 4

Botulinum cleaves SNARE proteins, inhibiting the release of NTs

Question 5

What is 1 quantum of neurotransmitter?

Answer 5

1 quantum of NT = 1 vesicle, which contains 1000s of NT molecules

Question 6

Miniature End Plate Potential (MEPP)

Answer 6

The post-synaptic potential caused by spontaneous (not induced) fusion and release of 1 NT vesicle.

Question 7

End Plate Potential (EPP)

Answer 7

The summation of the many MEPPs stimulated in response to an action potential

Question 8

How many vesicles are released per action potential?

Answer 8

Vesicles per action potential depend on how much Ca2+ influx occurs. At the neuromuscular junction, 1 AP = ~150 vesicles, much more than enough to trigger post-synaptic firing. In the CNS, 1 AP = 1-4 vesicles.

Question 9

Different types of neurotransmitter receptors

Answer 9

1. Ionotropic (1 NT opens 1 channel directly)
2. Metabotropic (GPCR or other secondary messenger)

Question 10

Ionotropic receptor

Answer 10

One type of post-synaptic receptor. Ligand gated ion channel that opens when 1 NT binds, fast response

Question 11

Metabotropic receptor

Answer 11

One type of post-synaptic receptor. GPCR that can open many ion channels in response to 1 NT, amplifying the signal. Response can be ~100 ms to minutes; plays role in modulating function or sustained effects.

Question 12

Chemical synapse properties

Answer 12

Neuronal tuning, synaptic strength modulation, signal amplification, excitatory/inhibitory signals

Question 13

Neuromuscular junction properties

Answer 13

No summation occurs; 1 action potential = 1 muscle activation (strong synapse)

1 AP -> ~150 vesicles, creating EPP up to 70 mV; ensures 1:1 transmission with 1.5-4 safety factor

Question 14

Typical EPSP value

Answer 14

~0.2-0.4 mV on average; ~25 summed for 10 mV threshold change

Question 15

Synapse

Answer 15

Anatomically specialized junction between 2 neurons

Question 16

Convergence

Answer 16

A single post-synaptic cell can receive signals from 100s or 1000s of synapses

Question 17

Divergence

Answer 17

1 pre-synaptic cell can branch to synapse on many post-synaptic cells

Question 18

What affects the excitability of a post-synaptic cell?

Answer 18

Number of active synapses and number of excitatory/inhibitory signals received

Question 19

Types of synapses

Answer 19

1. Electrical
2. Chemical

Question 20

Electrical synapse

Answer 20

Gap junction joining plasma membranes of 2 neurons, allowing local currents from arriving APs to flow directly through

Question 21

Chemical synapse

Answer 21

Synaptic vesicles containing NTs (can be multiple different) are released to the synaptic cleft and bind to the post-synaptic cell, stimulating a post-synaptic potential via ion channels. Prevents direct current propagation but permits signal summation.

Question 22

EPSP

Answer 22

Excitatory Post-Synaptic Potential. Created by opening Na+/K+ nonselective channels, allowing Na+ influx/small K+ efflux and local depolarization

Question 23

IPSP

Answer 23

Inhibitory Post-Synaptic Potential. Created by opening K+ or Cl- channels, reducing chance of reaching AP threshold. K+ hyperpolarizes. If Cl- is actively exported, Cl- influx hyperpolarizes. If not, Cl- stabilizes resting membrane potential and counters positive ion movement.

Question 24

Post-synaptic density

Answer 24

The post-synaptic membrane adjacent to the axon terminal is a specialized area with a high density of membrane proteins

Question 25

How are neurotransmitters stored and released?

Answer 25

1. NTs stored in vesicles docked at active zones or dispersed in axon terminal
2. AP opens voltage-gated Ca2+ channels, allowing Ca2+ influx.
3. Ca2+ binds synaptotagmins, causing SNARE protein conformational changes -> membrane fusion and NT release.

Question 26

Post-fusion vesicle fates

Answer 26

1. Endocytosis recycling beyond active zone.
2. “Kiss and run” fusion; resealing/withdrawal after brief fusion. Common in high frequency neurons.

Question 27

What happens to NTs after receptor binding?

Answer 27

Unbound NTs are removed by:
1. Reuptake; active transport and recycling to pre-synaptic axon.
2. Transport to nearby glial cells for degradation.
3. Diffusion away from receptors.
4. Enzymatic inactivation (some reused).

Question 28

Types of summation

Answer 28

1. Temporal
2. Spatial

Question 29

Temporal summation

Answer 29

Summation based on input frequency, where a new PSP is added on before the previous graded potential leaks away.

Question 30

Spatial summation

Answer 30

Summation of 2 inputs arriving simultaneously at different locations on the cell.

Question 31

Axon hillock

Answer 31

Most responsive area of neuron due to higher density of voltage-gated Na+ channels, = more negative threshold vs. dendrites or cell body. Synapses closer to the hillock will induce a greater EPSP.

Question 32

How long do post-synaptic potentials last?

Answer 32

Always longer than 1 AP duration; neuronal responses almost always come in bursts, not as single events.