06 Synapses

Question 1

Electrical synapse:

• structure
• drawbacks compared to chemical synapses

Answer 1

Gap junctions: 2 connexons, each made of 6 connexins

Drawbacks:

• no mech for inhibition
• no mech for neuromodulation

Question 2

Gray’s type I and II synapses:

• NT type?
• vesicle shape?
• shape?
• cleft size?

Answer 2

I: asymmetric

• excitatory
• Round vesicles
• big PSD (post synaptic density)
• wide cleft

II: symmetric

• inhibitory
• flat vesicles
• smaller PSD
• narrow cleft

Question 3

3 types of synapses

Answer 3

1. electrical - gap jxns
2. chemical - neurotransmitters
3. gaseous (NO, CO)

Question 4

Myasthenia Gravis

Answer 4

• Nicotinic Ach receptors blocked (at neuromuscular junction) by autoimmune antibodies
• severe muscle weakness
• tx with anticholinesterase inhibitors to keep Ach in synapse

Question 5

Lambert-Eaton Myasthenic syndrome

Answer 5

• presynaptic voltage-gated Ca+ channels blocked by autoimmune antibodies
• severe muscle weakness

Question 6

Proteins in vesicluar release in synapse, and purpose (8)

Answer 6

1. Synapsin I - binds vesicle to cytoskeleton until phosphorylated by Calmodulin
2. Rab - GTPase, propels vesicle to active zone

(bind vesicle to membrane)

3. v-SNARE - vesicular
   - VAMP, synaptobrevin
4. t-SNARE - target membrane
   - SNAP-25
   - syntaxin

(bind each other plus Ca+ to trigger fusion and fission)

5. Synaptotagmin
6. Neurexin

(bind to SNAREs to dissociate them)

7. SNAP
8. NSF

Question 7

Botulinum toxin

Answer 7

• binds to SNAREs to prevent interaction between them

- flaccid paralysis (no Ach release)

Question 8

Tetanus toxin

Answer 8

• binds to synaptobrevin (a t-SNARE) in inhibitory neurons

- Muscle spasms, lockjaw, from lack of inhibition

Question 9

Alpha-Latrotoxin

Answer 9

• Binds neurexin to cause massive NT release, leading to NT depletion
• Muscle spasms because muscles cannot relax
• black widow

Question 10

types of postsynaptic receptors

Answer 10

1. ionotropic

2. metabotropic

Question 11

Gaseous transmission NTs

Answer 11

NO, CO

Question 12

Nitric Oxide synthesis

Answer 12

L-Arg + NO synthase + Ca –> NO + L-citrulline

Ca+ activates NOS

Question 13

Nitric Oxide synapse types and their steps of release

Answer 13

1. Neuro-neuronal
   - Glu acts on postsynaptic NMDA receptors, bringing Ca+ into postsynaptic neuron
   - Ca+ activates nNOS via Calmodulin to produce NO
   - NO diffuses out
2. Neuro-effector (varicosity to effector organ)
   - AP causes Ca+ influx into varicosity
   - Ca+ activates nNOS
   - NO diffuses out

Question 14

Nitric Oxide effect on target cell

Answer 14

• NO binds to Fe in heme moiety attached to guanylate cyclase
• activation of guanylate cyclase produces cGMP

-ex: smooth muscle relaxation (Viagra)