Synapses

Question 1

Distance between pre and post synaptic cell membranes

Answer 1

Electrical: 3.5 nm

Chemical and gaseous synapses: 30-50 nm; 20 nm to 1-2 micrometers in smooth muscle

Question 2

Cytoplasmic continuity between pre and post-synaptic cell-membranes

Answer 2

Only in electric

Question 3

Ultrastructural components - Electrical

Answer 3

Gap junction pores
Connexon with diameter of 1.5 nm, made up of 6 connexins with 4 membrane spanning regions
Low resistance

Question 4

Ultrastructural components - Chemical

Answer 4

Presynaptic active zones; postsynaptic receptors and densities; axonal varicosities and intervaricose segments in smooth muscle

Question 5

Ultrastructural components - Gaseous

Answer 5

Presynaptic or postsynaptic cytosolic NO synthase (stored as nitrosothiols in secretory vesicles?); postsynaptic receptor is soluble guanylate cyclase with heme moiety

Question 6

Agent of transmission

Answer 6

Electrical: Ionic current
Chemical: Chemical transmitter molecule
Gaseous: Gas-like transmitter molecule

Question 7

Synaptic delay

Answer 7

Electrical: Negligible

Chemical and gaseous: 0.3 ms at least, typical: 1-5 ms

Question 8

Directionality of transmission

Answer 8

Elecctrical: Bi
Chemical: Uni
Gaseous: Either

Question 9

Events at presynaptic terminal

Answer 9

Depolarization
Ca2+ influx
Activates Ca2+/calmodulin kinase
Phosphorylates synapsin I
Vesicles released from cytoskeleton, fuse with presynaptic membrane
Neurotransmitter release
Vesicular membrane is recycled

Question 10

Events for neurotransmitters

Answer 10

Bind to receptors, diffuse away, hydrolyzed, reuptake into presynaptic terminal

Question 11

Events at postsynaptic

Answer 11

Receptors bind to neurotransmitters

Ion channels activated by direct or indirect gated receptors

Question 12

Signal transmission sites

Answer 12

Bridged junctions (Cell to cell channels)
Unbridged junctions (Synaptic clefts between cells)

Question 13

Electrical Transmission - Modulation

Answer 13

None: All-or-nothing

Question 14

Types of CNS synapses

Answer 14

Gray’s type I (Asymmetric synapse):
Round synaptic vesicles; wider cleft; more prominent postsynaptic densities than type II, excitatory synapses
Gray’s type II (Symmetric synapses):
Flat or pleomorphic vesicles; narrower cleft; and less prominent postsynaptic densities than type I; often associated with inhibitory synapses
Monoaminergic: Dense-core vesicles, wide synaptic cleft; no prominent pre/post synaptic densities
Peptidergic synapses: Large dense-core vesicles

Question 15

Neuromuscular synapse

Answer 15

Presynaptic dense bars
Calcium channels
Primary and secondary synaptic cleft
Acetylcholinesterase AChE and basal lamina in clefts
Postsynaptic junctional folds
ACh receptors face active zones
Voltage-gated Na+ channels along the sides and depths of folds

Question 16

Myasthenia gravis

Answer 16

Severe weakness of muscles
Autoimmune against AChR
Congenital and heritable: rare
Treatment: Inhibitors of acetylcholinesterase

Question 17

Synapsin I

Answer 17

Binds vesicles to cytoskeleton
Phosphorylated by Ca/calmodulin-depedent kinase
Phosphorylated synapsin I releases synaptic vesicles from cytoskeletal network

Question 18

Rab 3A

Answer 18

Neuron specific GTPases

Binds to vesicles and helps to propel vesicles toward the active zones

Question 19

Synaptotagmin

Answer 19

Has 4 calcium binding sites

Major calcium sensing protein that triggers vesicular exocytosis

Question 20

NSF: N-ethylmaleimide-Sensitive Factor

Answer 20

Cytosolic, hydrolyzes ATP to release the complex

Question 21

SNAPs: Soluble NSF Attachment Proteins

Answer 21

Cytosolic, binds to NSF

Question 22

v-SNAREs

Answer 22

Vesicular SNAP Receptor proteins

VAMP (Vesicle-Associated Membrane Protein): Synaptobrevin: binds t-SNARE

Question 23

Target membrane SNAP REceptor proteins or t-SNAREs

Answer 23

Syntaxin: Plasma membrane protein

SNAP-25: A plasma membrane protein of 25 kDa

Question 24

Neurexin

Answer 24

Plasma membrane protein: May also interact with vesicle proteins

Question 25

First steps in chemical transmission

Answer 25

Synthesis of neurotransmitter, channel proteins, synaptic vesicles and delivery to presynaptic site
AP propagates down trunk

Question 26

Preparation of vesicles for docking at active zone

Answer 26

Synapsin I normally anchors the vesicles to the cytoskeletal proteins
Once phosphorylated synapsin I releases the vesicles
Rab3A proteins bind to vesicles and help to propel vesicles toward the active zone

Question 27

Types of receptor-gated chemical transmission

Answer 27

Direct gating:
Fast time domain
Receptor is part of ion channel
5 subunits with 4 transmembrane domains
Indirect gating:
Slow time domain (several steps)
Receptor activates G proteins = metabotropic receptors
cAMP or cGMP cascade
7TM protein

Question 28

Gaseous Transmission

Answer 28

Mediated by the synthesis & diffusion of a gaseous messenger
No unique ultrastructural presynaptic or postsynaptic specializations
Gaseous messengers: Nitric oxide, Carbon monoxide
cGMP

Question 29

Neuro-neuronal synapse

Answer 29

Glutamate released from the presynaptic terminal to NMDA receptors
Entry to Ca through NMDA receptors activates neuronal NOS via its calmodulin-binding site
nNOS produces NO while converting arginine to citruline
NO acts in postsynaptic cell or diffuses to neighboring neurons, glial cells, or presynaptic terminal

Question 30

NO Transmission

Answer 30

Neuronal NOS activation
Passive diffusion of NO in postsynaptic cell and across the synapse to the presynaptic terminal and astrocytic processes
Binding of NO to iron in a heme moiety attached to soluble guanylate cyclase
Activation of guanylate cyclase and production of second messenger cGMP, activates cGMP -dependent protein kinase
Altered gating of ion channels in postsynaptic membrane
Inactivation and removal of NO occurs via rapid spontaneous breakdown of the gas molecule