Chemical synaptic transmission

Question 1

define synapse

Answer 1

the point of very close physical contact between a neuron and another cell.

Question 2

define chemical synapse

Answer 2

synapse in which a chemical neurotransmitter is secreted through the process of exocytosis by the presynaptic cell. (some exceptions exist)

Question 3

define electrical synapse

Answer 3

a synapse in which adjacent cells directly communicate through gap junctions

Question 4

What is a gap junction?

Answer 4

gap junctions are transmembrane protein connections formed by two connexons. Connexons are formed by six connexins. a gap junction interconnects two different cells and are found in virtually all solid tissues.

Question 5

what is a synaptic cleft?

Answer 5

the extracellular space between the presynaptic and postsynaptic cells. it is very narrow (12-20 nm)

Question 6

explain exocytosis

Answer 6

synaptic vesicles from inside the cell fuse with the plasma membrane so the vesicles bilayer becomes incorporated into the plasma membrane. the vesicles lumens cargo (neurotransmitters) is secreted into the synaptic cleft.

Question 7

two types of neurotransmitter receptor proteins

Answer 7

1. inotropic receptor proteins: contain a built in ion channel and neurotransmitter binding site.
2. metabotropic receptor proteins: have a neurotransmitter binding site but they lack an integral ion channel. they may open or close ion channels indirectly.

Question 8

who discovered the synaptic cleft?

Answer 8

Santiago Ramon y Cajal

Question 9

Who invented the Black Reaction? what is the black reaction?

Answer 9

Camillo Golgi. The reaction stains a few neurons and enables a clear visualization of isolated stained neurons and reveals details of its structure.

Question 10

what happens to neurotransmitters after exocytosis?

Answer 10

1. diffuse away out of the cleft
2. degraded by enzymes present within the synaptic cleft
3. reuptake into the presynaptic terminal
4. uptake into surrounding glial cells

Question 11

molecules that bind receptors

Answer 11

many molecules can bind receptors and be neurotransmitters.
Ligand: any molecule that binds to a receptor protein
-agonist: ligand that activates the receptor
-antagonist: ligand that prevents the receptor from being activated

Question 12

types of antagonists

Answer 12

competitive: binds to same site on receptor protein as agonist
non-competitive: binds to a different site on receptor protein than the agonist

Question 13

three modes of synaptic transmission

Answer 13

1. vesicular secretion by regulated Ca2+ triggered exocytosis
2. non-vesicular release of a water-soluble molecule through transmembrane channel
3. retrograde release of lipid-soluble molecule from the postsynaptic cell

Question 14

Steps in vesicular chemical synaptic transmission (7)

Answer 14

1. presynaptic depolarization (AP)
2. presynaptic Ca2+ influx through voltage gated Ca channels
3. exocytosis of neurotransmitter-containing synaptic vesicles
4. binding of neurotransmitter to postsynaptic receptors
5. change in postsynaptic membrane conductance and potential
6. diffusion, degradation or reuptake of neurtransmitter
7. synaptic vesicle membrane retrieval/recycing

Question 15

what makes a neurotransmitter inhibitory or excitatory?

Answer 15

it is not an intrinsic property of a neurotransmitter, it depends on which ionic conductances are activated or inactivated and which receptor proteins are activated or inactivated at a synapse

Question 16

what is the neuromuscular junction?

Answer 16

the NMJ is a chemical synapse between a motor neuron and a skeletal muscle fiber

Question 17

important features of the NMJ

Answer 17

1. Acetylcholine acts as an excitatory neurotransmitter
2. synaptic transmission is usually supra threshold and muscle fiber is reliably depolarized.
3. an excess of ACh is secreted (high safety factor)
4. acetylcholinesterase within the synaptic cleft rapidly hydrolyzes ACh into choline and acetate
5. liberated choline is taken up by the presynaptic terminal to synthesize more ACh

Question 18

where will AP triggering occur?

Answer 18

at the axon hillock. The cell has numerous presynaptic terminals (boutons) on the soma and dendrites so the responses to these multiple terminals are summed to determine if AP will fire or not. If AP will fire it will happen at axon hillock because threshold is lower at the axon hillock (many VGNCs)