Exam 1 - Synaptic Transmission

Question 1

what are the two general ways in which cells communicate?

Answer 1

• electrical synapses: gap jxns - direct electrical coupling

- chemical synapses: chemical diffuses from one cell to another

Question 2

where are the most significant electrical synapses in vertebrates?

Answer 2

• heart + smooth muscle
• certain cells of hypothalamus (to maximize burst of hormone secretion into the blood)
• liver
• lens
• myelin sheath

Question 3

structure of electrical synapses

Answer 3

• 3.5 nm diameter w/ 1.5 nm pore (not selective)

- connexons, one presynaptic + one postsynaptic

Question 4

under what conditions might a gap junction close?

Answer 4

when a cell is damaged:

• lowered cytoplasmic pH
• high cytoplasmic Ca2+

Question 5

function of electrical synapses?

Answer 5

• synchronization of electrical activity among populations of neurons
• myelin stabilization in the PNS (diffusion of nutrients)

Question 6

advantages and disadvantages of electrical synapses

Answer 6

advantages:
- connects to multiple cells
- fast conduction

disadvantages:
- affects only adjacent cells
- bidirectional - not good for a nervous system

Question 7

structure of chemical synapses

Answer 7

• synaptic cleft 50 nm
• usually vesicular secretion
• ligand-gated channels on postsynaptic membrane

Question 8

describe the steps that happen after an AP reaches a chemical synapse

Answer 8

1. depolarization of terminal membrane -> influx of Ca2+
2. elevation in internal Ca2+ -> causes neurotransmitter to be released from synaptic vesicles via exocytosis at release sites/active zones

Question 9

what are NSF and SNAPs?

Answer 9

NSF = NEM-sensitive fusion protein
SNAPs = soluble NSF-attachment protein

• prime synaptic vesicles for fusion
• regulate assembly of SNAREs which bind vesicles to membrane

Question 10

names of the three SNARE proteins and fxn of SNAREs

Answer 10

synaptobrevin - in membrane of synaptic vesicles
syntaxin and SNAP-25 - on plasma membrane

SNARE proteins form macromolecular complex that spans the two membranes and brings them into close apposition

Question 11

what is synaptotagmin and what is its function?

Answer 11

protein in synaptic vesicle membrane

when internal [Ca2+] is elevated, chemical properties of synaptotagmin change -> it inserts into the cell membrane and binds to other proteins -> actual fusion of vesicle membrane w/ presynaptic membrane + release of contents

Question 12

what is an EPP?

Answer 12

end plate potential

• AP in the presynaptic motor neuron -> transient depolarization of the post-synaptic muscle fiber
• normally large enough to bring membrane potential to threshold -> post-synaptic AP -> muscle contraction

Question 13

what is a MEPP?

Answer 13

miniature end plate potential

-spontaneously occurring depolarizations that are similar in shape and pharmacological behavior to EPPs

Question 14

how does the amount of Ca2+ influx into the presynaptic terminal affect the number of quanta released?

Answer 14

the greater the Ca2+ influx, the greater the number of quanta released

Question 15

what is the number of molecules in a quantum approximately?

Answer 15

each synaptic vesicle estimated to have [ACh] = 100 mM -> 10,000 molecules of neurotransmitter in one vesicle

(not all of these reach the post-synaptic membrane though - only about 70% do)

Question 16

what happens to ACh after it has bound to the post-synaptic membrane and then been released?

Answer 16

inactivated by AChE
-choline recycled into presynaptic terminal (sodium coupled choline transport system)-> made back into ACh by choline acetyltransferase

Question 17

what is spatial integration?

Answer 17

adding together of simultaneously occurring electrotonic inputs at different locations on the membrane

Question 18

what is temporal integration?

Answer 18

adding together of inputs occurring at different times, usually w/i 5-15 ms of one another

Question 19

what are EPSPs?

Answer 19

excitatory postsynaptic potentials

-increase Na+ permeability at postsynaptic membrane -> depolarization of membrane

Question 20

how can you get an EPSP that hyperpolarizes the cell?

Answer 20

EPSPs change permeability of both Na+ and K+ -> when membrane voltage is more positive than -11.5 mV (i.e., closer to the Na+ equilibrium potential), more K+ leaves the cell than Na+ enters -> hyperpolarization

Question 21

what are IPSPs?

Answer 21

inhibitory postsynaptic potentials

• inhibitory neurotransmitters increase Cl- permeability at the postsynaptic membrane -> hyperpolarization
• hyperpolarization also occurs if K+ permeability increases

Question 22

what is the reversal potential?

Answer 22

when membrane voltage is at -11.5 mV, equal amounts of Na+ and K+ cross the membrane and no net potential occurs

Question 23

describe information coding in the CNS

Answer 23

• place codes tell where information is coming from (internal maps)
• intensity coding (frequency and frequency modulation)
• recruitment (increase # of axons firing increases the system’s dynamic range)
• input organization