B6.036 Prework 1: NMJ

Question 1

how many motor neurons per muscle fiber

Answer 1

ONLY ONE

one NMJ per muscle fiber

Question 2

active zone of NMJ

Answer 2

portion of motor neuron axon that contains clusters of ACh filled vesicles
voltage gated Ca2+ channels are localized here

Question 3

where are AChRs located

Answer 3

muscle membrane directly opposite the active zone

tightly packed on junctional folds (infoldings of muscle membrane)

Question 4

how do AChRs work

Answer 4

ACh binds
AChR opens
cations flow through AChR into muscle cell and depolarize it

Question 5

how is ACh removed from the synapse

Answer 5

diffusion

hydrolysis by AChE

Question 6

dorsal horn

Answer 6

sensory neurons

Question 7

ventral horn

Answer 7

motor neurons

Question 8

root of motor neuron axons

Answer 8

axons extend from the cell body to the periphery by sending a process out the ventral root of the spinal cord

Question 9

speed of synapse formation

Answer 9

minutes

first step: aggregation of AChRs under motor neuron

Question 10

agrin

Answer 10

protein that directs aggregation of AChRs
made in motor neurons and shipped to muscle fiber
once released, binds to ECM and interacts with proteins on muscle fiber surface to induce aggregation of AChRs

Question 11

what specifically does agrin bind

Answer 11

basal lamina

LRP4 and MuSK

Question 12

structure of AChR

Answer 12

5 subunits
2 ACh binding sites
cationic channel

Question 13

function of AChR

Answer 13

allows Na+ ions into muscle
allows K+ ions out of muscle
allows Ca2+ ions into muscle
channel will desensitize

Question 14

endplate potential

Answer 14

results from release of ACh following a nerve action potential
amount of ACh releases is far in excess of what is needed to reach threshold in muscle fibers (high safety factor)

Question 15

quantal content

Answer 15

measurement of the synaptic strength of the synapse
# of vesicles of ACh that fuse with the nerve terminal membrane after each action potential

Question 16

safety factor

Answer 16

# of vesicles released in excess of what is needed to induce an action potential
very high at NMJ: every action potential in the motor unit will elicit an action potential in the muscle

Question 17

synapse specific translation

Answer 17

prior to innervation, all nuclei in muscle make AChR
once a synapse forms, electrical activity will turn off production of AChR at all nuclei
factors released by neuron will then turn production back on, but only in nuclei directly under motor neuron