Physiology of NMJ

Question 1

are the axons of motor neurones innervating skeletal muscles myelinated or unmyelinated

Answer 1

both starts myelinated then branches into unmyelinted branches near the muscle

Question 2

what is a terminal bouton

Answer 2

end of nerve branch that forms a synapse with the muscle membrane at the NMJ

Question 3

what causes the release of acetylcholine

Answer 3

the conduction of action potentials that arise in the cell via the axon to the boutons

Question 4

who transmits faster myelinated or unmyelinated

Answer 4

myelinated

Question 5

what is the neurotransmitter of all motor neurones of skeletal muscle

Answer 5

acetylcholine (ACh)

Question 6

what type of motor neurone innervates skeletal muscles

Answer 6

alpha

Question 7

what part of spinal chord does does myelinated alpha neurone arise from

Answer 7

ventral body of spinal chord

Question 8

where does the pre synaptic terminal bouton synapse

Answer 8

the endplate region of the skeletal muscle fibre

Question 9

what surrounds terminal boutons

Answer 9

schwann cell

Question 10

what are the components of the NMJ

Answer 10

terminal bouton

surrounding schwann cells

synaptic vessels

synaptic cleft

end plate region of the muscle cell membrane (sarcolemma)

Question 11

what is the sarcolemma

Answer 11

the end plate region of the muscle cell membrane

Question 12

what is the synaptic cleft

Answer 12

the space between neurones at a nerve synapse across which a nerve impulse is transmitted by a neurotransmitter (aka synaptic gap)

Question 13

what do synaptic vesicles contain

Answer 13

ACh

Question 14

where do synaptic vesicles cluster

Answer 14

active zones

Question 15

where are nicotinic ACh receptors located

Answer 15

at regions of the junctional folds that face the active zones

Question 16

what are the components of ACH

Answer 16

choline and acetylCoA (from acetate)

Question 17

when does the release of ACh from vesicles occur

Answer 17

when the action potential opens voltage activated Ca++ channel

Question 18

how is choline transported into the terminal

Answer 18

choline transporter (symport with Na+)

Question 19

describe the synthesis of ACh

Answer 19

happens in cytosol

from choline and acetyl coenzyme A (supplied by mitochondia) by the enzyme choline acetyltransferase (CAT)

Question 20

what concentrates ACh in the vesicles

Answer 20

vesicular ACh transporter

Question 21

what does arrival of the action potential at the terminal cause

Answer 21

depolarisation and the opening of voltage activated Ca++ channels - this allows calcium entry to the terminal

Question 22

what happens to the vesicles when calcium is released

Answer 22

fuse with the presynaptic membrane (exocytosis) causing ACh to diffuse into the synaptic cleft

Question 23

what does the diffusion of ACh in the synaptic cleft do

Answer 23

activates post synaptic nicotinic ACh receptors in the muscle endplate region

Question 24

what are nicotinic ACh receptors made of

Answer 24

a pentamer of glycoprotein subunits which surround a cation sensitive pore (opens in the presence of ACh)

Question 25

what happens when nicotinic ACh receptors open

Answer 25

Na+ enter the muscle cell whilst K+ exits = depolarisation called the end plate potential (e.p.p)

Question 26

what is the miniature endplate potential

Answer 26

the electrical response to one ‘quantum’ of transmitter released from a vesicle - summate to produce the e.p.p

Question 27

what does e.p.p do when it exceeds a threshold

Answer 27

triggers the opening of voltage activated Na+ channels which causes a propagated action potential that initiates contraction

Question 28

what type of response is the e.p.p

Answer 28

graded response

Question 29

where does action potential propagate in the skeletal muscle cell

Answer 29

sarcolemma (surface membrane) then enters transverse T tubules

Question 30

what does action potential arriving at T tubule trigger

Answer 30

release of calcium from SR- which in turn causes contraction by interacting with troponin associated with the myofibrils

Question 31

what terminates the action of ACh

Answer 31

acetylcholinesterase- hydrolyses ACh into choline (taken up by choline transporter) and acetate (diffuses from the synaptic cleft)

Question 32

Neuromytonia :
what are the symptoms
what causes it
treatments

Answer 32

cramps, stiffness, slow relaxation, muscle twitches

antibodies against voltage activated K+ channels

anticonvulsants

Question 33

lambert eaton myasthenic syndrome:
symptoms
cause
treatment

Answer 33

muscle weakness

antibodies against voltage activate Ca++ channels

anticholinesterases, potassium channel blockers

Question 34

myasthenia gravis:
symptoms
causes
treatment

Answer 34

progressive muscle weakness during periods of activity

antibodies against nicotinic ACh receptors

anticholinesterases, immunosuppressants

Question 35

what do anticholinesterases do

Answer 35

increase conc of ACh in synaptic cleft

Question 36

what does botox (botulinum toxin) do

Answer 36

acts at motor neurone terminals to irreversibly inhibit ACh release

Question 37

what do curare like compounds do

Answer 37

competitive antagonists of ACh receptors, reduce amplitude of e.p.p

used clinically to induce reversible muscle paralysis