U5 Lecture 40

Question 1

where does an action potential travel along

Answer 1

the axon of motor neuron to the presynaptic terminal on the muscle fiber

Question 2

what does depolarization of terminal open and trigger

Answer 2

it opens calcium channels to trigger release of acetylcholine (Ach)

Question 3

where does acetylcholine bind

Answer 3

to nicotinic Ach receptors on muscle

Question 4

what is excess acetylcholine destroyed by

Answer 4

acetylcholinesterase

Question 5

binding of Ach to receptors opens what

Answer 5

ligand gated sodium channel to cause large muscle depolarization which triggers muscle action potential in healthy individual

Question 6

action potential triggers calcium release from where which allows what?

Answer 6

release from sarcoplasmic reticulum which allows sliding of actin/myosin filaments due to cross bridges

Question 7

where does calcium return to after cross bridge formation

Answer 7

the sarcoplasmic reticulum

Question 8

what happens in motor neuron degeneration or conduction failure

Answer 8

efferent action potentials fail to reach effector, either due to motor neuron degeneration or loss of myelin on axons

Question 9

what is an example of motor neuron degeneration

Answer 9

amyotrophic lateral sclerosis

Question 10

what is an example of conduction failure

Answer 10

multiple sclerosis

Question 11

what happens in neuromuscular transmission failure

Answer 11

motor neuron action potential arrives at neuromuscular junction but fails to cause sufficient muscle excitation

Question 12

what is an example of neuromuscular transmission failure

Answer 12

lambert eaton syndrome or myasthenia gravis

Question 13

what happens in dystrophic myopathies

Answer 13

weakness and loss of muscle fibers

Question 14

what is an example of dystrophic myopathies

Answer 14

muscular dystrophy

Question 15

what happens in metabolic myopathies

Answer 15

a variety of inherited metabolic disorders in which genes encoding enzymes that aid glucose metabolism or ATP production in muscle are mutated

Question 16

what is an example of metabolic myopathies

Answer 16

mcardles myopathy

Question 17

what is problem #1 preliminary diagnosis

Answer 17

• patient presents with weakness
• difficulty standing up from seated position or lifting objects
• decreased street reflex
• minor atrophy of muscle tissue

Question 18

what are the take home messages for problem #1

Answer 18

1. motor neuron action potential normally leads to muscle fiber action potential because so much Ach is released and there are plenty of Ach receptors “high safety factor”
2. failure of neuromuscular transmission can prevent the activation of muscle contraction causing weakness. the resultant extended dishes can lead to muscle atrophy. note that this is the opposite of the training effects discussed in the first integration lecture

Question 19

what is the question of problem #2?

Answer 19

what is the specific problem with patient’s neuromuscular synaptic transmission?

Question 20

what is lambert eaton syndrome

Answer 20

• autoimmune disorder in which the body produces antibodies against voltage gated calcium channels in its own presynaptic terminals
• this results in decreased release of Ach from presynaptic terminals, failure to cause an actin potential in the muscle fiber and therefore muscle weakness

Question 21

what is myasthenia gravis

Answer 21

• autoimmune disorder in which body produces antibodies against nicotinic ACh receptors at its own neuromuscular junction
• this results in failure of ACh to open enough ligand-gated channels in postsynaptic membrane, failure of muscle action potential and therefore muscle weakness

Question 22

acetylcholinerase inhibitors are drugs such as

Answer 22

neostigmine, physostigmine or endrophonium

Question 23

how can Ach be applied directly to a muscle fiber

Answer 23

using a small pipette to spritz some out

Question 24

what are the take home messages for problem #2

Answer 24

1. autoimmune disorders can disrupt neuromuscular transmission by attacking presynaptic calcium channels or postsynaptic Ach receptors
2. using our knowledge of the steps required for normal synaptic transmission, we can devise a way to differentiate between lambert eaton and myasthenia gravis