USMLE Step 1 Physiology Review 53 05 Neuromuscular Transmission

USMLE Step 1 Physiology Review 53 05 Neuromuscular Transmission

 

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Play USMLE Audio MP3 53 05 Neuromuscular Transmission Below

Begin 53 05 Neuromuscular Transmission Transcription

Okay, now for some questions about neuromuscular transmission.

What is the neurotransmitter at the skeletal neuromuscular junction?

  • Acetylcholine.    

After acetylcholine is exocytosed and diffuses across the synaptic cleft, it binds to and activates a specific kind of receptor.  What is the name for the receptor type?

  • Nicotinic cholinergic receptors.

Where are the nicotinic cholinergic receptors located?

  • Muscle end plates.

There are two kinds of electrical events that take place at the neuromuscular postsynaptic membrane.  What are they called?

  • Miniature end plate potentials and end plate potentials.

What are the abbreviations for miniature end plate potentials and end plate potentials?

  • Capital MEPP and capital EPP.

So, miniature end plate potential may be referred to as MEPP, and end plate potential may be referred to as EPP.

What is believed to be the cause of a MEPP?

  • The random release of a single synaptic vesicle of acetylcholine.

What is the term for the amount of neurotransmitter stored in a single vesicle?

  • Quanta.

What is the approximate amplitude of a MEPP?

  • Point five (0.5) millivolts.

Student Doctor, please pause the tape and summarize the information on neuromuscular transmission discussed this far.

  • Acetylcholine is the neurotransmitter at the skeletal neuromuscular junction.  After acetylcholine is released via exocytosis and diffuses across the synaptic cleft, it binds to and activates nicotinic cholinergic receptors.  These receptors are in the muscle end plates.  There are two kinds of electrical events that take place at the neuromuscular postsynaptic membrane.  They are the miniature end plate potentials, known as MEPPs, and end plate potentials, known as EPPs. The random release of the single synaptic vesicle of acetylcholine is believed to cause a MEPP.  The term for the amount of neurotransmitter stored in a single vesicle is a quanta.  The approximate amplitude of a MEPP is point five (0.5) millivolts.

What causes an EPP, or end plate potential?

  • The synchronous release of more than one hundred synaptic vesicles in a nerve terminal.

What is the amplitude range of EPPs, or end plate potentials?

  • Fifteen to forty millivolts.

What does activation of the nicotinic receptors on the muscle end plate do to the postsynaptic membrane?

  • It causes an increase in membrane permeability to sodium and potassium.

The increased conductance of sodium and potassium results in a depolarization of the end plate.  What is the value for the equilibrium potential for the end plate potential, or EPP?

  • Minus fifteen (-15) millivolts.

What kind of current moves the membrane potential towards threshold?

  • Local circuit current.

When the action potential is generated, does it propagate in one direction or two directions along the muscle fiber?

  • The muscle moves in both directions along the muscle fibers, causing it to contract.

What happens to the acetylcholine after it dissociates from the receptors?

  • It is metabolized.

What is the enzyme that breaks down the acetylcholine?

  •  Acetylcholinesterase

What are the components that acetylcholine is broken down into?

  • Choline and Acetylene

What happens to the choline? You may want to pause the tape here.

  • About half of it is taken back up by the presynaptic neuron and used to make more acetylcholine.    

Student Doctor, please pause the tape and summarize the information on neuromuscular transmission discussed since the last summary.  We started with the cause of an end plate potential, or EPP.

  • The synchronous release of more than one hundred synaptic vesicles in a nerve terminal can cause an EPP.  EPPs range in amplitude from fifteen to forty millivolts.  Activation of the nicotinic receptors on the muscle end plate causes an increase in membrane permeability to sodium and potassium.  The increased conductance of sodium and potassium results in a depolarization of the end plate.  The equilibrium potential for the end plate potential, or EPP, is minus fifteen (-15) millivolts.  Local circuit currents move the membrane potential towards threshold.  The action potential moves in both directions along the muscle fibers, causing it to contract.  Acetylcholinesterase breaks down the acetylcholine into choline and acetylene after it dissociates from the receptors.  About half of the choline is taken back up by the presynaptic neuron and is used to make more acetylcholine.   

Under normal conditions in a healthy person, do motor neuron action potentials always result in muscle action potentials?

  • Yes.

Under pathological conditions end plate potentials are too small to generate action potentials.  In one such condition there is a decrease in the number of neuromuscular nicotinic acetylcholine receptors.  What is the name of this condition?

  •  Myasthenia gravis.

Student Doctor, please pause the tape and summarize the information discussed this far on motor neuron action potentials under normal circumstances and under pathological circumstances.

  • Under normal conditions in a healthy person, motor neuron action potentials always result in muscle action potentials.  Under pathological conditions end plate potentials are too small to generate action potentials.  In the condition known as myasthenia gravis, there is a decrease in the number of neuromuscular nicotinic acetylcholine receptors.

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