USMLE Step 1 Physiology Review 52 11 Bioelectric Phenomena

USMLE Step 1 Physiology Review 52 11 Bioelectric Phenomena

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Play USMLE Audio MP3 52 11 Bioelectric Phenomena Below

Begin 52 11 Bioelectric Phenomena Transcription

Now, for some questions about bioelectric phenomena.

Electric events in the human body are fast and weak.  For example, what is the duration of a typical synapse or nerve action potential?

  • One millisecond.

What is the typical amplitude for the QRS complex for the EKG?

  • One millivolt.

What is the QRS complex? You may want to pause the tape here.

  • It is a pattern on the EKG of a negative wave, followed by a positive wave, followed by a negative wave.  However, this signal is only about one percent of the value generated across cell membranes. 

Why is there such a difference?

  • Potential differences are attenuated when recorded at a distance from the generator.

What are the charged particles that form the current flow in the human body?

  • Ions.

Does an anode have a relatively positive charge or a relatively negative charge?

  • An anode has a relatively positive charge. 

Therefore, would cations flow away from or toward the anode?

  • Away from.

And would they flow toward or away from the cathode?

  • Cations would flow toward the cathode. 

Remember, the cathode attracts cations, and since opposites attract and cations are positive, the cathode must have a relatively positive or negative charge?

  • Relatively negative.

Student Doctor, please pause the tape and summarize the general information covered so far about bioelectrical phenomena.

  • Electric events in the human body are fast and weak.  For example, the duration of a typical synapse or nerve action potential is one millisecond and a typical amplitude for the QRS complex for the EKG is one millivolt.  However, this signal is only about one percent of the value generated across cell membranes because potential differences are attenuated when recorded at a distance from the generator.  Ions are the charged particles that form the current flow in the human body.  An anode has a relatively positive charge.  Therefore, cations flow away from the anode and towards the cathode.  Remember, the cathode that begins with “cat” attracts cations, and since opposites attract, the cathode must have a relatively negative charge.

How is membrane permeability for ions expressed?

  • It is expressed in terms of conductance, which is symbolized with a g, a lower case g.

What is the reciprocal of conductance?

  • Resistance.

What is the relationship between the flow of charges, or current, and resistance?

  • They are inversely proportional to one another.

What is the relationship between current and potential difference?

  • They are directly proportional to one another.

The flow of ions across a membrane is equal to conductance multiplied with what?

  • Potential difference.

What quality of cell membranes makes them good electrical insulators?  In other words, what ingredient in cell membranes makes them good insulators?

  • The lipid content.

What is this electrical insulating quality of the cell membrane called?  In other words, since charge differences can build up across the cell membrane, it is said to have what?

  • Capacitance.

Student Doctor, please pause the tape and summarize the information about bioelectrical phenomena discussed since the last summary.  We started talking about conductance.

Membrane permeability for ions is expressed in terms of conductance, symbolized as a lower case g.  The reciprocal of conductance is resistance.  The flow of charges, or current, and resistance are inversely proportional to one another. Current and potential difference are directly proportional to one another.  The flow of ions across a membrane is equal to conductance multiplied by the potential difference.  The lipid content of cell membranes makes them good electrical insulators.  Since charge differences can build up across the cell membrane, the membrane is said to have capacitance.

 

 

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