Electrophysiology:

• Properties--irritability and conductivity
  • Irritability--basic property of protoplasm that enables a cell to respond to a stimulus
  • Irritability--not a response
  • Nerve cells--maximize irritability and conductivity
• Characteristics of body which allow neurons to function:
  • Body is composed of ions in solution (electrolytes)
  • All neural events within (not between) cell bodies are directly related to the presence & characteristics of ions

Electrical Considerations

Electrical Force (ions):

• Ions have a charge because they have an excess or a deficit of electrons in their outer ring
  • Sodium chloride (NaCl) has no charge (shared)
  • NaCl will not conduct electricity in this form
  • When placed in water, Na+ & Cl- ions move apart & have a charge
• Ions with their charge; like magnets: unlike attract & like repell
• This is the basis for neural irritability and conduction
  • Electrical gradient - imbalance in charge
  • Equilibrium - move to reach balance
• Chemical force - like ions tend to be repelled until they reach equilibrium (e.g., ink crystals in water)

Recording of Electrical Activity

Biological systems produce voltage differences between two points which can be recorded.

All recording instruments measure voltage differences

• Voltage difference results from imbalance of ions
• May be from large groups of cells (EEG)
• Small areas or a single cell, (microelectrodes)
• Volume conductor; imbalance of ions moves out into distant body areas.

Recordings made by attaching wires to the subject:

• Conduct electricity because they contain electrons. Some ions will connect with the negative electrode and give up their electron, and some electrons will leave the electrode at the positive area.
• This movement is very slight, however.
• Whenever the electrons move in the wire, the amplifier in the instrument enlarges the effect so it can be recorded

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Requirements for Resting Potential

1. Ions in solution - Na+, Cl-, K+ A- (protein molecules)

2. Semipermeable membrane

• Membrane can vary its permeability to some ions and is selectively permeable to some ions
• A- ions cannot pass through membrane
• K+ and Cl- can pass freely through the membrane
• Na+ may or may not pass through membrane; this is crucial for the establishment and maintenance of the resting potential

3. Active transport (Na+ pump)

• Active metabolic process
• Probably a function of the cell membrane itself
• Na+ ions are moved from the inside to the outside
• There is a build-up of sodium (Na+) ions on outside
• Electrodes - outside is positive relative to the inside

4. Electrical and chemical gradients

• Na+ cannot reach equilibrium but remains outside
• Cl- and K+ ions are free to move thru membrane
• K+ moves in due to electrical gradient, Cl- electrical movement stops when the forces are equal

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The Membrane Potential

• Determined completely by ratio of K+ ions inside relative to outside
• -60 to -80 mV inside of cell relative to outside
• Measured with microelectrode or micropipette
• Na+ pump is a continuing process which keeps Na+ ions outside
• Na+ continually leaking into cell and must be excluded
• In this state (resting potential), the neuron is ready to conduct (like a loaded gun)

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TERMS you should know

irritability	electrolyte	resting potential
conductivity	electron	semipermeable membrane
ions	equilibrium	EEG
electrical gradient	microelectrode	Na+
chemical gradient	active transport	A-
volume conduction	K+	Cl-

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Links to Associated Areas

LINK Neurotransmission

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