Cellular Neuroscience (207) Ian Parker

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Cellular Neuroscience (207)Ian Parker

• Lecture 1 - Enough (but not too much!)
  electronics to call yourself a cellular
  neurophysiologist

http//parkerlab.bio.uci.edu
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Ohms Law
Current I
V IR
resistor R
battery V
V (Volts) - electrical driving force
(water pressure) voltage, potential, potential
difference, p.d. are all synonyms I (Amperes)
- electrical current flow (gallons per
minute) R (Ohms) - resistance (how
narrow the pipe is) R V/I so, if V 1
volt for R 1 W I 1A for R
1 k W I 10-3 A (1 mA)
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Charge
Charge amount of electricity (number of
electrons gallons of water) current
time 1 A 1 sec 1 Coulomb (C) How many
electrons are there in a Coulomb??
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Resistors in series and parallel
I
R1
Total R R1 R2 I V / (R1 R2)
V
R2
1/ total R 1/R 1/ R2
R2
R1
I1
I2
I I1 I2
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Conductance
Conductance is the reciprocal of resistance (i.e.
how easily something conducts electricity) Conduc
tance (G) 1/R Unit Siemen (S) 1/ 1W
total conductance G G1 G2 From Ohms law I
V / R So I V G
Itotal V (G1 G2)
G2
G1
I1
I2
I I1 I2
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Voltage dividers
E - V R2 /(R1 R2)
R1
V
R2
If V 1 V, R1 9 kW and R2 1 kW what is E?
what current flows through R1?
E
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Capacitance
Capacitor - two conductors separated by an
insulating gap (dielectric)
e.g. 2 metal plates separated by an air gap
Capacitance (C) increases as 1. The area of the
plates is increased 2. The separation between
the plates is decreased 3. The dielectric
constant of the insulator is increased
Capacitors store electricity, but cannot pass a
steady current Unit Farad (F) 1 F
capacitor that will store 1 Coulomb
when connected to 1 V Charge (q) stored on a
capacitor C V
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RC (resistor/capacitor) circuits
1. Low-pass RC circuit
switch
R
E
V
C

• Voltage rises exponentially from zero to V with
  time constant t
• t is time for change to 1/e of final voltage ( e
  2.71828)
• t (sec) R (W) C (F)
• what is t if R 1 MW, C 1 mF?

Switch closed
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The effect of a low-pass circuit is to pass
steady or slowly changing signals while
filtering out rapidly changing signals
brief change in voltage
longer change in voltage
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RC (resistor/capacitor circuits)
2. High-pass RC circuit
switch
E
C
R
V

• Output voltage instantly rises to match input
  voltage, then decays exponentially.
• Time constant of decay
• RC
• Effect is to block rapidly-changing voltages
  (capacitor is an insulator), but pass rapidly
  changing signals

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SI prefixes We will be using some very big and
some very small numbers