Download Cellular Neuroscience (207) Ian Parker

Cellular Neuroscience (207)
Ian Parker
Lecture # 1 - Enough (but not too
much!) electronics to call yourself
a cellular neurophysiologist
http://parkerlab.bio.uci.edu
Ohm’s Law
Current I
battery
V
V (Volts)
resistor R
V = IR
- 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
R = V/I so, if V = 1 volt
(how narrow the pipe is)
for R = 1 W
for R = 1 k W
I = 1A
I = 10^-3 A (1 mA)
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??]
Resistors in series and parallel
I
R1
Total R = R1 + R2
R2
I = V / (R1 + R2)
V
R1
I1
I = I1 + I2
R2
I2
1/ total R = 1/R + 1/ R2
Conductance
Conductance is the reciprocal of resistance (i.e. how easily
something conducts electricity)
Conductance (G) = 1/R
Unit : Siemen (S) = 1/ 1W
total conductance G = G1 + G2
G1
I1
I = I1 + I2
G2
I2
From Ohms law I = V / R
So
I =V*G
Itotal = V * (G1 + G2)
Voltage dividers
E - V * R2 /(R1 + R2)
R1
V
R2
E
[ If V = 1 V, R1 = 9 kW and R2 = 1 kW
what is E? : what current flows through R1?]
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
RC (resistor/capacitor) circuits
1. Low-pass RC circuit
switch
R
V
E
C
Voltage rises exponentially from
zero to V with time constant t
V
t is time for change to 1/e
E
of
final voltage ( e = 2.71828…)
tim e
Switch closed
t (sec) = R (W) * C (F)
[what is t if R = 1 MW, C = 1 mF?]
The effect of a low-pass circuit is to pass steady or slowly changing
signals while filtering out rapidly changing signals
B
brief change in voltage
longer change in voltage
RC (resistor/capacitor circuits)
2. High-pass RC circuit
switch
E
C
R
V
A
Output voltage instantly rises
to match input voltage, then
decays exponentially.
E
V
tim e
Time constant of decay
t = RC
Effect is to block rapidlychanging voltages (capacitor
is an insulator), but pass
rapidly changing signals
SI prefixes
We will be using some very
big and some very small
numbers