Ion	Color	[ion]in	[ion]out
Cations (+)
K+	orange	400	20
Na+	red	50	440
Ca2+		0.0001	125
Anions (-)
Cl-	blue	9	100
Anions-	green	156	30

Place the beads in the following pattern in- and outside your cell to model their relative concentration gradients.

Ion	Color	[ion]in	[ion]out	E
Cations (+)
K+	orange	9	1
Na+	red	1	9
Anions (-)
Cl-	blue	1	9
Anions-	green	9	1	x

Note that the inside and outside of the cell have an equal number of + and - charges. If you would stick an electrode into the cell to measure the potential difference between inside and outside then it would read 0 mV.

Exercises:

1. Consider what would happen if the cell membrane lets potassium ions (K⁺) pass through freely while none of the other ions could get through. As charges travel past the membrane the electrode would measure a potential between inside and outside. Would the potential be inside positive or negative?
2. Consider what would happen if the cell membrane lets sodium ions (Na⁺) pass through freely while none of the other ions could get through. As charges travel past the membrane the electrode would measure a potential between inside and outside. Would the potential be inside positive or negative?
3. Consider what would happen if the cell membrane lets chloride ions (Cl^-) pass through freely while none of the other ions could get through. As charges travel past the membrane the electrode would measure a potential between inside and outside. Would the potential be inside positive or negative?

Place the beads in the following pattern in- and outside your cell to model their relative concentration gradients.

Ion	Color	[ion]in	[ion]out	E
Cations (+)
K+	orange	9	1
Na+	red	1	9
Anions (-)
Cl-	blue	1	9
Anions-	green	9	1	x

Exercises:

1. At the resting potential the cell membrane is permiable to potassium ions (K⁺). What direction would the initial net flow of charges go? What will happen over time? As charges travel past the membrane what potential would an electrode measure between inside and outside?
2. How is it to disrupt the resting potential? How stable is it?
3. How expensive is it to maintain the resting potential?

Place the beads in the following pattern in- and outside your cell to model their relative concentration gradients during the resting potential.

Ion	Color	[ion]in	[ion]out	E
Cations (+)
K+	orange	+6	+4
Na+	red	+1	+9
Anions (-)
Cl-	blue	-1	-9
Anions-	green	-9	-1	x
Sum of Charges
		-3	+3

Exercises:

1. A cell membrane at rest is permiable to potassium ions (K⁺). The associated potential is negative inside of the cell relative to the surround. If the inside of a cells becomes less polarized and reaches a threshold level (e.g., -20mV), a set of voltage-gated sodium (Na⁺) channel open up. Which charges will flow when this happens? What direction will the net flow go initially? Why would the net flow point that way? Sodium channels close again as the potential becomes positive on the inside. What influence will the closing have on the flow of charges? As charges travel past the membrane what potential would an electrode measure between inside and outside?
2. How fast is the flow of charges for this transmission?
3. How high is the signal to noise ratio for this transmission?
4. How expensive is this type of signal transmission?
5. What type of electrical influence will make a neuron more likely to produce an action potential?
6. What type of electrical influence will make a neuron less likely to produce an action potential?

Most signalling from one neuron to another occurs across a gap at a dedicated signalling site (i.e., the synapse). It involves the opening or closing of membrane channels in the postsynaptic membrane through the excretion of neurotransmitters from the presynaptic membrane.

Ion	Color	[ion]in	[ion]out	E
Cations (+)
K+	orange	+6	+4
Na+	red	+1	+9
Anions (-)
Cl-	blue	-1	-9
Anions-	green	-9	-1	x
Sum of Charges
		-3	+3

Exercises:

1. A cell membrane at rest is permiable to potassium ions (K⁺). The associated potential is negative inside of the cell relative to the surround. What type of channels would you open or close in the postsynaptic membrane to bring about a rise in the postsynaptic cell's potential (i.e., a excitatory postsynaptic potential)?
2. What type of channels would you open or close in the postsynaptic membrane to bring about a further drop in the postsynaptic cell's potential (i.e., a inhibitory postsynaptic potential)?