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PSYCH 2230
Professor DeVoogd
Fall 2016
Week 2: Lecture 2 of 2
Wednesday, August 31st
Lecture Keywords: Neurophysiology, cell membrane, resting potential, action potential, concentration gradient, charge gradient, spike
Neurophysiology:
1. Neurons do actually look like the diagram. 2. Dendrites have bumps on them; they are synapses.
This lecture is focused on the cell membrane.
1. The cell membrane is highly specialized for electrical signaling.
a. It has a lipid bilayer (two fatty layers bound together by proteins).
b. It is studded with lots of “biological machinery” (proteins) that do different kinds of
jobs.
c. The lipid bilayer has selective permeability (semi-permeable).
i. This means it allows some ions to pass through channels, but not others.
Ion Channels:
1. Ions are charged particles.
2. There are pores in the cell membrane which ions can pass through.
a. Some channels require an “intervention to open.”
i. Selective channels (e.g. for Na+ in the diagram) are not always open.
b. Some stay open all the time (e.g. passive K+ channels in the diagram).
i. K+ channels are indiscriminate. Things go in and out as they wish.
The Resting Potential:
1. There is a difference in charge between the inside and outside of the neuron (i.e. there is a
voltage).
a. The inside of the cell is negative with respect to the outside (a healthy neuron at rest is
-50 to -80 mV).
b. Resting potential is established and maintained by ion flow across the cell membrane.
i. There is some active and some passive ion flow.
c. The neuron can be damaged if it is over-charged.
2. For example, salt sitting in a dish of water does nothing.
a. But, if you push an electrode into the water so it hits an ion, the charge in the water
changes.
3. The sodium/potassium pump (Na+/K+ pump) establishes the resting potential.
a. 3 Na+ ions are pumped out for every 2 K+ ions in.
b. This causes a net buildup of positively charged particles outside the neuron.
c. Because the active transport consumes energy, the nervous system is constantly using up
energy.
Forces that Contribute to Action Potentials:
1. The Na+/K+ pump establishes a “concentration gradient” and a “charge gradient”
a. Particles tend to diffuse down a concentration gradient.
b. In the charge gradient opposite charges attract and like charges repel
c. Once the Na+ is out, it can’t come back in.
d. Once the K+ is in, it CAN flow back out.
i. It does so until its concentration flow is balanced by the charge difference.
ii. As the concentration of K+ increases, they flow back out of the cell.
e. Chloride and calcium ions and large proteins (- charge) are present as well.
f. The proteins create a negative charge to start with.
g. The negative charge inside makes the sodium ions want to come back in, but they can’t
so they are hovering just outside the cell.
i. A -65mV charge is left behind in the cell.
Ion Channels:
1. Ion channels are pores in the membrane which ions can pass through.
a. Some stay open all the time.
b. Some open when the membrane voltage changes (e.g. voltage-gated Na+ channels).
c. Some open when a particular kind of molecule binds to them (e.g.ligand-gated channels). This type of ion channel will be discussed in a later lecture.
Voltage Gated Na+ Channels:
1. Voltage gated Na+ channels are normally closed.
2. They open when the voltage reaches a ‘threshold.’
a. Proteins that form the channel change shape to allow Na+ ions to pass through.
b. They stay open for approximately 1 millisecond.
c. After closing, there is a brief “refractory period” in which they cannot open again.
3. At -40mV the cell freaks out and there is a spike; there is a sudden shift from trying to depolarize to letting it just polarize then bouncing back.
a. The voltage gated Na+ channels open and a large flow of + ions come in.
b. The cell goes positive in charge, then it regulates itself again.
c. Sodium and potassium pumps are still working during this time.
4. This whole process has been in just a patch of an axon.
5. This is a chain reaction, so adjacent areas do the same thing.
a. If you did this to the center of an axon, it would spread up and down the axon in both
directions.
b. Under normal circumstances it starts at the beginning of axon and moves down in a
domino-like effect.
6. This process is faster in fatter axons.
a. It happens at rate anywhere between 2mph to 150/200mph.
The Action Potential:
1. The action potential is an electrical signal that travels down the axon.
a. It is caused by ions (mainly Na+ and K+) passing in and out of the neuron through ion
channels.
Measuring Action Potentials:
1. There is a slight overshoot in charge caused by extra K+ and Na+ on outside.
Properties of Action Potential:
1. Active propagation
a. The action potential does not diminish with distance (the charge does not weaken as it
travels).
2. Myelin is a crate-like structure.
a. It insulates a chunk of axon.
b. The insulation helps the charge not diffuse out.
c. Nodes of Ranvier allow the conduction to jump.
i. The charge jumps from one to the next.