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Psychology 312-1 . Physio logical Basis of Behavior jp-rosenfeld@northwestern.edu. We study how the brain makes mind and behavior. METHODS (in oversimplification terms): 1. Lesions (Ablation): Oldest Method; 2 ways:
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Psychology 312-1 Physiological Basis of Behavior jp-rosenfeld@northwestern.edu
We study how the brain makes mind and behavior METHODS (in oversimplification terms): 1.Lesions (Ablation): Oldest Method; 2 ways: a) Natural: Take them as they come (bullet holes, tumors; “Neuropsychology”.) b) You do it: chemicals, electricity (cheap). 2. Stimulation: “the opposite of lesions.” Again, electrical or chemical. 3. Recording: a)Imaging (PET, fMRI), b)Electrophysiology (EEG, ERP from neuron populations, or from single neurons)
Lesions and Stimulation… • ….. Assume that single structures are responsible for single behavior patterns or psychological functions. This is the height of naiveté, to wit……... “Structure A is substrate for anger.” Ergo… • a) If you lesion it, no more anger, as in lobotomy. (Usually no more lots of other stuff too.) • b) If you stimulate it, you get a guy angry.. Jose Delgado did the reverse….. http://www.youtube.com/watch?v=6nGAr2OkVqE
Of course, real neurons come in all shapes & sizes, like the following: • Drawing by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. (A) Denotes Purkinje cells, an example of a multipolar neuron. (B) Denotes granule cells which are also multipolar.
http://www.youtube.com/watch?v=ifD1YG07fB8&feature=related • The job of the nervous system is to transmit information from neuron to neuron, and to a target organ like a muscle which initiates behavior.
As we were saying… • The job of the nervous system is to transmit information from neuron to neuron, and to a target organ like a muscle which initiates behavior. • This happens by action potential (“spike”) propagation down a neuron, and, usually, via synaptic transmission to another neuron.
What’s a “potential?” • The name implies the ability to do work…electrical work, the ability to move charged particles from one level of potential to another, lower level.
All cells have a resting potential. • That is, a relatively constant difference in electrical potential across the cell membrane. Say +30 mV inside vs. – 10 mv outside. • This is where they spend most of their lives…at rest..it’s a good life. • “Irritable” tissue—muscle cells or nerve cells--- are different
Irritable cells….. • …..occasionally show sudden fast changes in cell membrane potential, before recovering the resting level. • In neurons, these changes are called “impulses” or action potentials, and they propagate from cell body to end of neuron.
Here is a neuron membrane passing from rest to action and back to rest…
In the next lecture or two…. • ….we will consider how the resting neuronal membrane potential and the action potential are generated. • To do this, we need to do some thought experiments…
OK, first, let us note that there are 3 passive phenomena mainly influencing the situation within a neuron: • (Passive means life is irrelevant to these phenomena.) • Membrane (semi-) permeability. • Chemical or concentration forces: Particles tend to move away from high and to low concentration. • Electrical forces. + “likes” – and “dislikes” +. – “likes” + and “dislikes” – OK, now, back to those thought experiments……
Consider a beaker of water divided by a semi-permeable membrane into 2 compartments, and I put a teaspoon of a monovalent salt into the left compartment. What does the salt ”want to” do? What forces are at work?
OK, the concentration forces drive ions to the right. No problem, both positive and negative ions can go through, so do. Now what forces act?
..So another pair go through, which puts system at balance or electrochemical equillibrium…with no difference in potential(=voltage)across the membrane
OK, only the cation can get through in the first instant. It does. Let’s now analyze forces and predict next moment.
The anion “wants to” follow the cation but is too big. (Good example of semipermeability.) What are forces now , and can we predict next moment?
We now have electrochemical equilibrium (There is still Fc but balanced by Fe) but with a residual voltage across the membrane
In the previous slide, the one permeable little cation is said to be:at its Equilibrium Potential. • This is the voltage across the membrane at which the electrical and chemical forces on the ion are in balance. • Fc + Fe = 0 or • Fe = -Fc
The situation (figure) in a resting neuron is more complicated: Given this situation what are forces on K+, the most permeable cation?
More complicated because more ions are involved, and they together affect electrical and chemical forces.
You should note that there is a relationship between Fc and Fe… • …i.e., the dis-proportionality between the left and right hand concentrations of permeable ions predicts the voltage across membrane at which system is at equilibrium. Thus we have an equation, the Nernst equation, which holds if K+ is sole permeable ion: • NERNST EQUATION: E =60 Log (K+o/K+i)
NERNST EQUATION with GOLDMAN EXTENTION : Derivation: • Total Force on an ion, say K+ = Electrical Force + Concentration (Chemical) Force. • Putting in units of Voltage, the total electrochemical force = • DV[K+] = ZFE + RT ln (K+i/K+o) • (Z= charge/mole,F= valence,E=membrance potential in voltage units, R= gas constant, T= temperature(absolute), ln = log to base e, K+i= inside Potassium concentration, K +o = outside Pot. Conc.) • When K+ is at electrochemical equilibrium, DV[K+] = 0 = C1E + C2 ln (Ki+ /K+o), where C1,C2….(all C) are constants. • So • C1E = - C2 ln (K+i/K+o) = C3 ln (K+o/K+i), and dividing both sides by C1 yields • E=C3/C1 ln (K+o/K+i) = C4 ln (K+o/K+i) =C5 log10(K+o/ K+i ). C5= about 60, so • NERNST EQUATION: E =60 Log (K+o/K+i) [Note: Log (x) = Log to base 10]
It is noted that this applies when only permeable ion is K+ • . Otherwise, one uses the Goldman equation (of which the Nernst is seen to be a special case). • E= 60 Log (PK [Ko+] + PNa [Nao+]+Pcl [Cli-]…….)/ • (Pk [Ki+] PNa [Nai+] + Pcl [Clo-]…….) • Pk= potassium permeability coefficient, Pna= perm coeff for Na, Pcl= perm coeff for Cl. Signs of ions omitted for clarity, but note, cation outside concentrations are in numerators, anion outside concentrations in denominator. Note what happens to equation if all coefficients but Pk go to zero. • Sample question: If K+o= 10000 and K+i=10, what is E if K+ is sole permeable ion? In other words, what is the K+ equilibrium potential?
This Nernst Equation…. • Allows us to see if a neuron is near/at equilibrium and if that ion is sole permeable one. • Thus if we calculate the equilibrium potential for K+, we use: E = 60 log ([K+]0/[K+]i) = -80 which is close to but not = to the actual -70. Meaning…?
The Equilibrium Potential of Na+.. • Using the concentrations of Na+ inside and outside in the equation yields the equilibrium potential for Na+: • E = 60 log ([Na+]0/[Na+]i) = +65 mV …but the real resting membrane potential is -70mV Is Na+ at equilibrium? We already guessed that it wasn’t due to Fc and Fe. What do we conclude about PNa or Na+ permeability?
Thus, the resting membrane potential is there because relatively permeable K+ moves as close to equilibrium as it can… • Is there proof? Yes. Scientists have manipulated interior and exterior [K+] and noted the effect on Em, the membrane resting pot. • They manipulate exterior [K+] simply by bathing neurons in solutions where [K+] systematically varies. Interior [K+] is manipulated as if neuron were a toothpaste tube. Both can be simultaneously changed.
Why the discrepancy?? • 90% is due to presence and influence of other ions. So if you used Goldman extension, most of discrepancy would go away: E= 60 Log (PK [Ko+] + PNa [Nao+]+Pcl [Cli-]…….)/ (Pk [Ki+] PNa [Nai+] + Pcl [Clo-]…….) = ~ -79 mV
The rest of the discrepancyis due to the… 2) Na+ & K+ Ion exchange mechanism, Or the “Sodium Pump.” This is a dynamic biochemical process that keeps Na+ out and K+ in, as the video will now demonstrate….. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html
The whole story • http://www.google.com/#q=hodgkin+experiment&hl=en&prmd=iv&source=lnms&tbs=vid:1&ei=kTyeTL2xKYKWnAftv-inDQ&sa=X&oi=mode_link&ct=mode&sqi=2&ved=0CAgQ_AU&fp=8a8ae2f39e51403c
You always see the overshoot… • …until you see the action potential!
Voltage Gated Ion Channels. • The first channels to open are Na+ channels. • The first critical finding by Hodgkin was that the greater the depolarization across the membrane, the greater the Na+ permeability (PNa), so Na+ rushes in (why?) which further depolarizes membrane.
Why doesn’t it inevitably lead to a spike? • Because although the first effect of depolarization in opens Na+ channels so Na+ rushes in…. • There is a delayed effect of the stimulation/depolarization, which is to more slowly open K+ channels. What does K+ “want to” do?
The outer wheel … • ….puts the brakes on the inner wheel. Whether you get a spike or re-polarization depends on the race between the inner and outer wheels. See James Stewart version of “Flight of the Phoenix.”
