C2006/F2402 -- 2008 --Outline of Lecture #14– Electrical Communication #2

Version A -- includes more space for taking notes

(c) Stuart Firestein, Columbia University, New York, NY. Last update 03/31/2005 04:46 PM .  

Notes by Chris Kelly Click here for a shorter version without the spaces.

Some links to animations and additional information on neuroscience are on the study guides for lectures 16 & 18  from 2004. See lecture 16 & 17 and lecture 18. If you need a back up text for the material that isn't in Becker, you might want to try Kimball's on line text.

I. Action Potential

• Brief Review

   

  • Action potentials are invariant and rapid changes in membrane potential used for signaling.

   

• Phases

   

  • Depolarization (initial)

     

    • Stimulus causes the ligand-gated sodium channels to open, causing a passive spread of depolarization inside the cell.

     

  • Rising phase

     

    • If that depolarization is sufficient (10 to 15 mV more positive), voltage-gated sodium channels at the axon hillock suddenly open, causing an enormous influx of sodium and the rapid depolarization spike.

     

    • This is essentially a positive-feedback cycle: sodium entering the cell causes it to depolarize. These depolarized conditions cause more sodium to enter the cell (because voltage-gated channels open). This depolarizes the cell even more, making more sodium channels open, and so on.

     

  • Falling Phase

     

    • Two factors contribute to the cell then racing back down toward negative potential:

       

      • 1) Voltage-gated sodium channels inactivate after being open for a certain time. So, those channels that opened, permitting the depolarization, slam shut and cannot be opened again until the cell has “reset” itself back to RMP.

       

      • 2) Voltage-gated potassium channels, which were tripped during the depolarization (around –20 mV), begin to open. Why did they not open as soon as the voltage hit –20 mV? They are slow to undergo the proper conformational changes.

       

    • The net effect is that the permeability to potassium once again far outweighs that to sodium, so the cell approaches E_K.

     

  • Undershoot

     

    • The potassium permeability is so high, in fact, that the cell goes closer to E_K than during RMP – it hyperpolarizes. This brief hyperpolarization can be divided into two phases.

       

      • Absolute refractory period (negative slope)

         

        • Voltage-gated sodium channels are still inactivated, and thus cannot be opened by anything. Since action potentials depend on these channels, no action potential is possible. Also, even if such an Na influx were possible, the K current is too high to be overcome.

         

      • Relative refractory period (positive slope)

         

        • Voltage-gated sodium channels begin to be released from inactivation, making action potential possible again, though more stimulus than usual is required since the cell is farther from threshold than usual.

         

• Recall that there are no appreciable changes in concentration at any point in this action potential. All that has changed are the relative permeabilties of potassium and sodium.

   

  • In fact, were you to poison the Na/K pump, eliminating the cell’s ability to restore those ions to the correct side of the membrane, the neuron could still fire another 150,000 action potentials before the concentrations started to become insufficient.

   

• Notice that there is little room for variety in this process, which is why action potentials are (1) invariant, and (2) all-or-nothing. You cannot have half of an action potential.

To review the action potential, try problem 8-2, A-C, & 8-3 to 8-4.

II. Propagation

• So what is the use of all this? An action potential is worthless if it does not travel somewhere.

   

  • Action potentials are designed for long-distance signaling, unlike graded changes in potential (i.e. depolarizations that do not reach threshold and do vary in amplitude, unlike action potentials).

   

  • The action potential is propagated along the axon.

     

    • Think of the axon like a fuse: when you light a fuse, the end heats up the next segment, which flares up and heats up the following segment, which then flares up. Unlike a piece of string, a fuse carries the flame all the way down to the other end.

     

• Where does the action potential begin?

   

  • At the beginning of the axon, where it first leaves the soma, there is a concentration of voltage-gated sodium channels. This area is called the initial segment, or axon hillock. The propagation down the axon begins with these channels opening.

   

  • When these channels open, positive charge enters the cell and spreads passively down the axon. This positive charge trips the next voltage-gated sodium channels on the axon, causing a sodium influx there, which then causes the next channels to open, and so on.

   

• Why doesn’t the signal go backward?

   

  • Remember the absolute refractory period. The positive charge will, in fact, passively spread in both directions in the axon. When it reaches the segment that just fired, however, those voltage-gated sodium channels will still be inactivated, and thus unable to fire an AP.

   

• How can one ensure that the propagation will make it?

   

  • Axons can be leaky, and concentration of voltage-gated sodium channels can vary. A situation could arise in which the positive charge that enters the axon is not able to trip the next channel in the series. This is overcome in two ways.

     

    • In invertebrates, the axons are wider, and thus the conductance is greater. So charge is more easily able to spread, and propagation is ensured.

     

    • In vertebrates, many axons are myelinated. Myelin is a fatty substance produced by glial cells that can wrap itself around the axon membrane. This produces an insulating effect, allowing charge to travel more easily in the axon.

     

    • In the first case, you have reduced the cable resistance. In the second case, you have increased the membrane resistance (and thus reduced “leakiness”). Both result in more conductance down the axon.

     

  • The myelin “sheath” is useless, however, unless it has gaps.

     

    • Ions clearly cannot pass through myelin. So having a myelin sheath around the axon would prevent signaling unless there are gaps in the myelin dotted along the axon. These are known as the nodes of Ranvier.

     

    • At each node, there is a high concentration of sodium and potassium channels. So when the positive charge reaches the node, the series of electrical events described by the action potential will occur there. The signal is regenerated. This kind of signaling is called saltatory conduction, since it appears the action potential is “jumping” from one node to the next.

     

    • Since action potentials take some time, having the myelin sheath accelerates the signaling process by reducing the number of action potential events along the axon. So signaling in myelinated axons is faster than in unmyelinated axons.

       

      • Conduction velocity can be up to 10 m/s.

       

    • There are several demyelinating diseases in the nervous system. Multiple sclerosis (MS) is a famous one, in which an inappropriate autoimmune response results in the destruction of myelin, thereby interrupting signaling in nerves. One can see these diseases in images of the nervous system because the characteristic white color of the myelin is missing in certain places.

     

    • Note on terminology: even though the cell is firing action potentials all along its axon, we say that it has fired a single action potential and simply propagated it.

To review propagation, try problem 8-2 D-E & 8-6.

III. Synapses

• So the action potential travels down the axon –– what then?

   

  • The end of the axon is a specialized area with characteristic proteins and biochemical processes. It is essentially independent from the soma, even though it gets its material from there.

   

  • This terminal area links one neuron to another, enabling intercellular communication. These areas of contact are called synapses, and you have about 10¹⁴ of them in your brain –– three orders of magnitude more than neurons.

   

• What is the nature of the synapse?

   

  • The big question is whether the link is an electrical or chemical one.

   

  • Otto Loewi showed that these links are primarily chemical by placing two frog hearts in bath solution, innervating only one of them, and then observing that both hearts are affected. In that particular case, the chemical was acetylcholine, and it was causing the hearts to slow down their beating. There was no electrical connection between the hearts, just a chemical one (the solution in which they were placed).

   

  • There are some electrical synapses in the body; they are created by gap junctions. The myocytes in the heart, for example, are connected by electrical synapses so that they can depolarize as a unit, giving a unified heartbeat.

   

• What is the structure of the synapse?

   

  • The presynaptic neuron is that which fired the action potential along its axon, to the axonal terminal. The postsynaptic neuron is the one that its axon contacts.

   

  • There is a space between the two cells known as the synaptic cleft. It is typically very narrow –– 20 to 40 nm –– but a definite gap.

   

  • The major players:

     

    • Presynaptic side: calcium and vesicles full of neurotransmitter

     

    • Postsynaptic side: neurotransmitter receptors

     

  • The process:

     

    • The action potential races down the axon and depolarizes the terminal. This causes voltage-gated Ca⁺⁺ channels at the terminal to open. Since the calcium concentration is 5 mM outside the cell and 0.1 µM inside, calcium rushes in. (FYI: lots of calcium inside the cell is a bad thing. That’s why the concentrations are the way they are.)

     

    • Calcium influx causes the vesicles full of neurotransmitter to dock with the presynaptic membrane and then fuse with it, causing exocytosis of neurotransmitter into the synaptic cleft.

     

    • The NT’s diffuse cross the synaptic cleft and contact receptors on the postsynaptic side.

     

• What are the kinds of neurotransmitters?

   

  • Many are amino acids: glutamate, GABA (g-amino butyric acid), glycine are examples.

   

  • There are also amino acid derivatives: serotonin, dopamine, epinephrine, etc.

   

  • Acetylcholine (ACh) is another.

   

• What are their overall effects?

   

  • Neurotransmitters are considered excitatory or inhibitory, based on whether they produce a depolarization or hyperpolarization (respectively) in the postsynaptic neuron.

   

  • Glutamate is the main excitatory NT in the CNS; glycine and GABA are the main inhibitory ones in the CNS. ACh is the primary NT in the PNS.

   

• How are these electrical effects produced?

   

  • When NT’s cross the cleft, they bind to receptors. These receptors are of two kinds:

     

    • Ionotropic: these receptors are also ion channels. When ligand binds, conformational changes in the channel subunits are induced, resulting in the opening or closing of the pore. This is a one-to-one effect: the ligand affects only one channel.

     

    • Metabotropic: the receptors are coupled to G-Proteins, and the cascade is initiated by the binding of ligand. The produced cascade usually results in the opening/closing of channels via second messangers. One ligand can thus affect many channels in this case.

     

  • In both cases, the opening of channels causes the flow of ions, which has an electrical effect.

   

  • If the net electrical effect is a hyperpolarization, than the NT has produced an IPSP (inhibitory post-synaptic potential). Depolarization -> EPSP (excitatory …).

   

• There are also gaseous neurotransmitters.

   

  • Nitric oxide, NO, is one example. Unlike solid neurotransmitters, these can freely diffuse from one cell to another.

   

  • NO initiates a cascade that results in guanalyl cyclase (GC) producing cGMP, whose net effect is vasodilation in many cases. Viagra inhibits PDE-3 from breaking down cGMP, thereby maintaining vasodilation and, hence, an erection.

   

  • GC and cGMP are analogous to AC and cAMP, which we have already studied. The essential difference is just the nucleotide involved.