Mind, Brain & Behavior Wednesday January 29, 2003

Ion Channels Chapter 7

Ion Channels  Found in all cells throughout the body.  Open and close in response to signals. Selectively permeable to specific ions High rate of flow (conductance)  Resting channels – usually open  Gated channels – open and close Refractory period – temporarily cannot be opened

Control of Gating  Binding of neurotransmitters, hormones, or second messengers from within the cell.  Phosphorylation – energy comes from a phosphate that binds with the channel. Dephosphorylation – removal of the phosphate.  Voltage-gated – responds to a change in the membrane potential.  Stretch or pressure gated – mechanical forces.

Effects of Drugs  Exogenous ligands – drugs that come from outside the body.  Endogenous ligands – naturally occurring  Agonist – binds with and opens a channel. Endogenous or exogenous (e.g., drug)  Antagonist – binds with and closes a channel. Reversible (curare) or irreversible (snake venom)

Importance of Calcium  Voltage-gated calcium (CA 2 ) channels permit CA to enter the cell.  As CA 2 rises, it binds with the neuron, preventing additional calcium from entering.  Increased calcium concentrations can cause dephosphorylation or permanent inactivation of a channel.  Calcium signals neurotransmitter release.

Goldman & Nernst Equations  Described in detail in Chapter 8 (optional)  Membrane potential: V m = V in – V out  Nernst Equation -- resting potential depends on the proportions of ions on the inside and outside of the cell and the temperature.  Goldman Equation – combines effects of different ions within the same neuron.

Propagation of an Action Potential  Axon diameter affects the speed with which an action potential travels down the axon. The larger the diameter of the axon, the faster the transmission.  Less permeability of the cell membrane increases transmission speed. Depolarization spreads down the axon instead of outward. Myelination increases the speed of transmission.

Diseases Causing Demyelination  Multiple Sclerosis (MS) Varied, unpredictable symptoms over entire brain functioning, not disabling, may be remitting Early intervention slows progression, chronic  Guillain-Barre syndrome Muscle weakness & paralysis on both sides of body, may affect breathing. May be related to auto-immune functioning. Goes away gradually within a few months.