Mind, Brain & BehaviorWednesday

January 29, 2003

Ion ChannelsChapter 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 (CA2) channels permit

CA to enter the cell. As CA2 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:

Vm = Vin – Vout

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.