Outline for Sept. 21, 2000

1) Announcements

Next Week is a quiz

2) III. Ion channels

IV. Synaptic transmission

V. Receptor Molecules

VI. Two types of Ion Channels

VII. Graded Potentials

Video: The Mind, 2nd edition "Endorphins: The Brain's Natural Morphine" 5 min.

III. Ion Channels

• Channels on the axon that let ions in are called- voltage-gated (as opposed to chemically gated) because they open and close depending on the electrical potential across the membrane.
• The cell body and dendrites don't have many of this kind (voltage-gated) of ion channel (so action potentials are restricted to axons, and graded potentials apply to dendrites and cell bodies).
• Ion channels are VERY small

IV. Synaptic Transmission

• Impulse arrives at axon terminal bouton
• This depolarization causes Ca²⁺ to rush into the presynaptic terminal
• This in turn causes the vesicles containing neurotransmitter to release it into the synaptic gap
• Neurotransmitter molecules bind to receptor molecules on postsynaptic membrane
• This causes (in 1 of 2 ways, stay tuned!) the chemically gated ion channels to open, causing depolarization (or hyperpol. if IPSP) of dendritic membrane or cell body membrane.
• These polarizations are graded; that is, they diminish the farther they have to travel.
• Neurotransmitters are eliminated by several mechanisms

V. Receptor Molecules

• On the postsynaptic cell's dendrites and cell body
• Lock-and-key analogy
• Neurotransmitters are either excitatory at a given synapse, or inhibitory.
• If excit., they open sodium (or calcium) channels (depolarization), if inhib, they open channels for chloride or potassium ions (tending to hyperpolarize membrane).

Agonists of the neurotransmitter mimic its effects; antagonists interfere with (inhibit) its effects (e.g., drugs, poisons).

VI. Two Types of (Chemically gated) Ion Channels

1. Fast (ionotropic, direct, ligand-activated)

2. Slow (metabotropic, indirect, G-protein-coupled)

1. Ionotropic

• Receptor is also an ion gate through which ions flow
• Fast because of this
• The 3-dimensional structure is changed when neurotransmitter binds, such that ions can enter. Example: acetylcholine (ACh) receptor at neuromuscular junction.

2. Metabotropic

• Receptor binds neurotransmitter, but there's a 2^nd messenger to effect the opening of the associated ion gate.
• Most receptors are metabotropic
• They're coupled with proteins called G proteins, which dissociate into subunits when the receptor is activated by neurotransmitter.
• The a -subunit of the G protein then migrates away, and causes an ion channel to open.

NMDA receptor for glutamate (the major excitatory neurotransmitter of the nervous system) - it's both a voltage-sensitive (like ion channels on the axonal membrane), and a ligand-gated receptor.

VII. Graded potentials

• EPSPs and IPSPs at
• transmission of subthreshold depolarizations is passive (but quick)
• signal gets weaker the farther it goes along membrane - decremental conduction
• myelin-covered axons display this along myelinated portions of the membrane (because there are no ion channels in myelinated portion of membrane)

• action pot. in myelinated axons jumps from one node of Ranvier to the next - SALTATORY CONDUCTION
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