CSB332 Exam

Topics: Action potential, Membrane potential, Electrophysiology Pages: 12 (3174 words) Published: April 13, 2015
Lecture 1

Cells of the Nervous System
Neuron – in the form of sensory, motor and interneurons -> transmit info using electrical signaling Glial Cells – metabolic support, protection and insulation for neurons Cell body – contains nucleus, other organelles – ex. mitochondria for ATP Dentrites – branches which incoming fibers make connections - receiving stations for excitation or inhibition

- covered with short dendritic spines that increase SA
- dendrites and their spines are constantly modified and can change rapidly in response to changes in synaptic transmission: #, size, shape, etc Axon – conducts electrical signals from cell body to terminal Transmission occurs from presynaptic to postsynaptic cell

Flow of information: synapse -> dendrite -> soma -> axon -> synapse Lecture 2 – Chapters 4 & 5
Ion channels and signalling
- ions are separated by cell membrane; when ion channels are opened, ions flow -> generating electrical signals -> so neurons can communicate info through these electrical signals - neurons generate a constant negative voltage across membrane: the rmp - an AP abolishes the –ve rmp, making membrane potential transiently positive - we can record and measure ion currents using electrophysiology - hyperpolarization is usually the movement of +ve ions moving outward, but it can also be the movement of –ve ions moving inward; while depolarization is the opposite How do these membrane potentials arise?

- electrical signals are generated through action of ion channels by opening and closing - the membrane bi-layer is impermeable to ions (polar/hydrophilic faces outward; nonpolar/hydrophobic faces inward) - ions cross membrane passively through ion channels (movement of ions driven passively by the difference of gradient across membrane); or actively through transport molecules (pumps/transporters) - both ion channels and transport molecules are membrane-spanning proteins (transmembrane) General features of an ion channel

- central water-filled pore
- selectivity filter: different ion channels are selective for different ion types - channel gate: closed or open to control ion movement
- size of channel varies
- mean open time varies from channel to channel
- probability of channel opening also varies
- when an ion channel is activated by stimulus, the prob. of channel opening is increased (vice versa) - Inactivation/desensitization: a conformation change has occurred in the channel, and it doesn’t work (won’t open) - inactivation for depolarization

- desensitization for chemical stimuli
* Activation/deactivation of a channel = increase/decrease in prob. of channel opening – not a change in the mean open time* Modes of Channel Activation
1. Voltage activated
- membrane potential; e.g. Na+ influx depolarizes membrane
2. Stretch activated
- mechanical distortion; e.g. mechanoreceptors (touch) in the skin 3. Ligand activated (by a chemical agonist)
- extracellular -> e.g. glu, ACh, etc
- intracellular -> e.g. Ca2+-activated K+ channel

- if activating stimulus present but conformational change prevents activity = inactivation (to depolarization/electrical stimuli) or desensitization (to chemical stimuli) - open channel block -> e.g. large molecule (toxin, Mg2+) binds to a channel and physically occludes the pore -> E.g. Pufferfish -> Bacteria within the fish generate a toxin called tetrodotoxin (TTX) which blocks Na+ channels, paralyzing its victim

In summary, open channel block:
-> channel is open but blocked;
-> e.g. TTX blocks Na+ channels (so no AP, no muscle movements); -> leads to death by respiratory paralysis
Electrophysiology: Intracellular Recording
- a recording electrode is inserted into a cell (neuron), so that the intracellular potential can be measured against the extracellular potential Patch Clamp Electrophysiology
- records the current passing through a single ion channel, and determines changes in response to voltage changes *review this...
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