Cell Membranes and Potentials Flashcards
Classic Properties of the Action Potential
• All or none, No decay in amplitude with propagation, or branching. • Propagated via spread of ionic current • Mediated via changes in ion conductance in the nerve cell membrane – Na+ ions and K+ ions – Different channels, different time courses, different characteristics of activation and inactivation • Ultimately dependent on cell metabolism(to maintain ion gradients)
“Classic” Action Potential
Action Potentials in Myelinated and Unmyelinated cells
In a myelinated axon, multiple layers of myelin sheath serve as an insulator preventing currents across the most of the cell membrane. As a result, depolarization only occurs at the unmyelinated areas of the axon—the nodes of Ranvier, which greatly increases conduction velocity in those nerves.
Action Potentials in Relation to Na and K pumps
Effect of AP with Toxins (TEA and TTX)
Difference in potential peaks of Na and K
the kinetics of the transient rise and fall of ion conductance is much faster for gNa when compared to that for gK. Thus the peak gNa occurs much earlier than that of gK.
Membrane Spanning Domains of Ion Channels
Figure 7 illustrates three of the four membrane-spanning domains (I - IV) in the Na+ and Ca2+ channels. Each domain is composed of six membrane-spanning amino acid sequences. The four domains are arranged in a circular geometry to form a central pore. The “protruding loop” in each quadrant represents the “P” region of the polypeptide chain that dips into the pore from the extracellular side - to form the ion selectivity filter. Each of the four S4 sequences is thought to be involved in the voltage controlled “gating” mechanism that “activates” (i.e. opens) the channel.
Gating charges in S4 sequence of membrane channel
Gating Mechanisms in the FAST Na Channel
Membrane Capacitance
– Storage of charge across cell membrane
– Very few ions involved
– Must be discharged prior to initiation of an action potential
Ca2+ Ions and Cell Excitability
– Ca2+ ions bound near membrane in extracellular space.
– Affect the strength of the electrical field across the cell membrane but not membrane potential
– Hypocalcemia increases membrane excitability by reducing electrical field strength; Hypercalcemia reduces membrane excitability by increasing electrical field strength, making it more difficult to discharge membrane capacitance.
Types of Exchange across membrane
Flux in diffusion versus carrier mediated transportation
Properties of Facilitated Diffusion
Three types of Glucose Transporter Proteins
Glut 1 - Constituitive
Glut 2- Mobilized in hepatocytes
Glut 4- Mobilized in erythrocytes, myocytes, and adipocytes
Mobilization of Glucose Transporters
Properties of Primary Active Transport
Basic Model of NaKATPase Pump
Subunits in NaKATPase
– Alpha-
• ATPase
• Pore function
– Na+ extrusion (3), K+ uptake (2)
– Beta
• Proper assembly of alpha subunit in membrane
Properties of Secondary Active Transport
Basic Model of Co-Transport
Basic Model of Anti-Transport
