Transmembrane Transport and Synaptic Transmission Flashcards
Two Pathways for Rapid Transmembrane Flux of Water
• Interlipid Pathway (10%) • Aquaporins (90%) (as easy as 4,3,2,1) – 4 monomers/aquaporin – 3 polypeptide chains/subunit – 2 subunits/monomer – 1 aquaporin
Importance of Aquaporins
• Maintenance of osmotic equilibrium in cells (AQP1)-Mutations in AQP1 gene lead to pulmonary and peripheral edema. • Important in kidney (AQP2, AQP3, AQP4) • AQP2 inserted into distal tubule in response to ADH
Osmosis through semi-permiable membrane
Osmosis in hydrostatic pressure column
Osmolarity vs. Osmolality
• Osmolarity: Concentration of osmotically active particles per liter of solution.
– Easier to measure-e.g. freezing point depression.
– Difference from osmolality is <1% for dilute solutions similar to those in the body
– Most widely used in physiological studies.
• Osmolality: Concentration of osmotic particles per kilogram of water.
– True determinant of osmotic pressure for dilute solutions (weight of solute not included).
– Clinical values expressed as osmolal concentration
Isosmotic vs. Isotonic
- Isosmotic = Same osmolality as extracellular fluid
- Isotonic = Sameosmolalityasextracellular fluid, but cell volume remains constant
Effect of NaCl Concentrations on RBC
Properties of Electrical and Chemical Synapses
Electrical and Chemical Synapses
NT Release
Ionotropic vs. Metabotropic
Synaptic Transmission at the skeletal neuromuscular junction (motor unit)
Action potentials (AP’s) are generated in the myelinated motor nerve axon and are propagated to the peripheral terminal of the axon. At the terminal, the axon is split into multiple terminal branches with each ending forming a “bouton” that synapses on a single muscle fiber. The branched axon together with its innervated muscle fibers is called a “motor unit”.
End Plate
Most important to recall from our basic electrophysiology lectures – each terminal branch of the motor neuron carries an AP to its synaptic terminal (so that all muscle fibers in a motor unit are activated simultaneously). The specialized structure (comprised of the pre- and postsynaptic components at the neuromuscular junction) is called the “end plate”.
Neuromuscular Transmission
Neuromuscular transmission is initiated by the depolarization at the pre- synaptic terminal during the presynaptic action potential. This depolarization opens voltage- sensitive Ca2+ channels in the presynaptic nerve terminal membrane and causes an
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influx of Ca2+ from the extracellular fluid in the synaptic cleft.
The increased [Ca2+]i is required for three functions: 1) the release of vesicles that are bound to reticular sites in the presynaptic cell cytoplasm; 2) the fusion of the synaptic vesicle membrane to the presynaptic terminal membrane; and 3) the opening of bound vesicles for release of neurotransmitter (ACh).
Active Zones
The vesicles are arranged in a double linear array in “active zones” with each vesicle ( ≈ 50 to 60 nm in diameter) and containing approx. (6 to 10) x 103 molecules of ACh). The active zones are directly above dense arrays of ACh receptors located on the crests of post-junctional folds in the postsynaptic muscle cell membrane.
Synaptic Cleft in neuromuscular synapses
The “synaptic cleft” between the pre- and postsynaptic membranes in the vertebrate skeletal neuromuscular junction is ≈ 50 nm wide and is filled with a meshwork of proteins and proteoglycans that are part of an extracellular matrix. The basal lamina portion of this matrix functions to maintain a stable junction between pre- and postsynaptic membranes.
Motor End Plate
Activation of the skeletal muscle nicotinic ACh receptor by ACh leads to generation of the end plate potential (EPP) which, in turn, leads to the muscle fiber AP. The excitatory EPP is a transient, local, non-propagated depolarization resulting from the greater influx of positive ionic current carried by the Na+ ion (compared to the efflux of positive ionic current K+) upon opening of the ACh channel in the muscle end plate.
Diversity of Synaptic Connections
Spacially Focused vs. Widely Divergent Synaptic
Spatially Focused Synaptic Network
Widely Divergent Synaptic Network
EPSPs and IPSPs
