Physiology Flashcards
Define simple diffusion and give an example.
the electrochemical gradient between compartments drives solute through the dividing membrane; solute moves from high to low concentrations
- happens via membrane channels or by passing directly through the matrix of the membrane
Define bulk flow and give an example.
the net difference in pressure on either side of a semi-permeable membrane drives the solvent through the membrane (even against a concentration gradient)
Define simple facilitated diffusion and given an example.
the electrochemical gradient drives solute from higher concentration to lower concentration (just like simple diffusion), BUT is dependent on a membrane protein carrier to get solute across
- characteristics not present in simple diffusion: specificity, competition, saturability
Define primary active transport and give an example.
a membrane carrier transports solute from high concentration to low concentration (against its concentration gradient)
- requires energy: directly from splitting ATP or other energy source
- e.g., Na+/K+ ATPase pump
Define secondary active transport and give an example.
- AKA Coupled or Facilitated Diffusion
- two solutes use the same carrier
- one is moving against its electrochemical gradient using the energy released by the other solute moving with its electrochemical gradient
Which types of cells are polarized? (4)
- Neurons
- skeletal muscle
- cardiac muscle
- smooth muscle
Polarized, meaning they have an electrical potential across their membrane
What are normal resting potentials for skeletal/cardiac and smooth muscle?
skeletal/cardiac: -90 mV
smooth: -30 mV
At resting potential, what are the relative concentrations of K+, Na+, Cl- and [A-] across the membrane?
K+: higher inside the cell
Na+: higher outside the cell
Cl-: higher outside the cell
[A-], meaning large negatively charged things inside cells: stuck inside cells
What are examples of the large, negatively charged substances stuck inside cells, which contribute to a membrane potential? (4)
- aspartate
- acetate
- pyruvate
- isethionate
What force drives ions across the membrane?
the difference between its concentration gradient and the cell’s elecrical gradient
What is the function of the Na+/K+ ATPase pump?
It maintains the concentration gradients (and the membrane potential) for the cell by pumping Na+ out and K+ in, both against their concentration gradient (consuming ATP). It pumps 2 K+ in for every 3 Na+ it pumps out.
Which way do ions move across the membrane?
an ion will move across the membrane whichever way brings the membrane potential toward the equilibrium potential for that ion.
How does a membrane potential change physiologically
- primarily by changing membrane permeabilities (basis for action, endplate and synaptic potentials)
- in a few cases by changes in ionic concentrations brought about by intense, prolonged stimulation of small cells
Increasing the conductance/permeability of a membrane for a particular ion always has what result?
ALWAYS moves the membrane potential toward the equilibrium potential for that ion.
For which ion is the membrane so permeable that it is always safe to assume that it is at equilibrium?
Chloride (Cl-)
Define depolarization, in general.
Decreasing membrane potential; making the inside of the cell less negative.
Define hyperpolarization, in general.
Increasing membrane potential; making the inside of the cell more negative.
In general, what is the cell’s resting membrane permeability for potassium, sodium, and chloride?
K+: high
Na+: low
Cl-: VERY high
What is the correct term to describe a cell at a resting potential of about -90 mV?
Steady state, not equilibrium; chloride is at equilibrium, but sodium or potassium
Which types of cells produce action potential? (4)
- Cardiac muscle
- skeletal muscle
- most neurons
- some smooth muscle
What is the driving force?
The difference between the membrane potential and the equilibrium potential for a particular ion?
What are the steps in producing an action potential? (5)
- The cell is at resting potential (-90 mV) and the cell receives a signal
- Sodium channel opens: sodium rushes in (down its concen gradient); cell depolarizes (gets much more positive inside)
- Potassium channel opens (triggered at same time but opens more slowly): potassium rushes out (down its concentration and electrochemical gradient); stops depolarization (peaked at +20 mV) and slowly re-polarizes the cell
- Hyperpolarization: Sodium channel resets outer and inner gates, then potassium channel resets;
- Cell re-establishes resting potential and is ready to produce another action potential
Describe the role of the inner and outer gates on the voltage-gated sodium channel throughout an action potential.
- At resting potential, the outer gate is closed & inner gate is open.
- When the voltage reaches threshold, the outer gate opens (lets sodium in)
- At the peak of depolarization, the inner gate closes (cuts off sodium passage)
- Both gates reset during re-polarization and hyperpolarization
What is the role of the sodium potassium ATPase pump during an action potential?
- Virtually nothing during any one action potential. It does NOT repolarize the cell.
- With each action potential, a cell gains a little Na+ and loses a little K+, which would kill the cell over time
- the pump keeps the ion concentrations normalized (and consumes energy)
What determines how soon the new action potential is created? (4)
How long it takes to get the voltage-gated Na+ channels upstream of the action potential to threshold, determined by:
- how much charge has to be neutralized by the inward Na+ currents
- how much inward Na+ currents the action potential has to do it
- how much current leaks out of the axon before reaching threshold
- how much the axon impedes axial current flow
Compare current speed and efficiently in myelinated vs. non-myelinated axons.
Non-myelinated: current leaks, meaning action potentials have to happen more frequently to propagate the signal (which is slow), and the peak of depolarization decreases over time; conduction velocity proportional to square root of axon radius
Myelinated: the membrane is insulated, maintaining the current; action potentials only happen at the Nodes of Ranvier; conduction velocity is proportional to axon radius
Saltory Conduction
AKA conduction in a myelinated axon
- faster and more efficient than non-myelinated conduction
- signal seems to “jump” from node to node
Describe the location of the sodium and potassium channels in myelinated axons, and what results when myelin is lost.
- Na+ channels are concentrated at nodes of Ranvier
- K+ channels are concentrated under the myelin, which prevents them from leaking K+ and repolarizing the cell; non-gated K+ at the nodes take over this function
- a loss of myelin results in slower, weaker signals that present as uncoordinated movement or, eventually, muscle weakness (if the signal doesn’t reach the muscle at all)
- commonly seen in multiple sclerosis
