Molecular Basis of Excitation Flashcards
Typical values of resting membrane potential are ___ for nerve cells and ___ for muscle cells
-70 mV; -90 mV
Normally the charge separation occurs in a region less than __ um wide on either side of the membrane
1
note: bulk of cytoplasm and ECF is electrically neutral
In this type of conduction, localized depolarization that spreads to neighboring membrane regions, but neighboring regions do not depolarize quite as much as the site of the NT deposition
decremental conduction
True or false? Hyper polarization is conducted decremental across dendrites and cell body
true
In this type of conduction, the membrane in one part of the cell reaches threshold, and the action potential is passed on to all parts of the cell. The magnitude of the AP is the same everywhere as it spreads down the nerve
non-decremental conduction
Chemical concentration differences across a membrane result in diffusive ___ energy driving forces
potential
Free energy is released by ions flowing down their concentration gradient. True or false?
true
note: free energy = RT ln [K+]out/[K+]in
Free energy is released when cations move toward the ___ side of a membrane or anions move toward the ___ side of the membrane
hint: electrical forces at play (think attraction)
negative; positive
The ___ force acting on a charged molecule is the sum of the electrical and diffusive forces
electrochemical
In regards to membrane selectivity, when a membrane is selectively permeable to ionic species of one charge, an ___ potential develops across that membrane. As this permeable cation moves down its concentration gradient, anions are left behind, resulting in charge ____.
electrical; separation
note: the gradually increasing charge separation limits makes it harder and harder for additional cations to leave
Net movement of ions ceases when the electrical and chemical forces are ___ in magnitude but ___ in direction
equal; opposite
The Nernst potential (Eion) measures what?
the membrane potential a given ion reaches electrochemical equilibrium (stops moving)
By convention, ionic current is ___ when a cation moves toward the negative side of membrane
positive
The electrical characteristic that makes ion movement difficult is called _____.
resistance
note: cell membranes have resistance in that they restrict the free flow of the ions from one side of the cell to the other
The driving force for ion movement across a membrane is what?
the difference between the existing membrane potential and the Nernst potential for the ion under consideration
What is the reciprocal of resistance?
conductance
A membrane with a high conductance for a given ion lets the ion move ___ for any given difference between membrane potential and the ion’s Nernst potential.
easily
note: high conductance means free ion movement through core of a protein channel
Based on Ohm’s Law, the ionic flux can be expressed as what?
I = g (Vm - Eion) = (Vm - Eion) / Rion
I = outward potassium current R = membrane potassium resistance g = membrane potassium conductance Vm-Eion = driving force, difference in membrane and potassium equilibrium (Nernst) potentials
Unique membrane conductance, gion, which is controlled by “____” that selectively let that type of ion across the membrane
channels
True or false? the relative permeability (conductance) of the various ion channels in a cell membrane determine the membrane potential.
true
note: the ion with the greatest conductance pulls the membrane potential to a value near its Nernst potential
Membranes are more permeable to ___ than to other ions.
potassium
Cells are slightly permeable to ___ so Vm is not quite as negative as it would be otherwise
Na+
Vm is generally closer to____ than ___ because cells are much more permeable to K+ than Na+
Ek; Ena
Give the Goldman-Hodgkin-Katz Equation which describes the relative permeability to Na+, K+, and Cl- and determine Vm
Vm = 61 log Pk [K]out + Pna [Na]out + Pcl [Cl]in
————————————————-
Pk [K]in + Pna [Na]in + Pcl [Cl]out
A decrease in the Pna/Pk ratio
An increase in [K+]in - [K+]out
When does this occur? What is the clinical correlation?
depolarizaiton; clinical correlation: hyperkalemia
The Na+/K+ pump, pumps __ Na+ out for every ___ K+ pumped in
3; 2
note: pump is electronegative
Ligand operated channels tend to be smaller and more ion specific than voltage-gated channels. True or false?
false; larger, less ion-specific
____-gated channels are responsible for the sequential movement of ions across the membrane during the generation of APs
voltage
Voltage-gated channels open in an “all or none” fashion. True or false?
true
Ion selectivity is based on what three things?
size, charge, energy of hydration
Voltage-gated Na+ channels have a ___ activation gate (m-gate) and a ___ inactivation gate (h-gate)
fast; slow
The Na+ fast activation gate opens rapidly in response to ___ and closes rapidly upon ____
depolarization; repolarization
The Na+ slow inactivation gate closes slowly upon ____ and opens slowly upon ____. It is open in the ___ state but closed in the ___ state.
depolarization; repolarization; resting; inactive
For the following phases of the sodium channel cycle, tell if fast gate and slow gate is open or closed:
resting state
fast gate: closed
slow gate: open
note: Pna or gna is low
For the following phases of the sodium channel cycle, tell if fast gate and slow gate is open or closed:
rising phase
fast gate: open
slow gate: open
note: Pna or gna is high
For the following phases of the sodium channel cycle, tell if fast gate and slow gate is open or closed:
falling phase
fast gate: open
slow gate: closed
note: Pna or gna is low
What is the rate limiting step in return to the resting conformation?
the rate at which the h gates open (recover from inactivation) - refractory period
This is the time when all h gates are closed. The cell in not excitable by any stimulus
absolute refractory period
This is the time when some but not all h gates have reopened. The cell is excitable with a supranormal stimulus
relative refractory period
Voltage-gated K+ channels open slowly in response to membrane ____
depolarization (Pk or gk increases)
Voltage-gated K+ channels close slowly in response to membrane ____
repolarization (Pk or gk decreases)
Voltage-gated K+ channels may remain open for a short time after completion of action potential, accounting for the after-_____ that is sometimes seen
hyperpolarization
The falling phase of the AP is due to what?
delayed inactivation of Na+ channels and the depolarization-induced delayed rise in K+ conductance
Does the amplitude of the AP change as it traverses the length of the axon
no
A faster neuron would have a ___ diameter and have ___ fibers
large; myelinated
In regards to motor neurons, these are large, myelinated fibers that innervate extrafusal skeletal muscle
Type A-alpha
In regards to motorn neurons, these are smaller, myelinated fibers that innervate intrafusal muscle. These have slower conduction velocities and are involved in reflex tone adjustments
Type A-gamma
These fibers are small and unmyelinated. Usually refer to sensory afferent fibers
Type C
In regards to somatosensory neurons, Type ___ afferents are large myelinated nerves carrying information like tactile sensation to the brain
I
In regards to somatosensory neurons, Types ___ and ___ sensory afferents are unmyelinated and cary information about sensations like proprioception, pain, and temperature
III; IV
