Neurons as Electrical Elements Flashcards
What is neuro-electricity due to?
ionic currents that are generated by the diffusion of 4 essential ions (Na+, K+, Ca2+, Cl-)
What is the difference between a passive and an active electrical signal?
- active (regenerative) response results from an action potential
- a passive response may be a hyperpolarization or depolarization but does not result in an action potential
Where does the non-zero voltage difference (membrane potential) come from?
uneven distribution of charged molecules inside and outside of selectively permeable cell surface membrane
When is the voltage at 0?
when depolarizing or dead
How are changes in voltage across the membrane produced?
changing the selective permeability of the membrane for different ions
What is the role of pumps/transporters in setting membrane potentials?
- responsible for ionic movments across neuronal membranes
- active transporters: move ions against concentration gradient
- ion channels: allow ions to diffuse down electrochemical gradient
When is there an actual energy source?
when we have a concentration gradient and if ion is out of its electrochemical “equilibrium condition”
What is the electrochemical “equilibrium condition” determined by?
concentration, charge, and voltage differences
What is needed to “harvest” the energy?
permeability is needed to generate “voltage”
At equilibrium chemical and electrical forces are:
equal and opposite and net diffusion is zero
What is the “equilibrium potential”?
- the particular voltage where the forces balance
- Ex = equilibrium potential of ion x
- also known as reversal potential
How do we calculate the equilibrium potential?
- calculated as “Nernst” potential
- outside over inside
- gives us V of inside relative to outside
- Veq or Eion
What does 2.303 RT/F equal at 18, 37, and 10 degrees celcius?
- 0.058 V (58 mV) @ 18
- 0.062 V @ 37
- 0.056 V @ 10
What is z?
ion’s valence (+1, +2, -1, -2)
What is the voltage change not accompanied by a measurable change in concentration?
only a small fraction of the total ions in the compartments have to re-distribute to create the gradient
What is the nernst equation?
Veq = +(2.303RT/zF) * log ([ion1]o/ +[ion1]i)
Why is there a need for the Goldman-Hodgkin Katz (GHK) equation?
because real neurons have more than just one permeant ion species
What is the GHK equation?
Vm = +(2.303RT/zF) * log((Pion1[ion1]o + Pion2[ion2]o) / (Pion1[ion1]i + Pion2[ion2]i)
What is P?
P refers to the permeability of the ion species relative to each other
When does the membrane potential change?
1) permeability changes for one or more ionic species (electrical and fast)
2) concentration of an ionic species changes
3) activity of electrogenic pump changes
What happens if the conductance (permeability) of the membrane for an ion(s) is increased?
- the membrane potential will change so that it is closer to the equilibrium (Nernst) potential for that ion(s)
- current will flow to move the membrane potential closer to Eion
What is the equation for ionic current?
Iionx = gionx * (Eionx - Vm)
- driving force: Ex - Vm
- gx: total conductance (# of open channels)
What is the driving force?
difference between the voltage at the moment and the equilibrium potential
What is another term for permeability?
conductance
If the conductance of a membrane for an ion(s) is increased, what happens to the membrane potential?
the membrane potential will change so that it is closer to (but never beyond) the (Nernst) potential for that ion
When the permeability of the membrane for an ion is increased, how will the current flow?
the current will flow so as to move the membrane potential nearer to Eion
If the conductance is decreased, what happens to the membrane potential?
the membrane potential will be less influenced by the ion whose permeability is reduced (move towards combined Nernst potentials of other ions)
What is Vm?
membrane potential at time 0
What is Eionx?
equilibrium potential for ion x
What occurs if there are “voltage” gated channels?
depolarization from pressure gated channels initiate further changes in permeability and thus voltage
What are the problems with fig 2.6?
1) peak should not reach ENa
2) mostly due to time dependent properties of channels
3) where the permeability of Na decreases, the permeability of K increases
What do the currents depend on?
depend on the driving force
What does the driving force change with?
Vm
What is the voltage-clamp technique used for?
to measure the current generated by the opening and closing of channels (even though voltage and current change at the same time)
What does the current flow for each ion depend on?
1) force driving the ions
2) conductance
What is the force driving the ions?
the difference between the voltage at the moment and the equilibrium potential
What is the conductance for an ion?
number of open channels*single channel conductance
When looking at an individual channel is there 100% probability it will open?
No, not identical action occurs every time
What are the possible actions of individual Na channels?
quick to open, quick to close even when membrane is still depolarized (won’t reopen - inactivation)
What are the possible actions of individual K channels?
slow to open, stay open until V is returned to negative potential
What accounts for the differences in the actions of Na channels vs K channels?
Na channels have an inactivation gate
An action potential is the result of:
fast activating inward Na conductance that turns off even though the membrane is depolarized and a slower activating K conductance that persists
What are the passive properties of membranes important for?
1) action potential conduction
2) determine how synaptic inputs summate and are integrated
What kind of conduction signals do passive membrane properties affect?
both active and passive conduction of signals
What two aspects of voltage spread result from the physical properties of neurons?
1) space dependent property
2) time dependent property
What is Rm and Cm and tau?
resistance and capacitance and membrane time constant
What do the values of Rm and Cm depend upon?
depend on size of the neuron (larger cells have lower resistance and larger capacitances, and brief time constants)
How does tau rely on the resistance and capacitance?
tau = Rm*Cm
What is saltatory action potential conduction?
propagation of action potentials along myelinated axons from one node to the next
How does myelinating the axon affect the resistance and therefore the time constant between nodes?
between the nodes there is very high resistance, so time constant is long and slow charging
How does myelinating the axon affect the capacitance in the nodes?
C is small, so charges up fast
What diameter of myelinated axon is equivalent to the conduction velocity of a squid axon (500 micrometer diameter)?
5-6 micrometer
What is MS?
multiple sclerosis: demyelination, lesions
What kind of sensory problems can a person with MS suffer from?
loss of sensitivity, prickling, numbness
What kind of motor difficulties can a person with MS experience?
muscle weakness, clonus, muscle spasms, difficulty moving difficulty with coordination, problems with speech or swallowing
What other symptoms can a person with MS experience?
visual, fatigue, acute or chronic pain, bladder and bowel difficulties, cognitive impairment, unstable mood
How does MS affect the channels and axons?
loss of myelin leads to Na+ channel redistribution and breakdown of axons
Ultimately, what does MS result in?
decreased conduction velocity and decreased ability to sustain firing
What drug has potential for treating MS and what is it currently used for?
beztropine, currently used for Parkinson’s
What effects does benztropine have that treat MS?
induces the differentiation of oligodendrocyte precursor cells (OPCs) and enhanced remyelination
