excitable cells 1 Flashcards
(17 cards)
What Does The Nervous System Do
-System of communication that allows an organism to react rapidly and modifiably to changes in its environment
-Neurones must:
=Collect
=Integrate
=Output
-neurones have excitable membranes
-electrical signals can travel very long distances
How Does The Nervous System work
-Electrical activity provides a rapid, reliable, and (flexible) means for neurones to receive, integrate and transmit signals.
-Chemical messengers (and receptors) between and within cells provide much more flexibility e.g. for inhibition
what are the electrical properties of neurones
-Electrical signals may be divided into
=Action potentials: fixed size, all-or-nothing signals that travel along (propagate) the axon
=Graded potentials: variable size, local signals not propagated over long distances
=Action potentials can pass either way along an axon, but tend to go one way (with important exceptions)
=Graded potentials pass both ways along the neuronal membrane
-Information coding
=APs are coded by frequency as they are of a unit size
=Graded potentials are coded by size and vary according to the strength of the stimulus
how do you measure the potential across a neurone
-have to connect the neurone to a voltmeter (a device that measures the potential difference between two electrodes)
-To do this, we insert a glass microelectrode (filled with KCl, to carry charge) into the neurone and another electrode (usually made of silver chloride) into the solution surrounding the outside of the neurone…before the glass electrode enters the neurone, the voltmeter reads Zero
-There is no potential difference within the extracellular solution…but as soon as the electrode enters a ‘resting’ cell, this value changes to somewhere between -65 and -90 mV (symbol commonly used is Vm)
-There is an uneven distribution of charge across the neuronal membrane
is it a positive or negative resting potential needed for a functioning nervous system
-negative
why do neurones have a resting potential
-Inevitable consequence of:
=Selectively permeable membrane
=Unequal distribution of charged molecules / ions
=Physical forces
-membrane consists of 2 layers of lipids
-membrane isn’t permeable by itself, only permeable when the electrical charge is present
why are membranes selective and unequal
-Channels confer selectivity- protein pores float in the membrane, can be complex systems allowing some ions to pass and some not too
-Pumps assist unequal charge distribution
-When a neurones is NOT generating these types of impulses it is said to be at rest
how does Physical Forces Controlling Ion Movements in Solution work
-The lipid bilayer provides a barrier to diffusion, so that we can end up with different concentrations of ions on either side of the membrane- concentration gradient
-passive diffusion when ion channels open and ions move down a concentration gradient
-Electric fields also cause ions to move- opposite charges attract and like charges repel.
-Because Ions are Charged, movement of ions gives rise to an electric current
-In a cell, bilayer provides a barrier to ion movement, thus if no channels are open, conductance will be zero and I = 0. Therefore to drive ions across the membrane electrically requires the membrane to have channels and a potential difference
what are the Two forces control movement of ions in aqueous solutions
-Diffusion
-Electrical field
How much current will flow is dependent upon…
1) Electrical Potential (voltage = force exerted on a charged particle), represented by symbol V and measured in volts, and
2) electrical conductance, relative ability (how easy) is it for charge to move from one point to another, symbol g and measured in siemens (S)….Electrical Resistance is the relative inability of an electrical charge to migrate., represented as R and measured in Ohms (R=1/g)
3) Ohms Law describes the relationship between potential, conductance and the amount of current that will flow, I=gV….so no current flows if g or v=0
what are ion pumps
-Ion pumps in the membrane set up the ionic concentration gradients found in neurones
-Important ion pumps:
=Na+ / K+ ATPase
=Ca2+ pumps (not just in the plasmamembrane)
-Without ion pumps, the resting membrane potential would not exist and the brain would not function!
-But the effect of pump inhibitors takes some time to work
what are Equilibrium Potentials
-selectively permeable to one ion
- Ionic gradients influence membrane potential by determining Equilibrium Potentials Eion
- Eion is the membrane potential that would be achieved in a neurone if the membrane were selectively permeable to that ion
whats the Nernst equation
-Used to calculate the equilibrium potential (Eion) for an ion
- Eion = 2.303 RT/zF log [ion]o/[ion]I
- at 37 degree c : Ek= 61.54 mV log [K+]o/ [K+]I
-where R= universal gas constant
-T= temperature
z= charge
-o= outside
-I= inside
what are the Four important points that should be kept in mind regarding the establishment of an equilibrium potential
1) Large changes in membrane potential are caused by miniscule changes in ionic concentration
2)the net difference in electrical charge occurs at the inside and outside surfaces of the membrane, not in the ‘interior’ solution- this is because the bilayer is so thin (~5nm) ions on either side of it interact electrostatically
3)Ions are driven across the membrane at a rate proportional to the difference between the membrane potential and the equilibrium potential
4) If the concentration difference across the membrane is known for an ion, the equilibrium potential can be calculated using the Nernst Equation
what are the Equilibrium Potentials of Relevant Ions
-intracellular:
K+= 100
Na+= 15
Ca2+= 0.0002
Cl-=13
-extracellular:
K+= 5
Na+= 150
Ca2+= 2
Cl-= 150
-Eion:
K+= -80 mv
Na+= 62mv
Ca2+= 123 mv
Cl-= -65 mv
-At rest the neuronal membrane is very permeable to K+. (Slightly permeable to others)
-At rest the real membrane potential is close to but not at EK
why are potassium channels so important in membrane potential
-because membrane is highly permeable to K at rest, changes in K concentration can have big effects.
-This increasing extracellular potassium causes a shift in Ek
-if increase it, Ek becomes more positive- it is depolarised
whats the Goldman equation
-Neurons do not have resting Vm at Eion for K+
-The resting membrane is also permeable to other ions e.g. Na+
much less
-To estimate real Vm you need the Goldman
-Vm= 61.54 mV log (Pk[K+]o+PNa[Na+]o) / (Pk[K+]i+ PNa[Na+]i
