W4 - Physiology of Nerve Cells (Lecture) Flashcards
What is the definition of membrane potential?
The difference in potential inside and outside the cell
How would you generally measure the membrane potential of a cell?
Put the cell in a saline solution, have a recording electrode inside the cell and a reference electrode in the solution, the difference is the MP
What is the general resting membrane potential of a cell?
-70mV
What are the 2 steps of forming an equilibrium potential?
- K+ flows down concentration gradient, 2. this continues until the electrical forces repel the flow of the ions
At equilibrium, the conc of K+ and charges on either side of the membrane aren’t equal on either side of the membrane, what IS equal (due to diffusion and electrical gradients)?
The K+ moving from left to right will equal those moving from right to left
What causes the repulsive electrical forces when forming a MP?
The inability of ions other than potassium to move across the membrane
What did Bernstein’s Hypothesis state?
MP is due to unequal conc of K+ across a membrane and memb only permeable to K+
What does the Na+/K+ pump do at rest and how?
Establishes gradient by moving 3Na+ out and only 2K+ in and maintains this gradient when
What does it mean for a cell to be polarised?
At rest the inside and outside of a cell is different in terms of potential (inside = -ve and outside = +ve)
What happens to the potential inside the cell when a cell undergoes hyperpolarisation and depolarisation?
Hyper - inside more -ve than resting, Dep - inside less -ve than resting
What type of signal is initially perceived by neurons then what does this turn into?
Electrical to chemical
Glial cells are metabolically and structurally supportive, name the four types
Astrocytes, ependymal, microglia and oligodendrocytes
Where are action potentials typically initiated?
Axon hillock
If concentrations of solutions on either side of a semi-perm membrane are equal, what does it mean about flow from left to right and vice/versa?
They will be equal
If concentrations of solutions on either side of a semi-perm membrane aren’t equal, how would the ions flow?
Along a diffusion gradient from high to low
What is the equilibrium potential determined by?
The difference in K+ across a membrane
What needs to be equal for an equilibrium potential to have been reached? (what determines behaviour of ions)
Diffusion and electrical forces
Why is the axoplasm of the giant squid axon so easily manually extracted?
It’s 100x larger than any other neuron
What 2 things are compared when testing Bernstein’s hypothesis?
MEasured and theoretical potential (which is based on how much K+ is found in the axoplasm of the giant squid axon)
The predicted potential using the giant squid axon (and only K+) was -93mV but something was making it more +ve than predicted, what was thee measured value?
-65mV
What type of equilibrium potentials would the involvement of Cl- or Na+ create? (more/less -ve or +ve), and what type of reasoning was used here?
Cl- more -ve (-55mV) and Na+ more +ve (+55mV), deductive reasoning
What do you add together to prove there’s not equal perm to K+ and Na+?
Do nernst equation for both and add them together, the answer doesn’t sum to -65mV
There is an unequal permeability to K+, Na+ and Cl-, what equation is used to measure the MP?
Goldman, Hodgkin and Katz
The pK value is 1.0, pNa is 0.04 and pCl is 0.45, what does this mean about how freely Na and Cl can move across the membrane?
Na moves only 4% as freely as K and Cl moves 45% as freely as K across the membrane
If the saline solution has a conc of 10X the required K+ conc, what happens to the resting membrane potential? (reference + recording electrode exp)
It becomes less -ve as the diffusion gradient will flatten
If the saline solution has a conc of 1000X the required K+ conc, what happens to the resting membrane potential? (reference + recording electrode exp)
It becomes positive as diffusion gradient and electrical gradient inverts
Overton identified that Na+ on the extracellular side of the membrane are important for APs, what did he suggest happens and what experiment did he do to prove an AP cannot occur without Na+?
Na+ enter the cell and without Na+, put cell in solution without any Na+ at all and no matter how stimulated the cell was there was no AP
How did Hodgkin and Huxley measure an AP?
Recording electrode in giant squid axon with reference in extracellular medium, axon is stimulated and AP can be measured
What does it mean for AP to be an all or nothing event?
Threshold value needs to be reached, depolarisation can happen with AP if it’s not reached
What are the 3 phases of the AP?
Rising (depolarisation), Falling (repolarisation), After-hyperpolarisation
Describe the process of changing pNa and pK during an AP
- pNa increases (depo) 2. pNa decreases as pK+ increases 3. perms return to resting state
What type of ion channels do Na+ and K+ move through during APs?
Voltage-gated ion channels
How would you show there is a traveling wave of APs?
Multiple electrodes along axon
What is the refractory period and what does this ensure and what does it prevent?
The period where no new action potentials can be initiated, ensures unidirectional APs, prevents bounceback from axon terminals
What happens to the voltage gated Na+ channels during the refractory period?
They are transiently inactivated by a protein ‘plug’ and then the activation gate takes over
The AP propagates to ensure unidirectional flow, how does this generally work?
Current flows along activated patch to depolarise adjacent patch, historic patch repolarises and undergoes the refractory period and the adjacent patch reaches the threshold, cycle repeats along axon
What are 3 things you can look at on the AP graphs/
Amplitude, frequency amd velocity
Give a short description for amplification, frequency and velocity for neurons (+APs)
Amp and velocity - same every time in a neuron, can differ between neurons, freq - variable but limited by refractory period
u = k x (sq rt of d) is used to measure the velocity of an AP, what increase do you need in d (axon diameter) to increase u by x10?
x100
Other than increasing axon diameter, how else do you increase velocity of AP?
Myelination for saltatory conduction
