Neural Signaling: Membrane and Action Potentials Flashcards
Charge separation (unequal conc. of ions across a membrane) give rise to membrane potential. The resting potential in cells are normally more ___________ inside than outside (varying from -9mV to -100mV). This is the opposite of osmolarity.
negative.
NOTE:
⚚ Osmole: To express the concentration of a solution in terms of numbers of particles, a unit called the osmole is used in place of grams.
One osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose because glucose does not dissociate into ions.
⚚ Osmolarity: Osmolarity is the osmolar concentration expressed as osmoles per liter of solution rather than osmoles per kilogram of water. Although, strictly speaking, it is osmoles per kilogram of water (osmolality) that determines osmotic pressure, the quantitative differences between osmolarity and osmolality are less than 1% for dilute solutions such as those in the body. Because it is far more practical to measure osmolarity than osmolality, measuring osmolarity is the usual practice in physiological studies.
True or False?
(a) Excitable tissues of nerves and muscles cells have lower resting membrane potentials than other cells (epithelial cells and connective tissue cells).
(b) The body as a whole is electrically neutral.
(a) True [excitable tissues, such as nerves and muscle cells typically have lower (more negative) resting membrane potentials compared to non-excitable cells like epithelial cells and connective tissue cells. This lower potential is crucial for their ability to rapidly respond to stimuli and generate action potentials, which is essential for their function in the body.]
(b) True.
A cell is said to be _________ when the intercellular fluid is relatively more negative than the extracellular.
polarized
Membrane potentials are due to the diffusion of ions down their concentration gradients, the electric charge of the ion, and any membrane pumps for that ion.
a) ____________ is the net movement of ions into the cell from the extracellular fluid.
b) ____________ (the movement of charges) is always measured in millivolts (mV).
a) Influx
b) Flux
[NB: 1V = 1000mV]
Lipid membrane has _______ electrical resistance since it has a few charged groups that can not carry current.
high
[But extracellular fluid and intracellular fluid both have low electrical resistance.]
In resting membrane potential, by convention, extracellular fluid is assigned a voltage of ____.
zero
Further notes on resting membrane potential:
⚚ In all cells a potential difference across the membrane exists
a. Inside is negative (Na+K+ATPase)
b. Membrane potentials usually within -40 to -90 mv
⚚ A cell with a resting membrane potential is said to be polarized.
⚚ Both the inside and the outside of the cell are electrically neutral.
_______ channels account for 95% of the resting membrane potential (RMP).
Leak
NOTE:
⚚ Leak channels are always open i.e. they have no gating mechanism.
⚚ The cell membrane is 75% more permeable to K+ than Na+.
The ________________ ATPase pump accounts for 5% of resting membrane potential.
Na+/K+
[this maintains the RMP. How, you ask? For every ATP molecule that this electrogenic pump uses, 3 Na+ are pumped out of the cell while 2 K+ are pumped into the cell.]
Why is the inside of the cell negative and why it is associated to K+?
- The cell membrane is more permeable to potassium ion movement than sodium ion movement, hence K+ easily leaves the cell leaving behind negative charges.
- The Na+/K+ ATPase. 3 Na+ out, 2 K+ in. So, outside will be relatively more positive than inside.
State the factors that determine the resting membrane potential (RMP).
- Selective permeability of the plasma membrane
- Leak channels (accounts for around 95% of RMP)
- Na+K+ATPase pump (5%)
- Differences in ion concentrations
The value of the equilibrium potential (Nernst potential) for any ion depends on the _________________________ across the membrane for that ion.
concentration gradient
[Video]
Given the ion concentration gradient the Nernst potential for any ion can be calculated. The Nernst equation is used to determine the electrochemical potential for any ion across the biological membrane. Write down the Nernst equation.
E(x) = -61 mV (log ([x]inside/[x]outside))
The membrane potential of a particular cell is at the K+ equilibrium. The intracellular concentration for K+ is at 150 mmol/L and the extracellular concentration for K+ is at 5.5 mmol/L. What is the Nernst potential?
E(x) = - 61 mV (log ([x]inside/[x]outside)) where x = K+
E = - 61 (log(150/5.5))
E = - 61 (1.436)
E = - 87.596
Remember:
The Nernst potential is the value for the voltage that must exist across the membrane in order to balance a chemical gradient that exists for the ion in question (in short, it’s the voltage required to maintain a chemical gradient for the ion).
The net movement of all ionic currents across the membrane determines the resting membrane potential. The net current flow (I) across the membrane is given by?
I(x) = g(x) {Em-E(x)}
where:
x - ion
g(x) - ion conductance
Em - resting membrane potential
E(x) - Nernst’s potential
To understand further:
At rest the membrane potential is not changing, then the sum of all currents must equal zero.
Thus
I (Na+) + I (K+) + I (Cl-) + … = 0
I(x) = g(x) {Em-E(x)}
where:
x: ion
g(x): ion conductance
Em: resting membrane potential (RMP)
E(x): Nernst’s potential
Solving for Em yields the ________ equation which gives the resting membrane potential (RMP).
Goldman
Note:
The resting membrane potential is a summation of all of the ion potentials times their percentage of the total ion conductance.
What is the difference between Nernst potential and resting membrane potential?
Nernst Potential: This is the membrane potential at which a particular ion would be in equilibrium, meaning there is no net movement of that ion across the membrane. It is calculated based on the concentration gradient of that specific ion across the membrane.
Resting Membrane Potential: This is the steady electrochemical state of the cell when it is not being stimulated or conducting impulses. It is determined by the relative permeabilities and concentration gradients of different ions (mainly sodium, potassium, and chloride) across the cell membrane.
[Video]
In essence, the Nernst potential applies to a single type of ion, while the resting membrane potential is a balance of the potentials of several different ions.
a) What helps to balance Na+ in the ECF?
b) What helps to balance K+ in the ICF?
a) Cl-
b) Proteins (-ve charged)
Electrochemical impulses are transient and rapid changes in Em. There are two forms of electrochemical impulses i.e.
graded potentials and action potentials
What is a graded potential?
A graded potential is a variable-strength, localized change in the membrane potential of a neuron in response to a stimulus, which can either excite or inhibit an action potential. Graded potentials can include diverse potentials such as receptor potentials, electrotonic potentials, subthreshold membrane potential oscillations, slow-wave potential, pacemaker potentials, and synaptic potentials.
Graded potential may be depolarising or hyperpolarising. What is depolarisation and hyperpolarisation?
Depolarisation: less polarisation = reduced magnitude of membrane potential e.g. from -70 mV to -50 mV.
Hyperpolarization: more polarization = increased magnitude of membrane potential (e.g. from -70 mV to -80 mV)
[Diagram]
Graded potentials can be excitatory or inhibitory. Explain.
Excitatory postsynaptic potentials (EPSPs) make the membrane potential less negative or more positive, thus making the postsynaptic cell more likely to have an action potential. [depolarization]
Inhibitory postsynaptic potentials (IPSPs) make the membrane potential more negative, and make the postsynaptic cell less likely to have an action potential. [hyperpolarization]
State five characteristics of graded potentials.
- They are local; changes in membrane potential are confined to relatively small regions of the plasma membrane.
- They are graded; this refers to the magnitude of the potential change and that the signal can be reinforced.
- Intensity of stimulus is directly proportional to magnitude of a graded potential.
- Graded events can be hypopolarizing (depolarizing - decrease in potential difference) or hyperpolarizing.
- They show spatial and temporal decay. Graded potentials are conducted with decrement. (conduction magnitude falls off the further you get from the point of origin)
State and briefly discuss four types of graded potentials.
(a) Receptor (generator) potentials: sensory receptors respond to stimuli from mechanoreceptors, thermoreceptors, nociceptors, and electromagnetic receptors; if the graded potential reaches the threshold an action potential is generated and sensory information is sent to the spinal cord and brain
(b) Pacemaker potential: specialized coronary cells known as pacemaker cells in the cardiac pacemaker region have leaky ion channels. This enables a slow positive increase in voltage across the cell’s membrane that occurs between the end of one action potential and the beginning of the next. The increase in membrane potential is what drives the self-generated rhythmic firing of pacemaker cells.
(c) Postsynaptic membrane potentials: these are graded potentials that develop on the postsynaptic membrane during synaptic transmission; they may be inhibitory or excitatory; if graded potentials reach threshold, an action potential develops
(d) End plate potentials: these are post-synaptic graded potentials that develop at the neuromuscular junction (they are always stimulatory and always reach threshold if generated by an action potential in the innervating alpha motor neuron).
How is an action potential generated?
An action potential is generated when graded potentials attain the set threshold.
Further notes:
Threshold value - the minimum voltage change required to open a voltage-gated channel.
