Lecture 3.1 Flashcards
How are membrane potentials expressed?
Inside relative to the outside and in mV
How is the MP measured?
Microelectrodes with tips of <1micrometer, that can penetrate cell membranes and are filled with KCl conducting solution - one on the inside of the cell and one on the outside, and a Voltmeter
Are animal MPs negative or positive?
What are the ranges for each type of cells?
Negative from -20mV to -90mV
Skeletal muscle -80mV to -90mV
Smooth muscle -40mV to -50mV
Neurones -50mV to -75mV
How are membrane potentials established?
Due to the selective permeability of the membrane to different ions. This is achieved through ion channels.
State 3 characteristics of an ion channel
- Selectivity - channel is permeable only to one or very few ions
- Gating - conformational change in protein molecule leads to opening and closing of the channel
- Fast rate of ion flow down electrochemical gradient for that ion
What is the hydrophobic interior of a bilayer permeable to?
Small uncharged molecules; O2, CO2, H2O, ethanol
What are the ionic concentrations of sodium?
Intracellular: 10mM
Extracellular: 145mM
What are the ionic concentrations of potassium?
Intracellular: 160mM
Extracellular: 4.5mM
What are the ionic concentrations for chloride?
Intracellular: 3mM
Extracellular: 114mM
What are the ionic concentrations of anions? What are they?
Intracellular: 167mM
Extracellular: 40mM
Negatively charged ions; phosphate bicarbonate, AAs
The action of which ionic channels dominate at rest?
Potassium
How is the membrane potential established at rest?
- Potassium channels in the membrane is open and the membrane is selectively permeable to potassium ions
- Potassium ions diffuse out of the cell down the concentration gradient
- Anions can’t follow so cell becomes negatively charged
- The membrane potential opposes the outward movement of potassium and the system will come into equilibrium
How is Ek reached?
When diffusion of potassium ions out of cells down the chemical gradient is opposite and equal to the flow of potassium ions into the cell down the electrical gradient (because as potassium ions flow out down chem gradient, cell becomes more negative and so positive potassium ions are attracted back into cell), and there is no net movement of potassium ions.
What is the Nernst equation and what does it tell us?
At 37 degrees, 61/valency of ion x log (outside concentration of ion/inside concentration of ion). It tells us the equilibrium potential of an ion and therefore, the MP for which that ion will be in equilibrium.
Why is the resting membrane potential close to Ek?
Open potassium ion channels dominate at rest
What is Ek?
-95mV
Why aren’t all resting membrane potentials equal to Ek?
Resting membrane is not perfectly selective for potassium ions as other types of channels are also open so the resting potential is usually less negative than Ek
Between which two ionic equilibrium potentials does the resting potential of skeletal muscle lie and why?
ECl and Ek as skeletal muscle resting membrane is highly permeable to both chloride and potassium ions
Define depolarisation
The inside of a cell becomes less negative and the membrane potential decreases
Define hyperpolarisation
Inside of membrane becomes more negative and the membrane potential increases
What will happen if you change the membrane permeability to a specific ion?
Changing the selectivity to an ion changes the membrane potential and moves it closer to that ion.
What will happen if you open more sodium and calcium channels?
Depolarisation
What will happen if you open chloride and potassium channels?
Hyperpolarization
What does the GHK equation prediction?
Membrane potential as an average of all the equilibrium potentials for all the ions of which there are ion channels open for.
If there are multiple ion channels open, what determines which one influences the membrane potential most.
- Number of channels open for that specific ion
2. Ease of which the ion can flow through the channel relative to the other ions
In what 3 ways can channels be gated?
- Voltage gated - channel opens/closes with changes in membrane potential
- Ligand gated - binding of a chemical molecule can open of close the channel. The chemical molecule can be an intercellular messenger or an extracellular transmitter.
- Mechanical gating - changes in membrane conformation causes change to ion channels
In fast synaptic transmission, what type of ion channel is the receptor?
Ligand-gated ion channel
What are the channels that depolarising transmitters open, selective for?
Sodium and calcium (and other cations) - these channels have positive reversal potential
What happens as a result of a depolarising transmitter binding to a receptor?
A change in membrane potential known as an EPSP (excitatory post synaptic potential) and excitation of cells
What types of channels do hyperpolarising transmitters open?
Channels with negative reversible potential i.e. Channels selective for Potsssium and chloride
What does binding of a hyperpolarising transmitter do?
Causes inhibition and change in membrane potential known as IPSP (inhibitory post synaptic potential)
Is the receptor is slow transmission an ion channel?
No, it is a molecule associated with a GTP-binding protein. It is a G protein receptor
In what two ways can a G protein receptor signal to a channel in slow synaptic transmission?
- Within membrane - transmitter binds to G protein receptor. G protein binds to channel. Channel opens.
- Via an Intracellular cascade - transmitter binds to G protein receptor. G protein binds to enzyme. Amplification by signalling cascade. Activates Intracellular messenger or a protein kinase. Channel opens.
What is the structure of a nicotinic ACh receptor?
- Ligand gated intrinsic ion channel
- ACh binds to it, channel opens
- Allows sodium and potassium to flow through but not anions
- Brings MP to 0mV - between Ek and ENa
What are the 4 places where synaptic connections can occur?
- Nerve-nerve
- Nerve-muscle
- Nerve-gland
- Sensory cell-nerve
