Flashcards in MR 3&4 The Resting Cell Membrane and electrical excitability Deck (32):
What is a membrane potential?
The electrical potential difference (voltage) across the plasma membrane
What is the Resting Membrane Potential expressed as?
Potential inside the cell relative to the extracellular solution
What is the range of RMPs in nerve cells?
-50 to -75mV
What is the RMP of smooth muscle cells?
What is the range of RMPs in cardiac and skeletal muscle?
-80 to -90mV
What sets up RMP in cells?
open K+ channels
What happens when chemical and electrical gradients are equal and opposite?
There is no net ion movement
What is the equilibrium potential for an ion?
The membrane potential at which there is no net movement of the ion across the membrane (conc grad=elec grad)
What is depolarisation and what channels may cause it?
Membrane potential decreases in size/ becomes more positive
may only be a few mV
Cell interior becomes less negative
e.g opening Na+ or Ca2+ channels
What is hyperpolarisation and what channels may cause it?
Membrane potential increases in size/ becomes more negative
Potential falls below resiting
e.g opening Cl- or K+ channels
What is fast synaptic transmission?
synaptic transmission where the receptor protein is also an ion channel. Binding of transmitter causes channel to open
What is slow synaptic transmission?
Where receptor protein and ion channel are separate proteins linked by either G-proteins or intracellular messengers
What occurs at excitatory synapses?
excitatory transmitters open ligand channels(Na+,Ca2+ or just general cations) causing membrane depolarisation
give an Excitatory Post-Synaptic Potential (EPSP)
longer time course than AP
graded with amount of transmitter
Give 2 excitatory transmitters
What occurs at inhibitory synapses?
Inhibitory transmitters open ligand-gated(e.g K+, Cl-) channels, causing hyperpolarisation
give an Inhibitory Post-Synaptic Potential
Give 2 inhibitory transmitters
Glycine, gamma-aminobutyric acid (GABA)
What is the absolute refractory period? (ARP)
Peroid in which another action potential cannot be fired because nearly all Na+ channels are in the inactivated state
What is the relative refractory period? (RRP)
Period in which it is difficult to initiate another action potential due to some Na+ channels still being inactive. Excitability returns to normal an number of inactivated channels decreases
What is accommodation?
Where slow depolarisation means that no action potential is fired even after the threshold is passed because sodium channels are inactivated
What is the basic structure of voltage gated Na channels?
Four repeats of 6 transmembrane domains, the 4th transmembrane domain being a voltage sensor.
1 alpa subunit
What is the inactivation particle of a channel?
section of 3 amino acids which when any of them are changed make the channel inactive by blocking the pore
What is the general structure of a voltage gated K+ channel?
4 alpha sub units, each having 6 transmembrane domains with the 4th being voltage sensitive
How do local anaesthetics like procaine act?
Blocking Na+ channels
What nerves does MS affect?
All CNS nerves
What nerves does Devic's disease affect?
Optic and spinal cord nerves only
Name two myelin affecting diseases of the peripheral nervous system
What is capacitance?
ability to store charge
How does channel density differ in myelinated and unmyelinated neurones?
Myelinated neurones have high Na+ channel density in the nodes of ranvier while the channels are evenly distributed in unmyelinated axons
How does myelin ensure faster action potentials?
Acts as an insulator allowing local current circuits to remain above threshold and depolarise the next node
Are myelinated axons always faster than unmyelinated axons?
No when the diameter is less than 1um unmyelinated neurones are faster
What happens when myelin is damaged?
The length constant is shorter and dnensity of action current reduced due to resistive and capacitive shunting causing failure to reach threshold