Lectures 1-5 Flashcards
What is the permiability of a membrane dependent on?
The number of channels for each ion
What two factors do ion flux depend on?
Chemical gradient
Electrical force
If a membrane were only surrounded by Na+, what would
happen
Na+ influx into the negatively charged area, down the chemical gradient
What is the equilibrium potential of an ion?
The voltage at which there is no net movement of a specific ion (when chemical and electrical forces are in equilibrium.
This is given by the Nernst equation.
What is the ionic driving force?
The net result of the chemical gradient and electrical force on ions
At rest, what are the relative permeabilities to Na+ and K+ across the axon membrane?
40 times more permeable to K+
What do ion pumps do?
THEY DON’T CONTROL MEMBRANE POTENTIAL
They top up the Em
catalyse ATP breakdown, efflux 2Na+ for 3K+ influx (electrogenic)
How does the influx of Na+ compare to the efflux of K+ at rest?
they are equal
Essentially, what is conductance?
permeability
It treats the membrane as an electrical resistor
g=1/R
When do Na+ Voltage gated ion channels open and close?
open gradually in depolarisation and close in repolarisation
What is the effect of depolarisation?
Em approaches E(Na)
What stops the influx of Na+ at the end of depolarisation?
Na+ channels undergo a conformational change
What causes repolarisation?
K+ VGC’s open as a delyaed response, allowing K+ efflux.
What causes hyperpolarisation?
conductance for K+ increases after Em returns to rest.
How is Em returned to -65mV?
K+ VGC’s close, using leak channels to establish resting potential
In what circumstances would Na+ influx lead to depolarisation?
If Na+ influx is greater than K+ efflux
The threshold potential is the Em value where this is established
What is the difference between the types of refractory periods?
Absolute refractory periods - impossible for another AP to be generated (most Na+ channels inactivated)
Relative refractory periods - a stronger stimulus is required to open enough Na+ channels to overcome the increased K+ conductance.
What prevents AP’s from travelling backwards?
local current flow to adjacent membranes
Are most neurones in the CNS or PNS?
They are normally straddling both, as the systems are in continuum
Where is the boundary between CNS and PNS?
the Pia mater (formally the Redlich-Obsteiner’s zone, where myelin sheathing changes)
What kind of state is the somatic system in?
bi-stable, it’s partially paralysed in REM sleep
What does the axon hillock do?
generate the action potential
What is the fastest type of synapse?
Why?
The Gap junction (electrical)
uses the direct transfer of ion currents
What is the function of the gap junction
It permits synchronous activity between cells, by allowing communication bidirectionally
What is the structure of the gap junction?
has one connexon made of 6 connexins, hemichannels form a pore junction
What triggers the exocytosis of neurostransmitter in the chemical synapse?
depolarisation oppens Ca2+ channel proteins, Ca2+ influx triggers exocytosis
What are the kinds of ionotropic signalling mechanisms at the synapse?
Excitatory - Na+ influx depolarises the membrane, forming a small excitatory post synaptic potential (EPSP)
Inhibitory - Cl- influx causes hyperpolarisation (small inhibitory post-synaptic potential (IPSP)
How does transmision and effect duration differ between ionotropic and metabotropic receptor signalling mechanisms?
metabotropic offer a slower transmission, but longer-lasting effects
what is special summation?
What is another kind of summation?
multiple EPSP may increase the likelihood of a threshold potential being reached, or EPSP and IPSP cancel each other out
Temporal summation
