WEEK 1 Flashcards
What is the nervous system made of?
Cells-neurones and glia
How do neurones work?
within the neurone=electrical activity between neurones (+other cells)=synapses
How is the nervous system organised?
Central vs Peripheral NS
Motor/Efferent vs Sensory/Afferent neurones
Somatic vs Autonomic NS
`What is an electrical signal?
A change in the balance of +ve and -ve charges due to ion transfer through ion channels
What are ions?
Charged particles (eg. Na+, K+, Cl-, Ca2+)
What controls ion movement in and out of a cell?
Cell membrane
General ion concentrations inside and outside of cells
K+=high conc. inside, low conc. outside
Na+=low conc. inside, high conc. outside
Ca2+=extremely low conc. inside, low conc. outside
Cl-=low conc. inside, high conc. outside
Basic electrical property of membranes
Inside of cell contains slight excess of anions->negative voltage
What is membrane potential (Em)?
The voltage inside a cell determined by the balance of charges
What are ion channels?
Membrane proteins which are essential for controlling Em
What is the difference between active and passive transport?
Active requires energy (ATP), passive doesn’t
Is ion movement active or passive?
Passive
Ion channel gating mechanism classification
Non-gated (leak): set Em of resting membrane
Voltage-gated: generate AP
Ligand-gated (chemical): generate Em changes at synapse
What is the resting membrane potential in neurones?
-65mV (excess -ve charge inside)
What is the permeability value of each ion dependent on?
The number of open channels for that ion
What factors influence ion movement (flux)?
Chemical gradient: unequal ion distribution (concentration gradient)
Electrical force: attraction/repulsion of ions by voltage inside the cell (Em)
How does the chemical gradient affect K+ and Na+ movement?
K+ drive to leave cell (efflux) due to high conc. inside, Na+ drive to enter cell (influx) due to high conc. outside
How does the electrical force affect K+ and Na+ movement?
both Na+ and K+ drive to enter cell (influx) due to negative Em
What is a property of chemical gradient?
It is ‘constant’-small changes are irrelevant fluctuations
When is an ion in equilibrium (no net flux)?
When the chemical gradient and electrical force are in balance
What is the Nernst equation used to define?
The equilibrium potential of an ion (Eion)
What is the equilibrium potential of K+ (Ek)?
-80mV
What is the equilibrium potential of Na+ (Ena)?
+62mV
What does Em>Ek tell us about K+ movement?
At -65mV, chemical influence (efflux)>electrical influence (influx)->K+ efflux (trying to reach -80mV)
What does Em
At -65mV, both chemical and electrical influences cause Na+ influx (trying to reach +62mV)
What is the ionic driving force?
The net force resulting from chemical and electrical influences, which is present whenever Em differs from equilibrium potential
Where does Em rest between?
Ena and Ek-closer to Ek as Pk is much higher (more open leak K+ ion channels-Pk=40xPna)
What is the Goldman equation used to define?
The membrane potential taking into account all ions permeant through that membrane
What does Em being at rest tell us?
Na+ influx=K+ efflux
What causes the maintenance of ionic gradients?
Ion pumps operating continuously (especially: Na+/K+ pump-Na+ efflux/K+ influx and Calcium pump-Ca2+ efflux)
What are the stages of an action potential?
Generator potential, depolarisation, repolarisation, hyperpolarisation and return to resting Em
What is required for action potential to occur?
Em less negative than threshold potential
How is depolarisation achieved?
Na+ influx due to increased Pna (opening of V-gated Na+ channels)
How is repolarisation achieved?
K+ efflux due to increased Pk (opening of V-gated K+ channels)-and terminating activity of ‘extra’ Na+ channels (decreased Pna)
What does conductance of ion channels relate to and why is it used?
Permeability and due to it being proportional to the number of ion channels open
Conductance (g)=
1/R(resistance due to membrane)
How do depolarisation and repolarisation affect V-gated ion channels?
Open due to depolarisation, Close due to repolarisation
Summary of AP events
Initial stimulus (depolarisation) which reaches threshold, opening of V-gated Na+ channels (increased gNa), Na+ influx-further depolarisation, Em approaches Ena, Na+ channels inactivate (decreased gNa), Na+ influx stops, delayed opening of V-gated K+ channels (increased gK), K+ efflux-repolarisation, but increased gK after Em returns to rest, Em approaches Ek-hyperpolarisation, V-gated K+ channels close (decreased gK), Em returns to resting potential via leak channels
What is threshold potential and what does it cause?
The Em value where Na+ influx (due to leak and V-gated channels)>K+ efflux (leak) and causes AP to be all-or-nothing event
What is the absolute refractory period?
The period after AP where no further AP can be caused by any stimulus
What causes the absolute refractory period?
Most V-gated Na+ channels being inactivated and many V-gated K+ channels being open
What is the relative refractory period?
The period after AP where a stronger stimulus is required to overcome increased gK/remaining K+ efflux
What causes the relative refractory period?
V-gated Na+ channels recovering from inactivation and some V-gated K+ channels still being open
Describe AP propagation along an unmyelinated axon
Electrotonic spread due to local current flow to adjacent membrane
Describe AP propagation along myelinated axon
Saltatory conduction-ion flow, therefore depolarisation, from node to node where V-gated channels are located-increasing speed of AP conduction
