Physiology of Neurons Flashcards
(45 cards)
how are electrical synapses different from chemical synapses?
faster always excitatory bidirectional smaller gap no plasticity no amplification coupled via gap junctions used for defensive reflexes, retina and brain
what does ‘no amplification’ mean?
signal is always weakened as it is transmitted from pre to post-synaptic cell
signal will not transmit if post-synaptic cell is much bigger than presynaptic cells
what is spatial summation?
a neuron determines whether to fire based on ‘addition’ of all tiny signals from several neurons synapsing on it
hence many small depolarisations can reach threshold
what is temporal summation?
when input neuron is firing fast enough, receiving neuron can ‘add’ many tiny signals to reach threshold
neurons ability to recover from tiny input is slow enough that when next signal arrives, neuron is still slightly depolarised
action potential summary
- at rest - K+ leaving the cell clamps the membrane potential negative (-70mV)
- an external factor (eg synaptic activity) causes membrane to depolarise slightly possibly reaching threshold
- Na+ conductance shoots up, Na+ goes into cell - membrane depolarises and voltage becomes positive
- with time delay, Na+ conduction diminishes, K+ conductance increases - K+ leaves the cell, voltage returns to resting potential
what is permeability?
how easy it is for a particle to move through the membrane
what is conductance?
how easy it is for charge to move across the membrane
what happens at initial depolarisation?
cells start at rest (-70mV), resting membrane potential is near Ek
inward rectifier K+ channels are open - K+ flowing out is dominant current
something occurs, causing cell to become less negative
what can cause a cell to become less negative?
nearby cell depolarising
synaptic transmission where neurotransmitter opens a ligand-gated channel
what is depolarisation?
inside the cell the voltage becomes less negative/more positive
describe the positive feedback of depolarisation
initial depolarisation causes a few Na+ channels to open
Na+ permeability increases, so Na+ current flows through channels into cell
additional current of Na+ entering cell causes greater depolarisation - membrane moves closer to 0mV
when voltage surpasses threshold (-50mV), cell is committed to AP
positive feedback of increasing Na+ conductance and increasing voltage until membrane becomes quite positive (+30mV)
what is repolarisation?
voltage becomes less positive/more negative inside the cell
what happens during repolarisation?
2 delayed-action events occur
Na+ channel inactivation: Na+ current going in decreases
delayed rectifier K+ channels open - K+ going out increases
what is the refractory period?
period of time in which a neuron is incapable of reinitiating an AP
occurs mostly during after-hyperpolarisation
amount of time t takes for membrane to be ready for a second stimulus, once it has returned to resting state following excitation
what is after-hyperpolarisation?
at the end of an AP, the voltage temporarily goes slightly more negative than at rest, followed by the return to resting membrane potential
why does after-hyperpolarisation occur?
when voltage drops below -60mV, inward rectifier K+ channels open, clamping voltage towards Ek
delayed rectifier K+ channels are still open (slow to close)
almost all Na+ channels are inactivated
coding of intensity by neurons
action potentials are ‘all or none’ - carry no information about size of stimulus
firing frequency represents intensity of activity
+ different neurons for different strength stimuli (light tough vs pain receptors)
what can affect firing frequency?
increasing threshold (more positive) lowers firing frequency
increasing excitatory synaptic activity increases firing activity
when lengthy synaptic currents are small, there is a greater threshold than there is for larger currents
this is due to Na+ current accommodation (channels are inactivated during slower, subthreshold depolarisation)
what is excitability?
how easy it is to start nervous signalling
‘sensitivity’ in sensory cells
‘ irritability’ in muscle/effector cells
changes in excitability are the basis of psychotropic pharmacology
what is threshold?
voltage above which an action potential fires
increased threshold lower excitability
changes have profound health and behavioural effects
what are channels?
proteins
sometimes conduct ions, sometimes don’t
have different conformational states
voltage-gated channels
change states based on transmembrane voltage
open when membrane becomes positive inside (increased permeability)
channels close when membrane repolarises
(inward rectifier channels are opposite)
what is the difference between an inactivated and a closed channel?
inactivated = when a channel stops conducting when membrane is positive inside closed = when a channel stops conducting when membrane is negative inside
what happens when Na+ channels open?
Na+ enters the cell
the membrane becomes more positive inside
([Na+] is higher outside the cell)
