Action potentials and nerve conduction Flashcards
graded potentials vs action potentials advantages:
- faster than APs (electrotonic spread)
- signal state w finer resolution (continuous analogue nature) vs binary/ digital nature of APs
- potentially higher info transfer than APs
graded potentials vs action potentials disadvantages:
- rapidly dissipate -> only short distances (few mm most)
- not regenerated along nerve/ axon
- susceptible to noise than APs
summation at axon hillock:
- graded electrical potentials generated by changes in membrane permeability (ligand-gated ion channels) on dendrites
- potentials propagate electrotonically from dendrites and cell body
- potentials are summed (spatial/ temporal) or integrated in axon hillock/ initial segment
initial segment=
trigger zone
define axon hillock:
where axon joints cell body
define initial segment:
thick unmyelinated section of axon that joins cell body at axon hillock,
- site of action potential generation
if depolarisation too weak and Vm doesn’t reach excitation threshold, how is it quickly dissapated?
- increase flow of K+ through leak channels returns cell to resting state
how is a cell like a capacitor?
- device which separates and stores charges
- but are ions (Na, K) rather than electrons
- can estimate no. of ions that have to move across membrane to take it from RMP to threshold
total charge on capacitor formula:
q = c x V
define capacitor formula:
q- total charge (coulombs)
V- voltage across membrane bilayer
c- capacitance in Farads (F)
- rapid depolarisation phase: voltage-gated Na channels have 2 gates
- activation gate (m gate) closed at rest
- inactivation gate (h gate) open at rest
- rapid depolarisation phase: depolarisation above threshold causes which gate to open?
m gate (activation)
- rapid depolarisation phase: m gate opens causing
influx of positive Na ions causes cell to depolarise more coz inward Na more than outward K
- rapid depolarisation phase: depolarisation causes more voltage gated..
Na channels to open
- more Na flows in, positive feedback
- Vm rapidly driven to Na equilibrium potential (E Na)
- repolarisation phase: after 1-2ms opening m gate..
inactivation (h) gate shuts
- voltage gated Na channels cannot be reactivated yet for short time afterwards
- repolarisation phase: absolute refractory period
where Na channels can’t be reactivated by depolarising impulses for short time
- eventually m gate closes, h gate reopens and voltage gated sodium channels (VGSC) can be reactivated
- repolarisation phase: which electrochemical force larger? Na or K
K+ driving it out of the cell larger
- Na channels closing, voltage gated K+ channels in membrane start to open
- repolarisation phase: K+ channel gates and features
only n gate
- triggered at threshold but open after short delay
- repolarises
- after-hyperpolarisation phase (undershoot):
- cell repolarise close to resting Vm
- but pK elevated as K+ channels slow to close
- Vm more negative than RVm
- h gate on Na channels open again and another action potential can be initiated
- after-hyperpolarisation phase (undershoot): relative refractory period
- as pK still elevated and some Na channels still inactive, will need slightly stronger stimulus to trigger anoterh action potential during this period
all or none principle:
- unlike graded potentials, action potentials can’t sum
- overlap is prevented by absolute refractory period
frequency (rate) coding of stimulus strength:
- stimulus strength is coded in frequency (firing rate) of action potentials
- no action potentials during absolute refractory period
- but stronger stimuli can during relative r.p.
local current flow:
electrotonic spread, same as how graded potential spreads
- activated nearby Na channels and ‘regenerates’ action potential in new part of membrane
unidirectional propagation:
- depolarisation causes opening of voltage gated Na channels ahead of action potential
- propagation back towards axon hillock prevented in absolute refractory period
action potential propagates along axon by:
- contiguous conduction (unmyelinated axon) = slower
- saltatory conduction (myelinated) = faster
oligodendrocytes:
- CNS
- forms several myelin sheaths
- myelinates sections of several axons
schwann cells:
- PNS
- forms one myelin sheath
- myelinates only 1 section of axon
myelination:
- schwann/ oligodendrocyte wraps around axon
- cytoplasm eventually squeezed out
- forms 50-150 layers of membrane
myelin sheath: features
- lipids
- insulator
- restricts movement of hydrophilic ions in/out of cell
- ions only leave in gaps called Nodes of Ranvier
- voltage gated Na and K channels concentrated there
- action potentials only at nodes (jump) along axon
unmyelinated axons: cons
- positive charges leak across membrane via ion channels
- but if too may +ve charges leak out, depolarisation too weak to trigger action potential in next part of membrane
- VGSCs must be distributed evenly along axon membrane to constantly regenerate action potential and allow to continue along axon - slow
myelinated axons: pros
- sheath increases electrical resistance of membrane (rm)
- reduces membrane capcitance (Cm)
- prevents +ve charges leaking out -> depolarisation can spread electrotonically further
- very fast
cable theory:
voltage decreases exponentially as it spreads along membrane electrotonically
membrane capacitance is:
measure of how strongly charges on either side of membrane attract each other
myelin decreases capacitance why:
- increase distance btw ICF and ECF
- not need as many ions to alter voltage = reduced time constant
nerve conduction: effect of diameter
large diameter:
- less resistance to movement of charges (ions) = faster (ri low)
smaller diameter: more resistance to movement of ions = slower (ri high)
nerve fibre classified by:
- axon diameter
- whether myelin is present
