Action Potential Mechanisms Flashcards
what is a resting membrane potential?
a potential difference that exists across the membrane of all cells
what is the resting membrane potential range for cells?
20-90mV
how does the ICF compare to the ECF
the inside (ICF) is NEGATIVE with respect to the outside (ECF)
what are the charges found within the ICF and ECF?
equal numbers of positive and negative charges in the ECF and ICF
if there are equal numbers of +ve and -ve charges in the ECF and ICF, why is there a difference in overall charge?
- the charges on each side of the membrane are not evenly distributed
- on the periphery of the ICF, the charges that lie there are negative
- on the edge of the ECF, the charges that lie there are positive
what does the resting. membrane potential arise from?
arises from the separation of charges on either side of the membrane
what is the resting membrane potential determined by?
- diffusion of K+ from cell interior through K+ channels*****
- the sodium/potassium pump also contributes by moving unequal amounts of Na and K
where are Na+ ions found?
mainly in the ECF
where are K+ ions found?
mainly in the ICF
where are Cl- ions found?
mainly in the ECF
how does the ‘resting’ membrane differ with respects to Na+ and K+ ions?
- the ‘resting’ membrane is impermeable to Na+
- the ‘resting’ membrane is very permeable to K+
what does diffusion of K+ ions out of the cell cause?
- an excess of negative charge inside the cell
- the potential gradient arising from diffusion is the ‘resting membrane potential’
how does the Na+/K+ pump contribute to the RMP?
- small traces of Na+ leak into cells
- the pump moves 3 Na+ outwards and 2 K+ inwards
when sodium is moved out of the cell and potassium is moved in using the pump, is this process passive or via active transport?
active transport
what is meant by active transport?
movement of ions that requires energy
how can the membrane potential be altered?
by applying an electric current to the cell (a ‘stimulus’)
what is the membrane potential ‘threshold’, after which depolarisation occurs?
around -55mV
what occurs once the membrane potential ‘threshold’ is reached?
- large depolarisation occurs
- opening of voltage-gated Na+ channels
- the MP ‘overshoots’ zero, so that the polarisation is reversed
- the action potential starts
what is the ‘rising’ phase of the action potential caused by?
- Na+ influx through voltage gated Na channels
what is the ‘falling’ phase of the action potential caused by?
- K+ efflux through voltage gated K channels
what occurs at around 35mV?
- Na+ channels shut (h gate closes)
- K+ channels open
- K+ leaves the cell, making the MP more negative
what are ion channels?
- transmembrane proteins
what type of ion channels exist in cells?
- gated channels
- ion selective (specific) channels
what types of gated ion channels are there?
- ligand
- voltage
what is the AP down stroke?
the ‘recovery’ phase
- refractory period in the Na+ channels
how can the AP event be described?
- an ‘all or nothing’ event
- amplitude is independent of stimulus
what is the refractory period of a neuron?
- period of inexcitability
- after an AP is initiated, the neuron cannot generate another AP until the first has ended
why does the refractory period occur?
it occurs due to the inactivation of voltage-gated sodium channels
what are the consequences of the refractory period?
- limits maximum firing frequency of action potentials in axons
- ensures unidirectional propagation of APs
- prevents summation of APs
what is unique about the cardiac AP?
it lasts as long as the ventricular contraction
how do action potentials travel?
APs travel along the axon as waves of depolarisation
how does the axon diameter effect the AP?
the speed of the action potential increases with the axon diameter
what can increase the speed of an AP in a smaller neuron?
- myelination which forms an insulating layer, reducing leakage of current from axon
what are glial cells?
cells that form myelin
what type of glial cells form myelin in the PNS?
- Schwann cells
what type of glial cells produce myelin in the CNS?
oligodendrocytes
what are the short unmyelinated specialised sections of the axon better known as?
Nodes of Ranvier
what is meant by saltatory conduction?
when an electrical impulse skips from node to node down the full length of an axon
what is the epineurium?
the outermost layer of dense, connective tissue that surrounds a peripheral nerve
what is the perineurium?
- protective sheathe that surrounds a nerve fascicle
- it bundles together axons that target the same anatomical region
what is the perineurium composed of?
mostly fibroblasts
what is the endoneurium?
a layer of delicate connective tissue that surrounds the myelin sheathe of each myelinated nerve fibre
are A fibres myelinated or unmyelinated?
myelinated
what type of nerve fibres are unmyelinated?
C fibres
what is the function of A-beta fibres?
mechanoreception
what are the functions of A-delta fibres?
- mechanoreception
- thermoreception (cold)
- nociception
- chemoreception
what is the function of C fibres?
- mechanoreception
- thermoreception (hot and cold)
- nociception
what type of fibres is the pulp composed of?
mostly unmyelinated C fibres
