action potentials Flashcards
action potential
it i a transient depolarisation of a cell
excitable cells
are cells that can generate action potentials
- for any particular cell an action potential is, within close limits, of fixed magnitude and duration.
what do action potentials do?
they transfer information
it is then coded by the frequency of action potentials passing along a nerve.
initiate cellular events
for example initiate muscular contraction (excitation-contraction coupling)
all or nothing law
once an action potential has been initiated varying the stimulus strength does not alter the configuration of action potential
threshold
to initiate an action potential it is necessary to depolarise the cell membrane to a critical (threshold) potential.
ionic basis of action potential
experiments showed that the action potential depolarisation was due to transient increase of membrane Na+ permeability.
membrane potential repolarises
back to resting potential and the action potential terminates.
repolarisation is aided by an even greater than normal increase of K+ permeability.
process of an action potential
resting potential
depolarising stimulus
sodium and potassium channels open
sodium entry depolarizes cell
sodium cannels start to close
K channels stay open
initiating a AP
cell must be depolarised from the resting membrane potential to a threshold potential
At threshold voltage- activated ion channels
they open and callow cations( usually Na+) to enter the cell, generating an action potential.
to initiate action potential, energy must be added to the system in a form of stimulus.
how to get to resting to threshold potential
- artificial application of electrical current
- at synapses between cells neurotransmitters bind to ligand-gated channels on target cell.
- spontaneously in pacemaker cells
- sensory cells- convert a stimulus to a change of membrane potential of the associated nerve, if stimulus is large enough it reaches threshold.
how is an action potential conducted(1)
a local current precedes the action potential wavefront in the cytoplasm.
trigger zone
the region of the cell that generates an action potential. it is represented by a reversal of the membrane potential polarity.
how is an action potential conducted(2)
some current leaks across the membrane, depolarising it.
if the depolarisation is sufficient to reach threshold an action potential is initiated.
factors affect AP conduction velocity
Cell diameter- CV increases as fibre diameter increases.
temperature- increases CV
myelination: vertebrate nerve fibres diameter>1um possess a myelin sheath which surrounded the axon with breaks about every millimetre.
saltatory conduction
when the AP jumps from node to node.
if the actions are thick they will be quicker
temperature will increase speed of ions.
coding information
As Stimulus increases number of action potentials generated increases.
the action potential frequency codes stimulus intensity.
Active transport
movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by proteins and requiring energy.
depolarisation
loss of the difference in charge between the inside and outside of the plasma membrane of muscle or nerve cell due to a change in permeability and migration of sodium ions to the interior.
myelinated fibres
an insulating layer(sheath) that forms around nerves which is made up of proteins and lipids. allows electrical impulses to transmit quickly and efficiently along nerve cells
permeable
allows molecules to pass through it
repolarisation
restoration of the difference in charge between the inside and outside of the cell membrane following depolarization.
