two types of axon and their features
- unmyelinated: small diameter, slow transmission of AP, continuous - myelinated: larger diameter, fast transmission of AP, saltatory
what are the 2 stages of action potential transmission, in both axon types?
- passive spread - generation of action potential
conventional current used in diagrams show
flow of cations (positive ions) from +ve to -ve terminals
3 stages in passive spread
1. subthreshold depolarisation at one region of the membrane 2. passive current flow (inside and outside the axon) 3. depolarisation of adjacent parts of the axon (both sides)
why is current only able to spread over a short distance? what is this distance
current dissipates quickly, restricting distance to less than 1mm
speed of unmyelinated axon AP transmission =
speed of myelinated axon AP transmission =
why is unmyelinated axon AP transmission so much slower
AP must be regenerated at every point in the membrane
is passive current flow between two points rapid or slow?
5 steps to AP transmission in an unmyelinated axon
1. action potential 2. passive current flow 3. depolarisation on both adjacent parts of the membrane to THRESHOLD 4. voltage-gated Na+ channels open 5. new full sized AP generated
glia cells function
wrap axon in myelin
which cells form myelin sheath in CNS
which cells form myelin sheath in PNS
branch off axon called...
gaps in myelin =
Nodes of Ranvier
Myelination increases efficiency of passive spread of current because...
due to the insulating properties of myelin, there is less current dissipation (current can only leave via the nodes of Ranvier)
Passive transmission occurs in one direction TRUE/FALSE
FALSE. Passive transmission occurs in BOTH DIRECTIONS
Current flows ________ between nodes
current flows _passively_ between nodes
in myelinated axons, where does action potential potential regeneration occur?
at the nodes of Ranvier
Myelination increases AP transmission speed because
increasing efficiency of passive spread, so regeneration of AP only has to occur at the nodes, as opposed to every part of the membrane
why are axons not always myelinated?
too large. unmyelinated can fit more per volume - important in the human brain
AP at adjacent node will try to move passively to depolarise the 'previous' node, but why will it fail?
this node will still be in the absolute refractory period (for 1-2ms). By the time this period is over, AP has already moved on
Can action potentials moving in opposite directions pass each other?
NO, like the reasons for APs moving in one direction, this is due to the refractory period
antidromic direction =
towards cell body along axon
orthodromic direction =
from cell body along axon
process where antidromic and orthodromic APs will cancel each other out, due to their absolute refractory periods
where are axons of: sensory neurons, motor neurons, and 'autonomic nervous system' found?
where on the axon would a high-density of voltage-gated Na+ channels be found?
trigger zone of axon
In sensory neurons, do the receptors immediately evoke an action potential?
NO. A receptor potential is first evoked
receptor potential =
a graded depolarisation which is first evoked by sensory neurons (instead of an AP)