Video Transcript
The flowchart provided outlines the
stages of an action potential. What element should replace the
gaps in the first four statements? Stage one, the axon membrane has a
resting potential of minus 65 millivolts; blank voltage-gated ion channels are
closed. Stage two, the energy of a stimulus
triggers some blank voltage-gated ion channels to open, and blank ions diffuse into
the axon cytoplasm. Stage three, more voltage-gated
blank ion channels open, and more blank ions diffuse into the axon. Stage four, the potential
difference across the membrane reaches plus 40 millivolts, and the voltage-gated
blank ion channels close. Stage five, the potassium
voltage-gated ion channels open, and potassium ions diffuse out of the axon. Stage six, more potassium ions
diffuse out of the axon until the membrane is hyperpolarized. The potassium channels close.
There are two key elements whose
positively charged ions regulate the stages of an action potential in an axon. These are potassium, represented
here as pink circles, and sodium, represented here as green circles. While potassium ions do play an
important role, it is the movement of sodium ions across the membrane of a neuron
which is crucial for initiating and propagating an action potential.
This diagram is a simple
representation of an axon membrane at rest, as described by statement one. During this stage of the action
potential, sodium ions are actively pumped out of the axon cytoplasm into the
extracellular space against their concentration gradient. In other words, they are being
transported from an area of low concentration inside the axon to an area of high
concentration outside. When a stimulus is present, as
described in statement two, energy is released which causes some voltage-gated
sodium ion channels to open.
Because there is a much higher
concentration of sodium ions outside the axon than inside, sodium ions diffuse down
their concentration gradient into the axon cytoplasm. This causes the inside of the axon
to become more positively charged than the outside, so more voltage-gated sodium ion
channels open and more sodium ions diffuse in, as described by statement three.
When a sufficient number of sodium
ions have diffused into the axon for the potential difference across the membrane to
reach plus 40 millivolts, the voltage-gated sodium ion channels close again, as
described by statement four. This prevents further sodium ions
from diffusing into the axon and allows the axon membrane to eventually return to
its resting potential. We have therefore demonstrated that
the element which should replace the gaps in the first four statements is
sodium.
