Video Transcript
The diagram provided shows the
stages of an action potential, with each stage assigned a number. State the correct sequence of
numbers. (A) 5, 2, 6, 3, 1, 4. (B) 1, 2, 6, 2, 5, 3. (C) 4, 2, 6, 1, 5, 3. (D) 4, 5, 1, 2, 3, 5.
This question asks us to list out
the steps of an action potential in a neuron. So to answer this question, let’s
review how these ions move during an action potential.
Prior to transmitting an action
potential, the membrane potential is at a resting potential between negative 65 and
negative 70 millivolts. At rest, the cytoplasm of the
neuron is much more negative than the extracellular space and the sodium ion
channels are closed, which prevents the sodium from moving down its concentration
gradient. When a neuron is not at rest, the
neuron is transmitting an action potential.
An action potential can be further
broken down into its separate stages: depolarization, repolarization,
hyperpolarization, and the refractory period.
Depolarization starts when a
stimulus, such as a neurotransmitter, reaches the dendrites of a resting neuron and
causes the membrane potential to flip from negative to positive. The membrane potential flips from
negative to positive because the voltage-gated sodium ion channels open, allowing
sodium ions to diffuse into the cytoplasm. When the membrane potential reaches
its peak positive value at positive 40 millivolts, it causes the voltage-gated
sodium ion channels to close again. As sodium ion channels close,
preventing any more sodium ions from entering the neuron, voltage-gated potassium
channels open, initiating repolarization.
In repolarization, the open
voltage-gated potassium ion channels allow potassium ions to diffuse out of the
neuron where it is more concentrated and into the extracellular space. When voltage-gated potassium ion
channels open, so much potassium diffuses out of the neuron that the membrane
potential becomes even more negative than the resting potential. This is called
hyperpolarization.
After hyperpolarization, the
membrane potential becomes so negative that the neuron cannot be stimulated to
transmit another action potential for a brief period of time. This is called the refractory
period. The refractory period resolves when
the sodium–potassium pumps return the membrane potential to the resting potential of
negative 70 millivolts. Only then will the neuron be ready
to transmit another action potential.
Now that we’ve reviewed the stages
of the action potential in neurons, we are able to answer our question. The correct sequence of numbers for
an action potential is 4, 2, 6, 1, 5, 3.
