Case 2 : Neural communication
- The Inside of the neuronal membrane at rest = negatively
charged in relation to the outside.
- An action potential = A brief fluctuation/change in membrane
potential caused by the rapid opening and closing of voltage-gated
ion channels; also, known as spike, nerve impulse, or discharge.
- Takes approximately 2 milliseconds.
- Action potentials are caused by depolarization of the membrane
beyond threshold
The depolarization that causes action potentials arises in different ways in different neurons
o depolarization can be caused by the entry of Na+ through specialized ion
channels that are sensitive to membrane stretching.
o In interneurons, depolarization is usually caused by Na+ entry through
channels that are sensitive to neurotransmitters released by other neurons.
o neurons can also be depolarized by injecting electrical current through a
microelectrode
Action potentials begin when voltage-gated ion channels open, altering membrane
permeability to Na+ and K+. Before and after the action potential, the neuron is at its
resting membrane potential of -70mV.
The action potential itself can be divided into three phases: rising phase, falling phase and
after hyperpolarization phase.
1. The neuron is at its resting membrane potential.
2. A stimulus leads to a depolarization.
3. Rising phase = due to a sudden temporary increase in the cell’s permeability to Na+
An action potential begins when a graded potential reaching the trigger zone depolarizes
the membrane to threshold (-55mV). As the cell depolarizes, voltage-gated Na+ channels
open, making the membrane much more permeable to Na+. Because Na+ is more
concentrated outside the cell and because the negative membrane potential inside the cell
attracts the positively charged ions, Na+ flows into the cells.
4. The addition of positive charge to the intracellular fluid depolarizes the cell membrane,
making it progressively more positive. Overshoot = the top third of the rising phase, the
inside of the cell has become more positive than the outside, and the membrane potential
has reversed polarity. It’s the portion of the action potential above 0mV.
5. The action potential peaks at +30mV. Na+ channels in the axon close and K+ channels open.
,6. Falling phase = increase in K+ permeability. Voltage-gated K+ channels open in response to
depolarization. The K+ channel gates are much slower to open → peak K+ permeability
occurs later than peak Na+ permeability.
When the Na+ channels close at the peak of the action potential, the K+ channels have just
finished opening, making the membrane
very permeable to K+. At a positive
membrane potential, the concentration,
and electrical gradients for K+ favor
movement of K+ out of the cell. As K+
moves out of the cell, the membrane
potential rapidly becomes more negative,
creating the falling phase of the action
potential and sending the cell toward its
resting potential.
7. After-hyperpolarization phase = when the
falling membrane potential reaches -70 mV,
the K+ permeability has not returned to its
resting state. Potassium continues to leave
the cell through both voltage-gated and K+
leak channels, and the membrane
hyperpolarizes, approaching -90mV.
8. The sow voltage-gated K+ channels close, and
some of the outward K+ leak stops.
9. Retention of K+ and leak of Na+ into the axon
bring the membrane potential back to -
70mV.
Propagation: AP moves along the axon by
saltatory conduction in myelinated neurons,
jumping between nodes of Ranvier (gaps
between myelin).
Generation of multiple action potentials
- the firing frequency of action
potentials reflects the magnitude of
the depolarizing current
- The relative refractory period follows
the absolute refractory period and is
characterized by the need for a
stronger-than-normal stimulus to
generate another action potential.
This occurs because some Na⁺
channels have recovered but K⁺
, channels are still open, leading to hyperpolarization. Therefore, reaching the
threshold requires a greater depolarizing current than usual. In other words, the
threshold value has temporarily moved to more negative, which requires a stronger
depolarization to reach it.
- absolute refractory period ARP= The period of time, measured from the onset of an
action potential, during which another action potential cannot be triggered. Because
Na+ is still open or inactivated
- The Inside of the neuronal membrane at rest = negatively
charged in relation to the outside.
- An action potential = A brief fluctuation/change in membrane
potential caused by the rapid opening and closing of voltage-gated
ion channels; also, known as spike, nerve impulse, or discharge.
- Takes approximately 2 milliseconds.
- Action potentials are caused by depolarization of the membrane
beyond threshold
The depolarization that causes action potentials arises in different ways in different neurons
o depolarization can be caused by the entry of Na+ through specialized ion
channels that are sensitive to membrane stretching.
o In interneurons, depolarization is usually caused by Na+ entry through
channels that are sensitive to neurotransmitters released by other neurons.
o neurons can also be depolarized by injecting electrical current through a
microelectrode
Action potentials begin when voltage-gated ion channels open, altering membrane
permeability to Na+ and K+. Before and after the action potential, the neuron is at its
resting membrane potential of -70mV.
The action potential itself can be divided into three phases: rising phase, falling phase and
after hyperpolarization phase.
1. The neuron is at its resting membrane potential.
2. A stimulus leads to a depolarization.
3. Rising phase = due to a sudden temporary increase in the cell’s permeability to Na+
An action potential begins when a graded potential reaching the trigger zone depolarizes
the membrane to threshold (-55mV). As the cell depolarizes, voltage-gated Na+ channels
open, making the membrane much more permeable to Na+. Because Na+ is more
concentrated outside the cell and because the negative membrane potential inside the cell
attracts the positively charged ions, Na+ flows into the cells.
4. The addition of positive charge to the intracellular fluid depolarizes the cell membrane,
making it progressively more positive. Overshoot = the top third of the rising phase, the
inside of the cell has become more positive than the outside, and the membrane potential
has reversed polarity. It’s the portion of the action potential above 0mV.
5. The action potential peaks at +30mV. Na+ channels in the axon close and K+ channels open.
,6. Falling phase = increase in K+ permeability. Voltage-gated K+ channels open in response to
depolarization. The K+ channel gates are much slower to open → peak K+ permeability
occurs later than peak Na+ permeability.
When the Na+ channels close at the peak of the action potential, the K+ channels have just
finished opening, making the membrane
very permeable to K+. At a positive
membrane potential, the concentration,
and electrical gradients for K+ favor
movement of K+ out of the cell. As K+
moves out of the cell, the membrane
potential rapidly becomes more negative,
creating the falling phase of the action
potential and sending the cell toward its
resting potential.
7. After-hyperpolarization phase = when the
falling membrane potential reaches -70 mV,
the K+ permeability has not returned to its
resting state. Potassium continues to leave
the cell through both voltage-gated and K+
leak channels, and the membrane
hyperpolarizes, approaching -90mV.
8. The sow voltage-gated K+ channels close, and
some of the outward K+ leak stops.
9. Retention of K+ and leak of Na+ into the axon
bring the membrane potential back to -
70mV.
Propagation: AP moves along the axon by
saltatory conduction in myelinated neurons,
jumping between nodes of Ranvier (gaps
between myelin).
Generation of multiple action potentials
- the firing frequency of action
potentials reflects the magnitude of
the depolarizing current
- The relative refractory period follows
the absolute refractory period and is
characterized by the need for a
stronger-than-normal stimulus to
generate another action potential.
This occurs because some Na⁺
channels have recovered but K⁺
, channels are still open, leading to hyperpolarization. Therefore, reaching the
threshold requires a greater depolarizing current than usual. In other words, the
threshold value has temporarily moved to more negative, which requires a stronger
depolarization to reach it.
- absolute refractory period ARP= The period of time, measured from the onset of an
action potential, during which another action potential cannot be triggered. Because
Na+ is still open or inactivated
