Welcome to Membrane Depolarization
Let’s uncover how cells create their electric voice
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Membrane Potential
Resting Potential & Action Potential
Action Potential
Action Potential
An action potential is defined is a rapid, temporary electrical signal that travels along the membrane of a neuron or muscle cell. It occurs when a cell's membrane potential quickly shifts from a negative resting state to a positive charge and back to negative. This process allows neurons to transmit information across long distances in the body, enabling communication between neurons and with other cells, such as muscle or gland cells.
Action potentials are essential for muscle contractions, sensory processing, and virtually all forms of cellular communication in the nervous system. The action potential proceeds through distinct steps: resting state, depolarization, repolarization, hyperpolarization, and a return to the resting potential.
Understanding the phases of an action potential is greatly simplified by examining its graph, which typically plots membrane potential (in millivolts, mV) on the y-axis against time (in milliseconds, ms) on the x-axis. This graph illustrates the dynamic changes in a neuron's membrane potential as it undergoes the action potential.
Let’s break down each phase with respect to the graph:
Resting Potential
On the graph, this appears as a flat line around –70 mV. At this stage, the neuron is at rest with all voltage-gated channels closed. The sodium–potassium pump and leak channels maintain this steady negative charge inside the cell.
Threshold Potential
On the graph, this is seen as a slight rise from –70 mV to about –55 mV. When depolarization reaches this level, voltage-gated sodium channels open, triggering the start of an action potential. Reaching threshold is essential—without it, no action potential occurs.
Depolarization and Peak
When the threshold (around –55 mV) is reached, voltage-gated sodium channels open, causing Na⁺ ions to rush into the neuron. This rapid influx makes the inside of the cell positive, and the membrane potential rises sharply to about +30–40 mV. At this peak, sodium channels close while voltage-gated potassium channels begin to open, marking the transition from depolarization to repolarization.
Repolarization
On the graph, this appears as a downward slope from about +30–40 mV back toward –70 mV. During this phase, voltage-gated K⁺ channels open, allowing K⁺ ions to flow out of the cell. This outward movement restores the membrane potential toward its resting negative value, counteracting the earlier Na⁺ influx.
Hyperpolarization (Undershoot)
On the graph, this is seen as a slight dip below –70 mV after repolarization. Because voltage-gated K⁺ channels close slowly, extra K⁺ continues to leave the cell, making the inside more negative than the resting potential. This brief phase creates a refractory period, ensuring the neuron cannot immediately fire another action potential.
Return to Resting Potential
On the graph, this is shown as a gradual return to –70 mV after hyperpolarization. During this phase, the sodium–potassium pump and leak channels restore the original ion balance, bringing the membrane back to its resting state and preparing the neuron for the next action potential.
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