Action Potentials in Neurons

• Neurons and Action Potentials

  • Neurons communicate through the generation and propagation of action potentials.

  • Action potentials typically begin near the axon hillock of the neuron cell body.

  • They propagate along the entire length of the axon.

• Voltage-Gated Channels

  • The action potential is characterized by the opening and closing of voltage-gated channels.

  • These channels alter the permeability of the plasma membrane to sodium ($Na^+$) and potassium ($K^+$) ions.

  • Action potentials require a threshold stimulus to initiate changes in voltage-gated sodium channel conformations.

• Phases of Action Potential

  • Phase One: Depolarization

  • Triggered by a threshold stimulus that opens voltage-gated sodium channels.

  • Sodium ions diffuse into the axon, making the membrane potential less negative.

  • Voltage approaches +30 millivolts at the peak of depolarization.

  • At this peak, two key events occur:

    • Inactivation gates of voltage-gated sodium channels close.

    • Voltage-gated potassium channels open, starting phase two.

  • Phase Two: Repolarization

  • Characterized by the outflow of potassium ions ($K^+$) as voltage-gated potassium channels remain open.

  • The membrane potential begins to return to a negative value.

  • The potential often goes below the resting state of -70 millivolts, leading to phase three.

  • Phase Three: Hyperpolarization

  • The membrane potential becomes more negative than the resting potential.

  • Voltage-gated potassium channels close and voltage-gated sodium channels are released from inactivation.

  • The neuron returns to resting state (-70 millivolts) through the activity of leak channels after hyperpolarization.

• Summary of Action Potential Generation

  • Threshold stimulus activates voltage-gated sodium channels.

  • Sodium ions diffuse into the axon, rapidly depolarizing it to +30 millivolts.

  • Sodium channels close, and potassium channels open, causing repolarization.

  • Membrane potential hyperpolarizes momentarily before returning to resting state (-70 millivolts).

  • The neuron is prepared to fire another action potential after the return to resting state.

Neurons and Action Potentials

Neurons communicate through the generation and propagation of action potentials, which are electrical signals fundamental to neuronal communication and information processing in the nervous system.

Action potentials typically begin near the axon hillock, a specialized region of the neuron cell body that integrates incoming signals. This initiation point is crucial, as it determines whether the action potential will be generated based on the summed input from synapses. Once initiated, action potentials propagate along the entire length of the axon, enabling rapid transmission of signals to other neurons, muscles, or glands.

Voltage-Gated Channels

The action potential is characterized by the opening and closing of voltage-gated ion channels, which play a significant role in the dynamics of membrane potential changes.
These channels alter the permeability of the plasma membrane to sodium ($Na^+$) and potassium ($K^+$) ions. When a stimulus reaches a certain threshold, it triggers conformational changes in voltage-gated sodium channels, allowing sodium ions to rush into the neuron. This influx is essential for the depolarization phase of the action potential, as it generates a positive feedback loop that rapidly changes the membrane potential.

Phases of Action Potential

Phase One: Depolarization

Triggered by a strong enough threshold stimulus, which can be a signal from another neuron or an environmental input, voltage-gated sodium channels open.
Sodium ions diffuse rapidly into the axon, making the interior of the neuron less negative (depolarizing it). This rapid influx of sodium ions continues until the membrane potential approaches around +30 millivolts at the peak of depolarization.
At this peak, two simultaneous key events occur:

• The inactivation gates of voltage-gated sodium channels close, preventing further influx of sodium ions.

• Voltage-gated potassium channels open, marking the transition to the next phase, and start allowing potassium ions to exit the neuron.

Phase Two: Repolarization

In this phase, the outflow of potassium ions ($K^+$) characterizes the action potential as voltage-gated potassium channels remain open for a brief period.
This outflow helps to return the membrane potential back towards a negative value. Beyond merely returning to resting potentials, the potential often goes below the resting state of -70 millivolts, a phenomenon known as hyperpolarization, leading to phase three.

Phase Three: Hyperpolarization

During hyperpolarization, the membrane potential temporarily becomes more negative than the resting potential. This function is crucial as it helps to reset the neuron’s ionic balance.
During this phase, voltage-gated potassium channels begin to close and the voltage-gated sodium channels are released from their inactivated state, which signifies the end of the action potential.
Following hyperpolarization, the neuron returns to resting state (-70 millivolts) primarily through the activity of leak channels, which allow a slow influx of sodium and efflux of potassium ions that help stabilize the resting membrane potential.

Summary of Action Potential Generation

The process of action potential generation is a finely tuned interplay of ion movements across the neuronal membrane.

• A threshold stimulus activates voltage-gated sodium channels, initiating a dramatic influx of sodium ions.

• The membrane rapidly depolarizes, reaching a peak potential of +30 millivolts.

• After this peak, sodium channels close, and potassium channels open, causing repolarization of the membrane.

• The membrane potential undergoes hyperpolarization momentarily before returning to the resting state of -70 millivolts.

• The neuron is then prepared to fire another action potential after the return to its resting state, ready for the next signal to transmit.