Neurons communicate through electrical signals called action potentials. These rapid, all-or-nothing events are triggered when the neuron's membrane potential reaches a threshold. Action potentials allow information to travel quickly along axons, forming the basis of neural signaling.

The resting membrane potential, typically around -70 mV, sets the stage for action potentials. It's maintained by ion concentration gradients and selective membrane permeability. When stimulated, voltage-gated ion channels open, causing a cascade of events that generate and propagate the action potential along the axon.

Resting membrane potential

Ionic basis of the resting membrane potential

• The resting membrane potential is the electrical potential difference across the cell membrane when the cell is at rest, typically around -70 mV in neurons
• Primarily determined by the concentration gradients of potassium (K+) and sodium (Na+) ions across the cell membrane
  • K+ is more concentrated inside the cell
  • Na+ is more concentrated outside the cell
• The cell membrane is more permeable to K+ than to Na+ at rest due to the presence of leak potassium channels, allowing K+ to diffuse out of the cell down its concentration gradient
• The Na+/K+ ATPase pump actively transports Na+ out of the cell and K+ into the cell, maintaining the concentration gradients of these ions across the membrane
• The unequal distribution of ions and the selective permeability of the cell membrane to these ions create the resting membrane potential, with the inside of the cell being more negative relative to the outside

Action potential generation and propagation

Steps involved in the generation of an action potential

• An action potential is a rapid, transient, and all-or-none electrical signal that propagates along the axon of a neuron
• Depolarization: When the membrane potential reaches the threshold (around -55 mV), voltage-gated Na+ channels open, allowing Na+ to rush into the cell, causing the membrane potential to become more positive
• Rising phase: As more voltage-gated Na+ channels open, the membrane potential rapidly rises towards the equilibrium potential for Na+ (around +40 mV)
• Peak: The membrane potential reaches its maximum value (around +40 mV) when the voltage-gated Na+ channels become inactivated
• Repolarization: Voltage-gated K+ channels open, allowing K+ to flow out of the cell, causing the membrane potential to become more negative. At the same time, voltage-gated Na+ channels close
• Hyperpolarization: The membrane potential becomes more negative than the resting potential due to the continued efflux of K+ through the open voltage-gated K+ channels

Propagation and refractory period of an action potential

• Propagation: The action potential propagates along the axon as the depolarization of one segment of the axon membrane causes the adjacent segment to reach the threshold, triggering a new action potential
• Refractory period:
  • During the absolute refractory period, the neuron cannot generate another action potential, as the voltage-gated Na+ channels are inactivated
  • During the relative refractory period, a stronger stimulus is required to generate an action potential
• The refractory period ensures unidirectional propagation of the action potential along the axon (from the axon hillock to the axon terminals)
• The presence of myelin sheaths and nodes of Ranvier in myelinated axons allows for faster propagation of action potentials through saltatory conduction

Graded vs Action potentials

Characteristics of graded potentials

• Graded potentials are local, variable-amplitude changes in the membrane potential that occur in response to stimuli
• Can be either depolarizing (excitatory) or hyperpolarizing (inhibitory), depending on the type of stimulus and the ion channels involved
• The amplitude of graded potentials depends on the strength of the stimulus
• Decay over short distances and do not propagate far from their site of origin
• Generated in dendrites and cell bodies

Characteristics of action potentials

• Action potentials are all-or-none, fixed-amplitude electrical signals that propagate along the axon
• Always depolarizing
• The amplitude of action potentials is constant and independent of the stimulus strength, provided the stimulus is above the threshold
• Can propagate over long distances without decaying
• Generated at the axon hillock and propagate along the axon

Voltage-gated ion channels in action potentials

Role of voltage-gated Na+ channels

• Voltage-gated Na+ channels are responsible for the rising phase of the action potential
• When the membrane potential reaches the threshold, these channels open, allowing Na+ to flow into the cell, causing depolarization
• Have two gates: an activation gate and an inactivation gate
  • The activation gate opens in response to depolarization
  • The inactivation gate closes with a slight delay, terminating the influx of Na+
• The opening and subsequent inactivation of voltage-gated Na+ channels contribute to the rising phase and the peak of the action potential

Role of voltage-gated K+ channels

• Voltage-gated K+ channels are responsible for the repolarization and hyperpolarization phases of the action potential
• Open in response to depolarization, but with a delay compared to the voltage-gated Na+ channels, allowing K+ to flow out of the cell
• The efflux of K+ through these channels brings the membrane potential back towards the resting state (repolarization) and even slightly below it (hyperpolarization)
• The coordinated opening and closing of voltage-gated Na+ and K+ channels shape the action potential waveform and enable the generation and propagation of electrical signals along the axon