The absolute refractory period refers to a period during the action potential. This is the time during which another stimulus given to the neuron (no matter how strong) will not lead to a second action potential. The absolute refractory period starts immediately after the initiation of the action potential and lasts until after the peak of the action potential. Following this period, the relative refractory period begins.

Relative refractory period

Neuronal Action Potential - Refractory Periods

The action potential is a rapid and reversible reversal of the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and some endocrine cells. In a neuronal action potential, the membrane potential rapidly changes from its resting level of approximately -70 mV to around +50 mV and, subsequently, rapidly returns to the resting level again. The neuronal action potential forms an important basis for information processing, propagation, and transmission. In muscle cells, the action potential precedes, and is necessary to bring about, muscle contraction. Some endocrine cells also exhibit action potentials, where the excitation leads to hormone secretion.

The action potential is also referred to as the electrical impulse or nervous impulse.

Graded potential

Neuronal Action Potential

All-or-nothing is usually used when describing the action potential. It refers to the well-known observation that an action potential always occurs in its full size (i.e., full magnitude of voltage change).

Many physiologists use all-or-nothing and all-or-none interchangeably.

Important Features of the Neuronal Action Potential

Proposed model for the inactivation of some voltage-gated ion channels. According to this model, after channel opening, the pore of the open channel is plugged by a globular cytoplasmic portion of the channel protein. The globular portion (ball of amino acids) is tethered to the rest of the protein by a linker part (chain of amino acids).

Channel inactivation

Neuronal Action Potential - Important Features of the Neuronal Action Potential

Refers to a conformational change of a channel protein by which the channel goes from the open state to the inactive state. The inactive state refers to a conformational state in which ions are not allowed to permeate the channel pore. Thus, with respect to ion permeability, the inactive state is similar to the close state of the channel. Ions cannot permeate the channel pore either in the closed or inactive state. However, the channel assumes very distinct and different conformations in the inactive state and closed state.

Neuronal Action Potential - Important Features of the Neuronal Action Potential

An electrophysiological technique in which the current passing across the cell membrane is controlled experimentally, and the membrane voltage is measured.

Voltage clamp

Neuronal Action Potential - Pharmacological Inhibition of Na⁺ and K⁺ Channels

Electrophysiology is the study of the electrical properties of biological macromolecules, cells, tissues, and organs. Electrical signals such as voltage and/or current are generally measured. Examples include measuring changes in the membrane voltage of excitable cells (e.g., neurons, muscle cells, and some endocrine cells) during an action potential. The current carrried by ions as they permeate the pore of ion channels can also be measured - both at the single-channel level (single-channel current), as well as the macroscopic current resulting from the activity of a population of channels. As another example, electrical measurements may involve recording voltage changes at the surface of the skin that result from the activity of skeletal muscles (electromyogram, EMG), cardiac myocytes (electrocardiogram, ECG), or neurons in the brain (electroencephalogram, EEG).

Refers to the ability of some cells to be electrically excited resulting in the generation of action potentials. Neurons, muscle cells (skeletal, cardiac, and smooth), and some endocrine cells (e.g., insulin-releasing pancreatic β cells) are excitable cells.

Resting Membrane Potential - Introduction

Refers to synaptic or receptor potentials that can vary in amplitude and direction. Graded potentials can be depolarizing or hyperpolarizing and do not have a threshold.

Action potential

Neuronal Action Potential - Introduction
Neuronal Action Potential - Graded Potentials versus Action Potentials

The Hodgkin cycle represents a positive feedback loop in neurons, where an initial membrane depolarization from the resting value (∼ −70 mV) to the threshold value (∼ −50 mV) leads to rapid depolarization of the membrane potential to approach the equilibrium potential for Na⁺ (V_Na ≈ +60 mV). The voltage-gated Na⁺ channels of neurons are responsible for the Hodgkin cycle.

See the figure depicting the Hodgkin cycle.

Important Features of the Neuronal Action Potential

Refers to the hyperpolarization phase of the action potential.

It is also referred to as undershoot.

See figure.

Hyperpolarization

Important Features of the Neuronal Action Potential

In electrophysiological convention, a negative current value or downward deflection of a current trace is typically referred to as an inward current. A negative current value (i.e., inward current) can reflect either the movement of positive ions (cations) into the cell or negative ions (anions) out of the cell.

Neuronal Action Potential - Pharmacological Inhibition of Na⁺ and K⁺ Channels

Lidocaine is a local anesthetic and an antiarrhythmic drug. It is a commonly used local anesthetic for minor surgery and in dental procedures. Lidocaine is also used topically to relieve itching, burning, and pain from skin inflammations.

Lidocaine's mechanism of action is to block fast voltage-gated Na⁺ channels of neurons and cardiac myocytes.

Other names used for lidocaine are xylocaine and lignocaine.

Pharmacological Inhibition of Na⁺ and K⁺ Channels

Lidocaine (Wikipedia)

Refers to the action potential of neurons. It is also referred to as the nerve impulse.

Action potential

Neuronal Action Potential

Neurotoxins are chemical molecules that have an adverse effect on neuron function and, thus, disrupt the normal function of the nervous system. Neurotoxins could be small molecules or peptides and can be derived from a variety of invertebrate and vertebrate animals, as well as plant species.

The following is a short list of some examples of neurotoxins:

α-Bungarotoxin: A peptide neurotoxin that inhibits the nicotinic acetylcholine receptor.

Chlorotoxin: A peptide neurotoxin that inhibits chloride channels.

α-Conotoxin: A peptide neurotoxin that inhibits the nicotinic acetylcholine receptor.

δ-Conotoxin: A peptide neurotoxin that inhibits voltage-gated sodium channels.

w-Conotoxin: A peptide neurotoxin that inhibits N-type voltage-gated calcium channels.

Picrotoxin: Inhibits GABA_A receptor chloride channels.

Tetrodotoxin: Inhibitor of neuronal voltage-gated sodium channels.

In electrophysiological convention, a positive current value or upward deflection of the current trace is typically referred to as an outward current. A positive current value (i.e., outward current) can reflect either the movement of positive ions (cations) out of the cell or negative ions (anions) into the cell.

Inward current

Neuronal Action Potential - Pharmacological Inhibition of Na⁺ and K⁺ Channels

Refers to that part of the action potential where the membrane potential is positive (inside with respect to the outside).

See figure.

Neuronal Action Potential - Important Features of the Neuronal Action Potential

A local topical anesthetic.

Procaine's mechanism of action is to block fast voltage-gated Na⁺ channels of neurons.

Pharmacological Inhibition of Na⁺ and K⁺ Channels

Procaine (Wikipedia)

The voltage difference across a cell plasma membrane in the resting or quiescent state. It is also simply referred to as the resting potential (V_rest). The value of the resting membrane potential varies from cell to cell. Depending on the cell type, it can range from −90 mV to −20 mV.

For example, V_rest is −90 mV in skeletal and cardiac muscle cells as well as in astrocytes. In a typical neuron, V_rest is approximately −70 mV. In many non-excitable cells, V_rest ranges from −60 to −50 mV. In photoreceptors, V_rest is about −20 mV.

Resting membrane potential

Refers to the rapid depolarization of the membrane early in the action potential. In neuronal, skeletal muscle, and cardiac muscle action potentials, the Hodgkin cycle is responsible for the spike phase of the action potential.

See figure.

Important Features of the Neuronal Action Potential

A rectangular signal waveform used in physiological studies to perturb (i.e., challenge) the system under study. The response of the system to the pulse is then studied carefully to learn about how the system responds to challenges.

Examples include pulses of voltage or current in electrophysiological experiments. Other examples include pulses of light, pressure, temperature, ligand, etc.

A square-wave pulse is defined by the amplitude and duration of the pulse, as well as by the frequency at which the pulse is applied to the system under study.

Neuronal Action Potential - Introduction

Square wave (Wikipedia)

Sub-threshold (or subthreshold) refers to a stimulus that is too small in magnitude to produce an action potential in excitable cells.

In general, a sub-threshold stimulus leads to the depolarization of the membrane, but the magnitude of the depolarization is not large enough to reach the threshold voltage. Therefore, sub-threshold stimuli do not elicit action potentials.

Threshold
Supra-threshold

Neuronal Action Potential - Introduction

Supra-threshold (or suprathreshold) refers to a stimulus that is large enough in magnitude to produce an action potential in excitable cells.

In general, a supra-threshold stimulus leads to the depolarization of the membrane, and the magnitude of the depolarization is larger than that necessary to simply reach the threshold voltage. Therefore, supra-threshold stimuli elicit action potentials.

Threshold
Sub-threshold

Neuronal Action Potential - Introduction

TEA

An inhibitor of voltage-gated potassium (K⁺) channels.

TEA is a quaternary ammonium cation (positively charged ion). It is also commonly used as a cation replacement for sodium (Na⁺) in physiological buffers used in ion replacement experiments.

Pharmacological Inhibition of Na⁺ and K⁺ Channels

TTX

Inhibitor of fast voltage-gated sodium (Na⁺) channels of neurons and muscle cells. It is an extremely potent and toxic neurotoxin.

Pharmacological Inhibition of Na⁺ and K⁺ Channels

Tetrodotoxin (Wikipedia)

The membrane voltage that must be reached in an excitable cell (e.g., neuron or muscle cell) during a depolarization in order to generate an action potential. At the threshold voltage, voltage-gated channels become activated. Threshold is approximately −50 to −40 mV in most excitable cells.

Sub-threshold
Supra-threshold

Neuronal Action Potential - Introduction

Refers to the hyperpolarization phase of the action potential.

It is also referred to as hyperpolarizing afterpotential.

See figure.

Hyperpolarization

Important Features of the Neuronal Action Potential

An electrophysiological technique in which the voltage of a cell membrane is controlled experimentally, and the current passing across the membrane is measured.

Current clamp

Neuronal Action Potential - Pharmacological Inhibition of Na⁺ and K⁺ Channels