depolarization

(noun)

Also called the rising phase, when positively charged sodium ions (Na+) suddenly rush through open voltage-gated sodium channels into a neuron.

Related Terms

(noun)

Depolarization is a change in a cell's membrane potential, making it more positive, or less negative, and may result in generation of an action potential.

Related Terms

Examples of depolarization in the following topics:

  • Electrical Events

    • Cardiac contraction is initiated in the excitable cells of the sinoatrial node by both spontaneous depolarization and sympathetic activity.
    • The SA node nerve cells are specialized in that they undergo spontaneous depolarization and generation of action potentials, without stimulation from the rest of the nervous system.
    • The SA node nerve impulses travel through the atria and cause muscle cell depolarization and contraction of the atria directly.
    • The AV node slows the neural impulse from the SA node by a slight amount, which causes a delay between depolarization of the atria and the ventricles.
    • The system of nerves that work together to set the heart rate and stimulate muscle cell depolarization within the heart.

  • Postsynaptic Potentials and Their Integration at the Synapse

    • At excitatory synapses, neurotransmitter binding depolarizes the postsynaptic membrane.
    • Since the electrochemical gradient of sodium is steeper than that of potassium, a net depolarization occurs.
    • If enough neurotransmitter binds, depolarization of the postsynaptic membrane can reach 0mV, which is higher than threshold of -30-50mV.  
    • This figure depicts the mechanism of temporal summation in which multiple action potentials in the presynaptic cell cause a threshold depolarization in the postsynaptic cell.

  • Mechanism and Contraction Events of Cardiac Muscle Fibers

    • The mechanism for CIRC is that receptors within the cardiomyocyte will bind to calcium ions when calcium ion channels open during depolarization, and will release more calcium ions into the cell.
    • Similarly to skeletal muscle, the influx of sodium ions causes an initial depolarization, however in cardiac muscle, the influx of calcium ions sustains the depolarization so that it lasts longer.
    • CICR creates a "plateau phase" in which the cell's charge stays slightly positive (depolarized) for longer before it becomes more negative as it repolarizes due to potassium ion influx.

  • Peripheral Motor Endings

    • These receptors open, allowing sodium ions to flow in and potassium ions to flow out of the muscle's cytosol, producing a local depolarization of the motor end plate known as an end-plate potential (EPP).
    • This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.
    • The action potential spreads through the muscle fiber's network of T-tubules, depolarizing the inner portion of the muscle fiber.
    • The depolarization activates L-type voltage-dependent calcium channels (dihydropyridine receptors) in the T-tubule membrane, which are in close proximity to calcium-release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum.

  • Electrocardiogram and Correlation of ECG Waves with Systole

    • The ECG works by detecting and amplifying tiny electrical changes on the skin that occur during heart muscle depolarization.
    • The P wave indicates atrial depolarization, in which the atria contract (atrial systole).
    • The QRS complex refers to the combination of the Q, R, and S waves, and indicates ventricular depolarization and contraction (ventricular systole).
    • The T Wave indicates ventricular repolarization, in which the ventricles relax following depolarization and contraction.
    • The ST segment refers to the gap (normally flat or slightly upcurved line) between the S wave and the T wave, and represents the time between ventricular depolarization and repolarization.

  • Membrane Potentials as Signals

    • In neurons, a sufficiently large depolarization can evoke an action potential in which the membrane potential changes rapidly.
    • This is called a depolarization if the interior voltage becomes more positive (say from –70 mV to –60 mV), or a hyperpolarization if the interior voltage becomes more negative (say from –70 mV to –80 mV).
    • In excitable cells, a sufficiently large depolarization can evoke an action potential , in which the membrane potential changes rapidly and significantly for a short time (on the order of 1 to 100 milliseconds), often reversing its polarity.

  • The Action Potential and Propagation

    • First is depolarization, followed by repolarization and a short period of hyperpolarization.
    • The depolarization, also called the rising phase, is caused when positively charged sodium ions (Na+) suddenly rush through open voltage-gated sodium channels into a neuron.

  • Microscopic Anatomy

    • Sarcomeres are connected to a plasma membrane, called a sarcolemma, by T-tubules, which speed up the rate of depolarization within the sarcomere.
    • Gap junctions, which connect proteins to the cytoplasm of different cells and transmit action potentials between both cells, and is required for cellular depolarization.

  • Fibrous Skeleton of the Heart

    • The separation allows the AV node and AV bundle to delay the wave of depolarization, such that the atria can contract and assist in ventricular filling before the ventricles themselves depolarize and contract.

  • Checking Circulation

    • During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventricular node down into the bundle of Hisand into the Purkinje fibers, spreading down and to the left throughout the ventricles.
    • This organised pattern of depolarization can be detected through electrodes placed on the skin and recorded as the commonly seen ECG tracing.