depolarization

Examples of depolarization in the following topics:

• Nerve Impulse Transmission within a Neuron: Action Potential

  • Signals are transmitted from neuron to neuron via an action potential, when the axon membrane rapidly depolarizes and repolarizes.
  • Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV.
  • Once the threshold potential is reached, the neuron completely depolarizes.
  • As soon as depolarization is complete, the cell "resets" its membrane voltage back to the resting potential.
  • The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.

• 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.

• Nerve Conduction and Electrocardiograms

  • This inrush of Na+ first neutralizes the inside membrane (called depolarization), and then makes it slightly positive.
  • The adjacent membrane depolarizes, affecting the membrane farther down, and so on.
  • Just as nerve impulses are transmitted by depolarization and repolarization of an adjacent membrane, the depolarization that causes muscle contraction can also stimulate adjacent muscle cells to depolarize (fire) and contract.
  • An electrocardiogram (ECG) is a record of the voltages created by the wave of depolarization (and subsequent repolarization) in the heart.
  • The P wave is generated by the depolarization and contraction of the atria as they pump blood into the ventricles.

• Stages of the Action Potential

  • Neural impulses occur when a stimulus depolarizes a cell membrane, prompting an action potential which sends an "all or nothing" signal.
  • Depolarization: A stimulus starts the depolarization of the membrane.
  • The refractory phase takes place over a short period of time after the depolarization stage.
  • During the refractory phase this particular area of the nerve cell membrane cannot be depolarized.
  • A neuron must reach a certain threshold in order to begin the depolarization step of reaching the action potential.

• 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.

• Synaptic Transmission

  • When an action potential reaches the axon terminal, it depolarizes the membrane and opens voltage-gated Na+ channels.
  • Na+ ions enter the cell, further depolarizing the presynaptic membrane.
  • This depolarization causes voltage-gated Ca2+ channels to open.
  • When the presynaptic membrane is depolarized, voltage-gated Ca2+ channels open and allow Ca2+ to enter the cell.
  • The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron.

• 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.

• Excitation–Contraction Coupling

  • This reduces the voltage difference between the inside and outside of the cell, which is called depolarization.
  • As ACh binds at the motor end plate, this depolarization is called an end-plate potential.
  • The depolarization then spreads along the sarcolemma and down the T tubules, creating an action potential.

• 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.