membrane potential

Examples of membrane potential in the following topics:

• Nerve Impulse Transmission within a Neuron: Resting Potential

  • The difference in total charge between the inside and outside of the cell is called the membrane potential.
  • For quiescent cells, the relatively-static membrane potential is known as the resting membrane potential.
  • The resting membrane potential is at equilibrium since it relies on the constant expenditure of energy for its maintenance.
  • Voltage-gated ion channels are closed at the resting potential and open in response to changes in membrane voltage.
  • The (a) resting membrane potential is a result of different concentrations of Na+ and K+ ions inside and outside the cell.

• Nerve Impulse Transmission within a Neuron: Action Potential

  • As an action potential travels down the axon, the polarity changes across the membrane.
  • Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV.
  • As K+ ions leave the cell, the membrane potential once again becomes negative.
  • The diffusion of K+ out of the cell hyperpolarizes the cell, making the membrane potential more negative than the cell's normal resting potential.
  • The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.

• Transduction and Perception

  • The change in electrical potential that is produced is called the receptor potential.
  • Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential .
  • In the nervous system, a positive change of a neuron's electrical potential (also called the membrane potential), depolarizes the neuron.
  • If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential.
  • In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.

• Synaptic Transmission

  • Neurotransmission at a chemical synapse begins with the arrival of an action potential at the presynaptic axon terminal.
  • 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.
  • These SNARE proteins are involved in the membrane fusion.
  • As long as it is bound to a post synaptic receptor, a neurotransmitter continues to affect membrane potential.

• Components of Plasma Membranes

  • The plasma membrane protects the cell from its external environment, mediates cellular transport, and transmits cellular signals.
  • The plasma membrane (also known as the cell membrane or cytoplasmic membrane) is a biological membrane that separates the interior of a cell from its outside environment.
  • The primary function of the plasma membrane is to protect the cell from its surroundings.
  • The membrane also maintains the cell potential.
  • The cell employs a number of transport mechanisms that involve biological membranes:

• Transduction of Sound

  • Above the basilar membrane is the tectorial membrane.
  • Lower frequencies travel farther along the membrane before causing appreciable excitation of the membrane.
  • Their bending results in action potentials in the hair cells, and auditory information travels along the neural endings of the bipolar neurons of the hair cells (collectively, the auditory nerve) to the brain.
  • When the hairs bend, they release an excitatory neurotransmitter at a synapse with a sensory neuron, which then conducts action potentials to the central nervous system.
  • Movement of stereocilia on hair cells results in an action potential that travels along the auditory nerve.

• Excitation–Contraction Coupling

  • Neurotransmitter release occurs when an action potential travels down the motor neuron's axon, resulting in altered permeability of the synaptic terminal membrane and an influx of calcium.
  • The Ca2+ ions allow synaptic vesicles to move to and bind with the presynaptic membrane (on the neuron) and release neurotransmitter from the vesicles into the synaptic cleft.
  • As a neurotransmitter binds, these ion channels open, and Na+ ions cross the membrane into the muscle cell.
  • 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.

• Water and Solute Potential

  • Water potential is the measure of potential energy in water and drives the movement of water through plants.
  • Water potential is a measure of the potential energy in water, or the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature).
  • Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water.
  • This is why solute potential is sometimes called osmotic potential.
  • In this example with a semipermeable membrane between two aqueous systems, water will move from a region of higher to lower water potential until equilibrium is reached.

• Chemiosmosis and Oxidative Phosphorylation

  • Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient.
  • The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient) owing to the hydrogen ions' positive charge and their aggregation on one side of the membrane.
  • However, many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels.
  • Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through a membrane protein called ATP synthase.
  • The turning of this molecular machine harnesses the potential energy stored in the hydrogen ion gradient to add a phosphate to ADP, forming ATP.

• Lysosomes

  • Lysosomes are organelles that digest macromolecules, repair cell membranes, and respond to foreign substances entering the cell.
  • The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle.
  • A lysosome is composed of lipids, which make up the membrane, and proteins, which make up the enzymes within the membrane.
  • The general structure of a lysosome consists of a collection of enzymes surrounded by a single-layer membrane.
  • A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium and then fuses with a lysosomes within the cell to destroy the pathogen.