Page 9 - Cell biology PDG 2024

Almost all plasma membranes have an electrical potential
across them, with the inside usually negative with respect to the
outside.
Many ions have a concentration gradient across the membrane,
including potassium (K+), which is at a high concentration
inside and a low concentration outside the membrane. Sodium
(Na+) and chloride (Cl−) ions are at high concentrations in the
extracellular region, and low concentrations in the intracellular
regions (resting potential) or polarized state

Action potential is a reversal of membrane potential in which the
membrane potential changes from -70mV to +40mV

The action potential has three main stages: depolarization,
repolarization, and hyperpolarization.

Stimulation of the neuronal cell causes depolarization state, when
positively charged sodium ions (Na+) suddenly enters through open
voltage-gated sodium channels into a neuron. As additional sodium
rushes in, the membrane potential reaches (+40 millivolts).

The repolarization is caused by the closing of sodium channels and
the opening of voltage-gated potassium channels. As a result, the
membrane permeability to sodium declines to resting levels. As the
sodium ion entry decreases, the slow voltage-gated potassium
channels open and potassium ions rush out of the cell thus restoring
again the negative membrane potential of the cell.

Hyperpolarization is a phase where some potassium channels remain
open and sodium channels reset. A period of increased potassium
permeability results in excessive potassium efflux before the
potassium channels close

Absolute refractory period: The period from the opening of the
sodium channels until the sodium channels begin to reset. During this
period, the neuron cannot respond to another stimulus, no matter its
strength

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