Once depolarization is complete, the cell must now “reset” its membrane voltage back to the resting potential. Action potentials are considered an “all-or nothing” event, in that, once the threshold potential is reached, the neuron always completely depolarizes. Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV. Na + channels in the axon hillock open, allowing positive ions to enter the cell (Figure 1). A stimulus from a sensory cell or another neuron depolarizes the target neuron to its threshold potential (−55 mV). At excitatory synapses, this opening allows positive ions to enter the neuron and results in depolarization of the membrane-a decrease in the difference in voltage between the inside and outside of the neuron. When neurotransmitter molecules bind to receptors located on a neuron’s dendrites, ion channels open. Transmission of a signal within a neuron (from dendrite to axon terminal) is carried by a brief reversal of the resting membrane potential called an action potential. Transmission of a signal between neurons is generally carried by a chemical called a neurotransmitter. Explain the stages of an action potential and how action potentials are propagatedĪ neuron can receive input from other neurons and, if this input is strong enough, send the signal to downstream neurons.During this period, the neuron cannot respond to another stimulus, no matter how strong. The period from the opening of the sodium channels until the sodium channels begin to reset is called the absolute refractory period.
The propagation of action potential is independent of stimulus strength but dependent on refractory periods. This results in hyperpolarization as seen in a slight dip following the spike. A period of increased potassium permeability results in excessive potassium efflux before the potassium channels close. Hyperpolarization is a phase where some potassium channels remain open and sodium channels reset.This expulsion acts to restore the localized negative membrane potential of the cell. As the sodium ion entry declines, the slow voltage-gated potassium channels open and potassium ions rush out of the cell. As a result, the membrane permeability to sodium declines to resting levels. The repolarization or falling phase is caused by the slow closing of sodium channels and the opening of voltage-gated potassium channels.During this change of polarity the membrane actually develops a positive value for a moment (+40 millivolts). As additional sodium rushes in, the membrane potential actually reverses its polarity. 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.The action potential is a clear example of how changes in membrane potential can act as a signal. These three events happen over just a few milliseconds.Īction potential: A. First is depolarization, followed by repolarization and a short period of hyperpolarization. This moving change in membrane potential has three phases. When the membrane potential of the axon hillock of a neuron reaches threshold, a rapid change in membrane potential occurs in the form of an action potential. Neurons typically send signals over long distances by generating and propagating action potentials over excitable axonal membrane.Īction potential is a brief reversal of membrane potential where the membrane potential changes from -70mV to +30mV.