action potential

Examples of action potential in the following topics:

• Nerve Impulse Transmission within a Neuron: Action Potential

  • As an action potential travels down the axon, the polarity changes across the membrane.
  • The action potential travels down the neuron as Na+ channels open.
  • Action potentials are considered an "all-or nothing" event.
  • Action potential "jumps" from one node to the next in saltatory conduction.
  • Action potentials travel down the axon by jumping from one node to the next.

• Excitation–Contraction Coupling

  • Excitation–contraction coupling is the connection between the electrical action potential and the mechanical muscle contraction.
  • It is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction .
  • Electrical signals called action potentials travel along the neuron's axon, which branches through the muscle, connecting to individual muscle fibers at a neuromuscular junction.
  • The depolarization then spreads along the sarcolemma and down the T tubules, creating an action potential.
  • The action potential triggers the sarcoplasmic reticulum to release of Ca2+, which activate troponin and stimulate muscle contraction.

• Signal Summation

  • Sometimes, a single excitatory postsynaptic potential (EPSP) is strong enough to induce an action potential in the postsynaptic neuron, but often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to be sufficiently depolarized to fire an action potential.
  • One neuron often has input from many presynaptic neurons, whether excitatory or inhibitory; therefore, inhibitory postsynaptic potentials (IPSPs) can cancel out EPSPs and vice versa.
  • The net change in postsynaptic membrane voltage determines whether the postsynaptic cell has reached its threshold of excitation needed to fire an action potential.
  • If the neuron only receives excitatory impulses, it will also generate an action potential.

• Transduction and Perception

  • If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential.
  • Thus, action potentials transmitted over a sensory receptor's afferent axons encode one type of stimulus.
  • The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor.
  • Thus, an intense stimulus will produce a more rapid train of action potentials.
  • Reducing the stimulus will likewise slow the rate of production of action potentials.

• 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.
  • As long as it is bound to a post synaptic receptor, a neurotransmitter continues to affect membrane potential.

• Control of Muscle Tension

  • The primary variable determining force production is the number of myofibers (long muscle cells) within the muscle that receive an action potential from the neuron that controls that fiber.
  • As mentioned above, increasing the frequency of action potentials (the number of signals per second) can increase the force a bit more because the tropomyosin is flooded with calcium.

• Transduction of Sound

  • 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.

• Synaptic Plasticity

  • Short-term synaptic enhancement results from more synaptic terminals releasing transmitters in response to presynaptic action potentials.
  • Synapses will strengthen for a short time because of either an increase in size of the readily- releasable pool of packaged transmitter or an increase in the amount of packaged transmitter released in response to each action potential.
  • Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection, which can last for minutes or hours.
  • The weakening and pruning of unused synapses trims unimportant connections, leaving only the salient connections strengthened by long-term potentiation.
  • Calcium entry through postsynaptic NMDA receptors can initiate two different forms of synaptic plasticity: long-term potentiation (LTP) and long-term depression (LTD).

• Nerve Impulse Transmission within a Neuron: Resting Potential

  • The resting potential of a neuron is controlled by the difference in total charge between the inside and outside of the cell.
  • For quiescent cells, the relatively-static membrane potential is known as the resting membrane potential.
  • As potassium is also the ion with the most-negative equilibrium potential, usually the resting potential can be no more negative than the potassium equilibrium potential.
  • The actions of the sodium-potassium pump help to maintain the resting potential, once it is established.
  • At the peak action potential, K+ channels open and the cell becomes (c) hyperpolarized.

• 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.
  • Solutes reduce water potential (resulting in a negative Ψw) by consuming some of the potential energy available in the water.
  • This is why solute potential is sometimes called osmotic potential.