action potential

Examples of action potential in the following topics:

• The Action Potential and Propagation

  • Action potential is a brief reversal of membrane potential where the membrane potential changes from -70mV to +30mV.
  • 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.
  • The propagation of action potential is independent of stimulus strength but dependent on refractory periods.
  • Schematic and B. actual action potential recordings.
  • The action potential is a clear example of how changes in membrane potential can act as a signal.

• Postsynaptic Potentials and Their Integration at the Synapse

  • Postsynaptic potentials are graded potentials and should not be confused with action potentials, although their function is to initiate or inhibit action potentials.
  • Unlike the action potential in axonal membranes, chemically-gated ion channels open on postsynaptic membranes.
  • Neurotransmitter binding at inhibitory synapses reduces a postsynaptic neuron's ability to generate an action potential.
  • A single EPSP at one synapse is generally far too small to trigger an action potential in the postsynaptic neuron.
  • 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.

• Membrane Potentials as Signals

  • In neurons, a sufficiently large depolarization can evoke an action potential in which the membrane potential changes rapidly.
  • In excitable cells, a sufficiently large depolarization can evoke an action potential , in which the membrane potential changes rapidly and significantly for a short time (on the order of 1 to 100 milliseconds), often reversing its polarity.
  • Action potentials are generated by the activation of certain voltage-gated ion channels.
  • Schematic and B. actual action potential recordings.
  • The action potential is a clear example of how changes in membrane potential can act as a signal.

• Microscopic Anatomy

  • Intercalated disks transmit electrical action potentials between sarcomeres.
  • Intercalated discs are gap junctions that link cardiomyocytes so that electrical impulses (action potentials) can travel between cells.
  • In cardiac muscle tissue, they are also responsible for transmission of action potentials and calcium during muscle contraction.
  • Gap junctions, which connect proteins to the cytoplasm of different cells and transmit action potentials between both cells, required for cellular depolarization.
  • Skeletal muscle contracts following activation by an action potential.

• Interactions of Skeletal Muscles

  • When stimulated by a single action potential a muscle contracts and then relaxes.
  • If an additional action potential were to stimulate a muscle contraction before a previous muscle twitch had completely relaxed then it would sum onto this previous twitch increasing the total amount of tension produced in the muscle.
  • For skeletal muscles, the force exerted by the muscle can be controlled by varying the frequency at which action potentials are sent to muscle fibers.
  • Action potentials do not arrive at muscles synchronously, and, during a contraction, only a certain percentage of the fibers in the muscle will be contracting at any given time.
  • If the frequency of action potentials generated increases to such a point that muscle tension has reached its peak and plateaued and no relaxation is observed then the muscle contraction is described as a tetanus.

• Peripheral Motor Endings

  • The highly excitable region of muscle fiber plasma membrane is responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract.
  • Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca2+ ions flow from the extracellular fluid into the presynaptic neuron's cytosol.
  • These receptors open to allow 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).
  • The action potential spreads through the muscle fiber's network of T-tubules, depolarizing the inner portion of the muscle fiber.
  • Skeletal muscle contracts following activation by an action potential.

• Mechanism and Contraction Events of Cardiac Muscle Fibers

  • The gap junctions spread action potentials to support the synchronized contraction of the myocardium.
  • Action potentials are the electrical stimulus that elicits the mechanical response in ECC.
  • An action potential, induced by the pacemaker cells in the sinoatrial (SA) and atrioventricular (AV) nodes, is conducted to contractile cardiomyocytes through gap junctions.
  • As the action potential travels between sarcomeres, it activates the calcium channels in the T-tubules, resulting in an influx of calcium ions into the cardiomyocyte.

• Principles of Electricity

  • This process, called action potential, underlies many nervous system functions.
  • Voltage is the measure of potential energy generated by separated charge.
  • Voltage may represent either a source of energy (electromotive force) or lost or stored energy (potential drop).
  • Differences in concentration of ions on opposite sides of a cellular membrane lead to a voltage called the membrane potential.
  • These concentration gradients provide the potential energy to drive the formation of the membrane potential.

• Types of Neurotransmitters by Function

  • Release of neurotransmitters usually follows arrival of an action potential at the synapse, but may also follow a graded electrical potential.
  • The neurotransmitters can also be classified based on function (excitatory or inhibitory) and action (direct or neuromodulatory).
  • The most prevalent transmitter in the human brain is glutamate, which promotes excitatory effects by  increasing the probability that the target cell will fire an action potential.

• Muscles that Cause Movement at the Hip Joint

  • Movement at the hip is similar to that of the shoulder joint, but due to increased weight-bearing requirements the range of potential movements is reduced.
  • Actions – Extends of the thigh and assists with rotation.
  • Actions: Adduction and flexing at the thigh at the hip joint.
  • Actions: Extends and laterally rotates at the hip.
  • The main action is flexing of the lower leg at the knee.