excitatory postsynaptic potential

Examples of excitatory postsynaptic potential in the following topics:

• Postsynaptic Potentials and Their Integration at the Synapse

  • Postsynaptic potentials are excitatory or inhibitory changes in the graded membrane potential in the postsynaptic terminal of a chemical synapse.
  • Postsynaptic potentials are changes in the membrane potential of the postsynaptic terminal of a chemical synapse.
  • Chemical synapses are either excitatory or inhibitory depending on how they affect the membrane potential of the postsynaptic neuron.
  • At excitatory synapses, neurotransmitter binding depolarizes the postsynaptic membrane.
  • This is an excitatory postsynaptic potential (EPSP) as it brings the neuron's potential closer to its firing threshold (about -50mV).

• 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.
  • A single neuron can receive both excitatory and inhibitory inputs from multiple neurons.

• Synaptic Plasticity

  • Short-term synaptic enhancement results from more synaptic terminals releasing transmitters in response to presynaptic action potentials.
  • Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection, which can last for minutes or hours.
  • Therefore, the next time glutamate is released from the presynaptic membrane, it will have a larger excitatory effect (EPSP) on the postsynaptic cell because the binding of glutamate to these AMPA receptors will allow more positive ions into the cell.
  • 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).

• Neurotransmitters

  • Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell (that is, decreasing the voltage gradient of the cell, thus bringing it further away from an action potential), while excitatory neurotransmitters cause depolarization (bringing it closer to an action potential).
  • The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein.
  • The depolarizing (more likely to reach an action potential) or hyperpolarizing (less likely to reach an action potential) effect is also dependent on the receptor.
  • When acetylcholine binds to the nicotinic receptor, the postsynaptic cell is depolarized.
  • Glutamate is considered an excitatory amino acid because Glu receptors in the adult cause depolarization of the postsynaptic cell.

• Types of Neurotransmitters by Function

  • They are released into and diffuse across the synaptic cleft, where they bind to specific receptors in the membrane on the postsynaptic side of the synapse.
  • 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).
  • NE, dopamine, and histamine can be excitatory or inhibitory depending on the receptor type.
  • 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.

• The Synapse

  • The neuron transmitting the electrical impulse away from the synapse is called postsynaptic neuron.
  • At a synapse , the presynaptic neuron sends information and postsynaptic neuron receives the information.
  • Most neurons, function as both as presynaptic and postsynaptic neurons.
  • Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together and carry out the signaling process.
  • Release of neurotransmitters usually follows arrival of an action potential at the synapse, but may also follow graded electrical potentials found in dendrites.

• Mechanics of the Action Potential

  • Resting potential.
  • If the membrane potential reaches -55 mV, it has reached the threshold of excitation.
  • Eventually, the cell potential reaches +40 mV, or the action potential.
  • This expulsion acts to restore the localized negative membrane potential of the cell.
  • Synaptic responses summate in order to bring the postsynaptic neuron to the threshold of excitation, so it can fire an action potential (represented by the peak on the chart).

• Classification of Neurons

  • The most straightforward classification is by the direction the action potentials (i.e. information) travel.
  • The combination of neurotransmitter and receptor properties results in an excitatory, inhibitory, or modulatory change to the target neuron.
  • Glutamate acts on several different types of receptors, and has effects that are largely excitatory.
  • Other types of neurons include excitatory motor neurons in the spinal cord that release acetylcholine, and inhibitory spinal neurons that release glycine.

• 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.
  • The neurotransmitter diffuses across the synaptic cleft, binding to receptor proteins on the postsynaptic membrane.
  • As long as it is bound to a post synaptic receptor, a neurotransmitter continues to affect membrane potential.
  • The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron.

• Stages of the Action Potential

  • The effect upon the postsynaptic (receiving) neuron is determined not by the presynaptic (sending) neuron or by the neurotransmitter itself, but by the type of receptor that is activated.
  • A neurotransmitter can be thought of as a key, and a receptor as a lock: the key unlocks a certain response in the postsynaptic neuron, communicating a particular signal.
  • Therefore, the neuron cannot reach action potential during this "rest period."
  • In other words, larger currents do not create larger action potentials.
  • The frequency of action potentials is correlated with the intensity of a stimulus.