Chapter 13

• A synapse is a specialized site of contact of a neuron with another neuron or with an effector.
• It is the locus where one cell influences the function of another cell.
• In synaptic transmission, a presynaptic signal has an effect on a postsynaptic signal.
• Synaptic plasticity is the ability to change the functional properties of synapses.
• Ionotopic synaptic action is fast and produces direct changes in ion permeability and thus membrane potential.
• Metabotropic synaptic action is slow and produces chemical signal transduction changes in the postsynaptic cell.
• Synaptic transmission is usually chemical but can be electrical.
• In fast synaptic transmission, an action potential in a presynaptic neuron leads to a rapid postsynaptic voltage change.
• An electrical synapse has electrical currents that flow from one cell directly onto the next, changing its membrane potential.
• An electrical synapse has essentially no delay.
• Electrical synapses are often not polarized.
• Electrical synapses are found in nervous systems where speed is most important.
• A gap junction is a specialized locus where protein channels bridge the gap between two cells, directly connecting their cytoplasm.
• Gap junctions provide a low-resistance path for current flow, electrically coupling the cells that they join.
• Depolarization or hyperpolarization of one cell produces a weaker corresponding change in the other cell. ']
• Chemical synapses can modify and amplify signals.
• Chemical synapses have a discontinuity between the cells because the synaptic cleft of a chemical synapse is a barrier to direct electrical communication.
• The presynaptic electrical signals ar first transduced into a chemical signal: the release of neurotransmitter molecules from the presynaptic terminals.
• The axon terminal of the presynaptic neuron contains neurotransmitter molecules stored in synaptic vesicles.
• At the synaptic cleft, both the pre- and postsynaptic membranes appear denser and thicker than elsewhere because of local aggregation of proteins at these membranes.
• Active zones of synaptic vesicles release neurotransmitters into the synaptic cleft.
• A presynaptic neuron releases neurotransmitter molecules in response to an arriving action potential.
• Neurotransmitter is synthesized in the presynaptic neuron and stored in synaptic vesicles until release.
• The released neurotransmitter molecules bind to receptor proteins imbedded in the postsynaptic membrane.
• Neurotransmitter receptors are transmembrane proteins that are effectors for change in the postsynaptic cleft, usually producing a change in postsynaptic membrane potential.
• Transmission at chemical synapses is necessarily slower than transmission at electrical synapses because the steps of transmitter release and receptor action take more time.
• Chemical synapses can amplify current flow.
• Chemical synapses can be either excitatory or inhibitory.
• Electrical synapses are nearly always excitatory.
• A synaptic potential that tends to depolarize the cell membrane is excitatory.
• A synaptic potential that tends to hyperpolarize the cell membrane is inhibitory.
• Excitation is an increase in the probability that a cell will generate an impulse or cause an increase in the impulse frequency.
• Inhibition is a decrease in the probability of impulse generation or a decrease in impulse frequency.
• Excitatory and inhibitory synapses summate their voltage effects to control action-potential generation of the postsynaptic.
• Each excitatory synapse usually produces very small excitatory postsynaptic potentials (EPSPs), one that depolarizes the membrane by less than 1 mV.
• If a nerve is stimulated rapidly and repeatedly, the resultant EPSPs combine in a process called temporal summation.
• Simultaneously occurring EPSPs produced by different nerves also combine in a process called spatial summation.
• Inhibitory synapses produce synaptic potentials called inhibitory postsynaptic potentials (IPSPs) that drive the membrane potential away from threshold.
• Synapses excite or inhibit a neuron by depolarization or hyperpolarization at the site of impulse initiation.
• A neurons output is an integral function of its input, called neuronal integration.