Anatomy & Physiology

Anatomy & Physiology Study Guide

13.9 Synapses Synaptic Activity

In the nervous system, messages move from one location to another in the form of action potentials along axons. These electrical events are also known as nerve impulses. In order to be effective, a message must not only be propagated along an axon but also transferred in some way to another cell. At a synapse between two neurons, the impulse passes from the presynaptic neuron to the postsynaptic neuron. A synapse may also include other types of postsynaptic cells. Properties of Synapses A synapse may be electrical, with direct physical contact between the cells, or chemical, involving a neurotransmitter. Electrical Synapses At electrical synapses, the presynaptic and postsynaptic membranes are connected together at gap junctions. The lipid portions of both membranes are held in position by membrane proteins called connexons . These proteins contain pores that permit the passage of ions between the cells. Because the two cells are linked in this way, changes in the transmembrane potential of one cell will create local currents that disturb the other cell as if the two have a combined membrane. The result is an electrical synapse propagates action potentials quickly and efficiently from one cell to the next. Chemical Synapses The situation at a chemical synapse is far more dynamic than that at an electrical synapse because the cells are not directly coupled. An incoming action potential may or may not release enough neurotransmitter to bring the postsynaptic neuron to the maximum limit. In addition, other factors may intervene and make the postsynaptic cell more or less sensitive to arriving stimuli. In essence, the postsynaptic cell at a chemical synapse is not a slave to the presynaptic neuron; its activity can be adjusted, or “tuned,” by a variety of factors. Chemical synapses are by far the most abundant type of synapse. Most synapses between neurons, and all communications between neurons and other types of cells, involve chemical synapses. Normally, communication across a chemical synapse can occur in only one direction: from the presynaptic membrane to the postsynaptic membrane. Based on their effects on postsynaptic membranes, neurotransmitters are often classified as excitatory or inhibitory. Excitatory neurotransmitters cause depolarization and promote the generation of action potentials, whereas inhibitory neurotransmitters cause hyperpolarization and suppress the generation of action potentials. Synaptic Fatigue Because ACh molecules are recycled, the synaptic knob is not completely dependent on the ACh synthesized in the cell body. But under intensive stimulation, resynthesis and transport mechanisms may be unable to keep pace with the demand for neurotransmitter. Synaptic fatigue then occurs, and the synapse weakens until ACh has been replenished.

©2018

Achieve Page 153

of 368