Figure 41.16 illustrates spatial and temporal summation. Most chemically gated synapses are fast, but some are slow. Figure 41.17 illustrates the transduction system utilized by slow synapses. Electrical Synapses Electrical synapses directly couple neurons. The impulse can travel in either direction.
Electrical synapses are more common in invertebrates.
Several types of receptors may exist for a given neurotransmitter. Hence, a given neurotransmitter can be excitatory in one tissue and inhibitory in a second tissue or even within a single tissue. For example, acetylcholine is excitatory in the CNS when it combines with nicotinic receptors. It is inhibitory when it combines with muscarinic receptors. Table 41.1 summarizes some of the best known neurotransmitters and their effects.