O

Ion / channel

Receptor

Ion / channel

Ion Channel Plasma Membrane

G protein O a subunit

2} The receptor activates a G protein.

^ A G protein subunit activates an ion channel directly, or indirectly through a second messenger.

G protein O a subunit

2} The receptor activates a G protein.

^ A G protein subunit activates an ion channel directly, or indirectly through a second messenger.

Inside of cell

44.16 Metabotropic Receptors Act through G Proteins

Metabotropic receptors activate G proteins, which can influence ion channels directly or through second messengers.

the presynaptic and postsynaptic cell membranes are separated by a space of only 2 to 3 nm, and specific membrane proteins called connexons link the two neurons by forming molecular tunnels between the two cells. Ions and small molecules can pass directly from cell to cell through the connex-ons. Transmission at electrical synapses is very fast and can proceed in either direction, whereas transmission at chemical synapses is slower and unidirectional.

Electrical synapses are less common in the nervous systems of vertebrates than are chemical synapses for several reasons. First, electrical continuity between neurons does not allow temporal summation of synaptic inputs. Second, an effective electrical synapse requires a large area of contact between the presynaptic and postsynaptic cells. This condition rules out the possibility of thousands of synaptic inputs to a single neuron—which is the norm in complex nervous systems. Third, electrical synapses cannot be inhibitory. Finally, chemical synapses appear to have considerable plasticity (modifiability), as we will see below, but electrical synapses do not. Thus, electrical synapses are useful for rapid communication, such as that between muscle cells (as we will see in Chapter 47), but they are less useful for processes of integration and learning.

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Essentials of Human Physiology

Essentials of Human Physiology

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