Synapses

As defined earlier, a synapse is an anatomically specialized junction between two neurons, at which the electrical activity in one neuron, the presynaptic neuron, influences the electrical (or metabolic) activity in the second, postsynaptic neuron. Anatomically, synapses include parts of the presynaptic and postsynaptic neurons and the extracellular space between these two cells. According to the latest estimate, there are approximately 1014 (100 quadrillion!) synapses in the CNS.

When active, synapses can increase or decrease the likelihood that the postsynaptic neuron will fire action potentials by producing a brief, graded potential there. The membrane potential of a postsynaptic neuron is brought closer to threshold at an excitatory synapse, and it is either driven farther from threshold or stabilized at its present level at an inhibitory synapse.

Thousands of synapses from many different pre-synaptic cells can affect a single postsynaptic cell (convergence), and a single presynaptic cell can send branches to affect many other postsynaptic cells (divergence, Figure 8-24). Convergence allows information from many sources to influence a cell's activity; divergence allows one information source to affect multiple pathways.

Convergence And Divergence Neurons

FIGURE 8-24

Convergence of neural input from many neurons onto a single neuron, and divergence of output from a single neuron onto many others. Presynaptic neurons are shown in green and postsynaptic neurons in purple. Arrows indicate the direction of transmission of neural activity. [Q|

FIGURE 8-24

Convergence of neural input from many neurons onto a single neuron, and divergence of output from a single neuron onto many others. Presynaptic neurons are shown in green and postsynaptic neurons in purple. Arrows indicate the direction of transmission of neural activity. [Q|

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

The level of excitability of a postsynaptic cell at any moment (that is, how close its membrane potential is to threshold) depends on the number of synapses active at any one time and the number that are excitatory or inhibitory. If the membrane of the postsynap-tic neuron reaches threshold, it will generate action potentials that are propagated along its axon to the terminal branches, which influence the excitability of other cells by divergence.

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