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| Across the Synapse |
How is information transferred from one neuron to the next? Neurons communicate at their meeting points, called synapses; the small gaps separating the neurons are referred to as the synaptic space. These synapses are not merely gaps but are functional links between the two neurons. Signals are transferred in only one direction across the synapse. The neuron that transmits information when it fires is called the presynaptic neuron. The synaptic terminals of the presynaptic neuron are on one side of the synapse; the dendrites of the other neuron, the postsynaptic neuron, are on the other side. Presynaptic and postsynaptic are relative adjectives; a postsynaptic neuron at one synaptic connection can be a presynaptic neuron at another synapse.
Synapses can be either chemical or electrical. An electrical synapse is what is often called a "gap junction," in which the membranes of two neurons are continuous at tiny spots, making the cells electrically contiguous. Gap junctions, which are not unique to neurons, allow for even more rapid communication. No chemical intermediary is involved in an electrical synapse. In the case of chemical synapses, however, chemicals called neurotransmitters are released from a presynaptic neuron, and dock with receptor proteins on the postsynaptic neuron. Such binding causes the shape of the protein to change and ion channels to open, much like the voltage-gated channels open in response to membrane potential changes (Fig. 3). We will discuss neurotransmitters in more detail below. Neurons are typically separated by about twenty to thirty nanometers in chemical synapses. Electrical synapses are more rapid than chemical ones but chemical synapses are easier to modulate. In vertebrates and many invertebrates, chemical synapses are more common than are electrical ones.
The action of the presynaptic neuron is referred to as an "all or none" response. A neuron can only fire or not fire; there is no "slightly activated" signal from a neuron. Whether or not a neuron will fire an action potential - that is, send a signal down its axon to be received by other neurons - depends on how many inputs it is receiving. It also depends on the nature of each input signal - excitatory or inhibitory - at each synapse. The sort of "net total" result of those signals determines whether the neuron will become excited, or depolarized, enough to fire an action potential and release neurotransmitter from its axon terminals.
Also recall that a signal traveling through the brain often involves many neurons, each making so many connections. This interconnectedness gives rise to the extraordinary complexity of the brain. The activation of a single sensory neuron could quickly lead to the activation or inhibition of thousands of neurons.