The processes of spatial and temporal summation of PSPs in passive den-drites underlie much of the integrative activity of neurons, but additional features can also play significant roles. First, some dendrites have voltage-sensitive channels that can boost the amplitude of their input signals. One place where this has been studied is in output neurons of the thalamus of the vertebrate brain, which have the job of relaying sensory inputs to particular processing areas in the cortex. Second, there is not necessarily a physical separation between input and output regions of a neuron. Sometimes dendrites make output synapses as well as receiving inputs, and in some amacrine cells, input and output synapses are intermingled. This produces local circuits in the nervous system, where information can flow through processes of a number of different neurons without travelling very far. In the operation of local circuits, we cannot necessarily consider a whole neuron as an integrative unit because individual dendrites can act independently, summing local inputs to regulate output from their own synapses. The operation of local circuits has been particularly studied in the vertebrate olfactory system and retina (see Box 5.1, p. 107), and in circuits that control insect legs (see Chapter 8). Finally, the way that inhibition opposes excitation is not always by simple arithmetic summation of EPSPs and IPSPs. Strategically placed inhibitory synapses can make it hard for excitatory synapses to generate large EPSPs because, when postsynaptic channels at the inhibitory synapse open, they provide pathways for current flow through the cell membrane which effectively short circuits it. This reduces both the amplitudes of PSPs and, by reducing the neuron's space constant, the distance that they can travel.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.